Control of the Polarity and Surface Morphology of GaN Films Deposited on C-Plane Sapphire
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steps during GaN growth. For example, sapphire substrates are cleaned in a H2 atmosphere at high temperature, the substrate temperature is ramped up after depositing the buffer layer, and the gas flow ratio (N2 , H2, NH3 ) in each step is changed to obtain high quality GaN films. Furthermore, the substrate is occasionally nitrided [4][5]. Each step involves the chemical reactions, which make the polarity issue complicated. The polarity structure should be determined at the initial stage of the growth, i.e., at the interface since the rest of the film growth can be considered as a kind of homoepitaxy. The insert of a buffer layer forms another interface and substrate nitridation moderates the surface state [6]. We have investigated the influence of a buffer layer and substrate nitridation on GaN film growth from the point of view of these interfaces [7]. As a result of this study, it was possible to control the polarity of GaN films on c-plane sapphire [8]. In this paper, GaN films with the different polarity were deposited under the same conditions except for the annealing time of the buffer layer. The systematic variation in the surface morphology will be presented. The influence of thickness and annealing time on the polarity of buffer layer was investigated in order to discuss the variation of surface morphology. (0001) Ga-face (+c)
Ga
(0001) N-face (-c)
N
c
c
c-plane sapphire
Fig.1 Schematic illustration of wrutzite GaN structure oriented along c-axis. Left side is (0001) Ga-face which is defined +c. Right side is (0001) N-face which is defined which is defined -c.
Experiment GaN films with 1.2µm thickness were deposited on c-plane sapphire by a two-step atmospheric MOCVD method. Prior to deposition, the substrate was thermally cleaned in a H2 gas flow at 1080°C for 10min. Substrate nitridation was carried out by heating at 1080°C in NH3 for 5min. After deposition of a 160nm GaN buffer layer at 600°C, the buffer layer was annealed at 1040°C for up to 100min under a gaseous mixture of N2, H2 and NH3. GaN films were deposited on the annealed buffer layer at 1040°C under the same conditions. The buffer layer as thick as 160nm was intentionally prepared for the purpose in this study. For comparison, high
Downloaded from https://www.cambridge.org/core. IP address: 5.189.201.104, on 04 Nov 2018 at 05:59:03, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300003173
quality GaN films with 20nm buffer layer were also deposited on sapphire with and without nitridation. Identification of the polarity was carried out by coaxial impact collision ion scattering spectroscopy (CAICISS). The polarity of the GaN films was determined by comparison with CAICISS results for ZnO which not only has a wurtzite
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