2DEGs and 2DHGs Induced by Spontaneous and Piezoelectric Polarization in AlGaN/GaN Heterostructures
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2DEGs and 2DHGs Induced by Spontaneous and Piezoelectric Polarization in AlGaN/GaN Heterostructures O. Ambacher*, A. Link, S. Hackenbuchner, and M. Stutzmann, Walter Schottky Institute, TU-Munich, Am Coulombwall, D-85748 Garching, Germany. R. Dimitrov, M. Murphy, J. Smart, J.R. Shealy, B. Green, W.J. Schaff and L.F. Eastman, School of Electrical Engineering, Cornell University, Ithaca, NY 14853, USA. ABSTRACT Two dimensional hole and electron gases in wurtzite GaN/AlxGa1-xN/GaN heterostructures are induced by strong polarization induced effects. The sheet carrier concentration and the confinement of the two dimensional carrier gases located close to one of the AlGaN/GaN interfaces are sensitive to a high number of different physical properties such as polarity, alloy composition, strain, thickness and doping. We have investigated the structural quality, the carrier concentration profiles and electrical transport properties by a combination of high resolution xray diffraction, Hall effect and C-V profiling measurements. The investigated heterostructures with N- and Ga-face polarity were grown by metalorganic vapor phase or plasma induced molecular beam epitaxy covering a broad range of alloy compositions and barrier thickness. By comparison of theoretical and experimental results we demonstrate that the formation of two dimensional hole and electron gases in GaN/AlGaN/GaN heterostructures both rely on the difference of the polarization between the AlGaN and the GaN layer. In addition the role of polarity on the carrier accumulation at different interfaces in n- and p-doped heterostructures will be discussed in detail INTRODUCTION AlGaN/GaN polarization induced high electron mobility transistors (PI-HEMTs) have been a subject of intense recent investigation and have emerged as attractive candidates for high voltage, high power operation at microwave frequencies [1-3]. Recent investigations of these heterostructures have shown that two dimensional electron gases forming the device channel are generated by positive polarization induced interface charges [4]. Kozodoy et al. observed an enhancement of hole concentration by over five orders of magnitude in Mg-doped AlGaN/GaN superlattices compared to Mg:GaN films. The high sheet hole concentration of the superlattices demonstrated the pivotal role of polarization in determining the band structure of the heterostructure and the accumulation of holes with high sheet carrier *
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concentration in the GaN wells [5]. In this paper we will focus on the formation of two dimensional electron (2DEGs) and hole gases (2DHGs) in n-type and p-type doped GaN/AlGaN/GaN heterostructures. Conduction and valence band profiles are calculated in order to determine the carrier distribution and the formation of two dimensional carrier gases at interfaces of heterostructures with N- and Ga-face polarity. The theoretical predictions for AlxGa1-xN/GaN based structures with different alloy composition of the barriers are compared to experimental results achieved
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