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Abstract
In this paper, fast response, zero-biased, solar-blind UV photodetectors based on graphene/β-Ga2O3 heterojunctions were fabricated by transferring a monolayer graphene onto fresh cleaved β-Ga2O3 (100) single crystal substrate. At zero bias, the photo responsivity at 254 nm and the UV/visible rejection ratio (R235 nm/R400 nm) and the response time are obtained to be 10.3 mA/W and 2.28 × 102 and 2.24 μs, respectively, for the graphene/β-Ga2O3 (100) detector. The fast response and the high sensitivity can be attributed to the high mobility and UV transparency of graphene top-electrode and the low defect density of the β-Ga2O3 (100) cleaved surface. Such zero-biased detectors are very promising for next-generation solar-blind UV photodetection.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to its low false alarm rate, solar-blind UV photodetectors have a wide range of applications such as environmental monitoring, flame detection, chemical analysis, short-wave communications and missile detection [1–3]. To date, various wide bandgap semiconductors, such as AlxGa1-xN, ZnxMg1-xO, diamond, SiC and β-Ga2O3 have been investigated for solar-blind UV detection purpose. Among them, AlxGa1-xN and ZnxMg1-xO ternary alloys have tunable band gaps from 3.4 to 4.8 eV, but their crystal qualities deteriorate rapidly and the phase segregation happens with the increase of Al and Mg content, which limits their performance as solar-blind UV detectors [4,5]. Diamond is an indirect-gap semiconductor, leading to a low quantum efficiency [6,7]. In contrast, monoclinic β-Ga2O3 has a direct band gap of 4.9 eV and high chemical and thermal stability, which makes β-Ga2O3 an ideal material for fabricating solar-blind UV detectors [8–10].
So far, both photoconductive (PC) and photovoltaic (PV) β-Ga2O3 solar-blind UV photodetectors were reported. Generally, PC detectors have high responsivities and simple structures, but large background current and slow response are two major impediments to their applications [11,12]. For instance, Luo et al. had fabricated a PC solar-blind β-Ga2O3 photodetector by using multilayer graphene as electrodes. The device has a high responsivity of 39.3 A/W at a bias of 20 V, but the response time is more than 200 s [13]. Therefore, PV (so called zero-biased) β-Ga2O3 photodetectors have attracted more and more attentions [14]. For example, Fang et al. synthesized single crystalline Ga2O3 microwires by a CVD method and fabricated a zero-biased ZnO/Ga2O3 heterojunction photodetector by coating the Ga2O3 microwires with ZnO layers, in which the responsivity was 9.7 mA/W and the rise/decay time are 100 μs/900 μs at a zero bias [11]. Liu et al. reported that a solar-blind UV photodetector was prepared by constructing a Schottky junction between Au and β-Ga2O3 nanowires array film and the decay time was as shorter as 64 μs. However, since the effective absorption of light occurs only at the edge of the gold electrode, the responsivity of this detector is only 0.01 mA/W [15].
Graphene has an excellent optical transmittance in a wide wavelength range from deep UV to NIR and a fantastic carrier mobility (field-effect hole mobility 8800 cm2/Vs at room temperature) [16]. Thus, graphene has been used as transparent top electrodes for various photodetectors to obtain high responsivity and fast response speed [17]. For example, by applying graphene top electrodes on the surface of semiconductor films or wafers, photodetectors can be constructed in a vertical structure, in which the light absorption area as well as the depletion region are exactly formed below the graphene electrode and the photogenerated carriers can be collection rapidly. To our knowledge, the zero-biased solar-blind UV photodetectors based on graphene/β-Ga2O3 vertical structures have been reported [18]. However, the rise/decay time of this device at zero bias are as longer as 0.62 s/0.67 s, respectively, which may be caused by large thickness (650 μm) and high surface defect density of the β-Ga2O3 wafer [19].
This paper presents that a zero-biased solar-blind graphene/β-Ga2O3 vertical photodetector can be formed by mechanically transferring monolayer graphene onto the cleaved (100) surface of 30 μm thick β-Ga2O3 single crystal substrate. At zero bias, the photodetector exhibited a response time of 2.24 μs and a maximum responsivity of 10.3 mA/W. The reasons for the fast response and high sensitivity were discussed in terms of the separation and the transportation of photogenerated carriers.
2. Experimental
2.1 Fabrication of the graphene/β-Ga2O3 photodetector
β-Ga2O3 (−201) (size: LxWxH = 5 × 2.5 × 0.6 mm3) single crystal wafer and monolayer graphene film were purchased from MTI Corporation and the Six Carbon Tech. Co. Ltd, respectively. First, the β-Ga2O3 (100) (size: LxWxH = 2.5 × 0.8 × 0.03 mm3) substrate were obtained by cleaving the β-Ga2O3 (−201) (size: LxWxH = 5 × 2.5 × 0.6 mm3) substrate. Second, an indium electrode was evaporated on the back side of β-Ga2O3 (100) substrate by thermal vacuum evaporation, then fixed on the top of a glass slide. Third, the monolayer graphene was mechanically transferred onto the top surface of the β-Ga2O3 (100). The procedure of the graphene transfer has been reported previously [20]. Finally, Au contacts were coated on one edge of the graphene electrode by thermal evaporation machine. A silicone insulating layer was inserted beneath Au contact between graphene and β-Ga2O3 (100) to reduce the leak current.
2.2 Device characterizations
Raman spectra of the graphene electrode were measured by a JY HR-800 LabRam Infinity Spectrophotometer (using a 488 nm laser). The crystal structure of β-Ga2O3 was characterized by a Rigaku D/max-2500 X-ray diffractometer (Cu Ka radiation, λ = 1.5406 Å). The I-V characteristics were measured by a Keithley 2461 sourcemeter. The photo-responsivity was tested by a system composed of a Xe lamp and a monochromator and an optical chopper. The response speed was measured using a damping light beam from a KrF excimer laser with the wavelength of 248 nm and the pulsed width of 20 ns. The photocurrent signals were recorded by a Keysight oscilloscope (DSOS404A).
3. Results and discussion
Figure 1(a) shows the schematic diagram of the graphene/β-Ga2O3 (100) detector. The UV light with the spot size of 0.5 mm2 illuminated perpendicularly on the surface of the graphene/β-Ga2O3 detector. As reported by Duan et al., the work function of graphene is in the range of 4.52 to 5.08 eV depending on the doping concentration [21]. The Fermi level of β-Ga2O3 is about 4.11 ± 0.05 eV corresponding to a net donor concentration of 6 × 1016 cm−3 [22]. Therefore, the energy band diagram at the interface of graphene and β-Ga2O3 under the UV illumination can be illustrated in Fig. 1(b). The band offset between graphene and β-Ga2O3 led to the formation of depletion region at the interface. When incident deep UV photons pass through graphene electrode, photogenerated carriers would be generated in the depletion region and then were separated under the effect of the built-in field, resulting in photoinduced voltage. Since no external driven bias was applied to the detector, such kind of detectors were named as the zero-biased photodetectors. When the zero-biased detectors were connected to a closed circuit, photocurrent could be measured by the sourcemeter [11,23].
Fig. 1 (a) The structure diagram of graphene/β-Ga2O3 vertical structure photodetector. (b) The energy band diagram under the solar-blind UV illumination at zero bias.
Figures 2(a) and 2(b) show 10 × 10 μm2 AFM images of the surface β-Ga2O3 (−201) substrate in top view and 45° perspective view, respectively. There are groove marks on the substrate surface and the root mean square (RMS) roughness is 0.918 nm. Figures 2(c) and 2(d) show 10 × 10 μm2 AFM images of β-Ga2O3 (100) substrate in top view and 45° perspective view, respectively. Apparently, the surface of β-Ga2O3 (100) substrate is smoother than β-Ga2O3 (−201) substrate and have a lower surface roughness (RMS) of 0.249 nm, which benefit to fabricating highly sensitive and quickly responsive photodetectors.
Figures 2 (a) and 2(b) are the top view and the 45° perspective view AFM images of β-Ga2O3 (−201) substrate, respectively. 2(c) and 2(d) are the top view and the 45° perspective view AFM images of β-Ga2O3 (100) substrate, respectively.
The X-ray diffraction patterns of the cleaved β-Ga2O3 substrate were shown in Fig. 3(a). The presence of sharp (400), (600) and (800) peaks of β-Ga2O3 confirmed that the surface of substrate belongs to the (100) plane of β-Ga2O3. The FWHM of (400) peak was 0.076°, consisting with the high crystalline quality of single crystalline β-Ga2O3 substrate. Raman spectra were taken from the graphene electrode after transferring to the surface of β-Ga2O3 (100) plane. As shown in Fig. 3(b) a weak D band and two strong bands corresponding to 2D and G bands can be observed. It is worth noting that the intensity ratio of 2D to G bands (I2D/IG) was about 2, indicating the graphene film is most likely monolayer [24]. In addition, the intensity ratio of D to G peaks (ID/IG) could be used to analyze the defects density in graphene because ID and IG are proportional to the defect density and the area of laser spot, respectively. The value of ID/IG was about 0.18, indicating that the structural defect density was relatively low in the graphene film [24].
Fig. 3 (a) The X-ray diffraction patterns of β-Ga2O3 (100) substrate. (b) The Raman spectrum of the graphene electrode on the β-Ga2O3 (100) surface.
Figure 4(a) shows typical I-V (logarithmic scale) curves of the graphene/Ga2O3 Schottky junction in dark and under the illumination of 254 nm light. A rectification behaviour in both dark and 254 nm light illumination suggesting that a Schottky junction was formed with the graphene/β-Ga2O3. Under 254 nm light illumination, the current at the reverse bias was about an order of magnitude higher than that of in dark. For thermionic emission, the I-V characteristics of the Schottky device can be given by the following equations:
where J(V,T) is the current density across the graphene/β-Ga2O3 interface, V is the applied voltage, T is the absolute temperature, e is the electron charge, kB is the Boltzmann constant, JS(T) is the saturation current density, n is the ideality factor, ΦB is the zero bias Schottky-barrier height, which is calculated from JS(T), A* is the Richardson constant (A* = 4πem*/h3), and m* is the effective mass of the charge carriers. JS(T) can be extracted by fitting the I-V curve with Eq. (1). For β-Ga2O3, A* is 41 Acm−2K−2 by taking the electron effective mass of 0.342 m0 [12,25]. Then ΦB value for the device was evaluated to be 0.78 eV, which is consistent with the previous reports [22,26].Fig. 4 (a) The typical I-V (logarithmic scale) curves of graphene/Ga2O3 Schottky junction in the dark and under 254 nm light illumination. (b) Spectral response obtained at zero bias.
Figure 4(b) showed the spectral response of the device at zero bias. The responsivity could be directly calculated by
where Iλ is the photocurrent, Id is the dark current, Pλ is the light power. The responsivity was measured at zero bias in the spectral range of 200 to 700 nm. The maximum responsivity of 20.6 mA/W happened at the wavelength of 235 nm and the cut-off of photocurrent happens at 280 nm. In general, a UV to visible rejection ratio defined as the ratio of the maximum responsivity in the UV range to the responsivity at 400 nm was used to evaluate the visible light exclusivity of UV detectors. Therefore, the UV to visible rejection ratio (R235 nm/R400 nm) of the solar-blind photodetector was obtained to be 2.28 × 102, indicating that the graphene/β-Ga2O3 Schottky junction could work properly as a solar-blind UV detector. In addition, since the bandgap of β-Ga2O3 is 4.9 eV (corresponding to 254 nm in wavelength), people conventionally cite the responsivity at 254 nm to evaluate the performance of solar-blind UV detectors. Thanks to the vertical structure and the high transparency of graphene top electrode, our detector presents a high responsivity of 10.3 mA/W under 254 nm light with the incident light power of 2.06 μW.The external quantum efficiency (EQE) is defined as the number of electrons probed per incident photon and can be calculated by
where Iλ is the photocurrent, Rλ is the responsivity, h is the Planck’s constant, c is the speed of light, q is the electronic charge, and λ is the incident light wavelength, respectively. Figure 5(a) shows the dependency of the responsivity and EQE at zero bias on the incident light power density under 254 nm light illumination. It was apparent that both the responsivity and EQE decreased slightly with the increasing light power intensity, which was common for photodetectors and could be ascribed to the self-heating-effect during the illumination [13]. Figure 5(b) shows the dependency of the responsivity and EQE on the bias voltage when the light power intensity of 254 nm light was fixed at 2.06 μW. At zero bias, the responsivity and EQE were 10.3 mA/W and 5%, respectively. With the bias voltage increasing from 0 to 4.5 V, the responsivity and EQE increased monotonously to 14.15 A/W and 6.89 × 103%, respectively. The significant increase of the responsivity and EQE with bias voltage was generally attributed to the depletion region widening and also the gain effect under a high electrical field [3,13].Fig. 5 (a) The responsivity and the EQE vs. light power intensities at zero-bias. (b) The responsivity and the EQE vs. the bias voltages (The light power intensity is kept at 2.06 μW). All the data were measured under 254 nm light illumination.
The response time is an important parameter for the photodetector. Hereon, we use 248 nm pulse light of a KrF excimer laser as the fast-optical signal and a Keysight oscilloscope as the signal collector to measure the response time of the device at zero bias. The equivalent circuit diagram is shown in the inset of Fig. 6(a), the photodiode is connected in parallel with a constant resistance (R1 = 1 MΩ) and the oscilloscope. Because the decay time is several orders of magnitude longer than the rise time, so we will focus on the decay time. The analysis of the voltage decay process involves the fitting of the photoresponse curve with a biexponential relaxation equation as
Where V0 is the steady state photovoltage, t is the time, C and D are constants, τ1 and τ2 are two relaxation time constants for the decay edges, respectively [27].Fig. 6 (a) The response of the graphene/β-Ga2O3 (100) photodetector, which was measured by Keysight oscilloscope under the illumination of the 248 nm pulse laser at zero bias. (b) The relationship between the decay time and laser energy density of the graphene/β-Ga2O3 (100) detector. (c) The response of the graphene/β-Ga2O3 (−201) detector. (d) The response of the graphene/β-Ga2O3 (100) photodetector. (a, b and c are measured with a series resistance of 1 MΩ, d was measured with a series resistance of 50 Ω.)
Figure 6(a) shows the time dependent response of the graphene/β-Ga2O3 (100) detector measured at a laser energy density of 7.5 µJ/cm2, and the decay edges of τd1 and τd2 are 4.95 µs and 124.85 µs. The relationship between the decay time and the laser energy density of the graphene/β-Ga2O3 (100) detector is shown in Fig. 6(b). As the laser energy density increases from 1 to 7.5 µJ/cm2 the variation of τd1 is small, but τd2 decreases monotonously from 200 to 125 µs. The decrease of τd2 can be attributed to that the high photogenerated carrier concentration increases the recombination probability and shortens the lifetime of electrons and holes. For comparison purpose, the graphene/β-Ga2O3 (−201) detector with β-Ga2O3 thickness of 0.6 mm is also fabricated and the response time is measured at the same laser energy density of 7.5 µJ/cm2. As shown in Fig. 6(c), the decay edges of τd1 and τd2 are 6.15 ms and 32.34 ms, respectively, which are much longer than that of the graphene/β-Ga2O3 (100) detector. It is known that the response time depends on RC constant of the circuit, where R is the equivalent series resistance of the circuit and C is mainly from the capacitance of the depletion layer in the photodiode. Considering that the thickness of the β-Ga2O3 (−201) substrate are 20 times larger than that of the cleaved β-Ga2O3 (100) substrate, a larger RC constant of the graphene/β-Ga2O3 (−201) detector leads to a longer decay time. To shorten the response time of the graphene/β-Ga2O3 (100) detector, the series resistance (R1) was reduced from 1 MΩ to 50 Ω. Figure 6(d) shows the time dependent response of the graphene/β-Ga2O3 (100) detector at zero bias with a laser energy density of 7.5 µJ/cm2. The rise time is less than 30 ns and the decay time (τd1/τd2) is 0.16 μs/2.08 μs, respectively.
Table 1 presents the key parameters of various solar-blind UV β-Ga2O3 detectors [11,13,15,18,28–32]. It can be seen from Table 1 that zero-biased photodetectors have shorter response time but lower responsivities than the detectors operating under biases. To our knowledge, the response time of our graphene/β-Ga2O3 (100) detector is the shortest among the reported β-Ga2O3 based photodetectors with a moderate responsivity and UV/Visible rejection ratio.
Table 1. Comparison of β-Ga2O3 solar-blind UV detectors.
The fast response of the graphene/β-Ga2O3 (100) detectors can be attributed to the following reasons: (1) because the depletion region is exactly under the graphene electrode, the photogenerated electrons and holes could be separated quickly by the built-in field [21,33]. (2) The β-Ga2O3 (100) substrate was obtained by cleaving the bulk crystal of β-Ga2O3 (−201) along (100) planes and the graphene electrode was transferred onto the surface of β-Ga2O3 (100) plane, immediately. This method minimized the contamination and the number of defects and impurities on the surface of β-Ga2O3 (100) substrate, which consequently reduced the probability of carrier trapping at the interface of junction [19]. (3) It has been reported that the electron mobility of β-Ga2O3 single crystal with a net donor concentration of high 1017/cm3 is as large as 130 cm2/Vs at room temperature [26], while graphene also has a hole mobility as high as 8800 cm2/Vs measured by field effect transistors [16]. Therefore, both electrons and holes will be collected quickly by the In and Au contacts.
Finally, we tested the stability of the photodetector by exposing the detector to ambient environment for three months and repeated the test every month, as shown in Fig. 7. The photocurrent maintained the original value even after 3 months, indicating a good stability of our zero-biased graphene/β-Ga2O3 photodetectors.
Fig. 7 The stability test of the graphene/β-Ga2O3 (100) photodetector. (measured by Keysight oscilloscope under the illumination of the 248 nm pulse laser at zero bias)
4. Conclusion
We fabricated a fast response and zero-biased solar-blind UV photodetector based on graphene/β-Ga2O3 (100) Schottky junctions. The height of Schottky-barrier of 0.78 eV was calculated by fitting the I-V curve in terms of the thermionic emission theory. The average responsivity at zero bias is as high as 10.3 mA/W under 254 nm light illumination and the UV/visible rejection ratio (R235 nm/R400 nm) is 2.28 × 102. Moreover, the detector also has a fast response with the decay time as shorter as 2.24 µs under zero bias, which has been ascribed to the quick separation of photogenerated carriers in the depletion region and the reduced defect states and impurity at the interface between graphene and β-Ga2O3 and also the high mobilities of electrons and holes in β-Ga2O3 and graphene. The performance of the detectors was kept stable during 3 months of storage in ambient atmosphere. The ultra-fast response and good stability make the zero-biased graphene/β-Ga2O3 photodetector very promising in future photoelectric applications.
Funding
National Natural Science Foundation of China (NSFC) (51872043, 51732003, 61574031); “111 project” (B13013); Key Research Program of Frontier Science, CAS, (QYZDB-SSW-SLH014); Open Research Fund of Key laboratory of UV-emitting materials and technology, (130028855, 130028856).
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