Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.778788
Title: Avalanche breakdown characteristics of thin Al0.85Ga0.15As0.56Sb0.44 avalanche photodiodes
Author: Abdullah, Salman
ISNI:       0000 0004 7964 5169
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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Abstract:
Currently Single Photon Avalanche Diodes (SPADs) based on InP multiplication layer dominate the market for single photon detection applications including autonomous driving and remote sensing applications. Major attractions of InP include its wide bandgap (1.46 eV at 296 K) and lattice matching with narrow bandgap absorber In0.47Ga0.53As (0.75 eV at 296 K) which facilitates photon detection at 1550 nm. However a problem with linear and Geiger mode avalanche photodiodes based on InP is their heavy dependence on temperature stabilisation mechanisms owing to higher temperature coefficient of avalanche breakdown of InP (6 mV/K for a nominally 100 nm thick avalanche layer). Moreover thin avalanching layers of InP can suffer from a significant band-to-band tunnelling current. A solution to this problem is an even wider bandgap material that is less susceptible to band to band tunnelling current (due to its wider bandgap) and more temperature robust (due to its temperature insensitive avalanche breakdown). A wider bandgap can reduce the band to band tunnelling currents at high operating fields (typical to Geiger mode detection) whereas a temperature insensitive avalanche breakdown can circumvent the operational complexity of temperature stabilisation circuitries (typical with linear and Geiger mode InP photodiodes). Al0.85Ga0.15As0.56Sb0.44 lattice matched to InP substrate is one such material system that provides a wider bandgap (1.59 eV at 296 K) and a reduce temperature dependence of avalanche breakdown (1.60 mV/K for a nominally 100 nm thick layer). This thesis reports fabrication and characterisation of avalanche photodiodes based on the thin avalanching layers of a novel material Al0.85Ga0.15As0.56Sb0.44 lattice matched to InP substrate. The primary objective is to understand the breakdown characteristics of Al0.85Ga0.15As0.56Sb0.44 and assess its potential as a replacement for InP Geiger mode APD. The temperature coefficient of avalanche breakdown of p-i-n mesa APD based on nominally 100 nm wide avalanche layer of Al0.85Ga0.15As0.56Sb0.44 (1.60 mV/K) is 1.56x and 3.75x times smaller than APDs based on InAlAs (2.5 mV/K) and InP (6 mV/K) avalanche layers respectively. As compared to wide bandgap InP and InAlAs, the indirect and wide bandgap of Al0.85Ga0.15As0.56Sb0.44 makes it less susceptible to band to band tunnelling currents. APDs based on 100 nm thick Al0.85Ga0.15As0.56Sb0.44 avalanche layer have also demonstrated a promising temporal stability of avalanche gain with a maximum fluctuation of ±1.33% at 353 K. The avalanche gain was to reduce by 15% when the temperature was increased from 294 K to 353 K, compared to 45% and 52% for commercial Si avalanche photodiodes S-5345 and S-6045 respectively. Stable dark current have been observed for Al0.85Ga0.15As0.56Sb0.44 avalanche layers and APDs employing these layers show no significant thermal degradation due to gain measurements at elevated temperatures. These attributes are also beneficial for single photon detection applications Owing to the small device capacitance of the Geiger mode APDs, excellent transient cancellation was achieved which facilitated the detection of weak avalanche signals at low overbias values. The maximum overbias in Geiger mode was limited to 2.5 - 4% due to device design which caused electric field confinement in the avalanche layer. The dark count rate was found to be insensitive to variation in the DC bias levels during the gateOFF time. A stable dark count rate was observed for Al0.85Ga0.15As0.56Sb0.44 Geiger mode APDs without relying on temperature stabilisation. A slight increase in dark count rate of 2.46% was recorded over 550 s which is attributed to variation of threshold level while under similar dark count rate conditions, the dark count rate of a Silicon Geiger mode APD decreased by 30%. Studies on dark count rate as function of pulse repetition frequency showed that the detector dead time should be greater than 700 ns to avoid any increase in the dark count due to possible afterpulsing effects. Al0.85Ga0.15As0.56Sb0.44 Geiger mode APD demonstrated a potential of room temperature photon detection for shorter overbias pulse durations with reduced dark count rate. An exponential time distribution was recorded in the dark count rate where majority of the breakdown events happen within a well-defined time duration and are registered close to the rising edge of the overbias pulse. The impact ionisation coefficients were extracted by fitting the experimental data for avalanche gain and excess noise using recurrence method by adjusting the threshold energies field dependence of impact ionisation coefficients. The ionisation coefficients have been extracted for an electric field in the range of 500-1250 kV/cm. using the set of extracted impact ionisation coefficients, breakdown probability was modelled as a function of overbias for Geiger mode APDs based on 100 nm thick Al0.85Ga0.15As0.56Sb0.44 avalanche layer. Modelling suggests that the recorded DCR increased at a significantly faster rate than predicted breakdown probability characteristics. This suggests either significant onset of tunnelling current, inaccuracies in ionisation parameters or influence of threshold level used in measurements.
Supervisor: Tan, Chee Hing ; Ng, Jo Shien Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.778788  DOI: Not available
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