Laser molecular beam epitaxy technology has been employed to deposit β-gallium oxide (β-Ga2O3) on (0001) sapphire substrates. After optimizing the growth parameters, -oriented β-Ga2O3 thin film was obtained. Ultraviolet-visible absorption spectrum demonstrates that the prepared β-Ga2O3 thin film shows excellent solar-blind ultraviolet (UV) characteristic with a band gap of 5.02 eV. A prototype photodetector device with a metal-semiconductor-metal structure has been fabricated using high quality β-Ga2O3 film. The device exhibits obvious photoresponse under 254 nm UV light irradiation, suggesting a potential application in solar-blind photodetectors.
© 2014 Optical Society of America
Due to the strong absorption of deep ultraviolet (UV) light by stratospheric ozone, the solar irradiation between 200 and 280 nm which is called solar-blind region does not exist at the surface of the earth [1, 2]. Consequently, the photodetectors operating in this region, the so-named solar-blind photodetectors, could detect very weak signals accurately under sun and artificial illuminations due to the “black background”. Solar-blind photodetectors have a vast and ever growing number of military and civil surveillance applications such as missile tracking, fire detection, ozone holes monitoring, chemical/biological analysis, and so on [3, 4]. Unfortunately, the currently commercially available solar-blind photodetectors (photomultiplier tubes) are bulky, fragile, and require large bias voltage, thus limiting their applications . In recent years, solar-blind photodetectors based on wide band gap semiconductors such as AlGaN, ZnMgO, diamond, and the monoclinic gallium oxide (β-Ga2O3) have attracted intensive attentions [4, 6–10]. Although AlGaN with high Al composition has been achieved solar-blind photodetection, the crystal quality of the AlGaN epitaxial layer deteriorates rapidly with increasing Al composition . Meanwhile, it is difficult to grow a single wurtzite ZnMgO with high Mg composition due to phase segregation between ZnO wurtzite and MgO rock salt . And the sensitivity range of diamond is restricted to wavelengths below 225 nm because its band gap is fixed at 5.5 eV .
β-Ga2O3 is well known as a transparent semiconductor compound with a band gap of ~4.9 eV, which is particularly suitable for solar-blind photodetection [4, 7]. Many techniques have been employed to prepare the Ga2O3 thin films, including sol-gel methods, spray pyrolysis, magnetron sputtering, metal-organic chemical vapor deposition, and plasma-assisted molecular beam epitaxy [12–17]. However, the film-type photodetectors show the poor photoresponse performance, which is insufficient for practical use [13, 14, 17]. One of the most important things may be due to the poor quality films. Laser molecular beam epitaxy (LMBE) is an ideal technology for the growth of high quality Ga2O3 thin films, because of: (1) the levels of barely impurity with an ultrahigh vacuum environment and high purity source materials, (2) the control of atomic-layer growth through the monitor of reflection high energy electron diffraction and by simple adjusting the laser repetition rate. In this paper, we explored the growth of β-Ga2O3 epitaxial thin films on (0001) sapphire (Al2O3) substrates under various temperatures and oxygen pressures by LMBE method. A prototype device with metal-semiconductor-metal (MSM) structure was fabricated using β-Ga2O3 thin film. The measurement results show that the device behaviors an excellent UV photoresponse characteristic behavior.
β-Ga2O3 thin film was grown on Al2O3 (0001) substrates using LMBE. The base pressure in chamber was 1 × 10−6 Pa. The growth temperatures ranging from 650°C to 850°C and oxygen pressure ranging from 5 × 10−3 Pa to 5 × 10−1 Pa were applied to explore the optimum growth conditions. The laser ablation was carried out at a laser fluence of 5 J/cm2 with a repetition rate of 2 Hz using a KrF excimer laser with a wavelength of 248 nm. The distance between target and substrate was 5 cm, and the substrate was rotated during deposition to improve the film uniformity. The thickness of the films was estimated by scanning electron microscope (SEM) to be ~200 nm, corresponding to a growth rate of 0.28Å/pulse. To fabricate a photodetector, an interdigital Ti/Au electrode was deposited on the β-Ga2O3 thin film using a shadow mask by radio frequency magnetron sputtering, and subsequently thermally annealed at 300°C for 10 min in Ar atmosphere. The electrode fingers were 200 μm wide, 2800 μm long, and with a 200 μm spacing gap. The schematic illustration of the interdigital electrodes of the device is shown in Fig. 1.
The crystal structure was measured by a Bruker D8 Advance X-ray diffractmeter (XRD) using Cu Ka (λ = 1.5405 Å) radiation. The surface morphology was characterized by a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) and a Bruker-Veeco atomic force microscopy (AFM). 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-based photodetector were measured by Keithely 2400. A UV lamp (~7 W) with the wavelength of 254 nm and 365 nm was fixed at a distance of ~5 cm away as the light source. The time-dependent photoresponse measurement was performed at a constant voltage of 10 V.
3. Results and discussion
Figure 2 shows the XRD patterns of the Ga2O3 thin films deposited at various substrate temperatures and oxygen pressures. For films grown at 5 × 10−3 Pa, when the substrate temperature is 650°C, there is no peaks appear except for Al2O3-related diffraction peaks, indicating that the prepared film is amorphous or microcrystalline. As the substrate temperature is 750°C, in addition to the diffraction peak of the substrate, three peaks located at 18.83°, 38.17° and 58.86° corresponding to β-Ga2O3 , and are observed. No other peaks apart from these are found. The result indicates that the film deposited on the (0001) Al2O3 substrate is pure β-Ga2O3 with single plane orientation. While the substrate temperature increases to 850°C, the intensities of and higher order diffractions peaks from Ga2O3 decrease. However, as the oxygen pressure increases, extra diffraction peaks of (110), and (113) of Ga2O3 phase appear and the intensity of the peaks corresponding to Ga2O3 plane decreases, featuring a polycrystalline nature. With the oxygen pressures of 5 × 10−1 Pa, these extra peaks are obvious. As a result, the thin film grown at 5 × 10−3 Pa and 750°C has the best crystallization with an out-plane relationship of β-Ga2O3parallel to Al2O3 (0001).
The results imply that the structure of the Ga2O3 films changes with different temperatures and oxygen pressures. For the molecules, atoms or ions composing the film do not have enough energy to transfer on the surface of the substrate at lower temperature. The growth of the crystal is constrained during the crystallization process. With the substrate temperature of 750°C, enough thermal energy is supplied to the molecules, atoms or ions on the substrate and increases the surface mobility, which leads to the plane family orientation. Figure 3 shows schematically a proposed geometrical epitaxial relationship between the (0001) Al2O3 substrate and the plane of β-Ga2O3 thin film. The oxygen atoms in the equivalent plane of β-Ga2O3 have the same arrangement as the oxygen atoms of the Al2O3 (0001) plane . In addition, 4 lattice planes of β-Ga2O3 in the  direction match very closely with 3 lattice planes of Al2O3 in the direction, and 2 lattice planes of β-Ga2O3 in the  direction match very closely with 3 lattice planes of Al2O3 in the direction . The lattice mismatches are −1.6% and + 3.13% respectively, which is accommodated by the domain variation principle . Therefore, when β-Ga2O3 is formed on the (0001) Al2O3 substrate, gallium can bond to the oxygen atom layer almost without feeling the difference between the (0001) Al2O3 and the plane of β-Ga2O3. The growth mode is suggested to be domain matching epitaxy. However, the high oxygen pressures probably destroys the arrangement of atoms, it causes the appearance of polycrystalline phase.
Figure 4(a) represents the plane-view FE-SEM image of the β-Ga2O3 thin films deposited at 5 × 10−3 Pa and 750°C. The grain size is ~150 nm, indicating a good crystallization property. Figure 4(b) shows the AFM image of surface morphology for a 2 µm × 2 µm scanning area. The mean surface roughness of the thin film is 3.42 nm.
The optical absorption measurements are extremely important in evaluating the optical parameters of thin films such as absorption coefficient and energy band gap etc. Figure 5 shows the UV-vis absorbance spectrum of the β-Ga2O3 thin film deposited at 5 × 10−3 Pa and 750°C. It is evident that β-Ga2O3 thin film has a significant absorption at wavelengths less than 250 nm, near the lower edge of the solar-blind region. A further analysis of the optical spectra is performed to calculate energy band gap. For β-Ga2O3 with a direct band gap, the absorption follows a power law of the form [21, 22]:Fig. 5, which is similar to the band gap of reported by others .
In order to check the UV photoresponse of the β-Ga2O3 thin films, a three-pair interdigital electrode was deposited through a shadow mask to serve as contact electrodes. The UV lamp with the wavelength of 365 nm and 254 nm were used as light sources, respectively. Figure 6 shows the room temperature I-V characteristics of the sample prepared under 5 × 10−3 Pa and 750°C with linear and logarithmic coordinate. It can be seen clearly that the current increases linearly as the applied bias increases both in dark and under different illumination conditions [Fig. 6(a)]. Such linear relationship suggests that good ohmic contacts between Ti/Au and Ga2O3 thin films. This is probably due to the large surface states at Ga2O3 surface so that carriers can tunnel through the barrier easily . The I-V curve measured under 365 nm light does not show significant increase as compared with the I-V curve measured in dark, which suggests the β-Ga2O3 thin films are not sensitive to 365 nm light [Fig. 6]. In contrast, the current shows a sharp jump as the device is exposed to the 254 nm light.
Figure 7(a) shows the time-dependent photoresponse of the detector to 365 nm and 254 nm illumination by on/off switching under an applied bias of 10 V, respectively. After multiple illumination cycles the device still exhibits a nearly identical response, indicating the high robustness and good reproducibility of the photodetector. Upon 365 nm UV illumination, the current slightly increases from approximately 128 nA of dark current to a non-stable value of approximately 300 nA extremely slow. Whereas, the current increases instantaneously by more than 1 order of magnitude to approximately 1460 nA under 254 nm UV illumination. At the same time, the recovery time for 365 nm or 254 nm UV illumination is not the same when the light is off. For a more detailed comparative study of the response time, the quantitative analysis of the current rise and decay process involves the fitting of the photoresponse curve with a biexponential relaxation equation of the following type :
Where I0 is the steady state photocurrent, t is the time, C and D are the constant, τ1 and τ2 are two relaxation time constants. As shown in Figs. 7(b) and 7(c), the photoresponse processes are excellently fitted. τr and τd are the time constants for the rise and decay edges, respectively. We note that the current rise to 254 nm illumination is steep with a τr of 0.86s [Fig. 7(b)]. In contrast, the rise edge to 365 nm illumination consists of two components (τr1 = 4.06 s, τr2 = 50.06 s) [Fig. 7(c)]. Also, the decay process usually consists of two components with a fast-response component and a slow-response component. The decay time constant τd1 / τd2 are estimated to be 1.02 s / 16.61 s and 3.46 s / 51.26 s to 254 nm and 365 nm illumination, respectively.
The photoresponse of a semiconductor to photon is a complex process of electron-hole generation, trapping, and recombination . In order to understand the photoconductance and decay processes in the β-Ga2O3 photodetector, a schematic diagram of carriers’ generation and recombination is shown in Fig. 8. The solid arrows represent the carriers’ generation, while the broken arrows show the pathways for the carriers’ recombination. Under the UV excitation, electron-hole pairs can be generated from valence-conduction band (process 1) and defect-conduction band (process 2) transitions. For 254 nm UV illumination, photogenerated carriers are mainly from process 1 with electrons injecting into the conduction band and holes leaving in valence band, and only a few from process 2 which should be assisted with defect states. However, carriers’ generation due to 365 nm excitation only occurs through process 2. For the semiconductor photoexcitation, process 1 is the main way for the generation of photogenerated carriers. So, the photocurrent under 254 nm UV illuminations is much larger than that of 365 nm UV light. Meanwhile, some of the photogenerated carriers are captured by the trapping states in β-Ga2O3 thin films. When the illumination is turned off, the electrons created in the conduction band recombine with the holes either through the recombination centers present in the material (process 3) or the band-to-band annihilation process (process 4). These processes are fast and responsible for the fast-response component of current decay. At the same time, the carriers captured by the trapping states would be released and recombined. Generally, traps in a wide band gap semiconductor are extremely deep . The time constant of the transient decay is governed by the depth of these traps and can be very long. This process is responsible for the slow-response component. In our case, the presence of numerous trapping states prevents carriers’ recombination may cause the slow recovery time . It should be noted that the performance of the photodetector is not optimized. We should be able to achieve a larger responstivity and higher photocurrent to dark current ratio by optimizing the fabrication parameters of the β-Ga2O3 thin film photodetector such as narrowing the spacing of the electrodes.
LMBE technology has been used to deposit β-Ga2O3 on (0001) sapphire substrates with various oxygen pressures and temperatures. The best crystallization of -oriented β-Ga2O3 thin layer is grown under 5 × 10−3 Pa and 750°C. The obtained β-Ga2O3 thin film has a significant absorption at wavelengths less than 250 nm, showing the characteristic of solar-blind sensitivity. In addition, a MSM structure photodetector was fabricated from the β-Ga2O3 thin film, which shows a strong sensitivity to the 254 nm UV light. Meanwhile, the device exhibits the high robustness and good reproducibility, indicating a potential application in solar-blind photodetectors.
This work was supported by Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), P. R. China, the National Natural Science Foundation of China (51072182, 51172208, 61274017), the Fundamental Research Funds for the Central Universities (Grant No. 2014RC0906), the Qianjiang Talent Program of Zhejiang Province (Grant No. QJD1202004), and National Basic Research Program of China (973 Program) (2010CB933501, 2010CB923202).
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