Fundamental research on 1.5μm quantum cascade lasers for optical communication

本课题旨在研究利用量子级联结构实现1.5μm波长半导体激光器的半导体异质结构的生长技术和物理特性。特别地，我们研究了具有大带不连续的GaN/AlGaN单异质结构的子带间跃迁。首先，我们利用MOCVD技术在A1203衬底上制备了AlGaN/GaN单异质结构，并在室温下获得了高达2000cm2/Vs的高电子迁移率，清晰地展示了高质量的异质界面。我们用FTIR进行了吸收测量，发现在270 meV处有一个吸收峰，对应于基态和第一激发态之间的能量差。与GaAs/AlGaAs单一异质结构的30 emV能量差相比，GaN/AlGaN的能量差大了一个数量级，这是由于强压电场效应造成的。此外，还研究了不同数量和厚度的量子阱的亚层间跃迁。随着量子阱总数的增加，吸收峰发生了位移。这是由于应变效应随着量子阱数量的增加而松弛。目前获得的最短波长为1.7μm。总之，该研究项目成功地证明了GaN/AlGaN异质结构或量子阱中子带间跃迁对15μm单极器件发光激光器应用的重要性。

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.