Mid-Infrared GaAs/AlGaAs Quantum Cascade Lasers Technology

Abstract

The quantum cascade lasers (QCLs) are unipolar devices based on intersubband transitions and tunnelling transport [1, 2]. The GaAs-based QCLs have proved to be an effective source of laser radiation in mid-infrared (MIR) as well as far-infrared (FIR) regions. They find application in the gas sensing systems (e.g., for detecting CO2, NO, CH4) [3], medical diagnostics [4] and environment monitoring [5]. The complex design and operating principle require the technology of quantum cascade lasers to ensure the highest accuracy, uniformity and repeatability of both epitaxy and device processing [6]. High requirements for epitaxy are connected with ultrathin layers of periodic active region, where the wave functions are to be properly engineered [7–9]. The thickness accuracy of the structure should in general be better than 3%. Another crucial issue is the optimisation of injector doping, which is necessary to assure a large enough dynamic range of laser operation [10, 11]. In order to restrict the lateral current spreading in the device it is necessary to form a good quality deep mesa structures. Since the available gain is usually limited, the device design should minimize the radiation losses. The mesa is etched through active region and reduction of the optical losses requires a smooth waveguide sidewalls [12]. There is a number of QCLs designs reported in the literature [6, 13]. Sirtori et al. [6]. have presented the ridge structure and double trench structure, in which ion implantation was used for electrical isolation instead of dielectric layers. The optimum one seems to be the double trench construction which insures a good overlapping of the lateral mode with the gain region and enables stable epi-down mounting necessary for efficient cooling of the device [14]. The construction in which the extended proton-implanted regions were applied in the area defined by the two trenches, was the another presented solution. In this case the channel for current injection was narrower than the ridge width, which minimizes absorption of generated radiation in dielectric layers insulating mesa sidewalls [6, 15]. As a rule the relatively high voltages as well as current densities are necessary to polarize QCLs. That indicates demand for low resistance ohmic contacts in order to reduce the device serial resistance. These contacts should be characterized by thermal stability, low depth of metal diffusion into semiconductor layers and lateral uniformity of metal–semiconductor interfaces. In this paper we discuss some issues fabrication technology of Al0.45Ga0.55As/GaAs QCL operating at λ ≈ 9.4 μm. The devices lased up to 262 K (−11 ◦C) with optical powers over 1 W at 77 K, threshold current density values of 7 kA cm−2 and differential efficiencies above 0.6 W/A. Since lasers were uncoated, the quoted values should be understood.