Optical studies of semiconductor heterostructures: measurements of tunneling times, and studies of strained superlattices

[摘要] This thesis describes experimental studies of semiconductor heterostructures, using optical techniques. The work presented concerns two topics in the study of semiconductor heterostructures: the escape of confined electrons and holes by tunneling, and the accommodation of lattice mismatch by strain. Time-resolved photoluminescence techniques have been used to measure the times required for electrons and holes to escape by tunneling through the A1As barriers of GaAs/A1As/GaAs/A1As/GaAs double barrier heterostructures. The effect of the indirect (X-point) levels in the AlAs barriers upon escape of confined electrons has been investigated using continuous (CW) photoluminescence. Time-resolved studies of electrically biased double-barrier heterostructures have been made, using both photoluminescence and photocurrent techniques. Finally, the accommodation of the large (6%) lattice mismatch in CdTe/ZnTe superlattices has been studied using Raman scattering.In Chapter 2 we describe the measurement of tunneling escape times for electrons and holes confined in the quantum well of undoped GaAs/A1As/GaAs/A1As/GaAs double-barrier heterostructures. Photoluminescence from carriers photoexcited in the quantum well by short optical pulses was used to study escape from the quantum well. By using the two-beam technique of photoluminescence excitation correlation spectroscopy (PECS), the first experimental measurements of the tunneling escape times for both the electrons and the holes were obtained. The tunneling escape times were seen to be exponentially dependent upon the barrier thickness for barriers between 16 and 34 A. Escape times for both electrons and holes were found to be fast, and were as short as 12 ps in structures with 16 A (6 monolayer) A1As barriers. The rapid escape of heavy holes that was observed experimentally was in disagreement with simple calculations of the heavy-hole tunneling escape times, which indicated that the heavy holes should escape on a time scale many orders of magnitude longer than the times observed experimentally. This drastic difference can be explained theoretically by considering a four-band model for holes in confined systems. For finite carrier densities and temperatures, mixing of the quantum well heavy hole levels with light hole levels, due to dispersion perpendicular to the growth direction, can explain the experimental observations. This observation that heavy holes can escape rapidly by tunneling is quite general, and is applicable to a wide variety of problems involving tunneling of holes in semiconductor heterostructures.Chapter 3 describes a study of the effect of indirect (X-point) levels in the A1As barriers on the tunneling escape of electrons in undoped GaAs/AlAs/GaAs/AlAs/GaAs double-barrier heterostructures. The X-point levels, thought to be important in the electrical characteristics of double-barrier heterostructures, were found to affect the escape of photoexcited electrons in devices where the energy of the electron state confined in the GaAs quantum well is nearly equal to, or higher than, that of the X-point levels in the AlAs barriers.In Chapter 4, we present time-resolved photoluminescence and photocurrent studies of tunneling in doped devices under electrical bias, in which current is flowing. Studies of the photoluminescence decay indicate that significant transport of photoexcited carriers from the electrodes into the quantum well occurs. This transport of photoexcited carriers constitutes a photocurrent, and the two-beam PECS technique for photoluminescence has been extended to a study of photocurrents in these devices. This technique may be useful for the study of tunneling in devices not amenable to photoluminescence techniques.Chapter 5 describes a study of the accommodation of lattice mismatch in CdTe/ZnTe strained layer superlattices. Using resonance Raman scattering, the energies of the ZnTe-like phonons were determined in a series of superlattices of varying average CdTe content. The ZnTe-like phonon energies decrease with increasing average CdTe content, indicative of the increasing strain of the ZnTe layers. The observed energies agree well with calculations of the strain shift of the phonons. The results indicate that the superlattice layers adopt a lattice constant independent of the buffer layer on which they are grown, and are coherently strained to a lattice constant that minimizes the strain energy of the superlattice.Finally, the Appendix describes operation of the colliding pulse mode-locked (CPM) dye laser used in the time-resolved photoluminescence and photocurrent experiments. Alignment of the laser, and routine operation are documented.

[发布机构] University:California Institute of Technology;Department:Engineering and Applied Science

[关键词] Applied Physics