Effect of Additional HCl on the Surface Morphology of High Quality GaN on Sapphire by HVPE
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Effect of Additional HCl on the Surface Morphology of High Quality GaN on Sapphire by HVPE X.Q. Xiu, R. Zhang, D.Q. Lu,.L. Gu, B. Shen, Y.Shi and Y.D. Zheng Department of Physics, Nanjing University, Nanjing 210093, P. R. China ABSTRACT The effect of introduction of additional HCl on the surface morphology and structural properties of hydride vapor phase epitaxy grown GaN during growth is investigated, and high quality GaN with smooth surface on sapphire is obtained by adding the additional HCl into the HVPE reactor. The result is attributed to the control of polarity of GaN films during growth. The additional HCl altered the equilibrium at the GaN growth front, and the reversible reaction decreased the nucleation density or growth rate. Further, lower growth rate promote the surface diffusion and the coalescence over (0001) plane. Additional HCl may improve the surface morphology by suppressing the (000-1) polarity growth in the initial stage of the growth. INTRODUCTION GaN and related III-nitrides materials have attracted a great deal of attention due to their potential usefulness in optical and electronic devices. GaN has the wurtzite structure and exhibits polarity along the c-axis due to the absence of center of inversion. The polarity of GaN corresponds to two surface structures: the (0001) Ga face and the (000ī) N face. Recently, it has been approved that the polarity of GaN has a great effect on the growth mechanism [1], surface morphology and device performance [2,3]. For the fabrication andof polarity dependent devices, such as piezo-electronic devices, it is desirable to grow films with flat surfaces. Precise control of the growth in polar direction is also necessary to obtain low defect density materials as well as a specular surface. Hydride vapor phase epitaxy (HVPE) growth of a thick GaN layer is an alternative and promising approach for providing thick GaN substrate for homoepitaxy. The growth rate in HVPE has reached as high as 100µm/h and a bulk-like GaN, with a thickness of several hundred micrometers has been easily achieved. However, GaN growth directly on sapphire by HVPE depends significantly on the growth conditions and shows poor reproducibility. It also shows a rather bad surface morphology containing hillocks and growth spirals [4]. According to E. S. Hellman’s work [5], GaN with hexagonal pyramidal morphology grown on sapphire are N-face. In contrast, Ga face polarity HVPE GaN has a smooth surface morphology. Since the polarity is known to be an important factor in determining the surface roughness and properties, it is important to obtain Ga-face HVPE GaN films on sapphire substrates. In this work, the effect of additional HCl flow on the surface morphology of GaN by HVPE is investigated and discussed from the polarity.
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EXPERIMENTS Undoped GaN films were grown by a conventional horizontal HVPE technology. These films were directly deposited on c-plane sapphire substrates at 1050 ˚C. Additional HCl flow of 5sccm was added into main N2 flow and simultaneously float towards to th
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