Global ETD Search

1	GaN heterojunction FET device Fabrication, Characterization and Modeling Fan, Qian 23 November 2009 (has links) This dissertation is focused on the research efforts to develop the growth, processing, and modeling technologies for GaN-based Heterojunction Field Effect Transistors (HFETs). The interest in investigating GaN HFETs is motivated by the advantageous material properties of nitride semiconductor such as large band gap, large breakdown voltage, and high saturation velocity, which make it very promising for the high power and microwave applications. Although enormous progress has been made on GaN transistors in the past decades, the technologies for nitride transistors are still not mature, especially concerning the reliability and stability of the device. In order to improve the device performance, we first optimized the growth and fabrication procedures for the conventional AlGaN barrier HFET, on which high carrier mobility and sheet density were achieved. Second, the AlInN barrier HFET was successfully processed, with which we obtained improved I-V characteristics compared with conventional structure. The lattice-matched AlInN barrier is beneficial in the removal of strain, which leads to better carrier transport characteristics. Furthermore, new device structures have been examined, including recess-gate HFET with n+ GaN cap layer and gate-on-insulator HFET, among which the insertion of gate dielectrics helps to leverage both DC and microwave performances. In order to depict the microwave behavior of the HFET, small signal modeling approaches were used to extract the extrinsic and intrinsic parameters of the device. An 18-element equivalent circuit model for GaN HFET has been proposed, from which various extraction methods have been tested. Combining the advantages from the cold-FET measurements and hot-FET optimizations, a hybrid extraction method has been developed, in which the parasitic capacitances were attained from the cold pinch-off measurements while the rest of the parameters from the optimization routine. Small simulation error can be achieved by this method over various bias conditions, demonstrating its capability for the circuit level design applications for GaN HFET. Device physics modeling, on the other hand, can help us to reveal the underlying physics for the device to operate. With the development of quantum drift-diffusion modeling, the self-consistent solution to the Schrödinger-Poisson equations and carrier transport equations were fulfilled. Lots of useful information such as band diagram, potential profile, and carrier distribution can be retrieved. The calculated results were validated with experiments, especially on the AlInN layer structures after considering the influence from the parasitic Ga-rich layer on top of the spacer. Two dimensional cross-section simulation shows that the peak of electrical field locates at the gate edge towards the drain, and of different kinds of structures the device with gate field-plate was found to efficiently reduce the possibility of breakdown failure. GaN HFETs Semiconductor Growth Fabrication Small Signal Modeling Quantum Drift-Diffusion Modeling Electrical and Computer Engineering Engineering
2	RAMAN SPECTROSCOPY CHARACTERIZATION OF PULSED LASER DEPOSITION GROWN ZNTE THIN FILMS ON SAPPHIRE SUBSTRATE / RAMAN CHARACTERIZATION OF PLD GROWN ZNTE FILMS ON SAPPHIRE Rezapoor, Fatemeh 06 1900 (has links) Compound semiconductors are the foundation of many electronic and optoelectronic devices. As a result semiconductor epitaxy can be viewed as the first significant step in device engineering. Accurate and reliable characterization methods are needed to measure semiconductor properties including optical, electrical, vibrational and crystal structure. In this thesis, the epitaxy of ZnTe thin films on sapphire substrate by Pulsed Laser Deposition system at different growth temperatures is studied. The texture analysis is inspected by Two Dimensional X-Ray Diffraction. The lattice constant of the films and strain studies are investigated by High Resolution X-Ray Diffraction. UV-Vis spectroscopy is applied to find absorption edge in ZnTe thin film in order to estimate optical bandgap. These common characterization methods reveal the great effect of growth temperature on crystalline and optical properties of ZnTe thin films. In addition, Raman spectroscopy is used for the first time in the Preston's group to examine vibrational modes in ZnTe thin films. This new characterization method, which is the main focus of this thesis, uncovers some new features of ZnTe thin films not accessible through other techniques. In this thesis, optimum experimental conditions, instrumentation and data analysis of Raman observations in thin films are studied in detail. The final results are in good agreement with other characterization methods and they can justify crystalline and optical observations. These results demonstrate that Raman spectroscopy is a non-destructive characterization method applicable to thin film analysis. / Thesis / Master of Applied Science (MASc) Raman Spectroscopy Epitaxy Thin Films Pulsed Laser Deposition ZnTe Crystal Structure Vibrational modes Semiconductor growth
3	Self organized formation of Ge nanocrystals in multilayers Zschintzsch-Dias, Manuel 05 June 2012 (has links) (PDF) The aim of this work is to create a process which allows the tailored growth of Ge nanocrystals for use in photovoltic applications. The multilayer systems used here provide a reliable method to control the Ge nanocrystal size after phase separation. In this thesis, the deposition of GeOx/SiO2 and Ge:SiOx~ 2/SiO2 multilayers via reactive dc magnetron sputtering and the self-ordered Ge nanocrystal formation within the GeOx and Ge:SiOx~ 2 sublayers during subsequent annealing is investigated. Mostly the focus of this work is on the determination of the proper deposition conditions for tuning the composition of the systems investigated. For the GeOx/SiO2 multilayers this involves changing the GeOx composition between elemental Ge (x = 0) and GeO2 (x = 2), whereas for the Ge:SiOx~ 2/SiO2 multilayers this involves changing the stoichiometry of the Ge:SiOx~ 2 sublayers in the vicinity of stochiometric silica (x = 2). The deposition conditions are controlled by the variation of the deposition rate, the deposition temperature and the oxygen partial pressure. A convenient process window has been found which allows the sequential deposition of GeOx/SiO2 or Ge:SiOx ~2/SiO2 without changing the oxygen partial pressure during deposition. For stoichiometry determination Rutherford back-scattering spectrometry has been applied extensively. The phase separation in the spatially confined GeOx and Ge:SiOx ~2 sublayers was investigated by X-ray absorption spectroscopy at the Ge K-edge. The Ge sub-oxides content of the as-deposited multilayers diminishes with increasing annealing temperature, showing complete phase separation at approximately 450° C for both systems (using inert N2 at ambient pressure). With the use of chemical reducing H2 in the annealing atmosphere, the temperature regime where the GeOx phase separation occurs is lowered by approximately 100 °C. At temperatures above 400° C the sublayer composition, and thus the density of the Ge nanocrystals, can be altered by making use of the reduction of GeO2 by H2. The Ge nanocrystal formation after subsequent annealing was investigated with X-ray scattering, Raman spectroscopy and electron microscopy. By these methods the existence of 2 - 5 nm Ge nanocrystals at annealing temperatures of 550 (GeOx) - 700° C (Ge:SiOx ~2) has been confirmed which is within the multilayer stability range. The technique used allows the production of extended multilayer stacks (50 periods ~ 300 nm) with very smooth interfaces (roughness ~ 0.5 nm). Thus it was possible to produce Ge nanocrystal layers with ultra-thin SiO2 separation layers (thickness ~ 1 nm) which offers interesting possibilities for charge transport via direct tunneling. Photovoltaik Strukturaufklärung Raman-Spektroskopie Rutherford Back Scattering XANES Röntgendiffraktometrie Nanokristall Mehrschichtsystem Germanium Siliciumdioxid Silicium Reaktives Sputtern Entmischung Tunneleffekt Durchstrahlungselektronenmikroskop Quantenpunkt Tempern annealing elemental semiconductors germanium germanium compounds nanofabrication nanoparticle nanocrystal quantum dot phase separation Raman spectra semiconductor growth semiconductor thin films silicon compounds silica reactive sputter deposition superlattice multilayer transmission electron microscopy tunnelling x-ray scattering photovoltaics rutherford back scattering XANES ddc:530 rvk:UM 3300 rvk:UQ 2000 Photovoltaik Strukturaufklärung Raman-Spektroskopie Rutherford Back Scattering XANES Röntgendiffraktometrie Nanokristall Mehrschichtsystem Germanium Siliciumdioxid Silicium Reaktives Sputtern Entmischung Tunneleffekt Durchstrahlungselektronenmikroskop Quantenpunkt Tempern
4	Self organized formation of Ge nanocrystals in multilayers Zschintzsch-Dias, Manuel 27 April 2012 (has links) The aim of this work is to create a process which allows the tailored growth of Ge nanocrystals for use in photovoltic applications. The multilayer systems used here provide a reliable method to control the Ge nanocrystal size after phase separation. In this thesis, the deposition of GeOx/SiO2 and Ge:SiOx~ 2/SiO2 multilayers via reactive dc magnetron sputtering and the self-ordered Ge nanocrystal formation within the GeOx and Ge:SiOx~ 2 sublayers during subsequent annealing is investigated. Mostly the focus of this work is on the determination of the proper deposition conditions for tuning the composition of the systems investigated. For the GeOx/SiO2 multilayers this involves changing the GeOx composition between elemental Ge (x = 0) and GeO2 (x = 2), whereas for the Ge:SiOx~ 2/SiO2 multilayers this involves changing the stoichiometry of the Ge:SiOx~ 2 sublayers in the vicinity of stochiometric silica (x = 2). The deposition conditions are controlled by the variation of the deposition rate, the deposition temperature and the oxygen partial pressure. A convenient process window has been found which allows the sequential deposition of GeOx/SiO2 or Ge:SiOx ~2/SiO2 without changing the oxygen partial pressure during deposition. For stoichiometry determination Rutherford back-scattering spectrometry has been applied extensively. The phase separation in the spatially confined GeOx and Ge:SiOx ~2 sublayers was investigated by X-ray absorption spectroscopy at the Ge K-edge. The Ge sub-oxides content of the as-deposited multilayers diminishes with increasing annealing temperature, showing complete phase separation at approximately 450° C for both systems (using inert N2 at ambient pressure). With the use of chemical reducing H2 in the annealing atmosphere, the temperature regime where the GeOx phase separation occurs is lowered by approximately 100 °C. At temperatures above 400° C the sublayer composition, and thus the density of the Ge nanocrystals, can be altered by making use of the reduction of GeO2 by H2. The Ge nanocrystal formation after subsequent annealing was investigated with X-ray scattering, Raman spectroscopy and electron microscopy. By these methods the existence of 2 - 5 nm Ge nanocrystals at annealing temperatures of 550 (GeOx) - 700° C (Ge:SiOx ~2) has been confirmed which is within the multilayer stability range. The technique used allows the production of extended multilayer stacks (50 periods ~ 300 nm) with very smooth interfaces (roughness ~ 0.5 nm). Thus it was possible to produce Ge nanocrystal layers with ultra-thin SiO2 separation layers (thickness ~ 1 nm) which offers interesting possibilities for charge transport via direct tunneling.:Contents 1 Introduction and motivation 1 2 Basic aspects 6 2.1 Microstructure of sub-stoichiometric oxides (SiOx, GeOx) 6 2.2 Phase transformations 9 2.3 Quantum confinement effect in nanocrystals 12 2.4 Applications of nanostructures in 3rd generation photovoltaics 17 3 Experimental setup 21 3.1 The magnetron deposition chamber 21 3.2 (Reactive) dc sputtering 22 3.3 Annealing processing 26 3.4 X-ray facilities 26 4 Analytical methods 30 4.1 Rutherford backscattering spectrometry (RBS) 30 4.2 Raman scattering 33 4.3 (Grazing incidence) X-ray diffraction (GIXRD) 35 4.4 X-ray reflectivity (XRR) 39 4.5 X-ray absorption near edge structure (XANES) 41 4.6 Transmission electron microscopy (TEM) 42 5 Properties of reactive dc magnetron sputtered Si-Ge-O (multi)layers 44 5.1 Deposition rate and film stoichiometry investigations 44 5.2 Stoichiometry dependent properties of GeOx/SiO2 multilayers 47 5.3 Lateral intercluster distance of the Ge nanocrystals in multilayers 51 6 Confined Ge nanocrystal growth in GeOx/SiO2 multilayers 54 6.1 Phase separation in GeOx single layers and GeOx/SiO2 multilayers 54 6.2 Crystallization in GeOx single layers and GeOx/SiO2 multilayers 58 6.3 Multilayer stability and smallest possible Ge nanocrystal size 60 6.4 Stacked Ge NC films with ultra thin SiO2 separation layers 66 7 Confined Ge nanocrystal growth in Ge:SiOx/SiO2 multilayers 71 7.1 Phase separation in Ge:SiOx/SiO2 multilayers 72 7.2 Crystallisation in Ge:SiOx/SiO2 multilayers 76 8 Summary and conclusions 79 List of Figures 83 List of Tables 85 Bibliography 86 info:eu-repo/classification/ddc/530 ddc:530

1

Page generated in 0.0654 seconds