以3ω法量測相變化薄膜熱傳導性質與其交流阻抗特性之研究

Abstract

本研究以自組之3ω法（3-omega Method）設備進行鍺銻碲（Ge2Sb2Te5,GST）相變化合金薄膜熱傳導係數之量測。以此設備先對二氧化矽（SiO2）薄膜進行量以驗證其可靠性，再對GST與摻雜Ce之GST（GST-Ce）進行熱傳導係數之量測，同時也利用自組之即時電性系量測統分析GST試片之交流阻抗（AC Impedance）性質，並搭配等效電路模型模擬分析GST試片中晶粒與晶界對電與熱性質影響之比重。實驗結果顯示結晶態GST之熱傳導係數（約0.8 W/mK）值皆較非晶態高（約0.35 W/mK），Ce摻雜則降低了GST的熱傳導係數。電性質分析顯示GST試片中之晶界為電阻性質變化之主要貢獻者，此由X光繞射分析顯示Ce掺雜導致晶粒細化，原子尺寸之差異亦引發應力場而成為晶粒成長之阻礙，從而造成相變化溫度與活化能之提高而獲得驗證。正切損失（Tangent Loss）特徵峰值對溫度之變化分析顯示Ce摻雜強化了界面極化（Interfacial Polarization）效應，晶界散射係數計算亦顯示GST-Ce中之細化晶粒結構造成較大程度的電子散射，從而降低了熱傳導係數與提高電阻特性。

This study investigates the thin-film thermal conduction properties of Ge2Sb2Te5 (GST) phase-change alloys by utilizing a self-assembly apparatus based on the 3-omaga (3ω) method. First, the thermal conduction of silicon dioxide (SiO2) was measured in order to identify the reliability of experimental tools. The pristine GST and cerium-doped GST (GST-Ce) thin films were then prepared and their thermal conductivities were measured by the 3ω apparatus. The AC impedance properties of these chalcogenide layers were also evaluated by an in-situ electrical measurement system. The data obtained were then implanted in an equivalent circuit model in order to distinguish the characteristics of grain and grain boundary in the electrical and thermal conduction properties of chalcogenide layers. Experimental results indicated that the intrinsic thermal conductivity of amorphous GST (= 0.35 W/mK) was lower than that of crystalline GST (= 0.8 W/mK) and the Ce doping causes the decrease of thermal conductivity in comparison with the GST of the same microstructure. Electrical analysis revealed grain boundary is the major contributor to the resistance property of GST. This was further confirmed by the grain refinement in GST-Ce sample as revealed by x-ray diffraction analysis as well as the increase of phase-change temperature and activation energy of doped GST layer caused by the stress-induced barrier due to the incorporation of alien atoms in the sample. Analysis of characteristic tangent loss peak shift as a function of temperature shows that Ce doping amplifies the interfacial polarization in GST. A calculation of grain-boundary scattering coefficient illustrates the fine grain structure in GST-Ce implies a more severe electron scattering, leading to the decrease of thermal conductivity and increase of electrical resistance property.