Influence of Partial Substitution of Te by Se and Ge by Sn on the Properties of the Blu-ray Phase-Change Material Ge8Sb2Te11

The opto-electronic properties of the Blu-ray phase-change material Ge8Sb2Te11 were investigated and compared to partially substituted compounds. Investigations were performed on Ge8Sb2Te6Se5 (I) and SnGe2Sb2Te2Se4 (II). To monitor the influence of substitution on both cation and anion sites onto the properties, in Ge8Sb2Te6Se5, only Te was substituted by Se, while in SnGe7Sb2Te7Se4, Ge also was partially replaced by Sn, yielding an equivalent degree of substitution. The crystallization of the amorphous compound to a rhombohedral phase was observed. The first transition temperature (T-c1) is 210 degrees C for I and 185 degrees C for II, while the second transition temperature (T-c2) cannot be clearly determined, because of the coexistence of both phases. Thin-film properties were examined and an increase of density and roughness was observed for both materials upon crystallization. In situ electrical resistance measurements show a huge electrical contrast between the amorphous samples and the crystalline samples, which amounts to 4 orders of magnitude for II and 5 orders of magnitude for I. Optical properties were characterized with variable incident-angle spectroscopic ellipsometry (VASE) and Fourier Transform infrared (FTIR) spectroscopy. Substitution leads to an enormous increase of the reflectivity contrast for blue light: The reflectivity contrast between the different phases increases from 20% to 50% for I and from 20% to 74% for II.