Micro-Nanoarchitectonics of Ga₂O₃/GaN Core-Shell Rod Arrays for High-Performance Broadband Ultraviolet Photodetection

Abstract

This study presents broadband ultraviolet photodetectors (BUV PDs) based on Ga₂O₃/GaN core-shell micro-nanorod arrays with excellent performance. Micro-Nanoarchitectonics of Ga₂O₃/GaN core-shell rod arrays were fabricated with high-temperature oxidization of GaN micro-nanorod arrays. The PD based on the microrod arrays exhibited an ultrahigh responsivity of 2300 A/W for 280 nm at 7 V, the peak responsivity was approximately 400 times larger than those of the PD based on the planar Ga₂O₃/GaN film. The responsivity was over 1500 A/W for the 270–360 nm band at 7 V. The external quantum efficiency was up to 1.02 × 10⁶% for 280 nm. Moreover, the responsivity was further increased to 2.65 × 10⁴ A/W for 365 nm and over 1.5 × 10⁴ A/W for 270–360 nm using the nanorod arrays. The physical mechanism may have been attributed to the large surface area of the micro-nanorods coupled with the Ga₂O₃/GaN heterostructure, which excited more photogenerated holes to be blocked at the Ga₂O₃ surface and Ga₂O₃/GaN interface, resulting in a larger internal gain. The overall high performance coupled with large-scale production makes it a promising candidate for practical BUV PD.

1. Introduction

Broadband ultraviolet photodetectors (BUV PDs) have attracted significant attention as an emerging technology, and they have been widely used in land invasion, forest fire and ozone hole security monitoring, satellite and security communications, and other fields [1,2,3,4]. Over the past few decades, significant developments in fabricating BUV PDs have been achieved based on wide-band-gap semiconductors, such as MgZnO/ZnO, AlGaN/GaN [5,6,7,8,9,10]. However, high Al/Mg-content AlGaN/MgZnO layers with high crystal quality are difficult to achieve by epitaxial growth, as the process of alloying makes the fabrication complex and introduces high defect density, thereby increasing the dark current and limiting the performance of PDs [11,12,13,14,15,16,17]. Alternatively, Ga₂O₃ can avoid the complex and unmanageable alloying process owing to its intrinsic solar-blind region band gap. Furthermore, Ga₂O₃ exhibits high thermal stability (MP 1730 °C), chemical stability and excellent thermal conductivity. Accordingly, Ga₂O₃-based PDs gained vast attention in the past few years [18,19,20,21,22,23,24,25,26,27,28]. Moreover, Ga₂O₃/GaN heterostructures have a broadband ultraviolet bandgap. More importantly, the intrinsic conduction band difference between Ga₂O₃ and GaN is 0.1 eV, which facilitates the flow of electrons. The intrinsic valence-band barrier between Ga₂O₃ and GaN is approximately 1.4 eV, which can effectively block hole transport, resulting in a large internal gain. Therefore, Ga₂O₃/GaN heterostructures are considered to be promising materials for BUV PDs. Guo et al. fabricated a super-high-performance self-powered UV PD based on the GaN/Sn:Ga₂O₃ p–n junction. The responsivity at 254 nm reached up to 3.05 A/W without consuming external power [29]. Lin et al. proposed a typical UV PD based on the “sandwich” structure (graphene/β-Ga₂O₃/GaN heterojunction). Using rectifying effect of the p-GaN/β-Ga₂O₃ diode, the photodetector showed an extremely low dark current density of 1.25 × 10⁻⁸ A/cm², and the responsivity is up to 12.8 A/W by the hot carrier multiplication in graphene under Solar-blind ultraviolet illumination [30]. Kalra et al. demonstrated epitaxial β-Ga₂O₃/GaN-based vertical metal–heterojunction-metal (MHM) broadband UV-A/UV-C PDs, the PD exhibited the responsivity (R) of 3.7 A/W at 256 nm, and the UV-to-visible rejection >10³, and a photo-to-dark current ratio of >100 [31].

Although there are many reports on BUV PD based on Ga₂O₃/GaN heterostructures, they are mostly thin-film structures [29,30,31,32,33]. The Micro-Nanoarchitectonics of Ga₂O₃/GaN rod arrays are featured with a higher surface-to-volume ratio display superior optical absorption ability and higher recovery efficiency. Specifically, the higher surface area induces more photogenerated carriers and enhances recombination of nonequilibrium carrier via the high surface states, leading to excellent response characteristics. He et al. [34] grew Ga₂O₃/GaN nanowires by partially thermally oxidizing GaN nanowires grown using molecular beam epitaxy (MBE). The fabricated PD based on vertical Ga₂O₃/GaN nanowire arrays showed a broadband spectral response from 276 to 366 nm with a high responsivity exceeding 550 A/W at −5 V bias and a fast response speed in the millisecond range. Wang et al. [35] fabricated monoclinic β-Ga₂O₃ vertically aligned nanorod arrays (NRAs) by hydrothermal and post-annealing methods. The fabricated solar-blind PD based on β-Ga₂O₃ NRAs exhibited a high responsivity of 550 A/W under a bias of 5 V. Moreover, the PD exhibited self-powered characteristics, that is, a responsivity of 10.80 mA/W at 0 V bias and response time of 0.38 s. Although significant developments have been achieved based on Micro-Nanoarchitectonics Ga₂O₃/GaN rod arrays, there are still many areas that could be further optimized. For example, most of the reported structures are based on relative complex vertical structures, and the preparation methods of Ga₂O₃/GaN heterostructures are mainly MBE [36,37,38,39], metal-organic chemical vapor deposition (MOCVD) [40,41,42], pulsed laser deposition (PLD) [43,44,45], atomic layer deposition (ALD) [46,47,48] and so on. These methods require precise control. It is worth noting that thermal oxidation, that is, the high-temperature thermal oxidation of GaN to obtain Ga₂O₃, is relatively simple and suitable for mass production [49,50,51,52,53,54,55,56,57,58,59,60,61]. Furthermore, the metal-semiconductor-metal (MSM) structure takes advantage of the high light–electron conversion of nano/micro-rod arrays and possesses the unique advantage of easier fabrication compared to other vertical sandwich structures. More importantly, such an MSM geometry constructed two electrodes on the nano/microarrays to increase the electrode effective area and thus effectively collect photogenerated carriers, further improving the performance of PDs.

In this study, Micro-Nanoarchitectonics of Ga₂O₃/GaN core-shell rod arrays were prepared by high-temperature oxidization of GaN M-NRAs. MSM PD based on Ga₂O₃/GaN core-shell microrod arrays (MRAs) was fabricated. The PD showed excellent performance; an ultrahigh responsivity of 2300 A/W for 280 nm and over 1500 A/W for the 270–360 nm band was obtained at a 7 V bias. The external quantum efficiency was approximately 1.02 × 10⁶% at 280 nm, respectively. The physical mechanism was attributable to the large surface area of the MRA arrays and Ga₂O₃/GaN heterojunction band structure, which blocked more holes at the surface of the M-NRA and Ga₂O₃/GaN heterojunction interface, resulting in a larger internal gain compared to the planar Ga₂O₃/GaN film-based PD. This mechanism is consistent with the results of PD fabricated using NRAs. It is worth highlighting that the features of inexpensive manufacturing and easy scalability are particularly attractive for mass production.

2. Materials and Methods

To take advantage of the internal gain brought by the special band structure of Ga₂O₃/GaN heterojunction, the Micro-architectonics of Ga₂O₃/GaN with core-shell structure were fabricated. Figure 1 is a schematic of the Ga₂O₃/GaN core-shell MRAs fabrication. GaN/sapphire samples with approximately 5 μm-thick unintentionally doped GaN epitaxial layer were used in this study. The GaN/sapphire wafers were ultrasonically cleaned in acetone and isopropyl alcohol for 15 min and then dipped into a diluted hydrochloric acid water solution (HCL:H₂O = 1:1) for 5 min to eliminate the native oxide. A SiO₂ film with a thickness of 200 nm was deposited on GaN using plasma-enhanced chemical vapor deposition (SENTECH SI500D). Subsequently, the polystyrene microsphere solution and alcohol were mixed in a 1:1 ratio, and then the polystyrene microspheres were transferred to the surface of the sample. A highly ordered self-assembled monolayer of microspheres with a diameter of 1.7 μm was arranged on the sample. To reduce the size of the microspheres and increase the distance between them, the samples were treated with oxygen plasma for 10 min. The polystyrene microsphere template was then transferred to the SiO₂ film by etching the SiO₂ film using inductively coupled plasma (ICP) with CF₄ gas for 120 s. After the ICP etching process, the polystyrene nanospheres were eliminated by dipping the sample in tetrahydrofuran for 2 h and sonicating for 30 s. Subsequently, the SiO₂ template was transferred to the GaN film through etching of the GaN film by ICP with Cl₂ and BCl₃ gases. Then, the sample was dipped in the BOE solution to eliminate the SiO₂ mask, and GaN MRAs with a cone morphology were fabricated. Subsequently, the sample was placed in an oxidizing furnace. The heating time from room temperature to 1000 °C was approximately 55 min in an oxygen-gas environment. GaN was oxidized in a quartz tube furnace injected with 50 sccm of oxygen gas at 1000 °C for 45 min at atmospheric pressure. Finally, the samples were removed from the oxidizing furnace when the temperature was reduced to 200 °C in a nitrogen protection environment. An interdigitated metal contact (Ti/Al) with a thickness of 30/100 nm was patterned using standard photolithography (Karlsuss MA6/BA6), DC magnetron sputtering, and lift-off processes. Each device comprised 10 pairs of interdigital electrodes with lengths, widths, and spacings of 300, 6, and 6 μm, respectively. Consequently, the effective illumination area was 3.42 × 10⁻⁴ cm². The fabricated PD was finally annealed by rapid thermal annealing under an N₂ atmosphere at 200 °C for 120 s. The contrast PD based on the planar Ga₂O₃/GaN film was fabricated using a direct high-temperature oxidization process from a GaN film, as described above.

To characterize the material properties, the surface and cross-sectional morphology, microstructure, and the elemental mapping were determined using a ZEISS SIGMA high-resolution field emission scanning electron microscope (FE-SEM). Phase formation and crystallinity were monitored using a Helios Nano-Lab 460HP high-resolution transmission electron microscope (HR-TEM). The I–V characteristics were measured using a Keithley 2410 source meter and Keithley 6514 programmable electrometer. A 450 W Xe arc lamp, mechanical chopper, and lock-in amplifier were applied to measure the photocurrent response spectra. The system was calibrated using a standard silicon detector. The devices were all illuminated perpendicularly to the front metal/semiconductor contact side. All measurements were performed at room temperature.

3. Results

Figure 2a shows the surface morphology of the polystyrene microspheres. A highly ordered self-assembled monolayer of microspheres with a diameter of 1.7 μm was arranged on the sample. Other sizes of the polystyrene microspheres can also be used to fabricate the GaN rod arrays with different sizes. The diameter of the microspheres was reduced to 1.3 μm and the spacing of the microspheres increased after oxygen plasma treatment, as shown in Figure 2b. GaN rod arrays with different sizes and distances can also be prepared flexibly by adjusting the power and duration of oxygen plasma processing. Figure 2c shows the surface morphology of the GaN MRAs, and the highly ordered GaN MRAs with a diameter of about 1.3 μm was obtained. This size is similar to the size of the polystyrene microspheres after plasma treatment. Figure 2d shows the surface morphology of the oxidized Ga₂O₃/GaN MRAs. The diameter of the Ga₂O₃/GaN MRAs was approximately 1.35 μm. The height of the Ga₂O₃/GaN MRAs was approximately 750 nm, as shown in the inset of Figure 2d. Clearly, the Ga₂O₃/GaN MRAs show a core-shell structure, in which the outer shell is Ga₂O₃ with a 200–300 nm thickness and the core is GaN with an 850–1050 nm thickness. Some Ga₂O₃ grains with several hundred nanometers in size were observed on the top surface of the MRAs, which was also confirmed in our previous study. The X-ray diffraction (XRD) spectrum of the sample and morphology of the planar Ga₂O₃/GaN film were similar to those previously reported and are not shown here [54,55,59,60,61].

To assess the photoresponse characteristics of the planar Ga₂O₃/GaN film and Micro-architectonics of Ga₂O₃/GaN core-shell rod arrays, MSM PD based on a planar Ga₂O₃/GaN film (sample A) and Ga₂O₃/GaN MRAs (sample B) were fabricated. The device structure diagram of sample B is shown in inset of Figure 3b. The MSM structure, that is, the two electrodes constructed directly on the MRAs, had a large electrode area, which enabled the efficient collection of photogenerated carriers. Moreover, owing to the intrinsic high valence-band barrier between Ga₂O₃ and GaN (1.4 eV), the holes generated in GaN will be blocked at the interface of Ga₂O₃/GaN heterojunction underneath the electrode, resulting in a greater internal gain. The larger metal electrode area will cause more holes to be blocked at the interface of Ga₂O₃/GaN heterojunction, resulting in greater internal gain. The spectral response from 200 to 400 nm at applied bias of 7 V was investigated. The spectral responsivity values via wavelength are calculated using the relation [29]:

$$\mathrm{R}=\frac{{\mathrm{I}}_{\mathrm{ph}}-{\mathrm{I}}_{\mathrm{Dark}}}{{\mathrm{p}}_{\mathsf{\lambda }}}$$

(1)

where I_ph and I_Dark are the currents measured under illumination and dark conditions, respectively, and P_λ is the incident power density at a specific wavelength of λ, which was calibrated using a standard silicon detector. The R dependences on wavelength under the applied bias of 7 V for samples A and B are shown in Figure 3a and Figure 3b, respectively. Both PDs exhibited a wide-spectrum response in the BUV region. When the illumination wavelength exceeds 370 nm, the responsivity of sample A decreases rapidly, which corresponds to the spectral response cutoffs of GaN material. Sample B has an obvious spectral response at 370–400 nm, which is attributable to the damage of GaN crystal structure by etching process during the preparation of GaN rod arrays, forming many surface defects in GaN and inducing the defect response. Therefore, it is necessary to further optimize the preparation process in the future. As shown in Figure 3a, the peak responsivity was 6.3 A/W at 260 nm for sample A, and the responsivity at the range of 250–360 nm exceeded 2 A/W. For sample B, the peak responsivity was 2300 A/W at 280 nm, the peak responsivity is enhanced by approximately 4 × 10² times compared with sample A (6.3 A/W), and the responsivity at the 270–360 nm band exceeded 1500 A/W, as shown in Figure 3b. It is also superior to some reported values [34,35]. Both of the two samples have high responsivity, indicating that the two PDs have a large external quantum efficiency (EQE), which is defined as the number of electrons generated per incident photon and can be obtained using the following equation [27]:

$$\mathrm{EQE}=\frac{\mathrm{Rhc}}{\mathrm{q}\mathsf{\lambda }}$$

(2)

where R, h, c, q, and λ are the responsivity, Planck’s constant, speed of light, electron charge, and wavelength of the incident optical light, respectively. It is observed that sample A exhibited an EQE of approximately 3 × 10³% for 260 nm and sample B exhibited an ultrahigh EQE of approximately 1.02 × 10⁶% for 280 nm. Compared with sample A, the internal gain of sample B is significantly increased. Obviously, it cannot be explained by the increase in light absorption efficiency owing to the large surface area of MRAs. This is assignable to the larger surface area of the MRA coupled with the Ga₂O₃/GaN heterostructure. This is discussed in detail later in this study.

Detectivity (D*), which usually describes the smallest detectable signal, is another key figure of merit of PDs. The detectivity (D*) can be expressed as [27]

$${\mathrm{D}}^{*}=\frac{\mathrm{R}}{\sqrt{2{\mathrm{eI}}_{\mathrm{dark}}/\mathrm{S}}}$$

(3)

where S (3.42 × 10⁻⁴ cm²) is the effective area under illumination with the shot noise from I_Dark regarded as the major component of the total noise. The detectivity D* of sample A was calculated as 4.47 × 10¹² Jones for 260 nm. The detectivity D* of sample B was calculated as 2.65 × 10¹³ Jones for 280 nm and shows that the ability to detect weak signals is improved.

Figure 4a,b shows the I–V characteristics of samples A and B under dark conditions and the illumination of 270 nm/360 nm light, respectively. For sample A, the dark current at 7 V bias is 1.14 × 10⁻⁹ A, and the photocurrents are 1.99 × 10⁻⁷ and 2.44 × 10⁻⁷ A at 270 nm and 360 nm light illuminations, respectively, while the photo-to-dark ratios for 360 nm and 270 nm/dark current both exceeded 10². Such low light-to-dark ratio is mainly due to the rather weak intensity of the 270 nm and 360 nm light illuminations, which is separated from the xenon lamp light passing through the monochromator. When the bias voltage is in the range of −3~3 V, the photocurrent at 360 nm light illumination is less than that at 270 nm light illumination, indicating that the depletion region is mainly in the Ga₂O₃ layer. As the bias voltage increases, the photocurrent for 360 nm increases rapidly, and exceeds the photocurrent for 270 nm, which indicates that the depletion layer of the device enters the GaN layer. The PD presents B-UV detection, which is consistent with the results of the response spectrum. For sample B, the dark current at 7 V bias is up to 10⁻⁶ A, which is three orders of magnitude higher than that of sample A. The significant increase in dark current is attributable to the following reasons. The larger electrode area of sample B not only increased the photocurrent, but also inevitably led to larger dark current. Meanwhile, the etching process of the GaN film also caused a deterioration of the GaN crystalline quality, resulting in a greater current leakage. Therefore, the photocurrent was evidently higher than that of dark current until the bias voltage exceeding 3.2 V. The photo-to-dark ratios for 360 nm and 270 nm/dark currents both exceeded 10. It is necessary to further optimize the process to improve the light/dark ratio.

To clearly explain the increased internal gain in sample B, the photodetection model and mechanism of sample B are schematically shown in Figure 5. As shown in Figure 5a,b, the microarchitectonics of Ga₂O₃/GaN core-shell rod arrays have a larger surface area, and more ambient oxygen molecules are adsorbed on the surface of the MRAs in the dark environment. The free electrons on the surface of the Ga₂O₃ film were trapped by oxygen molecules, forming a depletion layer and the energy band bending near the surface of the MRAs. When the device was illuminated under UV-C light, the photogenerated holes in Ga₂O₃ migrated to the surface of Ga₂O₃ due to the bending band. Compared with the photogenerated electrons in Ga₂O₃, the photogenerated holes in Ga₂O₃ that migrate to the surface of Ga₂O₃, were difficult to collect by the electrode, resulting in a large gain. When the device was illuminated under UV-A light, because negatively charged oxygen ions existed on the surface of the Ga₂O₃ film after the reaction with O₂, the photogenerated holes in the GaN film were aggregated at the interface of Ga₂O₃/GaN owing to the capacitance effect. These holes were also difficult for the electrode to collect, resulting in an enhanced gain. Therefore, sample B exhibited a greater internal gain than sample A in BUV band range.

Moreover, the Ga₂O₃/GaN core-shell MRAs with higher surface-to-volume ratios effectively improved the light absorption efficiency and thus excited more photogenerated carriers. Because of the larger electrode area, these additional photogenerated electrons can be effectively collected and increase the photocurrent. In addition, more holes were blocked at the Ga₂O₃/GaN interface underneath the electrode, resulting in high responsivity. Since the Ga₂O₃/GaN heterojunction has the low conduction band barrier (0.1 eV) and high valence band barrier (1.4 eV), the photogenerated electrons in the GaN layer can pass through the Ga₂O₃/GaN heterojunction and be collected by the electrode. Meanwhile, the photogenerated holes in GaN are effectively blocked in the Ga₂O₃/GaN heterojunction, noting that these blocked photogenerated holes are mainly located at the Ga₂O₃/GaN heterojunction interface below the electrode. A larger metal electrode area can block more holes, thus leading to greater photoconductance gain. The designed structure effectively coupled the optical field enhancement effect with the internal gain of the heterojunction, resulting in ultra-high responsivity of the PDs.

Whether the increased gain caused by the oxygen adsorption, or the large electrode area coupled by the internal gain of the Ga₂O₃/GaN heterojunction, both are related to the surface area of Ga₂O₃/GaN core-shell rod arrays. To further confirm the above mechanism, Ga₂O₃/GaN NRAs with larger surface areas were fabricated using the same method as described above, except that the polystyrene microspheres with a diameter of 1 μm was applied as the mask template and the thermal oxidation time was reduced to 30 min. Figure 6a,b shows the surface and cross-sectional topography of the NRAs. As shown in Figure 6a,b, the highly ordered Ga₂O₃/GaN NRAs were obtained, the diameter of the nano-column is approximately 700 nm and the height is approximately 400–450 nm. A thin layer of Ga₂O₃ was identified in the outermost layer of the NRA. In order to demonstrate the formation of the Ga₂O₃/GaN core-shell structures, high-resolution HR-TEM inspections were performed. As shown in Figure 6c–e, the outmost shell with a thickness of 10–50 nm is confirmed, which corresponds to the β-Ga₂O₃ (201) and β-Ga₂O₃ (400), and the core area corresponds to GaN (0001). These results indicate that the GaN NRAs were oxidized at high temperature and form the polycrystalline Ga₂O₃ film on the GaN surface.

To further confirm the formation of the nanoarchitectonics of Ga₂O₃/GaN rod arrays with core-shell structure by thermal oxidation, energy dispersive X-ray (EDX) and element mapping characterizations were performed on the cross section of Ga₂O₃/GaN NRAs. As shown in Figure 7a–e, it can be intuitively seen that Ga and O elements are mainly distributed in the outermost layer of the nanorods, which represents the formed Ga₂O₃ thin film layer. The Ga and N elements are mainly distributed inside the nanorods, which corresponds to the GaN layer inside. These results further demonstrate the formation of the Ga₂O₃/GaN core-shell structures. To assess the photoresponse characteristics of the nano-architectonics of Ga₂O₃/GaN core-shell rod arrays, the PD (sample C) was fabricated in these structures with identical preparation method. Figure 8 shows the responsivity spectrum of sample C. The PDs exhibited a wide-spectrum response in the BUV region. An obvious optical response at 370–400 nm is observed in sample C, which is the same as sample B, indicating the existence of interband response. The peak responsivity of sample C is in the UV-A band range, while the case for sample B is in the UV-C band as described above. This is mainly because the thickness of Ga₂O₃ in sample C is ultrathin, only 10–50 nm, and the sample C mainly exhibits UV-A band response induced from GaN material. As shown in Figure 8, the peak responsivity was 2.65 × 10⁴ A/W for 365 nm at 7 V applied bias, and the responsivity at the 270–360 nm band exceeded 1.5 × 10⁴ A/W, which is a further order of magnitude enhancement than sample B. These results show that larger surface area leads to greater internal gain, which is consistent with the proposed mechanisms as aforementioned. Nevertheless, it is possible to further optimize the thickness of Ga₂O₃ film on GaN by adjusting the oxidation time, and realize the tunable detection in UV band. Overall, we proposed a simple method to fabricate the B-UV PD with high responsivity, which enables the detection under weak light illuminations.

4. Conclusions

In this study, we designed and demonstrated a BUV PD based on Ga₂O₃/GaN core-shell MRAs and NRAs, which were fabricated using partially thermally oxidizing GaN MRAs and NRAs. The PD based on MRAs showed an ultrahigh responsivity of 2300 A/W for 280 nm and over 1500 A/W for the 270–360 nm band at 7 V. The detectivity and external quantum efficiency were approximately 2.65 × 10¹³ Jones and 1.02 × 10⁶% at 280 nm, respectively. The responsivity was further increased to 2.65 × 10⁴ A/W for 280 nm and over 1.5 × 10⁴ A/W for 270–360 nm using the nanorod arrays. Such high performance can be attributed to the blocking of photo-generated holes. Specifically, the large surface area of Ga₂O₃/GaN MRAs and NRAs led to the migration of photogenerated holes to the surface of Ga₂O₃ film, which was caused by the band bending of Ga₂O₃ surface induced by oxygen absorption mechanism and resulted in the large internal gains. Moreover, the larger metal electrode area can block more holes at the heterojunction interface of Ga₂O₃/GaN, which was caused by the larger valence-band barrier (1.4 eV). Overall, the proposed metal–semiconductor–metal structure enables a large surface area and is suitable for mass production.