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

五层非对称耦合量子阱（FACQW）结构与传统的矩形量子阱（RQW）结构相比，可以在远离吸收边的透明波长区产生很大的电折射率变化Δn。我们从理论和实验上研究了高质量GaAs/AlGaAs FACQW的制备及其在光调制器和光开关中的应用. FACQW结构的制备与表征采用精确快门控制的分子束外延（MBE）方法生长了FACQW结构，光致发光和光电流测试表明，所制备的FACQW结构符合设计要求，其Δn特性与计算结果一致.层厚起伏对FACQW结构电折射率变化的影响理论上研究了FACQW结构的1 ML厚度偏差和起伏对Δn的影响。 ...更多信息 FACQW中的所有层对Δn有相当大的影响。两个薄的Al（Ga）As势垒层的影响与其他GaAs阱层的影响相比很小。ii）两个较厚的GaAs层的厚度比显著影响Δn，只要层的厚度的比率保持恒定，Δn值就不会改变。iii）Δn中的急剧下降的物理起源。对电场特性进行了理论研究。通过改变薄AlGaAs势垒层的位置或Al含量来改变势垒层的电位，可以消除这种凹陷。（4）讨论了1 ML势垒层厚度的统计涨落对Δn特性的影响。即使当An随着层变厚或变薄的发生概率增加1 ML（具有偏离厚度的阱面积与层的总面积的比率）而减小时，FACQW仍然保持比常规矩形量子威尔斯（RQW）大得多的Δn。采用迁移增强外延法生长高质量量子阱结构迁移增强外延法（migration enhanced epitaxial，简称EPE）是一种很有前途的生长技术，它可以在较低的生长温度下获得更平坦、更陡峭的异质界面。我们将EPE方法引入到高质量量子阱结构的生长中。i）安装了一个计算机控制的K-单元快门系统，用于复杂的EPE生长序列。ii）K-单元快门系统，用于复杂的EPE生长序列。研究了AlGaAs生长的单元快门顺序。发现Al和Ga原子同时供给后，Ga和As原子交替供给的顺序可以实现更平坦的异质界面，且Al含量没有波动。利用该序列，在500℃下成功生长出了高质量的RQW.改进的FACQWA中的电致电阻效应具有与正常FACQW略微不同的结构，并且预期获得非常大的负Δn。从理论上详细研究了改进的FACQW中的电致电阻效应，讨论了改进的FACQW中大Δn的详细机理，并发现改进的FACQW比普通FACQW具有更好的层厚波动容限。少

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