On the Initial Growth Mechanisms of α-Phase Ga2O3 on c-Plane Sapphire by Mist CVD

Abstract

Abstract Body: Recently, α-Ga2O3 have been attracting great attentions as an ultra-wide bandgap semiconductor. However, the reason why meta-stable α-Ga2O3, which was originally synthesized at a pressure of 44 kbar and at a temperature of 1000 °C [1], is grown at around 500 °C under atmosphere by mist chemical vapor deposition (CVD) has not been elucidated yet. In this study, in order to analyze the growth mechanisms of mist CVD-grown α-Ga2O3, we investigated the growth processes at the initial stage. α-Ga2O3 with thickness (t) of 2.3, 4.7, 7.8, and 30 nm were grown on (0001) sapphire by the mist CVD method. The four samples were investigated by atomic force micrograph (AFM), transmission electron micrograph (TEM), and X-ray diffraction reciprocal space mapping (XRD RSM). High resolution (HR-) TEM images at the interface viewed along the [11-20] axis indicates that α-Ga2O3 is pseudomophically grown on sapphire for t=2.3 nm, however, relaxed for t=4.7, 7.8, and 30 nm. Therefore, we found that the critical thickness of mist CVD-grown α-Ga2O3 on (0001) sapphire is 2.3-4.7 nm. In addition, for t=4.7, 7.8, and 30 nm, α-Ga2O3 was successfully grown even after the lattice relaxation. In TEM images under two-beam diffraction condition of g=10-10 viewed along the [11-20] , dark and bright regions periodically introduced in the film near the interface are seen, but not of g=0001, for t=4.7, 7.8, and 30 nm. Thus, it is considered that regions strained along in-plane direction or/and misfit dislocations with the Burger’s vector parallel to the interface are periodically introduced near the interface after the α-Ga2O3 film relaxes. From the electron diffraction patterns and XRD RSM, it is confirmed that the films of t=4.7, 7.8, and 30 nm keep α-phase even after the lattice relaxation. XRD RSM also indicates that relaxation ratio gradually increases as the film thickness increases; for t=30 nm, the film is almost free standing. We also calculated the critical thickness based on the force-balance model and obtained the value of ~4.2 nm, which is consistent to the prediction (2.3-4.7 nm) derived from the HR-TEM. From the above coincidence, it is considered that the relaxation of mist CVD-grown α-Ga2O3 is caused by dislocations. On the other hand, by MBE, PLD, and MOCVD previously reported, the lattice relaxation is caused by phase transition from α to β [2,3], that is, the frustration of taking the α-phase causes the lattice relaxation so that the crystal structure changes from α to β. The lattice relaxation processes by mist CVD are different from those by other growth methods. This may be attributed to the low temperature growth and the association of water in the growth atmosphere. We will discuss the mechanism in more detail at the conference. [1] J. P. Remeika et al., Appl. Phys. Lett. 8, 87 (1966). [2] R. Schewski et al., Appl. Phys. Express 8, 011101 (2015). [3] Z. Cheng et al., Appl. Phys. Lett. 111, 162104 (2017).