Excess Noise and Avalanche Multiplication in InAlAs

In recent years, InAlAs avalanche photodiodes (APDs) have reported high sensitivities in high bit rate 10 Gb/s [1] and even 40 Gb/s [2] optical communication applications. InAlAs is able to supersede InP as the multiplication layer in SAM-structure APDs due to its superior ionization characteristics that include larger bandgap and very dissimilar electron and hole ionization coefficients (α and β respectively), while remaining lattice matched to InP substrates. Preliminary studies also suggest relatively small temperature dependence of breakdown voltages compared to InP, which reduces the need for temperature stabilization. The significance of a large electron and hole ionization coefficient ratio is lower excess noise and increased sensitivity, without sacrificing the gain-bandwidth product. Furthermore, InAlAs/InGaAs APDs are electron initiated, which further improves the excess noise performance [3] should there be low-field ionization in the InGaAs absorption layer [4]. Accurate characterization of the InAlAs ionization properties such as excess noise and ionization coefficients are essential when designing and optimizing the InAlAs APD performance using simulators and analytical models. It is therefore surprising that the only data on submicron avalanche structures is by Saleh et al. [5] and even this only reports on electron-initiated multiplication. We present a systematic study of avalanche multiplication and excess noise characteristics of InAlAs on a series of p
⁺
-i-n
⁺
and n
⁺
-i-p
^{+ diodes with nominal intrinsic region widths from 0.1 μm to 2.5 μm.}