BLUE-SHIFT OF EFFECTIVE BAND-GAP IN N-I-P-I DOPING SUPERLATTICES AS A FUNCTION OF OPTICAL EXCITATION INTENSITY
SUBAS BASTOLA
SUBAS BASTOLA
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Abstract
N-i-p-i doping superlattices are a special class of semiconductor superlattices which have been studied extensively and many devices have been realised. One of the important characteristics of the n-i-p-i doping superlattices is the tunability of their electronic structure by external excitation, either optical or electrical. It has been routinely reported that the effective band-gap of a n-i-p-i doping superlattice undergoes a blue-shift when the intensity of the external excitation light (pump) is increased. In other words, the luminescence peak arising from spatially indirect transitions (effective band-gap Eeffg) of a n-i-p-i structure shifts to a higher energy when the intensity of the excitation is increased. In this work, a quantitative relationship between the effective bandgap and the intensity of the excitation light has been obtained for the first time. Theoretical analysis showed that the effective band gap varies in direct proportion to the logarithm of the excitation intensity. In this study, n-i-p-i doping superlattices were grown using molecular beam epitaxy (MBE). Photoluminescence experiments were carried out on the samples at 4 K in order to enhance the luminescence arising from the indirect bandgap. Using AlGaAs based n-i-p-i samples, the plot of ln(Iw) versus Eeffg gave a slope of 4 x 10-21 eV-1 which agreed well with the theoretical value of 5.2 x 10-21 eV-1. It is found that this blue-shift is typically 50 meV for a doubling the magnitude of the optical excitation intensity from 50 mW cm-2 to 100 mW cm-2. A closely related phenomenon is the decrease of carrier lifetime with the increase in excitation intensity. N-i-p-i doping superlattices have tunable recombination lifetimes, which can, in theory, be varied up to infinity. With low optical excitation, electrons and holes are separated spatially and therefore the recombination lifetime is very long. However, as the excitation intensity is increased, the band-structure begins to resemble the bulk band-structure (i.e., band-structure of the host material). From the experiments, it was found that the carrier lifetime decreases exponentially by four orders of magnitude with increase in excitation intensity from zero to 500 mW cm-2 which also agrees well with our theoretical model.
Keywords
n-i-p-i doping superlattices, photoluminescence, optical excitation, blue-shift
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Date
1999
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