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. Unterrickera, M. Dietricha and D. Forkel-Wirthb

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 νQ, 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 νQ1 and νQ2 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 νQ2 is not oriented parallel to a principal crystal-axis. It is discussed if νQ2 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 νQ1. The frequency νQ1 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 νQ 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.