Strain evolution and phonons in AlN/GaN superlattices
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Strain evolution and phonons in AlN/GaN superlattices V. Darakchieva1, P. P. Paskov1, M. Schubert2, E. Valcheva1, T. Paskova1, H. Arwin1, B. Monemar1, H. Amano3 and I. Akasaki3 1 IFM, Linköping University, S-581 83 Linköping, Sweden 2 Fakultät für Physik and Geowissenschaften, Universität Leipzig, 04103 Leipzig, Germany 3 Department of Electrical and Electronic Engineering, Meijo University, Tempaku-ku, Nagoya 468, Japan ABSTRACT AlN/GaN superlattices (SLs) with different periods grown on GaN buffer layers were studied by infrared spectroscopic ellipsometry (IRSE), Raman scattering (RS) and highresolution reciprocal space mapping (RSM). The lattice parameters and the degree of strain in the GaN buffer and the SL constituents were determined. Phonon modes originating from the buffer layer and the SL sublayers were identified and their frequency shifts were correlated with the strain state of the films.
INTRODUCTION AlN/GaN and AlGaN/GaN SLs have been a subject of intense investigation due to their potential applications for high power transistors and ultraviolet laser diodes. However, only a few reports are devoted to their vibrational properties [1-4]. Experimental and theoretical studies in the dielectric continuum model [1, 2] of phonons in AlN/GaN SLs have revealed the presence of quasi-confined and interface modes at frequencies different from those of the SL constituents. Recently, Davydov et al. [4] have shown that these delocalised modes have A1(TO) and E1(LO) character with “normal” and “anomalous” behaviour. The “normal” modes gradually shift in frequency from the characteristic positions in GaN towards those of AlN, depending on the AlN mole fraction. The “anomalous” modes lie in the LO(GaN) – LO(AlN) frequency interval for the A1(TO) and in the TO(GaN) – TO(AlN) frequency interval for the E1(LO), respectively. On the other hand, the polar A1(LO) and E1(TO), and the non-polar E2 modes are predicted to be confined in the SL constituents and their frequencies are independent on the SL structure [4]. However, we note that the SL constituents are typically under strain resulting in a strain-induced shift of the SL phonon frequencies, which further complicates the mode identification. In addition, the formation of the SL induces a folding of the Brillouin zone in the growth direction leading to the appearance of new phonon modes. In this work we report on the strain evolution and vibrational properties of AlN/GaN SLs with different periods grown using a GaN buffer layer. The degree of strain in the GaN buffer and the SL constituents was determined and correlated with the observed phonon mode frequency shifts. EXPERIMANTAL DETAILS Crack-free SLs were grown by metalorganic vapor phase epitaxy on (0001) sapphire substrates using a 2-µm-thick GaN buffer layer. The structures consist of 10 period SLs with
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different well thicknesses, keeping the well-barrier thickness ratio as 3:1. The thickness of the AlN barrier and GaN well layers are listed in Table I. RSM was performed using a Philips triple
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