Wide Bandgap II-VI Compounds for Quantum Cascade Lasers

量子级联激光器(QCL)是一种基于量子阱(QW)内的子带跃迁的新型半导体激光器。QCL的更吸引人的特征是它们的单极传输、高功率发射、超快调制(100 GHz)以及在中红外和远红外产生前所未有的波长。在过去的几年里，QCL的发展取得了长足的进步，导致了第一批商用热电致冷QCL的出现。然而，这些激光器仍然存在许多局限性。一个关键的问题是QCL在室温(RT)下无法工作在连续波(CW)模式下。另一个原因是没有工作在短波长的QCL，如L=1.55 mm，这是发展超高速光通信所需的。这两个缺点都是由于目前使用的材料组合中电子限制不足所致。因此，需要基于宽带隙材料的新系统。本计划提出探索利用宽带隙II-VI材料来开发QCL。由II-VI半导体提供的大的导带偏移量和深量子阱为新一代超快光通信激光器提供了更高的电子限制(从而有机会获得工作在RT模式下的CW模式的QCL)和工作在1.55 mm的QCL。支持将II-VI半导体用于QCL的一个重要问题是，这些器件是单极的，不需要p型掺杂，而p型掺杂在大多数II-VI系统中一直是一个问题。虽然GaN基材料是一种替代材料，但与高质量的氮化物材料多层结构的生长相关的困难使得它们的吸引力不如近晶格匹配的、发展良好的II-VI材料系统。几种特别有希望用于低阈值电流、工作在RT下的II-VI材料系统将被研究，其中一些应该能够产生1.55 mm的发射。量子波特性，如可用于子带间跃迁的能级，将使用调制光谱和其他光学技术进行研究。这类技术在过去也被私人投资机构广泛使用。一旦一个系统被确定为具有QCL应用的潜力，量子波结构将被优化并结合到器件结构中，以研究发射和激光特性。

Quantum Cascade Lasers (QCLs) are a new kind of semiconductor laser based on intersubband transitions within a quantum well (QW). Among the more attractive features of QCLs are their unipolar transport, high power emission, ultrafast modulation (100GHz) and generation of wavelengths in the mid and far infrared, not ever obtained before. Considerable advances have been achieved in the development of the QCLs during the last few years, which have led to the appearance of the first commercial thermoelectrically cooled QCLs.However, there are still many limitations of these lasers. A critical one is the unavailability of QCLs operating in continuous wave (CW) mode at room temperature (RT). Another is the absence of QCLs operating at short wavelengths, such as l=1.55mm, needed to develop ultrafast optical communications. Both of these drawbacks are due to insufficient electron confinement in the materials combinations presently used. Therefore, new systems based on wide band gap materials are required.This program proposes to explore the use of wide bandgap II-VI materials for the development of QCLs. The large conduction band offsets and deep QWs offered by the II-VI semiconductors, such as ZnCdMgSe/ZnCdSe, provide the possibility of higher electron confinement (and consequently the opportunity to obtain QCLs operating at RT in CW mode) and of QCLs that operate at 1.55mm for a new generation of ultrafast optical communications lasers. An important issue in favor of using II-VI semiconductors for QCLs is the fact that these devices are unipolar and do not require p-type doping, which traditionally has been a problem in most of the II-VI systems. While GaN-based materials are an alternative, the difficulties associated with the growth of high quality multi-layered structures of the nitride materials make them less attractive than the near lattice-matched, well-developed II-VI material systems.Several II-VI systems that are particularly promising for QCLs with low threshold current, operating at RT and some of them that should be able to generate emission at 1.55mm will be investigated. The QW properties, such as the energy levels available for intersubband transitions, will be investigated using modulation spectroscopy and other optical techniques. Such techniques have also been extensively utilized in the past by the PIs. Once a system has been identified to have potential for QCL applications, the QW structures will be optimized and incorporated in device structures to investigate the emission and lasing characteristics.