Abstract
High-sensitivity avalanche photodiodes (APDs) are used to detect weak optical signals in a wide range of research, commercial, and military applications, including telecommunications, data centers, spectroscopy, imaging, light detection and ranging (LiDAR), medical diagnostics, and quantum computing. A widely used APD structure has separate absorption, charge, and multiplication (SACM) sections, where two different materials are employed as the multiplier and the absorber, respectively. The advantages of these two materials can be combined. This dissertation focuses on three research areas: (1) investigation of multiplier material with low excess noise and strong temperature stability, (2) investigation of absorber material with improved optical characteristics, and (3) investigation of SACM APDs incorporating the low-noise multiplier and the absorber that operate in the infrared spectral region.
Compared to conventional p-i-n photodiodes, APDs exhibit higher photodetection sensitivity due to their internal gain originating from serial impact ionization. However, the gain, M, is created by impact ionization which is a random process. This gives rise to detector noise expressed as an excess noise factor, F(M), which can degrade the signal-to-noise ratio and gain-bandwidth product. Therefore, a multiplier with low excess noise is always preferable. This dissertation demonstrates AlGaAsSb and AlInAsSb materials lattice matched to InP which exhibit low excess noise, comparable to that of silicon (Si) (k ~ 0.01). Subsequently, the optical constants of AlGaAsSb and AlInAsSb were extracted via variable-angle spectroscopic ellipsometry, and the impact ionization coefficients of AlGaAsSb were determined in a wide electrical field range via a series of wavelength-dependent multiplication gain curves. The extracted material parameters are necessary for future simulations of APDs using Sb-based multipliers. Additionally, these Sb-based material systems have exhibited significantly enhanced temperature stability of avalanche breakdown compared to commercially available Si-, InAlAs-, and InP-based APDs, reducing the cost and complexity of optical receiver circuits.
For APDs used in the short-wavelength infrared (SWIR) spectral region (900 – 1700 nm), Si and InGaAs are widely used as the absorption materials with the cut-off wavelengths of 1100 nm and 1680 nm, respectively. However, recent advances in applications such as imaging, optical communications, and LiDAR have created a need for photodetectors that operate in the extended SWIR spectral region (1700 – 2500 nm), which is beyond the cutoff wavelength of random alloy (RA) InGaAs. In this dissertation, a series of digital alloy (DA)-grown InAs/GaAs short-period superlattices were investigated to extend the absorption spectral range. The photoluminescence peak can be effectively shifted from 1690 nm (0.734 eV) for conventional RA InGaAs to 1950 nm (0.636 eV) for eight monolayer DA InGaAs at room temperature. The optical constants of DA InGaAs have been extracted via ellipsometry technique, showing absorption coefficients of 398 /cm, 831 /cm, and 1230 /cm at 2 µm for 6, 8, and 10 monolayer DA InGaAs. As the period thickness increases for DA InGaAs, a redshift at the absorption edge can be observed. These results pave the way for the future utilization of the DA-grown InAs/GaAs short-period superlattices as a promising absorption material choice to extend the photodetector response beyond the cutoff wavelength of RA InGaAs.
After the investigation of multiplication and absorption materials, this dissertation reports three Sb-based SACM APDs on semi-insulating InP substrates, InGaAs/AlInAsSb APDs, GaAsSb/AlGaAsSb APDs, and DA InGaAs/AlGaAsSb APDs. These APDs exhibit low excess noise and weak temperature dependence of avalanche breakdown due to the Sb-based multiplication regions. Using semi-insulating InP substrates removes the limitation on high-speed operation observed with Sb-based APDs on doped GaSb substrates. The InGaAs/AlInAsSb APDs and GaAsSb/AlGaAsSb APDs have conventional RA absorbers that operate in the SWIR spectral region. The absorber in the DA InGaAs/AlGaAsSb APDs can extend the operating wavelength into the extended SWIR spectral region. These SACM APDs exhibit excellent electrical and optical properties, making them ideal for use in high-sensitivity optical receivers in a wide range of applications within the SWIR and extended SWIR spectral regions.
