Photoluminescence in UHV/CVD tensile-strained Si type-II quantum wells on bulk crystal SiGe substrates
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Photoluminescence in UHV/CVD tensile-strained Si type-II quantum wells on bulk crystal SiGe substrates S.R. Sheng 1, N.L. Rowell 2 and S.P. McAlister 1 1 Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, ON K1A 0R6, Canada; 2 Institute for National Measurement Standards, National Research Council of Canada, Ottawa, ON K1A 0R6, Canada. ABSTRACT Near band-edge photoluminescence (PL) in high-quality UHV/CVD tensile-strained Si type-II quantum wells (QWs) with varying well width grown on bulk crystal SiGe substrates has been studied. In contrast to the blue-shifts observed in the PL lines of Si1-xGex QWs on Si, the PL lines of Si QWs exhibit red-shifts with increasing excitation density. The PL from the SiGe substrate shows no such shift. The PL red-shifts decrease as the well width decreases, and are essentially independent of temperature up to 15K where a transition from bound exciton emission to higher energy free exciton emission occurs. The rapid thermal annealing (RTA) was found to improve the crystal quality of the samples. RTA enhances the integrated PL intensity, results in narrowing and blue-shifting of PL bands, and reduces the exponent in the excitation power dependence as well as the amount of red-shifting at a given excitation density. Possible mechanisms for the observed shifts to lower energies of the PL lines with excitation density were examined, including band bending, band-filling, and binding of excitons to impurities. INTRODUCTION The recent development of high quality bulk single-crystal SiGe broadened the scope for SiGe devices [1-3]. Such substrates allow extended band engineering by permitting band offsets or band gaps to be varied [4], with strain adjustment in strained-layer quantum wells and superlattices [5] as well as higher potential for integrated optoelectronics. These substrates are also useful for the growth of thick lattice-matched or strained SiGe epilayers and heterostructures because of the reduced lattice mismatch [1]. Thick layers are important to optical devices such as detectors and solar cells with deep active regions required to optimize device quantum efficiency. We have reported [1] the growth of high-quality UHV/CVD SiGe epitaxial layers on commercial low-defect-density (~104 cm-2) bulk crystal SiGe substrates [1-3], and demonstrated that the use of bulk crystal SiGe substrates, combined with low-temperature epitaxy, leads to highquality SiGe epitaxial layers. In this paper, we report a study of near band-edge photoluminescence (PL) in high-quality UHV/CVD tensile-strained Si type-II multiple quantum wells (MQWs) with varying well width grown on similar bulk crystal SiGe substrates. Due to much larger conduction band (CB) offsets ∆Ec in tensile-strained Si QWs [3], the ultra-low excitation power densities are not necessary to study the type-II band alignment. In contrast, the excitation densities have to be low enough to reduce photo-induced band bending to insignificant levels for compressively strained Si1-xGex QWs on Si, othe
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