R. J. Nemanich

Raman scattering characterization of carbon bonding in diamond and diamondlike thin films

The atomic bonding configurations of carbon bonding in diamond and diamondlike thin films are explored using Raman scattering. The general aspects of Raman scattering from composites are presented. Effects are discussed due to crystalline or amorphous structures, large versus microcrystalline domains, and strong optical absorption and transparent regions. The Raman scattering from diamondlike films shows several features which are attributed to microcrystalline graphitelike structures which all originate from the same region in the sample. In contrast, the spectra of diamond films show features attributed to different components of a composite film. Components identified are crystalline diamond, and disordered and microcrystalline graphitic structures. The presence of precursor microcrystalline or amorphous diamond structures is also suggested.

Gold Schottky contacts on oxygen plasma-treated, n-type ZnO(0001̄)

Reverse bias current–voltage measurements of ∼100-μm-diameter gold Schottky contacts deposited on as-received, n-type ZnO(0001) wafers and those exposed for 30 min to a remote 20% O2/80% He plasma at 525±20 °C and cooled either in vacuum from 425 °C or the unignited plasma gas have been determined. Plasma cleaning resulted in highly ordered, stoichiometric, and smooth surfaces. Contacts on as-received material showed μA leakage currents and ideality factors >2. Contacts on plasma-cleaned wafers cooled in vacuum showed ∼36±1 nA leakage current to −4 V, a barrier height of 0.67±0.05 eV, and an ideality factor of 1.86±0.05. Cooling in the unignited plasma gas coupled with a 30 s exposure to the plasma at room temperature resulted in decreases in these parameters to ∼20 pA to −7 V, 0.60±0.05 eV, and 1.03±0.05, respectively. Differences in the measured and theoretical barrier heights indicate interface states. (0001) and (0001) are used in this letter to designate the polar zinc- and oxygen-terminated surfac...

Raman spectroscopy of diamond and doped diamond

The optimization of diamond films as valuable engineering materials for a wide variety of applications has required the development of robust methods for their characterization. Of the many methods used, Raman microscopy is perhaps the most valuable because it provides readily distinguishable signatures of each of the different forms of carbon (e.g. diamond, graphite, buckyballs). In addition it is non-destructive, requires little or no specimen preparation, is performed in air and can produce spatially resolved maps of the different forms of carbon within a specimen. This article begins by reviewing the strengths (and some of the pitfalls) of the Raman technique for the analysis of diamond and diamond films and surveys some of the latest developments (for example, surface-enhanced Raman and ultraviolet Raman spectroscopy) which hold the promise of providing a more profound understanding of the outstanding properties of these materials. The remainder of the article is devoted to the uses of Raman spectroscopy in diamond science and technology. Topics covered include using Raman spectroscopy to assess stress, crystalline perfection, phase purity, crystallite size, point defects and doping in diamond and diamond films.

Cleaning of AlN and GaN surfaces

Successful ex situ and in situ cleaning procedures for AlN and GaN surfaces have been investigated and achieved. Exposure to HF and HCl solutions produced the lowest coverages of oxygen on AlN and GaN surfaces, respectively. However, significant amounts of residual F and Cl were detected. These halogens tie up dangling bonds at the nitride surfaces hindering reoxidation. The desorption of F required temperatures >850 °C. Remote H plasma exposure was effective for removing halogens and hydrocarbons from the surfaces of both nitrides at 450 °C, but was not efficient for oxide removal. Annealing GaN in NH3 at 700–800 °C produced atomically clean as well as stoichiometric GaN surfaces.

Observation of a negative electron affinity for heteroepitaxial AlN on α(6H)‐SiC(0001)

This study demonstrates the presence of a negative electron affinity (NEA) surface on AlN was grown on α(6H)‐SiC. Heteroepitaxial AlN was grown on α(6H)‐SiC(0001) substrates by molecular beam epitaxy techniques. The surface electronic states were characterized by ultraviolet photoemission obtained at surface normal. The observation of a sharp spectral feature at the lowest energy of the emitted electrons is an indication of a surface with a negative electron affinity. In addition, the trend of the NEA feature was examined as a function of annealing. The surface Fermi level is found to be near the middle of the AlN gap, and a possible band alignment between the AlN and SiC is presented.

Morphology and phase stability of TiSi2 on Si

The formation mechanisms and properties of TiSi2 on Si are investigated. The particular emphasis is in relating the nucleation, morphology, and phase stability of the films. TiSi2 films were prepared by deposition of Ti on atomically clean silicon substrates in ultrahigh vacuum. The silicide formation was initiated either by in situ annealing or deposition onto heated substrates. The island formation of TiSi2 and surface and interface morphologies of TiSi2 were examined by scanning electron microscopy and transmission electron microscopy. The TiSi2 formation process was monitored with in situ Auger electron spectroscopy and low‐energy electron diffraction to analyze the surface concentration and the surface structures, respectively. Raman spectroscopy was used for phase identification of the TiSi2. Titanium film thicknesses from 50 to 400 A were examined. For all thicknesses studied, the C49 TiSi2 phase is observed to nucleate. Immediately after low‐temperature deposition, the interface morphology was smo...

Defects in plasma-deposited a-Si: H

The relationships between hydrogen vibrational spectra, electron spin densities and refractive index are investigated for a range of plasma-deposited amorphous silicon-hydrogen alloys. Results are also presented on the morphology of thick films as shown by scanning electron microscopy. A model is proposed for the structural origin of defects in these alloys based on voids that grow perpendicular to the film surface and are associated with coupled SiH2 units.

THE STRUCTURE AND PROPERTY CHARACTERISTICS OF AMORPHOUS NANOCRYSTALLINE SILICON PRODUCED BY BALL-MILLING

The structural transformation of polycrystalline Si induced by high energy ball milling has been studied. The structure and property characteristics of the milled powder have been investigated by x-ray diffraction, scanning electron microscopy, high-resolution electron microscopy, differential scanning calorimetry, Raman scattering, and infrared absorption spectroscopy. Two phase amorphous and nanocrystalline components contain some defects such as dislocations, twins, and stacking faults which are typical of defects existing in conventional coarse-grained polycrystalline materials. The volume fraction of amorphous Si is about 15% while the average size of nanocrystalline grains is about 8 nm. Amorphous elemental Si without combined oxygen can be obtained by ball milling. The distribution of amorphous Si and the size of nanocrystalline Si crystallites is not homogeneous in the milled powder. The amorphous Si formed is concentrated near the surface of milled particles while the grain size of nanocrystalline Si ranges from 3 to 20 nm. Structurally, the amorphous silicon component prepared by ball milling is similar to that obtained by ion implantation or chemical vapor deposition. The amorphous Si formed exhibits a crystallization temperature of about 660-degrees-C at a heating rate of 40 K/min and crystallization activation energy of about 268 kJ/mol. Two possible amorphization mechanisms, i.e., pressure-induced amorphization and crystallite-refinement-induced amorphization, are proposed for the amorphization of Si induced by ball milling.

Infrared active optical vibrations of graphite

The infrared refectivity of graphite has been measured for E⊥c and E□c and the frequencies of the “out-of-plane” A2u symmetry mode and the “in-plane” E1u symmetry mode have been determined to be 868 cm−1, respectively. Macroscopic effective charges for each mode have also been calculated, 0.08e for A2u and 0.41e for E1u. The assignment of the A2u frequency has enabled us to evaluate the magnitude of a four-body force that has been used to characterize the graphite lattice modes involving out-of-plane displacements.

Direct studies of domain switching dynamics in thin film ferroelectric capacitors

An experimental approach for direct studies of the polarization reversal mechanism in thin film ferroelectric capacitors based on piezoresponse force microscopy (PFM) in conjunction with pulse switching capabilities is presented. Instant domain configurations developing in a 3×3μm2 capacitor at different stages of the polarization reversal process have been registered using step-by-step switching and subsequent PFM imaging. The developed approach allows direct comparison of experimentally measured microscopic switching behavior with parameters used by phenomenological switching models. It has been found that in the low field regime (just above the threshold value) used in the present study, the mechanism of polarization reversal changes during the switching cycle from the initial nucleation-dominated process to the lateral domain expansion at the later stages. The classical nucleation model of Kolmogorov–Avrami–Ishibashi (KAI) provides reasonable approximation for the nucleation-dominated stage of switchi...