Thin AlGaAsSb Avalanche Photodiodes with Low Excess Noise

In photon-starved or high-speed applications, such as optical communications or medical imaging, where detection of weak light signals is required, avalanche photodiodes (APDs) are widely used. APDs with thin avalanche regions have shown low excess noise characteristics and high gain-bandwidth products.
In this work, gain and excess noise of thin Al0.85Ga0.15As0.56Sb0.44 (lattice-matched to InP substrates) p-i-n and n-i-p diodes with 100 and 200 nm avalanche regions have been measured for different carrier injection conditions. Very low excess noise values were obtained in p-i-n devices under pure electron injection, with effective ionisation ratios keff = 0.08-1. The AlGaAsSb electron ionisation coefficient was found to be higher than the hole ionisation coefficient�. A significant dead-space effect has been observed in such thin layers, reducing the excess noise. Recurrence equations were used to extract the ionisation coefficients and ionisation threshold energies for the electrons and holes in AlGaAsSb.
In addition, simulations using recurrence equations were carried out to simulate gain and excess noise characteristics in p-i-n diodes with keff = 0.1, 0.01, and 0.001, for different light injection profiles. F(M) characteristics were found to be higher for mixed injection conditions than for pure electron injection. However, for extremely low keff materials, excess noise remains low up to large gain values even for the most severe cases of mixed injection. Randomly-generated ionisation path lengths (RPL) simulations were also carried out to track the carriers initiating impact ionisation, and the sharp increase in F(M) characteristics was attributed to an increase in the number of hole-initiated ionisation events. Signal-to-noise (SNR) and noise-equivalent power (NEP) were calculated for APDs with keff = 0.1, 0.01, and 0.001, highlighting the interest of using avalanche gain to increase the sensitivity of optical detectors.
Finally, a Separate Absorption and Multiplication (SAM) APD combining a 100 nm-thin AlGaAsSb avalanche region and a 1 um InGaAs absorption region has been studied. Low dark currents and good photoresponse at 1550 nm wavelength have been demonstrated. The temperature dependence of gain was investigated for temperatures between 77 K and room temperature, and a temperature coefficient of breakdown voltage, Cbd, of -49 mV/K was obtained.