Ga2O3 Schottky Avalanche Solar‐Blind Photodiode with High Responsivity and Photo‐to‐Dark Current Ratio

Solar‐blind photodetectors have attracted extensive attention due to their advantages such as ultra‐low background noise and all‐weather. In this study, the planar Ti/Ga2O3/Au Schottky avalanche photodetector (APD) is fabricated based on β‐Ga2O3 epitaxial film on the sapphire substrate grown by metal–organic chemical vapor deposition. The Schottky APD exhibits a high responsivity of 9780.23 A W−1, an ultrahigh photo‐to‐dark current ratio of 1.88 × 107, an external quantum efficiency of 4.77 × 106%, a specific detectivity of 9.48 × 1014 Jones, with an ultrahigh gain of 1 × 106 under 254 nm light illumination at 60 V reverse bias, indicating high application potential for solar‐blind imaging. The superior photoresponse performances ascribe to the effective carrier avalanche multiplication, which contributes to the high photocurrent, and the high quality Schottky junction depletion, which leads to the low dark current.

Avalanche photodetectors (APDs) usually can promise orders of magnitude higher photocurrent than those of other type photodetectors due to avalanche photo carrier multiplication, and thus leads to high R. Reported Ga 2 O 3 APDs are with n-n junctions including -Ga 2 O 3 /SnO 2 , [22] -Ga 2 O 3 /ZnO, [23,24] -Ga 2 O 3 /ITO, [25] and -Ga 2 O 3 /MgO/ Nb:STO. [26]Due to the high dark current, these APDs show low PDCR (10 1 -10 4 ).Ideal APDs are with p-n junction or Schottky junction, which can form strong depletion at the junction and thus the APDs can offer very low dark current and high PDCR. [27,28]Ga 2 O 3 is an n-type semiconductor, and it is difficult to prepare p-type Ga 2 O 3 or epitaxial n-type Ga 2 O 3 on a p-type single crystal substrate, Ga 2 O 3 Schottky diode should be an ideal structure for realizing APDs with low dark current and high PDCR.Moreover, the Schottky diode type APDs usually have a lower avalanche breakdown voltage than the p-n diodes due to only one-side depletion.
In this work, a Schottky APD based on Ga 2 O 3 epitaxial film grown by metal-organic chemical vapor deposition (MOCVD) was fabricated.The Schottky APD offers a high R of 9780.23A W −1 , an ultrahigh PDCR of 1.88 × 10 7 , and an ultrahigh gain of 10 6 .The PDCR and gain are two orders of magnitude higher than those of the reported Schottky APD based on exfoliated Ga 2 O 3 single crystals. [27]

Results and Discussion
The X-ray diffraction (XRD) study illustrates that the Ga 2 O 3 epitaxial film on sapphire shows three peaks of 19.02°, 38.45°, and 59.17°, corresponding to the (2_01), (4_01), and (6_01) orientations, [29] as shown in Figure 1a.The top-view scanning electron microscope (SEM) image of the Ga 2 O 3 film, as shown in Figure 1b, shows clear grains and grain boundaries.The thickness of the Ga 2 O 3 film used to construct the Schottky APD is ≈310 nm estimated by SEM cross-section image as displayed in Figure 1c.The asymmertrical peak of O 1s in Figure 1d can be deconvoluted into two Gaussian-Lorentzian peaks, the peak OI located at 531.8 eV and peak OII located at 530.5 eV, corresponding to the oxygen vacancy and lattice oxygen atoms, respectively.The peak intensity ratio of OII/(OI + OII) is 15.6%, indicating a high oxygen vacancy density in the Ga 2 O 3 epitaxial film.
Figure 2a shows the structure of the fabricated Ga 2 O 3 Schottky APD. Figure 2b shows the energy band diagram of the Au-Ga 2 O 3 Schottky junction.A high Schottky barrier height (Ф b = 1.17 eV) can be extracted by fitting the forward I-V curve in dark with the following equations: [30] where I s is the saturation current, A is the contact area (9.4 × 10 −4 cm 2 ), A* is the effective Richardson constant, Ф b is the  Schottky barrier height, n is the ideality factor, k is the Boltzmann constant, q is the electron charge, and R s is the series resistance.
Figure 2c shows the current-voltage curves of the Ga 2 O 3 Schottky APD in dark and under 254 nm light illumination with various power densities P. In dark, the APD shows rectification characteristic with a low dark current of ≈10 −11 A at the reverse bias up to 60 V, due to the strong Schottky depletion.Under the light illumination, the APD show typical high avalanche photocurrent.The low dark current and high avalanche photocurrent lead to an ultrahigh PDCR of 1.88 × 10 7 at 60 V reverse bias with the P of 64 μW cm −2 .As shown in Figure 2d, the Ga 2 O 3 APD exhibits high photocurrent stability across nine continuous bias sweeps.After nine bias sweeps, the dark current of the APD show no clear change.The R of the APD is evaluated by the following equations:R = (I photo − I dark )/PS, where, S is the photosensitive area, P is the incident light power density.The higher light power density produces the larger responsivity and PDCR, as demonstrated in Figure 2e,f.The responsivity and PDCR of the APD achieves 141 A W −1 and 1.19 × 10 4 even at a light power density of 3 μW cm −2 , indicating a sensitive detection of weak light.Figure 2 g shows the reverse photocurrent (P = 49 μW cm −2 ) under different temperatures, showing that the breakdown voltage increases from 10.7 to 16.1 V with the temperature increase from 300 to 360 K.33] Figure 3a shows the current-reverse voltage curves of the Ga 2 O 3 APD in dark and under the light illumination in biexponential scale.Here, the dark current decreases with the increased reverse bias voltage, and this is because the increased reverse electric field results in an enlarged depletion width, and creates increased number of defects.While the former induces an enhanced depletion of free electrons in the semiconductor layer, the latter can also trap increased number of free electrons, and thus, it may lead to a decrease in the reverse current.The avalanche breakdown voltage, evaluated as the inflection point in the reverse current curves, decreases from 28.84 to 2.95 V with the increase of the P from 3 to 64 μW cm −2 , as shown in Figure 3b, due to the increased photo carrier density.The lateral depletion width in the Ga 2 O 3 APDs is estimated to be ≈0.72 μm, [34] and thus the avalanche breakdown electric fields of the APD with the P of 3-64 μW cm −2 are evaluated to be ≈0.04-0.40MV cm −1 , as shown in Figure 3b, which are comparable to those of the reported Ga 2 O 3 APDs with n-n junctions (0.05 and 0.15 MV cm −1 ). [22,25]To evaluate the avalanche intensity, avalanche gain estimated as [I L (V)-I D (V)]/[I L (1)-I D (1)] is studied.Here, I L (V) and I D (V) are the multiplied current under light illumination and dark current, respectively.I L (1) and I D (1) are the unmultiplied current under light illumination and dark current, respectively, at 1 V reverse bias.As the P increases, this gain at 60 V reverse bias increased to a maximum value of 1 × 10 6 at 49 μW cm −2 , and then decreases with further increased P, as shown in Figure 3c.The avalanche mechanism is shown in Figure 3d.The photogenerated electron carriers accelerate under the reverse bias (above the avalanche break voltage), when they obtain enough kinetic energy, additional electrons will be excited through the impact ionization process.
The normalized time-dependent photoresponse characteristics of the APD was studied, as shown in Figure 4a, to evaluate the influence of P on the photoresponse speed.The rise time and decay time are defined as the time with the current increases from 10% to 90% and drops from 90% to 10% of the peak value, respectively.The APD shows relatively long rise/decay response time in range of 0.79/0.0.64-6.60/3.85s due to the trapping and detrapping effects of the high structure defect density and high oxygen vacancy density which are revealed by the SEM and XPS, respectively.And, the APD shows decreased rise and decay time with increased P, as shown in Figure 4b, and this is attributed to that with the increased P, more trap states were filled with the photogenerated electrons, and this accelerates the light response speed of the APD.
To detect the UV-vis rejection ratio and the light response sensitivity to the sub-gap defects, the photocurrent response of the APD under light with various wavelength, , including 254, 405, 638, 785, and 808 nm were studied, as shown in where h is the Plank's constant, c is the velocity of light, and q is the elementary electric charge.
Our results show that increased reverse bias leads to increased PDCR, D*, R and EQE, as shown in Figure 5b,c, and this is because of that increased reverse bias can enlarge the depletion width and thus the impact distance of carriers, resulting in more carriers generated through the impact ionization.The APD achieves the maximum PDCR of 1.88 × 10 7 , D* of 9.48 × 10 14 Jones, R of 9780.235][26][27][28][35][36][37][38][39][40][41][42][43][44] Comparing with those reported Ga 2 O 3 photodetectors, the APD in this work shows the best overall performances of R and PDCR, as shown in Figure 5d, indicating high application potential for high contrast and high sensitivity solar-blind photodetection.

Conclusion
In conclusion, we have demonstrated a high-performance solarblind Schottky APD based on Ga 2 O 3 epitaxial film grown by MOCVD.While the high quality Schottky contact with large barrier height of 1.17 eV contributes to the low dark current of ≈10 −11 A, the avalanche breakdown contributes to a high photocurrent of 3.35 × 10 4 A at 60 V reverse bias with the light power density of 64 μW cm −2 .The APD shows a low avalanche break voltage of 2.95 V and breakdown electric field of 0.04 MV cm −1 with the light power density of 64 μW cm −2 , and achieves a high maximum avalanche gain of 10 6 at 60 V reverse bias.The APD shows a high R of 9780.23A W −1 , an ultrahigh PDCR of 1.88 × 10 7 , a high D* of 9.48 × 10 14 Jones, a high EQE of 4.77 × 10 6 % at 60 V reverse bias and high UV-vis rejection ratio with the light power density of 64 μW cm −2 , indicating great application potential in highly sensitive and high contrast solarblind imaging detection.

Experimental Section
The 300 nm thick Ga 2 O 3 epitaxial film was grown on the c-plane sapphire substrate by MOCVD (Emcore D180), with the electron concentration of ≈10 17 cm −3 .The substrate was ultrasonicated in acetone, ethanol, and deionized water in sequent, and dried with nitrogen gas.The substrate temperature and working pressure during film growth were 900 °C and 40 mbar, respectively, with the other growth details the same as reported. [45]Figure 1a shows the structure of the fabricated Ga 2 O 3 Schottky APD.To form ohmic contact, the Ga 2 O 3 under the cathode area was pretreated by inductively coupled plasma (ICP) using a mixture gas of Ar (5 sccm) and BCl 3 (15 sccm) for 4 min with the ICP/radio frequency power of 150/50 W, and then, 30/20 nm Ti/Au was deposited by electron beam evaporation as the cathode, and the sample was post-annealed in nitrogen atmosphere at 350 °C for 1 min.Finally, 50 nm Au was deposited by electron beam evaporation as the Schottky anode.The APD was patterned by photolithography with the interdigital electrodes with the finger length, finger width, and the spacing width of 400, 10, and 10 μm, respectively.The working area of the APD was 5.2 × 10 4 μm 2 .The electric properties were measured with Agilent B2902A.The light source used for photoresponse measurements was an LED (GS3535DUV-D-P20) with the peak wavelength of 254 nm and the spot radius of 20 mm which could cover the whole working area of the APD.The light power density (P) was calibrated by an optical power meter of Ophir (NOVA II header and PD300-UV probe).

Figure 1 .
Figure 1.a) XRD pattern of the Ga 2 O 3 epitaxial film on the sapphire substrate.b,c) Top-view and cross-section SEM images of the Ga 2 O 3 film.d) O 1s XPS spectra of the Ga 2 O 3 film.

Figure 2 .
Figure 2. a) The schematic structure of the Ga 2 O 3 Schottky APD.b) The band diagram of the -Ga 2 O 3 with Au. c) The current-voltage curves of the APD in dark and under light with various power density.d) The repeated nine photocurrent-voltage scans of the APD and the two dark current cans before and after such nine scans.e,f) The responsivity and PDCR of the APD as a function of the light power density.g) The reverse photocurrent-voltage curves at 300, 340, and 360 K with the light power density of 49 μW cm −2 .h) The dependence of the avalanche voltage on the temperatures.The incident light wavelength is 254 nm.

Figure 3 .
Figure 3. a) The current-reverse voltage curves in biexponential scale of the APD in dark and under the light with various power density.b) The APD avalanche voltage and breakdown electric field as a function of the light power density.c) The gain of the APD as a function of the reverse bias with various light power density.d) The avalanche mechanism of the APD.

Figure 4 .
Figure 4. a) The normalized time-dependent photoresponse characteristics of the APD at 60 V reverse bias with various light power density.b) The rise and decay time of the APD as a function of the light power density.

Figure 5a .
The APD shows very high UV-vis rejection ratio in range of 1.23 × 10 5 -4.43 × 10 6 at the bias of −60 V.This indicates that the photocurrent is dominated by the bandgap of the Ga 2 O 3 film, and the defects with the activation energy below 3.1 eV have little contribution to the photocurrent.The external quantum efficiency (EQE) and detectivity (D*) of the APD are evaluated by the following equations:

Figure 5 .
Figure 5. a) The current-voltage characteristics of the APD in dark and under the light with the wavelength of 254, 405, 638, 785, and 808 nm.b) The PDCR and D* of the APD as a function of the reverse bias with the 254 nm light power density of 64 μW cm −2 .c) The responsivity and EQE of the APD as a function of the reverse bias with the 254 nm light power density of 64 μW cm −2 .d) The responsivity versus PDCR of the reported Ga 2 O 3 solar-blind photodetectors and the APD in this work.