Abstract

We report the observation of a new avalanche phenomenon in superlattices. This effect is characterized by a large ratio of the multiplications for holes and electrons implying β ≫ α and is very different in nature from the previously studied band-edge discontinuity assisted ionization in quantum wells. The i region of our p⁺in⁺ structures, grown by MBE, consists of an undoped (n ≃10¹⁵ cm⁻³) Al_0.48ln_0.52As/Ga_0.47ln_0.53As superlattice. Six wafers with well and barrier thicknesses ranging from 100 to 470 Å and a number of periods varying from 20 to 50 were studied. In addition similar MBE structures with an AISb/GaSb superlattice were also grown. The electron and hole initiated gains (M_e and M_h) were determined using standard techniques. It was found that M_h ≫ M_e. Ionization ratios β/α in excess of 10² have been deduced which are the highest observed in any III–V material. All the experimental data can be explained in terms of a new mechanism, impact ionization across the band-edge discontinuity. There are concentrations of electrons and holes dynamically stored in the conduction and valence band wells which can easily approach 10¹⁶ cm⁻³ in our structures, at fields ≲ 10⁵ V/cm. Consider now a carrier (e.g., a hole) photoinjected into the structure; after gaining energy in the barrier layers it can impact-ionize one of the holes dynamically stored in the low gap layers out of the well across the valence band discontinuity. Electrons may of course impact-ionize electrons out of the wells but since ∆E_C(≃0.5 eV) is more than twice ∆E_v, the electron ionization rate is much smaller than the hole ionization rate which explains why the holes are multiplied much more than the electrons. New solid-state photomultipliers based on this phenomenon are discussed.

© 1986 Optical Society of America