Investigation of Buffer Layers for GaN Grown by MBE
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Investigation of Buffer Layers for GaN Grown by MBE Feng Yun1, Michael A. Reshchikov1, Paolo Visconti1,2, Keith M. Jones1, Dongfeng Wang1, Marc Redmond, Jie Cui1, Cole W. Litton3, and Hadis Morkoç1 1
Department of Electrical Engineering and Physics Department, Virginia Commonwealth University, Richmond, VA 23284, 2 Also with Istituto per lo Studio di Nuovi Materiali per l’Elettronica, CNR, Via Arnesano 73100 Lecce, Italy 3 Air Force Research Laboratory, Wright Patterson AFB, OH 45433, USA
ABSTRACT The structural quality of the buffer layer juxtaposed to the substrate is pivotal in attaining high quality GaN layers. In MBE deposition, low temperature, medium temperature and high temperature AlN buffer layers are at the disposal of the grower. There are quite a few reports, some discussing the benefits of high temperature buffer layers and others doing the same for low temperature buffer layers. The reports emanate from different laboratories; and due to stringent parameter control required, it is difficult to compare one type of buffer with another. To gain some insight, we undertook an investigation wherein these varieties of buffer layers were grown on nitridated sapphire substrate under similar conditions for a comparative analysis. In addition to the single buffer layers of both GaN and AlN varieties, some combinations of stacked buffer layers, including cases where these buffer layers were separated by GaN layers, were employed. Structural analysis by high resolution X-ray diffractometry and topological analysis by AFM were carried out to assess the quality of the epilayers grown on these buffers. Hall measurements at room temperature were carried out to characterize the electrical transport properties. INTRODUCTION: III-V nitride semiconductors are considered a favorable material system for applications in high power, high temperature electronic devices due to their wide band-gap and excellent thermal-conducting properties [1,2]. GaN, and its related ternaries, have also been very successful in optoelectronic devices such as emitters and detectors, covering a continuous spectrum from visible to ultraviolet. Thin films of GaN-based materials are most commonly grown by MOCVD and MBE [3]. Based on the consideration of availability and cost, large area, good quality crystals of sapphire are commonly used as the substrate for III-nitride growth. Though crystal orientations of sapphire and GaN (grown on c-plane sapphire) are nearly parallel, there is a 30° in-plane rotation between the unit cells of sapphire and GaN. Moreover, the lattice mismatch between GaN and sapphire is about 15%. In general, the quality of the films grown directly on any plane of sapphire without some sort of buffer layer is poor, although there has been a report on high quality GaN growth by plasma-induced MBE without any buffer layers[4]. The poor quality is the result of large mismatch in the lattice constants and coefficients of thermal expansion of GaN and sapphire. To be specific, the islands formed during the nucleation process and the
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