Fundamental research on 1.5μm quantum cascade lasers for optical communication

本研究项目旨在研究半导体异质结的生长技术和物理特性，以实现具有量子级联结构的1.5μm波长半导体激光器。首先，我们用MOCVD方法在A1203衬底上制备了AlGaN/GaN单异质结，在室温下获得了高达2000cm2/vs的电子迁移率，这清楚地表明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。GaN/AlGaN单层异质结的能量差为30 EMV，而GaN/AlGaN的能量差要大一个数量级，这归因于较强的压电场效应。此外，还研究了不同数目和不同厚度的量子阱的子间跃迁。随着量子阱总数的增加，吸收峰发生了移位。这是由于应变效应随量子井数的增加而松弛的结果。目前所得到的最短波长为1.7μm。综上所述，该研究项目成功地证明了GaN/AlGaN异质结或量子阱中的子带间跃迁对于单极器件的15μm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.