Yikai
Liao
,
Shujie
Jiao
*,
Shaofang
Li
,
Jinzhong
Wang
,
Dongbo
Wang
,
Shiyong
Gao
,
Qingjiang
Yu
and
Hongtao
Li
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, People's Republic of China. E-mail: shujiejiao@hit.edu.cn
First published on 24th November 2017
β-Ga2O3 films have been obtained by thermal annealing of amorphous thin films that were deposited by radio frequency magnetron sputtering. The influence of deposition pressure on the properties of the β-Ga2O3 films was investigated. The structural and optical properties of the β-Ga2O3 films were evaluated using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and optical spectrophotometric measurements. The full width at half maximum of β-Ga2O3 (600) X-ray diffraction peaks decreased firstly and then increased with increasing deposition pressure. A similar variation tendency was observed for the optical band gap of the β-Ga2O3 films. The reasons were attributed to joint action of the atoms' diffusion ability on the surface and oxygen partial pressure with deposition pressure. With increasing pressure, sputtered target species collided more and lacked energy to diffuse leading to poorly crystallized films. With high deposition pressure, oxygen partial pressure also increased in the deposition chamber, thus crystalline quality was improved due to the decrease in oxygen vacancies in the film, which was confirmed by investigating O 1s and Ga 3d core levels using X-ray photoelectron spectroscopy.
Ga2O3 has several different polymorphic structures, including α-Ga2O3, β-Ga2O3, γ-Ga2O3, δ-Ga2O3 and ε-Ga2O3.8 Among these, β-Ga2O3 is regarded as the most stable phase that could be converted from other metastable phases at high temperature.9 Because of its outstanding chemical stability, most device applications of Ga2O3 are based on β-Ga2O3. In recent years, various methods such as radio frequency (RF) magnetron sputtering,10–12 pulse laser deposition,13,14 molecular beam epitaxy,15 and chemical vapor deposition16,17 have been employed to fabricate β-Ga2O3 thin films. Among these methods, RF magnetron sputtering is widely used to deposit Ga2O3 thin films because of its low cost, easy manipulation and high film adhesion.18 Since β-Ga2O3 crystallization could only be realized at relatively high temperature, β-Ga2O3 is obtained at elevated substrate temperature during film deposition in the majority of the literature.19–21 Alternatively, there is another way to realize crystallization of β-Ga2O3 by post-annealing amorphous Ga2O3 films that have been deposited onto an unheated substrate at room temperature (RT) or low temperature, which is called solid-phase crystallization.22 This method has relatively low requirements for equipment and ensures sufficient reliability. In this method, room-temperature deposition leads to amorphous nanocrystalline films, which consist of a great amount of tiny crystalline grains with short-range order and chaotic orientation resulting from random atomic arrangements.23 Thermal post-annealing drives randomly arranged atoms to crystallize along the tiny crystalline grains, which could be considered as the nucleus,24 leading to the macroscopic crystallized films. The post-annealing parameters have been studied roundly and precisely in other studies25–27 including the annealing temperature, type of annealing and annealing gaseous environment. Not only the annealing process but also the properties of as-grown amorphous films have a great influence on the films. However, the properties of as-grown amorphous films and annealed films have been seldom studied under different sputtering conditions using the fixed annealing process.
In this paper, β-Ga2O3 thin films are obtained by a two-step method: thermally annealing the amorphous films deposited by radio frequency magnetron sputtering at RT (substrate temperature). The dependence of the structural and optical properties of as-grown films and annealed β-Ga2O3 films on the deposition pressure was studied in detail.
Film thickness was also evaluated using SEM cross-sectional images. Typical cross-sectional images of as-grown Ga2O3 films deposited at 0.5 Pa, 1.5 Pa and 3 Pa are shown in Fig. 2(a–c). The variation of film thickness with deposition pressure is summarized in Fig. 2(d). The thicknesses of the films deposited under different deposition pressures are in the range of 300 ± 10 nm. Considering film unevenness introduced by keeping the substrate unrotated during deposition, changing the deposition pressure from 0.5 Pa to 3 Pa did not affect the deposition rate significantly. Considering the sputtering process, more ionized gas particles impacting the target led to more sputtered species with increasing deposition pressure. On the other hand, more sputtered species result in more collision between them and less kinetic energy to diffuse on the surface. Meanwhile, low growth temperature also decreases energy-driven diffusion. Under these two conditions, the thickness of the film only changes slightly.
Fig. 2 SEM cross-sectional images of as-grown Ga2O3 films deposited at (a) 0.5 Pa, (b) 1.5 Pa and (c) 3 Pa and (d) summarized variation of film thickness with deposition pressure. |
A schematic diagram of the deposition process was proposed to represent the morphology evolution of the as-grown films with deposition pressure, as shown in Fig. 3. When the deposition pressure was 0.5 Pa, the amount of grown species was not enough, and the effect of energetic bombardment was significant, thus adatoms tended to fly away from the deposited surface, which perturbs the grain growth, leading to nonuniform grain sizes and defects, as shown in Fig. 3(a). As the deposition pressure increased up to 1.5 Pa, the particle bombardment effect was reduced because of increasing collision among sputtered species, thus leading to the decrease of grain size and the number of defects, as depicted in Fig. 3(b). When the deposition pressure was increased to 3 Pa, the effect of ion bombardment was further reduced and oxygen was increased, thus a smooth surface with uniform and finely packed grains was observed, as shown in Fig. 3(c).
Fig. 3 Schematic diagram of as-grown Ga2O3 films deposited at (a) 0.5 Pa, (b) 1.5 Pa and (c) 3 Pa in the sputtering process. |
Fig. 4 X-ray diffraction patterns of the as-grown films and annealed films deposited under different pressures. |
The β-Ga2O3 (600) peak showed the smallest peak width (FWHM) among the diffraction peaks. Fig. 5 shows the lattice structure of monoclinic β-Ga2O3. The (600) plane is considered to be the most stable plane with the largest atomic density and minimum surface free energy in monoclinic β-Ga2O3, thus crystallization along the (600) direction exhibited a fast growth rate compared to the other planes.34,35
Fig. 6(a) shows the amplified XRD patterns from 43° to 45.5°. Fig. 6(b) shows the influence of deposition pressure on the β(600) peak intensity and full width at half maximum (FWHM). It can be seen that the peak intensities decreased until the deposition pressure reached up to 2 Pa, then increased when the deposition pressure further increased. The FWHM of β(600) diffraction was found to gradually increase when the deposition pressure was varied from 0.5 Pa to 2 Pa but reduced from 2 Pa to 3 Pa. This indicated that only poorer plane texture and smaller grain size could be obtained as the deposition pressure increased to a certain extent, and better plane texture and larger grain size could be achieved again when furthering increasing deposition pressure.
Fig. 6 (a) XRD patterns from 43° to 45.5° of Ga2O3 thin films under different deposition pressures; (b) variation of the β(600) peak FWHM and intensity as a function of deposition pressure. |
Fig. 7 shows the amplified XRD patterns from 60° to 70°. With increasing deposition pressure, the intensity of the peaks remained high under relatively low deposition pressure from 0.5 Pa to 1 Pa, as shown in Fig. 7(a). Under intermediate deposition pressure from 1 Pa to 2.5 Pa (Fig. 7(b)), both β(04) and β(12) peak intensities gradually decreased. Under high deposition pressure from 2.5 Pa to 3 Pa, the β(04) peak intensity remained constant while the β(12) peak intensity increased (Fig. 7(c)). The plane texture of β(04) and β(12) changed with increasing deposition pressure.
Fig. 7 XRD patterns from 2θ = 60° to 70° of annealed films deposited under (a) low deposition pressure, (b) intermediate deposition pressure and (c) high deposition pressure. |
In a sputtering system, sputtered target species lacked energy to diffuse due to more collision with increasing the pressure, thus, leading to poorly crystallized films with small grain sizes. Further increasing the deposition pressure also increases the oxygen partial pressure in the deposition chamber, thus oxygen vacancies in the film could reduce, which improves crystallization.34,36 These two factors were supposed to be responsible for the change of XRD diffraction. From 0.5 Pa to 2 Pa, more sputtered species were loaded onto the surface with low energy, inducing small grains and poor crystallization. While from 2 Pa to 3 Pa, enough oxygen in the film increases the crystal quality and diffraction intensity, as shown in Fig. 6 and 7(c). The SEM images in Fig. 1 also confirmed this hypothesis. The film showed discontinuous particles at the surface under low pressure owing to the lack of energy of the sputtered species. With increasing pressure, the surface became smooth with oxygen partial pressure increased. Besides, the β(04) peak intensity kept decreasing from 1 Pa to 3 Pa, indicating that crystallization along this plane was suppressed with increasing deposition pressure. This was because the β(04) plane had the lowest surface atomic density and the other plane having a higher surface atomic density would be easier to form under high pressure.17
Fig. 8 Optical transmittance spectra of (a) as-grown and (b) annealed films. The insets in (a) and (b) show the enlargement of the wavelength region from 200 to 300 nm of the same spectra. |
Defects near the band gap edge have an important influence on the semiconductor optical properties. Since the formation energy of oxygen vacancies is much lower than that of gallium vacancies,37 oxygen vacancies are the main defects in the films and their amount has a great influence on the optical properties of β-Ga2O3 films. Higher sputtering pressure resulted in larger oxygen partial pressure, which reduced the amount of defects related to oxygen vacancies, leading to the increase of transmittance and blue-shift.
Further analysis of the optical transmittance spectra was realized to better understand the influence of deposition pressure on optical properties. The optical band gaps of the as-grown and annealed films are determined using a Tauc plot.38 For Ga2O3 with a direct band gap, the absorption obeys the following law:
(αhν) = B(hν − Eg)1/2 | (1) |
α = [1/d]In(T) | (2) |
The variation of optical band gaps of the as-grown and annealed films deposited under different deposition pressures is shown in Fig. 10. For the as-grown films, the band gap decreased at low pressure and increased with increasing deposition pressure. For the deposited annealed films, the band gap values also changed similarly to those of the as-grown films under different deposition pressures; however, the data are all in the range from 4.95 eV to 4.99 eV, which is consistent with the reported band gap value of β-Ga2O3 thin films.39,40 Increasing the deposition pressure could enhance the collision between sputtered species from the target, decrease the kinetic energy of arriving particles as well as the grain size of nanocrystals, and increase the band gap value.41 This was observed in other amorphous semiconductor materials like InSb.42 Also, the defect energy level near the band gap greatly influences the band gap values.43 With increasing deposition pressure, the amount of oxygen vacancies was reduced, thus leading to the increase of band gap value. The change of optical band gap with increasing the pressure was in good accordance with the SEM and XRD analyses.
Fig. 10 Variation of band gaps for as-grown and annealed films under different deposition pressures. |
Fig. 11 XPS spectra of as-grown and annealed samples, (a) O 1s core level at 0.5 Pa; (b) O 1s core level at 1.5 Pa; (c) O 1s core level at 3.0 Pa; (d) Ga 3d core level. |
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