Epitaxial Growth of High Quality GaN Films on Lattice Matched Metallic Layers

In recent years there have been great advances in wide bandgap electronic and optoelectronic devices based on GaN materials.  However, problems arising from growth on non-native substrates, such as sapphire or Si, continue to present major hurdles for development of higher performance and lower cost devices.  For example, complex layer structures and processes have been developed to successfully mitigate some of the problems arising from mismatch in both lattice constant (LC) and coefficient of thermal expansion (CTE) between GaN and foreign substrates.  However, the added complexity and costs reduce the utility of these techniques for many applications.  In the past, GaN growth on metallic substrates, buffer layers and interlayers have been studied by few groups for a number of applications including thermal management in high power devices, back contact for vertical-transport devices, backside mirror for light emitters and detector, and as threading dislocation filters.   The major obstacles for the growth of high quality GaN films in these studies have been 1) deposition of single crystal metal film with proper orientation on foreign substrates, 2) avoiding strong chemical reactions and interface mixing during the nucleation of GaN on the metal surface, 3) minimizing mismatch in LC and CTE to avoid high dislocation density, stress cracks or sever wafer bowing, and 4) achieving adequate thermal and chemical stability of the GaN/metal/substrate structure for the demanding growth conditions encountered in deposition of III-nitride films for example by hydride vapor phase epitaxy.  

 In this work we have studied high quality growth of GaN by RF-plasma assisted molecular beam epitaxy (PA-MBE) and metal organic chemical vapor deposition (MOCVD) on high-quality, single crystal refractory metals deposited by plasma sputtering on c-plane sapphire and Si(111) substrates.  We will first discuss the deposition and optimization of fully relaxed high quality metal layer structures that are properly oriented and lattice matched for the growth of low defect GaN films on c-plane sapphire and Si(111).  We will then compare the nucleation and growth of GaN by PA-MBE and MOCVD on these metal layers. Some initial results such as x-ray diffraction scans for GaN grown on these lattice-matched metal alloys are quite comparable to high quality GaN grown on c-plane sapphire.   Finally, we present some examples of improved wide bandgap devices based on GaN films grown on metal layers.