可撓式單晶鍺薄膜電晶體與高遷移率鍺金氧半場效電晶體之應變研究

Flexible Single-Crystalline Ge p-Channel Thin-Film Transistors and the Strain Response of High Mobility Ge n-MOSFETs

徐正一(William Hsu)

指導教授 ： 劉致為

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.10326

摘要

軟性電子具有可撓曲、重量輕和尺寸薄的特色，是繼半導體和平面顯示技術的下一個新興產業，而其最大的應用市場為軟性顯示技術。然而要製造使用在軟性顯示器中的可撓式薄膜電晶體，因為製程溫度受限於不耐高溫的塑膠基板，因此半導體材料主要使用可低溫生長的氫化非晶矽(a-Si:H)和有機半導體。這些材料的低遷移率和不穩定性使可撓式薄膜電晶體的發展因此受限。在論文中，我們利用聰明切的技術將單晶鍺薄膜轉移至可撓的聚亞醯胺軟性基板上，並且利用低溫製程形成蕭特基位能障源極和汲極，成功製作出可撓式單晶鍺薄膜電晶體。整個低溫製程不會破壞塑膠基板，並且提供了高遷移率的鍺通道和高飽和電流。
當傳統的矽 CMOS scaling 逐漸面對到其極限時，用各種新材料或新結構來增進CMOS性能並且繼續維持scaling的步調，現在這已經是針對未來CMOS發展的重要研究方向。其中同時擁有高遷移率電子和電洞的鍺已經受到很高矚目，並且有希望取代矽使用在22nm 節點製程。然而，鍺要實現在CMOS上還有一些困難需要克服，例如高品質的介面鈍化、低的源極和汲極電阻、適當的閘極介電層等等。
本論文中，我們研究且發展出鍺金氧半元件之結構。利用快速熱退火製程(rapid-thermal-oxidation)在鍺表面生長出高品質的二氧化鍺(GeO2)，並且在上面用低溫原子層沉積(atomic-layer-deposition)三氧化二鋁(Al2O3-O3)來確保二氧化鍺之品質，而且在沉積三氧化二鋁的過程中所提供的臭氧(O3)可再增進二氧化鍺之品質，所製作出的鍺金氧半電容特性曲線不會因頻率而改變(dispersion-free)。為了確認臭氧之效應，我們也製作了利用水(H2O)來當三氧化二鋁(Al2O3-H2O)之氧化物的鍺金氧半電容。另外，我們利用兩步驟退火來活化離子佈植成功製作高開關比率的鍺n+/p二極體，接近1的理想因子也代表此n+/p二極體的缺陷很少。
目前很多研究中鍺的n型電晶體都沒有顯現出其高遷移率的特性。然而，我們利用二氧化鍺鈍化、臭氧處理、兩步驟源極/汲極活化、閘極最後(gate-last)和高介電常數介電質(high-k)製程，在鍺(001)基板上製作出高遷移率之n型電晶體。使用臭氧處理之鍺n型電晶體之遷移率曲線和沒有臭氧處理的相比有較少的庫倫散射(Coulomb scattering)。最後，在施加適當方向的應力後，造成的應變可以造成能帶分裂(band splitting)和電子重新分佈(electron repopulation)，如此一來遷移率可以再進一步的提升。

關鍵字

可撓式薄膜電晶體 ； 蕭特基位能障源極/汲極 ； 鍺 ； 晶圓鍵合 ； 塑膠基板 ； 高遷移率 ； 表面鈍化 ； 應變 ； 應力 ； 金氧半場效電晶體

並列摘要

Flexible electronics is of great interest recently, because they enable classes of applications that lie outside those easily addressed with wafer-based electronics. The main applications of flexible electronics lie in flexible display technology. However, the performance of flexible thin-film transistors (TFTs) for flexible displays is limited due to low-temperature process required by plastic substrates. So far, the flexible TFTs are mainly based on amorphous Si (a-Si:H) and organic materials with low mobility and reliability. In this thesis, by a low-temperature process, including adhesion wafer bonding and Smart-cut techniques, high-quality of single-crystalline Ge thin film was transferred onto flexible polyimide substrates, and high-performance flexible Ge TFTs with Schottky-barrier S/D were then successfully integrated on it, preserving high mobility of Ge channel. The advantages of using Ge and Schottky-barrier S/D are addressed by making comparison between Ge and Si devices. Mechanical bending tests were applied to Ge TFTs to investigate its reliability.
When traditional Si CMOS scaling is approaching to its physical limits, introduction of performance boosters by alternative materials or novel architectures has become necessary to continue the scaling trend. High mobility materials like Ge or GaAs have been extensively investigated as channel material for replacing Si. With both high bulk electron and hole mobility, Ge has become a very promising candidate to be used in 22nm node. However, there are still several critical issues for Ge COMS. The major challenges to achieve high mobility Ge channels are the reduction of interface defect density, the increase of n-type dopant activation, proper strain configuration, and high-k integration.
In this thesis, GeO2 by rapid-thermal-oxidation using molecular oxygen is introduced as effective Ge interface passivation. Sequential Al2O3-O3 deposition by atomic-layer-deposition (ALD) using O3 as oxidant at low temperature can further improve the quality of metal-insulator-semiconductor (MIS) capacitors. The effect of O3 treatment is confirmed by the control samples where Al2O3-H2O was deposited by using H2O as oxidant rather than O3. Electrical characterizations are carried out and dispersion-free C-V curves are demonstrated. Besides, high on/off ratio n+/p diodes were fabricated by ion-implantation and two-step annealing for activation. The ideality factor of ~1.05 is attained, indicating low defects within the junction.
The performances of Ge n-MOSFETs do not reach its potential of high mobility in many previous studies. In this work, with GeO2 passivation, O3 treatment, two-step S/D activation, and gate-last integration with high-k, n-MOSFETs exhibiting high electron mobility for (001) Ge substrate and excellent transistor behaviors are fabricated. The effect of interface states and slow traps are addressed experimentally. A reduced Coulomb scattering was presented for Ge n-MOSFETs with O3 treatment, as compared to those without (Al2O3-H2O). Proper stress further boosts the mobility enhancement by band splitting and electron repopulation.

並列關鍵字

Flexible Thin-Film Transistor (TFT) ； Schottky-Barrier Source/Drain ； Ge ； Wafer Bonding ； Plastic Substrate ； High Mobility ； Surface Passivation ； Strain ； Stress ； MOSFET

參考文獻

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