We consider theoretically the optical spectra of electron-hole Stark-ladder transitions in superlattices under an electric field. Electron and heavy-hole subband energies and envelope functions are calculated using a scattering phase-shift treatment of the quasibound, Stark-localized states. Exciton binding energies and oscillator strengths are obtained as functions of the electric field for several pairs of Stark-localized electron and hole states. Accurate and rapid computation of these quantities is achieved by using the simple approximation developed in the preceding paper. We show that observed deviations of the Stark-ladder fan diagram from the expected linear dependence on electric fields result from excitonic effects. We also calculate the absorption coeScient as a function of photon energy for various electric fields, and we compare our results with recent experimental data on short-period GaAs/Al"Ga& "As superlattices.
We have designed and characterized an infrared spectrometer, which uses a linear array of quantum grid infrared photodetectors (QGIPs) as its spectral sensing elements. Each QGIP element shares the same detector material but has a different grid geometry. The detector material, which is based on a binary superlattice design, provides an 8–14 μm broadband absorption medium for the spectrometer. The geometry of the grid, which is the light coupling structure under normal incidence, selects individual absorption wavelength for each element. Using a linear array of QGIPs of different geometries, multiple wavelengths can be detected simultaneously, and the array thus forms a spectrometer. Multicolor infrared imaging can then be achieved by integrating such QGIPs in unit cells of a two-dimensional array.
We have observed an intensity-dependent shift in the exciton absorption features for a system of coupled quantum wells in which a thin barrier separates two wells of unequal widths. We found that for a fixed external bias on the sample, the normal quantum Stark shift of the excitons in the wells was suppressed when light of 1 μW/cm2 intensity was used to measure the absorption. The shift became systematically larger as the light intensity was decreased to as low as 100 pW/cm2. This extremely low-level nonlinearity may be due to internal fields that arise when free electrons and holes are separated within the coupled-well system.
We report the results of photocurrent measurements on Ino 53Gao 47As/Ino»Alo 4, As asymmetric coupled quantum wells lattice matched to InP substrates. We show that large shifts in excitonic absorptions with applied bias result from the resonant coupling of electron subbands. Results of our measurements agree well with calculations performed with use of a single-band envelope-function approximation that incorporates a scatteringphase-shift method for determining the quasibound electronand hole-subband states of the coupled-well system in an electric field. The rapid development of fiber-optic communications systems has stimulated a search for semiconductor electro-optic devices that operate in the 1.3and 1.55-pm wavelength bands. Effects that have been thoroughly characterized' in GaAs/Al Ga, "As multiple quantum wells (MQW's) are now being investigated in the Inp 53Gap 47As/Inp 5@Alp 4sAs (Refs. 5 -7) and Inp 53Gap 47As/InP (Refs. 8 and 9) quantum-well systems lattice matched to InP substrates. Many of the electrooptic devices being studied are based on an electric-fieldinduced shift in excitonic absorption peaks known as the quantum-confined Stark effect (QCSE). ' However, the magnitude of the QCSE depends strongly on the well thicknesses (for isolated, rectangular wells) and can be very small for the relatively thin wells (3 -8 nm) requiredfor implementation in devices that operate at 1.3 and 1.55 pm. The development of effective quantum-well modulators for these wavelengths will depend on identifying new electroabsorption mechanisms that are not subject to this limitation.In this Brief Report we show that asymmetric coupled quantum wells (ACQW's), in which two Inp 53Gap47As quantum wells of different thicknesses are coupled by a thin Inp5zAlp4sAs tunnel barrier (see Fig. 1), exhibit significant electroabsorption effects even when the wells are relatively thin. Previously, we demonstrated that the coupling of the lowest-energy electron subbands in a GaAs/Al Ga, As ACQW structure could be clearly observed in low-temperature photocurrent (PC) spectroscopy and resulted in enhanced intensity and phase modulation in waveguides. ' Here we report the results of roomand low-temperature PC measurements on two Inp 53Gap 47As/Inp 5pAlp 4sAs MQW samples: one containing uncoupled quantum wells and one containing the ACQW structure. We show that the shifts in the excitonic absorption peaks in the ACQW structure are large even for low biases, whereas the Stark shifts observed in the uncoupled-well sample are relatively small. The InQ 53GaQ 47As/Inp g2Alp 48As samples studied in this work were grown by molecular-beam epitaxy on n+-type InP substrates at a growth temperature of 520 C at a rate of 1 2 pm/h, with the group V (As4)togroup-III beam-flux ratio maintained at about 5:1. The layer sequence for the samples, starting from the substrate, is as follows: 0.35 pm n +-type (-2 X 10' /cm Si) Inp 5&Alp 4sAs; 0.15 pm undoped Inp 52Alp «As; 0.7 pm undoped MQW region; 0.15 pm undoped Inp &&Alp 4sAs; 0.2 pm p+-type ( -2X 10' /cmBe)...
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