Growth temperature dependence of strain in a GaN epilayer, grown on a c-plane sapphire substrate

Abstract

Despite the large mismatch in the lattice constants and thermal-expansion coefficients between the epilayer and the sapphire substrate, GaN epilayer is commonly grown on a c-plane sapphire substrate due to the lack of other suitable substrates [1–3]. The GaN epilayer on a c-plane sapphire substrate is widely grown with the use of the twostep metalorganic chemical vapor deposition (MOCVD), employing an AlN [4] or a low-temperature GaN buffer layer [5]. It was reported that a biaxial strain and a hydrostatic strain could coexist in the GaN epilayer [6, 7]. In GaN, the biaxial stress is due to thermal mismatch stress and growth [6–10]. The growth stress can be caused by lattice mismatch, island coalescence, grain growth, or surface stress [8–10]. Some in-situ measurements on the growth stress have been reported during MOCVD of GaN [8–10]. The internal hydrostatic strain was shown to have been introduced by the presence of point defects, which can be compressive or expansive depending on their size [6, 7]. A study was reported on a high-resolution X-ray diffraction (HRXRD) strain–stress analysis of a GaN/sapphire heterostructure grown through molecular beam epitaxy (MBE), particularly in the deformation state, depending on the relative content of N in the Ga1-xNx buffer layer by Harutyunyan et al. [11]. In the authors previous study [12], the strain analysis of a GaN epilayer with different growth times on a c-plane sapphire substrate via a two-step growth, using low-pressure, metalorganic chemical vapor deposition was conducted on the basis of precise measurement of the lattice parameters, using HRXRD. In this continuing study, the growth temperature dependence of the residual strain in a GaN epilayer at room temperature on a c-plane sapphire was studied. The final growth temperature in a fixed two-step MOCVD was varied from 850 to 1,050 C. The cand a-lattice parameters were measured using HRXRD, followed by the out-of-plane and in-plane strains. We have extracted the levels of biaxial and hydrostatic components of strain in the GaN films on sapphire. An undoped GaN epilayer was grown on a c-plane sapphire substrate in a horizontal MOCVD reactor at a low pressure of 300 Torr, as described in the authors’ previous articles [12, 13]. We employed a two-step growth method of GaN—consisting of the deposition of the low-temperature GaN buffer layer and high-temperature GaN epitaxial growth. The growth temperature of the high-temperature GaN was varied from 850 to 1,050 C so that the temperature dependence of the strain of the GaN epilayer induced after the two-step MOCVD growth process could be studied. The V/III ratios were 2,740 and 1,370 for buffer growth and the main high-temperature growth, respectively [12]. The growth rate was fixed at 0.07 lm/min; as such, the thickness of the GaN layer that was grown for 20 min was estimated to be 1.5 lm. The c-lattice and a-lattice parameters of the GaN epilayers were measured with a x-2h scan of the wurtzite GaN epilayer through triple-axis, HRXRD, to obtain precise measurements (PANalytical X’Pert PRO MRD), as described in the authors’ previous article [12]. S. I. Cho K. Chang Department of Chemical Engineering, University of Seoul, 90 Jeonnong-dong, Dongdaemun-gu, Seoul 130-743, South Korea