N-Type Ge/SiGe Multi-Quantum Wells for THz Light Emission: High Quality Growth and Material Parameter Calibration

The efficient integration of electronic and optoelectronic functions on a single chip within a CMOS-compatible material and technology platform is witnessing the next disruptive innovation in the microelectronics industry. Among the proposed approaches to “siliconize” photonics, the most ambitious is to develop a monolithic Si-based light source. The main challenge is the indirect bandgap of Si which hinders efficient light emission. A number of different solutions has been suggested to break down this material limitation, and the optimal strategy depends on the specific spectral range of application. In the THz range, quantum cascade laser (QCL) architectures based on the Group-IV material system have been predicted as viable potential sources up to room temperature 1-3 , by exploiting intersubband (ISB) transitions in the conduction band of n -type, Ge-rich Ge/SiGe multi-quantum well (MQW) heterostructures 3 . In this paper, we will show that the interface quality and threading dislocation density 4 in this material system have finally reached the level required for possibly enabling, within a wide temperature range, a material gain larger than the cavity losses 5 . From the structural standpoint, we report on the growth by ultra-high-vacuum chemical vapor deposition (UHV-CVD) of strain-compensated QCL stacks with a thickness up to 10 µm, demonstrating by X-ray diffraction and scanning transmission electron microscopy their high crystalline quality and remarkable growth reproducibility [Fig. 1(a)]. We will also describe optimized designs obtained in waveguide modelling with which waveguide losses comparable to III-V architectures are achieved 6 . In the model system of n -type Ge-rich asymmetric coupled quantum wells (ACQWs) 7,8 , being the building block of a QCL structure, we will prove a high degree of control on the engineering at the nanoscale the intersubband electronic spectrum and the wavefunctions relevant for tunneling processes, which allowed us the observation of photoluminescence at 4 THz after optical excitation using a free-electron laser (FEL) 9 . In addition, we will present a systematic calibration study of the material parameters controlling the interface roughness (IFR) and electron-phonon scatterings in n -type Ge/SiGe MQWs by combining experimental pump-probe data with a numerical model of ISB carrier dynamics including inelastic and elastic scattering channels. The time evolution of subband populations after FEL optical pumping is