Effect of Al pre-deposition on AlN buffer layer and GaN film grown on Si (111) substrate by MOCVD S.J. Bak 1 , D.-H. Mun 1 , K.C. Jung 1 , J.H. Park 1 , H.J. Bae 1 , I.W. Lee 1 , J.-S. Ha 1* ,T. Jeong 2 , T.S. Oh 3 1 Chonnam National University, Gwangju 500-757, Korea, 2 Korea Photonics Technology Institute, Gwangju 500-779, Korea, 3 Hongik University, Seoul 121-791, Korea. Abstract In this study, we investigated the effect of Al pre-deposition time on GaN crystal quality. The GaN layer was grown on a Si (111) substrate by metal organic chemical vapor deposition using an AlN buffer layer. Atomic force microscopy, scanning electron microscopy and X-ray diffraction were used to evaluate film growth. Consequently, we found significant differences in the epitaxial properties of the AlN buffer and GaN layer, which were dependent on the Al pre-deposition time. When the deposition time was 0s, the AlN buffer layer was very smooth, but the GaN layer did not have a high crystal quality. At a Al pre-deposition time of 60s, the AlN buffer started to be transformed into a rough surface and the GaN layer had both a mirror-like surface and better crystal quality. When the Al pre- deposition time was greater than 120s, GaN surfaces had a high roughness. In addition, the surface morphology of the GaN had not coalesced. Based on these results, the optimized Al pre-deposition time was found to be essential to achieve an appropriate surface roughness for high crystal quality of GaN. Keywords: MOCVD, GaN on Si, AlN buffer, Al pre-deposition * Corresponding Author: jsha@jnu.ac.kr 1. INTRODUCTION Because of the high crystallinity of GaN grown on sapphire with GaN buffer [1] , which has a band gap of 3.4eV, GaN is the most widely used material for lighting devices. Moreover, since this material holds great promise for use in outside illumination, GaN of high quality and large diameter is needed. To meet these needs, many researchers have focused on developing large-diameter substrates of sapphire and silicon carbide. However, these substrates cannot be readily commercialized because of their high costs. For this reason, the light emitting diode (LED) of GaN grown on a silicon wafer, which was first demonstrated by Guha et al. [2] and Dadgar et al. [3], has been receiving great attention. Because of the size availability and electrical and thermal conductivity of Si wafers, Si hold great promise for use as a substrate for GaN high power LEDs. However, there are several problems associated with the use of Si, such as the melt back etching phenomenon, differences in crystal structure, and the large lattice mismatch between the Si substrate and GaN layer, which can lead to low quality GaN crystals. Another crucial problem is the difference between the thermal expansion of coefficient (TEC) between GaN and Si substrate, which could result in cracks on the surface of the grown GaN layer when the growth temperature is changed during growth. [3] To suppress the formation of cracks and dislocations mentioned above, ZrN [4] , ScN [5] , TiN [6] , HfN [7] , SiC [8] buffer layers have been used. Among the many suggestions for buffer layers, AlN is commonly used as a buffer layer for the growth of GaN on Si due to its similarity to GaN material. [9, 10] Watanabe et al. used a high temperature AlN buffer layer to grow GaN on top of a Si substrate [11] . More recently, to eliminate the reaction between Si and nitride, Al pre-deposition prior to growth over a high temperature AlN buffer layer has been demonstrated to be an essential step for producing high quality GaN. [12, 13 , 14, 15, 16] Thus, in this study, we investigated the effect of Al pre-deposition times on GaN growth. 2. EXPERIMENTAL