以臨場原子層沉積術成長氧化釔於砷化鎵之結構及介面特性之研究

Abstract

以矽為基底的金氧半場效電晶體達到其物理極限的當前，尋找下一世代的半導體元件材料成為當前炙熱的議題。具有高載子遷移率的砷化鎵是一個常被探討的選擇。砷化鎵和矽之間的晶格差異比其他三五族半導體小，使得砷化鎵有較大的機會與矽基板做整合，再加上砷化鎵的半導體能隙是銦砷化鎵這類半討體之中最大的，此特性會抑制由微縮所造成的不良效應，更顯出砷化鎵的重要性。原子層沉積術可以在大面積基板上成長非常均勻且具保形性的薄膜，而且薄膜的厚度性極佳，因此這種成長方法已經被廣泛應用在半導體工業上，Intel發表的45奈米製程就是使用原子層沉積術成長氧化層，所以我們使用原子層沉積術成長氧化物是具有高度實用性的。稀土族氧化物之中的氧化釔擁有高的介電常數、相對高的導帶能差(與砷化鎵之間)以及高熱穩定性，最重要的是稀土族氧化物已經有許多成功的例子在砷化鎵表面形成低缺陷密度的介面；因此我們使用原子層沉積術成長氧化釔於砷化鎵上，也期許可以得到優異的介面特性。 在這個論文中，砷化鎵(001)-(4×6)和砷化鎵(111)-(2×2)被用來當作元件的基板；在沒有任何化學處理的前提下，我們以臨場的方式成長氧化釔於砷化鎵上。以反射式高能電子繞射和X光繞射分析術鑑定氧化釔薄膜的表面，以X光光電子能譜檢測薄膜的成分和推測氧化釔/砷化鎵之間價電帶的能量差異，以橢圓儀和X光反射率量測方式鑑定薄膜的厚度。電性方面，我們將試片做成金氧半電容元件以進行電容-電壓量測、漏電電流-電場量測以及準靜態電容-電壓量測。 我們得到了單相、單晶並且具相當優良晶向性的氧化釔。實現了高熱穩定性、高介電常數以及介面特性非常好的電容元件，這些結果顯示使用原子層沉積術成長氧化釔於砷化鎵上的元件的確有很大的機會可以成為下一代電晶體元件。

Owing to the demand in attaining electronic devices with higher speed and lower power consumption for CMOS, it is critically urgent to find another high k/high carrier mobility semiconductor to be employed in the MOSFETs. GaAs, being feverishly studied as a viable channel candidate for replacing Si, not only has higher mobility than Si but also has smaller lattice mismatch with Si than other III-V materials. Moreover, the bandgap of GaAs is widest among InGaAs semiconductors. This property can alleviate some adverse properties such as short channel effects. Atomic layer deposition (ALD) technique is widely employed in semiconductor industry for CMOS since 45 nm node due to its outstanding properties such as excellent conformity, uniformity, and precise thickness control. In this work, Y2O3, one of earth oxides, is used to passivate GaAs surface via ALD approach. Moreover, Y2O3 has high dielectric constant, relatively high conduction band offset, good thermal stability, and most of all, high potential for forming good oxide/GaAs interface to achieve low Dit. GaAs(001)-(4×6) and GaAs(111)A-(2×2) were used in this work. Without chemical surface treatment, we deposited in situ ALD-Y2O3 on GaAs surface using Y(Etcp)3 and H2O as precursors. In situ reflection high energy electron diffraction (RHEED) was applied to monitor the surface structure. We used X-ray photoelectron spectroscopy and X-ray diffraction to study Y2O3/GaAs band alignment and structure, respectively. Ellipsometry and X-ray reflectivity were utilized to estimate the film thickness and the growth rate per cycle (GPC) of ALD-Y2O3. Metal-oxide-semiconductor (MOS) capacitors were employed to investigate electrical characteristics such as capacitance-voltage (C-V), leakage current density-electric field (J-E), and quasi-static C-V (QSCV). Single-domain single-crystal ALD-Y2O3 was deposited on GaAs with excellent crystallinity. High thermal stability up to 900°C, very low frequency dispersion in both n- and p-type MOS C-V curves, and no discernible Dit peak through semiconductor band-gap have been achieved on GaAs(001)-(4×6). This work shows that ALD-Y2O3 has effectively passivated GaAs with low interfacial trap densities and excellent high temperature thermal stability. ALD-Y2O3 /GaAs(001) has been proved to be a vital candidate for beyond Si-based MOSFET.