Browsing by Author "Dr. Gerd Duscher, Committee Member"

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Development and Application of Electron Beam Induced Current and Cathodoluminescence Analytical Techniques for Characterization of Gallium Nitride-based Devices
(2005-02-27) Bunker, Kristin Lee; Dr. Phillip E. Russell, Committee Chair; Dr. Gerd Duscher, Committee Member; Dr. Carlton M. Osburn, Committee Member; Dr. Dieter Griffis, Committee Member
The focus of this research was the design, development, and implementation of Electron Beam Induced Current (EBIC) and Cathodoluminescence (CL) techniques on both a Scanning Electron Microscope (SEM) and high-resolution versions on a Scanning Transmission Electron Microscope (STEM). The EBIC and CL techniques were used to characterize electrical and optical properties of fully processed gallium nitride (GaN)-based and indium gallium nitride (InGaN)-based light emitting diodes (LEDs). SEM-EBIC experiments in a linescan configuration were used to determine the minority carrier diffusion lengths of electrons and holes in a fully processed GaN-based LED. A theoretical model with an extended generation source and a nonzero surface recombination velocity was used to extract the minority carrier diffusion length of the p-type and n-type layers. A minority carrier diffusion length of L[subscript n]=(80 ± 6) nm for electrons in the p-type GaN layer, L[subscript p]=(70 ± 4) nm for holes in the n-type GaN: Si, Zn active layer, and L[subscript n]=(54 ± 4) nm for electrons in the p-type Al[subscript 0.1]Ga[subscript 0.9]N layer were determined. The STEM-EBIC technique in a linescan configuration was used to determine the p-n junction location of an InGaN-based single quantum well LED with respect to the thin quantum well with nanometer precision. A novel sample preparation method using a Focused Ion Beam (FIB) technique and a custom STEM-EBIC sample holder were designed for these experiments. The relative position of the p-n junction with respect to the In[subscript x]Ga[subscript 1-x]N quantum well was found to be 19 ± 3 nm from the center of the In[subscript x]Ga[subscript 1-x]N quantum well. In addition, the simultaneous acquisition of Z-contrast, EBIC, and elemental aluminum and indium linescans was demonstrated. Following successful implementation of the STEM-EBIC technique, several advancements to the technique were implemented. A novel sample preparation method was developed involving a variation of the tripod wedge method in combination with the FIB technique to analyze any type of packaged or unpackaged optoelectronic device. The sample preparation is divided into several steps, including mechanical thinning, grid and wire attachment, and FIB milling to create an electron transparent membrane. In addition, a custom cross-sectional STEM-EBIC sample holder was designed to hold the fully prepared optoelectronic devices and allow for simultaneous STEM-EBIC experiments. A SEM-CL system with polychromatic spectroscopic and panchromatic imaging capabilities was designed and used to examine piezoelectric fields and indium composition fluctuations in an InGaN-based multiple quantum well (MQW) LEDs. The existence and direction of a piezoelectric field was determined with SEM-CL voltage dependence experiments and the magnitude was estimated to be 1.0 ± 0.2 MV/cm. Planar panchromatic CL imaging revealed inhomogeneous intensity on the same LED and spectral CL measurements were used to locally probe the intensity differences and identify any bandgap or indium composition differences. Finally, a STEM-CL system with polychromatic spectroscopic and panchromatic imaging capabilities was designed, constructed, installed and tested. The STEM-CL design consisted of a lens and fiber optic light collection system and a fiber optic vacuum feedthrough to direct the signal out of the microscope. The technique was demonstrated and STEM-CL spectra were obtained from InGaN-based MQW LEDs.
Diffusion Charateristics of Copper in Novel Metallic Films
(2003-10-23) Gupta, Abhishek; Dr. Gerd Duscher, Committee Member; Dr. Robert M Kolbas, Committee Member; Dr. J. Jerome Cuomo, Committee Member; Dr. Jagadish Narayan, Committee Chair
The goal of this work was to synthesize refractory materials like TiN, Ta and alloys of TiN-TaN in the form of thin films which are used as diffusion barriers in integrated circuits to prevent diffusion of Cu into the Si substrate. The primary emphasis of this research was to synthesize different microstructures of these films like amorphous, nanocrystalline, textured polycrystalline and single crystalline films, and to study the effect of these microstructures on their mechanical and electrical properties and on diffusion characteristics of Cu. Microstructures ranging from nanocrystalline to single crystalline TiN films on Si(100) substrates were synthesized by Pulsed Laser Deposition technique by varying the substrate temperature from 25°C to 650°C. Experimental techniques like XRD, TEM, HRTEM, STEM-Z, EELS, SIMS and four-point probe resistivity measurement were used for in-depth analysis. Effect of microstructures of these films on their mechanical and electrical properties, and on diffusion behavior of Cu was analyzed. An important finding of this research was that polycrystalline TiN films showed significantly more diffusion of Cu along the columnar grain boundaries, whereas nanocrystalline films exhibited significantly less diffusion of Cu comparable to that in single crystalline TiN films. Impurity induced amorphous Ta films stable up to high temperatures (~650°C) were synthesized by the Pulsed Laser Deposition and polycrystalline Ta films were processed by magnetron sputtering technique. Effects of different microstructures of these films on their electrical properties and on the diffusion characteristics of Cu were analyzed. Using the above experimental techniques along with RBS, stable amorphous Ta films showed insignificant diffusion of Cu. Polycrystalline Ta films showed significant diffusion of Cu along the grain boundaries. Recrystallization of amorphous Ta films and diffusion along the grain boundaries were observed at higher temperatures. The effect of alloying TiN and TaN to combine their useful properties as a diffusion barrier for Cu was analyzed. Different TaxTi1-xN alloys with x ranging from 0.3 to 0.7 were obtained by arranging the target in a special configuration and ablating it in a sequential manner by Pulsed Laser Deposition. Superlattice structures of TaN and TiN were achieved for x = 0.6. By using single crystalline cubic TiN as a buffer layer, cubic phases of all the alloys were obtained with low resistivity values. Nanoindentation measurements provided high hardness values of the alloys making them useful for hard coating applications. Insignificant diffusion of Cu was observed for all the concentrations after high temperature annealing and these studies proved TiN-TaN binary components to be a superior diffusion barrier for Cu.
Growth, Characterization and Device Processing of GaN Metal Oxide Semiconductor Field Effect Transistor (MOSFET) Structures
(2006-01-07) Saripalli, Yoganand Nrusimha; Dr. M.A.L. Johnson, Committee Chair; Dr. D.W. Barlage, Committee Member; Dr. Gerd Duscher, Committee Member; Dr. Zlatko Sitar, Committee Member; Dr John Muth, Committee Member
The physical properties of GaN, high saturation velocity, high breakdown fields, high electron mobility, wide bandgap energy and high thermal conductivity, make it a promising material for field effect transistor (FETs) devices for high speed, high power, and small channel length applications. Despite the success of GaN electronic devices such as heterojunction field effect transistors (HFETs), fabrication of GaN Metal Oxide Semiconductor (MOS) transistors remains a technical challenge. The primary reason for this is the non-availability of a gate dielectric with a low density of interface states and the simultaneous requirement of ohmic source/drain contacts which are compatible with enhancement mode structures. Unlike existing III-N HFET devices, which have a high free carrier density two dimensional electron gas (2DEG) in the semiconductor substrate, a MOSFET in either accumulation or inversion mode requires low free carrier concentration in the semiconductor channel, and a high density of free carriers in adjacent source and drain areas. This research explores the development, and demonstration of an enhancement mode (normally off) GaN MOSFET with highly doped source/drain ohmic contacts and compatible gate dielectric. Highly doped source/drain ohmic contacts were formed by selected area epitaxial regrowth of Si doped GaN by metalorganic chemical vapor deposition (MOCVD). The MOS gate dielectrics which have been investigated are Ga2O3/Gd2O3 and SiNx. To achieve uniform and highly doped GaN on reactive ion etched (RIE) and patterned GaN surfaces for source drain contacts, a low temperature regrowth (750-850oC) was developed. A model for growth morphology consistent with the low temperature regrowth of GaN on RIE patterned GaN surfaces is given. The detailed structural, optical, and chemical characterization of the low temperature regrown highly doped GaN for source and drain contacts has been provided. The structural characterization of GaN/Ga2O3/Gd2O3 interface used in MOS device fabrication is presented. Devices were fabricated based on III-N structures and epitaxial MOS dielectrics. The GaN MOSFET structures fabricated in this work exhibited enhancement mode (normally off) operation. This proof of concept demonstration of normally off GaN MOSFETs with epitaxially regrown source and drain contacts is a significant step in the development of enhancement mode III-N MOSFET devices for logic, high speed, high power and high temperature applications.
Intragrain Defect Characterization Of Solar Grade Silicon Using Near-Field Scanning Optical Microscopy
(2006-08-01) Peters, Jeremy Kelvin; Dr. George A. Rozgonyi, Committee Co-Chair; Dr. Hans D. Hallen, Committee Co-Chair; Dr. Gerd Duscher, Committee Member
Multicrystalline silicon (mc-Si) is a material used in the photovolatic (PV) industry because of its lower production cost in comparison to its single crystal or thin film silicon counterparts. Multicrystalline silicon grown by the block casting technique, in which molten silicon is cooled in a growth crucible, generates thermally induced stress, creating intragrain dislocation clusters. These dislocation clusters act as minority carrier recombination centers, reducing the overall cell efficiency. To gain understanding of the recombination behavior of these defects a characterization tool with submicron resolution is needed. Materials scientists have a number of microscopy options at their disposal to characterize the structural, chemical, and electrical properties of semiconductors. Optical microscopy, e.g., differential interference contrast (DIC) techniques such as Nomarski microscopy, is used to observe surface defects delineated by chemical etching. However, far-field optical spatial resolution is limited by the Abbe limit, which states that the minimum resolvable distance between two objects is limited by the wavelength of the incident radiation. Electron microscopy, including scanning electron microscopy (SEM) and transmission electron microscopy (TEM), is employed to provide electrical and structural information about bulk defect interactions. Although capable of sub-angstrom resolution, electron microscopy requires sample preparation that destroys the sample surface. Advances in scanning probe microscopy (SPM) have allowed scientists to break the far-field limit to produce images with nanometer and subnanometer resolution. Near-field scanning optical microscopy (NSOM) is a form of scanning probe microscopy. NSOM has the ability to image both surface and bulk properties of a material in a non-evasive manner with greater spatial resolution than far-field optical microscopy and electron microscopy. I intend to demonstrate NSOM as a characterization tool in photovoltaic (PV) silicon wafers, using carrier lifetime and photoinduced current variation as contrast mechanisms. The motivation for and development of NSOM as an characterization tool to map defect recombination behavior is first described. Then, an account of the carrier dynamics associated with the NSOM contrast modes is given. Next, the design challenges associated with the construction of the NSOM system are explained. An analysis of the intragrain defect lifetime and recombination behavior follows, using results from both existing characterization techniques and NSOM imaging. Finally, a summary of findings and description of areas for future study is given.
Laser Molecular Beam Epitaxial Growth and Properties of III-Nitride Thin Film Heterostructures on Silicon
(2006-03-01) Rawdanowicz, Thomas Adolph; Dr. Gerd Duscher, Committee Member; Dr. Robert Kolbas, Committee Member; Dr. J. Michael Rigsbee, Committee Member; Dr. J. Narayan, Committee Chair
The principal goal of this research was the investigation and process development of epitaxial growth mechanisms for the direct depositions of heteroepitaxial GaN thin films directly on Si(111) and Si(100) substrates without the incorporation or the formation of an interlayer at the GaN/Si interface. The research involved the design, development and implementation of a physical vapor deposition system based on a laser ablation process in an ultra high vacuum environment. Consideration is given to the role and control of substrate temperature as a function of elapsed deposition process time and its influence on lowering interfacial energies and limiting silicon nitride interlayer formation. The research results show that crack-free growth of 2 μm thick heteroepitaxial AlN and GaN thin films on Si(111) substrates can be achieved without the use of interlayer films. These thin film depositions resulted in atomically clean and chemically abrupt interfaces, while restricting the formation of silicon nitride at the interface. The resulting AlN and GaN epitaxial relationship on Si(111) is confirmed as [0002]║Si[111], [2110]║Si[110], and [0110]║Si[211]. The III-Nitride thin film on Si(111) is established by domain matching epitaxy (DME) exhibiting a ratio of (2110):(110) interplanar distances of 6:5 for GaN:Si and 5:4 ratio for AlN:Si with clean interfaces along silicon nitride free terraces of the Si(111) surface. Moreover, variations in the domain matching epitaxy were observed to result in the further reduction of residual interfacial strain with the incorporation of domain matching ratios of 5:4 for GaN/Si(111) and 6:5 for AlN/Si(111) occurring with a calculated frequency of nine 5:4 ratios for each 6:5 plane matching ratio for GaN/Si(111) and two 6:5 ratios for each 5:4 plane matching ratio for AlN/Si(111). For the case where silicon nitride (SiNx) is allowed to form at the interface, elemental analysis using electron energy loss spectroscopy provided evidence that the formation of SiNx occurs as a result of subsequent nitrogen diffusion to the GaN/Si interface after the GaN epitaxy is established.
Study of Si1-xGex Junction Formation for SOI Based CMOS Technology
(2009-01-08) Du, Yan; Dr. Mehmet C Ozturk, Committee Co-Chair; Dr. Veena Misra, Committee Chair; Dr. Carlton Osburn, Committee Member; Dr. Gerd Duscher, Committee Member
SiGe source/drain technology has been sucessfully applied to bulk metal oxide semiconductor field effect transistors (MOSFETs). Both channel mobility and source/drain contact resistivity are substantially improved with this technology. In this dissertation, SiGe junction formation for silicon on insulator (SOI) based CMOS technology was investigated. Strain in epitaxially grown films on SOI films and silicon nanowires is studied using Raman spectroscopy and transmission electron microscope (TEM). For epitaxially grown SiGe film on SOI, there is lower degree of strain development in the SOI layer due to the rigid interface between the SOI and the burried oxide as compared to bulk. However, for silicon nanowires on oxide, the situation is different since nanowires serve as compliant substrates. Part of the strain energy is transferred to silicon nanowires. The consistency between synthesized Raman peak shifts and the experimental measurements verified the strain sharing between the epitaxially grown SiGe films and the silicon nanowires. Splittings of high order Laue zone line (HOLZ) from a convergent beam electron diffraction (CBED) pattern was quantified to study the strain distribution in epitaxial SiGe films grown on silicon nanowires. It was found out in this study that elastic deformation of epitaxial SiGe at free surfaces leads to strain relaxation at these surfaces. This phenomenon is detrimental to strain engineering in a nanowire MOSFET and provides new challenges to develop smart designs for constraining strain in the nano-structures. Moreover, atomic layer deposition (ALD) Platinum is proposed for metal deposition on 3D epitaxial SiGe source/drain. The uniform deposition around 3D SiGe films effectively increases the contact surface area which is highly desired in the FinFET application.