The Effect of Doping in Split-Well Direct-Phonon THz Quantum-Cascade Laser Structures

Nathalie Lander Gower,Asaf Albo,Silvia Piperno

doi:10.3390/photonics8060195

Abstract

We have studied the effect of doping on the temperature performance of a split-well (SW) direct-phonon (DP) terahertz (THz) quantum-cascade laser (QCL) scheme supporting a clean three-level system. Achieving a system that is as close as possible to a clean n-level system proved to be the strategy that led to the best temperature performance in THz-QCLs. We expected to obtain a similar improvement to that observed in resonant-phonon (RP) schemes after increasing the carrier concentration from 3 × 1010 cm−2 to 6 × 1010 cm−2. Our goal was to improve the temperature performance by increasing the doping, ideally the results should have improved. To our surprise, in the devices we checked, the results show the contrary. Although an increase in doping had previously shown a positive effect in RP schemes, our results indicated that this does not happen with SW–DP devices. However, we observed a significant increase in gain broadening and a reduction in the dephasing time as the doping and temperature increased. We attribute these effects to enhanced ionized-impurity scattering (IIS). The observation and study of effects related to dephasing included in our experimental work have previously only been possible via simulation.

Highlights

Split-Well Direct-Phonon THzTerahertz (THz) quantum-cascade laser (QCL) are noteworthy because of their range of potential applications, which includes nondestructive detection of materials, imaging, and gas spectroscopy [1,2,3,4]
THz QCLs proved to be beneficial for wireless communication and imaging technologies [11]
It is known that the main mechanism limiting the performance of vertical-transition THz QCLs is nonradiative thermally activated longitudinal-optical (LO)-phonon scattering from the upper laser level (ULL) to the lower laser level (LLL) [16]

Summary

Introduction

Terahertz (THz) quantum-cascade laser (QCL) are noteworthy because of their range of potential applications, which includes nondestructive detection of materials, imaging, and gas spectroscopy [1,2,3,4]. It is known that the main mechanism limiting the performance of vertical-transition THz QCLs is nonradiative thermally activated longitudinal-optical (LO)-phonon scattering from the upper laser level (ULL) to the lower laser level (LLL) [16]. This is significantly reduced in diagonal transition structures [17,18]. To better understand the temperature performance of these devices, we focused on work carried out in 2016 that involved experimental results from three-well RP designs [32] These results led to the conclusion that high diagonality as a strategy cannot succeed on its own and must be paired with increased doping. The barriers’ composition and doping details are elaborated in the following text

Design

Findings

Discussion