Optimization of p-type AlGaN/GaN and GaN/InGaN Superlattice Design for Enhanced Vertical Transport
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Optimization of p-type AlGaN/GaN and GaN/InGaN Superlattice Design for Enhanced Vertical Transport M. Z. Kauser1,2, A. Osinsky1, J. W. Dong1, B. Hertog1, A. Dabiran1 and P. P. Chow1 1 SVT Associates, 7620 Executive Drive, Eden Prairie, MN 55344, USA. 2 Dept. of ECE, University of Minnesota, 200 Union St. SE. , Minneapolis, MN 55455,USA. ABSTRACT We report on p-type AlGaN/GaN and GaN/InGaN superlattice (SL) designs with significantly improved vertical and lateral electrical conductivity (σV and σL). Compositiongraded p−AlGaN layers were used to produce eight fold reduction in barrier height and a ~40% increase in the sheet hole density in the p-GaN wells compared to typical SL structures. Thirteen orders of magnitude and 35 times improvement is obtained for σV and σL compared to typical SL and bulk p-GaN, respectively. A similar approach for p-GaN/InGaN SL resulted in seven fold reduction in barrier height and a ~30% increase in the sheet hole density in the p-InGaN wells compared to a typical SL structures. σL is strongly dependent on hole mobility in the well and about 19 times improvement is obtained for the optimized design, SL-B, with µp=30 cm2 V-1s-1 compared to bulk-InGaN. More than 10 orders of magnitude improvement in σV is obtained for SL-B compared to modulation doped SL. INTRODUCTION One of the challenges in III-N research is to achieve the highly conductive p-type layers needed for vertical transport devices such as HBTs. The low p-type conductivity is due to the large Mg-acceptor ionization energy (~170 meV for GaN). Enhancement of lateral hole conductivity, σL, has been demonstrated in modulation doped AlGaN/GaN superlattice structures [1]. Vertical transport in SLs is severely reduced due to carrier capture in QWs. To maintain superior lateral conductance, and at the same time improve σV, modulation doped graded n−type AlGaN/GaN multi-channel structures have been investigated [2]. Recently, p-type AlGaN/GaN SLs with graded p-AlGaN layers have been proposed to improve vertical and lateral conductivities [3]. Researchers at NTT Basic Research Laboratory have recently demonstrated n-p-n GaN/InGaN HBTs with current gain as high as 3000 and DC powers of 10.4W for a 50 µm × 90 µm device [4]. This remarkable achievement was possible due to recent improvements in the growth of p-type InGaN with hole concentration near 1019 cm-3. The high hole concentration is due to a relatively low activation energy in InGaN, e.g., for 13% In mole fraction Ea~98 meV [5]. On the other hand, high doping leads to mobility reduction due to ionized impurity and alloy scattering. Low hole mobility in InGaN hinders realization of high current gains and high-powers in GaN based n-p-n HBTs. This can be offset by formation of a 2D hole gas channel, as shown in ref. [6]. P-type uniformly doped GaN/InGaN SLs have been demonstrated with higher hole concentration than bulk-InGaN although with reduced mobility [7]. In this paper band gap engineering is used to optimize vertical conductivity in p-type AlGaN/GaN and GaN/InGaN SLs. T
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