No abstract

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.

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.

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 CH 3 CN + 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( 3 P 0,2 ) + BrCN → Ar + Br + CN(A 2 i , B 2 + , 4 + , 4 ). 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.

The amorphous phase of hydrogenated carbon nitride, a-CN x :H (0 ≦x ≦1), films may have clusters consisting of a mixture of sp 2- and sp 3-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-CN x :H (hardness: 1–8 GPa), mechanically hard a-CN x :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-CN x :H, hard a-CN x :H and hard a-C:H, respectively.