Impact of Annealing on the Thermoelectric Properties of Ge 2 Sb 2 Te 5 Films
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Impact of Annealing on the Thermoelectric Properties of Ge2Sb2Te5 Films Jaeho Lee, Takashi Kodama, Yoonjin Won, Mehdi Asheghi, Kenneth E. Goodson Department of Mechanical Engineering, Stanford University, Stanford, California 94305, U.S.A. ABSTRACT Thermoelectric phenomena strongly influence the behavior of chalcogenide materials in nanoelectronic devices including phase-change memory cells. This paper presents the annealing temperature and phase dependent thermoelectric properties of Ge2Sb2Te5 films including the thermoelectric power factor and the figure of merit. The Ge2Sb2Te5 films annealed at different temperatures contain varying fractions of the amorphous and crystalline phases which strongly influence the thermoelectric properties. The thermoelectric power factor increases fom 3.2 μW/mK2 to 65 μW/mK2 as the crystal phase changes from face-centered cubic to hexagonal close-packed. The data are consistent with modeling based on effective medium theory and suggest that careful consideration of phase purity is needed to improve the figures of merit for phase change memories and potentially for thermoelectric energy conversion applications. INTRODUCTION Thermoelectric transport can have a large impact on the performance of semiconductor devices and related nanostructures [1-4]. The impact is possibly most pronounced in phasechange memories [5], which experience both large current densities and temperature excursions exceeding 600 °C. Recent measurements provided evidence of thermoelectric transport in phasechange cells by capturing a modification in the amorphous region [1] and the programming condition [2] with the bias polarity. However, there are relatively few data for the thermoelectric properties of phase-change materials at the film thicknesses relevant for contemporary devices. Past measurements of various chalcogenide materials revealed that holes are responsible for their positive Seebeck coefficient and the Seebeck coefficient strongly depends on phase and processing conditions [6-8]. However, the existing data for the Seebeck coefficient did not capture the impact of annealing and its impact on phase purity while the Ge2Sb2Te5 films can often contain a mixture of amorphous and crystalline phases, and the phase purity can strongly influence transport phenomena [9]. Our recent work reported the Seebeck coefficient of 25-nm and 125-nm-thick Ge2Sb2Te5 films between room temperature and 300 °C using an experimental structure in which a buried oxide layer induces lateral temperature fields [10]. Here we describe the impact of annealing on the thermoelectric properties of Ge2Sb2Te5 including the figure of merit for thermoelectric applications. The phase-dependent Seebeck coefficient is explained by different models, including the effective medium theory for treating phase impurities. EXPERIMENTAL DETAILS Samples are prepared on an SOI substrate with a 1.5- m-thick silicon layer and a 1- mthick buried oxide layer. A 70 nm-thick silicon dioxide film is deposited on the SOI substrate by plasma-enhanced chemical
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