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Raman scattering and ultraviolet-visible reflectivity have been used to characterize the structural and electronic changes that occur in GaAs during ion implantation and subsequent annealing. In this work, the damaged structure is modelled as an amorphous GaAs matrix embedded with GaAs microcrystals. The longitudinal-optic (LO) Raman-line characteristics were monitored to determine the amorphous volume fraction, the average microcrystal diameter and, for the annealed samples, the carrier concentration. An oscillator analysis of the reflectivity spectra, along with the effective medium approximation, was carried out to determine the linewidths of the interband peaks and the amorphous volume fraction in the damage layer. To determine damage depth profiles, spectra were taken as a function of the amount of material removed via chemical etch. A new method of interpreting reflectivity spectra was developed to deal with the etchant-induced roughness. This roughness reduced the reflectivity by a constant factor in the region between 4.5 and 5 eV. The ratio between reflectivities at 4.55 and 4.75 eV was monitored to determine qualitatively the amount of damage. The annealing studies show that structural recovery occurs at a lower temperature than that for which electrical activation occurs. The depth profile of a sample annealed at 400°C reveals that nucleation takes place not only at the boundary between the damaged and undamaged layers (i.e., "epitaxial regrowth") but also at the microcrystal/amorphous interfaces within the damage layer. The oscillator analysis of the dielectric properties was further developed, and a connection was established between the Strengths, positions, and linewidths of the interband oscillators and the shift in position of the LO Raman line. The results indicate that the static dielectric constant is independent of microcrystal size. A comparison between (211) and (100) oriented Si-implanted GaAs was done as well, showing greater near surface damage and a shallower total damage layer for the (211) samples. Finally, a method for characterizing damage, based on the observed shifts of the two-phonon ("2LO") Raman peak as the incident photon energy is varied around the E₁ interband energy (2.9 eV), has been developed. The results suggest that the total mass of the electron-hole pair involved in the scattering process increases even for large (>400 Å) microcrystals. The 525°C annealed sample had little damage, and was studied with this technique. / Ph. D.

published_or_final_version / abstract / toc / Physics / Master / Master of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

This dissertation describes the study of excitonic optical nonlinearities in semiconductors and semiconductor microstructures. The main emphasis is placed on the evolution of optical nonlinearities as one goes from bulk to quantum-confined structures. Included are experimental studies of molecular-beam-epitaxially-grown bulk GaAs and ZnSe, GaAs/AlGaAs multiple-Quantum-Wells (MQW's), and finally, quantum-confined CdSe-doped glasses. The microscopic origins and magnitudes of the optical nonlinearities of bulk GaAs and ZnSe were investigated and the exciton recovery time in ZnSe was measured. A comparison with a plasma theory indicates that in GaAs, band filling and screening of the continuum-state Coulomb enhancement are the most efficient mechanisms, while in ZnSe, exciton screening and broadening are the dominating mechanism for the nonlinearity. The maximum nonlinear index per excited electron-hole pair of ZnSe at room temperature is comparable to that of bulk GaAs and the exciton recovery times are of the order of 100 ps or less. A systematic study of the dependence of the optical nonlinearities on quantum well thickness for GaAs/AlGaAs MQWs and the results of nonlinear optical switching and gain in a 58 A GaAs/AlGaAs MQW are reported and discussed. The maximum change in the refractive index is greatest for the MQWs with the smallest well size and decreases with increasing well size, reaching a minimum for bulk GaAs. The maximum index change per photoexcited carrier increases by a factor of 3 as the well size decreases from bulk to 76 A MQW. A differential energy gain of 0.2 and the contrast of 4 are measured for a 58 MQW using 3 ns laser pulses. The linear and nonlinear optical properties of CdSe semiconductor microcrystallites grown under different heat treatments in borosilicate glasses are investigated. Pump-probe spectroscopic techniques and interferometric techniques were employed to study size quantization effects in these microcrystallites (quantum dots). Nonlinear optical properties due to the transitions between quantum confined electron and hole states are reported for low temperature and room temperature. A relatively large homogeneous linewidth is observed. Single beam saturation experiments for quantum confined samples were performed to study the optical nonlinearities as a function of microcrystallite size. Results indicate that the saturation intensity is larger for smaller size quantum dots.

This dissertation describes the study of various nonlinear optical effects in both bulk and quantum-confined semiconductors. Transverse effects in increasing absorption optical bistability are considered in bulk CdS for both single beam and wave mixing geometries. Measurement of the temporal response of BiI₃ quantum dots is described using degenerate four-wave mixing and explained theoretically. Finally, the experimental techniques developed to measure the one- and two-photon absorption coefficients of CdS quantum dots in glass are described along with the latest theoretical description and interpretation of the experimental spectra. The basic theory of increasing absorption optical bistability is presented along with experimental observation of this effect in CdS at low temperature. Transverse effects in increasing absorption optical bistability were observed in single beam experiments with CdS at low temperatures. The ring structures observed experimentally are explained theoretically. Degenerate four-wave mixing performed with this nonlinearity is theoretically shown to produce new scattering orders compared with a standard Kerr analysis. Experimental observation of these new scattering orders is presented. The temporal response of the nonlinearity in a solution of BiI₃ quantum dots in acetonitrile is determined using degenerate four-wave mixing. The independent contributions to the phase-conjugate signal are determined for both of the spatial gratings induced in the solution. The observed temporal responses indicated that a thermal mechanism was responsible for the nonlinearity. A theoretical analysis based on a thermal nonlinearity is presented which provides good agreement with the observed responses. The experimental techniques necessary to measure the one- and two-photon absorption coefficients of CdS quantum dots are described. The resultant measurements of quantum dot samples with microcrystallites ranging from 3.6 to 10.8 nm in diameter indicate no splitting of the energy levels associated with the hole. Theoretical spectra indicate this can be partially explained by the inclusion of Coulombic effects of the charged electron-hole pair.

High-order terahertz sideband generation (HSG), recently discovered experimentally in semiconductors, is an extreme nonlinear optical phenomenon with physics similar to high-order harmonic generation (HHG) but in a much lower frequency regime. A key concept in understanding the HSG and HHG is the quantum trajectories, where the quantum evolution of particles under strong fields can be essentially captured by a small number of quantum trajectories that satisfy the stationary phase condition of the Dirac-Feynmann path integral. However, in contrast to HHG in atoms and molecules, HSG in semiconductors can have interesting effects due to nontrivial “vacuum” states of band materials. A rich structure of the Bloch states in condensed matter systems would lead to a variety of phase effects in extreme nonlinear optics. / In this thesis, we show that in semiconductors with nontrivial gauge structures in the energy bands, the curved quantum trajectory of an electron-hole pair under a strong elliptically polarized terahertz field can accumulate a geometric phase. In particular, the geometric phase becomes the famous gauge invariant Berry phase for a cyclic trajectory. Taking monolayer MoS₂ as a model system, we show that the Berry phase appears as the Faraday rotation angle in the pulse emission from the material under short-pulse excitation. This finding reveals the Berry phase effect in the extreme nonlinear optics regime for the first time. / We further apply the Berry phase dependent quantum trajectory theory to biased bilayer graphene under strong elliptically polarized terahertz fields. The biased bilayer graphene with Bernal stacking has similar Bloch band features and optical properties to the monolayer MoS₂, such as the time-reversal related valleys and valley contrasting optical selection rule. However, the biased bilayer graphene has much larger Berry curvature than that in monolayer MoS₂, which leads to a large Berry phase of the quantum trajectory and in turn a giant Faraday rotation of the optical emission (∼ 1 rad for a THz field with frequency 1 THz and strength 8 kV/cm). This surprisingly big angle shows that the Faraday rotation can be induced more efficiently by the Berry curvature in momentum space than by the magnetic field in real space. It provides opportunities to use bilayer graphene and THz lasers for ultrafast electro-optical devices. / Finally, we study the geometric phase of a quantum wavepacket driven adiabatically along a trajectory in a parameterized state space. Inherent to quantum evolutions, the wavepacket can not only accumulate a quantum phase but may also experience dephasing, or quantum diffusion. We show that the diffusion of quantum trajectories can also be of geometric nature as characterized by the imaginary part of the geometric phase. Such an imaginary geometric phase results from the interference of geometric phase dependent fluctuations around the quantum trajectory. As a specific example, we again study the quantum trajectories of HSG in monolayer MoS₂. We find that while the real part of the geometric phase leads to the Faraday rotation of the linearly polarized light that excites the electron-hole pair, the imaginary part manifests itself as the polarization ellipticity of the terahertz sidebands which can be measured experimentally. The discovery of the geometric quantum diffusion extends the concept of geometric phases. / 最近，在實驗上發現了半導體中的一個極端非線性光學現象，即高次太赫茲邊帶產生(HSG)。它是原子与分子系统里的高次谐波产生(HHG)在太赫茲頻域的一個推广。HSG与HHG的關鍵物理過程均可用量子轨道理论解释，其中粒子的路徑積分描述的量子演化由若干滿足穩相近似條件的量子軌道主導。但是HHG与HSG之間存在着本質區別，即半導體的“真空態”可以具備一些非平凡的拓撲結構，從而給極端非線性光學领域帶來許多有趣的物理效應。 / 在這篇論文中，我們發現在強橢圓偏振太赫茲場作用下的具有非平凡规范結構的半導體中，電子空穴對的量子軌道可以積累一個非零的幾何相。特別地，如果我們考慮週期量子軌道，這個幾何相便成為著名的規範不變的Berry相。我們取單層MoS₂為模型系統，發現在光脉衝激勵下的材料中的光信號經歷一個法拉第旋轉，而且轉角由量子軌道的Berry相給出。這個發現首次揭示了極端非線性光學領域內的Berry相效應。 / 我們進一步將含Berry相效應的量子軌道理論應用于強橢圓偏振太赫茲場作用下的雙層石墨烯中。Bernal堆疊的雙層石墨烯与單層MoS₂具有某些相似的能帶結構与光學性質，例如兩者都具有兩個時間反演對稱的谷，且兩個谷內具有不同的躍遷選擇定則。但是雙層石墨烯有遠遠大於單層MoS₂的Berry曲率，從而其內的量子軌道也會積累一個遠遠大於單層MoS₂的Berry相。這個Berry相可以導致光信號巨大的法拉第旋轉（在頻率1THz以及場強8kV/cm的太赫茲場下約為1rad）。這個傳統方法下所無法產生的巨大法拉第旋轉說明比起實空間內的磁場，動量空間內的Berry曲率可以更加有效地誘發光信號的法拉第旋轉。我們的結果可以促使雙層石墨烯以及太赫茲激光在超快光電設備中的應用。 / 最後，我們考慮具有非平凡規範結構的參數空間內的量子波包在絕熱驅動下的量子演化。在演化過程中，這個波包不僅可以獲得一個量子相位，而且會經歷退相干（即量子擴散）。我們發現波包的一部分量子擴散具有幾何性質，而且這部分量子擴散可以表示為一個复幾何相的虛部。這個复幾何相可以通過量子軌道附近的帶有幾何相的量子路徑的相干來解釋。作為例子，我們研究了強橢圓偏振太赫茲場作用下的單層MoS₂中的量子軌道的复幾何相。我們發現此幾何相的實部誘發光的法拉第旋轉，而虛部則表現為邊帶光信號的橢圓偏振度，並且進而可以從實驗上進行測量。我們關於虛幾何相的研究拓展了幾何相這一概念的新領域。 / Yang, Fan = 強太赫茲場下半導體中的量子軌道的Berry相 / 楊帆. / Thesis Ph.D. Chinese University of Hong Kong 2014. / Includes bibliographical references (leaves 71-75). / Abstracts also in Chinese. / Title from PDF title page (viewed on 13, September, 2016). / Yang, Fan = Qiang tai he zi chang xia ban dao ti zhong de liang zi gui dao de Berry xiang / Yang Fan. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only.

A metal-oxide-silicon (MOS) tunneling diode is utilized to embed beta-FeSi 2 precipitates and give strong 1.5 tam electroluminescence at 80 K. And this simple MOS structure with beta-FeSi2 was fabricated by Fe ion implantation and rapid thermal oxidation (RTO) at 900°C, which is fully compatible with ultra-large scale integration (ULSI) processes. / beta-FeSi2 precipitates are also incorporated into a silicon-on-insulator (SOI) rib waveguide and a p+-i-n+ photodetector is monolithically integrated with this SOI rib waveguide. The photoresponse to 1550 nm laser of beta-FeSi2 precipitates was observed and compared to intrinsic silicon. / Beta-phase iron disilicide (beta-FeSi2) is a semiconductor that can act as a light emitting material at the wavelength of 1.55 mum and can also be grown epitaxially on Si substrates. In this thesis, Fe ion implantation into silicon using a metal vapor vacuum arc (MEVVA) ion source was performed to synthesize nano-scale beta-FeSi2 precipitates in silicon matrix. The implantation was performed at ∼-120°C and the effects of silicon substrate and conditions for the following thermal annealing on luminescence properties were studied. The samples were characterized by employing various analytical techniques including Rutherford backscattering spectrometry (RBS), transmission electron microscopy (TEM), atomic force microscope (AFM), photoluminescence (PL), and electroluminescence (EL). / It is found that the PL intensity is optimized in p-100 silicon substrates (with the resistivity of 15-25 O·cm) using Fe ion implantation at a voltage of 80 kV and dosage of 5x1015 cm -2. Formation of beta-FeSi2 can be completed after rapid thermal annealing (RTA) and strong photoluminescence is present. We also found that RTA could maintain the strain in beta-FeSi2 precipitates and there exists an epitaxial relationship between beta-FeSi2 and silicon. Additional furnace annealing at 850°C can relax the strain in beta-FeSi2 precipitates. / The development of both modern microelectronics and lightwave technologies has enabled the establishment of the Internet which has introduced a profound change in our everyday lives. Because of Moore's law, computing today is limited less by the computation ability of microprocessors than by the rate at which the processor can communicate with the outside world. Lightwave technology has had many successes in the long-haul communication field over the past decade. The advantages of lightwave technology over conventional electronics are becoming apparent for shorter and shorter reach applications and lightwave communications may eventually replace copper-based interconnects in microelectronics. To make possible optical interconnects, optical components, especially light emitters may be needed to be integrated on conventional silicon microchips. However, to date, no efficient on-chip silicon-based light emitter is fabricated in silicon photonics. / Sun, Caiming. / Adviser: Hon K. Tsang. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3703. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

New transparent p- and n-type semiconductors and luminescent materials have been prepared and characterized. Synthesis, structures, optical and electrical properties of new chalcogenide fluoride p-type transparent semiconductors MCuQF (M=Ba, Sr; Q=S, Se, Te) are described. Band-gap tuning and improvement in conductivity through p-type doping are demonstrated in the family. The new Ag sulfide fluoride BaAgSF has been prepared, and its optical and electrical properties have been examined. Phase stabilization as well as optical and electrical properties of the p-type conductors BaCu₂S₂ and BaCu₂Se₂ are considered. New n-type transparent conducting films of W-doped In₂O₃ and W-doped zinc indium oxide (ZIO) have been prepared by pulsed laser deposition, and their electrical properties have been examined. Results on new transparent thin-film transistors containing SnO₂ or ZIO are also presented. Strong near-infrared luminescence of BaSnO3 is described, and the emission brightness is correlated to the crystallite size of assembled nanoparticles. Syntheses, structures, and optical properties of (La,Y)Sc₃(BO₃)₄:Eu³⁺, (Ba,Sr)Sc₂(BO₃)₄:Eu²⁺, and LuAl₃(BO₃)₄:Ln³⁺ (Ln=Eu, Tb, Ce) have been considered with emphasis on relations between structures and optical properties. Finally, the synthesis and luminescence properties of new potential X-ray phosphors Lu₂O₂S:Ln³⁺ (Ln=Eu, Tb) are summarized. / Graduation date: 2005

The behavior and application of organic semiconductor materials depend strongly on their molecular structure, and molecular interactions. Several studies of intermolecular interactions in functionalized polyacene materials are presented. The degree and onset of aggregation of a functionalized anthradithiophene derivative was studied as a function of concentration in two different host matrices. The molecular environment was found to influence the degree and onset, but not the nature of aggregate formed. The effect of aggregation on photoconductivity was also studied. In a blend of two different anthradithiophene derivatives, the intermolecular separation was found to affect the nature of the interaction, transitioning from energy transfer at large intermolecular distances to the formation of an emissive excited state complex at smaller intermolecular distances. This complex was shown to have effects on both photoluminescence and photoconductivity. Finally, a single molecule fluorescence microscopy system was built and characterized. Software was written to process data produced from the system and several classes of functionalized polyacenes were studied at the single molecule level. In particular, the photophysics and molecular orientation of various derivatives were quantified. A new solution-processable, photoconductive, polycrystalline host material was found to be suitable for single molecule imaging, and the molecular orientations of individual molecules were found to depend on both their molecular structures and their local nano-environment. / Graduation date: 2013