Research on Five-Layer Asymmetric Coupled Quantum Wells and Their Application to Ultra-Fast and Ultra-Low Voltage Optical Modulators and Switches

点击翻译按钮获取中文摘要

The five-layer asymmetric coupled quantum well (FACQW) structure, in contrast to the conventional rectangular quantum well (RQW), can produce very large electrorefractive index change Δn in the transparency wavelength region far from the absorption edge. We theoretically and experimentally studied fabrication of high-quality GaAs/AlGaAs FACQWs and their application to optical modulators and switches.1. Fabrication and Characterization of the FACQW structuresThe FACQWs were grown by molecular beam epitaxy (MBE) with precise shutter control.Photoluminescence and photocurrent measurements of the samples revealed that the structures were fabricated as designed and their Δn characteristucs were consistent with calculation results.2. Influence of layer thickness fluctuation on electrorefractive index change of the FACQWThe influence of 1 ML thickness deviations and fluctuations from the designed FACQW structure on Δn was studied theoretically.i) The thickness variation of the thicker GaAs we … More ll layers in the FACQW has a considerable influence on Δn. The influence of the two thin Al(Ga)As barrier layers is small compared with that of other GaAs well layers.ii) The ratio of the thicknesses of the two thicker GaAs layers significantly affects the Δn, characteristics of the FACQW.The value Δn does not change as long as the ratio of the thicknesses of the layers is kept constant.iii) The physical origin of the sharp dip in the Δn-electric field characteristic was studied theoretically. The dip can be removed by changing the position or Al content of the thin AlGaAs barrier layer for potential modification.iv) The influence of the statistical fluctuation of the layer thickness by 1 ML on the Δn characteristics was discussed. Even when An decreases with the increase of the occurrence probability of a layer being thicker or thinner by 1 ML (the ratio of well area with deviated thicknesses to total area of the layer), the FACQW still maintains a much larger Δn than the conventional rectangular quantum wells (RQWs).3. Growth of high-quality quantum well structures with migration enhanced epitaxy methodThe migration enhanced epitaxy (MEE) method is a promising growth technique that can obtain flatter and steeper heterointerfaces at lower growth temperature than conventional MBE.We introduced the MEE method to growth of high-quality quantum well structures.i) A computer-controlled system of K-cell shutters was installed for complicated gowthsequence of MEE.ii) K-cell shutter sequence for AlGaAs growth was investigated. We found the sequence that after Al and Ga atoms are supplied simultaneously, Ga and As atoms are supplied alternatively can realize flatter heterointerface without Al content fluctuation. Utilizing this sequence, high-quality RQWs were successfully grown at 500℃.4. Electrorefrative effect in a modified FACQWA modified FACQW has a slightly different structure from the normal FACQW, and it is expected to obtain very large negative Δn. We theoretically studied in detail electrorefrative effect in the modified FACQW.We discussed the detailed mechanism of large Δn in the modified FACQW.In addition, we found that the modified FACQW has more tolerance of layer thickness fluctuation than the normal FACQW. Less