Breakdown Characteristics of In0.52Al0.48As–InP Heterojunction APDs

The breakdown characteristics of an avalanche photodiode (APD) are generally assessed by examining the steepness of the breakdown-probability curve as a function of the normalized excess breakdown voltage, ∆V /V BR , which is the voltage beyond the breakdown voltage, V BR , normalized by the breakdown voltage. This metric indicates how quickly the transition from stable operation (finite gain) to breakdown (infinite gain) occurs. For photon counting, it is most desirable to have the transition take place with as little excess voltage as possible. It is also desirable to have a low operational breakdown voltage. Unfortunately, homojunction APDs that have good breakdown characteristics are those that are thick (> 400nm) requiring high breakdown voltages [1,2]. Recently, we have demonstrated by means of analytical modeling that the breakdown-probability characteristics of Al 0.6 Ga 0.4 As–GaAs electron-injection heterojunction APDs can be optimized when the APD is operated in Geiger mode [3]. The optimization strategy is based on maximizing the benefits of the dead-space effect through the initial-energy and heterojunction-boundary effects [4,5]. In this paper we extend our results to hole-injection In 0.52 Al 0.48 As–InP heterojunc-tion APDs. In 0.52 Al 0.48 As and InP are lattice-matched with In 0.48 Ga 0.52 As, which is suitable for the wavelengths used in telecommunications (i.e., λ = 1.3 µm and 1.55 µm). In our structure, In 0.52 Al 0.48 As is used as a buildup layer for injected and offspring holes, allowing them to heat up prior to entering the InP layer while denying them ionization. This is due to the wide bandgap of In 0.52 Al 0.48 As, its high ionization threshold energy, and its relatively low ionization coefficients. On the other hand, the energized holes that enter the InP layer require a significantly reduced dead space before impact ionizing, thereby enhancing the breakdown probability [3]. We consider a separate-absorption-multiplication (SAM) APD structure in our calculations, as shown schematically as the inset in Fig. 1. We also consider the fact that the In 0.52 Al 0.48 As-buildup layer is a part of the multiplication layer, implying that carriers in this layer are in principle capable of impact ionization as long as they acquire sufficient energy. We intentionally make the electric field in the In 0.52 Al 0.48 As-buildup-layer smaller (about a half) than in that in the InP-multiplication-layer in order to suppress undesired ionization taking place therein. To characterize the breakdown probability, we …