# Novel Dilute Bismide, Epitaxy, Physical Properties and Device Application

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## Abstract

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## 1. Brief History of Epitaxial Growth of Dilute Bismides

^{19}years (the age of our universe is about 1.4 × 10

^{9}years for comparison) via-decay [1]. Bismuth has been used traditionally in fuse, thermoelectric products, cosmetics and medicine, etc. Surprisingly, the toxicity of Bi is even less than table salt (NaCl) [2] and it is therefore regarded as green element compared with its neighboring heavy metals like lead (Pb), antimony (Sb), arsenic (As) and mercury (Hg), etc. [3]. For this sustainability reason, Bi has recently attracted great attention to replace the toxic lead products in industry [4], and the price for Bi mine production has increased five times since 2000 [1]. While Bi is the last element in the group-V column in the Periodic Table, it has been largely neglected as a member of the III-V compound semiconductor family which plays a significant role in modern electronic and optoelectronic device applications nowadays. This is, to large extent, due to the difficulties in synthesizing high quality binary III-Bi crystals and alloying Bi into host III-Vs, which will be one of the main topics of this review paper. Synthesis of InBi alloy dates back to 1950s [5,6] and the X-ray crystallographic analysis revealed a PbO crystal structure, in contrast to the zinc-blende (ZB) structure found in majority III-V crystals. Perhaps the first interest to incorporate Bi into III-Vs was stimulated by the isoelectronic trap discovered in O-doped ZnTe in 1962 [7] and later in N-doped GaP in 1965 [8]. When the substituted Group-V element has a large difference in electronegativity (3.04, 2.19, 2.18, 2.05 and 2.02 in Pauling scale for N, P, As, Sb and Bi, respectively) with respect to the host Group-V element, isoelectronic traps can be formed. The incorporation of N in GaP forms traps for electrons with an impurity energy level close to the GaP conduction band (CB) [8]. The charged N

_{P}can bind a hole through Coulomb interaction forming a bound exciton emitting bright orange luminescence with energy below the bandgap of GaP. When Bi is incorporated in GaP, the Bi

_{P}isoelectronic atom traps a hole with a trapping energy close to valence band (VB) and becomes charged [9]. It then attracts an electron forming a bound exciton and showing the similar effect in luminescence. Such an isoelectronic trap effect was verified also in Bi-doped InP later [10]. In these samples, the Bi concentration is in the doping level of below 10

^{18}cm

^{−3}. Sharp bound exciton and phonon assisted photoluminescence (PL) signals are observed at low temperatures.

_{1−x}Bi

_{x}. The cutoff wavelength moves from 5.5 µm for InSb to 8.3 µm for InSb

_{1−x}Bi

_{x}with x = 2.2% at 77 K [14]. Polycrystalline InSbBi bulks were also demonstrated later from several groups [17,18,19]. Zilko and Greene used multitarget RF sputtering to synthesize single-crystal metastable InSbBi bulk on (110) GaAs substrates [20] and studied phase stability. The first InSbBi single crystal thin film evidenced by unambiguous X-ray diffraction was reported by Oe et al. from Japan on (100) InSb substrates using molecular beam epitaxy (MBE) in 1981 [21]. Bismuth was incorporated only under the In-rich growth condition and up to 5% Bi concentration was demonstrated when grown at 250 °C. However, crystal surface was covered by In/Bi droplets, and the growth window for achieving InSbBi with both good surface and structural quality is very narrow. Clear wavelength red-shift was observed from InSbBi alloy annealed at 420 °C in photovoltaic response at 100 K, confirming the bandgap reduction after Bi incorporation.

_{x}Sb

_{1−x}, which has the lowest bandgap of 0.1 eV and 0.145 eV at 300 K and 77 K for x = 0.4, respectively, and pushing toward long absorption wavelengths by adding Bi. Humphreys et al. from North Caroline State University in US reported the first MOCVD growth of InAsSbBi with a maximum Bi content of 4% in 1988 [24]. Ma et al. demonstrated the first successful growth of InAs

_{1−x}Bi

_{x}(x ≤ 2.6%) [25]. Clear PL peak shifts were observed with Bi composition at 10 K and a bandgap reduction of 55 meV/% Bi was deduced for InAsBi. Lee et al. from Northwestern University in US reported InSbBi/InSb heterostructure photodetectors [30]. The voltage responsivity at 10.6 µm was about 1.9 mV/W at 300 K, corresponding to the Johnson noise limited detectivity of 1.2 × 10

^{6}cmHz

^{1/2}/W. A comprehensive review on early work on III-Sb(Bi) and device applications can be found in reference [31]. Later, Oe and Okamoto reported the new semiconductor alloy of GaAs

_{1−x}Bi

_{x}with x = 2% by MOCVD in 1998 [32]. PL was observed at 10–300 K and the PL peak energy was found to be relatively insensitive to temperature: 0.1 meV/K. Meanwhile, there was also some interest in using Bi to compensate for a high and intrinsic p-type background in GaSb substrates due to the defect formation of Ga

_{Sb}V

_{Sb}in liquid phase epitaxy (LPE) [33,34,35,36]. For example, Chaldishev et al. from Ioffe Institute in Russia demonstrated a p-type background reduction from 10

^{17}cm

^{−3}to a few 10

^{15}cm

^{−3}by adding 10

^{19}cm

^{−3}Bi in GaSb during the LPE growth [35]. The mechanism is that Bi incorporation efficiently reduces Sb-vacancies and thus minimizes the antisite Ga

_{Sb}defects which are the origin of the high p-type background in GaSb.

_{1−x}Bi

_{x}(x ≤ 3.1%) grown by MBE and discovered a giant bandgap reduction of 84 meV/% Bi [44]. The promising results mimicking that found in GaNAs [38] and GaNP [39] has spurred research community to pay attention to this novel bismide alloys. As N and Bi mainly affect CB and VB of III-Vs, respectively, it is convenient to tune electronic property of electrons and holes independently. Compensation of atomic size of N and Bi in III-Vs mitigates the lattice mismatch problem and offers a large degree for strain engineering. The interest for GaAs(N)Bi alloy has increased since 2005, with extensive investigations of GaAs(N)Bi followed by searching for other dilute bismide in recent years. Figure 1 shows number of publications on dilute bismide since 1997. The number has increased at an accelerated pace in recent years. An annual dedicated international workshop on Bi containing compounds was initiated in 2010, and the first book specialized on Bi containing compounds was published in 2013 [45].

## 2. Theoretical Prediction

**k·p**band method, tight binding (TB) model and band anticrossing (BAC) theory, are also effective to give explanations in practice. All these methods and computational techniques have been used to various types of Bi-containing structures including surfaces, thin films, nanostructures and clusters. In Section 2.1, empirical models for electronic band properties are presented. In Section 2.2, we discuss structural, electronic and optical predictions by first principle calculations. In Section 2.3, some other methods are presented.

#### 2.1. Empirical Models for Electronic Band Properties

**k·p**band models, which are all based on solving the Hamiltonians. Briefly speaking, the BAC (VBAC) model is mostly used when impurity states are involved, and the TB model is constructed using the atomic orbitals while the

**k·p**band model includes the CB, heavy hole (HH), light hole (LH) and SO split-off band around some high symmetry points in reciprocal space, e.g., the Γ-point. More detailed descriptions of these models can be found elsewhere. A brief summary of the practical usage in Bi-containing III-V systems is given in Table 1.

#### 2.1.1. BAC/VBAC Model

_{SO}> E

_{g}condition, which is an optimal condition to suppress non-radiative CHSH (Conduction-Heavy hole Spin-orbit-Heavy hole) Auger recombination and IVBA, and to improve the performance in high temperature and thermal stability of light emitting devices [53,59,61,62]. Carrier mobility and concentrations for the In

_{y}Ga

_{1−y}Bi

_{x}As

_{1−x}system have also been computed with this model [62].

_{x}As

_{1−x}can be explained in terms of a VBAC interaction by Alberiet et al. [59]. Wu et al. [65] have investigated the bandgap bowing and SO split energy of InP

_{1−x}Bi

_{x}using the VBAC model and the results show a good agreement with the experimental data.

#### 2.1.2. TB Model

^{3}s

^{∗}TB methods to calculate dilute bismide alloys GaBi

_{x}As

_{1−x}and GaBi

_{x}P

_{1−x}. Their results show the evolution of the highest valence state in GaBi

_{x}Y

_{1−x}(Y = P, As). Strong bandgap reduction and change of CB edge energy could be predicted using a BAC model. Virkkala et al. [68] have investigated GaAs

_{1−x}Bi

_{x}alloys including the SO coupling in the TB model for directional Bi-Bi interaction. The results indicate that Bi states mix with host material states. The states near the VB edge agglomerate along the zigzag chains which are due to Bi-Bi interactions in a random alloy, an energy broadening in these states and the bandgap narrowing.

_{1−x}Bi

_{x}samples on GaAs (100) substrate. The atom positions in alloy are determined using a VFF strain energy minimization scheme. Their theoretical results are in good agreement with the experimental data and reproduce the crossover at 9% Bi composition, beyond which Δ

_{so}exceeds the bandgap, E

_{g}, and suggests a type-I band alignment at the interface between GaAs

_{1−x}Bi

_{x}and GaAs.

#### 2.1.3. The **k·p** Model

**k·p**model is used to analyze the electronic structure of conventional semiconductor lasers, which includes the CB, HH, LH and SO split-off bands. By adding additional spin-degenerate impurity-related states to the eight-band

**k·p**model, the 10-band, 12-band and 14-band

**k·p**models can be obtained, which are used for GaInNAs with 10 bands [71], (In)GaBi

_{x}As

_{1}

_{-x}with 12 bands [72] and GaBi

_{x}N

_{y}As

_{1−x−y}with 14 bands [73] models, respectively. Fluegel et al. [70] studied electron effective mass of n-type GaAs

_{1−x}Bi

_{x}in the range of 0% < x < 0.88% using magnetic field and temperature dependent resistivity measurements and the

**k·p**formalism. The overall decrease in electron effective mass is obtained and giant bandgap bowing and giant SO bowing effects agree with the perturbation theory.

#### 2.1.4. Combination of Empirical Models

**k·p**theory [58], a rapid reduction in bandgap with increasing Bi concentration and a very large SO split energy is predicted, and the gain and loss characteristics of ideal dilute nitride and bismide lasers should be at least as good as those of conventional InP-based telecomm lasers for GaAs

_{1−x}N

_{x}and GaAs

_{1-x}Bi

_{x}. Experimentally, optical spectroscopic techniques were used to explore the optical properties of In

_{0.53}Ga

_{0.47}Bi

_{x}As

_{1−x}/InP. Trying to achieve Δ

_{SO}> E

_{g}and a reduced temperature dependence of E

_{g}are of great significance for suppressing recombination losses and improving performance in mid-IR photonic devices.

**k·p**formalism to investigate VB structure of InAs

_{1−x}Bi

_{x}and InSb

_{1−x}Bi

_{x}alloy systems. The bandgap reduction results agree fairly well with the experimental data. The upward shifts in the heavy/light hole E

_{+}level and SO split-off level E

_{SO+}are observed in both cases. This upward movement of the E

_{+}band is primarily responsible for the bandgap reduction in these Bi containing alloys. Later they [74] used a mathematical model based on Quantum Dielectric Theory (QDT) to calculate the direct bandgap, E

_{0}, of Bi-containing ternary alloys. The variation of E

_{0}with x for InSb

_{1−x}Bi

_{x}and GaSb

_{1−x}Bi

_{x}is in a good agreement with the experimental results. The composition dependence of E

_{0}at different temperatures is also found for some other ternary alloys like InPBi and AlSbBi.

#### 2.2. Structural, Electronic and Optical Properties Predicted by First Principle Calculations

#### 2.2.1. Structural Property

#### Thin Films—Binary III-Bi Compounds

#### GaAs_{1−x}Bi_{x}

_{1−x}Bi

_{x}were also calculated by Abdiche et al. [78], where the strong nonlinear dependence of the Bi composition was also shown.

_{1−x}Bi

_{x}structure, the equilibrium solubility of epitaxial GaAs

_{1−x}Bi

_{x}could be greatly enhanced, yielding a Bi solubility of x = 0.083.

#### InAs_{1−x}Bi_{x} and InSb_{1−x}Bi_{x}

#### AlN_{1−x}Bi_{x}

_{1−x}Bi

_{x}[81,82] alloys using the WIEN2k code. With increasing Bi composition, the lattice constant deviates from the value obtained by Vegard’s law and the bandgap decreases. It remains a semiconductor until x = 0.5 and becomes a semimetal for x greater than or equal to 0.625. It displays a strong bandgap bowing parameter and a transition from indirect (AlN) to direct (AlNBi) band gap at x close to 0.02.

#### InP_{1−x}Bi_{x}

_{In}is energetically easier than Bi

_{P}and has not significantly contributed to the reduction of the bandgap of InP:Bi alloy [83].

#### Surface

#### Nanostructures

_{1−x}Bi

_{x}alloys and (GaAs)

_{m}/(GaBi)

_{n}superlattices (SLs) using full potential linear augmented plane wave (FPLAPW) method as implemented in the LmtART code and the full potential linear muffin-tin orbital (FPLMTO) method as implemented in the WIEN97 code. In both cases, they are possibly semiconductors when the proportion of GaBi is no more than that of GaAs and the semi-metallic property appears if the proportion of GaAs is lower.

_{x}Bi (x = 1–4) [88], BiGa

_{y}[89] and Bi

_{x}In

_{y}[90], using experimental results (photoelectron spectroscopy, velocity map imaging) and first principle theoretical methods. The results of structures, electronic energy levels, molecular orbitals and photoelectron spectra for all the Bi containing clusters are obtained. Thorough studies on Bi

_{n}(n = 2–12) clusters from both experiments and theoretical computations have been carried out by lots of researchers [91].

#### 2.2.2. Electronic and Optical Properties

#### III-Bi Compound

#### GaAsBi

_{1−x}Bi

_{x}NWs are investigated. The SO corrected bandgap of NWs decreases clearly with increasing Bi concentration. Optical properties, such as dielectric function, optical absorption spectra and reflectivity, enhance in the low energy range when the Bi concentration in GaAs is increasing. Besides, the Bi doped GaAs NW represents stability in the absorption coefficient at higher energies compared with the Bi doped GaAs bulk material [94].

#### GaNBi

_{1−x}Bi

_{x}alloys employing the FP-LAPW as implemented in the WIEN2k package. The lattice parameter of the alloys increases with increasing Bi concentrations. The band structure for different Bi compositions and effective masses of electrons are also calculated. It is found that the material remains a semiconductor until x = 0.5 and becomes a semimetal for x greater than or equal to 0.625. And GaNBi shows a giant and composition dependent optical bandgap bowing, and also has a phenomenal bowing of the SO split energy [95].

#### AlNBi

#### 2.3. Other Methods

## 3. Binary Bismides

#### 3.1. Theoretical Predictions

#### 3.2. Growth of Binary III-Bi

_{2}Bi and In appeared. Decreasing the growth temperature to 100 °C when the Bi:In flux ratio was fixed at 3.7, droplet formation was still not avoided. However, polycrystalline InBi began to form rather than previous In

_{2}Bi. Considering the excess Bi droplets on the surface, a reduced Bi flux was required to achieve good crystalline film. Further decreasing the growth temperature to 80–90 °C together with a reduced In:Bi flux ratio to 1.5, crystallinity of InBi was improved, accompanied with a poorly epitaxial InBi. Even at such a low Bi flux, Bi droplets on the surface were still not eliminated. Thus, it is concluded that InBi film growth should be operated at a low temperature below 100 °C and also at a Bi:In flux ratio <1. This growth parameter differs largely from traditional III-V compound growth, which needs a group-V overpressure condition. Keen et al. also attempted to grow GaBi and AlBi, but regrettably, no epitaxial growth was found.

## 4. Epitaxial Growth of GaAsBi

#### 4.1. Bi-Induced Surface Reconstruction, Segregation and Surfactant Effect

#### 4.1.1. Surface Reconstruction

_{2}:Ga flux ratio is around 0.5. With more Bi atoms incorporated, the as grown GaAsBi layer shows large uniformity and strong PL intensity [106]. Also, this huge difference in atomic size causes Bi atoms to form clusters, phase separation and atomic ordering during epiaxial growth [107]. For GaAsBi alloys with a Bi content up to 10%, CuPt-type ordering of As and Bi atoms on {111}B planes are observed through TEM, but when Bi content is increased to 13%, phase separation occurs instead of the pronounced atomic ordering.

#### 4.1.2. Segregation

#### 4.1.3. Surfactant Effect

#### 4.2. Bi Surface Droplets, Incorporation and Growth Model

#### 4.2.1. State of Bi Adatoms during Epitaxial Growth

#### Bi Desorption

_{As}e

^{-U1/KT}x, where P, F

_{As}, x and U

_{1}represent Bi desorption rate, As flux, incorporated Bi content and activation energy, respectively.

#### Bi Droplets

#### Bi Incorporation

_{2}:Ga of ~7:1 at 580 °C. The fact that Bi is isoelectronic with As makes Bi not to create any defects but a good surfactant with trace Bi incorporation [133]. As a general guideline already realized in the very early stage of InSbBi growth using MBE by Oe et al. in 1981, a low growth temperature and an V/III ratio close to unity are needed so as to obtain a significant Bi content using MBE [113,134]. The real picture of growth parameters on Bi incorporation is more complicated and we shall give more elucidation below. However, we will now introduce two growth models developed from Tiedje’s group on the modeled GaAsBi system under two typical growth conditions. Understanding the involved physical processes during MBE growth has great significance on precise control of growing desired films with specific compositions and material properties.

#### 4.2.2. GaAsBi Growth Model

_{Bi}, can be modeled with a Langmuir adsorption isotherm [133] and its model curve can be derived from reflection high-energy electron diffraction (RHEED) [133]. Langmuir isotherm assumes bonding of adsorbate atoms to themselves is much weaker than the bonding to surface. Incorporated Bi content is associated with the Ga-Bi bond.

#### Lu’s GaAsBi Growth Model

_{Bi}F

_{Ga}(1−x), where F

_{Ga}and x is Ga flux and Bi content, respectively. The second process presents that a Ga atom is inserted between two Bi atoms: one is bonded to a Ga atom and the other adheres on the surface. They assumed that the second process is forbidden due to that the form of Bi-Ga-Bi is not favorable because large strain may be induced in this kind of next-neighbor Bi atoms combination. That explains why the incorporation rate in the first process has a factor (1 − x) in it, meaning the second process is excluded. The third process is actually the Bi evaporation process which shows that the incorporated Bi is ejected back onto the surface and is replaced by an As atom. They assume that breakdown of the Ga-Bi bond in this process is thermally activated with a rate proportional to ${F}_{As}{e}^{-U1/KT}x$, where F

_{As}and U

_{1}is As flux and thermal activation energy for the replacement of a Bi atom by an As atom, respectively. Combining the first and the third processes together, they got the following rate equation:

_{Bi}, is given by a revised function based on Langmuir model [135]. In the present case, the Bi flux in the Langmuir isotherm is substituted by the net flux, in which the corresponding Bi surface coverage can be presented as follows:

_{Bi}is Bi flux, U

_{0}is desorption energy of Bi adatoms and b is a fitting constant. Equation (2) gives dependence of the incorporated Bi concentration on the growth parameters and fits very well with their experimental results. The various solid and broken lines in Figure 5 are computed from Equation (2) with a certain value of α, b, U

_{0}and U

_{1}which can be inferred from change in RHEED intensity [135]. The solid lines in Figure 5 show that Bi concentration increases with decreasing As flux, when holding Bi flux and growth temperature constant. This is easy to comprehend because As and Bi are always competing for Ga sites. Thus, too many As atoms will undoubtedly reduce the Bi concentration. Broken lines in Figure 5 reveal Bi concentration dependence on growth temperature and Bi flux. When keeping other parameters fixed, the Bi concentration initially increases with increasing Bi flux and finally saturates at a high Bi flux. In addition, the Bi content can also be enhanced by lowering growth temperature as shown in Figure 5, in agreement with the model.

#### Lewis’ GaAsBi Growth Model

_{2}:Ga surface coverage ratio on the growing film instead, and the As

_{2}:Ga flux ratio should be carefully controlled during GaAsBi growth since a little fluctuation of As

_{2}:Ga flux near 0.5 can cause a big change on Ga coverage. With this insight, Equation (4) is proposed to describe the Bi incorporation rate on the surface:

_{Ga}is Ga-terminated surface coverage, α

_{1}and a

_{2}are two constants. Interestingly, in both Lu’ and Lewis’ model, U

_{1}is used in the model but has a different meaning. From the physical process assumed in both model, U

_{1}should be considered as the difference of As-Ga and Bi-Ga bonding energy in Lu’s model and the activation energy for a Bi atom to break the Bi-Ga bond and to form a surfactant atom in Lewis’ model as will be discussed below.

_{Bi}θ

_{Ga}. In the second process an incorporated Bi atom is attached by a free Ga atom, thus the surface Bi content being reduced. This term was neglected in Lewis’ paper [136] as they claimed that it was a minor effect compared with the first and the third process. The third process represents the incorporated Bi atoms are thermally ejected back onto the surfactant layer. The activation energy, U

_{1}, is the energy difference between an incorporated Bi atom and a surfactant Bi atom. Analogy with Lu’s model, θ

_{Bi}is assumed to obey the Langmuir isotherm, which is described by Equation (3).

_{Ga}is important and can affect Bi incorporation. Firstly, a model which allows Ga and As atoms hopping on GaAs surface is proposed. Without Bi, As atoms will permanently attach with Ga if they land on a Ga site, otherwise will either be evaporated with a probability of P

_{A}or hopping to a new site if they land on an As site. Ga atoms experience a similar process that will attach when they land on an As site, or will be lost to form droplets with a small probability (P

_{G}). This becomes extremely important at relatively low As:Ga ratios since Ga atoms will experience more hopping events until they can find an As site. With the presence of Bi, As and Ga atoms can also stick to the incorporated Bi sites. It is assumed in this model that when an As atom lands on a Bi site, the Bi site will behave like an As which means As will not displace Bi. Similarly, Bi is assumed to behave like a Ga site when Ga lands on it. Considering these assumptions, the rate of change of As coverage, θ

_{A}, can be described by Equation (5):

_{G}can be derived from Equation (5) by substituting the normalization condition of the termination layer: θ

_{G}+ θ

_{A}+ x = 1. This is plotted as a function of As

_{2}:Ga flux ratio as represented by the dashed line in Figure 7 with P

_{A}= 0.12 and P

_{G}= 0.001.

_{2}:Ga flux ratio and Bi:Ga BEPR are presented in Figure 7a,b, respectively. Choosing appropriate values of P

_{A}, P

_{G}, U

_{1}, a

_{1}and a

_{2}, the Lewis’ model can well fit with experiments. Lewis’ model focuses on Bi incorporation under the detail variation of the As

_{2}:Ga flux ratio close to the stoichiometry condition. It is not valid for large As

_{2}:Ga flux ratios as the Ga coverage quickly approaches to zero. In this case, the first process in Lu’s model with a Ga atom inserted between a surfactant Bi atom and an As atom must be included.

#### 4.3. Influence of Growth Parameters on Bi Incorporation

_{2}:Ga flux ratio, Bi flux, growth temperature and growth rate, respectively. In addition, influence of MBE growth of GaAsBi using As

_{2}and As

_{4}will also be elaborated.

#### 4.3.1. As_{2}:Ga Flux Ratio

_{2}:Ga flux ratio can be qualitatively comprehended like this: both As and Bi are group-V atoms which tend to bond with Ga atoms. It is understandable that an excess As supply will outcompete for Bi since Ga-As bonding is stronger than that of Ga-Bi bonding. Hence, decreasing As

_{2}:Ga flux ratio will spontaneously enhance the possibility of Ga-Bi bonding and thus increase the Bi content. However, continuously decreasing As

_{2}:Ga flux ratio will not result in a monotonic increase of Bi content, rather risk of forming Ga droplets because excess Ga atoms on the growing GaAsBi surface cannot be evaporated at such a low growth temperature. Once the group-III rich surface is established, it is no longer energetically favorable for Bi atoms to bond with floating Ga atoms due to the weak bonding energy and the large strain introduced. Thus, the Bi content becomes saturated. A careful control of the As

_{2}:Ga flux ratio is essential to achieve a high Bi content and to avoid formation of Ga droplets.

_{2}:Ga flux ratio for GaAsBi grown at substrate temperatures of 220–230 °C, 265 °C and 330 °C, respectively. Both the 220–230 °C and the 265 °C samples are grown with Bi:Ga BEPRs of 0.47 and 0.35, respectively, while the 330 °C samples are grown with Bi:Ga BEPRs of 0.09 [136]. The Bi content is measured using high-resolution X-ray diffraction (HRXRD) and a similar behavior can be seen from the three data sets. Bi concentration increases sharply with decreasing As

_{2}:Ga flux ratio and then saturates for the flux ratio below 0.5.

#### 4.3.2. Growth Temperature

#### 4.3.3. Bi Flux

_{Bi}= 1. In Lu’s model, the theoretical value of maximum Bi content will be $=\frac{{e}^{U1/KT}{F}_{Ga}}{a{F}_{As}+{e}^{U1/KT}{F}_{Ga}}$. In Lewis’ model the maximum Bi content will be $x=\frac{{\theta}_{G}{e}^{U1/KT}}{{a}_{2}}$. In the former case, the As

_{2}:Ga flux ratio plays an important role and the maximum value can be reached when the flux ratio is close to 1. In the latter case, the saturated Bi content depends on the Ga surface coverage and will reach a maximum when the whole surface is covered by Ga. Therefore, surface coverage by one monolayer of Ga atoms will eventually stop Bi incorporation, if no access As atoms are provided.

#### 4.3.4. Growth Rate

_{G}. For a low growth rate, the Bi content can be kept the same if the As flux changes accordingly to keep the As:Ga flux ratio constant. It turns out to be extremely important in controlling the formation of Bi droplets. With a low growth rate, Bi droplets can be avoided since any excess Bi atoms have enough time to be evaporated. Using a low growth rate of 0.07 µm/h at 270 °C, Lu et al. was able to achieve x = 10% Bi incorporation with a low density of Bi droplets (1.7 × 10

^{6}cm

^{−2}). Lewis et al. tried two different growth rates: 0.13 and 1 µm/h, and found that the 1 µm/h sample had a higher Bi content than the 0.13 µm/h sample.

^{−8}Torr, lower than the situation studied in Lu’s and Lewis’ cases.

_{4}other than As

_{2}as the As source. Richards et al. [137] made a systematic study upon the influence of As

_{2}and As

_{4}on GaAsBi growth and showed that for the same Bi incorporation, the required As

_{2}:Ga atomic flux ratio is much lower than that of As

_{4}:Ga. As

_{2}dimers effectively provide As atoms that can be directly incorporated into GaAs, while for As

_{4}tetramers the thermal cracking efficiency into As is low and a much broad range of As:Ga flux ratio is permitted for Bi incorporation. This indicates that the optimum growth procedure is easy to achieve under As

_{4}condition. The measured recovery time for As reconstruction after the Bi shutter being closed is ~340 s for As

_{2}and ~800 s for As

_{4}. Considering typical growth time of ~1 s for one monolayer, much shorter than the As reconstruction recovery time, use of different As species will not influence Bi incorporation. In addition, samples grown under As

_{2}and As

_{4}exhibited almost no difference in PL spectrum.

#### 4.4. Thermal Stability and Bi diffusion

_{Ga}) [138] and Ga vacancies (V

_{Ga}) [139]. Upon post-growth thermal annealing, the As antisites are converted to As clusters [3] leading to high resistivity [140]. Incorporating Bi into GaAs is believed to diminish the As antisties [141] and enhance PL. Post-growth rapid thermal annealing (RTA) is a common way to minimize point defects and improve optical quality, and in dilute nitrides it can significantly enhance PL intensity by orders of magnitude [142]. It has also practical use for making dilute bismide devices such as laser diodes, as the upper waveguide layer and contact layer above the active QW region are often grown at a temperature much higher than that of GaAsBi. There are a number of annealing studies reported and they are summarized in Table 5 below.

_{1−x}Bi

_{x}QWs with x

_{Bi}= 2.2%–6.5% grown at 380–420 °C and annealed at 600–800 °C for 30 s. Pendellösung fringes are clearly observed for GaAs

_{0.935}Bi

_{0.065}up to an annealing temperature of 700 °C, indicating good crystal and interface quality. Puustinen et al. [141] carefully designed a set of 270 nm thick GaAsBi samples with the identical growth condition except using various growth temperatures between 220–315 °C. GaAs references samples were also grown at similar temperatures. Due to the large amount of As

_{Ga}antisites, the GaAs reference samples reveal residual compressive strain which decreases from 128 arcsec to 20 arcsec when increasing growth temperature from 220 °C to 315 °C. After the RTA at 500 °C for 30 s, the strain in the GaAs sample grown at 220 °C decreases to 11 arcsec, i.e., almost relieved. This is due to the diffusion of As

_{Ga}antisites to As clusters [3]. For GaAsBi, the estimated Bi content is 1.46%, 1.29% and 1.30% for the samples grown at 220, 270 and 315 °C, respectively. Upon RTA, the strain of the GaAsBi grown at 315 °C remains almost unchanged when increasing annealing temperature from 500 to 800 °C. However, the strain of the GaAsBi grown at 220 °C undergoes a large shift from 438 arcsec for the As-grown to 256 arcsec when annealed at 600 °C. The interference fringes disappear for annealing temperatures above 600 °C. Interestingly, Rutherford backscattering (RBS) confirms the same amount of Bi atoms before and after annealing at 600 °C. Cross section TEM images corroborate the improvement of material quality for both the annealed sample and the GaAsBi grown at 315 °C. These observations led the authors to conclude that the RTA of GaAsBi causes reorganization of Bi atoms towards GaAsBi clusters with a high Bi content grown at very low temperatures in addition to the removal of As

_{Ga}antisites, but has little effect on GaAsBi grown at a relatively high temperature of 315 °C. Grant et al. [147] studied RTA of GaAsBi/GaAs single QWs with x

_{Bi}= 3% and 3.5% reaching a similar conclusion that GaAsBi is stable up to an annealing temperature of 650 °C.

_{Bi}up to 5% and then decreases to nearly 1 for x

_{Bi}= 6.5%. The PL intensity increases with annealing temperature up to 700 °C and then decreases for x

_{Bi}= 4% and up to 600 °C for x

_{Bi}= 6.5%. Both the x

_{Bi}= 4% and the x

_{Bi}= 4.8% samples show an S-shape trend of PL peak energy as a function of temperature and the RTA at 700 °C for 30 s has almost no effect on removal of the S-shape for the x

_{Bi}= 4% sample but slightly mitigates the localization for the x

_{Bi}= 4.8% sample. In another study by Mazzucato et al. [146] with x

_{Bi}= 2.3% in a 200 nm thick GaAsBi, the S-shape behavior appears at a low excitation of 1 mW but disappears at an elevated excitation power of 10 mW, because a high excitation power can saturate the localized traps. After annealing at 750 °C, the S-shape disappears and the PL peak energy shows a blue-shift of 5 meV. This is attributed to the removal of localized states in the annealed sample as evidenced by a reduction of FWHM of about 10–25 meV. They also performed time-resolved PL and found the decay time increases from 130 ps for the as-grown sample to 180 ps for the annealed sample at 750 °C. This indicates a moderate reduction of non-radiative recombination upon thermal annealing. Time-resolved PL was also employed by Butkute et al. [144] to study thin and strained (30–100 nm) and thick and relaxed (0.5–1.5 μm) GaAsBi layers. For thick and relaxed GaAsBi layers, the x

_{Bi}= 3.5% sample shows no obvious change in decay time of tens ps after RTA at 600 °C for 30 s, while the decay time of the x

_{Bi}= 5% sample decreases from tens ps to less than 1 ps after the same RTA process. This is likely due to generation of dislocations in the x

_{Bi}= 5% sample sample upon annealing. For the thin layers, the decay time was measured to have several hundreds ps.

_{0.963}Bi

_{0.037}grown by MOCVD. The XRD pattern shows no change up to 700 °C for 900 s while the 10 K PL reveals an enhancement of integrated PL intensity by more than 10 times and a reduction of FWHM from 126 meV to104 meV. Later, the same group used PR to evaluate transitions in GaAs

_{0.965}Bi

_{0.035}and found a red-shift of 60 meV after RTA at 600 °C for 900 s [149]. No change in XRD is observed while the 10 K PL shows a peak at a constant energy of 1.36 eV when increasing the annealing temperature to 600 °C. This anomalous red-shift measured by PR is unexpected and the authors attribute it to an internal rearrangement of Bi atoms inside GaAsBi.

_{N}= 1.4% and x

_{Bi}= 3.2%) grown at 365 °C by MBE. The PL intensity for GaAsBi remains nearly unchanged up to 700 °C and then decreases, while for GaAsN, it increases nearly three orders of magnitude after being annealed at 600 °C and remains constant then. For GaAsNBi, it monotonically increases up to 700 °C and then decreases. The peak energy blue-shift is marginal for GaAsBi, up to 8 meV at 700 °C, but is large for GaAsNBi, about 27.5 meV at the same annealing temperature. No obvious change in XRD is seen in GaAsNBi for annealing up to 800 °C, indicating a very stable material. This implies that incorporation of Bi into GaAs has a minor effect on degradation of optical quality and thus RTA has also a minor effect for improving material quality. However, incorporation of N results in significant degradation of PL and RTA can remedy the crystal defects induced by N atoms. Ye et al. [151] reported RTA on 7.5 nm thick InGaAsBi QWs grown by MBE. Incorporation of Bi into InGaAs QW extends PL wavelength from 1.24 μm to 1.29 μm and increases PL intensity at the same time. Upon RTA, the PL intensity in InGaAs QW decreases with annealing temperature and no PL is observed at 700 °C. In the contrary, the PL intensity in InGaAsBi QW increases up to 700 °C and then decreases. However, a large red-shift of PL peak of 60 meV is observed already at 650 °C. This is due to the out-diffusion of Bi atoms from the QW. By cladding the QW with GaAsBi, the red-shift is reduced to be less than 10 meV under the same annealing condition as a result of suppression of Bi diffusion out of the InGaAsBi QW. Wu et al. [152] studied RTA on InP

_{1−x}Bi

_{x}thin films with x

_{Bi}= 0.1%–2.6% grown by gas-source MBE. Both XRD and PL show negligible changes in spectral features up to 600 °C. Above 600 °C, Bi content drops and PL shows a blue-shift. The integrated PL intensity is almost unchanged up to 600 °C and decreases afterwards. This critical annealing temperature is lower than 700–750 °C in the case of GaAsBi, which is expected because of the lower In-Bi bonding strength than that of Ga-Bi bonding.

## 5. Other Dilute Bismides

#### 5.1. III-N-Bi

#### 5.1.1. GaNBi

_{1−x}Bi

_{x}alloy phase because no other XRD peaks of a new crystalline phase are observed. Despite of the amorphous structure, dramatic absorption energy shift from 3.4 eV in GaN to below 1.2 eV is seen for GaN

_{1−x}Bi

_{x}alloy with x > 0.1. The decreased absorption energy tail is interpreted as two reasons: downward shift of CB and upward shift of VB resulted from Bi; interaction between the Bi-related states and the extended states of GaN matrix [155].

_{1−x}Bi

_{x}growth. The XRD spectra of GaNBi films grown at different growth temperatures attest that, with decreasing growth temperature, the intensity of GaN peaks decreases accompanied by a broad peak width. That is to say, decreasing growth temperature leads to a decreased GaN crystal fraction and size, meanwhile an enhanced Bi incorporation [155]. Microstructure study of GaN

_{1−x}Bi

_{x}film by high resolution TEM exhibites a dependence of microstructure on Bi composition and the variation from columnar growth to an almost amorphous structure. For the films with a high Bi content, small crystalline clusters are found to be embedded into the amorphous structure. As for Bi flux influence, electron probe microanalysis (EPMA) and RBS consistently confirm that Bi composition in GaN

_{1−x}Bi

_{x}alloy arises with increasing Bi flux. In addition, GaN

_{1−x}Bi

_{x}samples grown under different Ga:N ratios show no difference on Bi concentration, but an obvious different effect on optical properties. Under the N-rich grown samples, only a weak absorption tail is observed at low energy region about 1 eV, and at high energy region, a strong absorption edge corresponding to GaN matrix around 3.4 eV appeares. Nevertheless, under the Ga-rich growth condition, samples exhibite an enhanced absorption intensity in the low energy tail, indicating a rising substitutional Bi incorporation [156]. Therefore, significant Bi incorporation needs to be close to the stoichiometric or slightly Ga-rich conditions.

#### 5.1.2. AlNBi

_{1−x}Bi

_{x}alloys. Calculation results reveal that the lattice parameter of AlN

_{1−x}Bi

_{x}for x < 20% increases with increasing Bi content, but variation in lattice constant does not obey the Vegard’s Law. They also predicted a bandgap reduction with increasing Bi concentration and the change from an indirect to a direct bandgap at x ≅ 2%. The nonlinear decrease of the bandgap with increasing Bi content suggests an abnormal bandgap bowing effect of AlNBi. Furthermore, effective mass of AlN, AlBi and AlNBi are all calculated. However, no experimental results are available to testify the validity of the calculations.

#### 5.2. III-P-Bi

_{1−x}Bi

_{x}, InAs

_{1−x}Bi

_{x}and InP

_{1−x}Bi

_{x}in ZB structure, and predicted the miscibility gaps [80]. The bond lengths for InBi, InSb, InAs and InP are 2.880 Å, 2.805 Å, 2.622 Å and 2.541 Å, respectively, i.e., the bond difference between InBi and InP is the largest at 12%, resulting in the largest strain in InPBi. Mixing enthalpy calculation shows that ZB InPBi is the most difficult to be synthesized. However, once InPBi is successfully realized, it would be the hardest during these three materials due to the inverse relation of the hardness and the bond length.

#### 5.2.1. InPBi

#### Epitaxial Growth

_{3}pressure, growth rate and Bi flux are the most important parameters affecting the crystal qualities of InPBi. Figure 10 shows AFM images of InPBi films grown at 364, 324 and 275 °C, respectively. For high temperature growth (364 °C), Bi cannot be fully incorporated into InP matrix and form micrometer-size (In)Bi metallic droplets leading to a large RMS roughness value of 85 nm. For low temperature growth at 275 °C, the diffusivity of In, P and Bi atoms is significantly reduced, causing excess atoms accumulation on the surface and the growth dynamics will be more complicated. Even the InP reference sample grown at the same temperature showes similar morphology implying that Bi has a negligible effect on surface morphology of InP at this low growth temperature. Combining XRD results and AFM images, the optimum growth temperature for InPBi is about 300–330 °C.

_{3}pressure, it is usually excessive to avoid formation of In droplets on the surface. However, the case for growth by gas source MBE is different compared with solid source MBE. Experiments have shown that the incorporated Bi concentration is almost independent on PH

_{3}pressure in a certain range, but the PH

_{3}pressure will influence the crystal quality. Bi content is found to inversely proportional to the InP growth rate. Meanwhile, the low growth rate can inhibit the formation of surface droplets and improve the uniformity of the distribution of Bi atoms. As for Bi flux, the Bi content is approximately linear to Bi flux as long as no Bi droplets are accumulated on the surface.

#### Structure Property

^{−1}, deduced to be InBi like TO and LO phonon modes, respectively, while the other two are at 311 and 337 cm

^{−1}which are not found in the low temperature grown InP reference sample [159]. The integrated Raman intensity of the vibration modes at InBi-like TO, LO and 337 cm

^{−1}are linear to the Bi content at the reported range 0.3%–2.3%. Moreover, the InBi-like TO and LO modes show a strong Raman intensity comparable to that of the InP matrix, which is quite different from other dilute bismides. Wei et al. [160] presented a formula to quantify the relationship between Raman intensity of different vibration modes and the Bi content, which is useful to characterize the Bi concentration not only for InPBi, but also for Bi-related quaternaries such as InGaPBi and InAlPBi.

#### Optical Property

_{In}antisite related donor level below the CB and the other is Bi related acceptor level above the VB. The three peaks labeled as low energy (LE), middle energy (ME) and high energy (HE), respectively are attributed to recombination between the P

_{In}antisite level and the Bi related level, the P

_{In}antisite level and VB, and CB and the Bi related level, respectively.

#### InGaPBi and InAlPBi

#### N-Bi Co-Doping Phosphides

#### 5.3. III-As-Bi

#### 5.3.1. InAsBi

#### Epitaxial Growth of InAsBi

_{3}at 400 °C) yields a smooth InAsBi epilayer. Thus, the effective V/III ratio is equivalent to 1.2, approaching to the stoichiometry condition like the case for GaAsBi growth. In contrast, for MBE grown InAsBi, a haze-free surface of InAsBi alloys is realized at an As/In flux ratio of 1:1.1 [164]. This indicates that either MOCVD or MBE growth of InAsBi needs a critical control of V/III ratio near the stoichiometry of unity. Growth temperature is also a critical factor during growth and affect decomposition rate of As precursor. Commonly, the growth temperature of InAsBi is controlled within 350–400 °C by MOCVD [25,26]. At 375 °C, Ma et al. achieved InAsBi alloy with a Bi content of 4% in 1991 [26]. In the same year, they decreased the growth temperature down to 275 °C and obtained the highest Bi concentration of 6.1% [165].

#### Properties of InAsBi

_{0.037}the ΔE

_{g}/ΔT is 0.22 meV/K. In spite of the common tendency that a smaller Eg also exhibits a smaller ΔE

_{g}/ΔT in nature, the value of 0.22 meV/K is smaller than that of InSb (0.29 meV/K), whose Eg is smaller than that of InAsBi. This indicates that Bi incorporation has an effect on decreasing ΔE

_{g}/ΔT.

#### 5.3.2. InGaAsBi

_{x}Ga

_{1−x}As

_{1−y}Bi

_{y}with a narrower bandgap than GaAsBi by Bi incorporation. In addition, by adjusting the x, y compositions, one can obtain the desired bandgap suitable for device application and lattice matched to InP substrate at the same time. A theoretical cutoff wavelength of InGaAsBi is predicted to reach as long as 6 μm, further extending the wavelength to the mid-IR regime [64].

#### Epitaxial Growth of InGaAsBi

#### Properties of InGaAsBi

_{0}and E

_{0}+ Δ

_{SO}transitions of the same InGaAsBi samples [64], Kudrawiec et al. [175] found that the E

_{0}transition redshifts (~50 meV/% Bi) with Bi content. The observed transitions are explained by VBAC model [126] and common anion rule [131]. According to the VBAC model, Bi incorporation in InGaAs only has an influence on the VB. Thus, the observed transitions are interpreted as that the heavy/light hole band shifts upward, while both the CB and the SO split band keep unchanged as illustrated in Figure 15. These results are also in agreement with the common anion rules, based on which energy gap decreases when light anions are replaced by heavy Bi atoms. Incorporation of heavy Bi atoms into InGaAs host results in a decrease in energy bandgap and an increase in SO split energy, while the sum of the E

_{0}+ Δ

_{SO}transition is kept the same. Such modification of Bi on band structure of InGaAs can effectively eliminates Auger recombination sketched in Figure 15 and make InGaAsBi as an attractive gain medium suitable for temperature-insensitive telecom lasers.

#### 5.4. III-Sb-Bi

#### 5.4.1. InSbBi and Quaternary Alloys

_{2}Bi phases are formed on surface as the V/III ratio is slightly below the stoichiometry of unity, whereas BiSb phases are formed when it is slightly above the stoichiometry of unity. There are a few reports on the growth of InSbBi by LPE. For example, Iwanowski et al. [28] grew InSbBi layers by LPE with a Bi content of 0.5%. Dixit et al. [181] reported growth of InSbBi with a Bi content up to 4% by LPE on GaAs substrates.

_{1−x−y}Sb

_{y}Bi

_{x}alloy with x ≤ 0.017 and y ≤ 0.096. From PL spectra measured at 10 K, the bandgap reduction of InAsSbBi is determined as 55 meV/% Bi with increasing Bi concentration. Other quaternary alloys like GaInSbBi was reported by Du et al. [185] using MBE on InSb substrates. The introduction of Ga into InSbBi alloy can enhance Bi incorporation, improve lattice-matching to InSb, suppress multiple-phase growth and improve surface morphology. Infrared absorption measurements of Ga

_{0.04}In

_{0.96}Sb

_{0.97}Bi

_{0.03}film performed at 77 K indicate a cutoff wavelength as long as 10.7 μm.

#### 5.4.2. GaSbBi

_{1–x}Bi

_{x}layers with 0 < x ≤ 0.042 and observed two peaks. The high energy (HE) peak is related to the bandgap emission in GaSbBi and redshifts at a rate of 29 meV/% Bi at 150 K, in consistent with the bandgap shift determined from PR measurements. The low-energy (LE) peak is due to the optical transition between CB and the native acceptor states observed at low temperatures where acceptor states are not occupied by electrons. For high Bi content GaSbBi alloys, Rajpalke et al. [166] are the only researchers to have reported absorption and PL spectra with a Bi content up to 9.6%. A weak PL from the GaSbBi sample with 9.6% Bi is observed at 4 K with a transition energy of 490 ± 5 meV. Absorption spectrum measured at RT on the same sample reveals an absorption edge of 410 ± 40 meV (corresponding wavelength of 3.0 μm), indicating GaSbBi alloys as a promising candidate for mid-IR applications.

#### 5.5. Other Quarternary Bismides

#### 5.5.1. GaNAsBi

#### Epitaxial Growth of GaNAsBi

^{−6}Torr for a Ga flux of ~2 × 10

^{−7}Torr [134]. They also found a mirror surface without any Bi droplets for GaNAsBi epilayers grown with this optimized As flux at 350–400 °C. The GaBi molar fraction decreases linearly with increasing growth temperature when other growth parameters are held constant.

#### Properties of GaNAsBi

_{0.33}Bi

_{0.67})

_{z}As

_{1−z}to be lattice matched to GaAs substrates and the ratio of GaBi and GaN molar fractions is consistent with theoretical values based on Vegard’s Law [196] assuming the lattice constant of GaAs, GaBi [113] and cubic GaN as 5.65, 6.23 and 4.51 Å, respectively. The PL peak energy of Ga(N

_{0.33}Bi

_{0.67})

_{z}As

_{1−z}alloys at RT can reach 0.8 eV (1550 nm) for z = 6.5%.

_{0.68}Ga

_{0.32}As

_{0.69}P

_{0.31}reference alloy is also plotted. The temperature coefficient of the PL peak energy is determined to be 0.12, 0.14 and 0.16 meV/K for the Bi content of 4.7%, 3.7% and 2.0%, respectively, much smaller than that of In

_{0.68}Ga

_{0.32}As

_{0.69}P

_{0.31}. It is therefore deduced that the temperature dependence of bandgap for GaN

_{y}As

_{1−x−y}Bi

_{x}is governed by the GaBi molar fraction, irrelevant of the GaN molar fraction.

#### 5.5.2. BGaAsBi

_{y}Ga

_{1–y}As

_{1–x}Bi

_{x}lattice matched to GaAs by solid-source MBE [198]. B

_{y}Ga

_{1–y}As

_{1–x}Bi

_{x}was grown on semi-insulating (001) GaAs substrates at 320 ± 5 °C, favorable for growth of high quality of GaAsBi films [122,129], but not the optimal growth conditions for B incorporation [200]. The Bi content is kept the same as 1.6% while the B content increases from 0 to 1.2%. The compressive strain of BGaAsBi decreases with increasing B content. For y = 1.2%, the B

_{y}Ga

_{1−y}As

_{1−x}Bi

_{x}is lattice match to GaAs substrate. Distinguished fringes are observed in HRXRD indicating that all the epilayers are pseudomorphically strained to the GaAs substrate with a smooth interface. SIMS investigation combined with HRXRD results reveals that most B atoms are substitutional, while excess B atoms are probably interstitial [200], or possibly exist in forms of B-B or B-As complexes [201]. Room temperature PL measurements show no significant change in peak energy with increasing B content. The excess B atoms incorporated at non-substitutional sites lead to a high density of bandgap tail states which affect the temperature dependence of the bandgap in BGaAsBi. Specifically, the ability of Bi-related defects to trap carriers is diminished by increasing B atoms. Thus, a very weak temperature dependence of bandgap is presented in the lattice matched B

_{y}Ga

_{1−y}As

_{1−x}Bi

_{x}alloys.

## 6. Physical Properties of Dilute Bismides

#### 6.1. Surface and Structural Properties

#### 6.1.1. Surfactant Effect and Segregation

#### 6.1.2. Lattice Constant

_{0}(Å) = 5.6535 + 5.77 × 10

^{−3}x(%) [206]. The lattice constants for GaBi and InBi from different works are list in Table 6. Bismuth is widely believed to substitute group-V in the matrix. However, Kunzer et al. confirmed that about 10 % of the incorporated Bi had occupied the Ga sites in GaAsBi films [207].

#### 6.1.3. Lattice Structure

^{−1}in both spectra, and the intensity of this Bi-related vibration mode is much stronger in GaAsBi than that in InAs. Verma et al. theoretically calculated the GaBi-like TO and LO mode at 196 and 189 cm

^{−1}, respectively, and the InBi-like TO and LO mode at 161 and 155 cm

^{−1}, respectively, of which all the intensities enhance with increasing Bi content. For InPBi films, two modes at 149 and 171 cm

^{−1}are deduced to be InBi-like TO and LO phonon modes, respectively, which are linear to the Bi content at 0.3%–2.3% [159].

#### 6.2. Electronic and Transport Pproperties, Point Defects

#### 6.2.1. Electronic Properties

_{C,V}(GaAs) is the energy of the CB minima (CBM) or the VB maxima (VBM) of GaAs, E

_{Bi}is the energy of the Bi resonant level, x is the Bi concentration and C

_{Bi}is the coupling between the Bi resonant level and the GaAs VBM. The VBAC model treats Bi atoms as isoelectronic impurities to the host anion species. These impurities act as localized states in the host semiconductor. The interaction between these localized trap states and the host semiconductor band brings changes in the band structure. The VBAC model has been extensively used to study the band structures of the GaAs

_{1−x}Bi

_{x}alloys [58,67,153,213,214]. Prediction has indicated that dilute bismide alloys are promising to realize the SO split energy Δ

_{SO}exceeding the bandgap energy E

_{g}for suppressing non-radiative Auger recombination and IVBA [53].

_{1−x}Bi

_{x}alloys with x

_{Bi}< 5%. These calculations show that the incorporation of Bi atoms into GaSb host affects both the CB and VB. PR results are in good agreement with the theoretical calculations. The shift rate is −26.0 meV/% Bi for the CB and 9.6 meV/% Bi for the VB, respectively. So, the total bandgap reduction is 35.6 meV/% Bi as shown in Equation 8.

_{1−x}Bi

_{x}layers on GaSb with Bi concentrations between 2.3% and 10.4% has been measured by Batool et al. [63] using PR spectroscopy at RT. Their results show that the strained bandgap E

_{g}equals to Δ

_{SO}at a Bi concentration of 9.0% ± 0.2%. After decoupling the effect of strain, the Bi concentration is estimated as 10.5% ± 0.2% to meet the condition of E

_{g}= Δ

_{SO}in free-standing GaSb

_{1−x}Bi

_{x}layers.

_{y}Ga

_{1−y}Bi

_{x}As

_{1−x}and discovered that the interaction strength between the Bi resonant level and the VB of InGaAs not only depended on Bi content but also the In content as well. The VBAC model shows that In

_{y}Ga

_{1−y}Bi

_{x}As

_{1−x}lattice-matched to InP is possible by varying x and y compositions, with a theoretical cutoff wavelength of 6 μm. Energies of E

_{0}and E

_{0}+ Δ

_{SO}transitions in In

_{0.53}Ga

_{0.47}Bi

_{x}As

_{1−x}alloys with 0< x ≤0.036 have been studied by CER spectroscopy at RT [175]. The Bi related bandgap narrowing and the increase in SO splitting are equal. For In

_{0.53}Ga

_{0.47}Bi

_{x}As

_{1−x}layers with x > 3%, the SO splitting is very close to the energy gap. The VB nature of III-V semiconductors can be characterized primarily by the anion (V) species, while the CB can be explained primarily by the cation (III) species. The SO splitting, therefore, changes very significantly with changes in anion species. The Bi related changes in the band structure are caused by a shift of heavy- and light-hole bands towards the CB, resulting in the reduction of bandgap and the increase of the SO splitting.

_{1−x}Bi

_{x}alloys with 0˂ x ≤ 0.034 are studied using CER spectroscopy and ab initio calculations [161]. The E

_{0}transition shifts to long wavelengths very significantly (−83 meV/% Bi), while the E

_{0}± Δ

_{SO}transition shifts very weakly (−13 meV/% Bi) with increasing Bi concentration. These Bi-related changes in the band structure are in a good agreement with the VBAC and the ab initio calculations. Shifts of E

_{0}and E

_{0}± Δ

_{SO}transitions studied by the ab-initio calculations are −106 and −20 meV/% Bi, respectively. The incorporation of Bi atoms into an InP host affects both the CB and VB by −27 meV/% Bi and 79 meV meV/% Bi, respectively. By fitting the CER data with the bandgap function similar to Equation (8), the bowing parameter is determined as 8.5 ± 0.5 eV.

#### 6.2.2. Effective Mass

_{0.981}Bi

_{0.019}was derived as 0.088m

_{0}from magnetic field dependent PL. They considered that both hole and electron effective masses increased and inferred that the localized potential of Bi atoms strongly affected both VB and CB. They also measured the Bi composition dependence of the exciton reduced mass, μ

_{exc}, of GaAs

_{1−x}Bi

_{x}in a very large Bi concentration range (x = 0%–10.6%) [216]. The magnetic field dependent PL shows an unusual compositional behavior. The value of μ

_{exc}first increases rapidly by 50% for 0≤ x ≤1.5%, then fluctuates around 0.08m

_{0}(1.5% < x <6%), and decreases to below the GaAs value for x > 8%. Such a dependence reveals existence of different concentration intervals, which means that continuum states of the VB and CB hybridize with the Bi-related levels to different extents. The Bi hybridization presents an impurity-like behavior for x < 6% and a band-like character for x > 8%.

#### 6.2.3. Impact of Alloy Disorder on the Band Structure

_{1−x}Bi

_{x}alloys with x in the range of 0.2%–10.6%. The PL peak energy increases with PL pump intensity. Besides, the PL intensity is found to increase with Bi concentration for small x

_{Bi}, peaking at x

_{Bi}= 4.5%. The excitation intensity has a significant effect on the PL peak energy and linewidth. They attributed these phenomena to the shallow localized states induced by Bi clusters near the top of the VB. At low excitation intensity, only the localized states with the highest binding energies participate in PL. In this case, the PL peak energy is the smallest. With increasing excitation intensity, the recombination lifetime is reduced. In this case the holes have less time to thermalize down into the localized states before they recombine, so that the corresponding PL peak shifts higher in energy. The temperature dependent PL of GaAs

_{1−x}Bi

_{x}alloys shows an S-shape behavior due to the localized states [218].

_{v1}energy is shifted well above the pair/cluster state energies. CER and PR measurements for GaAsBi alloys show broadening of E

_{0}and E

_{0}+ Δ

_{SO}transitions in contrast to that of GaAs [175,219]. The broadening mainly results from the alloy inhomogeneity and tail of density of states that appears due to point defects, Bi pairs and other atom complexes.

#### 6.2.4. Transport Properties

_{1–x}Bi

_{x}films. The undoped GaAsBi epilayers demonstrates p-type conductivity in a wide range of Bi-concentrations (0.6% ≤ x ≤ 10.6%). The increase of conductivity with increasing x is paralleled by an increase in the density of free holes by more than three orders of magnitude. The p-type conductivity results from holes thermally excited from Bi-induced acceptor levels lying at 26.8 meV above the VBM of GaAs

_{1–x}Bi

_{x}with a concentration up to 2.4 × 10

^{17}cm

^{−3}at x = 10.6%. The free hole concentration increases with Bi concentration, while the free hole mobility decreases due to the hybridization of the Bi-induced localized states. Kini et al. [221] also studied the effect of Bi incorporation on hole mobility in GaAs

_{1–x}Bi

_{x}using Hall and PL techniques. Different from the results of Pettinari et al., they found that the hole concentration decreases with Bi concentration. They considered that Bi

_{Ga}antisite defects compensate the background acceptors, leading to the reduction of the effective hole concentration.

_{y}Ga

_{1−y}Bi

_{x}As

_{1−x}films grown on InP:Fe substrate, the electrical conductivity is n−type [62]. When the Bi concentration increases from 1.1% to 3.6%, electron concentration increases from 1 to 6 × 10

^{16}cm

^{−3}and electron mobility decreases from 7400 to 350 cm

^{2}/Vs.

#### 6.2.5. Impurity States

_{1−x}Bi

_{x}grown at 370 °C with x = 1.2% and x = 3.4%, respectively [8]. The deep-level trap concentration is on the order of 10

^{15}cm

^{−3}. Bismuth atoms facilitate migration of constituent atoms and prevent them from forming point defects. The possible origin of the hole traps is related to As antisite, As

_{Ga}, Bi antisite, Bi

_{Ga}, and relevant clusters of point defects.

_{In}defect, which is typical for low temperature grown InP. The acceptor trap contributes to the partial compensation of native free electron density in InPBi layers.

#### 6.3. Optical Properties

^{1/2}where E is photon energy for the direct gap interband transition under effective mass approximation [226]. Experimentally, the absorption edge is influenced by the shallow defect levels and phonon scattering. Photoluminescence is another important spectroscopy that plays a crucial role in optical characterization. The laser pumping produces electrons in CB and holes in VB. Those electrons and holes relax and distribute around CB bottom and VBM, respectively, and then recombine radiatively [227]. PL is sensitive to the band edge electronic transition and the shallow impurities and/or defect levels. For some narrow gap dilute bismides like InAsBi and InSbBi, the available PL is located in mid-IR region, which is overwhelmed by the strong environment black-body emission. Modulation PL technique based on the step-scan Fourier transform infrared (FTIR) spectrometer is therefore employed [228,229]. In addition, photoreflectance (PR) spectroscopy is a distinctive method for optical properties and electronic structure of dilute bismides. The photo-induced carriers modulate the built-in electric field and/or carrier density, which emphasize the weak transitions between critical points and eliminate the unwanted static background [58]. PR is sensitive to both the band edge structure and the above-bandgap transitions [230,231,232]. The optical transition characterization for the excited state is hence available. The FTIR-based PR technique covers a wide wavelength range from 0.5 to 20 μm [233,234], which is serviceable for most III-V-Bi alloys.

#### 6.3.1. Optical Bandgap

_{1−x}Bi

_{x}epitaxial film grown by multitarget sputtering system was studied by absorption at 20 K, which deduced a linear bandgap narrowing rate as about 17 meV/% Bi and suggested the semiconductor-semimetal transition of InSb

_{0.89}Bi

_{0.11}[20]. Later, Ma et al. performed the 10 K PL on InAsBi and As-rich InAsSbBi grown by MOCVD, to determine the bandgap narrowing rate as 55 meV/% and 46 meV/%, respectively [25]. However, recent reports for the MBE grown InSbBi and InAsBi showed very different bandgap narrowing rate. Svensson et al. obtained a narrowing rate of 38 meV/% for InAsBi by 77 K PL method [164] whereas the RT absorption by Rajpalke et al. gave a narrowing rate of about 35 meV/% for InSbBi [180], about twice as that in Reference [20]. In a report of Feng et al. [150], they investigated the thermal annealing influence on optical band edge for GaNAsBi. The results show that the PL energy shifts in a scale of tens meV with various annealing temperatures. They suggested that the PL energy shift is attributed to modification of the band tail induced by the micro-scale changes in bismides. This finding gives a hint that the effective optical bandgap of dilute bismides is remarkably affected by the distribution of band tail states. The determination of bandgap narrowing rate for dilute bismides hence disperses to a certain extent. We summarize the experimental bandgap narrowing rate for several III-V-Bi ternaries probed by different optical methods, as shown in Figure 18.

#### 6.3.2. Spectral Broadening in Dilute Bismides

#### 6.3.3. Photoluminescence Intensity

_{1−x}Bi

_{x}(x < 0.026) [245], which implied that the Bi incorporation increases the non-radiative recombination centers. But Lu’s work gave a different non-monotonous phenomenon: the RT PL intensity enhances more than two orders of magnitude with Bi content up to about 4.5% and significantly drops down for high Bi contents [217]. They suggested that the PL enhancement is attributed to the Bi induced localized electronic states near the VBM to promote formation of bound holes. The influences of the bound holes surpass the PL degradation due to the non-radiative centers. Later, Mohmad et al. reported a similar non-monotonous result at 10 K and confirmed the low-temperature PL originating from localization [243]. Their PL intensity at RT showed saturation rather than significant degradation. They attributed the saturation to the large GaAsBi/GaAs VB offset to avoid holes thermally escaping into GaAs substrate from the GaAsBi film.

## 7. Impact of Bismuth on Nanostructures

#### 7.1. Bismuth Surfactant Effect on InAs QDs

#### 7.2. Bismuth Catalyzed Growth of GaAsBi Nanowires

## 8. Device Application

#### 8.1. Telecom and MIR Lasers

_{SO}(2.2 eV for GaBi) and incorporation of Bi has the potential of achieving Δ

_{SO}≥ Eg. In this case, the non-radiative Auger recombination of CHSH type and IVBA are suppressed, and characteristic temperature of laser diodes is expected to be enhanced. This is of significant importance for designing Auger and leakage free long-wavelength lasers. For GaAsBiN/GaAs with Bi up to 12% and N up to 6%, which is within acceptable strain for making photonic devices, the optimum band structure with Δ

_{SO}≥ Eg is achievable for the energy range of ~450–850 meV (~1.5–2.7 μm). They determined that GaAs-BiN alloys have the potential to cover a wide spectral range from near- to mid-IR with a flexible control of the band offsets and SO splitting.

_{Bi}= 1.8% emitting at 987 nm [250]. From electroluminescence (EL) measurements at different temperatures, it is obtained that the peak wavelength is independent of temperature in the range of 100–300 K while the GaAs peak varies. The temperature insensitivity of the GaAsBi EL peak is explained by two competing processes: the bandgap change and the emission from lower energy states. GaAs

_{1–x}Bi

_{x}/GaA LEDs were also demonstrated by Sweeney et al. [261]. The device consists of a 50 nm GaAs

_{0.986}Bi

_{0.014}active layer between two 25 nm GaAs spacer layers, further sandwiched between a 1000 nm p-doped and a 1000 nm n-doped GaAs waveguide layers. The emission wavelength is measured to be ~936 nm at 260 K. The temperature dependence of the emission wavelength is measured to be 0.19 ± 0.01 nm/K in 80–260 K. The emission efficiency decreases rapidly with increasing temperature, implying that some non-radiative loss mechanism is significant.

_{0.975}Bi

_{0.025}active layer is grown at 350 °C by MBE. At RT, a narrow lasing spectrum of 982.8 nm is demonstrated with a pumping density above 2.5 mJ/cm

^{2}. The characteristic temperature of the laser is 83 K between 160 and 240 K. The lasing energy decreases at a constant rate of ~0.18 meV/K, which is only 40% of the temperature coefficient of the GaAs bandgap in the same temperature range. The lasing wavelength of photo-pumped GaAsBi lasers was later extended to 1204 nm at RT by the same research team in 2013 [263]. The characteristic temperature between 20 and 80 °C is T

_{0}~ 100 K, slightly larger than T

_{0}~ 66 K of the reported 1.3 µm InGaAsP FP lasers.

_{Bi}= 2.2%) QW between two 150 nm thick AlGaAs barriers for electrical confinement and further sandwiched between a 1400 nm p-doped and a 1400 nm n-doped AlGaAs waveguide layers. Electrically pumped lasing at RT is demonstrated under pulsed excitation with a threshold current density of 1.56 kA/cm

^{2}for a 1 mm long cavity at an emission wavelength of 947 nm. Later in 2014, Yoshimoto et al. reported lasing oscillation up to 1045 nm at RT from electrically pumped GaAs

_{1−x}Bi

_{x}FP lasers (x

_{Bi}≤ 4%) [264]. For GaAs

_{0.97}Bi

_{0.03}QW lasers, the characteristic temperature in the temperature range of 15–40 °C is 125 K, higher than that in a typical 1.3 μm InGaAsP FP laser. Electrically pumped GaAsBi/GaAs laser is also presented by Sweeney et al. [265] grown by MOCVD. For GaAs

_{0.956}Bi

_{0.044}FP lasers, lasing at 1038 nm is demonstrated at 180 K. In 2015, Mawst et al. reported GaAsBi QW lasers with GaAsP as strain compensated barriers grown by MOCVD [266]. The DFT calculations show that GaAsP is an effective barrier material for electron confinement in GaAsBi QWs with comparable band offset to that employing Al

_{x}Ga

_{1−x}As (x < 0.2) barriers. Ridge waveguide lasers are fabricated and broad EL emission at around 923 nm at 60 K is observed from the GaAs

_{0.97}Bi

_{0.03}active region. However, lasing is not observed up to the maximum current injection of 4 kA·cm

^{−2}.

_{0.53}Ga

_{0.47}As lattice-matched to InP possesses a direct bandgap of 0.74 eV (1.68 μm). Adding a small amount of Bi can push emission wavelength beyond 2 μm, thus entering mid-IR range. In 2011, Zide et al. presented studies on the optical and electrical properties of InGaAsBi (lattice-matched to InP) grown by MBE using solid sources of In, Ga and Bi, and a valved As cracker [62]. From optical transmission measurements, they estimated a bandgap reduction rate of about 56.1 meV/% Bi. The lowest bandgap energy of 0.496 eV is measured for x

_{Bi}= 3.18%, corresponding to a peak wavelength of about 2.5 μm and a strain value of about 0.75% with respect to InP substrate. The VBAC model is applied to simulate InGaAsBi lattice-matched to InP with varied In and Bi concentrations and a theoretical cutoff wavelength of 6 μm is obtained.

_{SO}≥ Eg, is estimated to occur for a Bi content between 2.5%~6%, smaller than about 10% for GaAsBi. The strain of InGaAsBi is also calculated and can be designed with zero or a small strain depending on In and Bi concentrations. Furthermore, they studied optical properties of In

_{0.53}Ga

_{0.47}Bi

_{x}As

_{1−x}/InP samples for x ≤ 3.2% [64,178]. The PR results resolve clear Eg and Eg + Δ

_{SO}features, with Eg ≈ Δ

_{SO}near x = 3.2% at RT.

#### 8.2. Photodetectors

^{6}cmHz

^{1/2}/W. The carrier lifetime is estimated to be about 0.7 ns from the bias voltage dependent responsivity measurements.

_{0.98}Bi

_{0.02}QWs. Absorption spectrum of InAsBi photodiode with the longest cutoff wavelength of 3.95 μm is obtained at 225 K, compared with a cutoff wavelength of 3.41 μm from the reference InAs photodiode at the same temperature. Temperature dependence of the bandgap is found to be 0.19 meV/K, smaller than that of InAs. In 2016, Gu et al. [268] reported an InGaAsBi detector, in which the whole structure is nearly lattice-matched to InP substrate. The Bi content is about 3.2% in the absorption layer and the cut-off wavelength is extended to 2.1 μm at RT, corresponding to a Bi-induced bandgap reduction of about 180 meV. The dark current is found to be lower than that in InGaAs detectors with a similar cut-off wavelength.

_{1−x}Bi

_{x}is characterized by XRD and EDX and reach unexpectedly high of about 10%. Compared with their previous GaAsBi photo-conductive THz detector, this one can be activated by a wide range of optical wavelength including 1.55 μm with a comparable detection sensitivity. In addition, this quaternary GaInAsBi detector exhibits good signal-to-noise ratio (SNR) of ~50 dB with the THz pulse spectrum extended to 3.5 THz.

#### 8.3. Other Devices

_{0.94}Bi

_{0.06}yields a direct bandgap at 1 eV with only 0.7% strain when grown on GaAs, it has been considered to be a promising candidate material for 1 eV sub-cells in multi-junction solar cells [271]. As GaAsBi has relatively high absorption coefficient, it could generate sufficient current to match the sub-cell photocurrent from the other sub-cells of a standard multi-junction solar cell.

^{18}cm

^{−3}, a 49.6% improvement in TPF is obtained from Si:InGaAs (1.37 × 10

^{−3}Wm

^{−1}·K

^{−1}) to Si:InGaAsBi (2.05 × 10

^{−3}Wm

^{−1}·K

^{−1}). The improvement results from a large Seebeck coefficient, which is coupled to a complex CB profile in these materials. A peak ZT-value of 0.23 is achieved for InGaAsBi (1.6% Bi), which is significant for III-V materials.

## 9. Summary

## Conflicts of Interest

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**Figure 2.**Schematic illustration of various physical processes of Bi atoms (surface is not fully covered with Bi adatoms).

**Figure 4.**Schematic illustration of three processes in Lu’s GaAsBi growth model. Reproduced from Reference [122].

**Figure 5.**Comparison of the experimental results (symbols) with the Lu’s model (dashed lines). Reproduced from Reference [122].

**Figure 6.**Schematic illustration of three processes in Lewis’ GaAsBi growth model. Reproduced from Reference [136].

**Figure 7.**(

**a**) Bi content as a function of As

_{2}:Ga flux ratio. The solid curves are fitted based on the Lewis’ model while the broken curve is a plot of θ

_{G}for P

_{A}= 0.12 and P

_{G}= 0.001; (

**b**) Bi content as a function of Bi:Ga beam equivalent pressure ratio (BEPR). Reproduced from Reference [136].

**Figure 8.**Bi content as a function of Bi flux grown at different growth rates. Reproduced from Reference [129].

**Figure 9.**Root-mean-square (RMS) roughnesss values obtained from atomic force microscope (AFM) measurements for different growth rates and Bi compositions. Reproduced from Reference [129].

**Figure 11.**(

**a**) Room temperature photoluminescence (PL) spectra of InPBi and InP. The red arrow is the band edge emission of InPBi bulk material. (

**b**) PL spectrum of InPBi measured at 10.5 K. The frame line is the measured spectrum and the red solid line is the overall fitted spectrum while the green, pink and blue solid lines are fitted spectrum for low energy (LE), middle energy (ME) and high energy (HE), respectively.

**Figure 13.**Photoluminescence peak energy and absorption edge vs. Bi composition at 10 and 300 K, respectively. The slopes are close to 55 meV/% Bi for both results. Reproduced from [168] [Photoluminescence of InAsBi and InAsSbBi grown by organometallic vapor phase epitaxy], with the permission of AIP publishing.

**Figure 14.**Temperature dependence of the bandgap of various InAsBi layers. Reproduced from Reference [169].

**Figure 16.**PL peak energy of GaNAsBi and In

_{0.68}Ga

_{0.32}As

_{0.69}P

_{0.31}as a function of measured temperature. Reproduced from Reference [134].

**Figure 19.**PL spectra of GaSb quantum well (QW) and GaSbBi QW in various magnetic fields. The detailed PL features in GaSbBi QW are present. A detailed PL feature being located at a higher energy than the effective bandgap of GaSb QW is shown in GaSbBi QW #2.

Method | Electronic Properties | Optical Properties |
---|---|---|

BAC/VBAC Model | GaAs_{1−x}Bi_{x} [58,59,60] | GaAs_{1−x}Bi_{x} [58,59] |

GaAsBiN_{=} | ||

GaAsBiN [53] | InGaAs_{1−x}Bi_{x} [61,62] | |

GaBi_{x}As_{1−x}/GaAs [63] | In_{0.53}Ga_{0.47}Bi_{x}As_{1−x}/InP [64] | |

InP_{1−x}Bi_{x} [65] | ||

InSb_{1−x}Bi_{x} [66] | ||

InGaAs_{1−x}Bi_{x} [61,62] | ||

In_{0.53}Ga_{0.47}Bi_{x}As_{1−x}/InP [64] | ||

TB Model | GaP_{1-x}Bi_{x}, GaAs_{1-x}Bi_{x} [67] | GaAs_{1−x}Bi_{x} [67] |

GaAs_{1−x}Bi_{x} [68,69] | ||

k·p Model | GaAs_{1−x}Bi_{x} [58,70] | |

GaBi_{x}As_{1−x}/GaAs [63] | ||

InGaNAs [71] | ||

InGaAs_{1−x}Bi_{x} [72] |

Structure | Structural Properties | Electronic Properties | Optical Properties |
---|---|---|---|

Thin Film | BBi/GaBi/AlBi [75,76,92] AlN _{1−x}Bi_{x} [81] GaN _{1−x}Bi_{x} [82] GaAs _{1−x}Bi_{x} [77,78,86,93,96] | BBi/GaBi/AlBi [75,76,92] AlN _{1–x}Bi_{x} [81] GaN _{1–x}Bi_{x} [82] GaAs _{1-x}Bi_{x} [60,68,69,77,78,79,86,93,96,97] InP _{1–x}Bi_{x} [73,83] InSb _{1–x}Bi_{x} [74,80] | GaAs_{1−x}Bi_{x} [93,96,97] |

Surface | Bi/GaAs(001)-c(4 × 4)surface [85] Bi/GaAs(100) (2 × 1) and (2 × 4) surfaces [84] | ||

Nanostructure | Bi-doped GaAs NWs [94] Bi ^{4+} ions [98] Bi-NWs, Nanotubes [87] Bicluster [88,89,90,91] | Bi doped GaAs NWs [94] Bi ^{4+} ions [98] Bi NWs, Nanotubes [87] Bi cluster [88,89,90,91] | Bi-doped GaAs NWs [94] |

**Table 3.**Calculation results for structural and electronic parameters of III-Bi compounds. I = indirect band, D = direct band and C = conductor.

Stable Phase | a (Å) | c (Å) | Eg (eV) | Band Type | |
---|---|---|---|---|---|

BBi | ZB [102] | 5.390 ~ 5.529 [102] | — | −0.085 ^{(FP-LAPW)} [102] | I [103] |

— | 1.134 ^{(PW-PP)} [103] | ||||

AlBi | ZB [102] | 6.266 ~ 6.460 [102] | — | −1.81 ^{(FP-LAPW)} [102] | D [103] |

— | 0.042 ^{(PW-PP)} [103] | ||||

— | 0.02 ^{(FP-LAPW)} [92] | ||||

GaBi | ZB [102] | 6.178 ~ 6.470 [102] | — | −2.91 ^{(FP-LAPW)} [102] | C [103] |

— | 0 ^{(PW-PP)} [103] | ||||

InBi | PbO [102] | 5.000 [102] | 4.800 [102] | −4.75 ^{(FP-LAPW)} [102] | C [103] |

0 ^{(PW-PP)} [103] |

Lu | Lewis | Ptak | |
---|---|---|---|

Growth Temperature | 300 °C | 330 °C | 315 °C |

As flux | 2.2 nm^{−2}·s^{−1} | As_{2}:Ga = 0.5–0.68 | As:Ga = 1.4 |

Growth rate | - | 1 µm/h | 0.16–2.0 µm/h |

Bi flux | Bi:As = 0.01–7 4 × 10 ^{−9} − 2.8 × 10^{−7} Torr | Bi:Ga BEPR = 0–0.3 (0–1.2) × 10 ^{−7} Torr | (0–3.3) × 10^{−8} Torr |

Reference | Material | x_{Bi} | Epitaxy | Growth Temp (°C) | RTA Temp (°C) | RTA Time (s) | PL (λ) | PL Intensity |
---|---|---|---|---|---|---|---|---|

[143] | GaAsBi (QW) | 0.6%–10.9% | MBE | 345–400 | 500–900 | 600 | No shift up to 700 °C | - |

[144] | GaAsBi (bulk) | 3.5%–6% | MBE | 220–330 | 550–750 | 30–180 | Almost no shift | 3× |

[145] | GaAsBi (QW) | 2.2%–6.5% | MBE | 380–420 | 600–800 | 30 | No shift up to 800 °C | 3× for x_{Bi} = 4% |

2× for x_{Bi} = 6.5% | ||||||||

[141] | GaAsBi (bulk) | 1.29%–1.46% | MBE | 220–315 | 500–800 | 30 | - | - |

[146] | GaAsBi (bulk) | 2.3% | MBE | 200–350 | 750 | No shift | 1.3× | |

[147] | GaAsBi (QW) | 3%–5.5% | MBE | - | 450–750 | - | No shift in PL | 2.2× |

60 meV red shift in PR | ||||||||

[148] | GaAsBi (bulk) | 3.7% | MOVPE | 420 | 500–750 | 60 | No shift | 10×@10 K |

[149] | GaAsBi (bulk) | 3.5% | MOVPE | 420 | 550–700 | 900 | No shift in PL | - |

60 meV red shift in PR | ||||||||

[150] | GaAsNBi (bulk) | 3.2% | MBE | 365 | 800 | 900 | 8 meV @GaAsBi | 1×@GaAsBi |

1.4%@N | MBE | 60 | 27.5 meV @GaAsNBi | >10×@GaAsNBi | ||||

[151] | InGaAsBi (QW) | 1% | MBE | 370–440 | 650–750 | 120 | 65 meV | 3× |

[152] | InPBi (bulk) | 0.1–2.6% | MBE | 325 | 400–800 | 120 | No shift up to 600 °C | 1× |

Alloy | GaBi | InBi |
---|---|---|

6.23 Å [204] | 6.5 Å [158] | |

6.272 Å [167] | 6.686 Å [203] | |

6.33 Å [141] | 7.024 Å [26] |

Ref. | Epitaxy Method | Growth Condition | Bi Influence | PL |
---|---|---|---|---|

[251] | MOCVD | 5 ML InAs @ 510–530 °C, Bi = 0.01–0.1 ML | Surfactant effect L _{D}↓ | 0.93 eV @ 77 K 1.46 µm @ 300 K |

[114] | MOCVD | InAs/InGaAs DWELL @400 °C calibrated T _{g} | Surfactant effect L _{D}↑ | I_{PL} & λ improved |

[112] | MBE | 2.5 ML@480 °C Bi = 8 × 10 ^{−8} Torr | Surfactant effect L _{D}↑ | I_{PL} improved |

[252] | MBE | 2.2 ML@400 °C Bi = 0.015–0.06 ML/s | Surfactant effect, Incorporation effect | I_{PL} & λ improved |

[253] | MBE | 2.3-3.3 ML@500 °C Bi = 5 × 10 ^{−8} Torr | Surfactant effect L _{D} ↓ | I_{PL}, FWHM & λ improved |

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Wang, L.; Zhang, L.; Yue, L.; Liang, D.; Chen, X.; Li, Y.; Lu, P.; Shao, J.; Wang, S.
Novel Dilute Bismide, Epitaxy, Physical Properties and Device Application. *Crystals* **2017**, *7*, 63.
https://doi.org/10.3390/cryst7030063

**AMA Style**

Wang L, Zhang L, Yue L, Liang D, Chen X, Li Y, Lu P, Shao J, Wang S.
Novel Dilute Bismide, Epitaxy, Physical Properties and Device Application. *Crystals*. 2017; 7(3):63.
https://doi.org/10.3390/cryst7030063

**Chicago/Turabian Style**

Wang, Lijuan, Liyao Zhang, Li Yue, Dan Liang, Xiren Chen, Yaoyao Li, Pengfei Lu, Jun Shao, and Shumin Wang.
2017. "Novel Dilute Bismide, Epitaxy, Physical Properties and Device Application" *Crystals* 7, no. 3: 63.
https://doi.org/10.3390/cryst7030063