稀磁半導體是通過摻雜磁性過渡金屬進入半導體基體而形成的。他們是一類部分磁性離子占居半導體本身離子態的新型半導體，擁有很多優異的特性，因而在工業和商業等方面具有廣泛的應用和巨大的潛能。磁性離子的出現使得他們擁有了有別于普通半導體材料的獨特磁性特性。室溫鐵磁性作爲特性之一已經在很多稀磁半導體中觀察到。然而，稀磁半導體中鐵磁序的來源一直備受爭議，因爲磁性摻雜劑本身擁有磁性性能。因此很難確定得到的産物確實是半導體與磁性過渡金屬摻雜劑的固溶體，還是半導體基包裹磁性過渡金屬團簇、沈析物或者二相，這些包裹的成分呈現了磁性特性。另外，絶大部分磁性過渡金屬在半導體基中溶解率都比較低。因此，人們越來越感興趣通過摻雜非過渡金屬摻雜劑來開發磁性系統。在這類系統中，可以排除鐵磁信號來源于摻雜劑，並且由于沒有鐵磁相互吸引力，可以實現較高的摻雜率。近來這樣的磁性系統已經被發展，例如碳摻雜的氧化鋅，碳摻雜的氧化钛等。 / 受這些想法的啓發，我們選擇非磁性元素鋁、鎢作爲摻雜劑，並將他們摻雜進入B₄C和SiC半導體基體中。利用高溫固相燒結法，以碳，硼和矽，鋁和鎢粉末作爲起始材料在2200ºC合成了鋁摻雜的B₄C，鋁摻雜的SiC和鎢摻雜的B₄C化合物。在鋁摻雜的B₄C體系中，鋁的摻雜沒有改變晶體結構。所有的摻雜樣品均保持與未摻雜樣品相同的晶體結構，即B₄C（ICDD-PDF：75-0424，空間群R-3m）。並且所有的結果，包括X射線衍射圖，拉曼譜，X射線光電子譜，都表明C-B-C鏈上的部分碳原子被摻雜的鋁原子所取代。替代導致這個系統中隨著鋁含量的增加而表現出鐵磁特性。更重要的是，在這個系統中觀察到了典型的自旋玻璃行爲。而在鋁摻雜的SiC系統中，所有的樣品，包括未摻雜樣品和摻雜樣品，都是包含了6H，4H和15R多型的混合物。相比之下，在這個系統中鐵磁序的變化與鋁摻雜的B₄C系統中的也有所不同。這表明在這兩種半導體中由于摻雜效應誘導的鐵磁序機制是不一樣的。爲了進一步的調查，嘗試以錳作爲摻雜劑。錳摻雜的SiC結果證實在SiC系統中鐵磁特征確實不依賴于摻雜劑。這兩個SiC系統證明了在摻雜的SiC系統中，鐵磁行爲與混合物中的一種SiC多型(15R-SiC)而非摻雜劑有關。鎢摻雜的B₄C是一個有趣的體系。 / 輕摻雜樣品呈現了鐵磁特性，但隨著鎢含量的增加鐵磁序減弱直至消失。在高摻雜的樣品中産生了B₂W₅二相，實驗證明它是沒有鐵磁性能的。作為比較，在相同燒結條件下合成了鎢和鋁共摻雜的B₄C。發現鋁和鎢各自發揮摻雜效應并促成共同的結果。一個虛弱的自旋玻璃特性在這個共摻雜的體系中被觀察到。 / 總之，四個不同的摻雜體系被合成。他們都表現出磁性特性的變化。其中鋁摻雜的B₄C體系還表現出自旋玻璃特性。這項工作證明非磁性摻雜劑確實能影響半導體的磁性特性，並促使摻雜的半導體表現出鐵磁性能。這使得這些摻雜的半導體表現的就像稀磁半導體材料一樣。這項工作提供了理想的系統來研究和闡明非過渡金屬摻雜的寬禁帶半導體中自旋序的來源問題。 / Diluted magnetic semiconductors (DMSs) are formed by doping magnetic transition metals (TMs) into semiconductor matrices. They are a type of new semiconductors with fraction of magnetic ions on the sites of semiconductor constituent ions. DMSs have some excellent properties which enhance their potentials for using in a wide range of industrial and commercial applications. The presence of substituted magnetic ions results in unique magnetic properties which distinguish them from the ordinary semiconductor materials. Room temperature ferromagnetic (FM) ordering，as one of the unique properties, has been observed in many DMSs. However, the origin of FM ordering in the DMSs is still controversial due to the presence of intrinsic magnetic properties possessed by the magnetic TM dopants. It is difficult to determinate whether the obtained products is indeed a solid solution of the semiconductor matrix and the magnetic TM dopant or it remains as semiconductor matrix engulfing a magnetic TM clusters, precipitates, or second phases that are responsible for the observed magnetic properties. Moreover, most of the magnetic TMs have extremely low solubility in the semiconductor matrix. Therefore, an increasing interest is provoked on developing magnetic systems by doping non-TM dopants. In this new type of system, the origin of the FM signal from the dopant can be eliminated, and higher doping ratio in the system can be expected due to no magnetic attraction. Recently, such magnetic systems have been obtained, for examples C-doped ZnO and C-doped TiO₂. / Inspired by these ideas, we had selected the non-magnetic element Al and W as the dopants, and introducing them to the semiconductor matrices, B₄C and SiC. The Al-doped B₄C, Al-doped SiC and W-doped B₄C compounds were synthesized by high temperature solid-state sintering of carbon, boron/silicon, and aluminum/tungsten powders at 2200ºC. In the Al-doped B₄C system, doping of Al had not altered the crystal structure. All of the doped samples retained the same structure as the un-doped sample, which corresponded to the B₄C (ICDD-PDF:75-0424, space group R-3m). All results, including results of XRD, Raman and XPS, indicated that the parts of the carbon atoms on the C-B-C chains were replaced by some of the doped Al atoms. The result of this replacement was that the system showed FM feature with increasing of the Al content. More important, a typical spin-glass behavior was observed in this system. While in the Al-doped SiC system, all of the samples, including un-doped and doped samples, were mixtures containing 6H-SiC, 4H-SiC and 15R-SiC polytype. By comparison, the changes in the FM ordering in this system were also different from those in the Al-B₄C system. This indicated that the induced mechanisms of FM ordering due to the doping effects on the two semiconductors were not the same. To further investigation, attempts were made by using Mn as dopants. The result of Mn-doped SiC confirmed that the FM feature in SiC systems indeed did not depend on with the dopants. The two doped SiC systems demonstrated that the FM behavior might be related to one type of SiC polytype (15R-SiC) in the mixture rather than dopants. / The W-doped B₄C system was an interesting one. The lightly doped sample showed FM feature, while the FM ordering became weak and disappeared with the increase of W content. A second phase B₂W₅ was produced in the highly doped samples. It was demonstrated that B₂W₅ had no FM feature. To compare, (W, Al) co-doped B₄C were also synthesized under the same heating condition. It was found that the doping effects of Al and of W played its own individual role, and facilitated a common outcome. A weak spin-glass feature was observed in the co-doped system but not in the W-doped system. / In summary, four systems were fabricated. All of them showed changes in magnetic behaviors. The Al-doped B₄C system showed spin-glass features for the first time. The results of this work demonstrated that the non-magnetic dopants indeed could affect the magnetic properties of semiconductors, and urged the doped-semiconductors to exhibit FM features. This made the doped-semiconductors behaved as DMS materials even there was no 3d electron in the whole of systems. This work provides an ideal system to illustrate the origin of spin order in non-TM doped wide-gap semiconductors. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Zhang, Caihong = 摻雜半導體中的誘導磁性特性 / 張彩虹. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references. / Abstracts also in Chinese. / Zhang, Caihong = Shan za ban dao ti zhong de you dao ci xing te xing / Zhang Caihong. / Abstract --- p.i / 摘要 --- p.iv / Acknowledgments --- p.vi / Table of contents --- p.vii / List of figure captions --- p.x / List of Table captions --- p.xiii / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Diluted magnetic semiconductors --- p.1 / Chapter 1.1.1 --- Origin --- p.1 / Chapter 1.1.2 --- Background --- p.2 / Chapter 1.1.3 --- Current development --- p.3 / Chapter 1.1.4 --- Present challenges --- p.5 / Chapter 1.2 --- Objectives of this work --- p.7 / References --- p.9 / Chapter Chapter 2 --- Magnetism and Semiconductors --- p.12 / Chapter 2.1 --- Magnetism --- p.12 / Chapter 2.1.1 --- Types of magnetism --- p.12 / Chapter 2.1.2 --- Spin glass --- p.15 / Chapter 2.1 --- B₄C --- p.18 / Chapter 2.2.1 --- Introduction --- p.18 / Chapter 2.2.2 --- Properties and applications --- p.19 / Chapter 2.2.3 --- Synthesis and phase diagram --- p.20 / Chapter 2.2.4 --- Structure and characterization --- p.21 / Chapter 2.2 --- SiC --- p.25 / Chapter 2.3.1 --- Properties and applications --- p.25 / Chapter 2.3.2 --- SiC polytypes --- p.26 / Chapter 2.3.3 --- SiC-base DMSs --- p.28 / References --- p.30 / Chapter Chapter 3 --- Methodology and Instrumentation --- p.34 / Chapter 3.1 --- Sample preparation --- p.34 / Chapter 3.2 --- Sample characterization --- p.36 / Chapter 3.3 --- Instrumentation --- p.37 / Chapter 3.3.1 --- High temperature furnace --- p.37 / Chapter 3.3.2 --- X-ray diffraction --- p.38 / Chapter 3.3.3 --- Raman spectroscopy --- p.39 / Chapter 3.3.4 --- X-ray photoelectron spectroscopy --- p.40 / Chapter 3.3.5 --- Scanning electron microscopy and Energy-dispersive X-Ray spectroscopy --- p.41 / Chapter 3.3.6 --- Transmission electron microscopy --- p.42 / Chapter 3.3.7 --- Physical properties measuring system --- p.43 / Chapter 3.3.7.1 --- Vibrating sample magnetometer --- p.43 / Chapter 3.3.7.2 --- AC measurement system --- p.44 / References --- p.46 / Chapter Chapter 4 --- Al-doped B₄C System --- p.47 / Chapter 4.1 --- Introduction --- p.47 / Chapter 4.2 --- Experiments --- p.48 / Chapter 4.3 --- Results and discussion --- p.49 / Chapter 4.3.1 --- Confirmation of the un-doped B₄C sample --- p.49 / Chapter 4.3.2 --- Confirmation of Al into the B₄C matrix --- p.51 / Chapter 4.3.3 --- Evaluation of magnetic properties --- p.59 / Chapter 4.3.4 --- Magnetism in the Al-B₄C --- p.62 / Chapter 4.4 --- Conclusions --- p.65 / References --- p.66 / Chapter Chapter 5 --- Al-Doped SiC System --- p.68 / Chapter 5.1 --- Introduction --- p.68 / Chapter 5.2 --- Experiment I --- p.69 / Chapter 5.2.1 --- Experimental condition 1 --- p.69 / Chapter 5.2.2 --- Results and discussion --- p.70 / Chapter 5.2.2.1 --- X-ray diffraction patterns --- p.70 / Chapter 5.2.2.2 --- Raman spectra --- p.72 / Chapter 5.2.2.3 --- Transmission electron microscopy images --- p.77 / Chapter 5.2.2.4 --- DC magnetization --- p.79 / Chapter 5.2.2.5 --- AC susceptibilities --- p.80 / Chapter 5.3 --- Experiment II --- p.81 / Chapter 5.3.1 --- Experimental condition 2 --- p.81 / Chapter 5.3.2 --- Results and discussion --- p.82 / Chapter 5.4 --- Experiment III --- p.88 / Chapter 5.4.1 --- Experimental condition 3 --- p.88 / Chapter 5.4.2 --- Results and discussion --- p.89 / Chapter 5.5 --- Mn-doped SiC --- p.91 / Chapter 5.5.1 --- Experimental condition --- p.91 / Chapter 5.5.2 --- Results and discussion --- p.92 / Chapter 5.6 --- Summary --- p.93 / Chapter 5.7 --- Conclusions --- p.94 / References --- p.95 / Chapter Chapter 6 --- W-doped and (W, Al)-codoped B₄C System --- p.98 / Chapter 6.1 --- Introduction --- p.98 / Chapter 6.2 --- Experiments --- p.99 / Chapter 6.3 --- Results and discussion --- p.101 / Chapter 6.3.1 --- X-ray diffraction patterns --- p.101 / Chapter 6.3.2 --- Raman spectra --- p.104 / Chapter 6.3.3 --- Scanning electron microscopy images --- p.106 / Chapter 6.3.4 --- DC magnetization --- p.108 / Chapter 6.3.5 --- AC susceptibilities --- p.111 / Chapter 6.4 --- Conclusions --- p.113 / References --- p.114 / Chapter Chapter 7 --- Summary and Suggestions for Future Work --- p.115 / Chapter 7.1 --- Summary --- p.115 / Chapter 7.2 --- Suggestions --- p.117 / References --- p.120

published_or_final_version / abstract / toc / Physics / Doctoral / Doctor of Philosophy

Future electronic devices having dimensions on the nanometer scale will rely on the resultant quantum effects for their operation. In this project, these quantum effects were investigated through the theoretical modeling and computer simulation of a confined twodimensional electron gas in a semiconductor heterostructure. Assuming hardwall boundaries and sharp geometrical features, the nanostructure conductance has been calculated by finding transverse eigenvalues and eigenfunctions, computing hopping integrals for a one-dimensional tight binding lattice, determining the Greens' propagators, and then finally evaluating the transmittance. From the transmittance, the conductance was determined.A structured and modular computer program in FORTRAN was developed to investigate the effects of geometrical modifications on the conductance of ballistic nanochannels. The program has been designed in such a way that the user need only supply the nanostructure specifications to an input data file. The program then uses this data file to perform the calculations. A separate, user-friendly program has been developed to form the data file. The program is such that additions and modifications can be easily made in the future. / Department of Physics and Astronomy

Thesis (Ph. D.)--University of Hong Kong, 2000. / Includes bibliographical references.

Thesis (Ph. D.)--Michigan State University. Physics, 2008. / Title from PDF t.p. (Proquest, viewed on Aug. 25, 2009) Includes bibliographical references (p. 153-158). Also issued in print.

by Pei-hsuon Chan. / Parallel title in Chinese characters. / Thesis (M.Ph.)--Chinese University of Hong Kong, 1988. / Bibliography: leaves 227-231.

Graduation date: 1994

Thesis (Ph. D.)--University of Washington, 2007. / Includes bibliographical references (leaves 181-186).

Thesis (Ph.D.) -- McMaster University, 1997. / Includes bibliographical references Also available via World Wide Web.

Graduation date: 1995