Dale Batchelor

Pendeoepitaxy of gallium nitride thin films

Pendeoepitaxy, a form of selective lateral growth of GaN thin films has been developed using GaN/AlN/6H–SiC(0001) substrates and produced by organometallic vapor phase epitaxy. Selective lateral growth is forced to initiate from the (1120) GaN sidewalls of etched GaN seed forms by incorporating a silicon nitride seed mask and employing the SiC substrate as a pseudomask. Coalescence over and between the seed forms was achieved. Transmission electron microscopy revealed that all vertically threading defects stemming from the GaN/AlN and AlN/SiC interfaces are contained within the seed forms and a substantial reduction in the dislocation density of the laterally grown GaN. Atomic force microscopy analysis of the (1120) face of discrete pendeoepitaxial structures revealed a root mean square roughness of 0.98 A. The pendeoepitaxial layer photoluminescence band edge emission peak was observed to be 3.454 eV and is blueshifted by 12 meV as compared to the GaN seed layer.

Insects groom their antennae to enhance olfactory acuity

Grooming, a common behavior in animals, serves the important function of removing foreign materials from body surfaces. When antennal grooming was prevented in the American cockroach, Periplaneta americana, field emission gun scanning electron microscopy images revealed that an unstructured substance accumulated on nongroomed antennae, covering sensillar pores, but not on groomed antennae of the same individuals. Gas chromatography analysis of antennal extracts showed that over a 24-h period nongroomed antennae accumulated three to four times more cuticular hydrocarbons than groomed antennae. Moreover, nongroomed antennae accumulated significantly more environmental contaminants from surfaces (stearic acid) and from air (geranyl acetate) than groomed antennae. We hypothesized that the accumulation of excess native cuticular hydrocarbons on the antennae would impair olfactory reception. Electroantennogram experiments and single-sensillum recordings supported this hypothesis: antennae that were prevented from being groomed were significantly less responsive than groomed antennae to the sex pheromone component periplanone-B, as well as to the general odorants geranyl acetate and hexanol. We therefore conclude that antennal grooming removes excess native cuticular lipids and foreign chemicals that physically and/or chemically interfere with olfaction, and thus maintains the olfactory acuity of the antennae. Similar experimental manipulations of the German cockroach (Blattella germanica), carpenter ant (Camponotus pennsylvanicus), and the housefly (Musca domestica), which use different modes of antennal grooming, support the hypothesis that antennal grooming serves a similar function in a wide range of insect taxa.

Effects of Arsenic Doping on Chemical Vapor Deposition of Titanium Silicide

This work examines the effects of implanted arsenic on nucleation and growth of TiSi 2 formed by rapid thermal chemical vapor deposition using SiH 4 and TiCl 4 as the precursors. In this study depositions were carried out in a temperature range of 750 to 850°C on Si substrates implanted with As atoms. The As implant doses ranged from 3 × 10 14 to 5 × 10 15 cm -2 . It is shown that heavy dose As can result in a barrier to TiSi 2 nucleation and enhance silicon substrate consumption. A surface passivation model is proposed to explain the effects. On Si, As provides a stable surface structure which inhibits adsorption of SiH 4 and TiCl 4 . Higher temperatures aid As desorption from the Si surface providing nucleation sites. With moderate implant doses, As results in an incubation time whereas very high doses (≥5 × 10 15 cm -2 ) almost completely suppress nucleation. During deposition, As diffuses through the TiSi 2 layer and plays a similar role on the TiSi 2 surface. Because TiCl 4 adsorption on TiSi 2 is favored, the substrate supplies the Si atoms for TiSi 2 formation resulting in enhanced consumption. Because this process relies on Si diffusion through TiSi 2 , beyond a threshold thickness the efficiency of the Si diffusion process drops resulting in suppression of the deposition process. The results indicate that the As dose also plays a role in grain size and surface morphology of the deposited layers. Higher As doses result in smaller grained TiSi 2 films which can be attributed to the role of As in nucleation.

Independent evaluation of conflicting microspherule results from different investigations of the Younger Dryas impact hypothesis

Firestone et al. sampled sedimentary sequences at many sites across North America, Europe, and Asia [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 106:16016–16021]. In sediments dated to the Younger Dryas onset or Boundary (YDB) approximately 12,900 calendar years ago, Firestone et al. reported discovery of markers, including nanodiamonds, aciniform soot, high-temperature melt-glass, and magnetic microspherules attributed to cosmic impacts/airbursts. The microspherules were explained as either cosmic material ablation or terrestrial ejecta from a hypothesized North American impact that initiated the abrupt Younger Dryas cooling, contributed to megafaunal extinctions, and triggered human cultural shifts and population declines. A number of independent groups have confirmed the presence of YDB spherules, but two have not. One of them [Surovell TA, et al. (2009) Proc Natl Acad Sci USA 104:18155–18158] collected and analyzed samples from seven YDB sites, purportedly using the same protocol as Firestone et al., but did not find a single spherule in YDB sediments at two previously reported sites. To examine this discrepancy, we conducted an independent blind investigation of two sites common to both studies, and a third site investigated only by Surovell et al. We found abundant YDB microspherules at all three widely separated sites consistent with the results of Firestone et al. and conclude that the analytical protocol employed by Surovell et al. deviated significantly from that of Firestone et al. Morphological and geochemical analyses of YDB spherules suggest they are not cosmic, volcanic, authigenic, or anthropogenic in origin. Instead, they appear to have formed from abrupt melting and quenching of terrestrial materials.

Optimization of Process Conditions for Selective Silicon Epitaxy Using Disilane, Hydrogen, and Chlorine

We have previously reported a process for low temperature selective silicon epitaxy using Si 2 H 6 , H 2 , and Cl 2 in an ultrahigh vacuum rapid thermal chemical vapor deposition reactor. Selective deposition implies that growth occurs on the Si surface but not on any of the surrounding insulator surfaces. Using this method and process chemistry, the level of Cl species required to maintain adequate selectivity has been greatly reduced in comparison to SiH 2 Cl 3 -based, conventional CVD approaches. In this report, we have extended upon the previous work and provide information regarding the selectivity of the silicon deposition process to variations in the growth conditions. We have investigated the selectivity of the process to variations in disilane flow/partial pressure, growth temperature, and system contamination. We demonstrate that increases in either the Si 2 H 6 partial pressure or flow rate, the process temperature, or the source contamination levels can lead to selectivity degradation. In regard to the structural quality of the selective epitaxial layers, we have observed epitaxial defects that have appeared to be a strong function of two basic conditions: the contamination level of the process and the chlorine flow rate or chlorine partial pressure. Overall, the results in this study indicate several process conditions that can inhibit the quality of a selective silicon deposition process developed for single-wafer manufacturing.

Deposition and characterization of polysilicon films deposited by rapid thermal processing

Low‐pressure chemical vapor deposition (LPCVD) of polycrystalline silicon in a cold‐wall, lamp‐heated, rapid‐thermal processor was investigated. Blanket polysilicon films were obtained by the pyrolysis of a 10% silane–argon gas mixture onto (100) Si wafers which were capped with a thermal SiO2 layer. The depositions were performed at a total pressure ranging from 1 to 5 Torr and in a temperature range from 575 to 850 °C. It was found that the deposition of films was controlled by a surface limited reaction at temperatures below ∼780 °C, and an activation energy of 39±2 kcal/mol was measured for this reaction. Above 780 °C, a decrease in activation energy was observed. To meet the throughput requirement of single wafer processing, deposition temperatures higher than 700 °C are needed. In this temperature range, deposition rates exceed 1000 A/min as compared to 20–300 A/min in conventional LPCVD furnaces. The structural characteristics of the films were assessed by ultraviolet surface reflectance, Raman spe...

Rapid thermal chemical vapor deposition of in situ boron-doped polycrystalline silicon-germanium films on silicon dioxide for complimentary-metal-oxide-semiconductor applications

In situ boron-doped polycrystalline Si1−xGex (x>0.4) films have been formed on the thermally grown oxides in a rapid thermal chemical vapor deposition processor using SiH4-GeH4-B2H6-H2 gas system. Our results showed that in situ boron-doped Si1−xGex films can be directly deposited on the oxide surface, in contrast to the rapid thermal deposition of undoped silicon-germanium (Si1−xGex) films on oxides which is a partially selective process and requires a thin silicon film pre-deposition to form a continuous film. For the in situ boron-doped Si1−xGex films, we observed that with the increase of the germane percentage in the gas source, the Ge content and the deposition rate of the film are increased, while its resistivity is decreased down to 0.66 mΩ cm for a Ge content of 73%. Capacitance-voltage characteristics of p-type metal-oxide-semiconductor capacitors with p+-Si1−xGex gates showed negligible polydepletion effect for a 75 A gate oxide, indicating that a high doping level of boron at the poly-Si1−xGex...

Rapid Thermal Chemical Vapor Deposition of Polycrystalline Silicon-Germanium Films on SiO 2 and Their Properties

In this work, polycrystalline SiGe has been viewed as an alternative gate material to polysilicon in single wafer processing for the deep submicrometer VLSI applications. We studied deposition of the silicon-germanium (SiGe) films with different germanium concentrations (up to 85%) on SiO 2 in a rapid thermal chemical vapor deposition reactor using GeH 4 and SiH 4 /H 2 gas mixture with the temperature ranging from 550°C to 625°C. Since the SiGe RTCVD process is selective toward oxide and does not form nucleation sites on the oxide easily, an in-situ polysilicon flash technique is used to provide the necessary nucleation sites for the deposition of SiGe films with high germanium content. It was observed that with the in-situ polysilicon flash as a pre-nucleation seed, the SiGe deposited on SiO 2 forms a continuous polycrystalline layer. Polycrystalline SiGe films of about 2000A in thickness have a columnar grain structure with a grain size of approximately 1000A. Compositional analyses from Auger Electron Spectroscopy (AES) and Rutherford backscattering (RBS) show that the high germanium incorporation in the SiGe films has a weak dependence on the deposition temperature. It is also noted that the germanium content across the film thickness is fairly constant which is a critical factor for the application of SiGe films as the gate material. Lastly, we found that the surface morphology of SiGe films become smoother at lower deposition temperature.

Reply to Boslough: Prior studies validating research are ignored

In PNAS, M. Boslough (1) raises issues about carbon spherules and nanodiamonds unrelated to our magnetic spherule focused research (2). Boslough should instead address the questions he raises to the appropriate investigators.