Physics of Strain Engineered Minigaps in III-V Digital Alloys

III-V short period superlattices called "digital alloys" are being grown using state-of-the-art growth techniques. These alloys consist of binary compounds stacked alternately in a periodic manner. Some of these digital alloys have small gaps called 'minigaps" in their energy bandstructure. These minigaps along with enhanced effective mass can be used to inhibit carrier multiplication of one carrier type in avalanche photodiodes (APDs). Thus, APDs with unicarrier multiplication can be designed that have low excess noise. This enhances the detector performance. It is possible to modulate the size of these minigaps by varying the biaxial strain in these superlattice structures. The strain alters the orbital overlaps which in turn can be used to vary the minigap sizes and their position relative to the band edges. We employ an Environment-Dependent Tight Binding model that is calibrated to Density Functional Theory bandstructure and wavefunctions to compute the strain-dependent electronic bandstructure. In this talk, we discuss the physics of these minigaps and their correlation with strain. Finally, using Monte-Carlo simulation results we look at the relationship between the minigap sizes and the excess noise performance of APDs.