Shiyu Xie

AlAsSb Avalanche Photodiodes With a Sub-mV/K Temperature Coefficient of Breakdown Voltage

The temperature dependence of dark current and avalanche gain were measured on AlAsSb p-i-n diodes with avalanche region widths of 80 and 230 nm. Measurements at temperatures ranging from 77 to 295 K showed that the dark current decreases rapidly with reducing temperature while avalanche gain exhibits a weak temperature dependence. No measurable band to band tunneling current was observed in the thinner diodes at an electric field of 1.07 MV/cm, corresponding to a bias of 95% of the breakdown voltage. Temperature coefficients of breakdown voltage of 0.95 and 1.47 mV/K were obtained from 80 and 230 nm diodes, respectively. These are significantly lower than a range of semiconductor materials with similar avalanche region widths. Our results demonstrated the potential of using thin AlAsSb avalanche regions to achieve low temperature coefficient of breakdown voltage without suffering from high band to band tunneling current.

Excess Noise Characteristics of Thin AlAsSb APDs

Characterization of AlAsSb avalanche photodiodes (APDs) with avalanche region widths w of 80 and 230 nm showed that electron ionization coefficient is slightly higher than that for hole. The avalanche gain at a given bias is marginally higher under pure electron injection, achieved using 442-nm laser, compared to those measured under mixed carrier injections using 542- and 633-nm lasers. Low-excess-noise factors were measured, corresponding to the lines of k = 0.1, k = 0.15, and k = 0.21, where k is the effective ionization coefficient ratio, for the APDs with w = 80 nm. Under the same carrier injection conditions, the APDs with w = 230 nm exhibit even lower noise corresponding to k = 0.05, k = 0.12, and k = 0.17. The lowest excess noise achieved, with k = 0.05, is significantly lower than those obtained in InP and InAlAs APDs with the same w.

Temperature dependence of impact ionization in InAs

An Analytical Band Monte Carlo model was used to investigate the temperature dependence of impact ionization in InAs. The model produced an excellent agreement with experimental data for both avalanche gain and excess noise factors at all temperatures modeled. The gain exhibits a positive temperature dependence whilst the excess noise shows a very weak negative dependence. These dependencies were investigated by tracking the location of electrons initiating the ionization events, the distribution of ionization energy and the effect of threshold energy. We concluded that at low electric fields, the positive temperature dependence of avalanche gain can be explained by the negative temperature dependence of the ionization threshold energy. At low temperature most electrons initiating ionization events occupy L valleys due to the increased ionization threshold. As the scattering rates in L valleys are higher than those in Γ valley, a broader distribution of ionization energy was produced leading to a higher fluctuation in the ionization chain and hence the marginally higher excess noise at low temperature.

Low Noise Avalanche Photodiodes Incorporating a 40 nm AlAsSb Avalanche Region

A separate absorption and multiplication avalanche photodiode incorporating a 500 nm InGaAs absorption layer, InAlAs bandgap grading/field control layers, an AlAsSb field control layer and a 40 nm AlAsSb multiplication layer was grown and characterized. Responsivity of 436 mA/W was measured at the punch-through voltage. A deviation of Be doping concentration in our AlAsSb field control layer leads higher than intended electric fields in the InGaAs absorption layer and the InAlAs field control layer. Calculation of avalanche gain suggests that the gain in the InGaAs is low and therefore the gain and excess noise characteristics are dominated by impact ionization in the InAlAs field control layer and the AlAsSb multiplication layer. Despite this low excess noise factors corresponding to an effective electron to hole ionization coefficient ratio between 0.1 to 0.15, were measured. This is lower than that from an InAlAs pin diode with a 100 nm avalanche region.

InGaAs/InAlAs single photon avalanche diode for 1550 nm photons

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.

InGaAs/InAlAs Avalanche Photodiode With Low Dark Current for High-Speed Operation

Waveguide InGaAs/InAlAs avalanche photodiodes (APDs) with high bandwidths (>40 GHz) and low dark current (<;50 nA at 90% of breakdown voltage) were demonstrated. The excess noise is low, corresponding to k ~ 0.2 line in the local excess noise model. Using these values bit error rate (BER) was calculated to assess the potential of our APDs. Calculated sensitivities of -21.5 dBm at 25 Gb/s and -14.2 dBm at 40 Gb/s are predicted for a BER of 10-10. Analysis showed that with lower amplifier noise, the low dark current and low excess noise from our APDs are necessary to optimize the sensitivity.

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

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.

High temperature and wavelength dependence of avalanche gain of AlAsSb avalanche photodiodes

The evolution of the dark currents and breakdown at elevated temperatures of up to 450  K are studied using thin AlAsSb avalanche regions. While the dark currents increase rapidly as the temperature is increased, the avalanche gain is shown to only have a weak temperature dependence. Temperature coefficients of breakdown voltage of 0.93 and 1.93  mV/K were obtained from the diodes of 80 and 230  nm avalanche regions (i-regions), respectively. These values are significantly lower than for other available avalanche materials at these temperatures. The wavelength dependence of multiplication characteristics of AlAsSb p-i-n diodes has also been investigated, and it was found that the ionization coefficients for electrons and holes are comparable within the electric field and wavelength ranges measured.

Temperature dependence of avalanche gain in Al 0.85 Ga 0.15 As 0.56 Sb 0.44 APD

We investigated the temperature dependence of dark current, avalanche gain and breakdown voltage in Al0.85Ga0.15As0.56Sb0.44 (hereafter AlGaAsSb) avalanche photodiodes (APDs) with avalanche region widths, w = 90 and 178 nm. There is negligible band to band tunnelling currents and a very weak temperature dependence of gain observed in both didoes. The temperature coefficient of breakdown voltage, Cbd = 0.41 mV/K in AlGaAsSb diode with w = 90 nm is the lowest value reported for different semiconductor materials with similar avalanche region width.

InGaAs/AlGaAsSb APD with over 200 GHz gain-bandwidth product

We investigate a novel In_0.53Ga_0.47As/Al_0.85Ga_0.15As_0.56Sb_0.44 (hereafter InGaAs/AlGaAsSb) avalanche photodiode (APD) for high speed operation. The fabricated APD with 20 μm diameter exhibits a maximum responsivity of 5.33 A/W at 1550 nm, a maximum 3-dB bandwidth (f_3dB) of 14 GHz, and a clear eye diagram at 10 Gb/s. A GBP value as high as 407.4 GHz was extracted. The results demonstrate the potential of using InGaAs/AlGaAsSb APDs for telecom/data communication system operating at or above 10 Gb/s.