Thermally-induced structural and electrical effects in GeTe-based amorphous alloys

Abstract

The thermal, structural electrical properties of bulk glasses based on GeTe compositions near the binary eutectic, Ge 15Te 85, are studied. Information regarding the non-crystalline state and the transformation from the non-crystalline to the crystalline state is reported. The particular alloys studied represent binary (Ge 17Te 83), ternary (Ge 15Te 80As 5) and quaternary (Ge 15Te 81Sb 2S 2) compositions. Structural information is obtained using X-ray diffraction techniques and density measurements. Thermal data are reported from differential scanning calorimetry (DSC), thermogravimetry (TGA) and mass spectrometry results. The electrical conductivity is measured as a function of temperature and, on the ‘as-prepared’ glasses, shows semi-conducting behavior with activation energies, E, of 0.43–0.48 eV. DSC, TGA and X-ray powder diffraction patterns indicate the samples crystallize as Te and GeTe in a two-step process, and melt at the binary eutectic temperature. The binary vaporizes as Te and GeTe in a two-step process. GeTeAs and GeTeSbS vaporize by essentially the same mechanism, with As evaporating (<300° C) before the Te, and Sb and S evaporating (420–480°C) after the Te but before GeTe. The results show that the properties of the bulk ‘as-prepared’ glasses are strikingly similar. Thermally-induced changes in the structural and electrical properties of bulk samples have been examined following a series of anneals (5 h, vacuum) at temperatures from 111°C to 190°C (glass transition temperature ⋍125−133° C ; crystallization temperature ⋍206−228° C as determined by DSC). DSC, TGA and mass spectrometry results have been correlated to electrical and structural changes. Results show that crystalline Te nucleates at the surface and forms a conductive surface layer. The conductivity of this surface layer is nearly temperature independent with E ≈ 10 −2 eV for all three alloys. Crystallization and the associated electrically conductive regions extend into the bulk material with further annealing. In these disordered alloys the additives As and Sb + S apparently do not act as electrical dopants in the sense of affecting the conductivity activation energy. The additives Sb + S however do retard crystallization of GeTe. The secondary crystallization product, GeTe, apparently changes the conduction mechanism to either a metallic or degenerate semiconductor type behavior.