Polarization induced 2D hole gas in GaN/AlGaN heterostructures
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Polarization induced 2D hole gas in GaN/AlGaN heterostructures S. Hackenbuchner1, J. A. Majewski, G. Zandler, O. Ambacher and P. Vogl Walter Schottky Institute and Physics Department, Technical University of Munich, Am Coulombwall 3, D-85748 Garching, Germany
ABSTRACT The generation of high density 2D hole gases is crucial for further progress in the field of electronic and optoelectronic nitride devices. In this paper, we present results of C-V profiling measurements for N-face GaN/AlGaN heterostructures and systematic theoretical studies of Mgdoped GaN/AlGaN gated heterostructures and superlattices. Our calculations are based on a selfconsistent solution of the multiband k.p Schrödinger and Poisson equation and reveal that the hole 2D sheet density is mainly determined by the polarization induced interface charges. For an Aluminium concentration of 30%, the induced hole density in the heterostructure can reach values up to 1.5x1013 cm-2. In the GaN/AlGaN superlattices, the hole sheet density increases with the superlattice period and saturates for a period of 40 nm at a value of 1.5x1013 cm-2.
INTRODUCTION High concentrations of holes in GaN and AlGaN layers are required for development of various electronic and optoelectronic nitride-based devices. The development of heterostructure bipolar transistors or laser diodes in UV regime has been hampered by the high ionization energy of the Mg acceptor commonly used for p-type doping in nitrides. Recently, it has been reported that the low doping efficiency of the deep Mg acceptor can be overcome in nitride heterostructures by utilizing the strong pyro- and piezoelectric character of these materials [1-5]. Unstrained nitride layers exhibit spontaneous polarization, whereas in strained layers both spontaneous and piezoelectric polarization are present. The differences of the polarizations in the adjacent layers induce interface charges. These, in turn, cause strong internal electric fields that can pull the acceptor energies below the Fermi level. In this paper, we report extensive theoretical studies of the electronic structure of holes associated with the degenerate valence band and confined within GaN/AlGaN quantum wells and superlattices. Our calculations clearly show that the density of the two-dimensional hole gas (2DHG) is mainly controlled by the polarization charge at the GaN/AlGaN interface. Other factors, such as the widths of barriers and wells, the temperature, and the Mg-concentration profile play a minor role. EXPERIMENT Our theoretical studies have been motivated by performed experiments, which demonstrate the accumulation of holes at the interface in p-type doped GaN/AlGaN/GaN heterostructures. Such heterostructures with N-face polarity have been obtained with plasma induced molecular beam epitaxy, by depositing on O-terminated and AlN covered sapphire substrates. Through 1
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Figure 1. Carrier concentration versus depth determined by C-V profiling in p-type GaN/AlGaN/GaN hetero-structures with N-face polarity.
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