Optical Characterization and Third-Order Optical Susceptibility of Uniformly Doped Short-Period Gallium Arsenide Doping Superlattices
The optical properties of short-period semiconductor doping superlattices make them attractive materials for nonlinear integrated optical applications. We report fabrication (by molecular beam epitaxy), optical characterization, and modeling of uniformly doped GaAs doping superlattices with periodicities between 300 and 100 A. Optical characterization by nearly-degenerate four wave mixing at lambda = 10.6 μm is accomplished for the first time in a doping superlattice: low temperature photoluminescence and room temperature photoreflectance is also performed. We observe excitation-dependent photoluminescence in doping superlattices with period lengths as short as 100 A where the degree of luminescence tunability decreases with decreasing period length. At low photoexcitation intensity, the photoluminescence peak energies and intensities increase with decreasing superlattice period, providing evidence of impurity layers. An anomalous red-shift in the low excitation luminescence energies is also observed for periodicities of 200 A and less. The photoreflectance spectra display Franz-Keldysh oscillations at the bulk band gap with an impurity-related feature appearing for superlattices with periods of 200 A and less. We find radiative recombination between spatially localized donor and acceptor impurity states dominates for periods less than 200 A in uniformly doped superlattices. The occupied spatially segregated impurity states, which are associated to potential fluctuations arising from the random distribution of dopants, lead to screening of an effective superlattice potential and thus the observed photoluminescence tunability. Greater luminescence efficiency observed for short periods may allow lasing at carrier densities below that which will completely screen the superlattice potential, making uniformly doped short-period doping superlattices promising materials for tunable coherent light sources. We also theoretically show short-period doping superlattices are suitable for enhancement of a nonresonant third-order optical susceptibility. This optical nonlinearity arises from carriers in nonparabolic energy subbands and can be optimized by engineering the space-charge superlattice potential profile. While four wave mixing experiments show a larger isotropic third-order susceptibility than expected, we do not observe superlattice enhancement because of the lack of dispersive minibands in uniformly doped superlattices. Short-period planar doped GaAs superlattices are suggested as better materials to achieve enhancements in this optical nonlinearity.
- Pub Date:
- January 1990
- GALLIUM ARSENIDE;
- Engineering: Materials Science; Physics: Optics