Characterisation of GaAsBi based heterojunction and MQW photodiodes

This dissertation focuses on incorporating dilute bismuth (Bi) into a GaAs substrate to enable avalanche photodiodes (APDs) operating in the near-infrared range while reducing excess noise at high gain.
GaAsBi epitaxial layers grown by molecular beam epitaxy (MBE) incorporated small amounts of Bi, tuning the bandgap and enhancing spin–orbit splitting. XRD (with consistency checks against the optical response) determined Bi composition and layer thickness, while SIMS provided a qualitative and semi-quantitative indicator of dopant profiles, axial layer placement, and relative Bi incorporation trends.
By fabricating p–i–n and n–i–p heterojunction devices and multiple quantum well (MQW) structures, this work systematically investigated relationships among lattice strain, optical response, dark current, and avalanche gain–noise performance. Moderate Bi content effectively suppresses hole-initiated impact ionization at high reverse biases, reducing the β/α ratio and lowering the APD excess noise. Multi-period MQWs, maintaining controllable strain, achieve noise reduction similar to thick single GaAsBi layers while exhibiting stronger sensitivity around 1 µm.
Excessive Bi content or MQW period number leads to strain accumulation and defects, increasing dark current and highlighting the need for further optimization of epitaxial growth and strain compensation. The dissertation also explores potential applications and limitations, and proposes future directions including temperature-dependent characterization, strain management, advanced multi-region designs, and large-scale integration.
In summary, GaAsBi shows promise for high-gain, low-noise near-infrared detection, and with continued refinement of epitaxial processes and device architectures, could enable high-speed optical communications, lidar, and highly sensitive sensing systems.