N. Jiang

Synthesis and structural study of nano/micro diamond overlayer films

Abstract The nano/micro diamond overlayer films have been successfully fabricated by means of bias growth technique using microwave plasma-enhanced chemical vapor deposition method. During the diamond growth, as a negative bias (−100xa0V) is applied midway on the substrate side, the nanocrystalline diamond layer can be controllably deposited on the as-grown microcrystalline diamond to form a nano/micro overlayer structure. Transmission electron microscopy images reveal that the nanocrystalline layer consists of ultra-fine diamond crystallites, of which the average grain size is only about 4–5xa0nm. The formation mechanism of diamond nanocrystals on diamond micrograins under the bias conditions is discussed. Moreover, by the repeat-biasing way, three or more layered diamond films can be produced as well.

Reducing the grain size for fabrication of nanocrystalline diamond films

Abstract Microwave plasma-enhanced chemical-vapor-deposited (MPECVD) diamond films grown under various bias conditions applied to the substrates have been examined by electron microscopy. It is noticed that in a certain range, increasing the bias voltage can effectively reduce the diamond grain size. Transmission electron microscopy (TEM) images distinctly reveal that the diamond films consisting of ultrafine crystallites of about 7–10xa0nm can be successfully fabricated by applying an appropriate bias voltage. Comparing the conventional diamond films deposited without bias and the nanocrystalline diamond films grown with a bias, one can see that the latter have better field emission properties.

Growth and structural analysis of nano-diamond films deposited on Si substrates pretreated by various methods

Nanocrystalline diamond lms have been deposited on Si substrates by microwave plasma-enhanced chemical-vapordeposition (MPECVD) method. The microstructure and morphology of the as-deposited carbon lms are found to be strongly in#uenced by substrate pretreatment methods. Under higher methane concentration conditions, nanocrystalline diamond lms could be grown on Si substrates which were ultrasonically scratched both by diamond and SiC powders, but their surface topographies were rather di!erent. On the substrates which were mechanically abraded by the Al 2 O 3 grits, nano-diamond particles are formed in the amorphous matrix, that is considered to be the diamond-like carbon (DLC). The pretreatment e!ects on the formation of nano-diamonds are discussed related to the nucleation processes. The study also indicates that by a two-step (high methane concentration followed by a low methane concentration) growth process, even on the substrates scratched by the low-cost nondiamond abrasives, fabrication of polycrystalline diamond lms with high grain density is possible. ( 2000 Elsevier Science B.V. All rights reserved.

Carbon nanofibers synthesized by decomposition of alcohol at atmospheric pressure

In the present study, we fabricated the carbon nanofibers (CNFs) by decomposition of methyl alcohol at atmospheric pressure. The CNFs were grown on Ni/Si substrates using simplified hot-filament chemical vapor deposition equipment. The deposits mainly consist of the semicrystalline CNFs, in which a few of carbon nanotubes are included. On the 30-nm-thick Ni/Si substrates, the mean length of the CNFs is 2–3 μm, and their average diameter is less than 100 nm. The as-deposited CNFs were evaluated by both scanning and transmission electron microscopes. The field-electron-emission properties of CNFs were characterized as well.

Structural characteristics and field electron emission properties of nano-diamond/carbon films

Nano-diamond/carbon (NDC) films were fabricated by the microwave plasma-enhanced chemical-vapor-deposition method. It is found that such NDC films exhibit much better electron emission properties. The turn-on field is only ≈2.5 V/μm, which is smaller by a factor of eight than that of the conventional microcrystalline diamond films. Raman spectroscopy and curve fitting technique were employed to investigate the bonding structure of the NDC films. The films are confirmed to consist of sp2–sp3 bonded amorphous matrix in which the high-density diamond crystallites are embedded. Transmission electron microscopy observation indicates that the average size of the diamond crystallites is less than 5 nm. The good field emission properties of NDC films are discussed relating to film microstructure and electrical conductivity.

0 0 0 1) oriented GaN epilayer grown on (112̄0) sapphire by MOCVD

Abstract Since the major advantage of (1 1 2 0) sapphire substrate is the ability to easily form cleaved facets, which is important for the fabrication of edge-emitting lasers, it is meaningful to investigate the GaN epilayer grown on (1 1 2 0) sapphire substrate. X-ray diffraction (XRD) measurement indicates that the GaN growth is still along (0xa00xa00xa01) direction even on (1 1 2 0) sapphire substrate. However, compared with the GaN grown on (0xa00xa00xa01) sapphire substrate, the GaN layer on (1 1 2 0) sapphire substrate shows a larger c -axis lattice constant and smaller a -axis lattice constant, which indicates that there exists an enhanced lattice-mismatch compared with the case on (0xa00xa00xa01) sapphire substrate. The detailed XRD measurement indicates that the strain exerted on the GaN on (1 1 2 0) sapphire substrate is increased by 0.03% compared with that on (0xa00xa00xa01) sapphire substrate. Furthermore, the low-temperature photoluminescence indicates that the GaN on (1 1 2 0) sapphire substrate shows a 4.5xa0meV blue-shift compared with the GaN on (0xa00xa00xa01) sapphire substrate, which is in a good agreement with our calculation based on the lattice-mismatch induced strain model. A crystallographic depict is proposed for a good explanation of a GaN layer growth on (1 1 2 0) sapphire substrate. By selective-area diffraction (SAD) and high-resolution transmission electron microscopy measurement (HREM), the in-plane orientation relationship is determined to be (1 1 2 0) GaN ∥(1 1 0 0) sapphire and (1 1 0 0) GaN ∥(0 0 0 1) sapphire , which also shows that a larger lattice-mismatch between (0xa00xa00xa01) GaN and (1 1 2 0) sapphire substrate occurs and thus gives rise to an enhanced compressive strain. In turn, it supports our above discussion.

Field electron emission of diamond films grown on the ultrasonically scratched and nano-seeded Si substrates

In the present study, we compare the field emission properties of diamond films grown on ultrasonically scratched and nano-seeded Si substrates. The diamond films were fabricated in a microwave plasma chemical vapor deposition system. It is confirmed that these two kinds of pretreatment methods, scratched or nano-seeded, result in rather different field emission properties. The diamond films grown on the ultrasonically scratched Si substrates present much higher emission current and lower threshold field than those of the films grown on the nano-seeded substrates. Cross-sectional transmission electron microscopy has been employed to evaluate the diamond films, and the field electron emission behaviors are analyzed in relation to the interface structures.

Interactions between inversion domains and InGaN/GaN multiple quantum wells investigated by transmission electron microscopy

The propagation properties of inversion domains (IDs) in InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition have been investigated by transmission electron microscopy (TEM). The majority of the IDs, originating from the sapphire and/or buffer layer, propagate through the MQWs with normal wurtzite structure retaining their original structural features. Some of IDs could induce V-shaped pits in the MQW structures proposing a new formation mechanism for the so-called V-shaped defects. Detailed measurements show that a few IDs are found to be stopped in abnormal MQW regions, where In droplets appear due to phase separation. We presented direct evidence of pure In-phase droplets by means of high-resolution TEM. The above results provide new information on the structural defects in InGaN/GaN-based materials.

Influence of Inversion Domains on Formation of V-Shaped Pits in GaN Films.

The influence of inversion domains (IDs) on the formation of V-shaped pits in GaN films grown by metalorganic chemical vapor deposition has been investigated using transmission electron microscopy. It was found that IDs can induce V-shaped pits which have a large size distribution. Upon introducing InGaN/GaN multiple-quantum wells to observe various growth stages, our results showed that the origin of these ID-induced pits may be a delayed formation of the IDs during island-island coalescence at the initial stage of film growth, thus we need not adopt a general explanation based on the different growth rates for the two opposite polarities.