Photoluminescence study of Ge containing crystal defects

Abstract

Multi-crystalline Ge (mc-Ge) and Ge layers on Si substrate grown by MBE and CVD are studied. The photoluminescence (PL) spectra are analysed by comparison with those obtained for bulk Ge. We present a scheme which explains the temperature dependence of the peak positions of the direct and indirect lines observed in the PL spectra. The complex fine structure of the indirect band, consisting of different sub-peaks, can be understood in terms of phonon assisted processes. Around 80 K emission of longitudinal acoustic (LA) phonons is the decisive process whereas at room temperature absorption of LA phonons dominates. Moreover, PL spectra measured in material with crystal defects suggest that multi-phonon absorption is involved in the formation of the indirect sub-peaks. As compared to mono-Ge the overall luminescence intensity of MBE and CVD grown Ge layers on Si substrate is strongly reduced. In the CVD grown Ge layers the direct line dominates at room temperature whereas in the MBE grown layers the indirect lines are found to be decisive. A shift of the peak positions for the Ge layers to lower energy (10 meV and more) is caused by tensile strain. In the Ge layers on Si and in mc-Ge, respectively, at 300 or 80 K several lines are observed, ranging roughly between 0.45 and 0.6 eV, which are originated by crystal defects/dislocations. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.