title: Simulation, fabrication and characterisation of III-V photodetectors
Creators: name: Martínez Oliver, Cristina
Publisher: name: University of Glasgow
Year: 2024

description: Photonic Integrated Circuits (PICs) are set to revolutionise optical communication and sensing technologies by enabling the integration of optical interconnects directly into electronic chips, which can significantly reduce the power required to transmit data. Currently, electrical interconnects suffer losses ranging from 50% to 80% due to thermal dissipation, which can be mitigated with existing Mach-Zehnder optical modulators that are already on the market. These modulators are millimetre-scale devices, consuming energy in the picojoule per bit range. However, by leveraging devices in the micrometre to nanometre scale, power consumption can potentially be reduced by a factor of 1,000, achieving levels in the attojoule per bit range [1]. A key challenge in the development of PICs is the integration of efficient and high-performance devices such as photodetectors, particularly those based on III-V semiconductors for near-infrared (NIR) applications, onto silicon platforms.

This thesis explores the integration of III-V nanowire photodetectors on silicon substrates using Template-Assisted Selective Epitaxy (TASE), a technique that offers precise control over nanowire placement and geometry, making it highly suitable for PIC applications. The research presented herein covers the entire spectrum of work from simulation to fabrication, followed by a detailed electrical and optical characterisation to evaluate device performance.

This project began with device simulations using the Sentaurus TCAD software, with which a comprehensive study of III-V materials was conducted. The heterostructure (n-InP/i-InGaAs/p-InP) emerged as the most suitable material combination for photodetectors' optical performance optimisation, outperforming simpler heterostructures such as pure InP and InGaAs. A secondary study examined the effects of mid-bandgap traps within the devices, and showed how the variability in current-voltage (I-V) curves is highly dependent on the position and concentration of these defects, highlighting their significant impact on device characteristics. The simulations were then used to analyse the electrical behaviour of the fabricated devices and address the observed variability in their I-V characteristics.

Following the simulation phase, photodetectors were fabricated with a focus on optimising critical steps such as material growth and doping techniques. The electrical characterisation of these devices revealed substantial variability in I-V characteristics across different devices, a consequence largely attributed to the crystallographic orientation of the silicon substrate used in their fabrication. This variability presents significant challenges for achieving reliable comparisons between devices, highlighting the need for meticulous control over fabrication steps and the importance of selecting the appropriate substrate to achieve consistent and reliable device performance.

The following optical characterisation aimed to study the influence of design variations on device responsivity after light-coupling through a waveguide, marking a step toward their integration into a full optical link. However, the high variability found in the electrical characterisation led to a shift in focus for this work. The subsequent optical study, where the devices were optically characterised at low temperatures for potential use in quantum technologies, a thermal anomaly was observed in which measurements on a 300 nm wide device, down to 5 K, showed an unexpected increase in dark current below 200 K. This was explained through the presence of defects in the devices. An optical anomaly was also observed, requiring further research and explanation which is outside the scope of this thesis.

The findings of this work provide important insights into the challenges and potential solutions for integrating III-V photodetectors into silicon-based PICs, contributing to the advancement of optical communication.