Photoluminescence and electrical characterization of SiGe/Si heterostructures grown by rapid thermal chemical vapour deposition

Abstract

A systematic photoluminescence (PL) spectroscopy study on fully strained and partially relaxed SiGe layers grown on silicon by rapid thermal chemical vapour deposition (RTCVD) is reported. The recent observation in fully strained SiGe single layers of a well-resolved excitonic luminescence transition consisting of a no-phonon line followed by phonon-mode replicas is accurately analysed and is interpreted as a free excitonic transition subject to localization by a fluctuating random alloy potential. The clear observation of such a PL excitonic transition with a measured lifetime of 9 μs shows that our thin strained SiGe layers grown by RTCVD exhibit high quality electronic properties. This corresponds to a low residual impurity content, freedom from crystal defects and low concentration of non-radiative band gap deep levels. We point out in this study the usefulness of this excitonic transition measurement for accessing directly and with accuracy the fundamental optical band gap energy of the tetragonally distorted strained SiGe material, and for probing alloy fluctuation in the layer and at the hetero-interface. The relaxation process which can occur when the thickness increases has been investigated by measuring the change in the PL features. This work shows that, when the PL excitonic transition vanishes, defect-related deep PL bands assumed to be assigned to the presence of dislocations in the SiGe layer begin to appear. These defect-related deep PL bands are tentatively connected to deep levels observed by transient capacitance spectroscopy in the SiGe layer and at the hetero-interface.