A photoluminescence study of the effect of well thickness in strained InGaAs/AlGaAs heterostructures grown by molecular
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Low temperature photoluminescence measurements were carried out on pseudomorphically strained InjGa1-jAs-Alo.28Gao.72As ternary-on-ternary heterostructures grown by molecular beam epitaxy to investigate the change in the transition energy, linewidth, and intensity as a function of InGaAs well thickness at two different indium compositions, x = 0.10 and x = 0.15, respectively. Sharp exciton peaks as narrow as 4 - 6 meV were observed from the InGaAs wells grown at 530 °C with 1 min of growth interruption at the top and bottom heterointerfaces. The linewidth decreases as the well thickness is increased up to 300 A. In addition, there are signs of linewidth broadening and sharp decrease in the photoluminescence intensity at higher well thicknesses that may indicate the onset of plastic relaxation. Relatively small variations in the transition energy were observed at well thicknesses that are up to «=3 times the theoretical critical layer thickness calculated by the Matthews-Blakeslee model,10 suggesting that the small density of dislocations that may be present may not have a significant effect on the band structure of the well.
I. INTRODUCTION One of the main goals of strained-layer epitaxy is to create metastable thin films with properties different from those of the corresponding bulk material. Typically, such films are pseudomorphic compound semiconductor structures grown from lattice-mismatch materials by molecular beam epitaxy (MBE). Such structures offer nearly complete flexibility in tailoring their electronic and optical properties and have proven to be highly successful in novel microelectronic1 and optoelectronic2 devices. In the case of lattice-mismatched systems, extensive theoretical3 and experimental4 results on bandgap discontinuity, among other parameters, have been reported for the strained In^Gai-^As-GaAs system, which is a mix of two binary constituents, InAs and GaAs. The attempt to achieve greater bandgap discontinuity leads to investigation into more advanced heterostructures that incorporate two ternary compounds such as In^Gai-jAs and Al^Gai-yAs, where two bowing parameters in the bandgap variation with composition and strain influence the band lineup. This ternary-on-ternary heterointerface has been addressed from a theoretical standpoint by Arent,5 who, using the virtual crystal approximation, calculated its bandgap discontinuity as a function of indium and aluminum composition. Techniques such as deep-level transient spectroscopy6 (DLTS), charge transfer,7 and photoluminescence8 (PL) have previously 1834
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J. Mater. Res., Vol. 9, No. 7, Jul 1994
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been used to estimate the conduction band offset in this strained alloy system. These studies showed that the band offset, A £ e / A £ g , lies in the range from 0.60-0.70 for indium and aluminum compositions ranging from x = 0.07-0.35 and v = 0.20-0.35, respectively. Generally, no further discussions on the variation of the PL transition energy, linewidth, and intensity as a function of well t
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