Our goal is to elucidate the basic processes that govern hydrogenated amorphous silicon (a-Si:H) film growth. Fundamentally different deposition sources, i.e., reactive sputtering and remote SiH₄ glow discharges, are used to grow thin films. The physics and chemistry of deposition processes are studied by in situ measurements, including modulated beam mass spectroscopy, Langmuir probes, and isotope exchange. Bulk film properties are examined by visible and infrared absorption spectroscopy, drift mobility, capacitance-voltage, photoconductivity spectral response, photothermal deflection, and p-i-n devices.

The reactions between group IV hydride molecules and surfaces of heteroepitaxial Si_1-xGe_x and Si_1-xC_x films on Si substrates are studied using in situ IR and thermal desorption spectroscopies. The goal is to provide fundamental data on self-limiting surface reactions, such that group IV semiconductor growth can be controlled in an atomic layer-by-layer fashion for advanced device fabrication.

We are determining the relationship between the atomic motion of hydrogen in amorphous silicon films and the formation of metastable electronic defects during free carrier injection by light illumination. The thin-film samples are prepared with various initial hydrogen contents, including isotopically labelled diffusion couples, subjected to annealing or post-hydrogenation, and analyzed.

This project explores the use of reactive magnetron sputtering to deposit thin-film hydrogenated amorphous silicon (a-Si:H) and silicon nitride (a-SiN_x:H) layers for use in thin-film transistors. The goal is to demonstrate that sputtering is an industrially practical, low-cost, and environmentally friendly process. The metrics include electronic-quality film properties, high deposition rates, uniformity over large areas, and the use of doped targets to eliminate the need for toxic gases.

Our goal is to develop a low-temperature growth process for thin-film hydrogenated amorphous silicon (a-Si:H) and silicon nitride (a-SiNx:H) layers, which are used as the semiconductor and dielectric materials in thin-film transistors, respectively. The films are grown using reactive magnetron sputter deposition, fabricated into field-effect devices, and analyzed. Proof-of-principle experiments suggest that sputter deposition will succeed at low temperatures and open up the use of lightweight plastic substrates for the portable computer market.

We are interested in studying solid-state reactions involving thin films of metals deposited on semiconductor substrates (Si or GaAs). The samples are characterized using standard thin-film analysis tools such as TEM and x-ray diffraction. In order to study the kinetics of the reaction, the sheet resistance of the film is monitored in situ during the annealing process using a four-point probe assembly. By monitoring the changes in the sheet resistance versus time we can attempt to understand the rate-determining mechanisms of the reaction. We plan to use this technique in both traditional furnaces and rapid thermal annealing (RTA) systems.

We plan to explore several areas related to small metal structures in a typical integrated circuit. The first area is the measurement of metal-to-metal contact resistance of small-area interconnect metallization systems. The second area of interest explores the reactions induced by local heating using a small resistance heater (microheater) fabricated with photolithography techniques. These heaters act as ``fuses'' in some IC systems. We are also exploring the possibility of building a scanning tunnelling potentiometer to electrically characterize these small structures.

Low-temperature processing for devices is a key direction in future electronic processing. We have been investigating the low-temperature (<250°C) SPE growth of Si-Ge films on Si(100). This is accomplished by thermally annealing a-Ge/Au bilayers deposited on Si(100) using thermal evaporation. RBS, TEM, ion channeling, and AES were used to characterize the microstructure of the films. The process of the growth is studied using XTEM and in situ resistivity measurements.

Ti metallization plays an important role in state-of-the-art MOS processing technology. Not only is it used extensively for SALICIDE formation for source/drain contacts, but it is also used to form conductive TiN diffusion barriers needed for reliable contacts with aluminum metallization. We investigate the reaction sequence in this system using both in situ measurements such as differential resistivity measurements and ex situ microstructure analysis, including TEM and XRD.

We are developing a new technique that is potentially a very powerful method for directly obtaining quantitative values for small enthalpy of reactions at interfaces, surfaces, and near surface regions. This microcalorimeter is expected to have high sensitivity, capable of measuring extremely small amounts of heat generated during solid/solid reactions and surface processes as well as low enthalpy of internal microcrystalline processes. This technique will be useful for measuring the kinetics of interface reactions, such as the nucleation of silicides at buried interfaces, and the study of near-surface processes, such as point-defect annihilation and coalescence of vacancies in Si following ion implantation.

A wide variety of techniques for the deposition of thin films utilize bombardment of the growth surface by low-energy ions to enhance film properties. We are studying the deposition of metal films using a low-energy metal ion beam with a well-defined energy that can be varied between 5 eV and 200 eV. In situ scanning tunneling microscopy is used to characterize the surface morphology. When we combine this technique with ex situ x-ray diffraction and transmission electron microscopy studies, we form a nearly complete picture of the dependence of film microstructure on the energy of depositing atoms.

Ceramic coatings are widely used as thermal barriers to increase the maximum operating temperature of metal alloys. Our research project seeks a greater understanding of the role of atomic-sized defects, internal interfaces, and porosity on the thermal conductivity of oxide coatings. Experimental coatings are deposited using magnetron sputtering. Our newly developed measurement technique enables us to measure the thermal conductivity of films only 100 nm thick over a wide temperature range, 80-800K.

Thermal plasma processing shows great promise as a technology for the remediation of toxic wastes. Unfortunately, the extremely high temperatures produced in plasma-arc melting can lead to significant loss of metals from the molten slag by evaporation. The Plasma Arc Facility at the University of Illinois is extensively instrumented to allow real-time monitoring and control of plasma-arc processing of model systems for contaminated wastes. We are studying the kinetics of metal evaporation and particle generation using optical probes.

Thin-film synthesis methods show great promise of producing novel thermoelectric materials with improved thermal efficiencies. We are studying the lattice thermal conductivity and Seebeck coefficient in semiconductor multilayers with the goal of evaluating the merit of this approach for achieving improved figures of merit for thermoelectric cooling and energy conversion.

Our experiments on the fundamentals of crystal growth by molecular beam epitaxy (MBE) and etching by low-energy ions take place in the EpiCenter, a collaborative facility for research in physics, electrical engineering, and materials science. We use in situ scanning tunneling microscopy (STM) to quantify the surface structure and morphology of a wide variety of semiconductors and metals. The STM studies include atomic resolution imaging as well as systematic studies of morphology evolution during low-temperature processing.

The proposed work encompasses the fabrication and characterization of ferroelectric Pb(Sc _0.5 Ta _0.5 )O₃ (PST) thin films by means of metal-organic chemical vapor deposition (MOCVD) techniques. We will also perform spectral response measurements, frequency response measurements, blackbody responsivity measurements, and noise measurements. The performance of the PST detectors will then be compared with commercial state-of-the-art pyroelectric detectors. The ultimate goal of this work is to develop a room-temperature infrared radiation detector. The proposed IR detector development is based upon a different class of materials, the ferroelectric PST system, and a novel detection approach which utilizes both the ferroelectric and the pyroelectric properties of PST.

The objective of the project is to use metal-organic chemical vapor deposition (MOCVD) technology to develop lead-selective membranes that are capable of sensing lead to below 0.2 ppm in water. The MOCVD work will entail fabrication of pure and mixed metal oxide and sulfide membranes that will be tested for their sensitivity and their susceptibility to common interfering ions in water. Simple and layered structures as well as mixed metal anion films will be fabricated using MOCVD at UIUC. Microstructural characterization will also be made at UIUC, while sensing tests will be carried out at Motorola.

This program focuses on the relationship between processing, structure, and properties of MOCVD (metal-organic chemical vapor deposition) grown epitaxial oxide thin films in single and multilayer configurations. Besides the fundamental interests, this program is also driven by technological applications of epitaxial ceramic thin films to electrooptical devices. Systems under study include ferroelectric thin films (PT, PZT, LN), laser host materials (YVO₄), and sensor materials (SnO₂). Epitaxial thin films are grown using a cold wall, horizontal MOCVD chamber. Microstructure and microchemistry are characterized using XRD, SEM, TEM, AES, and XPS. Electrical and optical properties are also carried out on grown films.

Multitarget sputtering techniques have been developed to allow the sequential deposition of ultrathin (10-100 Å) alternating layers of metastable (GaAs)_1-x(Si₂)_x and GaAs. Preferential sputtering has been investigated and excellent control over film chemistry has been demonstrated. The structural perfection and electronic properties of such intercalated structures are being studied. Superlattice x-ray scattering techniques are also being used to study thermal and ion bombardment enhanced diffusion in these materials.

The growth of high-quality, mixed, III-V ternary and higher order single-crystal thin films by MBE incorporating reactive species using low-energy accelerated ion beams is being investigated. Structural perfection, optoelectronic properties, and composition of deposited alloy films are related to growth conditions. An understanding of this relationship may allow the development of new classes of multicomponent materials by exploiting the fact that sputtering is basically a physical rather than a chemical growth technique.

The primary objective of this research is to develop a detailed understanding of energetic particle/surface interactions for controllably altering nucleation and growth kinetics, microchemistry, and physical properties of metal and semiconductor films during deposition from the vapor phase by a variety of techniques including ion-assisted MBE, plasma-assisted CVD, sputter deposition, and primary-ion deposition. Low-energy ion/surface interactions allow the crystal grower additional dynamic control, at the atomic level, over microchemistry and microstructural evolution. Kinetic energy can be efficiently coupled to the growth surface thereby altering surface reactivity as well as adsorption, adatom diffusion, and nucleation kinetics.

We are developing a general model for the prediction and analysis of elemental incorporation probabilities and depth distributions of dopants in vapor phase deposited films as a function of experimental parameters such as film material, dopant, film growth temperature, growth rate, and the flux and kinetic energy of dopant species incident at the growing film surface. The model accounts for dopant diffusion, surface segregation, and low-energy ion bombardment effects. Model predictions are tested using accelerated-beam MBE as well as glow discharge and ultrahigh vacuum ion beam sputter deposition in which actual doping profiles will be determined by secondary ion mass spectrometry (SIMS) and, where applicable, capacitance/voltage and Hall effect measurements.

Chemical reaction paths can be affected by laser-induced photochemistry. We are investigating semiconductor crystal growth using focused laser beams to initiate heterogeneous reactions at substrate surfaces as well as by gas-phase photodissociation. LCVD offers a unique opportunity to probe the thermodynamics and kinetics of controlling gas phase and gas-surface reactions during film growth due to the spatial, temporal, and wavelength selectivity of coherent light sources.

Transition-metal nitrides such as TiN are used extensively as hard and wear-protective coatings for mechanical components, as abrasion-resistant optical coatings, and as diffusion barriers in integrated circuits. We have grown the first single-crystal TiN layers, using ultrahigh vacuum reactive magnetron sputter deposition on MgO, and have investigated their mechanical, electrical, and optical properties. We have also grown the first epitaxial nitride strained-layer superlattices, TiN/VN, and have found that they have mechanical and electrical properties that are a strong function of the superlattice period. Recently, we have grown metastable NaCl-structure (Ti,Al)N alloys and have found that they have greatly enhanced high-temperature oxidation resistance.

This program addresses three fundamental issues in CuInSe₂ (CIS) solar cell technology: (1) grain boundaries and their effects on second-phase populations, bulk grain conductivity, and heterojunction behavior; (2) stoichiometry and point defects; and (3) control of electronic properties and carrier collection of the heterojunction through the CIS chemistry. Experiments involve (1) growth of single-crystal and polycrystalline layers as a function of thickness, composition, substrate, and growth parameters; (2) testing of the structural, chemical, electronic, and optical properties of the fabricated materials; and (3) modification of the chemistry of the layers.

CuInSe₂ semiconductor coatings are being deposited by combining sputtered fluxes of Cu and In with an evaporated flux of Se. This approach takes advantage of the strengths of both the sputtering and evaporation processes while removing the requirement for using H₂Se as a working gas. The project also involves studies of CuInSe₂ deposition where a portion of the evaporated Se flux is ionized and accelerated in the direction of the substrate. Emphasis is on the basic mechanisms of coating growth and the resultant electronic properties of the coatings. The coatings are examined both by direct measurement and by fabricating and testing CuInSe₂/CdS heterojunction devices.

* Denotes principal investigator.