Germanium and GeSn based Quantum Well Lasers and Nanoscale Multi-gate FETs

The incredible technological advancements over the last century have been possible due to tiny trinkets designed using semiconducting crystalline materials, especially Silicon and III-V compounds. Silicon, a group IV element has become the first choice in developing microchips serving an ever-growing set of applications including, computation, RF communications, solar cells, power electronics, quantum computing and its periphery, optoelectronics, IOT sensors, and lately artificial intelligence. Billions of Si-based complementary transistors (CMOS) are present at the center of most computing devices used today such as HPC servers, compute farms, laptops, and smartphones. The astonishing rise in transistor count, performance, and functionality as well as the exponential reduction in cost has been possible over the past decades due to a singular idea: shrinking the device. However, this rule, also called Moore's Law has been slowing over the past two decades and has eventually come to a standstill in its traditional definition. Moore's law has since been sustained by ingenious innovations such as high-k gate dielectrics, vertical scaling, lattice strain engineering, novel material developments and, lately chiplets as well as multi-die vertical packaging. As conventional Si CMOS approaches a roadblock, this work presents research on Germanium-based multi-gate devices providing promise for faster and low-power operation. This work discusses how Ge grown on a GaAs substrate can be tuned and utilized to form a virtually defect-free channel for ultra-scaled multi-gate transistors. Calibrated solvers informed using in-house materials and devices as well as literature are used to predict device performance for advanced structures. Further, a hybrid CMOS system with the high hole mobility p-channel device formed using tensile strained Ge, and the high electron mobility n-channel device formed using the underlying InGaAs layer is proposed and simulated. As scaling approaches Gate-all-around Nanosheet FETs in 2024 and complementary-FETs (CFETs) around 2034, Ge-on-AlAs based transistors can offer unique process simplifications, defect reduction, yield improvement, and high-performance advantages showing promise for future IRDS nodes. The process design, material stack, device, and circuit performance for this novel Ge-based NSFET is presented in this work. The lack of large strain or strain relaxation in the NS multilayer starting stack is seen to be a great process advantage for the Ge-AlAs NSFET system. To a certain extent, Si seems omnipotent for all things electronics. However, one exception is on-chip light generation. A coherent electrically controllable on-chip light source is a central component critical for optoelectronics, quantum technologies, fiber communications, and sensing. Due to the indirect bandgap, Si cannot produce light hence direct bandgap materials such as GaAs and GaN have been the primary choice for off-chip light sources integrable on the platform. Interestingly, Ge has a pseudo-direct bandgap, i.e., unlike Silicon, it can be manipulated to produce light using heavy doping, tensile strain, and Sn alloying. Similar to conventional III-V light sources, reduction in the dimensionality of the gain medium, i.e., Ge can enable a drastic reduction in the current required to produce light, among other performance considerations. This reduced dimensionality can be achieved by forming quantum wells and quantum dots. In this work, two new types of Ge-based quantum well lasers are introduced and analyzed along with qualitative and quantitive benchmarking. The first QW laser uses a small epitaxial biaxial tensile strain to improve the direct-ness of the Ge gain medium. The internal quantum efficiency, net gain, and threshold current can be improved drastically by choosing the right tensile strain while staying within a certain critical thickness value. For the first time, the impact of biaxial tensile strain on the optical properties of Ge is analyzed and reported through a systematic study of the dielectric spectra and optical constant using VASE. The changes in the band structure due to tensile strain are correlated with the critical points to uncover various optical transitions. An even better QW laser architecture is possible by utilizing a GeSn QW. This QW laser uses Sn-alloying to form a GeSn active region which is further lattice matched to the waveguide (InGaAs) and the optical confinement layers (InAlAs) around it. This completely lattice-matched laser structure can offer unique advantages such as virtually defect-free active region, tunability as well as improved efficiency and threshold current density. The absence of strain and consequently strain relaxation in the laser stack enables one to steer away from the critical thickness limitation while opening doors to designing multiple quantum well lasers among other complex architectures. The impact of Sn alloying on the atomic structure, lattice coherence, and relaxation is analyzed through XRD reciprocal space maps and rocking curves as a function of Sn concentration. Further, this lattice-matched system, GeSn-InGaAs-InAlAs has the potential to mirror the benefits of the mature GaAs-AlGaAs system which led to many great technological innovations over the past decades such as lasers and LEDs.