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Abstract
The growth of single crystal ε-Ga2O3 films under the assistance of Sn element was studied using pulsed laser deposition (PLD). The crystal structure, optical properties and chemical state of the element were investigated to analyze the influence of tin during the film epitaxy. There is a transition layer at the substrate and the ε-Ga2O3 film interface. Increase in Sn atomic ratio will cause a rise in the transition layer thickness and the growth rate. In addition, due to the Sn atoms aggregation and the formation of clusters, the higher dark current (Idark), photocurrent (Iphoto) and responsivity (R) were achieved for the enhanced electron transportation in the ε-Ga2O3 metal-semiconductor-metal (MSM) photodetectors. The optical bandgap Eg determined from R increased from 4.81eV to 4.88eV and 4.94eV with Sn contents increasing from 0.9% to 1.2% and 1.5%, consistent with the transmittance results.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, Ga2O3 has attracted great attention for high power electronic and ultraviolet (UV) or deep-ultraviolet (DUV) applications because of its exceptional properties, such as large bandgap (Eg), high theoretical breakdown electric field and physical and chemical stabilities [1–5]. A lot of research activities have been focused on the growth of β-Ga2O3 film and the fabrication of β-Ga2O3 devices. β-Ga2O3 is a thermodynamically stable polymorph and its single crystal is easy to obtain by conventional melt growth methods such as Czochiralski, floating zone and edge-defined film-fed growth [6–9]. However, researches have shown an increasing interest in a metastable form of ε-Ga2O3. As an experimental result indicates, the crystal structure of ε-Ga2O3 belongs to the high symmetry hexagonal system with space group P63mc [10]. A theoretical research studied by Maccioni and Fiorentini also predict that the metastable form of ε-Ga2O3 possesses a high spontaneous polarization along the <001> direction. This property allows ε-Ga2O3 to form a two-dimensional electron gas (2DEG) in heterostructures as well as AlGaN/GaN [11]. Up to now, the single phase ε-Ga2O3 film can be obtained by halide vapor phase epitaxy (HVPE) [12], metal organic chemical vapor deposition (MOCVD) [13–16], mist chemical vapor deposition and molecular beam epitaxy (MBE) [17–19]. However, few studies have been carried out on the fabrication of ε-Ga2O3 device and indepth analysis of device performance [20,21].
In this paper, we report the epitaxial growth of (0001)-oriented ε-Ga2O3 films on c-plane (0001) sapphire substrates under the assistance of Sn element by PLD. First, we discuss the effects of Sn atomic percent on structural, optical properties and chemical state of the element. Then, the MSM deep ultraviolet (DUV) photodetectors using comb electrodes were fabricated and demonstrated the higher Iphoto and R in ε-Ga2O3 samples. The influence of the Sn content on the photodetector performance has not been reported before. Further, the existence of polycrystalline Ga2O3 transition layer at the interface between substrate and ε-Ga2O3 was also discussed.
2. Experimental details
The Sn assisted Ga2O3 films, ε-Ga2O3 thin films with (0001) orientation were grown by PLD equipment on double-polished (0001)-oriented sapphire substrates with Ga2O3: Sn targets. The Ga2O3 ceramic targets with different Sn atomic percent of 0.3% (Sample B), 0.5% (Sample C), 0.7% (Sample D), 0.9% (Sample E), 1.2% (Sample F) and 1.5% (Sample G) were used for the film growth. The growth temperature were fixed at 650°C, and the laser pulse irradiated the target at 3Hz frequency with the energy density of 2.0 J/cm2. The base chamber pressure was Torr before film deposition and the oxygen partial pressure was kept at 0.01mbar during film growth for 6000 pulses. The undoped β-Ga2O3 epilayer was also grown as a reference (sample A). The structural properties of the films were investigated by means of the high resolution x-ray diffraction (HRXRD) in the 2θ mode with Cu Kα line of 0.15406nm. The transmission electron microscopy (TEM) measurements were carried out with HT7700 equipment, transmittance was obtained with a Lambda 900 double-beam UV-vis-NIR spectrophotometer, and the x-ray photoelectron spectroscopy (XPS) measurements were performed with a Kratos Analytical Axis Ultra XPS using monochromatic Al Kα radiation. The secondary ion mass spectroscopy (SIMS) analysis was performed by Cameca 6f using caesium ion bombardment to obtain the Sn concentration depth profiles. The acceleration voltage was 10kV. As for the photodetectors fabrication, standard photolithography and lift-off techniques are used to define the electrodes, and the ohmic contacts were interdigital Ti/Au (10nm/100nm) metal stacks with the total length of 17.5mm and the finger spacing of 100μm. The electrical characteristics of the photodetectors were obtained with an Agilent B1500A Semiconductor Device Analyzer. And the responsivity R were investigated by measuring the photocurrent with the scanning monochromator under the illumination of Xe lamp spectrum from 200nm to 400nm.
3. Results and discussion
3.1 Material characterization
As shown in Fig. 1, the peak at 41.68° is the diffraction of the sapphire (0006) planes, and the other three peaks for sample A and B are attributed to diffractions of , and β-Ga2O3 planes. With the Sn content increasing from 0.3% to 0.5%, there is a small shoulder at the higher angle side of the diffraction peak and the and diffraction peaks can both be resolved into two separate peaks, respectively, as presented in Fig. 1(b)-1(d). The lower angles of 38.27° and 58.93° correspond to the β-Ga2O3 diffraction position while the higher angles are the diffraction positions of (0004) and (0006) planes of ε-Ga2O3 (PDF#06-0509). As the Sn atomic percent increasing further (sample D-G), the peak at about 19.11° is the corresponding diffraction position of (0002) planes of ε-Ga2O3, indicating the disappearance of β-Ga2O3 reflexes and the formation of phase-pure ε-Ga2O3 [15–19]. Compared with the powder diffraction results of 18.988°, 38.644° and 59.597°, all of the ε-Ga2O3 diffraction peaks are shift to the high-angle side with different degrees. According to Bragg's equation, the interplanar spacings d of (0002), (0004) and (0006) planes of ε-Ga2O3 are 0.4644nm, 0.232nm and 0.1547nm, respectively. The magnitude of out-of-plane compressive strain is calculated to be about 0.56%, 0.34% and 0.19%. Thus the ε-Ga2O3 films contain an in-plane tensile strain for the smaller lattice constants of a = b = 0.29nm than those of sapphire, a = b = 0.4758nm. On the other hand, the undoped Ga2O3 film (sample A) is monoclinic phase with three diffraction peaks corresponding to the, and planes of β-Ga2O3 planes, respectively.
Fig. 1 (a) HRXRD diffraction curves of Sn assisted growth of ε-Ga2O3 and undoped β-Ga2O3 samples on sapphire. The peaks of ε-Ga2O3 are located at the right side of β-Ga2O3 peaks, (b-d) HRXRD diffraction curves of sample B-D and fitted results.
Figure 2 shows the TEM images of β-Ga2O3 (sample A) and ε-Ga2O3 (sample E and G) films respectively. In contrast to the β-Ga2O3 sample, the ε-Ga2O3 films exhibits columnar contrasts, indicating a columnar growth of the material. The interplanar spacing d is determined to be 0.464nm, corresponding to the (0002) planes spacing of ε-Ga2O3, so the ε-Ga2O3 (0004) plane is parallel to the α-Al2O3 (0006) plane. Furthermore, as shown in Fig. 2(f) and 2(g), the selective area electron diffraction (SAED) pattern of sample A and G also confirm the crystal structure of the β and ε phase determined by the HRXRD measurement results. As shown in Fig. 2(b), some contrast can be observed at the interface between sapphire substrate and β-Ga2O3 epilayer, indicating that there might be a small amount of α-phase [22]. However, ε-Ga2O3 film does not grow directly from the surface of substrate, and the irregular atom arrangement of polycrystalline Ga2O3 transition layer can be observed at the interface, as shown in Fig. 2(e) [14]. The thickness of transition layer varies from 4nm to 10nm as the Sn atomic percent increases from 0.9% to 1.5%. With TEM measurement, we also calculated the thickness of the film varies from 112nm to 445nm, 510nm and 570nm for sample A, E, F and G respectively. In other words, the existence of Sn atoms enhances the film growth rate. During film epitaxy, the volatile Ga2O can be formed and desorbs from the surface under the poor oxygen condition [23,24], so the growth rate is much lower, such as sample A. However, under the assistance of Sn element, a significant increase of the growth rate was obtained, which can be explained according to the following reactions:
Thehas negative values, which indicates that the above reactions have a high probability to occur on the surface: Tin atoms can be easily oxidized into SnO2 or SnO, and the tin oxides react with gallium suboxide Ga2O to form Ga2O3 while the SnO2 and SnO are reduced to SnO or metallic Sn, then the reduced species can be reoxidized to SnO2 and SnO [19].This process occurs again and again. For the reoxidation of Ga2O, the adsorption of Ga2O is restrained and the growth rate is improved. The intermediate tin species might act as oxygen trap to provide oxygen to Ga2O. The larger the Sn percent, the less the desorption of Ga2O and the faster the film growth.Fig. 2 TEM images of (a) sample A, (c) sample E, (d) sample G and (b), (e) HRTEM image of selected regions in dotted box and dashed box in (a) and (d), showing a small amount of α-phase Ga2O3 and a polycrystalline Ga2O3 layer under β-Ga2O3 and ε-Ga2O3,respectively. SAED patterns of (f) sample A and (g) sample G.
As can be seen in Fig. 3, the slightly higher Sn concentrations were obtained near the surface, around 2.25 × 1019, 1.74 × 1019 and 1.32 × 1019cm−3 and interior concentrations were 2.08 × 1019, 1.62 × 1019 and 1.20 × 1019cm−3 for sample E-G, respectively. While the Ga intensity (counts/second) increases with Sn atomic percent decreasing from sample E to G. In order to analyze the chemical states of the elements, XPS measurements were carried out for sample A and E-G. Before XPS measurement, the in situ Ar+ plasma treatment was used to etch several atomic layers on the sample surface and the binding energy were referenced to the C1s line (284.6 eV) from adventitious carbon. The Ga2p3/2, O1s and Sn3d spectra are illustrated in Fig. 4. The Ga2p3/2 peak can be resolved into two separate peaks, the higher binding energy of 1118.3eV and the lower binding energy of (1116 ± 0.4)eV correspond to the Ga-O bonding of Ga2O3 and Ga2O, respectively. The area ratiosof sample A and E are almost the same of 4.7%, and that of sample F and G decreases to 3.1% and 1.37%. Therefore, the presence of Sn atoms can promote the formation of Ga2O3 and the effects are much more evident with the Sn atomic percent increasing, as the film thickness determined by TEM. The binding energy peak of O1s is about at 530.3eV for ε-Ga2O3 samples, similar to that of undoped β-Ga2O3 [25,26]. Moreover, the Sn3d peaks also can be deconvoluted into three distinct peaks at (486.6 ± 0.1)eV, (495.4 ± 0.2)eV and (491.2 ± 0.2)eV, the first two corresponding to Sn4+ 3d5/2, 3d3/2 in the Sn4+ oxidation state [27], in accordance with the Sn3d spectra of pure SnO2, while the last one attributed to the 3d levels of metallic Sn0 [28,29]. Based on the area of the peaks and the corresponding sensitivity factors, the ratios between metal Sn to Sn4+ are calculated to be 9.9:90.1, 7.3:92.7 and 5.1:94.9 for three ε-Ga2O3 epilayers, respectively. Even taken the Sn atomic ratio into account, the percent of metal Sn also monotonically decreases with the increment of Sn percent. Therefore, most Sn atoms in ε-Ga2O3 mainly exist in the tetravalent Sn4+ oxidation state, but a few of them exist in the metallic Sn0 state. As there is no crystalline Sn phase detected in the HRXRD, metal Sn could exist in the film by the form of clusters [30].
Fig. 3 SIMS spectra of ε-Ga2O3 samples E-G, showing the Sn concentration and Ga intensity as a function of sputter depth.
Fig. 4 (a) XPS spectra of the undoped β-Ga2O3 (sample A) and ε-Ga2O3 (sample E-G) Ga2p3/2, (b) O1s (sample E-G), (c-e) Sn3d (sample E-G)
With respect to the crystal phase transformation from β-Ga2O3 to ε-Ga2O3 under the presence of Sn element, we should consider the following points: (a) The orderings of atoms are different in β-Ga2O3 and ε-Ga2O3 crystal structure. (b) During the film growth, much more tin atoms accumulate on the film surface, illustrated by SIMS measurements. (c) For the XPS results of Sn4+ oxidation state and no existence of crystalline SnOx in XRD measurement,we suppose that although above three reactions, Eq. (1)-(3), can occur first, Sn atoms finally enter into the Ga2O3 crystal lattice as a substitutional or interstitial impurity except a few of metal Sn clusters. As described above, during film epitaxy, the Sn atoms can be oxidized into SnO2 and the tin and the oxygen atoms show octahedral coordination in rutile structure. The bond length is larger than 0.2nm in SnO2 and that in octahedral position of Ga2O3 is larger than 0.19nm while that in tetrahedral position is less than 0.19nm. Thus, when tin atoms enter into the lattice, they will occupy the octahedral lattice site first, see [31]. As much more tin atoms accumulate on the film surface, on one hand, they can increase the growth rate by oxidation cycles and on the other hand, they might promote the formation of and stabilize the phase with more octahedral lattice site, like the catalyst. By comparing the coordination spheres for gallium lattice sites, we find that half of the gallium atoms present a tetrahedral coordination and the other half present an octahedral coordination in β-Ga2O3 while only 1/4 gallium atoms show tetrahedral coordination and 3/4 atoms show octahedral coordination in ε-Ga2O3. Under the assistance of the tin element, more octahedral structure than in beta-Ga2O3 can be formed, leading to the ε-Ga2O3 [19].
Fig. 5 shows the optical transmittance spectra of ε-Ga2O3 and β-Ga2O3 films. For the wavelength larger than 300nm, the average transmittance is beyond 70% and a sharp absorption edge at about 250nm can be observed. According to the equations [32]:
we can obtain the absorption coefficient α, and the optical bandgap Eg can be evaluated from thevs plots, where t, Rc and T are the film thickness, the reflectance and transmittance, respectively. The thicknesses of the epilayers were determined by high resolution transmission electron microscope (HRTEM). For the λ larger than 300nm, the transmittance is over 70% and we adopt the Rc of 30% to calculate α. Then the Eg are 5.02, 4.83, 4.87 and 4.91eV for sample A and E-G respectively. Generally, the optical Eg of ε-Ga2O3 and β-Ga2O3 are comparable [33]. However, in this work, the difference is increased to 0.19eV, and the greater tin atomic percent, the larger the Eg. As the lattice constants of ε-Ga2O3 is smaller while the β-Ga2O3 is larger than those of sapphire, there is an in-plane tensile strain in ε-Ga2O3 and compressive strain in β-Ga2O3 [34], so the Eg is widened in β-Ga2O3 than that in ε-Ga2O3. On the other hand, with tin atomic percent increasing, the film thickness also increases to relieve the strain and thus the Eg increases.Fig. 5 (a) Transmittance spectra and (b) vs curves and linear extrapolation for estimating the optical bandgap of Sn assisted ε-Ga2O3 (sample E-G) and undoped β- Ga2O3 sample on sapphire.
3.2 Photoelectric characteristics of the devices
To investigate the characteristics of the UV photodetectors, we carried out the current-voltage (I-V) measurements both in the dark (Idark) and under the normal incidence of 254nm with various UV-light illumination intensities, 300, 600 and 900μW/cm2 (Iphoto). As demonstrated in Fig. 6, it is obvious that the Idark and Iphoto increase linearly with bias voltage (Vbias), and the Idark and Iphoto of ε-Ga2O3 devices are higher than those of β-Ga2O3 devices. With the Sn atomic ratio increasing from 0.9% to 1.2% and 1.5%, they both decrease gradually, as illustrated in Fig. 6(a)-6(e). The photo to dark current ratio (PDCR) of sample E reaches beyond 1 × 105 under the 600μW/cm2 intensity, which is much higher than the other two ε-Ga2O3 samples. The larger Idark in ε-Ga2O3 may be related to the enhancement of conductivity. As discussed above, some metal Sn atoms could exist in the ε-Ga2O3 film by the form of clusters, confirmed by the XPS measurement results, and might enhance the carrier conduction through percolative tunneling though the energy levels introduced by the Sn ions in the ε-phase are too deep to release electrons [32]. With the increase of Sn percent, the concentration of Sn4+ increases while the Sn0 decrease, leading to the reduction of the film conductivity and thus the Idark.
Fig. 6 (a) characteristics of Idark versus voltage, (b-e) characteristics of Iphoto versus voltage and (f) characteristics of PDCR for the β-Ga2O3 and ε-Ga2O3 photodetectors.
According to the formula on responsivity of
where IL is the photocurrent, ID is the dark current, Pλ is the power intensity of incident light and S is the effective illuminated area. The dependence of responsivity R on the illumination wavelengths λ for the fabricated photodetectors is shown in Fig. 7. Sample E achieves a higher R in comparison with other Ga2O3 devices. The maximum R (Rmax) are 0.052A/W, 3.74A/W, 0.49A/W and 0.29A/W at the Vbias of 20V for sample A and E-G, respectively. By means of cutoff wavelength is λ atRmax, and the bandgap can be obtained to be 4.96eV (250nm), 4.81eV (258nm), 4.88eV (254nm) and 4.94eV (251nm) which are in consistence with the transmittance results. In addition, the external quantum efficiency (EQE), calculated byare 25.4%, 1825%, 239% and 142% for four samples, respectively, which means there are larger gains in ε-Ga2O3 devices than in β-Ga2O3.Fig. 7 The dependence of R versus illumination wavelengths λ for the different photodetectors at Vbias = 10 and 20V.
The time-dependent photoresponse measurement was performed at a constant voltage of 20V and by a 254 nm illumination square-wave light with a Plight of 600 μW/cm2, as shown in Fig. 8. The response edges and recovery processes usually consist of a fast response and a slow response component. Generally, the fast-response can be ascribed to the carrier transition from band to band and the slow-response is associated with the carrier trapping and detrapping from the defects bands, such as oxygen vacancy and gallium-oxygen vacancy pairs in Ga2O3 films. The quantitative analysis of the current response and recovery processes was performed with a biexponential relaxation curve fitting, based on the following equation [35]
where I0 is steady state photocurrent, A1 and A2 are the constants, t is the time, τ1 and τ2 are two relaxation time constants. The values of the time constants for Sample A and E-G are summarized in Table1. With Sn atomic percent increasing from 0.9% to 1.5%, the decay time constants τd2 decreases from 3.80s (sample A) to 3.25s, 3.12s and 2.21s, indicating less VO and other deep defects in ε-Ga2O3 film.Fig. 8 (a) Time-dependent photoresponse of the Ga2O3 photodetectors under a Vbias of 20 V, (b-e) the current response and recovery biexponential relaxation fitting curve of sample A and sample E-G.
Table 1. The rise time constants (τr1 and τr2) and values of decay time (τd1 and τd2).
4. Conclusion
The single crystal ε-Ga2O3 films were deposited on c-plane sapphire substrates using PLD under the assistance of Sn element. The HRXRD peaks shift to higher angle side, demonstrating the in-plane compressive strain in ε-Ga2O3,and the TEM images illustrate the presence of transition layer between the substrate and the ε-Ga2O3 film. The increase of ε-Ga2O3 growth rate with Sn percent can be ascribed to the formation of tin-oxides, oxidizing the gallium suboxides and restraining the desorption of Ga2O. However, much more Sn atoms enter the crystal lattice and substitute the Ga3+, but the introduced energy level are too deep to increase the conductivity and some atoms aggregate to form the clusters, improving the electron transportation. In addition, the ε-Ga2O3 MSM photodetectors demonstrate the improved Iphoto and R in comparison with β-Ga2O3 device. According to the R results, the Eg increased from 4.81eV to 4.88eV and 4.94eV, in consistence with the transmittance results.
Funding
National Natural Science Foundation of China (61774116 and 61334002); National 111 Centre (B12026).
Acknowledgments
We acknowledge Associate Professor Zhenping Wu of Beijing University of Posts and Telecommunications for the photocurrent of photodetectors measurements and we also acknowledge Associate Professor Yingjie Lu of Zhengzhou University for the responsivity of photodetectors measurements.
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