Date of Completion

3-20-2019

Embargo Period

9-16-2019

Keywords

thermoelectric effects, high temperature, semiconductor, minority carrier, GTR, activation energy, phase change memory, vacuum

Major Advisor

Helena Silva

Co-Major Advisor

Ali Gokirmak

Associate Advisor

Zlatan Aksamija

Associate Advisor

Tai-Hsi Fan

Associate Advisor

John Ayers

Field of Study

Electrical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Under extreme thermal gradients (~> 1 K/nm) significant asymmetric melting of current-carrying highly-doped silicon micro-wires was observed. This bizarre phenomenon was explained by the generation-transport-recombination (GTR) process of the minority carriers in semiconductors at elevated temperatures. The finite element simulation model, using effective media approximation, supported this explanation; however, the asymmetry observed in the experiments is significantly larger than that obtained from the computational results. As the calculation based on conventional non-isothermal semiconductor physics underestimates the exponential growth of carrier generation near melting, the carrier concentration, hence the shift of the hottest spot because of the thermoelectric Thomson effect, was underestimated.

In this work, a high temperature semiconductor model is developed where the continuity equations for electrons and holes are solved and the energy exchange associated with generation-recombination processes are duly accounted for. Modified generation-recombination mechanisms included in the model sharply increase the carrier concentration to ~1022 cm-3 at melting for silicon, and the shift of hottest spot is closer to that observed in the experiments. This approach allows us to calculate a more accurate melt profile in non-equilibrium conditions, which is also critically important for designing phase change memory (PCM) cells. PCM is an emerging non-volatile memory technology where electrical pulses are used to transition between high and low resistance states and where the thermoelectric effects are significant due to extreme thermal gradients (~> 50 K/nm) at high operating temperatures.

To adopt this model for PCM, characterization of material properties at elevated temperature is required. In this work, the effective activation energy of metastable Ge2Sb2Te5 (GST) is calculated from device level high speed resistivity measurements data. The extracted temperature dependent activation energy shows parabolic shape with maximum at ~450 K, and is expected to correspond to a distribution of trap levels that contribute to conduction. Also, from the simultaneous Seebeck-temperature and resistivity-temperature measurements of amorphous-fcc (face centered cubic) mixed-phase GST thin-films, the Fermi energy of single crystal fcc GST was calculated, which is 0.16 eV bellow the valence band edge.

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