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Abstract
High-sensitivity, rapid response β-Ga2O3 film-based metal-semiconductor-metal (MSM) solar-blind photodetectors were obtained by replacing conventional Au/Ti electrodes with single-layer graphene (SLG) nanosheets. Compared with the photodetector with Au/Ti electrodes, the photodetector using SLG electrodes demonstrates enhanced deep ultraviolet photoresponse properties, including a more rapid response speed (0.078 s), a higher light-to-dark current ratio (~1.41 × 104%), a higher R254/R365 rejection ratio (~1020), and the larger detectivity (~6.29 × 1011 Jones). The integration of SLG with β-Ga2O3 offers a large transparent area to the incident photons, and narrows the barrier at the contact interface upon ultraviolet illumination, strongly suggesting an unlimited potential for deep ultraviolet optoelectronic devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
β-Ga2O3 is a material holding great promise for numerous deep ultraviolet (DUV) optoelectronic applications owing to its wide energy bandgap (~4.8 eV), high breakdown electric field (~8 MV/cm), and high thermal and chemical stability [1,2]. The progress in high performance β-Ga2O3 based optoelectronic devices, like metal-semiconductor field-effect transistors, p-n diodes and so forth, have increasingly lured attentions for their tremendous potential applications [1,3]. Among those applications, β-Ga2O3 is, in particular, suitable for solar-blind photodetectors within the continuous wavelength coverage from 200 nm to 280 nm and capable of detecting a very faint signal under intense background radiations [4,5]. So far, solar-blind photodetectors have been widely investigated by many groups in terms of MSM structure, heterojunctions and Schottky barrier diodes etc [6–12]. Chen et al. fabricated DUV photodetectors with the diamond/β-Ga2O3 heterostructures, and the maximum responsivity of the fabricated photodetector (PD) was 0.2 mA/W and their applicability as solar-blind photodetectors had a short response time with tens of milliseconds [13]. Yang et al. investigated the device performance of Schottky barrier diode photodetectors based on Cu/β-Ga2O3/Ti/Au vertical structure. These photodetectors demonstrated a high rectification ratio of 5 × 107 under ± 2 V bias and a fast response speed of 0.7 s [14]. Singh Pratiyush et al. demonstrated the satisfactory responsivity of 3.3 A/W in β-Ga2O3 based ultraviolet (UV) MSM photodetectors, along with an extremely small dark current about 4 nA and a response time of 3.3 s [15]. In those devices, MSM PDs possess noticeable strengths including low dark current, high responsivity and compatibility with conventional fabrication process. Nevertheless, the poor response speed of MSM PDs is indeed the major drawback [16,17]. Attempts are conducted to optimize the response speed by replacing that electrode materials [18,19].
Graphene has long been a cutting-edge issue of intensive study for promising electronic and optoelectronic applications [20,21]. Especially, graphene has the exclusive integration of high carrier mobility and optical transparency ranging from the deep-UV to infrared spectral range, facilitating it to become an eminent candidate for high quality transparent conducting electrodes [22,23]. Qu et al. fabricated β-Ga2O3/SiC heterojunctions photodetectors by means of graphene transparent electrodes and also investigated their device performance [24]. Ai et al. fabricated vertical solar-blind photodetectors in the sandwich structure by utilizing graphene as well as β-Ga2O3 thin films and the fabricated devices exhibited the large responsivity [25]. In the current study, graphene is applied as the UV-transparent conductive electrodes in β-Ga2O3 MSM PDs in an attempt to speed up their response. In this paper, the fabricated MSM PD using Au/Ti electrodes is also compared with the devices using SLG electrodes in terms of photoresponse performances. β-Ga2O3 DUV MSM PDs with SLG electrodes demonstrated superior photoresponse properties, such as the more rapid response speed, higher Ilight/Idark ratio, higher rejection ratio and higher detectivity.
2. Experimental
β-Ga2O3 thin films were deposited via radio frequency magnetron sputtering technique. Sapphire (0001) was employed as substrates. The deposition chamber was evacuated to the pressure of ~10−5 Pa. Argon was admitted up to the pressure of 1 Pa. The β-Ga2O3 films were generated at 750 °C and 80 W for 4h. Also, the SLG was grown by MOCVD on 50 μm copper foil with a surface polymethyl methacrylate (PMMA) layer, extricated then by etching the copper foil in Ferric chloride solution. After a certain period of the time, two separated floating SLG sheets were transferred on the surface of the obtained β-Ga2O3 thin film using standard procedures. For comparison, a pair of Au/Ti electrodes were deposited on the left side of the SLG films by radio frequency magnetron sputtering technology (Fig. 1).
Fig. 1 Schematic diagram of the β-Ga2O3 wafer with SLG and Au/Ti electrodes, together with the optical microscopy image of electrode/β-Ga2O3/electrode hybrid structure.
3. Results and discussion
Figure 2(a) shows the Raman spectrum of the SLG. It is clearly found that a high G peak (1593 cm−1) and a higher 2D peak (2692 cm−1), with a Raman intensity ratio of about 2.71, indicating the graphene is single-layer and of great quality. Furthermore, the defect-related D peak is negligible, demonstrating that there are barely defects inside of the SLG [26,27]. Figure 2(b) demonstrates the XRD patterns of Ga2O3 thin films deposited on (0001) sapphire substrates. All the peaks are identified as β-Ga2O3 (01) and higher order diffractions, apart from the substrate diffraction peaks. Besides, no redundant peaks are found, revealing the single (01) plane orientation growth of that β-Ga2O3 thin film. Figure 2(c) shows the optical absorption spectrum of the β-Ga2O3 thin film and it is obvious that β-Ga2O3 thin film has a cut-off wavelength near 260 nm. The optical bandgap for direct allowed transition is estimated to be 4.8 eV [inset of Fig. 2(c)] [28].
Fig. 2 (a) Raman spectrum of the SLG. (b) XRD pattern of the β-Ga2O3 thin film. (c) UV-vis absorbance spectrum of the β-Ga2O3 thin film with the plot of (αhν)2 versus hν for the sample in inset.
In order to check the UV photoresponse of the β-Ga2O3 photodetectors, Fig. 3 shows the room temperature I-V characteristics of the sample. It can be observed that the devices with Au/Ti and SLG electrodes alike are only sensitive to 254 nm light illumination. Moreover, the photocurrent is found to increase with increasing voltage intensity under 254 nm light [Fig. 3(a)]. Figure 3(a) shows the photodetector with SLG-Au/Ti electrodes exhibits rectifying characteristics, which means that the barrier height is different at SLG/β-Ga2O3 and Ti/β-Ga2O3 interfaces upon 254 nm light illumination. Besides, something obvious is that I-V characteristics curve of the photodetector with SLG-SLG electrodes is more linear than that of the photodetector with Au/Ti-Au/Ti electrodes, suggesting the barrier at SLG/β-Ga2O3 interface is lower and narrower than the barrier at Ti/β-Ga2O3 interface while 254 nm light illuminating [29]. It is also noticed that under (200 μW/cm2) 254 nm light illumination at + 5 V bias, the Ilight/Idark ratio is ~0.07 × 102 for the photodetector with Au/Ti-Au/Ti electrodes, while the photo-to-dark current ratio of the photodetectors with SLG-SLG and SLG-Au/Ti electrodes are ~1.41 × 102 and ~1.29 × 102, respectively. The rejection ratio is regarded as the ratio of the responsivity under 254 nm and 365 nm lights illumination, which reveals the ratio of signal/specific background radiation noise. The rejection ratios, under (200 μW/cm2) 254 nm and (200 μW/cm2) 365 nm illumination, of those devices with Au/Ti-Au/Ti, SLG-Au/Ti and SLG-SLG electrodes are 89, 136 and 1020, respectively [Fig. 3(b)].
Fig. 3 I-V characteristics curves on a linear scale toward the β-Ga2O3 photodetectors in dark and under (200μW/cm2) 365 nm and (200μW/cm2) 254nm light illumination: (a) The devices with Au/Ti-Au/Ti electrodes, SLG-Au/Ti electrodes and SLG-SLG electrodes, respectively. I-V characteristics curves on a logarithmic scale toward the β-Ga2O3 photodetectors in dark and under (200μW/cm2) 365 nm and (200μW/cm2) 254nm light illumination: (b) The devices with Au/Ti-Au/Ti electrodes, SLG-Au/Ti electrodes and SLG-SLG electrodes, respectively.
For evaluating the performance of the devices with different electrodes comparatively, their room temperature time-dependent photoresponse to 254 nm light illumination is investigated by controlling the on/off switching of 245 nm light source under an applied bias of 30 V, and the results are shown in Fig. 4. Figure 4(a) shows that after multiple illumination cycles, the devices still exhibit the nearly identical response, showing the strong robustness as well as great reproducibility of the three photodetectors. Besides, the remarkable characteristic in Fig. 4(a) is that the Ilight/Idark ratios of those devices increase successively, in which the Ilight/Idark ratio of the device with SLG-SLG electrodes is much larger than the device with Au/Ti-Au/Ti electrodes.
Fig. 4 Time-dependent photoresponse of the β-Ga2O3 thin films photodetectors to 254nm light illumination curves on a logarithmic scale: (a) The devices with Au/Ti-Au/Ti electrodes, SLG-Au/Ti electrodes and SLG-SLG electrodes, respectively. Enlarged view of the rise/decay edges and the corresponding exponential fitting curves on a linear scale: (b) The devices with Au/Ti-Au/Ti electrodes, SLG-Au/Ti electrodes and SLG-SLG electrodes, respectively.
The rise edge as well as decay edge of photocurrent corresponds to two components with initial fast response and slow response later in general [7,30]. For a more detailed comparison of the response time of that three devices, the quantitative analysis of the photocurrent rise and decay processes refers to the fitting of the photoresponse curve with a bi-exponential relaxation equation of the following formula [31]:
where I0 is the steady state photocurrent, t is the time, A as well as B is constant, both τ1 and τ2 are relaxation time constants. What can be observed is that the current rise for the devices with SLG-SLG and SLG-Au/Ti electrodes is steep with the τr of 0.078 s and 0.21 s, respectively. In contrast, the rise edge for the device with Au/Ti-Au/Ti electrodes consists of two components (τr1 = 0.32 s, τr2 = 2.9 s) [Fig. 4(b)]. The decay time constant τd is estimated to be 0.060 s and 0.13 s for the PDs with SLG-SLG and SLG-Au/Ti electrodes, respectively. As a contrast, the decay edge for the device with Au/Ti-Au/Ti electrodes consists of two components (τd1 = 0.41 s, τd2 = 7.7 s). It is evident that the photodetectors with SLG electrodes present the much faster response speed to UV light and darkness than the photodetectors with Au/Ti-Au/Ti electrodes. The fast rise and fall of photocurrent in the SLG/β-Ga2O3 device has been attributed to the effect of SLG as large area transparent conductive electrodes [32].To quantitatively assess the device performance of the present three PDs, the detectivity (D*) was calculated. The detectivity, used for describing the weakest detectable signal in general, can be defined by the following formula [33]:
where R is the responsivity, Pλ is the light intensity, S is the effective area of the photodetector channel, e is the electronic charge and Ilight is the photocurrent. Based on Formula (2) and those constants resulting from the experiment, the D* under the 254 nm light illumination of 200 μW/cm2 is calculated to be 2.66 × 1011 Jones, 6.29 × 1011 Jones and 6.16 × 1011 Jones, for the PDs with Au/Ti-Au/Ti, SLG-Au/Ti, SLG-SLG electrodes, respectively. Figure 5 shows the detectivity obtained under illumination of diverse deep ultraviolet light intensities. What is obvious is that the detectivity is found to decrease with the increasing light intensity. The phenomenon is possibly attributed to the self-heating at strong light intensity, which can lead to the increase in charge carrier scattering as well as recombination rate [16,34].Fig. 5 The detectivity of the β-Ga2O3 thin films photodetector under light illumination with diverse intensities: The photodetectors with Au/Ti-Au/Ti electrodes, SLG-Au/Ti electrodes and SLG-SLG electrodes, respectively.
The photoresponse of β-Ga2O3 towards photon is a complicated process including photocarrier generation, electron-hole separation and recombination etc. In order to comprehend the differences of interface contact between the electrodes and β-Ga2O3 thin films, a schematic energy band diagram at forward bias voltage in the β-Ga2O3 thin films MSM photodetectors is shown in Fig. 6. When the metal was contacted with β-Ga2O3 thin film, the electrons moved to the metal and thus the energy levels near the n-type semiconductor β-Ga2O3 bent upward, causing the formation of built-in electric field near the metal/β-Ga2O3 contact, which is called the “depletion region”. For the interface contact between metal Ti and β-Ga2O3, the barrier height (qΦM) of ~0.27 eV could be formed, according to the Schottky-Mott rule, in which the work functions of Ti and the electron affinity of β-Ga2O3 are ~4.27 eV and ~4 eV, respectively [29,35]. In such the case, the barrier is relative low, making it susceptible to electron tunneling [Fig. 6(c)]. Consequently, the device solely with Au/Ti electrodes has larger dark current and lower photo-to-dark current ratio. In contrast, the work function of SLG is ~4.5 eV, yielding a barrier height (qΦG) of ~0.5 eV at contact interface between the SLG and β-Ga2O3, according to the Schottky-Mott rule [29,36]. In that case, the transport processes are mainly dominated by thermionic emission of carriers and it is difficult for electrons to through the barrier relative to electron tunneling [Fig. 6(a)]. Hence, the dark current of the devices with SLG electrodes is extremely lower than that of the device with Au/Ti electrodes at forward bias voltage.
Fig. 6 Schematic energy band diagram of the electrode/β-Ga2O3 heterojunction at forward bias voltage: (a) The photodetector with SLG electrodes in dark conditions; (b) The photodetector with SLG electrodes under UV illumination; (c) The photodetector with Au/Ti electrodes in dark conditions; (d) The photodetector with Au/Ti electrodes under UV illumination.
While 254 nm UV light is illuminated on the device with Au/Ti electrodes, the photoexcited electron–hole pairs will be separated and then the photogenerated holes are mostly trapped in the depletion region. The rearrangement towards the spatial charge gives rise to an increase in positive charge density in the depletion region, thus causing the barrier to shrink. The narrowed depletion region allows the electrons to tunnel more easily in the MS interface, leading to the relatively large photocurrent [Fig. 6(d)]. Similarly, the device with SLG electrodes, the bulk of the photogenerated holes are likewise trapped in the depletion region due to the forward bias. Besides, the surge of positive charge density in the depletion region narrowed the depletion region dramatically due to SLG offering large highly transparent area to the incident photons, resulting in nearly barrier-free and quick tunneling at the SLG/β-Ga2O3 interface [Fig. 6(b)] [15,37,38]. Therefore, the high response speed of the devices with SLG electrodes is created by such a transport mechanism. The high photo-to-dark current ratio of the fabricated UV MSM PD using SLG electrodes is because of the low dark current attributing to the existence of a relatively high barrier in dark condition and the high photocurrent that created by tunneling effect under 254 nm light illumination.
Table 1 lists some important photoresponse properties of several β-Ga2O3-based and graphene-related photodetectors. It can be observed that the devices with SLG electrodes present the high photo-to-dark current ratio, detectivity and rapid response speed. Such a rapid response speed and high performance, with the simple structure and low fabricating costs offer tremendous potential in high performance optoelectronic system and photodetectors in years to come.
Table 1. Summary of the key photoresponse parameters of the solar-blind PDs fabricated using β-Ga2O3 thin films.
4. Conclusions
In summary, we fabricated high-sensitivity and rapid response DUV photodetectors based on β-Ga2O3 thin films with SLG electrodes. By introducing graphene as transparent conductive electrodes, the response as well as recovery time is significantly reduced for the narrowed barrier at SLG/β-Ga2O3 interface. The MSM PD with SLG-SLG electrodes that exhibits a response time of 0.078 s and a recovery time of 0.060 s, is much faster than the photodetector with ordinary Au/Ti-Au/Ti electrodes. Besides, the fabricated β-Ga2O3 device using SLG-SLG electrodes exhibits the higher light-to-dark current ratio (~1.41 × 102), rejection ratio (~1020) and detectivity (~6.16 × 1011 Jones), which is superior to the device with Au/Ti-Au/Ti electrodes. Our research shows that the β-Ga2O3 based photodetectors in MSM structure with SLG electrodes have an infinite development prospect for the applications in high-performance DUV optoelectronic devices.
Funding
National Natural Science Foundation of China (61604100, 11404029, 51572033, 51172208), Beijing Municipal Commission of Science and Technology (SX2018-04), the Fund of State Key Laboratory of Information Photonics and Optical Communications (BUPT), and the Fundamental Research Funds for the Central Universities.
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