臺灣博碩士論文加值系統

在本文中,我們測量並分析了硒化鋅鎘磊晶薄膜的介電性質。在溫度200 K~460 K 及外加電場頻率 20 Hz ~ 1M Hz 下，對硒化鋅鎘磊晶薄膜電容(capacitance)及耗散值(dissipation)作量測。在測量中我們觀察到了硒化鋅鎘磊晶薄膜Debye-like relaxation 的現象，這個現象可以從載子重新分佈的行為來了解，這些在缺陷位置上的載子重新分佈時所需的活化能是由熱能所提供，因此外界的熱能，就成了載子重新分佈的能量來源。介電常數隨頻率的關係可由Cole-Cole 函數，和Fuoss-Kirwood 方程式去分析。本實驗利用電容方法與導電係數的求法可測得硒化鋅鎘磊晶薄膜活化能大小。兩種方法均可得到活化能大約為0.65 eV。另外，我們也發現，在這一系列測量的樣品中，活化能的大小與樣品中鎘的濃度有關，當鎘的濃度增加時，活化能會相應的減少。對於這個現象，我們應用了 four-center model 來解釋，意思是指當硒化鋅鎘磊晶薄膜中鎘的濃度改變時，以缺陷為中心周圍的原子組態會相應變化，因此活化能會有差異。此外，我們也探討了載子的傳輸機制，藉由資料的分析我們認為古典概念的 correlated barrier hopping (CBH) 較符合載子的傳輸機制。
除了介電性質的研究外，我們也做了光學的測量。在室溫Raman scattering 的實驗方面，我們觀察了樣品中聲子系統的振動模式，以及不同的鎘濃度對聲子LO mode的影響。我們進一步也發現硒化鋅鎘磊晶薄膜具有單一振動模式。我們也觀察到ZnSe-like 縱向聲子模式隨著鎘濃度增加有紅移的現象，我們把這種紅位移歸因於鎘在硒化鋅鎘中所產生的無序(disorder)與應力(strain)效應。為了分析硒化鋅鎘磊晶薄膜的無序現象，我們引入了spatial correlation model，從實驗數據的分析中，我們的phonon correlation length LZnSe 隨著鎘的含量的增加而有變小的趨勢，這個結果符合理論預期。

Abstract

Dielectric and conductivity properties of Zn1-xCdxSe epifilms were studied as a function of temperature from 200 K to 460 K and frequency from 20 to 1M Hz. A Debye-like relaxation in the dielectric response has been observed, which is explained in terms of the presence of charge redistribution due to electron hopping among defects. The frequency dependence of dielectric relaxation is analyzed by the Cole-Cole function and Fuoss-Kirwood equation. The activation energy of dielectric relaxation is estimated to be 0.65 eV, which is in good agreement with the values obtained from dc conductivity as well as capacitance measurements. It is found that the activation energy decreases with increasing Cd content and this behavior is interpreted in terms of the four-center model, in which the number of Cd atoms appearing in the nearest-neighbor sites of a defect can have four possible configurations. In addition, we demonstrate that the transport mechanism of the carrier conduction in Zn1-xCdxSe epilayers can be well described by the correlated barrier hopping model.
We also report on the vibrational properties of Zn1-xCdxSe determined by Raman scattering measurements. We found that Zn1-xCdxSe exhibits single mode behavior. The broadening in linewidth and asymmetry can be interpreted in terms of the spatial correlation (SC) model.

Content

摘要-III
Abstract-IV
List of Figures-V
List of Table 1-VII
Chapter 1 Introduction to ZnxCd1-xSe epifilms
Introduction -1
Reference -3
Chapter 2 Dielectric Properties of Zn1-xCdxSe epifilms
2.1 Theory
2.1.1 The complex permittivity-4
2.1.2 The frequency dependence of dielectric constant-8
2.1.3 The Cole-Cole plot and Fuoss-Kirkwood description-11
2.1.4 Case of point dipole in crystal lattice-15
2.1.5 Correlated-barrier-hopping (CBH) model-17
2.1.6 Reference-20
2.2 Sample Preparation
2.2.1 Introduction-21
2.2.2 Growth conditions of samples-22
2.3 Experimental setup-23
2.4 Results and Discussion-24
2.5 Reference-45
Chapter 3 Optical Properties of Zn1-xCdxSe epilfilms
3.1 Theory and Experimental Apparatus-47
3.1.1 Introduction- 47
3.1.2 Stokes shift and Anti Stokes shift- 47
3.1.3 Raman Scattering Apparatus-50
3.1.4 Spatial Correlation Model -55
3.2 Experimental Detail-57
3.3 Results and Discussion-58
3.4 Reference-67
Chapter 4 Conclusion
Conclusion -68

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