Browsing by Author "Carlton Osburn, Committee Member"

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Defect and Impurity Distributions in Traditionally Cast Multicrystalline and Cast Monocrystalline Silicon for Solar Substrates
(2008-08-26) Witting, Ian Thomas; Gerd Duscher, Committee Member; Carlton Osburn, Committee Member; George Rozgonyi, Committee Chair
Deposition of Metal and Metal Oxide Thin Films from Metal Organic Precursors in Supercritical Carbon Dioxide Solution
(2005-04-22) Barua, Dipak; Ruben G. Carbonell, Committee Member; Gregory N. Parsons, Committee Chair; Carlton Osburn, Committee Member
Thin films of metals and metal-oxides are deposited in batch (Chemical Fluid Deposition) and cyclic (Atomic Layer Deposition) processes from metal organic precursors in supercritical carbon dioxide solutions. New materials have been introduced in the deposition processes. Deposited films are analyzed in details in order to evaluate their quality and chemical composition. Analyzing techniques, X-ray photoelectron spectroscopy (XPS), ellipsometry, Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and auger electron spectroscopy (AES) are adopted to characterize the films. Capacitance-voltage measurements are performed to prove the device quality deposition of metal oxide films. The process establishes a new approach in metal oxide deposition, and controllable growth of metal and metal oxide films in supercritical carbon dioxide.
Formation of Stacked SiGe Nano-bridges
(2007-11-08) Kwak, Byung-Il; Veena Misra, Committee Co-Chair; Mehmet C. Ozturk, Committee Chair; Carlton Osburn, Committee Member
We have successfully demonstrated a novel method to form a three-dimensional array of Si or Si1-xGex nanowires that are horizontally aligned to the Si substrate. Unlike previous attempts, the nanowires of desired diameters and lengths can be readily formed at desired locations on a standard Si substrate. The process has been accomplished by epitaxial growth of Si and Si1-xGex layers, conventional lithography, reactive ion etching and selective etching of Si with respect to Si1-xGex. Therefore sensors and thermoelectric devices of nanowires can be readily integrated on Si chips, and nanowires themselves can serve as the channels of MOSFETs allowing three dimensional integration of MOSFETs for increased current drive. Among the four steps of the whole process, the thesis focuses on selective etching Si over Si1-xGex. TMAH, which is an anisotropic etchant and does not contain any alkali metals, was adopted for selective etching. It has been shown that the Si vertical etch rate is at least 300 times faster than the etch rate of Si1-xGex at 73 °C. For lateral selective etching, it turned out to be crucial to align the patterns along a certain direction. When the patterns are aligned to <110> direction, are exposed f111g planes which have the lowest etch rate and the etching proceeds only until two f111g planes meet at the center of Si layer sandwiched between the top and bottom Si1-xGex layers. After this, etching continues very slowly. On the other hand, if the patterns are aligned to [100], [110] planes are exposed but etched faster than {100} planes: the desired etch rate and the selectivity to Si1-xGex nanowires can be obtained.
Germanosilicide Contacts to Ultra-shallow P+N Junctions of Nanoscale CMOS Integrated Circuits by Selective Deposition of In-situ Doped Silicon-Germanium Alloys
(2003-04-21) Liu, Jing; Mehmet C. Ozturk, Committee Chair; John R. Hauser, Committee Member; Gregory N. Parsons, Committee Member; Carlton Osburn, Committee Member
One of the key challenges for future CMOS technology nodes is to form source/drain junctions with very small parasitic series resistance values. This requires fundamentally new junction and contact formation technologies to produce ultra-shallow junctions with super-abrupt doping profiles, above equilibrium dopant activation and contact resistivity values near 10⁻⁸ ohm-cm². Recently, this laboratory demonstrated a new junction formation technology based on selective deposition of heavily doped Si[subscript 1-x]Ge[subscript x] alloys in source/drain regions isotropically etched to the desired depth. The results to date indicate that the technology has the potential to meet all junction and contact requirements of future CMOS technology nodes. Of particular interest to this thesis is the smaller bandgap of Si[subscript 1-x]Ge[subscript x] resulting in a smaller metal-semiconductor barrier height, which is a key advantage in reducing the contact resistivity of a metal-semiconductor contact. In this work, formation of germanosilicide contacts to heavily boron doped Si[subscript 1-x]Ge[subscript x] alloys was studied with the intention to find a contact solution for future CMOS technology nodes beyond 100 nm. During the early stages of the research project, germanosilicides of Ti, Co, Ni, Pt, W, Ta, Mo and Zr were studied to identify the most promising candidates as source/drain contacts. The first set of experiments showed that Zr, Ni and Pt may have advantages over other candidates. Of the three germanosilicides, zirconium di-germanosilicide, Zr(Si[subscript 1-x]Ge[subscript x])₂ exhibited the best thermal stability but suffered from a high resistivity and excessive substrate consumption. Ni and Pt germanosilicides were considered attractive because they were both mono-germanosilicides and consumed much less Si[subscript 1-x]Ge[subscript x] than Zr(Si[subscript 1-x]Ge[subscript x])₂. Additionally, both had resistivity values lower than that of Zr germanosilicide which could be reached at temperatures as low as 300 °C. Of the three germanosilicides, NiSi[subscript 1-x]Ge[subscript x] was found to be the only one capable of yielding the desired contact resistivity of ˜ 10⁻⁸ ohm-cm² on both p⁺ and n⁺ Si[subscript 1-x]Ge[subscript x]. Unfortunately, NiSi[subscript 1-x]Ge[subscript x] was found to suffer from Ge out-diffusion, which had a direct negative impact on its thermal stability. NiSi[subscript 1-x]Ge[subscript x] formed at temperatures above 450 °C exhibited high sheet resistance and a rough germanosilicide⁄Si[subscript 1-x]Ge[subscript x] interface. Below this temperature, ultra-shallow p⁺-n juntions with self-aligned NiSi[subscript 1-x]Ge[subscript x] contacts were formed with excellent reverse bias junction leakage characteristics. It was also observed that the thermal stability of NiSi[subscript 1-x]Ge[subscript x] formed on heavily boron doped Si[subscript 1-x]Ge[subscript x] was noticeably better. A new approach was proposed to form ultra-thin Ni Si[subscript 1-x]Ge[subscript x] layers with enhanced thermal stability. By inserting a thin Pt interlayer between Ni and Si[subscript 1-x]Ge[subscript x], the thermal stability of NiSi[subscript 1-x]Ge[subscript x] was found to be significantly improved. On boron doped Si[subscript 1-x]Ge[subscript x], the material was found to be stable at least up to 700 °C with a total starting metal thickness of 10 nm. Pt incorporation was also found to result in better surface and interface roughness. This work has shown that high quality boron doped Si[subscript 1-x]Ge[subscript x] source⁄drain junctions with NiSi[subscript 1-x]Ge[subscript x] contacts can be formed. The junctions exhibit contact resistivity values near 10⁻⁸ ohm-cm², which satisfies the requirements of future CMOS technology nodes.
Light Element Impurities and Related Defects in Polycrystalline Silicon for Photovoltaic Application
(2004-11-04) Lu, Jinggang; David E. Aspnes, Committee Member; George A. Rozgonyi, Committee Chair; Carlton Osburn, Committee Member; Gerd Duscher, Committee Member
This thesis examines light element impurities and related defects in polycrystalline sheet and RGS (Ribbon Growth on Substrate) ribbon silicon. The interaction dynamics between oxygen, carbon, nitrogen, and intrinsic point defects, as well as the role of grain boundaries (GBs) on oxygen and carbon precipitation have been investigated. It is found that a high concentration of interstitial oxygen (Oi) will precipitate readily in polycrystalline sheet and ribbon silicon, and the precipitation of substitutional carbon (Cs) is mainly controlled by oxygen precipitation. By monitoring the Cs reduction by infrared absorption and the precipitate density by preferential etching, it is concluded that formation of interstitial carbon by trapping silicon self-interstitials is an indispensable step for the observed fast Cs precipitation. It is concluded that a low oxygen content is vital important to prevent extensive oxygen precipitation. On the contrary, a high carbon content (~ 1x 10¹⁸ cm⁻³) can be tolerated as long as the initial Oi concentration is low. The impact of GBs on oxygen precipitation in sheet silicon has been investigated. Infrared microspectroscopy shows nitrogen gettering at GBs and preferential etching reveals a precipitate denuded zone near GBs. The gettering of nitrogen at GBs is likely to be responsible for the denuded zone formation, considering the enhancement of nitrogen impurities on oxygen precipitation. The impact of GBs on carbon precipitation in RGS ribbons has also been studied. Infrared microspectroscopy indicates a higher remaining Cs concentration in the intra-grain region and preferential etching reveals a 20 to 30 μm wide precipitation band near GBs. Assuming that the tensile strain associated with carbon precipitates must be relaxed in order for the precipitation to proceed, it is shown that the precipitation band formation is mainly controlled by diffusion of vacancies from the intra-grain region to GBs.
Optimization of Metal Gate Electrode Stacks for Work Function Tuning
(2007-08-22) Lee, JaeHoon; Doug Barlage, Committee Member; Veena Misra, Committee Chair; Carlton Osburn, Committee Member; Gregory Parsons, Committee Member
SIMS Quantification of Matrix and Impurity Species in III-Nitride Alloys
(2006-02-09) Gu, Chunzhi (Jitty); Dieter P. Griffis, Committee Co-Chair; Mark A L Johnson, Committee Member; Carlton Osburn, Committee Member; Fred A. Stevie, Committee Member; Phillip E. Russell, Committee Chair
New applications in optoelectronic devices and high power electronic devices continue to be developed using III-Nitride. In the case of Al[subscript x]Ga[subscript 1-x]N, the quantification of matrix and impurity species is essential for matrix composition analysis, dopant control, and impurity control. Dynamic SIMS quantification in Al[subscript x]Ga[subscript 1-x]N is challenging because of matrix and charging effects. The secondary ion yield of matrix and impurity species varies in Al[subscript x]Ga[subscript 1-x]N with different AlN mole fraction (x). Al[subscript x]Ga[subscript 1-x]N also shows charging effects when the material becomes more insulating with increasing AlN mole fraction. In this work, a SIMS quantification method is developed for the Al[subscript x]Ga[subscript 1-x] N system over the range of x = 0 to 1. A set of Al[subscript x]Ga[subscript 1-x]N films prepared on SiC or sapphire substrates with AlN mole fraction ranging from 0 to 0.58 were implanted with ¹⁶O, ²⁴Mg and ²⁹Si. Very high Al concentration Al[subscript x]Ga[subscript 1-x]N samples were created using high dose ion implantation of Ga into AlN. With these samples, calibration curves of matrix ion intensity ratio for quantification of Ga and Al matrix constituents, Relative Sensitivity Factors (RSF) for impurity species, and sputter rate as a function of AlN mole fraction were obtained. Using these calibration curves, the matrix and impurity concentrations of an unknown Al[subscript x]Ga[subscript 1-x]N sample can be determined, and the elemental composition of multi-layer Al[subscript x]Ga[subscript 1-x]N samples can be measured. Electron beam charge neutralization methods for high Al content Al[subscript x]Ga[subscript 1-x]N are shown. The calibration curves in Al[subscript x]Ga[subscript 1-x]N using O₂⁺ bombardment with positive secondary ion detection, using Cs⁺ bombardment with negative secondary ion detection and MCs⁺ detection are developed. The ionization mechanisms under these conditions are rationalized. Using the sputtering conditions stated above, the sputter yield decreases with AlN mole fraction in Al[subscript x]Ga[subscript 1-x]N and the rate of decrease in the sputter rate versus x declines as x increases. In the range of x=0 to 0.58, the matrix ion intensity ratios of Al-containing ions over Ga-containing ions appear to increase linearly with the corresponding matrix mole fraction ratio or AlN mole fraction. For higher x, the inverse plots of the ratio of Ga-containing ions over Al-containing ions as a function of GaN mole fraction or mole fraction ratio appear to increase linearly in the range of x=0.39 to 1. The RSFs for Si and Mg normalized to the appropriate Ga-containing matrix ions decrease exponentially with x in the range of x=0 to 0.58; those normalized to the N-containing matrix ions have smaller variation with x in the range of x=0 to 0.58. The exponential correlation of RSFs with x is consistent with that of ion yield with the surface work function. Based on the calibration curves developed in this work at multiple analysis conditions, the matrix elements in Al[subscript x]Ga[subscript 1-x]N can be quantified in the range of x=0 to 1, and the impurity species can be quantified over x=0 to 0.58. The technique can be employed for impurity control, composition and growth rate determination, as well as structural analysis of the finished optoelectronic and electronic devices. The ionization yields of both positive and negative ions are studied when x is changed in Al[subscript x]Ga[subscript 1-x]N. The yield variation is mainly caused by the increase of surface concentration of primary species due to the sputter yield reduction as x is increased.
Spectroscopic Study of Hafnium Silicate Alloys prepared by RPECVD: Comparisons between Conduction/Valence Band Offset Energies and Optical Band Gaps
(2003-12-31) Hong, Joon Goo; Gerald Lucovsky, Committee Chair; Carlton Osburn, Committee Member; Gerd Duscher, Committee Member; Klaus Bachmann, Committee Member
Aggressive scaling of devices has continued to improve MOSFET transistor performance. As lateral device dimensions continue to decrease, gate oxide thickness must be scaled down. As one of the promising high k gate oxide material, HfO₂ and its silicates were investigated to understand their direct tunneling behavior by studying conduction and valence band offset energies with spectroscopy and electrical characterization. Local bonding change of remote plasma deposited (HfO₂)[subscript x](SiO[subscript 8322;)[subscript 1-x] alloys were characterized by Fourier transform infrared (FTIR) spectroscopy, x-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) as a function of alloy composition, x. Two different precursors with Hf Nitrato and Hf-tert=butoxide were tested to have amorphous deposition. Film composition was determined off-line by Rutherford backscattering spectroscopy (RBS) and these results were calibrated with on-line AES. As deposited Hf-silicate alloys were characterized by off-line XPS and AES for their chemical shifts interpreting with a partial charge transfer model as well as coordination changes.Sigmoidal dependence of valence band offse energies was observed. Hf 5d state is fixed at the bottom of the conduction band and located at 1.3 ± 0.2 eV above the top of the Si conduction band as a conduction band offset by x-ray absorption spectroscopy (XAS). Optical band gap energy changes were observed with vacuum ultra violet spectroscopic ellipsometry (VUVSE) to verify compositional dependence of conduction and valence band offset energy changes. 1 nm EOT normalized tunneling current with Wentzel-Kramer-Brillouin (WKB) simulation based on the band offset study and Franz two band model showed the minimum at the intermediate composition matching with the experimental data. Non-linear trend in tunneling current was observed because the increases in physical thickness were mitigated by reduction in band offset energies and effective mass for tunneling. C-V curves were compared to each other, and more hysteresis was observed with increasing x. Localized Hf 5d state as a trap site was the reason for hysteresis and its reverse direction with temperature-dependent C-V curves. Temperature-dependent I-V study located Hf 5d state. For the integration issue, nitridation study was performed at the interface, surface and both. Interfacial nitridation gave more effective reduction in EOT.