AACVD of Cu₃N on Al₂O₃ Using CuCl₂ and NH₃

Abstract

Cu₃N has been grown on m-Al₂O₃ by aerosol-assisted chemical vapor deposition using 0.1 M CuCl₂ in CH₃CH₂OH under an excess of NH₃ at 600 °C, which led to the deposition of Cu that was subsequently converted into Cu₃N under NH₃: O₂ at 400 °C in a two-step process without exposure to the ambient. The reaction of CuCl₂ with an excess of NH₃ did not lead to the growth of Cu₃N, which is different to the case of halide vapor phase epitaxy of III-V semiconductors. The Cu₃N layers obtained in this way had an anti-ReO₃ cubic crystal structure with a lattice constant of 3.8 Å and were found to be persistently n-type, with a room temperature carrier density of n = 2 × 10¹⁶ cm⁻³ and mobility of µ_n = 32 cm²/Vs. The surface depletion, calculated in the effective mass approximation, was found to extend over ~0.15 µm by considering a surface barrier height of ϕ_B = 0.4 eV related to the formation of native Cu₂O.

1. Introduction

Cu₃N is a novel semiconductor in which crystal imperfections such as Cu interstitials (Cu_i) and nitrogen vacancies (V_N) give rise to states that are energetically located inside or very close to the conduction and valence bands, respectively [1], but do not give rise to any mid-gap states. Consequently, it has been suggested to be suitable as a solar cell absorber in view of the fact that it has an indirect energy band gap of ~1.0 eV [2], but also due to the fact that n- and p-type doping are possible. However, despite the fact that Cu₃N has been described as a defect-tolerant semiconductor, so far no one has fabricated a working p-n junction solar cell using Cu₃N. In the past, Chen et al. [3] fabricated a Cu₃N p-n homojunction on indium tin oxide, and Yee et al. [1] fabricated an Al: ZnO/ZnS/Cu₃N p-n heterojunction, both of which exhibited rectifying behavior but no photogenerated current. This has been attributed to the large concentration of Cu_i defects, which capture electrons and result into substantial Shockley–Read–Hall recombination and quenching of the steady-state minority carrier concentration under illumination. In other words, crystal imperfections such as V_N and Cu_i can still reduce the minority carrier lifetime and prevent the extraction of photogenerated electron–hole pairs in Cu₃N. Nevertheless, Cu₃N has been used successfully for energy storage as it has a cubic anti-ReO₃ crystal structure, belonging to the Pm3m space group (number 221) with a lattice constant of 3.8 Å [4], and a vacant body center that can readily act as a host for Li ions in batteries [5]. Cu₃N has been obtained by many different methods including reactive sputtering [6], molecular beam epitaxy [7], atomic layer deposition [8,9] and pulsed laser deposition [10,11]. Recently, we converted Cu into Cu₃N under NH₃: O₂ between 400 °C and 600 °C, and observed distinct spectral features and maxima in differential transmission at 500 nm (≡2.48 eV), 550 nm (≡2.25 eV), 630 nm (≡1.97 eV) and 670 nm (≡1.85 eV) on a ps time scale by ultrafast pump–probe spectroscopy (UPPS) [12]. These correspond to the M and R direct energy band gaps of bulk-relaxed and strained Cu₃N in excellent agreement with density functional theory (DFT) calculations of the electronic band structure [12]. This observation of the M and R direct energy band gaps in fact confirmed that Cu₃N has a clean energy band gap. More recently, we also showed that iodine-doped Cu₃N, i.e., I: Cu₃N, is a p-type semiconductor and that the extensive incorporation of I in Cu₃N can be used to convert Cu₃N into γ-CuI, which is a p-type transparent semiconductor that was used in conjunction with n-type Cu₃N, for the fabrication of a γ-CuI/TiO₂/Cu₃N p-n heterojunction that exhibited rectifying current–voltage characteristics [13]. In the past, most have focused on n-type doping of Cu₃N, such as Gao et al. [14], who showed that the incorporation of Zn resulted into n-type Cu₃N and increased the carrier density from n = 10¹⁷ to 10²¹ cm⁻³ with a resistivity of 10⁻³ Ω cm. In contrast, only a few have considered p-type doping of Cu₃N, such as Matsuzaki et al. [15], who used NF₃ for the growth of F-doped p-type Cu₃N. The ability to obtain p-type Cu₃N is important, of course, for the realization of p-n junctions, but it should be noted that I-VII γ-cuprous halides such as γ-CuCl, CuBr and CuI are p-type transparent semiconductors with a zinc blende crystal structure and direct energy band-gaps of 3.3, 2.9 and 2.95 eV, respectively. In addition, they have lattice constants close to that of Si [16] and may be readily combined with Cu₃N for the realization of novel cuprous electronic and optoelectronic devices.

Here Cu₃N has been obtained by aerosol-assisted chemical vapor deposition (AACVD) using CuCl₂ in CH₃CH₂OH and NH₃. AACVD is a low-cost growth method [17] that has been used for the growth of a broad range of semiconductors [18] including III-V semiconductors such as InN, GaN and In_xGa_1−xN [19]. The growth of these III-Vs is carried out using anhydrous N₂ and NH₃, i.e., O₂ and H₂O are generally avoided and eliminated. In the past, McInnes et al. [19] used 0.1M GaCl₃ and 0.1M InCl₃ in acetonitrile (CH₃CN), N₂ as carrier gas and anhydrous NH₃ to grow In_xGa_1−xN. The total flow rate through the 0.1 M GaCl₃ and 0.1 M InCl₃ solutions was maintained at 529 mL min⁻¹, and anhydrous NH₃ was used at a high flow rate of 862 mL min⁻¹, which assisted in promoting the formation of smaller droplets whilst also ensuring an excess of NH₃ for the deposition of In_xGa_1-xN. The deposition was carried out at 600 °C, which exceeds the melting point of both GaCl₃ and InCl₃ and gave layers with a thickness of ~2 μm. Both GaCl₃ and InCl₃ react directly with NH₃, leading to the deposition of GaN and InN, respectively, while CH₃CN [20] breaks into HCN and CH₄ at elevated temperatures [21,22].

AACVD has also been used for the growth of Cu₃N by Yamaguchi et al. [23], who obtained Cu₃N on α-Al₂O₃ by AACVD at 300 °C using copper (II) acetylacetonate Cu(O₂C₅H₇)₂ that was dissolved in aqueous NH₃. No CuO or Cu₂O was detected in the Cu₃N despite the fact that Cu(O₂C₅H₇)₂ was used in aqueous NH₃. Others such as Park et al. [8] used metal organic sources of copper such as C₁₄H₃₂CuN₂O₂, which contains oxygen in conjunction with NH₃ for the atomic layer deposition of Cu₃N, but metal organic sources are expensive [9].

No one has previously attempted to grow Cu₃N using CuCl₂ and NH₃ by AACVD or tried to grow Cu₃N on m-Al₂O₃, which is ideally suited for the growth of cubic and tetragonal crystals. It is found that the reaction of CuCl₂ with NH₃ will not give Cu₃N as in the case of halide vapor phase epitaxy (HVPE) of III-V semiconductors such as In_xGa_1−xN. In contrast, the reaction of CuCl₂ with an excess of NH₃ resulted into the deposition of polycrystalline Cu layers consisting of oriented grains on m-Al₂O₃, which have a higher crystal quality compared to Cu obtained by sputtering [12] or electron beam evaporation [15] used previously to obtain Cu₃N under NH₃: O₂. Consequently, the Cu layers on m-Al₂O₃ obtained via the reduction of CuCl₂ under NH₃ at elevated temperatures were converted into cubic Cu₃N under NH₃: O₂ at a lower temperature without exposure of the Cu to the ambient. The Cu₃N layers on m-Al₂O₃ have an anti-ReO₃ cubic crystal structure with a lattice constant of 3.8 Å and are n-type with carrier density n = 2 × 10¹⁶ cm⁻³ and mobility µ_n = 32 cm²/Vs at room temperature. The electrical properties are described in conjunction with theoretical calculations of the conduction band potential profile, surface band bending and depletion in the effective mass approximation.

2. Materials and Methods

Initially, 1.34 mg of CuCl₂ (Aldrich 99.999%, 134.45 gmol⁻¹), which has a rusty-brown color, was dissolved in 100 mL ethanol CH₃CH₂OH and stirred at 1000 rpm for 10 min at room temperature. CuCl₂ is soluble in water (75 g/100 mL H₂O at 25 °C) and ethanol (53 g/100 mL CH₃CH₂OH at 25 °C) but less so in acetonitrile CH₃CN (1.6 g/100 mL at 20 °C). In contrast, CuCl has a considerably lesser solubility than that of CuCl₂. More specifically, CuCl is slightly soluble in water (0.0047 g/100 mL H₂O at 20 °C) and insoluble in ethanol CH₃CH₂OH and acetone (CH₃)₂CO. In order to obtain a satisfactory growth rate, a 0.1 M solution of CuCl₂ in CH₃CH₂OH was prepared that has a dark green color due to the (CuCl₄)²⁻ ions that are yellow and Cu⁺² ions that are blue. The 0.1 M CuCl₂ liquid precursor was turned into a mist using a Venturi nebulizer and Ar as a carrier gas. Square samples of ~8 mm × 8 mm m-Al₂O₃ with a thickness of ~0.3 mm were cleaned sequentially in trichloroethylene, methanol, acetone and isopropanol at 80 °C, after which they were rinsed in deionized water at 20 °C and dried with nitrogen, followed by a dehydration bake at 120 °C. The clean m-Al₂O₃ was loaded in a quartz boat that was positioned at the center of a 1” hot wall, single zone AACVD reactor, capable of reaching 1100 °C that was fed by a manifold consisting of four mass flow controllers connected to Ar, NH₃, O₂ and H₂ and a separate side manifold for controlling the flow of Ar through the Venturi nebulizer, as shown in Figure 1a. The reactor was purged with 1000 mL/min of Ar for 10 min from the main manifold, after which the temperature was ramped at 30 °C/min under a flow of 90 mL/min Ar: 10 mL/min H₂ at one atmosphere. Upon reaching 600 °C, the flow of Ar: H₂ was interrupted and a flow of 800 mL/min NH₃ was initiated, while at the same time a flow of 1000 mL/min Ar was established through the nebulizer. A visible flow of the aerosol was observed on the upstream side that was maintained for 30 min. Subsequently, the flow of Ar through the Venturi nebulizer was interrupted and the reactor allowed to cool down to 400 °C under a flow of 300 mL min⁻¹ NH₃. Upon reaching 400 °C, the Cu was converted into Cu₃N under a flow of 300 mL min⁻¹ NH₃ and 15 mL min⁻¹ O₂ for 30 min. At the end of the growth period, cool down took place under a flow of 300 mL/min NH₃ supplied from the main manifold until the temperature fell below 100 °C. A typical temperature–time profile is shown in Figure 1b. The Cu₃N layers were removed after purging with 1000 mL min⁻¹ of Ar at room temperature and were stored in a desiccator under vacuum.

The morphology and crystal structure of the Cu₃N layers were determined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The carrier density and mobility of the Cu₃N layers were determined by the Hall effect in the van der Pauw configuration by using a Keithley 2635A constant current source in conjunction with a Keithley 2182 voltmeter controlled by LabView.

3. Results and Discussion

The reaction of CuCl₂ in CH₃CH₂OH with an excess of NH₃ did not lead to the direct deposition of Cu₃N, as in the case of HVPE of III-V semiconductors such as In_xGa_1−xN, but resulted into the deposition of metallic Cu on m-Al₂O₃ that had a shiny, reflective surface and metallic conductivity. A typical SEM image of the Cu layer obtained on m-Al₂O₃ at 600 °C is shown in Figure 1c, from which one may observe that the Cu layer is polycrystalline and consists of grains oriented along a single direction. A higher magnification image is also shown in Figure 1d, from which it is evident that the grains have sizes of ~5 µm, while a side view of the Cu on m-Al₂O₃ is shown in Figure 1e, showing that columnar growth occurs. The epitaxial growth of Cu on c-Al₂O₃ and a-Al₂O₃ has been investigated extensively for the growth of high-quality graphene [24,25], but only a few have considered the growth of Cu on m-Al₂O₃ [26]. The deposition of Cu on m-Al₂O₃, which contains grooves or steps along specific crystallographic directions, as shown in Figure 2a, will lead to instabilities and ruptures of the Cu layer at elevated temperatures [27]. These ruptures occur at high curvature sites, i.e., peaks and ridges, which act as retracting edges leading to a net flux of atoms away from the high positive curvature regions. For sufficiently thin layers, this process will lead to a self-assembly of the Cu grains along a specific direction [28].

The Cu layers exhibited clear peaks in the XRD, as shown in Figure 2a, corresponding to the face-centered cubic (fcc) crystal structure of Cu with a lattice constant of 3.6 Å. No peaks belonging to CuO, Cu₄O₃ or Cu₂O are observed in Figure 2a. Likewise shown are the peaks corresponding to the underlying m-Al₂O₃, which has an oxygen-terminated surface with tetragonal crystal symmetry that is suitable for the epitaxial growth of semiconductors with a cubic crystal structure. It is worthwhile to point out that the deposition of Cu on n-type Si (001) resulted in columnar growth, as shown in Figure 3a,b. The Cu pillars have a height of ~20 µm, but they are not ordered in any way. No Cu₃N was obtained under an excess of NH₃ by varying the temperature between 300 °C and 800 °C. Instead, the reaction of CuCl₂ in CH₃CH₂OH with NH₃ always led to the deposition of Cu on m-Al₂O₃, which occurs via the reduction of CuCl₂ to CuCl and then into Cu by the H₂ evolving from NH₃.

More specifically, the 0.1 M solution of CuCl₂ is initially converted into a mist of liquid drops and mixed with NH₃, which is soluble in CH₃CH₂OH [29]. Subsequently, the liquid drops are vaporized at an elevated temperature, and the CH₃CH₂OH gives C₂H₄ and H₂O according to the reaction C₂H₅OH → C₂H₄ + H₂O. No carbon is released from the pyrolysis of C₂H₄ between 500 °C and 800 °C [30]. Upon vaporization, CuCl₂, which has a melting point of 498 °C, will be reduced to CuCl, which has an even lower melting point of 423 °C [31], and finally into metallic Cu by the H₂ evolving from the breakdown of NH₃. Before elaborating further, it is useful to note that the thermal breakdown of NH₃ into N₂ and H₂ was investigated as early as 1905 by White et al. [32], who showed that it depends on the gas flow, i.e., residence time as well as the temperature. In particular, White et al. [32] showed that a flow of 200 mL/min NH₃ resulted in a dissociation of 5% NH₃ at 600 °C and 10% at 700 °C. However, the breakdown of NH₃ is also promoted catalytically by the deposited Cu at elevated temperatures [33]. In other words, the Cu deposited on the m-Al₂O₃ will participate actively in the dissociation of NH₃ near the surface, thereby further promoting the reduction of CuCl₂ and deposition of Cu, which has a melting point of 1085 °C. A schematic representation of the proposed reaction mechanism is shown in Figure 3c. For completeness, it must also be pointed out that the NH₃ will react with CH₃CH₂OH and give ethylamine (CH₃CH₂NH₂) and acetonitrile (CH₃CN), which have boiling points of 20 °C and 82 °C, respectively. CH₃CH₂NH₂ and CH₃CN will dissociate into HCN and CH₄ depending on the temperature and residence time, but they are not expected to influence the overall reaction governing the deposition of Cu. It is also important to mention that the Cu will tend to react with H₂O supplied from the CH₃CH₂OH and give CuO and Cu₂O. However, no oxides are detected in Figure 2a, so it is very likely that they are reduced to metallic Cu due to the H₂ evolving from the NH₃ over the Cu. This is consistent with the findings of Kim et al. [34], who showed that CuO is converted into metallic Cu under an excess of H₂ without the formation of intermediate Cu₄O₃ or Cu₂O.

In short, CuCl₂ is reduced to CuCl and then into Cu by the H₂ evolving from NH₃, according to: CuCl₂ + H₂ → CuCl + 2HCl and 2CuCl + H₂ → 2Cu + 2HCl. The HCl reacted in turn with the excess NH₃, giving NH₄Cl, i.e., NH₃ + HCl → NH₄Cl, which solidified below its sublimation temperature, i.e., ~340 °C near the cool end of the reactor, very similar to what occurs during conventional HVPE of III-Vs.

The reduction of CuCl₂ into Cu may also be achieved by using H₂ as opposed to NH₃. In order to show this, the 0.1 M solution of CuCl₂ in CH₃CH₂OH was used to deposit a layer of CuCl₂ on 15 mm × 30 mm soda lime glass (SLG) slides by drop-casting, as shown in Figure 4a. The CuCl₂ layer had a light green color and good uniformity, and a typical SEM image is shown in Figure 4b. This was converted into Cu under a flow of (i) 10 and (ii) 50 mL.min⁻¹ pure H₂ at 400 °C for 30 min, as shown schematically in Figure 4c. The CuCl₂ as-deposited on SLG displayed a crystalline structure and multiple peaks in the XRD, as shown in Figure 4d, but all the peaks were eliminated after the reduction of the CuCl₂ into Cu.

The Cu deposited on m-Al₂O₃ at 600 °C by AACVD using CuCl₂ and NH₃ has a higher crystal quality compared to the Cu obtained by sputtering, which was nonetheless successfully converted into crystalline Cu₃N under a flow of 300 mL/min NH₃ and 15 mL/min O₂ between 400 °C and 600 °C, as shown previously [12]. The Cu₃N obtained in this way had an anti-ReO₃ cubic crystal structure, and we observed the M and R direct energy band gaps of Cu₃N by UPPS in excellent agreement with DFT calculations of the electronic structure, confirming that it has a clean energy gap [12]. Consequently, the polycrystalline Cu layer that was obtained by AACVD on m-Al₂O₃ at 600 °C was converted into Cu₃N under a flow of 300 mL/min NH₃ and 15 mL/min O₂ at 400 °C. The Cu₃N had an olive-green-like color, and a typical SEM image of the Cu₃N layer on m-Al₂O₃ is shown in Figure 1f. This exhibited peaks in the XRD, as shown in Figure 2b, corresponding to the anti-ReO₃ cubic crystal structure of Cu₃N with a lattice constant of 3.8 Å.

The reaction of Cu with NH₃ containing O₂ and the formation of Cu₃N can be understood by considering the catalytic oxidation of NH₃ by O₂ in the presence of a catalyst, e.g., Cu, Pt, etc., at elevated temperatures, as described by Carley et al. [35], who investigated the catalytic reactivity of Cu (110) metal surfaces with coadsorbed NH₃ and O₂. More specifically, Carley et al. [35] proposed that the oxidation of NH₃ leads to the formation of a stabilized N monolayer on the Cu metal surface, which in turn is responsible for the conversion of the bulk Cu layer into Cu₃N. It should be noted that the reaction of NH₃ with O₂ also gives H₂O according to the reaction NH₃ + O₂ → NO + H₂O, which was observed to condense near the cool end of the reactor upon increasing the gas flow of O₂. The reaction mechanism of the conversion of Cu into Cu₃N is depicted schematically in Figure 3d. No Cu₃N was obtained from Cu by using only NH₃, in accordance with Matsuzaki et al. [15]. Moreover, no CuO or Cu₂O peaks are detected in Figure 2b, but Cu₂O will nevertheless form as native oxide on the surface of the Cu₃N upon exposure to the ambient, as we have shown previously by using Raman spectroscopy [36]. Before considering the electrical properties of the Cu₃N layers, it is useful to point out that the reaction of CuCl₂ with a smaller flow of 100 mL/min NH₃ at 600 °C mainly led to the deposition of Cu₂O, not Cu₃N.

In order to measure the Hall effect, Ag ohmic contacts were deposited at the four corners of the Cu₃N layers on m-Al₂O₃. It has been shown that Au, Ag and Cu in Cu₃N give rise to a semiconductor-to-metal transition and remarkably constant electrical resistivity over a very broad range of temperatures [37]. Consequently Ag, Au and Cu may be used for the formation of ohmic contacts on Cu₃N, and in the past, we have shown that Au and Ag deposited on n-type Cu₃N results in the formation of contacts with linear IVs [13]. The Cu₃N layers on m-Al₂O₃ were found to be n-type and had room temperature carrier densities of 2 × 10¹⁶ cm⁻³ with a maximum mobility of 32 cm²/Vs. The Cu₃N layers are n-type as they are Cu-rich, but also due to the fact that oxygen may be included in the Cu₃N by the preferential formation of interstitial oxygen (O_i) that acts as donors, not as acceptors [36]. Furthermore, the Cu₃N layers obtained here were found to be persistently n-type, and the carrier density and mobility did not exhibit any changes upon illumination with light of λ = 450 nm under ambient conditions. In other words, the n-type Cu₃N layers did not exhibit any photoconductivity, which may be attributed to recombination via Cu_i and V_N states, in accordance with Yee et al. [1].

It is worthwhile pointing out here that Matsuzaki et al. [15] showed that epitaxial Cu₃N layers with a thickness of 25 nm on SrTiO₃ were p-type, due to the upward surface band bending mediated by the chemisorption of O₂⁻, but switched to n-type upon exposure to ultraviolet (UV) light and reverted back to p-type after terminating the irradiation. In contrast, they observed that the Cu₃N layers remained n-type after exposure to UV light under vacuum, confirming that the adsorbed O₂⁻ is responsible for the surface inversion observed under ambient conditions in air. However, the epitaxial Cu₃N layers of Matsuzaki et al. [15] were found to be persistently n-type under ambient conditions, with a carrier density of the order of 10¹⁴ cm⁻³ and mobility of 100 cm²/Vs after annealing under NH₃ between 125 and 350 °C, suggesting a change in the composition of the surface and overall band bending. The Cu₃N layers obtained here were found to be persistently n-type and had a room temperature carrier density of 2 × 10¹⁶ cm⁻³, perhaps due to the fact that after the conversion of Cu into Cu₃N under NH₃: O₂, the flow of NH₃ was maintained for at least 30 min until the temperature fell well below 100 °C.

However, the properties of Cu₃N layers with a thickness of a few tens of nm will depend strongly on the properties of the surface but also the properties of the underlying substrate that is often overlooked. The Cu₃N layers obtained here are persistently n-type with a carrier density of 2 × 10¹⁶ cm⁻³, most likely due to the fact that the thickness of the Cu₃N layers is greater than 1 µm, so it is bulk-like and will not be strongly influenced by properties of the surface or underlying m-Al₂O₃. In thermodynamic equilibrium, the Fermi level (E_F) with respect to the conduction band minimum (E_C) away from the surface and deep in the bulk is determined from:

$$n=N_C{e}^{-\frac{E_C-E_F}{kT}},$$

where N_C is the conduction band effective density of states, k is Boltzmann’s constant and T the temperature taken to be equal T = 300 K. The electron density is equal to n = 2 × 10¹⁶ cm⁻³, and the conduction band effective density of states in Cu₃N is given by:

$$N_C=2{\left(\frac{2\pi m_{n}kT}{h^2}\right)}^{\frac{3}{2}},$$

where m_n is the electron effective mass in Cu₃N taken to be m_n= 0.16 m_o [6], m_o is the free-electron mass and h is Planck’s constant. This gives N_C = 1.6 × 10²⁴ m⁻³ or 1.6 × 10¹⁸ cm⁻³, so E_C − E_F = 0.11 eV in the bulk where a flat band condition exists. On the other hand, the energetic position of the Fermi level with respect to the conduction band edge, i.e., E_C − E_F, at the surface is dependent on the local density and energetic position of any surface states that will be occupied by electrons, which in turn may pin the Fermi level at the surface. According to Navío et al. [38], the Fermi level at the surface of ultrathin Cu₃N layers is pinned at the middle of the gap, which will give rise to a barrier height of ϕ_b = 0.5 eV. The surface depletion region will extend into the Cu₃N, and the depletion width is:

$$w=\sqrt{\frac{2{ϵ}_s{\phi }_b}{e{N}_D}},$$

where ε_S = ε_Rε_o, ε_R is the static dielectric constant of Cu₃N, ε_o the permittivity of free space, e the electron charge and N_D the donor density taken to be equal to n = 2 × 10¹⁶ cm⁻³. Considering that the static dielectric constant of Cu₃N is ε_R ~10 [39], the depletion width is found to be w = 0.16 µm, taking into account that the Fermi level at the surface of Cu₃N layers is pinned at the middle of the gap, according to Navío et al. [38]. However, despite the fact that we did not detect any CuO or Cu₂O in the XRD, a thin layer of Cu₂O will exist on the surface of Cu₃N. According to Hodby et al. [40], the Fermi level at the surface of Cu₂O is pinned at states residing energetically in the upper half of the band gap ~0.4 eV below the conduction band edge. The native Cu₂O layer of Cu₃N is expected to have a thickness of only a few nm and will be completely depleted, so the depletion width taking ϕ_b = 0.4 eV is found to be w = 0.15 µm. The conduction band potential profile of the Cu₃N layer including the native Cu₂O layer at its surface is shown in Figure 5a, where the work function and electron affinity of Cu₃N i.e., ϕ(Cu₃N) = 5.0 eV and χ(Cu₃N) = 3.5 eV [41] have been considered as well as the work function and electron affinity of Cu₂O, i.e., ϕ(Cu₂O) = 4.8 eV and χ(Cu₂O) = 3.2 eV [42]. The formation of p-type Cu₂O over the n-type Cu₃N will lead to the confinement of photogenerated electron–hole pairs at the Cu₂O/Cu₃N heterojunction, which will inadvertently result into recombination via states at the interface, thereby suppressing the photoconductivity. This mechanism is different to that put forward by Yee et al. [1], who fabricated an Al: ZnO/ZnS/Cu₃N p-n heterojunction that exhibited rectifying behavior but no photogenerated current, which was attributed to the large concentration of Cu_i defects that capture electrons and result in substantial Shockley–Read–Hall recombination and quenching of the steady-state minority carrier concentration under illumination. While it is possible that both mechanisms are responsible for the suppression of the photocurrent and photoconductivity in Cu₃N, it is imperative that the surface recombination should be suppressed via the deposition of suitable layers that prevent the formation of Cu₂O that was originally suggested to act as a suitable passivation layer for Cu₃N, similar to that of SiO₂ for Si p-n junction solar cells [2].

4. Conclusions

Cu₃N layers have been grown on m-Al₂O₃ by aerosol-assisted chemical vapor deposition using 0.1 M CuCl₂ in CH₃CH₂OH under an excess of NH₃ at 600 °C, which resulted in the deposition of epitaxial Cu layers consisting of oriented grains with a face-centered cubic crystal structure that were subsequently converted into Cu₃N under NH₃: O₂ at 400 °C in a two-step process without exposure to the ambient. The reaction of CuCl₂ with an excess of NH₃ did not give Cu₃N, which is different to halide vapor phase epitaxy of III-V semiconductors such as In_xGa_1−xN. The Cu₃N layers obtained in this way have an anti-ReO₃ cubic crystal structure and persistent room temperature carrier density of n = 2 × 10¹⁶ cm⁻³ and mobility of µ_n = 32 cm²/Vs, but they did not exhibit any photoconductivity due to recombination via surface states in the Cu₂O or via indirect recombination via Cu_i defects, which capture electrons and result into substantial Shockley–Read–Hall recombination