Quantum-Confined Stark Effect in Artificially Made Quantum Well Structures
Doctoral thesis, 1993
The objective of the work presented in this thesis is to study the quantum-confined Stark effect in differently shaped quantum well structures. This effect has shown great interest and significant applications in optoelectronic devices. Most of the studies and applications have focused on square quantum wells. The modification of the potential shape offers us a new freedom to create new optical properties and improve device performances. For this purpose, we established a simple, reliable and widely usable mathematical tool to simulate the quantum-confined Stark effect in arbitrarily shaped QWs. Improvement was made to Bloss' numerical method by including the material dependence of effective mass and nonparabolicity effect. Simulations gave a whole comparison of Stark shifts for several simple QWs, and also led to the new findings of novel QW structures. For the first time, we studied the QCSE for a two-step and an inverse parabolic QWs, and found, by photoluminescence measurements, much larger interband Stark shifts for the two-step and inverse parabolic QWs than conventional square QWs; more than two times larger for the former and about two times for the later. As for an additional purpose for studying the limitations of growing non-square QWs, we fabricated the inverse parabolic QWs by MBE using both digital and analog techniques. The digital well showed a good agreement between the experimental and theoretical results, and deviations were observed for the analog well, that were attributed to fluctuation in QW parameters. Moreover, the concept of local-to-global state transitions was, by experiments, confirmed to be a useful rule for searching novel QW structures of the QCSE. In the aspect of intersubband Stark shift, a thin layer inserted QW showed much flexibility of tuning intersubband transitions and the largest intersubband Stark shift was obtained as the well was adjusted to a step-like well. There was a wide range of wavelength tunability, e.g. 8-25 µm within Â±100 kV/cm. This advantage was also found through a comparison with other non-square wells.