Studies of Vacancies in Dislocation-Free Ge Crystals

Abstract

Dislocation‐free Ge crystals of 50 g weight have been grown from the melt by the Dash technique. We have diffused Cu into slabs of these crystals for 0.5 to 2 hr at 700–850°C. Acceptors with an ionization energy indistinguishable from that of the lowest substitutional Cu level were observed. Their concentration ranged from ∼0.5×10¹⁵/cm³ for samples taken from the tops of crystals to ∼3×10¹⁵/cm³ for samples from near the bottom. Insufficient time was available for the dissociative diffusion of substitutional Cu into the interior from the surface to account for the observed acceptor concentrations. The data may, however, be interpreted as indicating that vacancies trapped into the crystal during normal growth combine with rapidly diffusing interstitial Cu atoms to make substitutional Cu atoms. According to this interpretation, the data show that the concentration of vacancies trapped into the bottom of a dislocation‐free Ge crystal is larger than at the top, presumably as a consequence of the heat treatment the crystal receives during normal growth. The vacancy concentration at the melting point is 2.9–3.9×10¹⁵/cm³, on this model. Methods of altering the vacancy content of the crystals by further heat treatment are discussed. Some experimental details of Cu diffusion into dislocation‐free Ge are given in an Appendix.