Effect of Buffer Design on AlGaN/AIN/GaN Heterostrucutres by MBE
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Effect of Buffer Design on AlGaN/AlN/GaN Heterostrucutres by MBE Gon Namkoong, W. Alan Doolittle Georgia Institute of Technology, Microelectronic Research Center, 791 Atlantic Dr, Atlanta, GA, USA A.S. Brown Department of Electrical and Computer Engineering, Duke University, Durham, NC 27709, USA M. Losurdo, M.M. Giangregorio, G. Bruno Institute of Inorganic Methodologies and of Plasmas, IMIP-CNR and INSTM, via Orabona 4 – 70126 Bari, Italy
ABSTRACT The effect of the buffer layers on the subsequent GaN epitaxial layers and electrical properties of AlGaN/AlN/GaN heterojunction structures nitrided at various temperatures was investigated. For AlN buffer layers, two different growth conditions of AlN buffer layers were introduced to avoid Al droplets. We found that etch pit density and structural quality of GaN epitaxial layer strongly depends on the growth conditions of AlN buffer layers. When using a double buffer layer (low temperature GaN on high temperature AlN) for 200 °C nitridation, the etch pit density was measured to high 107 cm-2 in GaN epitaxial layers. Furthermore, we observed that electrical properties of AlGaN/AlN/GaN heterostructures depend on growth conditions of buffer layers and nitridation temperatures. The mobility in Al0.33Ga0.67N/AlN/GaN structures grown on single AlN buffer layers for 200 °C nitridation were 1300 cm2/Vs at a sheet charge of 1.6×1013 cm-2. Using the double buffer layer for 200 °C nitridation, the mobility increased to 1587 cm2/Vs with a sheet charge of 1.25×1013 cm-2.
INTRODUCTION AlGaN/GaN heterostructure field effect transistors (HFET) have attracted intense attention, because they have emerged as promising candidates for high-voltage, high-power, and hightemperature microwave applications. For further improvement in device performance, the comprehensive understanding on initial growth condition is necessary because of the lack of high quality lattice-matched substrates for III-Nitride material growth. A previous study of the effect of the initial growth conditions and layer design (i.e nitridation and types of buffer layer) of the subsequent GaN epitaxial layers was previously reported [1]. In this work, this study is expanded to examine high temperature (800-850°C) AlN and combined AlN/GaN buffer layer effectiveness to improve the electrical properties of AlGaN/1nm AlN/GaN heterostructures grown by radio-frequency (rf) plasma Molecular Beam Epitaxy (MBE). Recently, high temperature AlN buffers have been used to grow AlGaN/GaN heterojunction structures. However, it is difficult to grow high quality AlN buffer layers due to the relatively low desorption rate and surface diffusion of Al, leading to a three-dimensional (3D) growth [2]. In this work, two different approaches were used to grow AlN buffer layers to
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avoid Al droplets by monitoring in-situ reflection-high energy electron diffraction (RHEED). Furthermore, the effect of these two growth conditions on the subsequent GaN epitaxial layers was investigated by X-ray diffraction, and etch pit density s
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