Properties of vacancy-hydrogen defects in group-IV semiconductors

B. Bech Nielsen

Department of Physics and Astronomy, University of Aarhus

In crystalline silicon and germanium, a vacancy defect V_m consisting of m vacancies may interact with n hydrogen atoms and form V_mH_n complexes, in which each hydrogen atom is bound to one of the host atoms neighbouring the vacancy defect by a strong covalent bond. Infrared absorption spectroscopy has previously been applied to identify the local vibrational modes of several of these complexes, and information about their electronic properties has been obtained from electron paramagnetic resonance (EPR) and deep level transient spectroscopy. Based on these investigations, it appears that the electronic states within the band gap essentially represent combinations of the "free" dangling-bond orbitals residing on the neighbouring host atoms that do not bind a hydrogen atom. Hence, a covalent bond between a hydrogen atom and a host atom appears to be electronically "inert". In this talk, I will review some of our previous work and discuss the vibrational and electronical properties of V_mH_n complexes in simple terms. As an example, our findings for the VH₃ defect in silicon will be described in some detail. At low measuring temperatures this defect gives rise to an EPR signal displaying trigonal symmetry and hyperfine interactions from three equivalent hydrogen atoms. As the measuring temperature is increased the signal broadens and disappears and a new signal displaying cubic symmetry appears. From correlated EPR and infrared absorption studies two local modes at 2156 and 2186 cm^-1 are identified as the E and A1 Si-H stretch modes of VH₃, respectively. Uniaxial stress applied along the [110] or [111] direction during cooling of the sample induces optical dichroism of the E mode absorption. From the thermal decay of the dichroism, we have determined the activation energy for reorientation of VH₃ to be 0.50 eV. Thus, hydrogen atoms in VH₃ may well below room temperature jump from one site to another inside the vacancy. The implication of this finding will be discussed at the end of the talk.