Annual Report 1994

Report 2 / 3

PAC-investigations of the electric field gradient with ⁷⁷Br and ^111mCd in ternary semiconductors

A. Möller, R. Vianden, S. Unterricker^a, M. Dietrich^a and D. Forkel-Wirth^b

^a Bergakademie Freiberg, Fachbereich Physik, D-09596 Freiberg/Sachsen
^b ISOLDE-Collaboration, CERN, Genf

Contents:

First measurements with ¹¹¹In located at the Ga-sites of AgGaS₂, AgGaSe₂ and AgGaTe₂ resulted in a small quadrupole coupling constant ν_Q, which had shown an unusually strong temperature-dependence caused by the large tetragonal compression τ=2-c/a of these three compounds. In order to investigate the EFG at the A- and the C-sites of the chalcopyrite-structure measurements with ^111mCd and ⁷⁷Br were carried out. The isotope ^111mCd was implanted at ISOLDE (CERN) into AgGaS₂, AgGaSe₂, ZnSiP and CdGeP₂. The isotope ⁷⁷Br was implanted correspondingly into AgGaSe₂ and CuInSe₂. Additionally some experiments with As-containing samples like CdGeAs₂ and CdSnAs₂ were carried out. In these cases the probe atoms were produced via the nuclear reaction ⁷⁵As(α,2n)⁷⁷Br by irradiating the samples with an α-particle-beam of 33 MeV. The quadrupole interaction was measured at RT and in some cases at higher temperatures.

In the case of AgGaS₂ and AgGaSe₂ the R(t)-spectra of ^111mCd show two different quadrupole coupling constants ν_Q1 and ν_Q2 at RT. The first one belongs to an axially symmetric EFG, which was predicted for the substitutional Ag-site. The second frequency is due to a non axially symmetric EFG. Measurements with a single cyristal of AgGaS₂ in different orientations show that the EFG causing ν_Q2 is not oriented parallel to a principal crystal-axis. It is discussed if ν_Q2 is caused by a sulphur-vacancy (that is trapped by the Cd-probe in one of four possible directions) because at 400°C this frequency disappears completely in favour of ν_Q1. The frequency ν_Q1 shows no temperature-dependence. The frequencies observed with ¹¹¹In in CdGeP₂ and ZnSiP₂ are more or less the same like the frequencies observed with ^111mCd in these compounds. That means in these compounds both probe atoms are located at the A-sites.

First measurements with the isotope ⁷⁷Br were carried out in GaAs, where the probe atoms were produced by irradiating the sample with an alpha particle-beam of 33 MeV. The same procedure was tried for CdGeAs₂ and CdSnAs₂. The R(t)-spectra had no good statistics, nevertheless a small fraction of a large frequency was seen, belonging to an axially asymmetric EFG. The R(t)-spectra of AgGaSe₂ and CuInSe₂ with implanted ⁷⁷Br similarly show a frequency that is due to such an EFG. In this case the fraction is approximately 50 percent. That means the probe atoms are located substitutionally at the C-sites of the ternary compounds. No temperature dependence of ν_Q was observed. The EFG at the C-sites is very large and axially asymmetric, caused by the different ionicity of the A-C- and the B-C- bondings.