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Title: | 分子束磊晶成長鉍薄膜的電性量測與霍爾效應分析 Electrical Properties and Hall measurement Analysis of Bismuth thin film grown by MBE |
Authors: | Ding-Lun Wu 吳定倫 |
Advisor: | 林浩雄 |
Keyword: | 鉍,反應式離子蝕刻,傳輸線模型,尺寸效應,霍爾效應,表面態, Bismuth,reactive ion etching (RIE),transmission line model (TLM),size effect,Hall effect,surface states, |
Publication Year : | 2019 |
Degree: | 碩士 |
Abstract: | 本論文利用變溫霍爾效應量測、解析鉍薄膜的塊材(bulk)與表面(surface)電性。鉍薄膜係以分子束磊晶法成長在SOI (Silicon on insulator)基板之上。以XRD、EBSD檢驗,薄膜大部分的domains為(0003)垂直晶向。薄膜之後度為54 nm,除了薄膜之量測外,我們以反應式離子蝕刻(Reactive-Ion Etching, RIE)技術將鉍薄膜逐步減薄,並分別於38、30、16 nm的厚度下進行變溫霍爾效應量測。
我們首先對零磁場下的電導進行分析,援用文獻上金屬態表面與半導體態塊材模型 [14],對四片樣品的電導進行解析擬合。我們發現四片樣品在T<50K時,均為表面態的特性,塊材的特性可以忽略。其次我們觀察這個溫度下的Rxx與Rxy磁阻。由於在此溫度下的Rxx磁阻均不為零,由古典金屬態傳導理論,我們推斷表面金屬態傳導應屬two-band model。我們採用DFT計算 [16],以及ARPES對鉍表面態的費米表面量測結果 [17],估計表面電洞、電子濃度比值為6.4;並假設此比例不隨溫度改變。除此之外,我們也假設塊材內部為本質半導體特性:n=p=ni。基於上述的假設,我們以表面、塊材均為two-band model的四通道模型對四片樣本的變溫霍爾量測結果進行解析。 解析的結果顯示表面電子的移動率約為電洞的兩倍。三片經RIE減薄的樣品之移動率相當接近;其值約為500至1000 cm2/Vs。而T<50K下,表面電洞濃度隨厚度減薄而降低;其範圍在0.6×1013至1.2×1013 cm-2。但原始成長樣品的特性較為不同,其移動率則比這三片要高三倍,濃度只有0.4×1013 cm-2。三片經RIE處理樣本的塊材移動率則顯示半導體的特徵,在高溫區有明顯的T-1.5特性,推測是受到acoustic phonon scattering。在低溫區移動率明顯隨溫度下降而降低,推測是低載子濃度顯現粒子性質受到缺陷散射所致。三片樣品的塊材載子濃度顯現類波茲曼溫度分布的半導體性質。在低溫區的活化能與零磁場電導模型的能隙值相當;我們認為是厚度減薄量子能階化把鉍由半金屬轉變為低能隙半導體。原始成長樣品的塊材特性則與三片樣品相近。 文獻上對鉍薄膜的電性研究多限於零磁場電導的解析,本論文則更進一步提出假設模型,利用磁阻的量測對鉍的表面態與塊材進行移動率與載子濃度的完整闡述。 In this thesis, the variable temperature Hall effect is used to measure and analyze the bulk and surface electrical properties of the Bismuth film. The Bismuth thin film is grown on a SOI (Silicon on insulator) substrate by molecular beam epitaxy. According to XRD and EBSD analysis, most of the domains of the Bismuth film are (0003) vertical crystal orientation. The as grown thickness of the film is 54 nm. In addition to the measurement of this film, we use the Reactive-Ion Etching (RIE) technique to gradually thin down the Bismuth film to 38, 30 and 16 nm and use variable temperature Hall measurement to analysis their electrical properties. We first analyze the conductance of Bismuth under zero magnetic field and use two parallel conduction model [14] of metallic surface states and semiconductor bulk states to analyze and fit the conductance of four samples. First, we found that our four samples are all surface states dominate at T < 50K, and the properties of bulks can be neglected. Second, we observe the behaviors of Rxx and Rxy at this temperature range and the value of magnetoresistance below this temperature is not zero. From the classical metal states conduction theory, we conclude that the metallic surface states conduction should belong to the two-band model. We use the DFT calculation [16] and ARPES [17] which used to measure surface states of Bismuth and to estimate the ratio of surface states’ hole and electron concentration. The value is 6.4 as shown and we assume that this ratio does not change with temperature. In addition, we also assume that the interior of bulk is an intrinsic semiconductor property that behaves n = p = ni. Based on above assumptions, we analyze the variable temperature Hall measurement results of four samples with a four - channel model via two-band model fit. The results of the analysis show that the surface electrons have a mobility of about twice as high as holes. The mobility of three RIE-thinned samples is quite close and its value is approximately 500 to 1000 cm^2/Vs. At T<50K, the surfaces hole concentration decreases with thickness reduction and its range from 0.6×10^13 to 1.2×10^13 cm^-2. However, the characteristics of as-grown samples are different from those RIE-treated samples. The mobility is three times higher than the other three samples, and the concentration is only 0.4×10^13 cm^-2. The bulk states mobility of RIE-treated samples shows the characteristics of semiconductor, and has a distinct proportional to T^-1.5 characteristic in the high temperature region, presumably subject to acoustic phonon scattering effect. Moreover, the mobility in low temperature region decreases significantly with decreasing temperature, and it is presumed that due to low carrier concentration which shows the particle properties and caused by defect scattering. The bulk carrier concentration of three RIE- treated samples exhibit the semiconducting properties which are Boltzmann temperature distribution. The activation energy in low temperature region is equivalent to the energy gap of the zero magnetic field conductance model. We believe that because of thickness thinning, the quantum energy grading transforms Bismuth from semi-metal to a low-gap semiconductor. In other literatures, the electrical study of Bismuth films is mostly limited to the analysis of zero-field magnetic conductance. In this thesis further proposes a hypothetical model, using the measurement of magnetoresistance to fully describe surface states transport properties of Bismuth and the mobility and carrier concentration of the bulk Bismuth. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/74484 |
DOI: | 10.6342/NTU201902771 |
Fulltext Rights: | 有償授權 |
Appears in Collections: | 光電工程學研究所 |
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