1. Introduction

Recently, deep ultraviolet (UV) solar blind photodetectors have a great significance in civil and military applications, such as missile tracking, short-range secure communication, UV astronomy, ozone holes monitoring, corona detection and so on [1–4]. Several wide band gap materials have been employed to fabricate solar-blind photodetector, such as AlGaN, MgZnO, c-BN, diamond and β-Ga₂O₃ [5,6]. Challenge for AlGaN to apply for DUV PDs is high defects in AlGaN with high Al-content to achieve large bandgap [7]. Synthesizing high quality MgZnO faces a big challenge due to the significant phase segregation problem with a band gap large than 4.5eV [8]. c-BN is an indirect band gap material [9]. Diamond is high cost and UV defection range is constrained due to the fixed bandgap [10]. β-Ga₂O₃ as a stable oxide of Ga [11], with a band gap of ~4.2-5.3 eV, is considered as one of the ideal candidates to fabricate deep UV solar blind photodetector [12,13].

Recently, many research studies Ga₂O₃ based PDs, like nanomaterials, single crystal and thin film. Li et al. constructed a bridged β-Ga₂O₃ nanowires for solar-blind photodetection, which shows a fast decay time (<<20 ms) [14]. Chen et al. fabricated a self-powered solar-blind photodetector based on Au/β-Ga₂O₃ nanowires array film schottky junction, which exhibits a very low dark current of 10 pA at −30 V [15]. Kwon et al. prepared an excellent photoresponsivity and spectrum selectivity using exfoliated Ga₂O₃ single crystal [16]. Oh et al. made photodetectors by transferred graphene electrodes onto exfoliated Ga₂O₃ single crystal film, which also shows a high photoresponsivity of 29.8 A/W [17]. However, the preparation processes of nanomaterials are complicated and the photodetectors based on nanomaterials are unstable. While single crystal is very expensive. Thin film has huge advantages in applications due to the simple preparation process, less time consuming and lower cost.

There are many methods to grow β-Ga₂O₃ thin films in the past few years, including molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), halide vapor phase epitaxy (HVPE), atomic layer Deposition (ALD), pulsed laser deposition (PLD), low pressure chemical vapor deposition (LPCVD), radio frequency magnetron sputtering (RMFS). High purity, uniformity, and composition of Ga₂O₃ thin films can be synthesized by using MBE and MOVPE. And a low vacuum required for ALD can synthesize films with good uniformity and large area. Films deposited by using PLD can control the thickness effectively and get a good membrane component. However, the growth rates of these growth methods are slow [18–20]. Although HVPE has a high growth rate, while it needs a high growth temperature to prepare high purity material. LPCVD [21] is a growth technique for vertical power electronic devices. RFMS is a simple and affordable equipment to fabricate Ga₂O₃ films with a relatively low deposition temperature and fast growth rate, while can also a large area fabrication [22,23]. Recently, RFMS is one of the most popular deposition methods to grow Ga₂O₃ thin films for many researchers. The deposition rate of 3~4.5 nm/min and the average grain size of 50 nm can be obtained by RFMS [22].

At present, there are several structures were fabricated for Ga₂O₃ thin film solar blind photodetectors, such as metal-semiconductor-metal (MSM) photodetectors, Schottky barrier photodetectors, p-n junction photodetectors, p-i-n photodetectors, and phototransistors [24,25]. Among these structures, the MSM structure was widely used due to its simple production process, low growth cost, easy industrialization, easy integration, low capacitance and low dark current. There are several factors affecting the performance of MSM structure, such as the thickness and quality of thin film, the material and spacing of electrode, the interface between metal-electrode, and so on [26–32]. Among them, the active layer thickness of β-Ga₂O₃ thin film is one of the key factors, which affects the absorption of UV light and the distribution of electric field, playing an important role on the photoelectric properties of the solar-blind UV photodetector [33]. An active layer cannot absorb all light with a thin thickness, which it would impact the transport of photo-generated carriers and increase the production cost. Thus, to optimize the thickness of the thin film is instructive for the further exploitation of the high performance MSM structure β-Ga₂O₃ thin film photodetectors.

In this paper, β-Ga₂O₃ thin films with different thickness (90 nm-540 nm) were grown on (0001) sapphire substrates using radio frequency magnetron sputtering. The MSM structure β-Ga₂O₃ thin films photodetectors were fabricated, and the photoelectric performances were investigated. The optical properties of β-Ga₂O₃ thin film and photoelectric properties of the β-Ga₂O₃ MSM photodetector were tuned obvious by the film thickness. And the optimization thick of β-Ga₂O₃ thin film is 303 nm.

2. Experimental

The β-Ga₂O₃ thin films were grown on (0001) oriented single crystal α-Al₂O₃ (10 mm × 10 mm × 0.5 mm) substrates by radio frequency magnetron sputtering. Radio frequency power was set at 70 W, with a base pressure of 1 × 10⁻⁴ Pa and a high purity (99.99%) Ga₂O₃ disk as the target. The flow rate of Ar (99.99%) gas was fixed to 25 sccm by a mass flow controller. The Ga₂O₃ thin films were deposited at a working pressure of 1 Pa. Firstly, the growth temperature was set 550 °C, 650 °C and 750 °C respectively for exploring the optimum deposition temperature. The thickness of the film is about 360 nm. Then, the Ga₂O₃ films with the thickness ranged from 90 nm to 540 nm by varying the deposition times at the optimum deposition temperature of 750 °C. The deposition time of Ga₂O₃ films are 30 min, 60 min, 90 min, 101 min, 120 min, 150 min and 180 min for the film thickness of 90 nm, 180 nm, 270 nm, 303 nm, 360 nm, 450 nm and 540 nm respectively. The crystal structures characterization was measured by a Bruker D8 Advance X-ray diffractometer (XRD). Ultraviolet-visible (UV-vis) absorption spectrum was taken using a Hitachi U-3900 UV-visible spectrophotometer. The current-voltage (I-V) and time-dependent photoresponse of the β-Ga₂O₃ thin films based photodetector were measured by Keithely 2450. The time-dependent photoresponse measurement was performed with the UV lamp. A 6 W lamp was used as light source to provide light of 254 nm wavelength for the UV response measurement. (All the characterizations were carried out at room temperature.)

3. Results and discussion

Figure 1(a) shows the XRD patterns of Ga₂O₃ thin films deposited on Al₂O₃ substrates at various substrate temperatures. When the substrate temperature is between 550 °C and 750 °C, three peaks correspond to β-Ga₂O₃ ($\overline{2}01$), ($\overline{4}02$) and ($\overline{6}03$) are observed except for the diffraction peak of the substrate. With the increase growth temperature of substrates, the intensity of the diffraction peaks increases. The optimal deposition temperature is found to be at 750 °C. Figure 1(b) gives the UV-visible absorbance spectrum of β-Ga₂O₃ thin film. The spectra of the measured samples exhibit a sharp absorption edge at wavelengths about ~260 nm. The plot (ahv)² versus hv, as shown in the light inset of Fig. 1(b), where h is Planck’s constant, a is the absorption coefficient, and v is the frequency of incident photon. The band gaps of β-Ga₂O₃ thin films are 4.99 eV, 4.97 eV and 4.95 eV for the deposition temperature of 750 °C, 650 °C and 550 °C fitted by extrapolating the linear region. The observed narrowing of the band gap can be attributed to the better crystalline quality, which is consistent with the XRD results. Figure 1(b) show that the high deposition temperature leads to large band gap of β-Ga₂O₃ thin films. This can be explained by the following reasons. Generally, the high the deposition temperature will lead to the better crystallinity of the thin film and the close to the theoretical value of the energy gap. However, low deposition temperature results the film with many defects, such as oxygen vacancy and gallium vacancy. Defects can produce intermediate energy levels in band gaps [34]. Therefore, high deposition temperature leads to a larger the band gap of β-Ga₂O₃ thin films. Consistent experimental results can be found in the literatures [35,36].

 

Fig. 1 Ga2O3 thin film: (a) XRD patterns at various substrate temperatures.(b)The UV absorbance spectrum and compared with the plot of (ahv)2 versus hv for the sample in inset. (c) Cross-sectional SEM image.

Download Full Size | PDF

The thickness of β-Ga₂O₃ thin films is about 360 nm for exploring the optimum deposition temperature here, as shown in Fig. 1(c).

In order to investigate UV photoelectric properties of the β-Ga₂O₃ thin film, a three-pair interdigital Ti/Au electrodes were deposited on top of the β-Ga₂O₃ thin film using a shadow mask to construct a MSM structure device. The thickness of Ti layer and Au layer are about 30 nm and 100 nm, respectively. The electrode fingers were 100 μm wide, 2800 μm long, 100 μm spacing gap and 3 pair fingers. The 365 nm and 254 nm wavelength were used as light sources, respectively. The effective irradiated area was ~0.03 cm². Figure 2(a) shows the schematic configuration of β-Ga₂O₃ solar blind photoelectric detector MSM structure. Figure 2(b) displays the I-V characteristic curves of the sample prepared at 750 °C with a thickness of 360 nm, which shows a Schottky-type contact. The I-V curve measured under 365 nm light does not show increase as compared with the I-V curve measured in dark, which suggests that the β-Ga₂O₃ thin film is not sensitive to 365 nm light. In contrast, the current shows a sharp jump when the device is exposed to the 254 nm light. Notably, the photocurrent increases linearly with the increase of the light intensity.

 

Fig. 2 The MSM structure β-Ga₂O₃ solar blind photodetector: (a) the schematic diagram of MSM structure. (b) I-V characteristics curves, the current as a function of the light intensity in the inset. (c) Time-dependent photoresponse to 254 nm light with a light intensity of 1 mW/cm² under an applied bias of 5 V. (d) Experimental and fitted curve of the rise and decay process photoresponse to 254 nm light.

Download Full Size | PDF

To test the repeatability of the photodetectors, the optical response was studied by used dynamic response time measurements. Figure 2(c) shows the on-off switching characteristics of the photodetector device with an intermittent 254 nm UV light (with a light intensity of 1 mW/cm²) at 5 V. Under 254 nm UV light, the photocurrent instantaneously increases to a stable value of approximately 1 μA. When the UV light turns off, the current decreases rapidly down to ~0.1 nA, which is quite close to the initial dark value. The time-dependent photoresponse curve displays a good repeatability. The rise time and fall time for photodetector were measured at various bias voltages. The quantitative analysis of the current rise and decay process involves the fitting of the photo-response curve with a exponential relaxation equation of the following type [13,37]:

Figure 3(a) shows the typical XRD pattern of β-Ga₂O₃ film with various thicknesses ranging from 90 nm to 540 nm. Only ($\overline{2}01$) direction diffraction peaks of β-Ga₂O₃ is observed except the peak of substrates, which indicates that the β-Ga₂O₃ film is grown along with the crystal plane family of ($\overline{2}01$). The intensity of diffraction peaks increases as the film thickness increases. With the increase of thickness, the absorption edge exhibits a rad shift. The bandgaps are estimated to range from 4.6 eV to 5.2 eV with various thicknesses (inset of Fig. 3(b)). Figure 3(c) shows optical bandgaps with various thickness and fitted curve of exponential fitting. The bandgap present exponential decrease with the increase of film thickness.

 

Fig. 3 (a) XRD patterns of the Ga₂O₃ thin film with various thickness ranging from 90 nm to 540 nm. (b) UV visible absorption spectra and the plot of ${(ahv)}^2$versus$hv$in inset. (c) Optical bandgaps with different thickness and the corresponding exponential fitting. (d) Time-dependent photoresponse of the β-Ga₂O₃ photodetector to UV light under an applied bias of 5 V with various thickness.

Download Full Size | PDF

Figure 3(d) shows the time-dependent photoresponse of photodetectors with various thickness film to 254 nm light (1mW/cm²) by on/off switching under an applied bias of 5 V. The photocurrent first rises and then decreases as the film thickness increases from 90 nm to 540 nm. The optimal thickness of β-Ga₂O₃ thin film in MSM structure photodetectors is 303 nm. The UV light cannot be absorbed completely when the thickness of β-Ga₂O₃ film is too thin. While for the thick film, the distribution of the electric field lines is sparse inside the film between the positive and negative electrode in the electrostatic field.

Transmission method is used to get the optical absorption coefficient of β-Ga₂O₃ thin film for 254 nm wavelengths, which can measure the attenuation of light. The current is measured under 254 nm light with the light intensity of 0.3 mW/cm² at 5 V. Figure 4(a) shows the current with various thickness film and the fitting of the curve. 85% of 254 nm wavelength can be absorbed when the film thickness is 303 nm. Transmission characteristics of the incident light in absorption medium is that the light decay exponentially with the increase of the propagation distance. Light attenuation is proportional to the light intensity in the medium, as shown in the following formula:

 

Fig. 4 (a) The incident light of UV light intensity P = 300 μW/cm², transmitted light intensity decrease with the increase of β-Ga₂O₃ thin film thickness. (b) Schematic without light: The vertical distribution of electrostatic field schematic in the film based on the MSM structure is shown under an applied bias of 5 V. (c) (I_light-I_dark)/I_dark with different thickness. (d) The 303 nm thickness β-Ga₂O₃ thin film: Time-dependent photoresponse of the MSM photodetector to UV light under an applied bias of 5 V.

Download Full Size | PDF

Figure 4(b) shows the vertical distribution of electrostatic field schematic in the film based on the MSM structure. It is shown under an applied bias of 5 V in dark [38–40]. Under the illumination of 254 nm light, the non-equilibrium carriers are generated on the surface and inside of β-Ga₂O₃ thin film. The non-equilibrium carriers decay exponentially from the surface to the interior of the film.

When the thickness is less than 303 nm, the optical absorption rate is the main factor for the photocurrent. When the thickness is greater than 303 nm, the distribution of the electric field line plays a major role. Considering the light absorption and the distribution of electric field lines, the parameters of the active layer thickness can be optimized. The active layer thickness is avoided too thick or too thin. It is suitable to be around 303 nm thickness seen from the Fig. 3(d). The sensitivity of photodetector defined as (I_photo-I_dark)/I_dark in percent (I_photo is the current of the device illuminated with a 254 nm light source and I_dark is the dark current) [41]. Figure 4(c) shows the (I_photo-I_dark)/I_dark of the various thickness β-Ga₂O₃ thin film UV photoelectric detector. The (I_photo-I_dark)/I_dark of the 303 nm thick β-Ga₂O₃ thin film UV photoelectric detector is approximately as high as 16000, suggesting a good application in UV photodetectors. Figure 4(d) shows the time-dependent photoresponse of 303 nm thick film photodetector. The time-dependent photoresponse of the photodetector to 254 nm light by on/off switching under an applied bias of 5 V. For the time-dependent photoresponse, the device still exhibits a nearly identical response after multiple light cycles, which indicates the high robustness and good reproducibility of the photodetectors. Approximately, About 303 nm thick is recognized as a suitable candidate for a solar-blind photodetector.

4. Conclusions

In conclusion, β-Ga₂O₃ thin films with different thickness have been prepared on the (0001) sapphire substrates by radio frequency magnetron sputtering technique. Meanwhile, photodetector based on the thin films have been fabricated. Different thickness of active layer can affect photoabsorption and electric field distribution of the β-Ga₂O₃ photodetector. The thickness of the film is optimized by combining the light absorption and the distribution of the electric field. The appropriate film thickness can be used to make high performance and low cost devices. The experiment result shows that the β-Ga₂O₃ photodetectors with 303 nm thickness grown at 750 °C takes the highest ratio of (I_light-I_dark)/I_dark upon exposure to 254 nm light. Furthermore, the device demonstrated excellent stability over time. It showed fast response and recovery time. Our study offers a great potential in the choice of thickness of β-Ga₂O₃ solar-blind UV photodetector applications.