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Up-converted band to band luminescence generated by photons with energy lower than the band gap has been observed in high purity GaAs and GaAlAs epitaxial layers as well as in undoped melt-grown GaAs. This is explained by assuming a two-step excitation process involving a deep center as intermediate state. Since ecah crystal investigated has shown this effect, we conclude that intrinsic defects in pure GaAs and GaAlAs crystals provide the deep levels necessary for the up conversion.
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Quagliano et al. (1984) studied this question.
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