Shigeo Ohshio

Epitaxial Growth of Zinc Oxide Whiskers by Chemical-Vapor Deposition under Atmospheric Pressure.

ZnO whiskers were epitaxially grown by a chemical-vapor deposition technique employed at atmospheric pressure. Highly oriented ZnO whiskers grew at a substrate temperature of 550°C on (0001)α-Al2O3 substrates with a growth rate of 3.7 nm/s. X-ray diffractometry revealed that the epitaxial relationship between the whiskers and the substrate was determined as ZnO[1010](0001)//Al2O3[1210](0001) or ZnO[1210](0001)//Al2O3[1010](0001). In addition, the full-width at half maximum value of the (0002) reflection was as low as 0.43°. Images obtained using a scanning electron microscope were analyzed and it was found that the whisker tip likely has a radius of curvature of approximately 20 nm. The typical number density of the whiskers has reached 1.3×105mm-2.

Conductive indium-doped zinc oxide films prepared by atmospheric-pressure chemical vapour deposition

Abstracts are not published in this journal

Homogeneous Growth of Zinc Oxide Whiskers

Several common modes of crystal growth provide particularly simple and elegant examples of spontaneous pattern formation not only in nature but also under artificial circumstances. We have already reported that well-organized ZnO whiskers are epitaxially grown using a chemical vapor deposition technique [Satoh et al..: Jpn. J. of Appl. Phys. 38 (1999) L586]. One aim of this study is to determine the optimum growth conditions for obtaining the structure containing homogeneous whiskers grown with a relatively high growth rate. A substrate temperature of 550°C and a vaporizing temperature of 125°C are the most appropriate for obtaining homogeneous whiskers. Whiskers are highly oriented in the a-and c-axes directions of the hexagonal structure. The growth rate reached a maximum value as high as 7.5 nm/s.

Quantitative Analysis of Hydrogen in Amorphous Films of Hydrogenated Carbon Nitride

The amorphous phase of hydrogenated carbon nitride, a-CNx:H (0 x 1), films may have clusters consisting of a mixture of sp2- and sp3-hybridized materials with cluster sizes of 0.2–2 nm. The hydrogen termination limits the size of the carbon and carbon nitride clusters. It also influences the mechanical properties of the sample. In this experiment, the relationship between the hydrogen content and the mechanical properties of carbon and related materials was investigated using elastic recoil detection analysis (ERDA), nanoindentation techniques and Raman spectroscopy. The samples were classified into three categories of hardness: mechanically soft a-CNx:H (hardness: 1–8 GPa), mechanically hard a-CNx:H (8–30 GPa) and hard hydrogenated amorphous carbon (a-C:H) (more than 30 GPa). The hydrogen contents of the sample were 10–50 at.%, 5–40 at.%, and less than 3 at.% for soft a-CNx:H, hard a-CNx:H and hard a-C:H, respectively.

Synthesis of Amorphous Carbon Nitride Films Using Dissociative Excitation Reaction

In this investigation, we aim to produce highly nitrogen-doped carbon, so-called carbon nitride, films without the incorporation of hydrogen. In the physical vapor deposition process, irradiation by energetic nitrogen ions increases nitrogen content without the incorporation of hydrogen. In the chemical vapor deposition process, hydrogen should be included into the film due to the use of a hydrocarbon reactant. In this study, the synthesis of carbon nitride films having high nitrogen and low hydrogen contents was attempted using a chemical-vapor-deposition apparatus. First of all, a CH3CN+Ar mixture was selected as a reactant including hydrogen. Dehydrogenation of the reactant was carried out by plasma decomposition. Second, as a reaction system without hydrogen, BrCN+Ar was also selected for starting materials. The dissociative excitation reaction of cyanides with argon metastable atoms produces CN radicals, Ar(3P0,2)+BrCN →Ar+Br+CN(A2Πi, B 2Σ+, 4Σ+, 4Π). This finally proceeds to the deposition of CN radicals to form the carbon nitride film on a solid-state surface. When using the former reactant, large amounts of hydrogen remained in the amorphous carbon nitride films, although the amount of hydrogen varied with deposition conditions. The sample formed using the latter reactant was amorphous carbon nitride with very little hydrogen. The nitrogen fraction [N]/([N]+[C]) of the sample using the latter rectant is as high as ~0.3, higher than those obtained from the samples synthesized with the former reactant.

PREPARATION OF NITROGEN CONTAINING CARBON FILMS USING CHEMICAL VAPOR DEPOSITION ENHANCED BY ELECTRON CYCLOTRON RESONANCE PLASMA

A chemical vapor deposition apparatus enhanced by electron cyclotron resonance plasma was employed to deposit nitrogen containing carbon films. In the apparatus, negative dc bias voltage was applied to the substrate for acceleration of positive ions toward the substrate. The deposition rate and nitrogen content of the film was found to be mainly dependent upon the deposition conditions. Although a large N2 flow rate and bias voltage contribute to inhibit film growth through surface sputtering of the substrate, an optimum [N2]/([CH4]+[N2]) flow rate of 0.67 and a bias voltage of 50 V promote nitrogen implantation into the growing films through possible nitrogen ion bombardment.

Solid Solubility of Nitrogen in Amorphous Carbon Films Deposited in Electron Cyclotron Resonance Plasma

Electron cyclotron resonance plasma is used in the formation of nitrogen-containing carbon films with a CH4+N2 gas mixture. To accelerate cations from the plasma room, rf-induced negative dc bias voltage is applied to the Si substrate. An increase in negative bias is effective for implantation of nitrogen, however, the nitrogen fraction saturates at [N]/([N]+[C])=0.09 at 300 V. The results of X-ray photoelectron spectroscopy suggests that the films have the C–N–C bond or nitrogen terminates the carbon dangling bond. In addition, ion acceleration causes a G-band and a D-band to appear on IR spectrum, which would normally appear on the Raman spectrum obtained from typical diamond-like carbon films. Furthermore, the hardness and maximum Youngs modulus of nitrogen-containing carbon films are comparable to those of most diamond-like carbon films.

Dehydrogenation of Nitrogen-Containing Carbon Films by High-Energy He2+ Irradiation

It is well known that amorphous carbon and related films consist of nano-sized carbon clusters. With the incorporation of hydrogen into these films, the hydrogen termination limits the cluster size and decreases the bond strength among clusters. In this study, dehydrogenation from amorphous hydrogenated carbon nitride films was accomplished using 3.75 MeV-He2+ irradiation. The hydrogen atoms forming methyl and ethylene groups were mainly removed from the film during the irradiation procedure. With the progress of dehydrogenation from methyl and ethylene groups, the degree of order of the atomic configuration became large, resulting in an increase of the cluster size. The mechanical properties of amorphous films were improved due to cluster growth.

Field emission property of Al:ZnO whiskers modified by amorphous carbon and related films

A new type of ceramic field emitter was designed using the chemical vapor deposition process. There are two major features of the materials used to construct the emitter: an electrically conductive Al:ZnO whisker with an extremely sharp tip and amorphous carbon and related coating materials with a relatively low electron affinity at the tip region. Some aggregations of whiskers were synthesized as a function of the radius of curvature at the whisker tip. In addition, four types of amorphous coatings were provided for this experiment: C:H, CNx:H with C–H and N–H terminations, CNx:H with N–H termination and CNx films. The field emission property was influenced not only by the radius of curvature at the tip but also by the termination structure of the amorphous carbon and related films.

Hardness and Structure of a-CNx Films Synthesized by Chemical Vapor Deposition

Mechanically hard a-CNx films were synthesized using a combination of ion bombardment and the chemical vapor deposition process using the dissociative excitation reaction of BrCN with Ar metastable atoms. Nanoindentation tests disclosed that the indentation hardness, Youngs modulus and elastic recovery increased with increasing ion-accelerating voltage. Moreover, the degree of flow among clusters decreased in the ion-bombarded sample. The D (disordered)-band absorption on an infrared absorption spectrum was replaced by a C–N absorption assigned to the tertiary aromatic amine. These results suggest that the internal and external structures of the carbon nitride cluster change from the two-dimensional order to the three-dimensional order of C–N. The structure of hard a-CNx is clearly distinguishable from nitrogen-containing diamond-like carbon.