Fundamental research on 1.5μm quantum cascade lasers for optical communication

本研究计画主要探讨半导体异质结构的成长技术与物理，以实现1.5μ m波长的量子级联结构半导体雷射。首先，我们采用MOCVD方法在Al_（120）_3衬底上制备了AlGaN/GaN单异质结，并在室温下获得了高达2000 cm ~ 2/Vs的高电子迁移率，清晰地展示了高质量的异质界面。我们使用FTIR进行吸收测量，发现在270 meV处的吸收峰对应于基态和第一激发态之间的能量差。与GaAs/AlGaAs单异质结的30 emV能量差相比，GaN/AlGaN异质结的大能量差大了一个数量级，这归因于强的压电场效应.此外，我们还研究了不同数量和厚度的量子威尔斯阱的子带间跃迁.随着量子威尔斯阱总数的增加，吸收峰发生了位移。这是由于应变效应随量子威尔斯数增加而松弛的结果。本研究成功地证明了GaN/AlGaN异质结构或量子威尔斯阱中的子带间跃迁对于15μm单极性发光激光器的重要性。

This research project is organized to investigate growth technique and physics of semiconductor heterostructures for realizing 1.5μm-wavelength semiconductor lasers with quantum cascade structures. In particular, we investigate intersubband transition in GaN/AlGaN single heterostructures with large band discontinuity.First, we fabricated AlGaN/GaN single heterostructures ob A1203 substrates by MOCVD and achieved high electron mobility up to 2000cm2/Vs at room temperature, which clearly demonstrates the heterointerface of high quality. We performed absorption measurement using FTIR and found an absorption peak at 270 meV corresponding to the energy difference between the ground and the first excited states. In contrast to the energy difference of 30 emV at the GaAs/AlGaAs single heterstructure, the big energy difference in GaN/AlGaN is larger by one order of magnitude, which is attributed to strong piezoelectric field effects.In addition, intersuband transition is investigated for various quantum wells with different number and well-thickness. With the increase of the total number of quantum wells, the shift of absorption peak was observed. This is due to the relaxation of strain effects with the increase of the numer of quantum wells. AT present stage the shortest wavelength so far obtained is 1.7μm.In summary, the research project has succeed in demonstrated importance of intersubband transition in GaN/AlGaN heterostructures or quantum wells for 15μm light emitting laser applications with unipolar devices.