鍺與鍺錫量子井二維電洞氣的磁導特性與等效質量

Magneto-Transport and Effective Mass of Two-dimensional Hole Gases in Ge and GeSn Quantum Wells

邱柏元(Po-Yuan Chiu)

指導教授 ： 李峻霣

國立臺灣大學/電機資訊學院/電子工程學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202000684

摘要

自旋電晶體等自旋元件可藉由Rashba自旋軌道耦合效應來調控自旋方向，可作為超低功耗的邏輯元件。另一方面，利用自旋軌道耦合效應操控量子位元之自旋亦可以得到更快的量子運算。在四族材料中，鍺(錫)具有強大的自旋軌道耦合效應且與CMOS技術相容，此外，透過形成二維電洞氣及其輕電洞質量特性，鍺(錫)電洞遷移率可大幅提高。因此，本論文將研究鍺(錫)中的二維電洞特性如磁導特性與電洞等效質量。

目前，大部分的鍺二維電洞系統都是透過調變摻雜方法而形成。本論文中，我們使用離子佈值來調變鍺/鍺矽異質結構中的二維電洞濃度，使電洞濃度與佈值劑量呈線性相關。較高的劑量會導致較高的二維電洞濃度，遷移率將因離子佈值引起的較強雜質散射而降低。透過變溫的Shubnikov-de Haas (SdH)振盪，我們萃取二維電洞的等效質量，此等效質量隨著電洞濃度的增加而增加，可能是價帶的非拋物線性所致。在樣品中觀察到弱局域效應，而相位退相干的主要機制是載子-載子散射。

在論文的第二部分，我們研究調變摻雜鍺錫量子井。我們通過減壓化學氣相沉積法成長三種調變摻雜鍺錫量子井結構，其鍺錫量子井的錫濃度分別為6.1％，7.5％和11.1％。這些樣品在低溫 (1 - 10 K)下皆觀察到明顯的SdH振盪和量子霍爾平台。透過變溫的SdH振盪得到上述三種結構的電洞等效質量分別為0.103 m0、0.091 m0和0.083 m0。Dingle比分析顯示主要的散射機制是大角度散射如合金散射或介面粗糙散射。

我們使用Sentaurus TCAD套件以六能帶k·p法來模擬鍺錫能帶結構，並使用Nextnano3套件以自洽薛丁格-卜松方程來計算鍺錫量子井中二維電洞的等效質量，以研究量子井厚度、錫濃度和應變對電洞等效質量的影響。因為鍺錫量子井的壓縮應變隨錫濃度增加而增強，導致等效質量隨之降低，初步實驗數據與模擬結果吻合。

關鍵字

鍺/鍺矽異質結構 ； 鍺錫/鍺異質結構 ； 二維電洞氣 ； 離子佈值 ； 調變摻雜 ； 等效質量 ； 自洽薛丁格-卜松方程式

並列摘要

Spintronic devices such as spin-FETs are considered the next-generation logic devices for ultra-low power applications by Rashba spin-orbit coupling (SOC) to manipulate the spin orientations. On the other hand, the SOC was also used to manipulate the spins for fast-operation qubits. Among group-IV materials, Ge(Sn) are quite promising due to their strong SOC and they are compatible with the CMOS process. Besides, the mobility in Ge(Sn) can be high by the formation of two-dimensional hole gases (2DHGs) and their light effective hole masses. Therefore, the 2DHG characteristics in Ge(Sn) such as magnetoresistance and effective mass is investigated in this thesis.

Most of the prior Ge 2DHG system were formed by modulation-doping technique. Here we use ion implantation to modulate the 2DHG density in a Ge/GeSi heterostructure. The hole density is linearly related to the implant dose. While a higher dose leads to a higher 2DHG density, the mobility is reduced due to the stronger impurity scattering induced by ion implantation. The effective masses of 2DHGs are extracted from temperature-dependent Shubnikov-de Haas oscillations and increase with the hole density, probably due to the non-parabolicity of the valence bands. Weak localization effect in the samples were observed. The main mechanism of phase decoherence is carrier-carrier scattering.

In the second part of this thesis, modulation-doped GeSn quantum well (QW) are investigated. Three modulation-doped GeSn QW structures were epitaxially grown by reduced pressure chemical vapor deposition. The Sn fraction of the GeSn QWs are 6.1%, 7.5%, and 11.1%. Clear SdH oscillations and quantum Hall platueas were observed for those samples at cryogenic temperatures (1 - 10 K). By analyzing temperature-dependent SdH oscillations, the effective hole masseswere extracted to be 0.103 m0, 0.091 m0, and 0.083 m0, for the GeSn devices with Sn fractions of 6.1%, 7.5%, and 11.1% respectively. The Dingle ratio analysis showed that the main scattering mechanism is large-angle scattering such as alloy scattering or interface roughness scattering.

Sentaurus TCAD software and a six-band k·p method were used to simulate the GeSn band structures. Nextnano3 software and a self-consistent Schrödinger-Poisson equation were used to calculate the 2DHGs effective mass in GeSn QWs to investigate the effects of QW thickness, Sn fraction, and its strain on the effective hole mass. The simulation results suggested the effective mass decreases as the Sn fraction in the GeSn QW increases since the compressive strain of the GeSn QW layer increases with its Sn concentration. The preliminary experimental data followed the simulation predictions.

並列關鍵字

Ge/GeSi heterostructure ； GeSn/Ge heterostructure ； two-dimensional hole gas ； ion implantation ； modulation-doped ； effective mass ； self-consistent Schrodinger-Poisson equation

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