Robert J. Culbertson

Atom‐probe field‐ion microscopy of a high intensity gallium ion source

The emission characteristics of a liquid‐metal ion source have been investigated with a magnetic‐sector atom‐probe field‐ion microscope. The emission is classified into low, intermediate, and high intensity emission regimes. In the high intensity emission regime the emission is approximately 99% Ga+, but Ga++ and molecular gallium ions are also present. The relative intensities of Ga+, Ga++, Ga2+, and Ga3+ were measured as a function of total current in the high intensity emission regime. Energy distributions of these four species were also measured at various total currents. Spectral analysis of the optical emission from the ion source indicate that the light is due to radiation from neutral atomic gallium. It is concluded that a considerable amount of gas phase neutral gallium is in the vicinity of the emitter. Also, the ionization of atomic gallium takes place at the emitter surface, while the molecular ions are formed far from the surface.

The formation of ordered, ultrathin SiO2/Si(1 0 0) interfaces grown on (1 × 1) Si(1 0 0)

Ordering is observed at SiO2/Si(1 0 0) interfaces when 2–40 nm thick SiO2 films are grown on passivated, ordered (1 ×1) Si(1 0 0) surfaces produced by a novel wet chemical cleaning. A mechanism is proposed for the occurrence of this ordering. The thin oxides are grown by a variety of conventional oxidation techniques or by rapid thermal oxidation between 750 and 1100 °C. The evolution of oxygen, carbon, hydrogen and silicon coverages are detected by ion beam analysis (IBA) using a combination of ion channeling, nuclear resonance, elastic recoil detection and time-of-flight secondary ion mass spectrometry. IBA detects Si surface peak areal densities lower than that of a disorder-free, bulk-terminated (1 × 1) Si(1 0 0) crystal calculated by Monte-Carlo methods. This result indicates that Si substrate atoms are shadowed by Si atoms located i na2n mordered region on the oxide side of the interface. Beyond 2 nm, the oxide becomes amorphous. Reflection high-energy electron diffraction (RHEED) at 10 keV confirms the presence of order: a (1 ×1) streaky pattern commensurate with Si(1 0 0) is observed instead of an amorphous surface. Infrared (IR) spectroscopy shows that the ordered SiO2/Si(1 0 0) interfaces exhibit a constant, well-defined frequency of optical absorption across a 1 nm thickness in the interfacial oxide region near Si. This is in contrast to a rapidly changing frequency found for conventional oxides in the same region. Thus, IR supports the presence of a well-defined bond-length and stoichiometry as detected by IBA and RHEED.

Relaxation and H coverage of ammonium fluoride treated Si(111)

Using medium energy ion scattering and elastic recoil detection, we have studied silicon surfaces prepared by ex situ NH4F wet etching. We report direct measurements of relaxation and hydrogen coverage of the passivated Si(111)‐(1×1) surface. For Si(111), nearly ideal, unreconstructed surfaces are obtained, terminated by a single atomic layer of hydrogen. Silicon backscatter yields agree closely with simulations of a bulk truncation, with an inward relaxation of the outermost layer of 0.075±0.03 A. On the other hand, Si(001) prepared by NH4F solution shows severe roughening.

Strain measurements of SiGeC heteroepitaxial layers on Si(001) using ion beam analysis

The strain in SiGeC heteroepitaxial films grown on Si(001) substrates by chemical vapor deposition is quantified using ion channeling. Rutherford backscattering spectrometry was used to quantify the Ge concentration as well as the film thickness, nuclear resonance elastic ion scattering was used to quantify the C concentration, and ion channeling was utilized to measure film quality and C substitutionality. Channeling angular scans across an off‐normal major axis were used to quantify the strain. The results confirm that addition of C compensates for the strain introduced by Ge.

A graduate program for high school physics and physical science teachers

The Modeling Instruction Program at Arizona State University has demonstrated the feasibility and effectiveness of a university-based graduate program dedicated to professional development of in-service physics teachers. The program culminates in a Master of Natural Science degree, although not all students in the program are degree candidates. The findings draw from nine years of experience in developing, implementing, and evaluating the effects of the program (2001–2009).

Abstract: Study of semiconductor surfaces using an atom‐probe field ion microscope (1) Hydrogen chemisorption on silicon

Semiconductors, particularly silicon, are some of the most important materials in technology today. Surface properties of Si have been intensively investigated by many techniques, such as low‐energy electron diffraction (LEED)1,2 and ultraviolet photoemission spectroscopy (UPS).3,4 Most recently Sakurai and Hagstrum have studied surface electronic structures for hydrogen chemisorption on various silicon surfaces using ion neutralization spectroscopy (INS) and UPS.5 Their study suggested the formation of new hydride surface phases, such as SiH5b and SiH3,5c in addition to a well known monohydride phase SiH.5a However this conclusion was drawn without observation of those surface molecules. Here we present a new approach to the study of hydrogen chemisorption on Si surfaces. We applied atom‐probe field ion microscopy to observe directly these surface compounds. We have also succeeded in achieving a new atomic resolution of silicon in field ion microscopy.

Atomic displacement free interfaces and atomic registry in SiO2∕(1×1) Si(100)

We use ion beam analysis to probe the structure and interface of ultrathin thermal oxide films grown on (1×1) Si(100) surfaces prepared using the Herbots-Atluri [U.S. patent No. 6,613,677 (Sept. 2, 2003)] wet chemical clean. We discover that these oxide layers are structurally registered with the substrate lattice with no interfacial structural disorder. Registry of Si atoms is most pronounced along ⟨111⟩ directions relative to the Si substrate, consistent with a β-cristobalite epitaxial phase. This structurally registered phase transitions to an amorphous structure approximately 2nm from the interface.

Microstructure and stoichiometry dependence of ion beam nitrides as a function of energy and temperature: A comparative study between Si and SiGe

The microstructure and stoichiometry of nitrides formed by direct low‐energy ion beam nitridation has been investigated as a function of ion energy and substrate temperature for Si(100) and SiGe/Si(100) films. Cross‐sectional transmission electron microscopy, Rutherford backscattering spectroscopy combined with ion channeling and in situ x‐ray photoelectron spectroscopy were used. It was established that a substrate temperature of 700 K produces a homogeneous amorphous nitride layer, whereas lower substrate temperatures decrease the incorporation of nitrogen in the film, while causing the formation of a nitrogen‐poor amorphous layer beneath the nitride film. The N‐to‐Si or N‐to‐(Si+Ge) atomic ratio is found be close to 1.33 at 1 keV and decreases with ion energy. Effects of chemically enhanced physical sputtering of germanium are observed.

PHOTOILLUMINATION EFFECT ON SILICON FIELD ION MICROSCOPY.

The photoillumination effect which occurs during the field ion microscopy of silicon is investigated in detail using an UHV magnetic‐sector atom‐probe field ion microscope equipped with a retarding potential energy analyzer. It is shown that the observed enhancement of image intensity and field evaporation rate which occurs upon illumination with red light is due to the presence of an oxide layer. Measurements of energy deficits indicate that (1) the oxide layer causes a large potential drop at the emitter cap and (2) red light illumination drastically reduces or completely eliminates the reduction in potential.

Ionization of liquid metals, gallium

Field ionization of a liquid metal, such as Ga or In, has attracted considerable interest since it could become a source of strong focused ion emission. We have studied this phenomenon using the improved magnetic‐sector atom‐probe field ion microscope. The energy distributions of field ionization of Ga and critical energy deficits are measured as a function of surface temperature and electric field. The field ionization of Ga can be divided into a low and a high emission region. In the low emission region Ga+ ions are emitted at a field of approximately 0.5 V/A when the emitter is heated to approximately 600 °C. The intensity of this Ga+ emission is sufficient to yield a desorption image of the emitter surface. This implies that the ion flux is ∠103 ions/atomic site/second. When the emitter is kept at low tempertures (78 K) the emission is predominately Ga++. In both cases the ions display a sharp energy distribution (?1.5 eV FWHM) and a small energy deficit (?2.1 eV). In the high emission regime Ga++ ions are produced with a broad energy distribution (?12 eV FWHM). The total current ranges from 10−8 A or less in the low emission regime up to or greater than 10−5 A in the high emission regime.