Impact of In content on the structural and optical properties of (In,Ga)N/GaN multiple quantum wells grown by plasma-ass
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Impact of In content on the structural and optical properties of (In,Ga)N/GaN multiple quantum wells grown by plasma-assisted molecular beam epitaxy Patrick Waltereit and James S. Speck Materials Department, University of California, Santa Barbara, California 93106 ABSTRACT We have studied the structural and optical properties of a series of (In,Ga)/GaN multiple quantum wells with identical thicknesses but varied In content grown by plasma-assisted molecular beam epitaxy. Careful choice of the growth parameters returns samples with smooth and abrupt interfaces. The shift of the photoluminescence transition energy with externally applied biaxial tension was investigated. We observed a red-shift for small In contents while a blue-shift was detected for higher In contents. This result is in qualitative agreement with band profile calculations taking into account both the band gap deformation potentials and the piezoelectric polarization in these structures. However, the magnitude of the shift is well in excess of the calculated one. We attribute this finding to a substantial deviation of the piezoelectric constants from those calculated for unstrained material. Finally, we estimate the piezoelectric polarization of InGaN/GaN for linear and non-linear terms in strain. INTRODUCTION One of the most distinguishing materials properties of group-III nitrides are their polarization fields of a magnitude not found in other III-V semiconductors. [1] In the case of thin layers, polarization discontinuities at heterointerfaces lead to the formation of electrostatic fields. The resulting quantum-confined Stark-effect (QCSE) is accompanied with a red-shift of the transition energies and a decrease of the electron-hole wavefunction overlap with respect to flat-band conditions. [2] The total polarization of a layer is composed of spontaneous and piezoelectric contributions. Recently, it has been proposed that the discontinuities in GaN/AlGaN spontaneous [3] and InGaN/GaN piezoelectric [4] polarization are significantly smaller and larger, respectively, than the theoretically predicted values. [1] Usually, changes in thicknesses or compositions are made for investigating polarization discontinuities. However, this procedure prevents a clear separation of spontaneous and piezoelectric polarization unless the layers are unstrained. In contrast, one can utilize externally applied stress to unambiguously examine the piezoelectric polarization. [4, 5] The aim of this paper is two-fold. First, we report on the structural and optical properties of a series of (In,Ga)N/GaN multiple quantum wells (MQWs) having identical structural parameters except for the In content in the well. Second, the piezoelectric polarization in strained (In,Ga)N wells is investigated by studying the photoluminescence under external modulation of the strain state. EXPERIMENTAL The samples under study were grown by plasma-assisted molecular beam epitaxy in a Varian Gen-II system equipped with effusion cells for the group-III species (two for Ga, one for In) and
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