Quantum efficiency enhancement of germanium-on-insulator photodetectors for integrated photonics on silicon

Optical interconnects, enabled by electronic-photonic integrated circuits (EPICs), has been proposed as a potential solution to the latency, bandwidth and power density bottlenecks of the conventional metal interconnects in complementary metal-oxide-semiconductor (CMOS) integrated circuits. Optoelectronic components such as photodetector is one of the key building blocks, converting optical signals into electrical ones, to realize such a platform. Germanium (Ge), thanks to its CMOS-compatibility and considerable optical absorption at tele-communication wavelengths, has been widely studied for integrated photodetectors on silicon (Si). In recent years, germanium-on-insulator (GOI) has attracted significant attention. The intermediate insulator layer can play several vital roles: 1) preventing carrier cross-talk and material inter-diffusion between Ge and the underlying Si; 2) enabling a stronger optical confinement in the top Ge for sensing and communication at mid-infrared wavelengths (3 to 14 µm); 3) enhancing electrostatic control for Ge CMOS; 4) facilitating a dense three-dimensional back-end-of-line (BEOL) electronic-photonic integration. The quality of Ge can also be generally improved compared to that from direct epitaxy, which could help reduce the dark current noise from the photodetectors. However, state-of-the-art photodetectors on GOI are reported with limited quantum efficiency (QE). This is due to their planar deployment of the contacts at the Ge surface that is inefficient in collecting the deep photon-generated carriers. In this thesis, a method to realize a vertical p-i-n structure in GOI, using silicon dioxide (SiO2) as the insulator layer, was developed by ion-implanting boron at one side of an epitaxial-grown Ge layer, followed by arsenic at the other side, along with a bonding and layer transfer process for the GOI fabrication. The vertical p-i-n structure builds a vertical electric field across the Ge layer for an efficient carrier collection. Abrupt doping profiles were verified in the transferred high-quality Ge layer. The photodetectors exhibit a dark current density of∼47 mA/cm2 at −1V and an optical responsivity of 0.39 A/W at 1550 nm, which are prominently improved compared to state-of-the-art GOI photodetectors. An internal quantum efficiency (IQE) of ∼97% indicates an excellent carrier collection efficiency. The experimental 3-dB bandwidth of ∼1 GHz agrees well with the theoretical calculation including series resistance and parasitic capacitance of the device. The bandwidth is expected to reach ∼32 GHz with an optimized contact resistance as well as mesa diameter. In addition, an improved QE was also demonstrated in uniformly tensile-strained GOI photodetectors. The tensile strain reduces the Ge bandgap and increases its band-to-band absorption coefficient, enabling an enhanced QE particularly at the tele-communication L-band (1,565 to 1,625 nm) and beyond. The uniform tensile strain is achieved by a patented recessed approach in placing silicon nitride (SiNx) sidewall stressor. A self-aligned dry etching (SADE) method was introduced to remove the insulator (SiO2) layer during the Ge waveguide-on-insulator (WGOI) patterning, followed by placing the tensile-stressed SiNx stressor at the waveguide sidewalls. A tensile strain of ~0.7% with an enhanced uniformity was observed from the Ge WGOI with a 580 MPa-tensile SiNx stressor, using micro-Raman measurements, along the transverse direction of the waveguide. The corresponding metal-semiconductor-metal (MSM) photodetectors exhibit a ~70nm extension of the absorption coverage towards longer wavelengths and a ~2× enhancement on the QE (as well as the absorption coefficient) at 1,620 nm. An additional ~37% QE improvement was also established, compared to the detectors without employing SADE. Theoretical calculation tells that a 1.5 GPa-tensile stressor could extend the Г-valley-heavy-hole and -light-hole bandgap edges towards ~1,665 and ~1,710 nm, respectively, covering the entire tele-communication bands. The results potentially facilitate a high-bandwidth and high-efficiency GOI photodetector at BEOL for integrated photonics. Suggestions have also been given to further improve the device performance. Furthermore, the concept of uniformly-straining Ge can be extended for the bandgap engineering of other semiconductor materials such as III-V and GeSn for a wide range of optoelectronic applications including light sources, electro-absorption modulators and photodetectors.Doctor of Philosoph