Collective Behavior during Ion Beam Processing of Materials
D. G. Cahill,* R. S. Averback,* J. Kim, M. Ghaly
National Science Foundation, DMR-96-32252 (Conducted in the Coordinated Science Laboratory)

We use molecular dynamics calculations and scanning tunneling microscopy to study collective behavior during energetic ion impacts with solid surfaces. Collective behavior such as thermal spikes can produce qualitative differences in the morphology and microstructure produced by ion implantation in comparison to the predictions of binary collision models. The experiments take advantage of the EpiCenter facility to produce clean surfaces of a wide variety of materials and quantify the morphologies produced by ion impacts.

Crystal Growth by Laser-stimulated Chemical Vapor Deposition (LCVD)
J. E. Greene,* R. Tsu, T. Bramblett, Q. Lu
U.S. Office of Naval Research, N00014-90-J-1241 (Conducted in the Coordinated Science Laboratory)

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.

Crystal Growth of Mixed III-V Semiconducting Thin Films by Accelerated-Beam MBE
J. E. Greene,* R. Powell, G. Tomasch
Joint Services Electronics Program, N00014-96-1-0129 (Conducted in the Coordinated Science Laboratory)

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.

Development of Thin-Film Scanning Calorimetry with 0.2 nJ Sensitivity
L. H. Allen,* S. Lai
National Science Foundation, DMR 94-19604; Joint Services Electronics Program, N00014-96-1-0129 (Conducted in the Coordinated Science Laboratory)

We have developed a new thin-film differential scanning calorimetry (TDSC) technique that has extremely high sentitivity of 0.2 nJ, by combining two calorimeters in a differential measurement configuration by using SiN membrane technology. The TDSC has successfully measured the heat capacity and melting process of Sn nanostructures formed via thermal evaporation with deposition integral thickness of only 1 A. We have observed a decrease of up to 120?C in the melting point of Sn nanostructures, which agrees with the liquid-shell model describing the size-dependent melting point depression.

Direct Deposition of Polycrystalline Silicon for Thin-Film Electronics
J. R. Abelson,* J. E. Gerbi
University of Illinois (Conducted in the Coordinated Science Laboratory)

We study the nucleation and growth of polycrystalline silicon (px-Si) thin films on glass substrates at relatively low substrate temperatures (?500?C) using real-time, in situ spectroscopies. The goals are to determine the kinetics of hydrogen and silicon hydride adsorption, diffusion, reaction and elimination from the growing surface, and the effects of fast particle bombardment on these processes; to develop realistic models of px-Si film deposition; and to improve px-Si growth methods based on this knowledge.

Electron Microscopy Studies of Thin Films
J. M. Gibson,* J. C. Yang, P. Miller, M. Yeadon, M. Kleinschmidt, M. Marshall
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

We use ex situ and in situ electron microscopy to study thin films. Thin films are grown by molecular beam epilaxy, or gas reaction, in specially built ultrahigh-vacuum transmission electron microscopes. Quantitative measurements from images are used to extract stress magnitudes or examine ordering in films.

Fate of Heavy Metals in Plasma Arc Vitrification
D. G. Cahill,* T. Yamamoto
U.S. Army Construction Engineering Research Laboratories, 93-D-0018-17 (Conducted in the Coordinated Science Laboratory)

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.

Fundamental Studies of the Effect of Crystal Defects on CuInSe2/CdS Heterojunction Behavior
A. Rockett,* L.-C. Yang, H. Z. Xiao, G. Berry, G. Kenshole
National Renewable Energy Laboratory, DOE NREL XAD-3-12114-1 (Conducted in the Coordinated Science Laboratory)

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.

Growth and Physical Properties of Epitaxial Transition-Metal Nitrides and Nitride Superlattices
J. E. Greene,* F. Adibi, I. Petrov
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

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.

Growth of Superlattice Structures by Multitarget Sputtering
J. E. Greene,* Y. W. Kim
Joint Services Electronics Program, N00014-96-1-0129 (Conducted in the Coordinated Science Laboratory)

Multitarget sputtering techniques have been developed to allow the sequential deposition of ultrathin (10-100 A) 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.

Heat Transport in Oxide Thin Films
D. G. Cahill,* S.-M. Lee
National Science Foundation, CTS 94-21089 (Conducted in the Coordinated Science Laboratory)

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, 80-800 K.

High-Speed, Monolayer-Sensitivity Scanning Microcalorimetry for Solid-Solid Interface and Surface Studies
L. H. Allen,* G. Ramanath, S. Lai
National Science Foundation, DMR 94-19604 (Conducted in the Coordinated Science Laboratory)

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.

In Situ Measurements during Thermal Annealing
L. H. Allen,* M. Y. Zhang
IBM Corp.

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.

In Situ Studies of GaN Growth
J. M. Gibson,* M. Yeadon
U.S. Office of Naval Research, N00014-95-1-0324 (In cooperation with the Materials Research Laboratory)

We use in situ transmission electron microscopy to understand the growth of Ga and Al nitrides on sapphire substrates. The growth method is reactive molecular beam epitaxy.

Investigations of Sequential Interface Reaction Products of Ti during SALICIDE and TiN Processes
L. H. Allen,* Z. Ma, G. Ramanath, S. Lai
NCR Corp. (Conducted in the Coordinated Science Laboratory)

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.

Low-Energy Ion/Surface Interactions during Metal and Semiconductor Crystal Growth from the Vapor Phase-Control of Microstructure and Microchemistry on the Atomic Scale
J. E. Greene,* D. Lubben, T. Bramblett, L. Markert
Joint Services Electronics Program, N00014-96-1-0129 (Conducted in the Coordinated Science Laboratory)

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.

Low-Temperature Growth of Si, Si-Ge, and Si-C Compounds
L. H. Allen,* Z. Ma, G. Ramanath, S. Lai
Joint Services Electronics Program, N00014-96-1-0129 (Conducted in the Coordinated Science Laboratory)

Low-temperature processing for devices is a key direction in future electronic processing. We have been investigating the low-temperature (¨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.

Metal Structures in Integrated Circuit Technology
L. H. Allen,* Z. Ma, M. Y. Zhang
University of Illinois

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.

Microscopic Mechanisms of Assisted MBE Crystal Growth
D. G. Cahill,* L. Liu
U.S. Department of Energy, DOE-PL0017293SJ (Conducted in the Coordinated Science Laboratory)

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.

Molecular Beam Epitaxy Assisted by Low-Energy Ions
D. G. Cahill,* B. W. Karr
U.S. Department of Energy, DE-FG02-96ER45439 (Conducted in the Materials Research Laboratory)

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.

New Spectroscopies of Medium Range Order in Plasma-deposited Silicon Films
J. R. Abelson,* J. M. Gibson, J. E. Gerbi, P. M. Voyles
National Renewable Energy Laboratory

We deposit nominally amorphous and fine-grained polycrystalline silicon films using low-temperature plasma chemical vapor deposition or reactive magnetron sputtering; analyze the amorphous-to-polycrystalline phase transformation during growth using real-time spectroscopic ellipsometry and reflection IR absorption; and evaluate the medium range atomic order using ex situ variable coherence or variable resolution TEM. This work provides the fundamental understanding necessary to use mixed phase films in photovoltaic (solar) cells.

Semiconducting CuInSe2 Deposition by Hybrid Sputtering/Evaporation and Ion-assisted Deposition Process
A. Rockett,* L. C. Yang
Electric Power Research Institute (Conducted in the Coordinated Science Laboratory)

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.

Superhard Coating
H. H. Chen,* C. H. Ma, in collaboration with J. H. Huang of Tsing Hua Univ., Taiwan
U.S. Department of Energy, DE-FG02-96ER45439; NSC Grant, Taiwan (In cooperation with the Materials Research Laboratory)

For improved hardness, corrosion resistance, and decorative purposes, titanium nitride has received a wide range of applications. Fundamental problems involving adhesion, interface strength, effect of residual stress on strength, texture effect on properties, etc., are some of the significant topics that still require systematic investigation. Moreover, refining coating procedures to improve the aforementioned properties by introducing new microstructure remains a great challenge. This program involves the deposition and characterization of transition metal nitride thin films as well as boron nitrides obtained by ion beam assisted deposition (IBAD) and laser ablation depostion (LAD) methods. A variety of microstructure/microchemistry techniques along with mechanical and corrosion testing are carried out.

Surface and Gas Phase Reaction Chemistries in the Remote Plasma Deposition of Boride and Nitride Thin Films for Microelectronic Diffusion Barriers
J. R. Abelson,* G. Girolami, J. Sung, D. Goedde
University of Illinois

Remote plasma processing is used to deposit ZrB2, HfB2, and other refractory thin films for use as advanced diffusion barriers in VLSI and high-temperature electronic devices. The remote plasma approach combines the best features of the chemical and physical vapor deposition routes: the high growth rate, low temperature, and conformal coverage characteristic of CVD, and the dense, polycrystalline microstructure of the PVD process. This research includes analysis of the growth chemistry using in situ reflection IR spectroscopy and modulated mass spectroscopy with isotope labeling experiments; and evaluation of the diffusion barrier properties on group IV and III-V semiconductors.

Theoretical and Experimental Investigations of Thermal and Accelerated Dopant/Surface Interactions during Vapor Phase Film Growth in VLSI Device Fabrication
J. E. Greene,* L. Markert, G. Xue
Semiconductor Research Corp. (Conducted in the Coordinated Science Laboratory)

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.

Thin-Film Electronics on Curved Substrates
J. R. Abelson,* R. Nuzzo (Chemistry), H.-C. Jin, M. Erhardt
Defense Advanced Research Projects Agency

Thin-film transistor arrays are deposited on curved substrates for use in focal plane imaging devices. Silicon and silicon nitride layers are deposited using low-temperature plasma chemical vapor deposition or reactive magnetron sputtering, and the active layers are patterned using block-printing methods with polymer or self-assembling monolayer inks in place of photolithography.