Improved Mobilities and Resistivities in Modulation-Doped P-type AlGaN / GaN Superlattices
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Improved Mobilities and Resistivities in Modulation-Doped P-type AlGaN / GaN Superlattices Erik L. Waldron, John W. Graff, E. Fred Schubert, and Amir M. Dabiran1 Boston University, Department of Physics and Electrical and Computer Engineering Department, Boston, MA 02215, U.S.A. 1 SVT Associates, 7620 Executive Drive, Eden Prairie, MN 55344, U.S.A.
ABSTRACT
P-type AlGaN / GaN superlattice structures have demonstrated higher acceptor activation due to a modulated valence band resulting from the superlattice as well as spontaneous and piezoelectric polarization fields. The polarization effects are due to the wurtzite structure of AlGaN and the strain present in AlxGa1-xN / GaN heterostructures. Variable temperature Hall effect studies of Mg doped Al0.20Ga0.80N / GaN superlattices reveal an improvement in resistivity and mobility for modulation-doped structures versus uniformly doped structures. Very low resistivities less than 0.1 Ω cm and hole mobilities ~ 36 cm2/V s are demonstrated. This improvement is attributed to a reduction of neutral and ionized impurity scattering for the twodimensional hole gas present in the GaN layers of the modulation-doped superlattice. The improvement is greatest at temperatures below ~ 150 K. The doped regions of the superlattices have Mg concentrations of ~1019 cm-3. Two modulation-doped samples were grown by MBE: a standard scheme with dopants only in the AlGaN barriers, and a shifted scheme with dopants concentrated near the AlGaN / GaN interfaces. The standard sample has mobilities of 8.9 and 36 cm2/V s at 300 and 90 K, respectively. Resistivities of the standard sample are 0.21 and 0.068 Ω cm at 300 and 90 K, respectively. Carrier concentrations for this sample are 3.4 and 2.5 x 1018 cm-3 at 300 and 90 K, respectively. Capacitance-voltage profiling on the samples shows a clear indication of a two-dimensional hole gas as well as the periodicity of the superlattice.
INTRODUCTION
Mg acceptors in GaN and AlxGa1-xN have an activation energy of 150-250 meV [1-3], considerably larger than kT at 300 K, resulting in low activation and therefore low conductivity in p-type GaN. This results in degraded performance of many devices including LEDs, lasers, and heterojunction bipolar transistors. The strong temperature dependence of the transport properties is problematic for device operation at both high and low temperatures where carrier freeze-out occurs. P-type AlxGa1-xN / GaN doped superlattices have been demonstrated to have higher acceptor activation [4-7] resulting in low resistivity. This can be further improved by minimizing ionized and neutral impurity scattering mechanisms through modulation doping. In this work, the effect of modulation doping in Al0.20Ga0.80N / GaN superlattices is investigated using Hall-effect and C-V profiling techniques. We show the modulation-doped I12.11.1
(MD) and shifted-modulation-doped (SMD) samples to have superior electrical properties compared to uniformly-doped (UD) samples, especially at low temperatures. Furthermore, a C– V profile is presente
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