Zero-Power-Consumption Solar-Blind Photodetector Based on Î²

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Zero-Power-Consumption Solar-Blind Photodetector Based on #-Ga2O3/NSTO Heterojunction Daoyou Guo, Han Liu, Peigang Li, Zhenping Wu, Shunli Wang, Can Cui, Chao-rong Li, and Weihua Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13771 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 25, 2016

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ACS Applied Materials & Interfaces

Zero-Power-Consumption Solar-Blind Photodetector Based on β-Ga2O3/NSTO Heterojunction Daoyou Guo,†,‡ Han Liu,† Peigang Li,*,†,‡ Zhenping Wu,‡ Shunli Wang,† Can Cui,† Chaorong Li,† and Weihua Tang*,†,‡

†

Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech

University, Hangzhou 310018, China. ‡

State Key Laboratory of Information Photonics and Optical Communications & Laboratory of

Optoelectronics Materials and Devices, School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China.

1

ACS Paragon Plus Environment


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ABSTRACT: A solar-blind photodetector based on β-Ga2O3/NSTO heterojunctions were fabricated for the first time, and its photoelectric properties were investigated. The device presents a typical positive rectification in the dark, while under 254 nm UV light illumination, it shows a negative rectification, which might be caused by the generation of photo-induced electron-hole pairs in the β-Ga2O3 film layer. With zero bias, i.e., zero power consumption, the photodetector shows a fast photoresponse time (decay time τd= 0.07 s) and the ratio Iphoto/Idark ~ 20 under 254 nm light illumination with a light intensity of 45 µW/cm2. Such behaviors are attributed to the separation of photogenerated electron-hole pairs driven by the built-in electric field in the depletion region of β-Ga2O3 and the NSTO interface, and the subsequent transport toward corresponding electrodes. The photocurrent increases linearly with increasing the light intensity and applied bias, while the response time decreases with the increase of the light intensity. Under -10 V bias and 45 µW/cm2 of 254 nm light illumination, the photodetector exhibits a responsivity Rλ of 43.31 A/W and an external quantum efficiency of 2.1×104 %. The photo-to-electric conversion mechanism in the β-Ga2O3/NSTO heterojunction photodetector is explained in detail by energy band diagrams. The results strongly suggest that a photodetector based on β-Ga2O3 thin-film heterojunction structure can be practically used to detect weak solar-blind signals because of its high photoconductive gain. KEYWORDS: β-Ga2O3/NSTO heterojunction, thin film, rectification, solar-blind, photodetector, zero-power-consumption.

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1. INTRODUCTION Deep ultraviolet (DUV) photodetectors have attracted extensive attention recently due to their potential applications in many significant fields, such as missile tracking, flame detection, non-line-of-sight optical communication, biomedicine, ultraviolet radiation monitoring below the ozone hole, etc.1-3 In particular, DUV photodetectors working in the solar-blind spectrum region (wavelength shorter than 280 nm)—the so-named solar-blind photodetectors—have been intensively studied for their high precision and accuracy weak signal detection ability, even in sunlight, due to the absence of solar-blind region irradiation at the Earth’s surface.4-8 Commercial solar-blind photodetectors are mainly based on the photoelectric effect (photomultiplier tubes), and their applications are limited due to the bulky structure, the fragility, along with the requirement of a large bias voltage.6 Current researches in solar-blind photodetectors focus mostly on wide bandgap semiconductors such as AlGaN, ZnMgO, diamond, and monoclinic gallium oxide (β-Ga2O3).4-9 However, there are obvious disadvantages in these structures, such as that the epitaxial quality of the AlGaN film deteriorates dramatically with increasing Al concentration, the single wurtzite phase of the MgZnO film breaks down with increasing Mg content, and the bandgap of diamond cannot be tuned.10-11 β-Ga2O3 is considered as an ideal candidate for its wide bandgap (4.9 eV), which lies sharply in the solar-blind spectrum region and also exhibits a flexible tunability in bandgap by alloying with different materials. Moreover, due to its high chemical and thermal stability, β-Ga2O3 is favorable for robust devices that can work in harsh environments.10-12

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To date, β-Ga2O3 materials in the forms of single crystal,13-15 thin film,4,10-11,16-17 and nanostructure3,6-7,18-20 have been exploited as the active layer of solar-blind photodetectors. Different types of photodetector structures based on β-Ga2O3, such as metal-semiconductor-metal (MSM),4,10-11,17 Schottky,13-14,21 and heterojunction,16,19,22-23 have been reported. An ideal photodetector should satisfy the 5S requirements, i.e., high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability.19 However, most detectors currently are based on photoconductive type, and their performance (especially the response speed) is worse than desired due to the persistent photoconductivity effect. Furthermore, considering the potential applications in secure communication and space detection, self-powered and zero power consumption type photodetector are strongly desirable characteristics in a solar-blind photodetector. The vertical heterojunction structure photodiode can satisfy the above requirements. Zhao et al. reported a solar-blind avalanche photodetector based on a ZnO-Ga2O3 core-shell microwire heterojunction with satisfactory performance in terms of high responsivity, high detectivity, and fast response time.19 Chen et al. obtained an self-powered Au/β-Ga2O3 nanowires array Schottky junction solar-blind photodetector with a fast response time.21 However, the heterojunction structure-based thin-film type is more favored by device manufacturers because of its low cost, ease to fabricate, and excellent reproducibility. Although thin film-type solar-blind photodetectors based on β-Ga2O3/SiC and β-Ga2O3/p-Si heterojunctions have been investigated in recent years,16, 22-23

their performance is non-ideal and poorer than that of nanostructure or

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single-crystal types.14,19,21 The intrinsic β-Ga2O3 is an insulator with an energy bandgap of 4.9 eV, an electron affinity of 4.0 eV, and a Fermi level (EF) midway between the conduction band and valence band.16,24 The commercial single crystalline 0.7 wt% Nb:SrTiO3 (NSTO) has an energy bandgap of 3.2 eV, an electron affinity of 4 eV, and an EF below the conduction band bottom of 0.08 eV.25-27 A staggered energy band diagram can be formed between the β-Ga2O3 and NSTO, which would promote the separation and collection of photogenerated carriers in the depletion layer by built-in potential. Recently, gallium oxide thin films were grown on NSTO substrates at a low temperature, and a negative differential resistance phenomenon was observed in an amorphous gallium oxide/NSTO heterostructure by our group.28 In this work, we prepared crystalline β-Ga2O3 thin film on a single crystalline NSTO substrate and constructed the β-Ga2O3/NSTO heterojunction structure for the first time. The β-Ga2O3/NSTO characteristic

heterojunction and

solar-blind

photodetector

exhibits

photoelectric

property.

a

typical

More

rectification

importantly,

the

photodetector can work in a zero-power-consumption mode and demonstrates excellent performance.

2. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of the β-Ga2O3/NSTO Heterojunction. The β-Ga2O3/NSTO heterojunction was fabricated by radio frequency magnetron sputtering of a β-Ga2O3thin film on a (100) Nb:SrTiO3 substrate. Before loading into the deposition chamber, the commercial single crystalline 0.7wt% Nb-doped SrTiO3 substrate, with the

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dimensions of 10mm×5mm×0.5mm, was ultrasonically cleaned in acetone and ethanol. The base pressure in the sputtering chamber was 1×10-4 Pa. The growth temperature and Ar gas pressure were fixed at 750 oC and 0.8 Pa, respectively. The power applied to the Ga2O3 disk target was set at ~80 W. The crystallinity of the as-grown thin film was investigated by X-ray diffraction (XRD). The surface and interface morphology of the heterojunction was characterized by a field emission scanning electron microscope (FE-SEM). The optical bandgap was measured by using ultraviolet-visible (UV-VIS) optical absorption spectrum with a sample directly deposited on a (0001) α-Al2O3 substrate under the same growth conditions. The element content was analyzed by X-ray photoelectron spectroscopy (XPS). 2.2. Fabrication and Characterization of the Photodetector. An Au/Ti point electrode (~ 3 mm diameter) was deposited on the β-Ga2O3 thin film as a semi-transparent electrode through a metal-mask by radio frequency magnetron sputtering, consisting of an ultra-thin (3 nm) Ti layer following an Au layer (7 nm). A small point electrode (~ 0.2 mm diameter) of In metal was pressed onto the thin Au/Ti electrode as a region to connect Cu wire. Another In metal with a diameter of about 2 mm was pressed on the back of the NSTO substrate with an Ohmic contact as the bottom electrode. The cross-sectional schematic of the β-Ga2O3/NSTO heterojunction structure photodetector is shown in Figure 1. The current-voltage (I-V) characters, time-dependent photoresponse, and pulse laser photoresponse of the heterojunction photodetector were measured by Keithely 2450. The time-dependent photoresponse measurement was performed by a low-pressure mercury lamp of 254 nm wavelength

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with various light intensity and applied bias. All the characterizations and measurements were performed at room temperature.

3. RESULTS AND DISCUSSION The Ga2O3 thin film layer formed on the NSTO substrate is a monoclinic β phase

( )

determined by XRD (JCPDS Card no. 43-1012). The measured lattice planes of 201 , (400), (110), and (002) indexed in Figure 2a indicate that the as-grown β-Ga2O3 thin film is a poly-crystal. The obtained β-Ga2O3 thin film has a thickness of about 1.324 µm estimated from the cross-sectional FE-SEM image of β-Ga2O3/NSTO heterojunction, as shown in Figure 2b. The inset of Figure 2b is the top view FE-SEM image, displaying the surface of the thin film. The grain size of β-Ga2O3 is ~300 nm, indicating a good crystallization property. The β-Ga2O3 thin film prepared on Al2O3 substrate under the same conditions shows a significant absorption edge at a wavelength of about 260 nm, as shown in Figure 3a. As a direct bandgap semiconductor, the bandgap of the β-Ga2O3 thin film is calculated by extrapolating the linear region of the plot of (αhν)2 versus hν and taking the intercept on the hν-axis. The estimated bandgap of ~ 4.80 eV, as shown in the inset of Figure 3a, is less than our previous results due to the different thin film deposition methods.10,29-30 XPS analysis was carried out to verify the oxygen deficiency in the as-grown thin film. The surface of the as-grown β-Ga2O3 thin film was etched by Ar+ bombardment before the XPS measurement. The charge-shift spectrum was calibrated using the C 1s peak at 284.8 eV. As shown in Figure 3d, the O1s core level peaks of β-Ga2O3 thin film can be divided into two components: I and II, located at 530.9 eV and 532.1 eV, respectively.

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The peak I is assigned to lattice oxygen ions, and the peak II is assigned to the oxygen ions in the oxygen vacancies region.10,31-34 The peak ratio of II/I is about 1/4, implying the existence of abundant oxygen vacancies in the film. Figure 3b presents the XPS spectrum with the binding energy in range of -10 ~ 1200 eV and the enlarged view of -5 ~ 10 eV (inset). The valence band maximum (VBM) position was determined by linear extrapolation of the leading edge of the valence band (VB) spectrum to the base lines to account for any instrument resolution-induced tails.35 The energy difference between the EF and the VBM is calculated to be about 2.20 eV in the as-grown β-Ga2O3 thin film. The energy peak for Ga 3d is centered at 20.4 eV(Figure3c), which is attributed to the presence of gallium oxide and not gallium metal. Before the investigation of the electrical and photoelectric properties of β-Ga2O3/NSTO heterojunction, the In-NSTO interface between the bottom electrode and NSTO substrate is confirmed as Ohmic contact by the linear I-V relationship of the In-NSTO-In structure in Figure 4a. According to our previous reports,4,10 the contact between the top electrode (Au/Ti) and β-Ga2O3 thin film here is also quasi-Ohmic contact due to the presence of the large oxide vacancies as surface states at the β-Ga2O3 surface. The inset of Figure 4b shows the fresh, dark I-V characteristic curve of the β-Ga2O3/NSTO heterojunction structure photodetector. Herein, a positive (forward) bias applied to the device is defined as the current flowing from the β-Ga2O3 thin film into the NSTO substrate, as shown in Figure 1. The photodetector presents obvious rectification characteristics. The dark current (Idark) increases sharply following a turn-on voltage of about 3 V under positive bias, exhibiting an asymmetric property.

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The asymmetric ratio, Idark(10 V)/Idark(-10 V) = 40.0 nA /-3.1 nA, is about 13 in dark conditions. When the photodetector is exposed to UV light illumination with a wavelength of 254 nm, the photocurrent (Iphoto) shows a significant change for both the negative and positive biases. Such changes increase with increasing the light intensity. It is interesting to note that the rectification direction reverses compared to the dark condition, showing negative rectification. Under the illumination of light with an intensity of 45 µW/cm2, Iphoto is - 93.0 µA at the applied bias of -10 V, while it is 17.0 µA at the applied bias of 10 V, showing an asymmetric ratio of about 5.5. These ratios of Iphoto/Idark are about 3×104 and 4.25×102 for -10 and 10 V bias, respectively. When the illumination is cut off, the β-Ga2O3/NSTO heterojunction returns to positive rectification. However, the decay process of the photocurrent is slow. Figure 4c shows the consecutive sweeps of dark I-V curves after the 254 nm light was turned off, and the insert displays the change of sweep voltage. The current decreases quickly during several initial I-V sweeps, and then decreases slightly. Nevertheless, the starting point of fresh dark current cannot be obtained again. Figure 4d shows the dark I-V characteristic curve of the β-Ga2O3/NSTO heterojunction, which was measured one day after the 254 nm light was turned off. The values of Idark are about 344.2 and - 25.6 nA at the voltage of 10 and -10 V, respectively, both values are about 8 times higher compared to the initial dark current. To evaluate the performance of the β-Ga2O3/NSTO heterojunction structure photodetector, we investigated the time-dependent photoresponse by periodically turning on and off UV illumination under different conditions. For the applied bias of 0

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V (Figure 5a), the dark current is approximately -0.42 nA, which should be lower in fact because the current limit of our electrical test system is about ± 0.4 nA. Under 254 nm illumination with the light intensity of 45 µW/cm2, the current instantaneously increases to a stable value of approximately -8.6 nA, with an Iphoto/Idark ratio of about 20, exhibiting the characteristic of working with zero-power-consumption. The current decreases rapidly down to the initial dark value when the UV light is turned off. However, for applied biases of -10 V (Figure 5d) and 10 V (Figure 5g), the photoresponse time to UV illumination is slower than that with 0 V bias. Figure 5d shows consecutive illumination cycles of 50 times with a nearly identical response at -10 V, indicating high robustness and good reproducibility. While it exhibits remnant photoconductivity for 10 V bias, the values of both the dark current and the photocurrent are continuously higher than that of the original values after repeated exposure. For the applied bias of -10 V, under 45 µW/cm2 254 nm light illumination, the current increases from approximately -13.5 nA of dark current to a non-stable value of approximately -151.2 nA of photocurrent, with Iphoto/Idark ~ 11. The recovery time is also long after the light is turned off. Such slow recovery might be attributed to the photogenerated carrier trapping states, which would release the captured electrons and holes gradually under the applied bias. For a more detailed comparison of the response time, the quantitative analysis of the current rise and decay process is expressed by the fitting of the photoresponse curve with

a

bi-exponential −t

I = I 0 + Ae

τ1

−t

+ Be

τ2

relaxation

equation

of

the

following

type4,10:

,where I0 is the steady-state photocurrent, t is the time, A and B

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are constants, and τ1 and τ2 are two relaxation-time constants. As shown in Figures 5b, 5e and 5h, the photoresponse processes are well fitted. The time constants for the rise edge and decay edge, respectively, are denoted by τr and τd. The detailed parameters of the response time are listed in Table 1. It is noteworthy that both the current rise and decay processes consist of two components, i.e., a fast-response component and a slow-response component for a fixed non-zero bias, while there is only a fast-response component for 0 V bias. Generally, the fast-response component can be attributed to the rapid change of photogenerated carrier concentration as soon as the light is turned on or off, while the slow-response component is corresponding to the carrier trapping/releasing process because of the existence of defects in β-Ga2O3 thin films, such as oxygen vacancies measured by XPS. For example, for 254 nm illumination with a light intensity of 45 µW/cm2, the decay process is rapid, with a τd of 0.07s for 0 V bias, while the decay process for -10 V bias is slow, consisting of two components (τd1 = 0.65 s, τd2 = 6.87 s). Considering the possible influence of the delay time of the 254 nm low-pressure mercury lamp during the process of turning the light on and off, the decay process of the β-Ga2O3/NSTO photodetector was also measured while excited by the 248 nm KrF laser with a repetition rate of 1 Hz. Figure 5c shows the normalized temporal pulse photoresponse for 10 pulses with the applied bias of 0 V, and the enlarged view of the decay edge and exponential fitting (inset). For the 0 V bias, the photodetector exhibits a fast decay time of τd= 0.07 s, which is consistent with that under the illumination of a 254 nm low-pressure mercury lamp. Nevertheless, for the -10 V bias, the decay time

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constants of τd1 and τd2 under the laser excitation are estimated to be 0.05 s and 1.06 s (Figure 5f), respectively, which are much shorter than that under the mercury lamp illumination. The shorter of the τd1 can be attributed to the larger Iphoto/Idark ratio due to the strong light intensity of the laser, while the shorter τd2 should be attributed to the less capture of photogenerated carriers by trapping states as a result of the short duration time of the laser irradiation. Figure 6a shows the time-dependent photoresponses of the photodetector to 254 nm illumination with an applied bias of -10 V under various light intensities (0, 5, 10, 15, 20, 30, 40, 45 µW/cm2). The photocurrent increases linearly with increasing the light intensity (Figure 6b), but the response time (i.e. the rise and decay time) decreases with the increase of the light intensity (Table 1). Furthermore, the effect of the applied biases on the photoresponse is investigated. Time-dependent photoresponses shown in Figure 6c were measured under 254 nm light illumination with the light intensity of 45 µW/cm2 at various applied biases. Both the dark current and photocurrent increase with the increase of the applied bias, because more carriers are transported to the corresponding electrodes under the stronger electric field. The key parameters to evaluate the performance of a photodetector are the sensitivity, defined as (Iphoto-Idark)/Idark and expressed as a percentage; the spectra responsivity (Rλ), defined as the photocurrent generated per unit power of incident light on the effective area of a photodetector; and the external quantum efficiency (EQE), defined as the number of electrons detected perincident photon.36-37 The larger the values of Rλ and

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EQE, the higher the performance a photodetector has. We can calculate Rλ and EQE using the following equations5,36-37: Rλ=(Iphoto-Idark)/PS and EQE= [(Iphoto-Idark)/e]/(P/hν) = hcRλ/eλ, where P is the intensity of the light shining on the devices, S is the effective illuminated area, e is the electronic charge, h is Planck’s constant, ν is the frequency of the light, c is the velocity of light, and λ is the wavelength of the light. The sensitivity values, Rλ and EQE, of the β-Ga2O3/NSTO heterojunction photodetector under various light intensities and applied biases are summarized in Table 1. In our system, the maximum sensitivity of 19.48 (also the fastest response time) is obtained at 0 V under a 45 µW/cm2 254 nm light illumination. The value of Rλ increases with the increase of the applied bias (Figure 6d). Herein, the maximum Rλ value of 43.31A/W is obtained at -10 V with a light intensity of 45 µW/cm2, which corresponds to a maximum EQE ~ 2.1×104 %, while the sensitivity is only 10.20. The above phenomenon can be explained as followed: on one hand, the applied bias could promote the separation and collection of photogenerated carriers, which is beneficial for the enhancement of photocurrent, leading to the improvement of Rλ and EQE; while on the other hand, the bias also improves the dark current, which remarkably increases in our system due to the presence of a large number of oxygen vacancies in the as-grown β-Ga2O3 thin film, leading to a decreasing sensitivity. Table 2 compares the characteristic parameters of the β-Ga2O3/ NSTO heterojunction photodetector with the Ga2O3/SiC and Ga2O3/Si heterojunction. The values of Rλ and EQE for the β-Ga2O3/NSTO heterojunction photodetector are better than those of the Ga2O3/SiC heterojunction.16 The response time of the β-Ga2O3/NSTO heterojunction

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photodetector (0.07 s) is better than that of Ga2O3/Si (1.79 s).22 Considering the energy levels of Ec and Ev of Ga2O3, Si, and NSTO,24-27 the Ga2O3/Si heterojunctions would be straddling type, but it would form a staggered energy band diagram between the β-Ga2O3 and NSTO, which would promote the separation and collection of photogenerated carriers in the depletion layer by built-in potential. From the view of feasibility of fabrication, the advantage of β-Ga2O3/NSTO heterojunction photodetector is that both components are oxides, which makes the fabrication process easy. In contrast, we have to consider the oxidation of Si or SiC during the deposition of Ga2O3 for Ga2O3/SiC and Ga2O3/Si based photodetectors. The photoresponse of a semiconductor in the heterojunction is a complex process, including the generation, separation, transportation, trapping, and recombination of electron-hole pairs. In order to understand the microscopic mechanism of the photo-to-electric conversion process in our β-Ga2O3/NSTO heterojunction structure photodetector, we show schematic energy-band diagrams under different conditions in Figure 7. Many oxygen vacancies existed in our as-grown β-Ga2O3 thin film according to the O1s XPS result in Figure 3d. Varley et.al's calculations indicated that oxygen vacancies in β-Ga2O3 were deep donors with ionization energy (Ed) >1 eV.38 Wang et.al estimated the donor level of oxygen vacancies to be 1.82 eV below the conduction band minimum through cathodoluminescence measurements.39 Therefore, it suggests that the level of oxygen vacancies should be close to the middle energy level between the conduction band and valence band. For the high density of oxygen vacancies, the Fermi level will be normally pinned close to the oxygen vacancies level. In fact, our as-grown

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β-Ga2O3 films are highly insulating. In our previous reports,10 the Fermi level of the as-grown sample supports others' research work on n-type samples. Herein, for the as-grown β-Ga2O3 thin film, the bandgap is 4.80 eV, as calculated from the UV-Vis absorbance spectrum, and the energy difference between the EF and the VBM is 2.20 eV, as obtained by XPS, as described above. Figure 7(0) shows the energy band diagram of isolated β-Ga2O3 and NSTO semiconductors. After contact, electrons would flow until Fermi levels lined up. The equilibrium band diagram of the β-Ga2O3/NSTO heterojunction is shown in Figure 7a. In this diagram, an ideal interface is assumed where the traps at the interface are ignored. Ideally, the built-in potential (Vbi) can be found as the difference between the work functions,Vbi = Φs(NSTO)−Φs (β-Ga2O3) = 2.52 eV. The band offset of the valence band, ∆Ev, is 1.60 eV, and that of the conduction band, ∆Ec, is 0 eV. The Fermi level of β-Ga2O3 is close to but below the middle of the energy bandgap, and that of NSTO is close to the bottom of the conduction band. Numerous electrons exist in the NSTO because of its n-type high conductivity. The depletion layer in the β-Ga2O3 region is very thick compared with the depletion layer in NSTO because the resistivity of the β-Ga2O3 layer is much higher than that of the NSTO substrate. When a negative voltage is applied to the β-Ga2O3 (i.e., the top electrode of the β-Ga2O3/NSTO heterojunction structure) (Figure 7b), there are almost no electrons flowing from β-Ga2O3 to NSTO due to the rare free electrons in the β-Ga2O3. Therefore, the exhibiting current is small (inset of Figure 4b). When a positive voltage is applied to the β-Ga2O3(Figure 7c), the current increases slowly as the voltage increases and then

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increases rapidly at the applied voltage of about 2.5 V (inset of Figure 4b), corresponding to the value of the built-in potential, Vbi. This is because the free electrons in the NSTO will cross the barrier and flow to β-Ga2O3 under a high bias. When 254 nm UV light illuminates the β-Ga2O3/NSTO heterojunction, the light will traverse the semi-transparent top electrode, excite the β-Ga2O3 thin film, and generate electron-hole pairs. For the 0 V bias, the photogenerated electron-hole pairs in the depletion region will separate under the built-in electric field (Figure 7d) and subsequently transport toward corresponding electrodes, while the photogenerated electron-hole pairs away from the depletion region in the β-Ga2O3 thin film will recombine again quickly and will not contribute to the photocurrent. When a negative bias is applied to the β-Ga2O3 (Figure 7e), all of the photogenerated electron-hole pairs in the β-Ga2O3 thin film will separate rapidly, and subsequently the electrons will transport toward NSTO and the holes will transport toward β-Ga2O3, leading to a substantial increase of current (Figure 4b). Although the photogenerated electron-hole pairs can also separate with a positive bias (Figure 7e), the increase of photocurrent is less, because of the barrier and the relatively larger dark current. Due to the presence of many oxygen vacancies in the β-Ga2O3 thin film (Figure 3d), the photogenerated electrons will be captured by oxygen vacancies under illumination and will be released with a bias when the light turns off. In fact, the process of the carrier trapping/releasing is extremely slow.40-41 Therefore, under a fixed bias, the photoresponse time, which consists of two components, with a fast-response component attributed to the rapid change of photogenerated carrier concentration as soon as the light is turned on/off and

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a slow-response component caused by the carriers trapping/releasing owing to the defect states, is always slow due to the existence of oxygen vacancies. However, without an extra electric field (under a 0 V bias), the trapped carriers cannot be released from the oxygen vacancies traps and they will recombine immediately, radiatively or nonradiatively; therefore, they do not contribute to the current. As a result, the photoresponse time only has the fast component, exhibiting a fast rise/decay process.

4. CONCLUSIONS The β-Ga2O3/NSTO heterojunction was fabricated by radio frequency magnetron

sputtering of a β-Ga2O3 thin film on a single crystal NSTO substrate. The as-grown β-Ga2O3 thin film exhibits a solar-blind UV characteristic. The β-Ga2O3/NSTO heterojunction presents a positive rectification in the dark, and a negative rectification under 254 nm UV light illumination. The photodetector based on a β-Ga2O3/NSTO heterojunction shows a fast photoresponse time and an Iphoto/Idark ratio of about 20 under a 45 µW/cm2 254 nm light illumination with no bias applied. The obtained zero-power-consumption characteristic can be attributed to the separation of the photogenerated electron-hole pairs in the depletion region under the built-in electric field and the subsequent electron-hole transport toward corresponding electrodes. With the increase of the light intensity, the photocurrent of the photodetector increases, while the response times decrease. When the applied bias increases, the photocurrent increases because the stronger electric field drives more carriers to the corresponding electrodes. With a bias of -10 V, the photodetector exhibits a maximum responsivity Rλ of 43.31 A/W and an EQE of 2.1×104 % under a 45-µW/cm2 254 nm light illumination. In

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summary, we demonstrated that the β-Ga2O3/NSTO heterojunction can be practically used to fabricate a zero-power-consumption solar-blind photodetector.

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AUTHOR INFORMATION Corresponding author: *P. Li. E-mail: [email protected]. *W. Tang. E-mail: [email protected].

Notes The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS We thank D. Murray and P. Lin for helpful discussion. This work was supported by the National Natural Science Foundation of China (No. 51572033, 61274017, 51172208, 11404029), Science Foundation of Zhejiang Sci-Tech University (ZSTU) (No. 16062190-Y), Science and Technology Department of Zhejiang Province Foundation (Grant No. 2017C37017).

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Figure and Table Captions Figure 1. Schematic illustration of β-Ga2O3/NSTO heterojunction photodetector. Figure 2. (a) XRD patterns of β-Ga2O3/NSTO heterojunction; (b) Cross-sectional view and top view (inset) FE-SEM images of β-Ga2O3/NSTO heterojunction. Figure 3. (a) UV-Vis absorbance spectrum of the β-Ga2O3 thin film with the plot of (αhν)2 vs. hν in the inset. (b) XPS survey spectrum and valence band edge spectrum (inset) of the as-grown β-Ga2O3 thin film. (c) Core level of Ga 3d in the 15-30 eV region. (d) Core level of O 1s in the 529-534 eV region. Figure 4. (a) I-V characteristic of the In-NSTO-In structure. (b) I-V characteristic curves of the β-Ga2O3/NSTO heterojunction structure photodetector in the dark and under 254 nm light illumination with various light intensities; the inset is the enlarged view of the fresh dark I-V characteristic curve. (c) Consecutive sweeps of I-V curves of the β-Ga2O3/NSTO photodetector when the 254 nm illumination is turned off; the change of sweep voltage is depicted in the inset. (d) I-V characteristic curve comparison of the original dark condition and the dark-again condition after turning off the 254 nm light for a day. Figure 5. Time-dependent photoresponse of the β-Ga2O3/NSTO heterojunction structure photodetector measured under different conditions: (a) Illuminated by 254 nm light with an intensity of 45 µW/cm2 by on/off switching at 0 V; (b) Enlarged view of the rise/decay edges and the corresponding exponential fitting; (c) Normalized temporal pulse response excited by KrF excimer pulse laser for 10 pulses at 0 V, and the enlarged view of the decay edge and exponential fitting (inset); (d) Illuminated by

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254 nm light with the intensity of 45 µW/cm2 by on/off switching at -10 V; (e) Enlarged view of the rise/decay edges and the corresponding exponential fitting; (f) Normalized temporal pulse response excited by KrF excimer pulse laser for 1 pulse at -10 V, and the exponential fitting of the decay edge; (g) Illuminated by 254 nm light with the intensity of 45 µW/cm2 by on/off switching at 10 V; (h) enlarged view of the rise/decay edges and the corresponding exponential fitting. Figure 6. Time-dependent photoresponse of the β-Ga2O3/NSTO heterojunction structure photodetector measured under different conditions: (a) Illuminated by 254 nm light with various light intensities by on/off switching at -10 V; (b) Current as a function of the light intensity; (c) Illuminated by 254 nm light with a light intensities of 45 µW/cm2by on/off switching under various applied bias; (d) Responsivity as a function of applied bias. Figure 7. Schematic energy band diagrams of the β-Ga2O3/NSTO heterojunction. (0)Before contact. Dark conditions (after contact): (a) Under 0 V bias; (b) Under reverse bias; (c) Under forward bias, under a 254nm UV illumination (after contact): (d) Under 0 V bias; (e) Under reverse bias; (f) Under forward bias. Table 1. Comparison of the performance parameters of the β-Ga2O3/NSTO heterojunction structure photodetector under various light intensities and applied biases. Table 2. Comparison of the characteristic parameters of the β-Ga2O3 thin film photodetectors based on heterojunction.

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TABLE 1

β-Ga2O3/NSTO Heterojunction Photodetector Sensitivity [(Iphoto-Idark)/Idark];

Response Time(τ)

Rλ = (Iphoto-Idark)/PS;

Various Test

EQE= hcRλ/eλ

τr

τd

Conditions Light Intensity 2

(µW/cm )

45

45

Bias (V)

5

Rλ

EQE

τr1

τr1

τd1

τd1

(A/W)

(%)

(s)

(s)

(s)

(s)

10

2.27

10.43

5.1×103

1.81

14.51

0.96

11.80

5

2.41

2.08

1.0×103

1.82

13.81

0.80

11.60

2

2.58

0.35

1.7×102

1.76

11.71

0.84

10.92

0

19.48

2.6×10-3

1.3

-2

3.58

0.36

1.8×102

0.99

9.51

0.56

8.73

-5

4.22

3.37

1.6×103

0.93

10.49

0.73

9.17

-10

10.20

43.31

2.1×104

1.08

8.78

0.65

6.87

5.38

36.77

1.8×104

1.35

9.51

0.92

8.71

3.20

39.04

1.9×104

1.98

12.56

1.17

10.70

2.26

42.34

2.1×104

2.91

18.99

1.44

12.42

30 15

Sensitivity

-10

0.21

0.07

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TABLE 2 Heterojunction (Thin film)

Responsivity (A/W)

EQE (%)

I254/Idark

Response Time

Ref.

Ga2O3/SiC

0.068

34

103

1.2ms

[16]

Ga2O3/Si

370

1.8×105

9.4×102

1.79s

[22]

Ga2O3/NSTO

43.31(-10V)

2.1×104(-10V)

20(0V)

0.07s(0V)

This work

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Fig.1 181x128mm (96 x 96 DPI)



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Fig.2 314x132mm (96 x 96 DPI)


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Fig.3 318x259mm (96 x 96 DPI)



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Fig.4 321x255mm (96 x 96 DPI)


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Fig.5 328x530mm (96 x 96 DPI)



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Fig.6 335x267mm (96 x 96 DPI)


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Fig.7 520x451mm (96 x 96 DPI)



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A graphical abstract 464x241mm (96 x 96 DPI)


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Zero-Power-Consumption Solar-Blind Photodetector Based on Î²

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