In the first paper of this conference, Prof. Stuke presented a detailed review of the optical and electrical properties of the basic materials, Se, Ge and Te, which can be formed in both the amorphous and crystalline form1). In the final day of this meeting we would like to tie the basic physics learned from these discussions to more material ends — that is to the design of materials which can be utilized for the basic differences between crystal and amorphous forms and, in particular, the differences in optical properties.

Reflectivity studies of the Te(Ge, As)-based amorphous semiconductor in the conducting and insulating states

The thermal, structural electrical properties of bulk glasses based on GeTe compositions near the binary eutectic, Ge15Te85, 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 (Ge17Te83), ternary (Ge15Te80As5) and quaternary (Ge15Te81Sb2S2) 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 043–048 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 ( ⋍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

Thermally-induced structural and electrical effects in GeTe-based amorphous alloys

Observations of Surface‐Nucleated Crystallization in Memory‐Switching Glasses

Valence electron configuration of Te in amorphous TeGe alloys

This chapter surveys transitions between the viscous liquid state and the glass, phase-separated, or crystalline states and some of the uses of these transitions in altering the structure and properties of solids In addition to its intrinsic interest, the liquid ↔ glass transition is important for kinetically limiting the other two types of transition It is these limiting effects which make possible the formation of quite unique structures in the transitions We will begin by reviewing the liquid ↔ glass transition

Transitions in Viscous Liquids and Glasses

Transport measurements have been performed on the conducting state of a memory-type chalcogenide glass, of composition Te81Ge15As4. DC resistivity was measured from 4°K to 300°K and indicates no carrier freeze-out occurs down to 4°K. The resistance ratio, ϱ(300°K)/ϱ(4°K), was found to be 2.5. AC conductivity measurements at room temperature showed only an 18% increase in σ(ω) up to 150 khz. Hall effect experiments at 100 Hz show no dependence of carrier concentration on temperature from 77°K to 300°K, and indicate a room temperature mobility of 85 cm2/V-sec. The sign of the Hall constant implies that the carriers are holes. Resistance was measured as a function of magnetic field from 0 to 140 kOe, at 1.6°K. The magnetoresistance was positive and proportional to H2 at low fields, with saturation beginning to set in above about 80 kOe. The results of these studies, taken in conjunction with the X-ray and NMR data, can be used to suggest a plausible model for memory behavior in chalcogenide glasses.

B.P. Kraemer

Papers

Transport properties of a memory-type chalcogenide glass☆