量子效應對矽與鍺奈米電晶體之電荷傳輸特性影響研究

Quantum Effects on Charge Transport Properties of Si and Ge Nanostructured Transistors

賴威廷(Wei-ting Lai)

指導教授 ： 李佩雯

國立中央大學/資訊電機學院/電機工程學系/博士(2012年)

摘要

本論文主題在於製作及研究矽與鍺奈米結構 (奈米線和量子點) 及其相關電晶體元件。首先，藉由電子束微影搭配 C4F8/SF6 電漿蝕刻及熱氧化製程技術，設計製作了寬度由 7 至 60 奈米之一維矽奈米線。在矽奈米線金氧半場效電晶體中，實驗發現載子傳輸行為與通道大小及奈米線尺寸息息相關：在固定奈米線線寬為 24 奈米下，當通道長度 (Lg) 與電子波波長 (le) 滿足 mle/2 = Lg 關係式，如通道長度為 52 及 34 奈米，元件在臨限電壓附近呈現電流平台及震盪電流特性；反之，當通道長度為 42 奈米時，元件行為即如同傳統金氧半場效電晶體。再者，當奈米線線寬縮減至 10 奈米以下時，元件電流則呈現更加明顯之震盪行為。經由變溫量測分析，量子干涉及次能帶散射效應應為所觀察電流行為之來源機制。這實驗結果也顯示，當通道由三維平面結構縮減至零維量子點時，載子傳輸特性將會有本質上的改變。
在製作零維鍺量子點方面，由於鍺析出及鍺濃縮效應，高溫熱氧化絕緣層上單晶矽鍺合金結構，未氧化完的矽鍺合金之鍺含量會隨氧化時間漸變，並同時逐漸形成鍺量子點。直至所有矽緩衝層被氧化消耗，鍺原子甫全部析出並聚集形成鍺量子點。因此，熱氧化絕緣層上單晶矽鍺合金平面可形成小顆且緊密之鍺量子點；而氧化矽鍺合金奈米線可形成單一或是少數鍺量子點排列在完全氧化之奈米線中間，並且自我對準至兩旁未完全氧化之矽鍺/矽墊層區。
陰極激發光之波峰能量隨量子點尺寸縮小而藍移之現象說明，由於量子侷限效應，鍺量子點具有準直接能隙特性。同時，我們也實驗驗證了矽鍺量子點/矽奈米柱異質介面光偵測器。藉由矽鍺與矽在價電帶的偏移量，元件在照光下，具有高達 10^4 之光電流增益。再者，若將鍺量子點埋進複晶矽薄膜電晶體以形成鍺量子點光電晶體，初步的模擬結果說明，經由光波導進入元件之光電場會集中在鍺量子點彼此之間，進而增強鍺量子點之吸光效率及光響應。這說明鍺量子點在光纖通訊或光積體電路上有其發展潛力。另一方面，在包含鍺量子點之氮化矽光子晶體共振腔中，計算之 quality factor (Q) 可接近 10^5，提供了實現鍺量子點光源的可能性。
另一方面，藉由鍺量子點共振穿隧二極體以及單電子電晶體，我們探討了鍺量子點之電子能結構及電荷傳輸特性。在高摻雜矽/二氧化矽/鍺量子點/二氧化矽/高摻雜矽之共振穿隧二極體中，由於高摻雜矽電極之電子分布能帶不足以同時對應多個量子點能階，故可直接由量測之穿隧電流譜線解析鍺量子點內部之單粒子電子能結構。在適當的照光條件下，量子點內部多出的電洞，使我們經由實驗觀察到額外之光激發細微電流結構。同時，我們也藉由電壓脈衝激發來探討通過鍺量子點之暫態載子傳輸行為：於電壓改變瞬間出現之大幅震盪電流響應是源於鍺量子點內部之位移電流；而當元件逐漸回到穩定態後，則改由穿隧電流所主導。
本論文的最終目標在於實現具有自我對準電極之高效能鍺量子點單電洞電晶體。在有效抑制穿隧位能障隨閘極偏壓而降低之寄生效應下，元件於室溫具有明顯之庫倫阻斷震盪電流，而最大峰對谷電流比值高達 750。

關鍵字

鍺 ； 量子點 ； 單電子電晶體 ； 奈米線 ； 矽

並列摘要

In this thesis, fabrication and characterization of both Si and Ge nanostructures (nanowire (NW) and quantum dot (QD)) as well as the associated transistors were investigated. Si NWs of various width ranging from 7 to 60 nm were generated using a combination of electron-beam lithographic, C4F8/SF6 plasma etching, and thermal oxidation processes. For one-dimensional Si NW metal-oxide-semiconductor field-effect-transistors (MOSFETs), there appears to be strong channel and width size dependence on the carrier transport wherein. For a NW with a given wire width (W) of 24 nm, clear current plateau/oscillation features were observed around the threshold regime once the channel length (Lg) satisfied the relation of mle/2 = Lg, where le is the electron wavelength, for instance, Lg = 52 and 34 nm. Whereas the NWFETs behaved like a conventional MOSFET when Lg = 42 nm. On the other hand, as the wire width was reduced less than 10 nm, NWFETs exhibited much strong current oscillation behaviors. Temperature-dependent analysis suggests the interplay of quantum interference and intersubband scattering effects being the plausible mechanisms for the observed current behaviors. It also implies there should be dramatic different carrier transports properties when the channel is scaled from three-dimensional planar structures into zero-dimensional QDs.
As for the generation of Ge QDs, thanks to Ge segregation and condensation effects during high temperature thermal oxidation of SiGe/Si-on-insulator (SGOI) structures, Ge QDs are ultimately formed by a progressive concentration of the Ge content within the remaining unoxidized SiGe until the entire Si is used up. As a consequence, tiny and dense (size and density) Ge QDs were formed after thermal oxidation of SGOI planar structures, whereas a single or a few Ge QDs line up along the core of an oxidized SGOI NW which self-aligns with adjacent unconsumed SiGe/Si pads.
The blue shift of the cathodeluminescence (CL) peak energy with a reduction of the QD size revealed the quasi-direct bandgap properties of Ge QDs because of quantum confinement effects. SiGe-QD/Si-pillar heterostructured photodetectors were experimentally demonstrated. Thanks to the effective confinement of holes within the QD by the valence band offset between SiGe and Si, a current enhancement up to 10^4 was achieved under light illumination, suggesting potentials of Ge QDs for fiber-optical communications or optoelectronic applications. On the other hand, incorporating the Ge QDs into the gate stacks of poly-Si thin-film-transistors, preliminary simulation results pointed to a possible promising enhancement in the light absorption efficiency and photoresponsivity by the concentration of the electric field between adjacent Ge QDs. Meanwhile, the calculated quality factor (Q) up to almost 10^5 is obtained if Ge QDs are embedded in L-type Si3N4 photonic crystal cavities, giving an opportunity for the realization of Ge-QD light emitting devices.
The electronic structures of the Ge QDs and the charge transportation wherein were investigated in terms of Ge-QD resonant tunneling diodes (RTDs) and single-electron transistors (SETs). In a designed n+-Si/SiO2/Ge-QD/SiO2/n+-Si RTD, we are able to resolve the one-particle electronic structure of the Ge QD directly from the steady-state tunneling current spectroscopy for the sake that the electron bandwidth of n+-Si electrodes could not cover more than one energy levels of the Ge QD simultaneously. Additionally we observed a salient photogenerated fine structures under suitable light pumping owing to excess holes dwell in the QD. We gained the insight of the transient carrier transport through the Ge QD by applying different voltage trains to a Ge-QD RTD, and found out that the displacement current played a major role in the pulse transient region whereas the tunneling current dominated when the device reached to the steady state. The ultimate goal of this thesis is the experimental demonstration of high performance Ge-QD single-hole transistors with self-aligned gate and source/drain electrodes. As a result of effective suppression of gate-induced tunneling barrier lowing, clear Coulomb blockade oscillations with a large peak to valley current ratio up to 750 is achievable at room temperature.

並列關鍵字

Single-elecron transistor ； Ge ； Si ； Nanowire ； Quantum dot

參考文獻

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