Emission and Excitation Spectra of ZnSex S1-xSingle Crystals
Shirley Tiong-Palisoc
Pysics Department, De La Salle University
2401 Taft Avenue, 1004 Manila, Philippines
Considerable attention has been paid to the evaluation of the fundamental properties of the II-VI semiconductors and their application in devices. To date, the material quality of these semiconductors remains inferior to that of Si and III-V semiconductors. However, in spectral regions, where Si and III-V devices cannot provide the required bandgap, the II-VI semiconductors are of potential importance in optoelectronic devices such as short wavelength (visible) light-emitting diodes, electroluminescent panels, optical waveguides and photovoltaic solar cells. Recently, major efforts have been devoted to realizing visible light-emitting devices from the green to near-uv region, with the wide bandgap Zn-chalcogenides. The first lasing of ZnSe based laser diodes occurred in 1991 (Haase et al. 1991). However, its degradation still limits the commercial use of this structure. The degradation is thought to be dominated by built-in defects (Ghua et al. 1993, Haugen et al. 1995, Tomiya et al. 1995, Spahn et al. 1997) acting as scattering centers for charged carriers. The Zn-chalcogenides have a direct wide energy bandgap between 2.26 and 3.76 eV. There is an efficient direct band-to-band recombination in these materials, which implies that efficient light-emitting devices can be expected with effective injection of minority carriers. ZnSe and ZnS are especially important for blue light-emitting devices since the near bandgap emission of ZnSe occurs at 460nm and that of ZnS at 340nm. However, control and understanding of intrinsic doping in such bulk II-VI compounds is far from complete. For example, the degree of control of conductivity has been hampered by the self-compensation effect (Pantrat et al. 1982, Ohishi et al. 1992) and the residual impurities in these materials which cause difficulties in fabricating low resistivity pn-junctions – a prerequisite for efficient minority carrier injection devices. While the interpretation of the compensation effect is still contentious, the electrical and photoluminescence properties can be controlled effectively by intentional impurity doping.
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