Deep level impurities in germanium

Abstract

This paper will review the properties of deep level impurities in germanium. With the exception of the well known hydrogen-like impurities, all of the electrically active impurities which have been studied in detail act as multiple acceptors or donors introducing deep impurity levels (>/·03 eV). The number and type of levels are consistent with the tetrahedral bonding model. As examples, Zn (with a 4 s 2 outer orbit configuration) acts as a double acceptor in Ge ( 4 s 2 4 p 2); Cu (4 s 1) acts as a triple acceptor and Te ( 5 s 2 5 p 4) acts as a double donor. However, no theoretical treatment has proven adequate for calculation of the positions of energy levels for the deep level impurities. Hall coefficient studies have determined the positions of deep acceptor levels introduced into Ge by the elements Zn, Cd, Mn, Fe, Co, Ni, Cu, Ag and Au. Deep donor levels are introduced by S, Se and Te, For most of these impurities the positions of the energy levels have been confirmed by studies of extrinsic photoconductivity or optical absorption. Values of diffusivity and the temperature dependence of solubility are also known for many of these impurities. The interpretation of both steady state and transient photoconductivity studies has demonstrated the importance of the charge state at deep level impurities in determining recombination processes. At low temperatures, multiply charged negative centers are effective hole traps; multiply charged positive centers act as electron traps. Some quantitative values of capture cross sections have been obtained for different charge states of multiple acceptor impurities. Reference will be made to other electrical, optical and magnetic experiments made possible because of the present knowledge of deep level impurities in Ge and the control now possible over crystal preparation.