Electrical characterization of II–VI compounds and devices

Abstract

The recent progress in making blue-green semiconductor double-heterostructure (DH) injection lasers has come about directly from advances in p-type doping of ZnSe and related compounds, initially with Li, and more recently with N. This paper focuses on techniques we have used for characterizing these p-type II–VI epilayers. The main electrical characterization technique employed is a small-signal AC admittance measurement as a function of both voltage and frequency. We use this to extract the net doping ( N A − N D) and also to determine the epilayer conductivity as a function of temperature (σ( T)). We use this and the general physical form of the mobility vs temperature to extract information about both the mobility and the compensation for our specific samples. The technique is shown to work well for p-type ZnSe : Li. We obtain the result that all such samples are substantially uncompensated, regardless of the ratio ( N A − N D)/[Li]. From this we conclude that excess Li does not cause compensation, but rather passivation by the formation of Li substitutional- interstitial complexes. We measure the dissociation energy of the complex, which is about 0.7 eV, in good agreement with theory. For ZnSe : Li samples with few complexes, the maximum room-temperature hole mobility was about 36 cm 2/V s and the peak mobility was about 155 cm 2/V s at about 125 K; for samples with a high concentration of complexes, the transport was degraded. In the case of p-type ZnSe : N, transport results were similar to ZnSe : Li with a high concentration of complexes. This may be due to either complexes or other point defects in the epilayers and offers an explanation of the high turn-on voltage of DH lasers.