Electron and Hole Confinement in GalnN/GaN and AlGaN/GaN Quantum Wells
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Electron and Hole Confinement in GaInN/GaN and AlGaN/GaN Quantum Wells A. Hangleiter, S. Lahmann, C. Netzel, U. Rossow, P. R. C. Kent , A. Zunger Institut f¨ur Technische Physik, Technische Universit¨at Braunschweig, Mendelssohnstr. 2, D-38106 Braunschweig, Germany National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401-3393, U.S.A. E-mail: [email protected] ABSTRACT We show that the strong bowing of the bandgap of GaInN, which is primarily due to bowing of the valence band edge, translates into a strongly composition dependent ratio of the conduction band offset to the valence band offset with respect to GaN. For common In mole fractions of 020 % this leads to a reversal of the band offset ratio and to very weak electron confinement. This theoretical picture is verified by comparing results of time-resolved spectroscopy on asymmetric AlGaN/GaInN/GaN and AlGaN/GaN/AlGaN quantum wells. Since electron confinement is much stronger for GaN/AlGaN wells than for GaInN/GaN wells, the effect of asymmetry is very weak for the former and fairly strong for the latter. INTRODUCTION Carrier confinement in quantum well structures is a crucial issue for light emitting devices like LEDs and lasers. Nevertheless, the band offsets for GaInN/GaN heterostructures are still insufficiently understood. Both for InN and AlN the band offsets relative to GaN are considered to much larger in the conduction band than in the valence band. For quantum wells made of such heterostructures, this should lead to fairly strong electron but rather weak hole confinement. In particular, it is commonly assumed [1] that the small valence band offset of InN with respect to GaN of only 0.3 eV scales linearly to GaInN with relatively low In mole fraction of less than 20 %, which is used in typical light-emitting devices. Together with strong polarization fields present in nitride heterostructures any small band offset severely affects carrier confinement. In this paper, we show that the strong bandgap bowing of GaInN results in a strongly composition-dependent discontinuity ratio. For low In contents of less than 20 % this leads to strong hole and weak electron confinement in quantum wells. The resulting strongly different behavior to GaN/AlGaN quantum wells is demonstrated by comparing asymmetric quantum wells from both material systems. THEORETICAL AND EXPERIMENTAL APPROACH The theoretical basis of our considerations is formed by calculations using a 4000 atom periodic supercell, where Ga and In atoms randomly occupy cation sites and where all atoms are permitted to relay to their minimum strain energy positions [2]. The strain energy is modeled via the valence force field approach [3]. The electronic structure is calculated by solving a Schr¨odinger equation using empirical pseudopotentials for each relaxed supercell [2]. Experimentally, we have studied asymmetric AlGaN/GaN/AlGaN in addition to AlGaN/GaInN/GaN [4] single quantum well structures. The AlGaN/GaN/AlGaN structures were grown by low-pressure metal-organic vapor phas
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