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Article

Investigation and Comparison of the Performance for β-Ga2O3 Solar-Blind Photodetectors Grown on Patterned and Flat Sapphire Substrate

by
Zuyong Yan
1,*,
Shan Li
2,
Zeng Liu
3,
Jianying Yue
4,
Xueqiang Ji
4,
Jinjin Wang
4,
Shanglin Hou
1,
Gang Wu
1,
Jingli Lei
1,
Guobin Sun
5,
Peigang Li
4,* and
Weihua Tang
2,*
1
School of Science, State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China
2
College of Electronic and Optical Engineering & College of Microelectronics, National and Local Joint Engineering Laboratory for RF Integration and Micro-Packing Technologies, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
3
School of Electronic Information Engineering, Inner Mongolia University, Hohhot 010021, China
4
School of Integrated Circuits & State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China
5
Cloud Network Operations Center, China Unicom Gansu Branch, Lanzhou 730050, China
*
Authors to whom correspondence should be addressed.
Crystals 2024, 14(7), 625; https://doi.org/10.3390/cryst14070625
Submission received: 19 June 2024 / Revised: 4 July 2024 / Accepted: 5 July 2024 / Published: 7 July 2024
(This article belongs to the Special Issue Epitaxial Growth and Application of Metallic Oxide Thin Films)
Browse Figures
Versions Notes

Abstract

:
Ga2O3, with its large band gap, is a promising material suitable for utilization in solar-blind photodetection. Sapphire with a higher lattice match with Ga2O3 was used as the substrate for epitaxial growth of Ga2O3. Here, the epitaxial layers of Ga2O3 were deposited by MOCVD on patterned sapphire substrates. The structure of epitaxial Ga2O3 layers on patterned substrates has been identified by X-ray diffractometry. To investigate the influence of the patterned substrates on the formation of epitaxial layers, thin Ga2O3 layers were grown on a flat sapphire substrate under the same conditions. Both types of samples were β-phase. However, no improvement in the layers’ crystalline quality was discovered when utilizing patterned sapphire substrates. In addition, the performance of the obtained two types of Ga2O3 photodetectors was compared. The photoelectric properties, such as responsivity, response speed, and detection capability, were different in the case of flat samples.

1. Introduction

Ultra-wide bandgap (UWBG) semiconductors have attracted extensive attention over the past few years because of their ultra-wide optical bandgap, excellent electronic transport properties, and photoelectric performance [1,2,3]. As a promising UWBG semiconductor material, Ga2O3 has exceeding characteristics, such as large bandgaps (4.7–5.2 eV), adjustable doping, rational electron mobilities, large-area bulk substrates, and excellent solar-blind ultraviolet (SBUV) photosensitivity [4,5]. These advantages make Ga2O3 promising for the fabrication of high-power rectifiers and switches, X-ray detectors [6,7], and solar-blind photodetectors (SBPDs) [8,9,10,11,12,13,14].
To date, the development of Ga2O3 films growth has been studied on flat sapphire [15,16], GaN [17], ZnO [18], diamond [19], and SiC [20] by various film preparation methods, for example, Molecular Beam Epitaxy (MBE) [21], Pulsed Laser Deposition (PLD) [22], Sol–Gel [23], mist Chemical Vapor Deposition (mist CVD) [24,25], Halide Vapor Phase Epitaxy (HVPE), plasma-enhanced chemical vapor deposition (PECVD) [26,27], radio frequent magnetron sputtering (RFMS) [28], and Metal Organic Chemical Vapor Deposition (MOCVD) [29].
Recently, research has shown that replacing flat sapphire substrates (FSSs) with patterned sapphire substrates (PSSs) provides an alternative, simplistic, and efficient way to improve the properties of Ga2O3 thin films [30,31,32,33]. The introduction of PSS is regarded to be an effective method to improve the photoelectric performance of Ga2O3. It is attributed to the surface-periodic array structure of the PSS which can efficiently enhance the light scattering [34,35,36]. With the advantages of transparency, wafer-level patterning, and high-throughput production, PSS has been commonly used as the substrate for light-emitting diodes (LED) to date [34]. Lee et al. have proposed that PSS can improve the efficiency of light extraction from the interface by enhancing photon scattering at the substrate–film interface, which in turn improves the overall photon absorption of the film, because of the roughness and patterning of the PSS surface [37]. Therefore, this PSS-based film fabrication method can pave the way for the development of SBPDs by changing the structure and size of the PSS. It is expected that this technology will also be used for other optical applications, e.g., metamaterials, spectroscopy, and plasma optoelectronics.
The application of PSS during epitaxial lateral overgrowth (ELO) reduces the density of threading dislocations in Ga2O3 heterogeneous epitaxial. Dang G T et al. prepared ELO α-Ga2O3 layers on an FSS with a silica mask using mist CVD, where the α-phase has a higher lateral growth rate [38]. Yang D et al. reported that α-Ga2O3 films with low threading dislocation density and residual stress can be obtained on microcavity-embedded sapphire substrates by mist CVD [30]. Nonetheless, the growth rate of mist CVD was much lower than that of HVPE. Son et al. found that Ga2O3 films grown on PSS by (HVPE) have a lower dislocation density compared to films grown on FSS [39]. Shapenkov et al. reported that the polycrystalline structure of Ga2O3 films grown via HVPE was affected by the morphology of the substrate surface [40]. Ga2O3 films grown on FSS are pure α-phase, in contrast to the HVPE deposition of Ga2O3 films on PSS, which produces a mixture of α-phase and ε-phase (or κ-phase). Nevertheless, ε (or κ)-Ga2O3 is formed at the top of the mask, which needs to be buried under a thick α-Ga2O3 layer and therefore requires a longer time of growth.
Comparisons of different types of PSS used for SBPD are insufficient, although the results of several studies have shown the advantages of PSS over FSS. In addition, the same types of PSS using different methods need to be investigated further. For the fabrication of Ga2O3 photodetectors, MOCVD is considered as an effective film preparation method to prepare large-scale continuous Ga2O3 layers, which reduces Ga2O3 defects effectively and demonstrates better optical–electrical performance [41,42]. Given that the standard ELO method of mask-selective growth has been shown to result in improved crystal quality of α-Ga2O3 films prepared by the HVPE method, it is hoped that epitaxial growth on PSSs with MOCVD will also prove to be useful. As far as we know, no systematic study on the sensing performance of Ga2O3-based PDs using MOCVD on PSS has been reported.
Based on the above, the Ga2O3 films were grown on PSS by the MOCVD method. The scanning electron microscope (SEM), X-ray diffraction (XRD), and UV-vis spectroscopy were used to analyze the Ga2O3/PSS sample. Then, Ga2O3/PSS photodetector and Ga2O3/FSS detector were prepared using the same semiconductor processing technology. Additionally, the photoelectric characteristics of the two photodetectors under 254 nm illumination, including responsivity, response speed, and detection capability, were investigated under the same measured conditions.

2. Materials and Methods

Device fabrication: The MOCVD method was used to prepare Ga2O3 films on cone-type PSS purchased from HeFei Crystal Technical Material Co., Ltd., HeFei, China. Trimethylgallium (TEGa) and oxygen (O2) gas are used as Ga and O precursors, respectively. During the preparation of the Ga2O3 samples, argon (Ar) was used as a carrier gas in the deposition system with a flow rate of 2000 sccm. Meanwhile, the growth process was maintained at 500 °C under the chamber pressure of 25 Torr. Ga2O3 thin films were obtained on FSS by the same method. Finally, square Ti/Au electrodes with a side length of 1000 μm and a thickness of 50 nm were prepared on Ga2O3 samples with an electrode distance of 1200 μm by a magnetron sputtering system to obtain two types of photodetectors.
Characterization: The surface and cross-section morphology of PSS and as-prepared samples were studied by SEM. X-ray diffraction (XRD) curves were measured for crystalline quality using a high-resolution XRD system. The optical absorption is characterized using a UV-visible spectrophotometer. Measuring the optoelectrical performance of photodetectors with the Keithley 4200-SCS parameter analyzer combined with the 254 nm UV light. The spectral response characteristics were analyzed by a monochromator (7IMS30, SOFN INSTRUMENTS CO., Ltd., BeiJing, China). The light source coupled to the monochromator is a xenon lamp (7ILX150A-UVC, SOFN INSTRUMENTS CO., Ltd.). The light power was measured using the optical power and energy meter (PM100D, Thorlabs Inc., Newtown, CT, USA).

3. Results and Discussion

SEM images of PSS show that the nanocones are arranged in a hexagonal pattern over the entire surface of the substrate (Figure 1a,b). The height and diameter of the cone pattern were approximately 600 nm and 700 nm, respectively, and the pitch was 1.0 µm between each cone pattern. SEM images of Ga2O3 samples grown on PSS are shown in Figure 1c,d. The island shape was similar to that of the cone-type substrates, with a well-developed plane on the top. Then, a monoclinic crystal habit became clear on the side facets. However, the surface of the side facet was still bumpy, and the boundaries of each island could easily be identified. The Ga2O3 film grown on FSS, which is relatively flatter, is shown in Figure 1e,f. The distribution of the grains can be seen in the samples.
Figure 2a plots the XRD spectra of the Ga2O3 layers grown on PSS. The clear major sharp peak of the sample is from sapphire. The main peaks observed are the β-Ga2O3 (−201), (201), (−402), (−510), (−602), and (−603), and no ε- or α-Ga2O3 peaks were observed, which were obtained in previous reports via HVPE and mist CVD methods [33,38]. Furthermore, the quality of the films on patterned substrates was polycrystalline, while single crystal samples were on flat substrates, which are shown in Figure 2b. The sample grown on FSS are (−201) oriented single crystal with stronger diffraction intensities. The optical absorption properties of the Ga2O3 samples grown on PSS were analyzed by a UV-vis spectrometer as shown in Figure 2c, with a strong absorption edge around 230–240 nm. However, absorption peaks also appear at other long wavelength bands compared to the Ga2O3 sample grown on a conventional sapphire substrate (Figure 2d). Absorption at long wavelengths is not a result of a change in the absorption properties of the Ga2O3 thin film sample itself, but rather due to the periodic structure of the PSS affecting the test results. During the testing process, light is scattered in the periodic structure of the sample grown on the PSS, making the actual tested absorbance increase.
The photodetector array on a PSS is schematically presented in Figure 3a. The size of the array photodetector is 1 × 1 cm2. The Ga2O3 films work as the SBUV illumination absorbing layer deposited on the PSS. Meanwhile, the Ga2O3/PSS device shows color light scattering observed at the electrodes when the device was irradiated with natural light due to the ordered nanocones. These bright colors result from the selective reflection of light from periodic microstructures, which are called structural colors [43,44]. From a physical point of view, structural color is color produced by purely physical structures that do not contain any pigment elements. This property has a wide range of applications in fields such as displays, printing, anti-counterfeiting, sensors, and biological and optical devices. On the surface of the sample that is not covered by metal, the selective reflection of light by the periodic structure is destroyed because the surface is relatively rough and unsmooth, as shown in the SEM results. In addition, more light was transmitted into the sample. Therefore, structural color could not be observed. After the sample is coated with metal, it makes the surface smoother and easier to reflect light and produce color. Its long-range ordered structure causes the viewer to see different colors at different angles.
To evaluate the function of the substrates for Ga2O3 film quality and device photodetection performance, the devices fabricated from Ga2O3 films grown on FSS are also prepared. To investigate and compare the optoelectronic characters of two devices, the I–V characteristics of dark current and photocurrent for both devices in logarithmic coordinates are shown in Figure 3b. At 5 V, the Ga2O3/PSS device shows a dark current of 3.4 × 10−12 A and a photocurrent of 1.7 × 10−8 A; meanwhile, the Ga2O3/FSS device shows a dark current of 6.3 × 10−12 A and a photocurrent of 8.0 × 10−8 A. In comparison with the Ga2O3/FSS device, the Ga2O3/PSS device shows a slight decrease in photocurrent and an insignificant change in dark current. Figure 3c illustrates that the on/off ratio increases as the bias increases. Obviously, the Ga2O3/FSS device shows a higher on/off ratio than the device based on Ga2O3/PSS. The optoelectrical performance of the samples on PSS observed a smaller characteristic than for samples on FSS, which is due to the poorer quality of the films.
The switching characteristics of two photodetectors versus light intensity are shown in Figure 4a,b, in which, similar trends can be seen within 2–80 μW/cm2. In a dark situation, the UV lamp is switched on, and the current is increased to obtain a photocurrent. After 20 s, the UV lamp is switched off, and the current decreases rapidly, gradually returning to the initial dark current value, and after 20 s, the previous process is repeated. The transient curves of the above period-varying currents are measured using the Keithley 4200-SCS semiconductor analyzer. The photocurrent periodically varies depending on the light intensity. It can be observed that the photocurrent gradually increases with the light intensity, and a higher photocurrent is seen in the FSS.
Figure 4c,d show the time-dependent photoresponse of two photodetectors under alternating On and OFF cycles under 254 nm illumination with voltage bias from 1 V to 5 V. It is observed that both the devices exhibit steady periodic switching characteristics at each voltage bias condition. As the bias changed from 1 V to 5 V, the photocurrent of Ga2O3/PSS increased from 3.3 to 16.0 nA, while the photocurrent of Ga2O3/FSS increased from 10.3 to 83.5 nA.
The relationship between the photocurrent and the power density is fitted by Iph∝Pθ, where Iph is the photocurrent, P is power density, and θ is the empirical coefficient, respectively. The θ constants are extracted to be 0.536 and 0.614 for the Ga2O3/PSS device and Ga2O3/FSS device, respectively (Figure 5a,b). The R2 values of the best fits are 0.96 and 0.99, respectively. Experimental data highlight that both behaviors are sublinear. The difference in θ is probably due to a lower correlation factor for Ga2O3/PSS.
The comparison of the response speed of both devices is shown in Figure 5c,d. The transient curves can be fitted with the following equations to obtain the rise/decay (τr/τd) times.
I ( t ) = I 0 + A e − t / τ
where I0 is the steady-state photocurrent, A is the constant, t is the time, and τ is the relaxation time constant. τr and τd are the rise edge and decay edge of the time constants, respectively.
The results of the fit are represented by the dashed red lines. Both devices have the same rise time (0.30 s). Device decay time reduced from 0.653 s (Ga2O3/PSS) to 0.094 s (Ga2O3/FSS). This may be due to the low crystalline quality of the Ga2O3 film grown on the PSS and the discontinuity and grain separation.
Responsivity (R) is a parameter that measures the performance of photoelectric conversion, which can be defined as follows [45]:
R = I ill − I d P × S
where Iill is current under illumination, and Id is dark current, and P and S are the power density of the incident light and the active area of the device exposed to incident light, respectively.
The R dependence on different incident powers was measured under 254 nm UV light at 5 V, as shown in Figure 6a. It can be observed that all the devices exhibit an obvious photoresponse, which is even more considerable for the Ga2O3/FSS device. The maximum responsivity of the device incident light has increased from 20.5 mA W−1 (Ga2O3/PSS) to 65.7 mA W−1 (Ga2O3/FSS), respectively. In particular, the relationship between responsivity with the incident power was investigated. The R first reaches a peak and then decreases with increasing incident power. This may be the lower carrier recombination and longer carrier lifetime under lower intensity illumination light and the saturation of the trap state [46].
To further investigate the influence of PSS on detectors’ photoelectronic properties, specific detectivity (D*) and noise equivalent power (NEP) were also calculated, which can be defined using the following formulas
D * = S R 2 e I d
N E P = 2 e I d R
where e is the elemental charge. Figure 6b,c show D* and NEP of two photodetectors against incident power density. It is well known that the higher the D* value, the lower the noise and the better the detection ability. In contrast, D* of the Ga2O3/PSS device is smaller, suggesting that compared to the Ga2O3/FSS device, the detection capability of Ga2O3/PSS structure for the incident light decreases with the deterioration of film quality. NEP represents the minimum incident optical power required to generate a photocurrent equal to the noise current at a given wavelength. As the power increases, NEP decreases and reaches the minimum value, then increases, as shown in Figure 6c, which shows the opposite curve to that in Figure 6b.
Additionally, the performance of the two devices was investigated at different voltages. Under 254 nm UV illumination (power density: ~600 μW cm−2), the R, and D* increase as the applied voltages increase, as shown in Figure 6d,e. The R and D* of the Ga2O3/FSS device have increased from 1.7 mA W−1 and 9.9 × 108 Jones to 14.0 mA W−1 and 56.3 × 108 Jones, respectively. Obviously, the Ga2O3/PSS device has similar tendencies and lower performance compared to the Ga2O3/FSS device. The R and D* of the Ga2O3/PSS device at 5 V are 2.9 mA W−1 and 17.2 × 108 Jones, respectively, which are larger than those of 0.60 mA W−1 and 4.6 × 108 Jones at 1 V. Furthermore, the NEP value of the two detectors was investigated at different voltages. The NEP decreases as the applied voltages increase, as shown in Figure 6f.
To further investigate the wavelength selectivity, the R of two devices for incident light wavelength ranging from 250 to 600 nm with a voltage of 5 V was studied (Figure 7). It can be seen that the R value of Ga2O3/PSS increases firstly from 0.74 mA W−1 (200 nm) to 1.5 mA W−1 (240 nm) and then reduces to 7.6 × 10−5 mA W−1 (600 nm) with increasing light wavelength, which indicates that the PDs show a distinct peak centered at ~240 nm. Meanwhile, it can be clearly seen that the spectral response characteristics (the maximum response value of 5.3 mA W−1) of the Ga2O3/FSS devices are better due to the higher film quality. Spectral response is consistent with the results of absorption spectra (Figure 2c,d).

4. Conclusions

In conclusion, Ga2O3/PSS solar-blind UV detectors have been prepared based on large-area Ga2O3 layers that were grown from PSS using MOCVD. The expected improvement in the quality of the thin film by growing Ga2O3 thin films on PSS was not successful. These expectations were derived from previous experience growing GaN films on PSS. XRD results confirm the deterioration of crystal quality. Meanwhile, the epitaxial lateral overgrowth (ELO) of Ga2O3 via MOCVD has not been demonstrated. In addition, under the same measurement conditions, the photoelectronic properties of both photodetectors were compared when illuminated under solar-blind light, indicating that the photoresponse performance of the films on PSS is degraded to that on FSS, including the deterioration of photocurrent, response speed, R, and D*. The ratios of R and D* are 3.2 and 2.3 for FSS and PSS devices, respectively. In conclusion, the growth of Ga2O3 thin films on PSS has not yet met the expected. The above results show that MOCVD is not favorable for the growth of β-Ga2O3 on patterned substrates, leading to the degradation of the quality of β-Ga2O3 films and even the formation of interrupted incomplete films. It may be due to the low thickness of the lateral overgrowth region over the nucleation layer in the thinner β-Ga2O3 film grown in this work.

Author Contributions

Conceptualization, Z.Y.; methodology, S.L.; software, G.S.; formal analysis, X.J. and J.W.; investigation, J.Y.; resources, S.H.; data curation, G.W. and J.L.; writing—original draft, Z.Y.; writing—review and editing, Z.L.; supervision, P.L.; project administration, W.T.; funding acquisition, W.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant No. 62204125), the National Key Research and Development Program of China (Grant No. 2022YFB3605404), and HongLiu Outstanding Young Talents Funding Scheme of Lanzhou University of Technology. The APC was funded by HongLiu First-Class Disciplines Development Program of Lanzhou University of Technology.

Data Availability Statement

The original contributions presented in the study are included in the article material, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Top-view SEM images of (a) PSS, (c) Ga2O3 film deposited on PSS, and (e) Ga2O3 film deposited on FSS. Cross-sectional SEM micrograph of (b) PSS, (d) Ga2O3 film deposited on PSS, and (f) Ga2O3 film deposited on FSS.
Figure 1. Top-view SEM images of (a) PSS, (c) Ga2O3 film deposited on PSS, and (e) Ga2O3 film deposited on FSS. Cross-sectional SEM micrograph of (b) PSS, (d) Ga2O3 film deposited on PSS, and (f) Ga2O3 film deposited on FSS.
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Figure 2. XRD pattern from θ-2θ scans of Ga2O3 layers grown on (a) PSS and (b) FSS. UV-vis absorbance characterizations of the as-prepared Ga2O3 film grown on (c) PSS and (d) FSS.
Figure 2. XRD pattern from θ-2θ scans of Ga2O3 layers grown on (a) PSS and (b) FSS. UV-vis absorbance characterizations of the as-prepared Ga2O3 film grown on (c) PSS and (d) FSS.
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Figure 3. (a) The schematic diagram of the Ga2O3/PSS PDs. (b) Typical I–V curves of Ga2O3/PSS and Ga2O3/FSS device in the dark and under 254 nm illumination with an intensity of 600 μW/cm2. (c) The on/off ratio of the Ga2O3/PSS and Ga2O3/FSS devices.
Figure 3. (a) The schematic diagram of the Ga2O3/PSS PDs. (b) Typical I–V curves of Ga2O3/PSS and Ga2O3/FSS device in the dark and under 254 nm illumination with an intensity of 600 μW/cm2. (c) The on/off ratio of the Ga2O3/PSS and Ga2O3/FSS devices.
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Figure 4. Photo-switching characteristics of (a) Ga2O3/PSS detector and (b) Ga2O3/FSS detector under solar-blind UV illumination with different intensities under 5V. Photoresponse curves of (c) Ga2O3/PSS detector and (d) Ga2O3/FSS detector versus voltage bias.
Figure 4. Photo-switching characteristics of (a) Ga2O3/PSS detector and (b) Ga2O3/FSS detector under solar-blind UV illumination with different intensities under 5V. Photoresponse curves of (c) Ga2O3/PSS detector and (d) Ga2O3/FSS detector versus voltage bias.
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Figure 5. Photocurrents of (a) the Ga2O3/PSS device and (b) the Ga2O3/FSS device with different power densities. Error bars are related to the nonlinear fitting of the experimental data. (c) The rise and (d) the decay time of two devices at 5 V.
Figure 5. Photocurrents of (a) the Ga2O3/PSS device and (b) the Ga2O3/FSS device with different power densities. Error bars are related to the nonlinear fitting of the experimental data. (c) The rise and (d) the decay time of two devices at 5 V.
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Figure 6. Comparison of Ga2O3/PSS and Ga2O3/FSS devices’ R, D*, and NEP (a~c) under different power densities and (d~f) at various biases (λ = 254 nm).
Figure 6. Comparison of Ga2O3/PSS and Ga2O3/FSS devices’ R, D*, and NEP (a~c) under different power densities and (d~f) at various biases (λ = 254 nm).
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Figure 7. Spectral response characteristics of the Ga2O3/PSS device (@5V).
Figure 7. Spectral response characteristics of the Ga2O3/PSS device (@5V).
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MDPI and ACS Style

Yan, Z.; Li, S.; Liu, Z.; Yue, J.; Ji, X.; Wang, J.; Hou, S.; Wu, G.; Lei, J.; Sun, G.; et al. Investigation and Comparison of the Performance for β-Ga2O3 Solar-Blind Photodetectors Grown on Patterned and Flat Sapphire Substrate. Crystals 2024, 14, 625. https://doi.org/10.3390/cryst14070625

AMA Style

Yan Z, Li S, Liu Z, Yue J, Ji X, Wang J, Hou S, Wu G, Lei J, Sun G, et al. Investigation and Comparison of the Performance for β-Ga2O3 Solar-Blind Photodetectors Grown on Patterned and Flat Sapphire Substrate. Crystals. 2024; 14(7):625. https://doi.org/10.3390/cryst14070625

Chicago/Turabian Style

Yan, Zuyong, Shan Li, Zeng Liu, Jianying Yue, Xueqiang Ji, Jinjin Wang, Shanglin Hou, Gang Wu, Jingli Lei, Guobin Sun, and et al. 2024. "Investigation and Comparison of the Performance for β-Ga2O3 Solar-Blind Photodetectors Grown on Patterned and Flat Sapphire Substrate" Crystals 14, no. 7: 625. https://doi.org/10.3390/cryst14070625

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