Investigation of the Effects of the Multiplication Area Shape on the Operational Parameters of InGaAs/InAlAs SPADs

A 2D model of an InGaAs/InAlAs single photon avalanche photodiode has been developed. The influence of the active area structure in the multiplication region on the diode's operating parameters has been studied. It was found that changing the diameter of the structure's active region leads to a change in the dark current in the linear part of the current-voltage curve and a change in the breakdown voltage. Reducing the diameter of the active region from 25 $\mu$m to 10 $\mu$m allowed decreasing the dark current in the linear mode by about $10$ dB. It has been shown that the quality of the SPAD device can be assessed by knowing the avalanche breakdown voltage and the overall current-voltage curve plot if we consider structures with the same multiplication region thickness and different remaining layers. The higher the breakdown voltage, the better the structure's quality due to smaller local increases in the field strength. Following this statement, we conclude that for further use in single-photon detectors, it is reasonable to pick specific SPADs from a batch on the sole basis of their current-voltage curves

Investigation of temperature and temporal stability of AlGaAsSb avalanche photodiodes

Since avalanche gain and breakdown voltage in most semiconductor materials change with temperature, instruments utilizing Avalanche Photodiodes (APDs) for their avalanche gains need to incorporate either temperature stabilization or voltage adjustment in the APD operation circuits. In this work we evaluated the temperature and temporal stability of avalanche gain in Al 0.85 Ga 0.15 As 0.56 Sb 0.44 , a wide bandgap semiconductor lattice-matched to InP substrates. We investigated the temperature and temporal stability of the gain and breakdown voltage at temperatures of 24 °C (room temperature) to 80 °C. The breakdown voltage varies linearly with temperature with a temperature coefficient of 1.60 mV/K. The avalanche gain reduces from 10 to 8.5, a reduction of 15%, when the temperature increases from 24 to 80°C. The temporal stability of gain was recorded when the APD was biased to achieve an avalanche gain of 10. Fluctuations are within ± 0.7% at 24°C, increasing to ± 1.33% at 80°C. The temperature and temporal stability of avalanche gain indicates the potential of using Al 0.85 Ga 0.15 As 0.56 Sb 0.44 APDs grown on InP substrates to achieve high tolerance to temperature fluctuation

Low-Photon Detection Using InGaAs/InAlAs and InAs Avalanche Photodiodes

Single-photon avalanche photodiodes (SPADs) based on the InGaAs/InAlAs material system are designed, fabricated, and characterised for 1550 nm light detection. The two designed wafers reduce the electric field across the InGaAs absorber to a minimum in order to minimise dark current. The first wafer is designed to punch-through at the point of high breakdown probability (above breakdown voltage), while the second is designed to punch-through just under breakdown voltage. The first wafer is found to be unsuitable for single-photon counting due to an uncharacteristically fast rise in dark count rate, likely caused by the onset of punch-through during breakdown. Low photon levels are detected using diodes fabricated from the second wafer, however the diodes were found to not fully punch-through, preventing single-photon counting. Peak laser pulse detection probabilities at 150 K were 73, 71, and 46 % for 100, 30, and 10 photons, respectively. At room temperature, pulse detection probabilities were 39, 35, and 30% for the respective photon levels. This informs future SPAD designs; crucially that full punch-through must occur before breakdown voltage. A simulation model for the sensitivity of electron APDs (e-APDs) is developed and applied to InAs e-APD based optical receivers. The model simulates bit-error rate (BER), and captures the effects of inter-symbol interference (ISI), dark current, current impulse duration, avalanche gain, and amplifier noise. With a target BER of 10 −12 , the receivers’ sensitivities were -30.6, -22.7, and -19.2 dBm for 10, 25, and 40 Gb/s data rates. The simulated InAs APDs offer improvements over existing InAlAs APDs at 10 and 25 Gb/s, however SOA-PIN based receivers outperform both types of APD for 40 Gb/s for 1550 nm operation. Utilising the newly developed e-APD sensitivity model, and a previously developed sensitivity model for standard APDs, simulations are performed comparing InAs, InAlAs, and InP based optical receivers. The simulations utilise a common parameter set where appropriate, allowing for a direct comparison between the three avalanche materials for high-speed operation. Simulated InAs APDs achieved the best performance for 10 Gb/s operation (-29.4 dBm for InAlAs), while the simulated InAlAs APDs were found to perform better at 25 and 40 Gb/s, achieving a sensitivity of -23.5 and -21.0 dBm, respectively. InP APDs showed sensitivities of -27.9, -22.5, and -19.9 dBm, for 10, 25, and 40 Gb/s operation, respectively. These simulations demonstrate significant performance benefits to replacing InP with InAlAs as an APD avalanche region. Additional simulations of InAs APDs were performed, exploring how to further optimise InAs based optical receivers

Impact ionization in AlAs0.56Sb0.44 photodiodes

The aim of this work is to characterize the impact ionization characteristics of AlAs0.56Sb0.44 towards its use as the avalanche medium in separate absorption and multiplication avalanche photodiodes (SAM-APDs) based on InP substrate for optical communication systems. The previous studies of the AlAs0.56Sb0.44 material were only undertaken on very thin p-i-n structures where we cannot accurately estimate the impact ionization coefficient and excess noise behavior due to “dead-space” effects. In this work, much thicker AlAs0.56Sb0.44 homojunction diodes were investigated systematically. The absorption coefficient was fitted by 1-D quantum efficiency model. Comprehensive multiplication and excess noise measurements based on AlAs0.56Sb0.44 homojunction diodes over a wide range of thickness were performed at room temperature. The bulk electron and hole ionization coefficients, α and β respectively, were found to be very disparate and ‘silicon like’ at low electric fields and α > β over the whole electric field range. The ionization coefficients were determined from 220-1250 kV/cm for α and from 360-1250 kV/cm for β. The β was found to rapidly drop at the low electrical field, but the α was similar to that of InP and InAlAs. Noise measurements carried out on the thickest p-i-n structure exhibits the best reported excess noise based on InP substrate, k = 0.005 at room temperature. The SAM-APDs using AlAsSb show potentially better performance than those using InP/InAlAs as the multiplication layer

Avalanche noise characteristics of single Al/sub x/Ga/sub 1-x/As(0.3

Avalanche multiplication and excess noise have been measured on a series of Al/sub x/Ga/sub 1-x/As-GaAs and GaAs-Al/sub x/Ga/sub 1-x/As (x=0.3,0.45, and 0.6) single heterojunction p/sup +/-i-n/sup +/ diodes. In some devices excess noise is lower than in equivalent homojunction devices with avalanche regions composed of either of the constituent materials, the heterojunction with x=0.3 showing the greatest improvement. Excess noise deteriorates with higher values of x because of the associated increase in hole ionization in the Al/sub x/Ga/sub 1-x/As layer. It also depends critically upon the carrier injection conditions and Monte Carlo simulations show that this dependence results from the variation in the degree of noisy feedback processes on the position of the injected carriers

A theoretical comparison of the breakdown behavior of In0.52Al0.48As and InP near-infrared single-photon avalanche photodiodes

We study the breakdown characteristics and timing statistics of InP and In0.52Al0.48As single-photon avalanche photodiodes (SPADs) with avalanche widths ranging from 0.2 to 1.0 mu m at room temperature using a random ionization path-length model. Our results show that, for a given avalanche width, the breakdown probability of In0.52Al0.48As SPADs increases faster with overbias than InP SPADs. When we compared their timing statistics, we observed that, for a given breakdown probability, InP requires a shorter time to reach breakdown and exhibits a smaller timing jitter than In0.52Al0.48As. However, due to the lower dark count probability and faster rise in breakdown probability with overbias, In0.52Al0.48As SPADs with avalanche widths <= 0.5 mu m are more suitable for single-photon detection at telecommunication wavelengths than InP SPADs. Moreover, we predict that, in InP SPADs with avalanche widths <= 0.3 mu m and In0.52Al0.48As SPADs with avalanche widths <= 0.2 mu m, the dark count probability is higher than the photon count probability for all applied biases

Impact Ionization In AlGaAsSb Avalanche Photodiodes

This work aims to demonstrate a separate absorber, charge, multiplication (SACM) avalanche photodiode (APD) with GaAsSb/AlGaAsSb grown on InP. AlAsSb shows very dissimilar ionization coefficients between electrons(α) and holes(β) and extremely low excess noise. The temperature dependence breakdown coefficient (Cbd) in AlAsSb was found to be very, small 8.5mV/K in a 1μm p-i-n diode, and the electron and hole impact ionization coefficients increase at about the same rate as the temperature decreases, significantly less so than in InP and InAlAs. However, this material suffers from oxidization and surface leakage current. This is significantly improved by employing the AlGaAsSb quaternary alloy system, enabling low dark current while maintaining low excess noise and a large α/β ratio. The extraction of ionization coefficients from avalanche multiplication measurements has clarified this material's characteristics and optimized the avalanche region thickness in SACM APD design. It is the first report of a room temperature, ultra-high gain (M=278, λ=1550 nm, V=69.5 V, T=296 K) linear mode avalanche photodiode, grown on an InP substrate using a GaAs0.5Sb0.5/Al0.85Ga0.15As0.56Sb0.44 separate absorption charge and multiplication (SACM) heterostructure. This design employs a novel GaAsSb absorber that is graded to wider bandgap charge and multiplication layers with several AlxGa1-xAsSb grading layers. This represents a ~10× gain improvement over commercial, state-of-the-art InGaAs/InP-based APDs (M ∼30) operating at 1550 nm. The excess noise factor is extremely low (F<3) at M=70 and this design gives a quantum efficiency of 5935.3% at maximum gain. A 200 µm diameter device gives a capacitance limited 3 dB bandwidth of 0.7 GHz (M=25, V=65 V). Furthermore, this SACM APD shows an extremely low-temperature-dependent breakdown coefficient (Cbd) of ~11.83 mV/K, which is ~10× lower than equivalent InGaAs/InP commercial APDs. This demonstration opens a pathway to realize high sensitivity receiver systems at eye-safe, infrared wavelengths (1400 - 1650 nm) for a variety of applications

Infrared time-correlated single-photon counting

This Thesis investigates near infrared ( ~ 1550 nm) time-correlated singlephoton counting, studying the single-photon detectors and some of the potential application areas. Custom designed and fabricated InGaAs/InP single-photon avalanche diode detectors were characterised. Our devices yielded single-photon detection efficiencies of ~10 %, timing jitter of 200 ps, and noise equivalent power comparable to the best commercially available avalanche photodiodes operated in Geiger-mode. The afterpulsing phenomenon which limits the maximum count rate of InGaAs/InP single-photon avalanche diodes has been investigated in detail and activation energies calculated for the traps that cause this problem. This was found to be ~250 meV for all the devices tested, despite their differing structures and growth conditions, and points to the InP multiplication region as the likely location of the traps. Ways of reducing the effects caused by the afterpulsing phenomenon were investigated and sub-Geiger mode operation was studied in detail. This approach enabled freerunning, afterpulsing-free operation at room temperature of an InGaAs/InP singlephoton avalanche diode detector for the first time. Finally, time-of-flight photon counting laser ranging was performed using both singlephoton avalanche diodes and superconducting nanowire single-photon detectors. The use of the latter resulted in a surface to surface depth resolution of 4 mm being achieved at low average laser power at an eye-safe wavelength of 1550 nm