Chemical Vapor Deposition of Diamond Films on Patterned GaN Substrates via a Thin Silicon Nitride Protective Layer Y. S. Zou, † Y. Yang, † Y. M. Chong, † Q. Ye, † B. He, † Z. Q. Yao, † W. J. Zhang,* ,† S. T. Lee, † Y. Cai, ‡ and H. S. Chu ‡ Center of Super-Diamond and AdVanced Films (COSDAF), and Department of Physics and Materials Sciences, City UniVersity of Hong Kong, Hong Kong SAR, China, and Hong Kong Applied Science and Technology Research Institute Company Limited, Hong Kong SAR, China ReceiVed March 19, 2007; ReVised Manuscript ReceiVed January 9, 2008 ABSTRACT: Integrating diamond films with GaN-based devices may enhance heat dissipation and thus improve device performance for high power loading. Direct deposition of diamond films on GaN layers has been hampered by GaN degradation in the chemical vapor deposition environment for diamond growth. In this work, three approaches were introduced to grow high-quality diamond films on patterned GaN substrates via a thin silicon nitride protective layer, that is, (i) a two-step process involving an initial rapid growth step, (ii) addition of nitrogen to hydrogen-based plasma to suppress reactions between GaN and hydrogen, and (iii) deposition in argon-based plasma. Continuous, adherent, and high-quality micro- and nanocrystalline diamond films were successfully deposited. All three approaches were effective in reducing plasma-induced GaN decomposition and etching, and in eliminating film cracks and delamination. 1. Introduction GaN is widely used to fabricate optoelectronic devices such as light-emitting diodes (LEDs), photodetectors, and laser diodes working in the blue and violet light regions. 1–3 GaN-based devices are usually constructed on sapphire substrates. However, excessive heating due to high thermal resistance of sapphire and high operation current densities may reduce device perfor- mance and lifetime. Therefore, improvement in thermal man- agement of GaN-based devices is desired. One approach to reduce device temperature during operation is to spread the heat over a large area with the assistance of high heat dissipation materials. Diamond, with its outstanding physical and chemical proper- ties, is a promising material for mechanical and electronic applications. 4–11 Because of its highly insulating nature and highest thermal conductivity (20 W cm -1 K -1 at room tem- perature) diamond is arguably the best candidate for heat dissipation applications, in particular in high-power electronic devices. Thick hexagonal GaN layers have been grown on (110) single-crystal diamond wafers via an AlN transition layer by metal organic chemical vapor deposition (MOCVD). 12 However, the delicate GaN-based devices cannot endure the harsh environment during chemical vapor deposition (CVD) of diamond, where the substrates are exposed to hydrogen plasmas and high temperature. Although the melting point of GaN is around 2500 °C, GaN reacts with hydrogen around 800 °C. 13 In addition, GaN is decomposed and etched under the conditions for CVD diamond growth. The poor chemical stability of GaN under typical diamond CVD conditions has hampered the application of diamond films for GaN-based devices. There are relatively few reports on the deposition of continuous and high-quality diamond films on GaN. By combining carburization with bias- enhanced nucleation and growth, oriented diamond was depos- ited on hexagonal GaN layers via conventional microwave plasma CVD. 14,15 However, the nucleation density of diamond was very low, and only isolated diamond crystals were obtained on GaN surface. Continuous diamond films have been grown on epitaxial GaN films via hot filament CVD where the reactivity of hydrogen is much lower than in microwave plasma. 16 However, the diamond films deposited were porous with low crystal quality and poor adhesion. Recently, deposition of continuous boron-doped diamond films on p-GaN substrate by hot-filament CVD was reported. 17 In this work, we report three approaches to grow high-quality diamond films on patterned GaN layers in a microwave plasma CVD system. All three approaches are effective in restraining the decomposition and etching of GaN substrates in the plasma. Continuous, adherent, and high-quality micro- and nanocrys- talline diamond films were grown on GaN via a thin silicon nitride transition layer. 2. Experimental Procedures GaN epitaxial layers used in the present study were grown on c-face (0001) sapphire substrates by MOCVD. Trimethylgallium and ammonia were used as the Ga and N sources, respectively. The thickness of GaN epitaxial layer was about 5 μm. To improve heat dissipation, patterns with different size and geometry including circles, grooves, and crosses were formed on GaN surfaces by lithography-assisted chemical etching. To protect GaN surfaces against abrasion and plasma CVD environ- ment, a thin SiN x layer of about 120 nm in thickness was precoated on GaN by using rf-assisted CVD. Diamond films were then deposited on SiN x -coated GaN layers using a commercial 1.5 kW ASTeX microwave plasma CVD system. 18 The substrates were ultrasonically cleaned with acetone and ethanol for 15 min successively, then rinsed with deionized water. To achieve a high nucleation density of diamond, the substrates were ultrasonically abraded for 60 min in a suspension of nanodiamond powder (grain size ∼ 5 nm) in ethanol. After abrasion, the diamond seeds were actually embedded in the SiNx layer. Observations via cross-sectional scanning electron microscopy (SEM, Philips 30 XL FEG) revealed that the SiN x layer remained intact after abrasion and diamond growth (as shown below), indicating that the underlayer GaN should remain nondamaged during the diamond abrasion process. In the reactor, the substrates were mounted on a Mo substrate holder located on an inductively heated plate. The substrate temperature was directly measured by an optical pyrometer through a sapphire window. Growth of microcrystalline and nanocrystalline diamond films was conducted in two gas systems, that * Corresponding author. E-mail: apwjzh@cityu.edu.hk. † City University of Hong Kong. ‡ Hong Kong Applied Science and Technology Research Institute Company Limited. CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1770–1773 10.1021/cg070267a CCC: $40.75  2008 American Chemical Society Published on Web 04/18/2008