Crystallisation mechanism in Te rich GeSbTe thin films

The crystallisation kinetics of nonstoichiometric GeSbTe films is significantly different from that of stoichiometric GeSbTe compositions. The purpose of this paper is to study the mechanism of crystallisation in Te rich thin films using optical reflection at 650 nm, four probe electrical resistivity and x-ray diffraction measurements. We have shown that the optical reflection and four probe electrical resistivity in nonstoichiometric samples, with lower Sb content and higher Te content, in relation to the stoichiometric materials, have three steps at measuring temperature of about 130, 180 and 200°C. X-ray measurements in these samples show that the first step is related with the crystallisation into the fee Ge₁Sb₂Te₄ stochiometric phase, the second step appears due to the formation of an additional crystalline Te phase and the third step occurs when the fee transform into the hexagonal Ge₁Sb₂Te₄ stochiometric phase, preserving the crystalline Te. Crystalline Te has higher reflection and lower resistivity than the fee Ge₁Sb₂Te₄ crystalline phase. This explains the appearance of the second step in reflection and resistivity measurements. Large deviation from stoichiometric compositions increases the onset temperature for crystallisation.