Lifetime and leakage current considerations in metal-doped germanium

Abstract

This paper discusses the impact of transition metal (TM) impurities in germanium on the electrical material and device characteristics, i.e., the recombination and generation lifetime and the leakage current in a p–n junction. As will be shown, most relevant data have been obtained in the fifties and sixties from Hall effect and lifetime measurements, while there is a lack of accurate Deep Level Transient Spectroscopy (DLTS) results on metal-doped Ge. It will be shown that the most efficient lifetime killers are Fe, Ni and most likely Co, while Cu, which is the most notorious contaminant, has little impact on the minority carrier lifetime. Owing to the asymmetry in the capture cross sections, n-type material will generally have a lower lifetime than p-Ge for the same concentration of substitutional metal centers. Due to the presence of near mid-gap acceptor levels, most TMs will be efficient leakage current generators at room temperature. However, the expected maximum values for the leakage current density are one to two decades smaller than what is typically found in state-of-the-art ion-implanted shallow p–n junctions in Ge.