Design and characteristics of GaAsSb Photodiodes

This thesis focuses on the design, characterization, and performance optimization of GaAsSb/AlGaAsSb Separate Absorption Charge and Multiplication (SACM) avalanche photodiodes (APDs) grown on InP substrates, targeting applications in the 1550 nm telecom wavelength range, as well as short-wave infrared (SWIR) systems for LiDAR and free-space communication.
The absorption coefficients of GaAsSb and InGaAs p-i-n photodiodes were measured, showing strong absorption near 1550 nm, with coefficients of approximately 8400 cm⁻¹ for GaAsSb and 8100 cm⁻¹ for InGaAs. Both materials demonstrated low dark current and favourable photocurrent responses, validating their suitability for photodetection in this wavelength range.
Impact ionization measurements revealed that GaAsSb has a lower electron ionization coefficient (α) than InGaAs at fields below 210 kV/cm, which results in reduced multiplication but significantly lowers noise. This makes GaAsSb ideal for low-noise applications. At higher fields, GaAsSb has smaller inter-valley separations allow competitive electron ionization rates, making it suitable for high-field applications such as long-range optical communication and LiDAR.
Electroabsorption effects in GaAsSb and InGaAs were studied, examining the Franz-Keldysh effect. A theoretical model was developed to predict the electroabsorption behaviour in both materials, providing practical insights for optimizing high-speed optical modulators and photodetectors under applied electric fields.
GaAsSb/AlGaAsSb SACM APDs demonstrated excellent performance, achieving high multiplication (M > 1200) and low excess noise (F < 7 at M = 200) at room temperature. Compared to commercially available InGaAs/InP devices, these APDs exhibited a 40-fold improvement in maximum achievable multiplication and 6.5 times lower excess noise at M = 25. Photocurrent spectral measurements were utilized to optimize the device design by providing accurate information on the electric field within the absorber region.
In conclusion, GaAsSb/AlGaAsSb SACM APDs offer a compelling alternative to InGaAs-based devices, delivering lower noise at moderate fields and strong multiplication performance at higher fields. These characteristics make GaAsSb-based APDs highly suitable for telecommunications, LiDAR, and other infrared sensing systems, where high sensitivity, low noise, and optimized field management are crucial for performance.