Auger Recombination in Antimony-Based, Strain-Balanced, Narrow-Band-Gap Superlattics
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EXPERIMENT The structure under investigation was grown for optical characterization measurements using a Perkin-Elmer 430P MBE machine on a nominally undoped GaSb substrate. A schematic of the epitaxial structure is shown in Figure l(a). Forty periods of a four-layer superlattice are grown between A1GalnAsSb barriers. The barriers serve to increase the absorption of the optical excitation source and keep the photogenerated carriers in the superlattice. Figure l(b) shows one period of the superlattice. Room temperature photoluminescence from this structure indicates a band gap near 4.0 •m and good structural quality. A four-layer superlattice using the same alloys but with slightly different layer thicknesses has been described earlier.• An optically-pumped 5.2 •tm laser using a similar superlattice has operated at 185 K.2
Mat. Res. Soc.
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83 Proc. Vol. 484 ©1998 Materials Research
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Figure 1: (a) Schematic of the epitaxial structure of the sample and (b) schematic of one period of the four-layer superlattice. Two optical methods have been used to measure recombination rates in this sample: timeresolved differential transmission3 and time-resolved photoluminescence upconversion. 4 A schematic of the differential transmission measurement is shown in Figure 2(a). A pulse from a Ti:sapphire laser is used to create a dense distribution of nonequilibrium carriers in the sample. The transmission of the sample is then probed with the mid-infrared output of an optical6 5 parametric oscillator. The wavelength of the mid-infrared probe is tunable from 2.6 - 4.4 jim. After passing through the sample, the probe is directed through a monochromator which increases the spectral resolution of the measurement. The transmitted probe intensity is measured with a liquid nitrogen cooled InSb detector. By chopping the pump and probe separately, we can extract a signal proportional to the change in transmission of the probe induced by the nonequilibrium carriers and a signal proportional to the equilibrium transmission of the probe. These values are used to calculate the differential transmission, which is the change in transmission induced by the presence of the non-equilibrium carriers normalized by the equilibrium transmission.
A schematic of the time-resolved photoluminescence upconversion experiment is shown in Figure 2(b). Each pulse from the Ti:sapphire laser is split into two. One is used to excite the sample, which is mounted in a variable temperature dewar. The mid-infrared photoluminescence
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from the sample is collected and imaged onto a nonlinear crystal. The remaining portion of the Ti:sapphire pulse is focused onto the same spot on the nonlinea
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