無鉛(Bi0.5Na0.5)TiO3-BaTiO3薄膜與ZrO2-HfO2超薄膜之製備與其鐵電和結晶性質分析

Abstract

鐵電材料的壓電性質可被應用於制動器、能量擷取器、感應器等元件。對於能量擷取器元件應用，不鏽鋼基板相較於矽基板擁有許多優點，例如：能提供較大的形變量而有較大的功率輸出，並且製作成本較低，然而目前以不鏽鋼基板製作的能量擷取器並未有使用無鉛(Bi0.5Na0.5)TiO3-BaTiO3 (BNT-BT)作為壓電層的研究報導。本研究嘗試以溶膠凝膠法(sol-gel)搭配旋轉塗佈法(spin-coating)製作BNT-BT薄膜和不鏽鋼基板的單層壓電片(unimorph)，並深入探討BNT-BT薄膜製備於不鏽鋼基板上的表面形貌、表面微成份偏析、結晶性質，以及討論表面形貌與結晶性質和其鐵電性質的關係，最後總合以上結果作為開發單層壓電片的方針。藉由使用快速熱退火(rapid thermal annealing; RTA)令BNT-BT薄膜結晶與添加LaNiO3薄膜作為不鏽鋼基板和BNT-BT薄膜的介面緩衝層，我們成功製備了漏電流小且鐵電性質顯著的BNT-BT薄膜和不鏽鋼基板的單層壓電片，BNT-BT薄膜的兩倍殘餘極化量(2 times remanent polarization; 2Pr)達59.4 µC/cm2。 ZrO2-HfO2由於其薄膜厚度極小且和互補式金屬氧化物半導體(complementary metal-oxide-semiconductor; CMOS)相容，因此極有潛力成為下一個世代電子元件的關鍵材料，例如：鐵電隨機存取記憶體(Ferroelectric random access memory; FeRAM)與負電容電晶體(Negative capacitance field-effect transistor; NC-FET)等元件。除了元件的尺寸，電子元件對於熱預算(thermal budget)的要求也越來越高，我們的研究團隊已成功開發了不需經熱退火即展現出鐵電特性的ZrO2超薄膜，本研究延續研究團隊之成果嘗試進一步了解如何調控ZrO2-HfO2超薄膜之鐵電性質。本研究系統性地討論以電漿反應式原子層層積技術(plasma-enhanced atomic layer deposition; PEALD)製作的ZrO2、HfO2、ZrxHf1-xO2固溶體超薄膜在沈積完狀態(as-deposited)與經快速熱退火後的結晶和鐵電性質，ZrO2與ZrxHf1-xO2固溶體超薄膜的鐵電性質經過wake-up循環(wake-up cycling)後的變化，以及以熱反應式原子層層積技術(thermally driven atomic layer deposition; thermal ALD)或電漿反應式原子層層積技術製作之ZrO2的結晶和鐵電性質差異。本研究結果指出：以電漿反應式原子層層積技術製作的ZrO2只需經wake-up循環而不需經快速熱退火即具有顯著的鐵電性質，其兩倍殘餘極化量達28.1 µC/cm2，適合用於熱預算要求較高的元件，而以電漿反應式原子層層積技術製作的Hf0.5Zr0.5O2經快速熱退火後不需經過wake-up循環即具有顯著的鐵電性質，其兩倍殘餘極化量達25.8 µC/cm2，則適合用於可承受高溫但不可承受多次且高強度電場的元件。

The piezoelectricity of ferroelectric materials can be applied on actuators, energy harvestors, and sensors devices. For the applications on energy harvestor devices, stainless steel substrates possess several advantages compared to silicon substrates. For exemple, stainless steel substrates can provide larger strains resulting in larger power output and lower the cost of fabrication. However, no research has reported that lead-free (Bi0.5Na0.5)TiO3-BaTiO3 (BNT-BT) was chosen as piezoelectric layers for energy harvestors fabricated on stainless steel substrates until now. In this study, we tried to fabricate unimorphs composed of BNT-BT thin films and stainless-steel substrates by appling sol-gel and spin-coating technique. We derived the research guidelines with the investigation of the surface morphology, surface micro-segregation, and crystalline properties of BNT-BT thin films coated on stainless steel substrates and the discussion of the relationship between its ferroelectric properties and both surface morphology and crystalline properties. By appling rapid thermal annealing (RTA) to crystallize BNT-BT thin films and introducing LaNiO3 thin films between BNT-BT thin films and stainless steel substrates as interface buffer layer, we successfully prepared unimorphs composed of stainless steel substrates and BNT-BT thin films having low leakage current and prominent ferroelectric properties. Thanks to the ultrathin film thickness and the complementary metal-oxide-semiconductor (COMS) compatiblility, ZrO2-HfO2 showed its potential to be the key material of next generation electronic devices, for exemple, ferroelectric random access memory (FeRAM) and negative capacitance field-effect transistor (NC-FET). Except for dimensions, the requirement of low thermal budget has become more and more demanding for electronic devices. Our research group has successfully developed the ZrO2 ultrathin films who show ferroelectric properties without annealing. In this study, we try to understand how to manipulate the ferroelectric properties of ZrO2-HfO2 ultrathin films based on the preceding results of our group. We systematically discussed the crystalline and ferroelectric properties of as-deposited and rapid thermal annealed ZrO2, HfO2, and ZrxHf1-xO2 solid solution ultrathin films prepared by plasma-enhanced atomic layer deposition (PEALD). We also discussed the evolution of the ferroelectric properties of ZrO2 and ZrxHf1-xO2 solid solution ultrathin films after wake-up cycling. Moreover, the difference of crystalline and ferroelectric properties between ZrO2 deposited by thermally driven atomic layer deposition (thermal ALD) and PEALD was studied. Results indicated that ZrO2 deposited by PEALD showed prominent ferroelectric properties without rapid thermal annealing and was suitable for those devices requiring low thermal budget. On the other hand, rapid thermal annealed Hf0.5Zr0.5O2 deposited by PEALD showed ferroelectric properties without wake-up cycling and was suitable for those devices that can withstand high temperature but are fragile under high and multiple electric field cycling.