Microhardness study of CdZnTeSe crystals for X-ray and gamma ray radiation detectors

It is known that dislocations can act as electrically active shallow levels and influence the charge collection efficiency. The goal of the presented work is to increase our understanding of key factors influencing the crystal quality and its suitability for radiation detection by a study of its mechanical properties, namely the microhardness and its dependence on crystal composition. Microhardness is closely connected to dislocations and their mobility. We analyzed the Cd
_1-x
Zn
_x
Te
_1-y
Se
_y
(CZTS) crystal prepared by THM method at Brookhaven National Laboratory measuring ellipsometry, photoluminescence, Zn and Se composition and Vickers microhardness along the crystal growth axis. The concentration of Zn in the studied samples was between 7-10% and the concentration of Se was approx. 4%. We found out that the Zn composition is nearly constant in the first half of the crystal and has tendency to slightly decrease towards the end of the crystal. The Se composition is nearly constant along the ingot. The room temperature bandgap measured by ellipsometry follows the Zn distribution, i.e. it is approx. 1.53eV in the region between the tip and middle of the ingot and decreasing to 1.52 eV. We observed that the Vickers microhardness varies from 65 HV 0.05 to 85 HV 0.05 at a load of 50g. It correlates with the measured bandgap. This result shows that stable distribution of Zn and Se resulting in constant bandgap is important to stabilize its mechanical properties and this way also the impact of dislocations on crystal quality and charge collection efficiency of radiation detectors.