Xinxin Zhou

A simple Monte Carlo model for prediction of avalanche multiplication process in Silicon

A Simple Monte Carlo model has been developed to model the avalanche characteristics of Silicon. Good agreement with experimental results from Silicon p+-i-n+ diodes with i-regions ranging from 0.082 to 0.26 ?m, an n+-i-p+ diode with an i-region of 0.82 ?m and a p+n diode was obtained. Therefore our model can be used to model the avalanche process in diodes with varying electric field profiles. We also studied the competing effects of the ratio of electron to hole ionization coefficients and the dead space on excess noise factor, by varying these parameters in our simulations of ideal p+-i-n+ diodes with avalanche regions width of 0.05 to 0.3 ?m to cover the electric field range in the measured devices. As avalanche region width reduces from 0.3 to 0.05 ?m, the electron to hole ionisation coefficient ratio decreases from 3.42 to 1.23 while the dead space to avalanche width ratio increases from 0.19 to 0.49 for electrons. The former increases the excess noise while the latter suppresses the avalanche noise such that on balance, a weak dependence of excess noise on the avalanche width for w < 0.3??m was observed in these p+-i-n+ diodes, consistent with the excess noise results reported in thin Silicon p+-i-n+ diodes.

An InGaAlAs–InGaAs Two-Color Photodetector for Ratio Thermometry

We report the evaluation of a molecular-beam epitaxy grown two-color photodetector for radiation thermometry. This two-color photodetector consists of two p⁺in⁺ diodes, an In_0.53Ga_0.25Al_0.22As (hereafter InGaAlAs) p⁺in⁺ diode, which has a cutoff wavelength of 1180 nm, and an In_0.53Ga_0.47As (hereafter InGaAs) p⁺in⁺ diode with a cutoff wavelength of 1700 nm. Our simple monolithic integrated two-color photodetector achieved comparable output signal and signal-to-noise (SNR) ratio to that of a commercial two-color Si-InGaAs photodetector. The InGaAlAs and InGaAs diodes detect blackbody temperature as low as 275°C and 125°C, respectively, with an SNR above 10. The temperature errors extracted from our data are 4°C at 275°C for the InGaAlAs diode and 2.3°C at 125°C for the InGaAs diode. As a ratio thermometer, our two-color photodetector achieves a temperature error of 12.8°C at 275°C, but this improves with temperature to 0.1°C at 450°C. These results demonstrated the potential of InGaAlAs-InGaAs two-color photodetector for the development of high performance two-color array detectors for radiation thermometry and thermal imaging of hot objects.

InAs Photodiodes for 3.43

We report an evaluation of an epitaxially grown uncooled InAs photodiode for the use in radiation thermometry. Radiation thermometry measurements was taken using the photodiode covered blackbody temperatures of 50 °C-300 °C. By determining the photocurrent and signal-to-noise ratio, the temperature error of the measurements was deduced. It was found that an uncooled InAs photodiode, with the peak and cutoff wavelengths of 3.35 and 3.55 μm, respectively, measured a temperature of 50 °C, with an error of 0.17 °C. Many plastics have C -H molecular bond absorptions at 3.43 μm and hence radiate thermally at this wavelength. Our results suggest that InAs photodiodes are well suited to measure the temperature of plastics above 50 °C. When tested with a narrow bandpass filter at 3.43 μm and blackbody temperatures from 50 °C-300 °C, the InAs photodiode was also found to produce a higher output photocurrent, compared with a commercial PbSe detectors.

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A single photon avalanche diode (SPAD) with an InGaAs absorption region, and an InAlAs avalanche region was designed and demonstrated to detect 1550 nm wavelength photons. The characterization included leakage current, dark count rate and single photon detection efficiency as functions of temperature from 210 to 294 K. The SPAD exhibited good temperature stability, with breakdown voltage dependence of approximately 45 mV K−1. Operating at 210 K and in a gated mode, the SPAD achieved a photon detection probability of 26% at 1550 nm with a dark count rate of 1 × 108 Hz. The time response of the SPAD showed decreasing timing jitter (full width at half maximum) with increasing overbias voltage, with 70 ps being the smallest timing jitter measured.

Radiation Thermometry

We report the evaluation of an InAs/GaSb type-II superlattice (T2SL) as a potential uncooled or thermo-electrically cooled midwave infrared photodiode for use in radiation thermometry. A T2SL structure was grown and analyzed. Temperature-dependent radiation thermometry measurements were performed at reference blackbody temperatures of 25 °C-100 °C, in terms of photocurrent, signal-to-noise ratio (SNR), and temperature error. Despite using an unoptimized T2SL diode, we found that an SNR >1 was obtained using an uncooled diode, even at a target temperature of 25 °C. Cooling increased the photocurrent and reduced the dark current, leading to increased SNR across the blackbody temperature range measured. For instance at a target temperature of 50 °C, cooling the diode to 77 K increases the SNR by a factor of 614, while a more modest SNR enhancement factor of 19 was obtained when the diode was cooled to 200 K. At the thermo-electric cooler compatible temperature of 200 K, a target temperature of 50 °C with a temperature error of ±1 °C could be successfully measured, demonstrating the potential of T2SL.

InGaAs/InAlAs single photon avalanche diode for 1550 nm photons

When using avalanche photodiodes (APDs) in applications, temperature dependence of avalanche breakdown voltage is one of the performance parameters to be considered. Hence, novel materials developed for APDs require dedicated experimental studies. We have carried out such a study on thin Al1–xGaxAs0.56Sb0.44 p–i–n diode wafers (Ga composition from 0 to 0.15), plus measurements of avalanche gain and dark current. Based on data obtained from 77 to 297 K, the alloys Al1−xGaxAs0.56Sb0.44 exhibited weak temperature dependence of avalanche gain and breakdown voltage, with temperature coefficient approximately 0.86–1.08 mV K−1, among the lowest values reported for a number of semiconductor materials. Considering no significant tunnelling current was observed at room temperature at typical operating conditions, the alloys Al1−xGaxAs0.56Sb0.44 (Ga from 0 to 0.15) are suitable for InP substrates-based APDs that require excellent temperature stability without high tunnelling current.

InAs/GaSb Type-II Superlattice for Radiation Thermometry

We report the fabrication of InAs planar avalanche photodiodes (APDs) using Be ion implantation. The planar APDs have a low background doping of 2 × 1014 cm-3 and large depletion widths approaching 8 μm. The thick depletion width enabled a gain of 330 to be achieved at -26 V at 200 K without inducing a significant tunneling current. No edge breakdown was observed within the APDs. The surface leakage current was found to be low with a gain normalized dark current density of 400 μAcm-2 at -20 V at 200 K.

Thin Al 1− x Ga x As 0.56 Sb 0.44 diodes with extremely weak temperature dependence of avalanche breakdown

We designed and demonstrated an InAs avalanche photodiode (APD) for X-ray detection, combining narrow band gap semiconductor materials and avalanche gain from APDs. The InAs APD (cooled by liquid nitrogen) was tested with a 55Fe X-ray source. Full width at half maximum (FWHM) from the spectra decreases rapidly with reverse bias, rising again for higher voltages, resulting in a minimum FWHM value of 401 eV at 5.9 keV. This minimum value was achieved at 10 V reverse bias, which corresponds to an avalanche gain of 11. The dependence of FWHM on reverse bias observed is explained by the competition between various factors, such as leakage current, capacitance and avalanche gain from the APD, as well as measurement system noise. The minimum FWHM achieved is largely dominated by the measurement system noise and APD leakage current.

High-Gain InAs Planar Avalanche Photodiodes

Silicon-based single photon avalanche diodes (SPADs) are widely used as single photon detectors of visible and near infrared photons. There has, however, been a lack of models accurately interpreting the physics of impact ionization (the mechanism behind avalanche breakdown) for these devices. In this paper, we present a statistical simulation model for silicon SPADs that is capable of predicting breakdown probability, mean time to breakdown, and timing jitter. Our model inherently incorporates carriers’ dead space due to phonon scattering and allows for nonuniform electric fields. Model validation included avalanche gain, excess noise factor, breakdown voltage, breakdown probability, and timing statistics. Simulating an n-on-p and a p-on-n SPAD design using our model, we found that the n-on-p design offers significantly improved mean time to breakdown and timing jitter characteristics. For a breakdown probability of 0.5, mean time to breakdown and timing jitter from the n-on-p design were 3 and 4 times smaller compared to those from the p-on-n design. The data reported in this paper are available from the ORDA digital repository (DOI: 10.15131/shef.data.4823248).

InAs avalanche photodiodes as X-ray detectors

The electron and hole avalanche multiplication characteristics have been measured in bulk AlAs0.56Sb0.44 p-i-n and n-i-p homojunction diodes, lattice matched to InP, with nominal avalanche region thicknesses of ~0.6 μm, 1.0 μm and 1.5 μm. From these and data from two much thinner devices, the bulk electron and hole impact ionization coefficients (α and β respectively), have been determined over an electric-field range from 220–1250 kV/cm for α and from 360–1250 kV/cm for β for the first time. The α/β ratio is found to vary from 1000 to 2 over this field range, making it the first report of a wide band-gap III-V semiconductor with ionization coefficient ratios similar to or larger than that observed in silicon.