Low Effective Electron Mass InGaAs/InAlAs for High Power Terahertz Quantum Cascade Lasers

Abstract

Summary form only given. Quantum cascade lasers (QCLs) are powerful sources of coherent radiation covering the frequency range from mid-infrared to terahertz. In the terahertz frequency range the active region is normally realized using a GaAs/Al x Ga 1-x As semiconductor heterostructure. This material system enables a variable conduction band offset by changing the Al-content in the barrier layers without introducing a significant lattice mismatch between the barrier and well material. In comparison to the standard GaAs-based material system, active regions based on material systems with a lower effective electron mass are highly beneficial for the design of terahertz QCLs as the optical gain increases for a lower effective electron mass [1]. Promising material systems are based on InGaAs or InAs with an effective electron mass of 0.043 and 0.023, respectively, compared to that of GaAs (0.067) [2, 3].In this work we present a systematic study of growth related asymmetries for terahertz QCLs based on the InGaAs/InAlAs material system lattice matched on InP. A nominally symmetric active region enables the comparison of the positive and negative bias direction of the very same device [4]. With such bias dependent performance measurements asymmetries like dopant migration and interface roughness, which play a crucial role in this material system, are studied and result in a preferred electron flow in growth direction. A structure based on a three well optical phonon depletion scheme is optimized for this bias direction. Depending on the doping concentration the performance of the QCLs shows a trade-off between maximum operating temperatures and high output powers. While a peak output power of 151 mW is achieved for a sheet doping density of 7.3 x 1010 cm-2, the highest operation temperature of 155 K is found for 2 x 1010 cm-2. By further attaching a hyperhemispherical GaAs lens to a laser facet, the peak output power could be improved and reaches a record output power ...