Date of Award

5-2008

Degree Name

MS in Electrical Engineering

Department

Electrical Engineering

Advisor

Fei Wang

Abstract

Silver doped chalcogenide glasses, such as Ag-Ge-Se, display resistance switching behavior under low threshold voltages, which makes these materials very promising in non-volatile memory applications. In this thesis, ternary glasses of composition (GeS3)1-xAgx (x = 0.1 and 0.2) are studied. Non-volatile memory cells based on the above compositions are fabricated and electrically characterized.

Thin films are fabricated at 3 different evaporation angles (45°, 60°, and 90°) in a vacuum thermal evaporator. Finished films are thermally annealed at 150°C and 200°C respectively. Both virgin and thermal annealed samples are examined using Raman scattering. Raman line-shapes of both virgin and thermal annealed samples display signature peaks corresponding to Ge-S tetrahedral and Ag-attached thiogermanate units, which confirms the composition of the thin films. Extra elemental sulfur peaks are recorded in all films because of sulfur's low vaporization temperature. Thermally annealed films show a dependency on deposition angle. We found elemental sulfur peaks (at ~200 cm-1 could be removed by annealing at lower temperatures (150°C) from films deposited at smaller angles, while higher temperature (200°C) is needed to remove sulfur peaks from films deposited at 90°. This result indicates that the vertically deposited films are more condensed than the obliquely deposited ones. A condensed network needs more thermal energy for any structural adjustment, in this case, the release of elemental sulfur.

Non-volatile memory devices were fabricated with different film thickness from about 10nm up to 120 nm. Electrical characterizations of these devices show clear current switching behavior. The transition threshold voltage is around 0.353 Volts to 0.733 Volts with the average being around 0.526 Volts. While the transition threshold voltage does not rely on the film thickness, the current does. We observe that the peak writing current increases as a function of film thickness.

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