製備鍺銻碲單晶奈米線及其電遷移行為之研究

Abstract

本研究使用氣液固相法（Vapor-Liquid-Solid，VLS），以金（Gold，Au）奈米粒子為催化晶種，藉由調變爐管壓力及基板溫度成長鍺銻碲（Ge2Sb2Te5，GST）硫族合金（Chalcogenides）單晶奈米線（Nanowire，NW）。掃描式電子顯微鏡（Scaning Electron Microscopy，SEM）、能量散佈分析儀（Energy Dispersive Spectroscopy，EDS）及X光繞射儀的分析顯示，在爐管壓力為1 torr、載流氣體流量為50 sccm、基板溫度為415℃的條件下可長成單晶之GST NW。 完成NW製備後，接著在施加直流偏壓的條件下進行平均破壞時間（Mean-time-to-failure，MTTF）分析，探討GST NW之電遷移（Electromigration，EM）行為。經200℃、不同電流密度（4x10^3至3.2x10^5 A/cm^2）之量測後，以Black方程式計算得電流密度加速因子（即Black方程式中的n值）約為0.98，此意味著表面擴散為EM破壞之主導機制；以6x10^4 A/cm^2之電流密度、不同加熱溫度（200至300℃）之MTTF測試發現EM之活化能約為1.23 eV，與薄膜試片比較，此顯示單晶結構之GST NW的抗EM破壞能力較佳。EDS分析得知在外加偏壓下，GST NW中的碲（Tellurium，Te）原子會朝陽極端遷移，鍺（Germanium，Ge）與銻（Antimony，Sb）原子則往陰極端方向擴散；SEM之觀察發現，在EM破壞之末期時，靠近GST NW之陰極端會形成局部頸縮（Necking）之區域，此顯示電子風力效應主導EM過程之質流遷移，推斷局部頸縮現象為表面擴散造成空位累積所致，此使得該區域焦耳熱效應增強而導致GST NW之EM破壞。

In this study, gold (Au) nanoparticles are served as the catalyst to synthesize the Ge2Sb2Te5 (GST) chalcogenide nanowires (NWs) via the vapor-liquid-solid (VLS) method. Analytical results of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray diffraction indicated that the single-crystalline GST NWs may form at the conditions of ambient pressure of 1 torr, vehicle gas flow rate of 50 sccm and temperature of 415℃. After the completion of GST NW preparation, this study further performs the mean-time-to-failure (MTTF) test under DC bias in a self-assembly electronical measurement system to evaluate the electromigration (EM) behaviors of GST NWs. The MTTF test carried out at the temperature of 200°C and current density ranging from 4x10^3 to 3.2x10^5 A/cm^2 found that the n value or, the current density exponent of Black’s equation, is about equal to 0.98. This implied that the surface diffusion is the dominant transport mechism of EM failure. Moreover, the MTTF test performed at the current density of 6x10^4 A/cm^2 and temperatures ranging from 200 to 300℃ revealed that the activation energy of EM is 1.23 eV. This implied a better resistance to EM failure for GST NW in comparison with thin film samples. EDS analysis indicated that Te elements migrate to the anode side whereas Ge and Sb elements move to the cathode side of GST NWs subjected to the DC bias. SEM observed the occurrence of necking in cathode side of GST NW when EM failure occurred. This implied the electron wind force effect dominates the mass transport during EM process. The necking is ascribed to the vacancy accumulation caused by surface diffusion, consequently escalating the Joule heating effect and eventually leading to the EM failure of GST NWs.