Wenlu Sun

Enhanced low-noise gain from InAs avalanche photodiodes with reduced dark current and background doping

We reduced the room temperature dark current in an InAs avalanche photodiode by increasing the p-type contact doping, resulting in an increased energetic barrier to minority electron injection into the p-region, which is a significant source of dark current at room temperature. In addition, by improving the molecular beam epitaxy growth conditions, we reduced the background doping concentration and realized depletion widths as wide as 5 μm at reverse biases as low as 1.5 V. These improvements culminated in low-noise InAs avalanche photodiodes exhibiting a room temperature multiplication gain of ∼80, at a record low reverse bias of 12 V.

High-Gain InAs Avalanche Photodiodes

We report two InAs avalanche photodiode structures with very low background doping in the depletion region. Uniform electric fields and thick depletion regions have been achieved. Excess noise measurements are consistent with k~0 and gain as high as 70 at room temperature is observed. The measured gain-bandwidth product is >; 300 GHz. All measurements are consistent with Monte Carlo simulations.

Common-Mode Cancellation in Sinusoidal Gating With Balanced InGaAs/InP Single Photon Avalanche Diodes

We demonstrate a sinusoidal-gated InGaAs/InP single photon avalanche diode (SPAD) pair with high photon detection efficiency (PDE) and low dark count rate (DCR). The photodiode pair is biased in a balanced configuration with only one of the SPADs illuminated. The advantage of balanced detectors is cancellation of the common component of the output signal, which in this case arises from sinusoidal gating. In conventional sinusoidal gating, narrow-band RF filters are used to eliminate the gating signal while imparting minimal change to the avalanche pulses. A disadvantage of this approach is that the requisite filters fix the operating frequency, whereas the balanced SPAD receiver is frequency agile. At a laser repletion rate of 1 MHz and a temperature of 240 K, the DCR and PDE are 58 kHz and 43%, respectively. The afterpulse probability is lower than a single sinusoidal-gated SPAD. Jitter of 240 ps is achieved with one photon per pulse and an excess bias of 1.6%.

Monte Carlo Simulation of InAlAs/InAlGaAs Tandem Avalanche Photodiodes

Monte Carlo simulation is performed on a low-noise, three-stage tandem avalanche photodiode with InAlAs/InAlGaAs impact-ionization-engineered multiplication region. The simulated excess noise factor agrees well with experimental measurements. A modified structure to further reduce the excess noise is proposed.

Study of bandwidth enhancement and non-linear behavior in avalanche photodiodes under high power condition

A self-consistent Monte Carlo simulation tool based on physics-level description of carrier transport is developed to study the frequency responses of avalanche photodiodes working under high optical power conditions. Simulation is performed on an InGaAs/InAlAs SACM avalanche photodiode. Significantly enhanced bandwidth is observed at high photocurrent, which results in high gain-bandwidth product. The associated nonlinearity is characterized by measuring the mixing conversion efficiency.

Improved sinusoidal gating with balanced InGaAs/InP Single Photon Avalanche Diodes

We report balanced InGaAs/InP single photon avalanche diodes (SPADs) operated in sinusoidal gating mode with a tunable phase shifter to reduce common mode noise. This technique enables detection of small avalanche pulses, which results in reduced afterpulsing. For laser repletion rate of 20 MHz at 240 K, the dark count rate for photon detection efficiency of 10% is 8.9 kHz.

Monte Carlo Simulation of

In this paper, a Monte Carlo model is used to investigate the impact ionization properties of Al_xGa_1-xAs with a high composition of Al (x ≥ 0.6). Two Geiger-mode avalanche photodiode (APD) structures using Al_xGa_1-xAs as the multiplication region are studied. Simulations show a strong link between the APD properties and the electric field profile. It was also found that Al_9.6Ga_0.4As APD structure has higher excess noise and more abrupt breakdown probability compared to Al_0.8Ga_0.2As.

{\rm Al}_{x}{\rm Ga}_{1-{x}}{\rm As}~(x\geq 0.6)

Avalanche photodiodes (APD) are important components in short-wave and mid-wave infrared detection systems (imaging, laser radar, communications, etc.) because their internal gain can improve receiver sensitivity and enables the detection of weak photon fluxes. The statistical nature of impact ionization in APDs contributes to excess shot noise, however. The excess noise factor, F(M), is related to the ratio of the electron and hole ionization coefficients, k, and multiplication, M, by F(M) = <;M<;sup>2<;/sup>>/<;M><;sup>2<;/sup> = k<;M> + (1-k)(2-1/<;M>). In the mid-infrared, HgCdTe APDs represent the current state-of-the-art; at liquid nitrogen temperatures, advanced devices offer excellent low noise characteristics with F(M) ~ 1, gains of >1000, and excellent dark currents. Unfortunately, devices that operate at shorter wavelengths exhibit degraded noise characteristics and significantly lower maximum gains <;30. This, when combined with the growth and fabrication challenges associated with II-VI compounds has motivated the search for alternative APD materials. InAs APDs, which offer a shorter wavelength cutoff wavelength of ~3 μm, have recently been found to exhibit F(M) ~ 1, with moderately low dark current at room temperature. Increasing the multiplication region thickness increases the gains achievable at low bias, which is beneficial for integration with Read Out-Integrated Circuits (ROICs). Therefore, to design a high-gain InAs APD, a thick multiplication region is required. This, in turn, necessitates extremely low background doping (<;1015 cm-3), or appropriate counter doping, in order to realize complete depletion and a uniform electric field profile. In this paper, we report record high-gain InAs APDs employing 6 μm-thick and 10 μm-thick intrinsic regions with low background doping of ~4×1014 cm-3, as determined by C-V profiling. An AlAsSb blocking layer was used in both structures to suppress electron diffusion current, the dominant dark current mechanism at room temperature. Significant reduction in dark current were critical to characterize these thicker multiplication regions at high gain, resulting in record high room temperature multiplication gain of ~300 at 15 V bias. This is ~6× the previous record for InAs of 50 at 15 V, as well as significantly higher than the maximum gain of 126 reported at higher biases, and represents a significant step forward in InAs APD performance.

Avalanche Photodiodes

We report sinusoidal gating of InGaAs/InP single photon avalanche diodes (SPAD) operated at wavelength of 1310 nm with high photon detection efficiency (PDE) and low dark count rate (DCR). At a gating frequency of 80 MHz and temperature of 240 K the DCR and PDE were 15.5 kHz and 55%, respectively. The slope of DCR versus PDE increases with higher laser repetition rate. There are two mechanisms that contribute to this trend. The first is due to the lower afterpulse probability associated with a lower laser repetition rate. The other is due to the RC effect, which is illustrated by an equivalent circuit that includes a model of the SPAD. We also show that relative to gated passive quenching with active reset (PQAR) for fixed PDE, sinusoidal gating yields lower afterpulsing rates for the same hold-off time. This is explained in terms of the integrated pulse shape and the resultant charge flow. The afterpulse probability, Pa, is related to the hold off time, T, through the power law, Pa∝T-α where α is a measure of the detrapping time in the multiplication region.

Record high gain from InAs avalanche photodiodes at room temperature

Natural lithography with 100-nm-diameter SiO(2) spheres followed by inductively coupled plasma etching was used to texture the surface of 4H-SiC for a wide-spectrum large-acceptance-angle anti-reflective layer. The surface showed low normal-incidence reflectance of < 5% over a wide spectrum from 250 nm to 550 nm. Photodiodes fabricated from the surface-textured SiC showed broader spectral and angular responsivity than SiC photodiodes with SiO(2) antireflective coating. The textured SiC photodiodes showed peak responsivity of 116 mA/W, large angle of acceptance angle (< 2% decrease in responsivity at 50° incident angle) and low dark current at 10V.