We compare experimental fluctuation electron microscopy (FEM) speckle data with electron diffraction simulations for thin amorphous carbon and silicon samples. We find that the experimental speckle intensity variance is generally more than an order of magnitude lower than kinematical scattering theory predicts for spatially coherent illumination.

We hypothesize that decoherence, which randomizes the phase relationship between scattered waves, is responsible for the anomaly. Specifically, displacement decoherence can contribute strongly to speckle suppression, particularly at higher beam energies. Displacement decoherence arises when the local structure is rearranged significantly by interactions with the beam during the exposure. Such motions cause diffraction speckle to twinkle, some of it at observable time scales.

We also find that the continuous random network model of amorphous silicon can explain the experimental variance data if displacement decoherence and multiple scattering is included in the modeling. This may resolve the longstanding discrepancy between X-ray and electron diffraction studies of radial distribution functions, and conclusions reached from previous FEM studies.

Decoherence likely affects all quantitative electron imaging and diffraction studies. It likely contributes to the so-called Stobbs factor, where high-resolution atomic-column image intensities are anomalously lower than predicted by a similar factor to that observed here.

Diamond is a unique material that often exhibits extreme properties compared to other materials. Discovered about 30 years ago, the use of hydrogen in plasma-enhanced chemical vapor deposition (CVD) has enabled the growth and coating of diamond in film form on various substrate materials. CVD diamond research has been actively continued subsequently to develop new understanding and approaches for the growth and processing of this fascinating material. Currently, the study and development of diamond films has enabled a wide range of applications based on the combination of unique and extreme properties of diamond and the variety of film properties obtainable through tuning the microstructure, morphology, impurities, and surfaces. This issue of MRS Bulletin introduces the latest research, recent applications, and the challenges ahead for CVD diamond films.

Carbon nanotubes (CNTs) have been grown by catalytical chemical vapor deposition (CCVD) with a thin iron layer as the catalyst. High surface tension metal, tantalum (Ta), and low surface tension, SiO2, have been deposited as the supporting layers before depositing the catalysts. SEM, TEM, STEM and EELS have been used to examine the morphology, structure, and chemical profile of iron nanoparticles and CNTs. The results have shown that the catalyst nanoparticle morphologies were distinctly different on two supports. In particular, Fe nanoparticles on SiO2 were found to follow a Vollmer-Weber (VW) growth mode and a Stranski-Krastanov (SK) growth mode on Ta. It was also found that CNT growth varied significantly on two supports in terms of morphology, growth rate and growth mode. Dense CNTs were grown on Ta with fast growth rates (> 1µm/min) and vertical alignment for the iron thicknesses of 1.5-9 nm. In contrast, CNTs grown on SiO2 exhibited a slow growth rate (< 100 nm/min) with all deposited iron thicknesses, indicating a severe catalyst poisoning. The results suggested that the catalyst morphology in combination with the presence of an iron wetting layer contributed to the enhanced CCVD growth of CNTs on Ta.

UV Raman scattering is employed as a nondestructive structure sensitive probe to investigate the vibrational properties of the wide bandgap, machined and indented surfaces of 6H-SiC and β-Si3N4. In these materials, the short absorption depth of UV light allows for accurate probing of the surface, and the transparency to visible light allows for analysis of the bulk material. The study on 6H-SiC (0001) included measurements of indentations, and of machined circular (0001) wafer edges. The indentation analysis indicates the response of the material to localized pressures. Machined 6H-SiC wafer edges and machined β-Si3N4 surfaces indicate a ductile response and ductile material removal for machining at cutting depths on a nm and μm scale. Raman scattering measurements of the ductile surfaces and ductile material removed indicate residual structure changes. The residual surface structures could indicate that a high-pressure phase transformation is the origin of a ductile response on machined brittle materials.

Polycrystalline diamond films, single crystal bulk diamonds, and diamond powder were treated in microwave plasma of hydrogen at 1.6 torr under a negative direct-current bias of −150 to −300 V without metal catalyst. It was found that fibrous structures, uniformly elongated along the direction normal to the specimen surface, were formed on the diamond surfaces. Similar experiments for glasslike carbon resulted in conical structures with frizzy fibers at the tops. Transmission electron microscopy measurements indicated that the fibers formed on diamond consisted of randomly oriented diamond nanocrystals with diameters of less than 10 nm, while the conical structures formed on glasslike carbon consisted of graphite nanocrystals. Field emission measurements of the fibrous specimens exhibited better emission efficiency than untreated ones. The field emission electron microscopy of the fibrous glasslike carbon showed a presence of discrete electron emission sites at a density of approximately 10,000 sites/cm2.

Wide band gap semiconductors exhibit a low electron affinity and may prove suitable for cold cathode applications. We introduce a simple closed-form analytic approximation for the stability of electrons in the electron accumulation layer of planar Low Electron Affinity (LEA) semiconducting cathodes. This analysis extends our previous results, which used Runge-Kutta numerical integration of the linearized equations of motion for the electric potential and quasi Fermi level. The model shows conditions in which the electrons in the accumulation layer form a two dimensional array of regions of higher and lower electron density. This instability could lead to field enhancement without surface roughness and could account for observed electron emission at low applied fields.

AlN layers were grown on 6H-SiC(0001) by molecular beam epitaxy using ammonia as the nitrogen source. Clean (√3×√3)R30° SiC surfaces was prepared by in-situ annealing alone and also by in situ annealing consisted of followed by Si deposition and subsequent annealing. The surface morphology of the AlN films observed by AFM was significantly changed by the nucleation procedure. When the AlN growth was initiated with Al flux exposure on a SiC surface prepared by thermal annealing, the surface roughness of the AlN was significantly reduced. Two-dimensional growth of AlN was observed with reflection high-energy electron diffraction from the very beginning. Atomically flat AlN surfaces with a RMS-roughness of ∼0.3 nm were obtained. On the other hand, when film growth was initiated with an ammonia flux exposure on a Si rich SiC surface, a high density of bumps was observed. The bumps seemed to originate from SiNx formation at the heteroepitaxial interface. It was found that control of the Si composition and the V/III ratio at the growth interface is crucial for the AlN film quality.

The 4% lattice mismatch between Si and Ge creates strain in epitaxial layers of SiGe alloys on Si, and this strain can manifest itself in the morphological structure of the surface of the epitaxial layer. This study explores the relationship of the evolution of the surface morphology of SiGe layers grown on a range of Si surface orientations. We have grown thin, strained and thick, relaxed layers of Si0.7Ge0.3 by solid source molecular beam epitaxy on substrates with surface normals rotated from [001] towards [111] by angles of θ = (0, 2, 4, 10, 22) degrees. The surface morphology was investigated by atomic force microscopy, which showed considerable ordering of surface features on relaxed samples. These features evolve from hut-like structures at 0 degrees to large mesa-like structures separated by pits and crevices at 22 degrees. The organization of these features is also shown to vary with the substrate orientation. Each surface has characteristic directions along which features are aligned, and these directions vary continuously with the angle of rotation of the substrate. Transmission electron microscopy confirmed that misfit dislocations had formed along those same directions. The state of relaxation of each layer is quantified by Raman spectroscopy in order to make a direct correlation between residual strain and surface morphology.

We report on UV Raman spectroscopy of InxGa1−xN thin films grown on (0001) sapphire substrates using a specially designed metal-organic chemical vapor deposition (MOCVD) reactor. Eight films were examined in the compositional range 0<×<0.50. Mid and deep-UV Raman spectroscopy was done with the 325.2 nm line of the HeCd laser and the 244 nm line of a double frequency Ar ion laser. The mode behavior of the A1(LO) and E2 phonons was also investigated. We have found compelling evidence for one-mode behavior for the A1(LO) phonon mode, while our data for the E2 mode deviates from the predictions for one-mode behavior. The results for the A1(LO) mode are consistent with the previously reported phonon mode behavior in AlGaN alloys. Also, evidence regarding the presence of compositional inhomogeneities and spinodal decomposition in InGaN thin films is presented.

Cobalt disilicide contacts to silicon–germanium alloys were formed by direct deposition of pure cobalt metal onto silicon–germanium films on Si(001) substrates. Segregation of germanium was observed during the reaction of the cobalt with the silicon–germanium alloy. The nature of the Ge segregation was studied by transmission electron microscopy, energy dispersive spectroscopy, and x-ray diffraction. In the case of cobalt films deposited onto strained silicon–germanium films, the Ge segregation was discovered to be in the form of Ge-enriched Si1−xGex regions found at the surface of the film surrounding CoSi and CoSi2 grains. In the case of cobalt films deposited onto relaxed silicon–germanium films, the Ge segregation was dependent on formation of CoSi2. In samples annealed below 800 °C, where CoSi was the dominant silicide phase, the Ge segregation was similar in form to the strained Si1−xGex case. In samples annealed above 800 °C, where CoSi2 was the dominant silicide phase, the Ge segregation was also in the form of tetrahedron-shaped, Ge-enriched, silicon–germanium precipitates, which formed at the substrate/silicon– germanium film interface and grew into the Si substrate. A possible mechanism for the formation of these precipitates is presented based on vacancy generation during the silicidation reaction coupled with an increased driving force for Ge diffusion due to silicon depletion in the alloy layer.

The quasi-LO and quasi-TO modes of AlN crystallite were investigated. The analysis indicates that the Raman mode behavior concurs with Loudons’ model of mode-mixing in wurtzite (WZ) structure crystals which is due to the long-range electrostatic field. Phononlifetimes of GaN and AlN crystallites were studied via Raman lineshape. It was found that the low energy E2 mode lifetime is about an order of magnitude longer than that of the other modes, and that impurities impact significantly the phonon-lifetimes.

The quasi-LO and quasi-TO modes of AIN crystallite were investigated. The analysis indicates that the Raman mode behavior concurs with Loudons’ model of mode-mixing in wurtzite (WZ) structure crystals which is due to the long-range electrostatic field. Phonon-lifetimes of GaN and AIN crystallites were studied via Raman lineshape. It was found that the low energy E2 mode lifetime is about an order of magnitude longer than that of the other modes, and that impurities impact significantly the phonon-lifetimes.

Raman analysis of the E2 mode of AlxGal-xN in the composition range 0 ≤ x ≤ 1 is presented. The lineshape was observed to exhibit a significant asymmetry and broadening toward the high energy range. The spatial correlation model is discussed, and is shown to account for the lineshape. The model calculations also indicate the lack of a long-range order in the CVD (chemical vapor deposition) alloys. These results were confirmed by X-ray scattering: the relative intensity of the superlattice line was found to be negligible. The line broadening of the E2 mode was found to exhibit a maximum at a composition x∼0.5 indicative of a random disordered alloy system. The stress state of the alloys was found to be tensile and was attributed to the difference in the thermal expansion coefficients of the SiC substrate and the film.

Cold cathodes have been fabricated using two different nitride structures as a thin film emitting layer. The A1N and graded AlGaN structures are prepared by metalorganic chemical vapor deposition (MOCVD) on an n-type 6H-SiC substrate. Individual aluminum grids are perforated with an array of either 1, 3, or 5μm holes through which the emitting surface is exposed. After device fabrication, a hydrogen plasma exposure was found to be necessary to activate the cathode. The devices have displayed a limited lifetime and a small percentage of the devices operate, although half of the devices with 5μm holes functioned. The highest measured collector currents are 0.1μA for A1N and l0nA for AlGaN at grid voltages of 110V and 20V, respectively. The grid currents are typically 10 to 104 times the collector currents.

Cobalt disilicide films have been formed on strained epitaxial Si0.80Ge0.20/Si(100) alloys via co-deposition of silicon and cobalt. Co-deposition is shown to improve the epitaxy and prevent the phase segregation commonly observed with the formation of Co/SiGe contacts using other methods such as the direct deposition of cobalt onto SiGe or the sequential deposition of a silicon sacrificial layer and cobalt onto SiGe. EXAFS measurements at the cobalt K edge indicate that co-deposited films annealed at 500–700° C are indeed crystalline CoSi2 throughout this temperature range. The XRD patterns of the co-deposited films do not exhibit any of the CoSi2 (111), (220) or (311) peaks normally associated with other preparation methods. The sheet resistance and r.m.s. roughness of the CoSi2 films increase monotonically with annealing temperature. These results indicate that co-deposited films are epitaxial to the (100)-oriented SiGe substrate and suggest that low thermal budget, low resistivity contacts to strained SiGe can be grown with this method. Issues related to the presence of Ge at the CoSi2/substrate interface will be discussed.

Comparisons between the wetting characteristics of (0001)si 6H-SiC and (111) Si surfaces in various acids and bases were made. It was found that 10:1 HF dipped Si (111) surfaces were hydrophobic where as the (0001)si 6H-SiC surfaces were hydrophilic. (0001)si 6H-SiC surfaces capped with a 20Å Si layer, however, were hydrophobic after HF dipping and exhibited outgassing levels on annealing which were several orders of magnitude lower than SiC wafers dipped in HF without the capping layer. Annealing the Si capped (0001)si 6H-SiC surfaces in UHV at 1100°C for 5 min. was found to be sufficient to thermally desorb the Si capping layer and produce a (3×3) Si rich, oxygen free (0001)si 6H-SiC surface.