金屬-鐵電薄膜-Al2O3絕緣層-半導體(MFIS)結構之SrxBiyTa2O9薄膜的電性和微觀結構研究

Abstract

本研究主要探討不同Sr/Bi含量比的SBT鐵電薄膜覆鍍於Al2O3超薄膜的矽基板上，經過超高溫度熱處理後(>800oC)，模擬MFIS結構的單一電晶體(one-transistor)形式其電性上的變化情形。 SBT薄膜的製備採用金屬有機分解法方式(MOD)，Al2O3超薄膜是使用電子束蒸鍍系統方式製備。記憶視窗(Memory window)主要是MFIS結構電性量測的重要指標，其形成的原因是SBT鐵電薄膜矯頑電場所致，而SBT矯頑電場的變化與Sr/Bi含量比有關，當Bi含量越趨近於2.0時，矯頑電場會越大，因而記憶視窗也會越大。當燒結溫度越高時，高含量Bi的SBT記憶視窗會有變大情形，可能是Bi高溫過量揮發使成份比趨近於2.0所致。C-V量測時，當外加電壓超過10V以後，記憶視窗會因絕緣層界電崩潰所形成的電荷穿透而有變小的現象。在Bi含量越高的SBT會因高溫擴散至Si介面而增加了雜質濃度，使空乏區的電荷量局部提高，造成C-V曲線有偏移和變形的情形。另一方面，漏電流密度會隨著Bi含量的增加而越來越低，這可能是Bi在Pt或Al2O3介面處形成氧化物提高了電阻所致。除此之外，本實驗還使用反應式電漿濺鍍方式置備ZrO2薄膜的矽基板做為對照組，然而電性結果卻比Al2O3差，可能是高功率的電漿造成ZrO2薄膜受損，增加了陷阱電荷密度因而降低了薄膜的品質。

We have investigated the electrical properties of Metal – Ferroelectric – Insulator - Semiconductor (MFIS) structure, in which SrxBiyTa2O9 (SBT) and Al2O3 were used as ferroelectric and insulator layers, respectively. When fabricating the MFIS structure, SBT thin films were deposited by metal organic deposition (MOD) method and Al2O3 buffered layers were prepared by dual E-gun evaporation and post oxidation. It was found that memory window measured from Capacitance-Voltage (C-V) properties would be related to the coercive field of SBT with different Bi ratios. It has been reported that the coercive field would increase when Bi contents in SBT were closed to 2.0. Therefore, the larger memory window after high temperature annealing is related to the factor that the originally higher Bi composition tend to 2.0 after vaporization. Moreover, the width of memory window decreases with applied the voltage over 10V, indicating the charge injection caused by dielectric breakdown of Al2O3 insulator. On the other hand, the reduced leakage current density with the higher Bi content is attributed to the formation of interfacial Bi-oxide. However, the bending and shift in C-V curve with the higher Bi content may be the result of larger Si surface donor concentration derived from Bi diffusion after annealing. For comparison, ZrO2 films were deposited by r.f. sputtering and also used as the buffered layers in MFIS structure. However, its electrical properties were inferior to those with Al2O3