Characterization of bias-enhanced nucleation of diamond on silicon by invacuo surface analysis and transmission electron microscopy

Ultrathin Diamond Nanofilms-Development, Challenges, and Applications

Diamond is a highly attractive material for ample applications in material science, engineering, chemistry, and biology because of its favorable properties. The advent of conductive diamond coatings and the steady demand for miniaturization in a plethora of economic and scientific fields resulted in the impetus for interdisciplinary research to develop intricate deposition techniques for thin (≤1000 nm) and ultra-thin (≤100 nm) diamond films on non-diamond substrates. By virtue of the lowered thickness, diamond coatings feature high optical transparency in UV-IR range. Combined with their semi-conductivity and mechanical robustness, they are promising candidates for solar cells, optical devices, transparent electrodes, and photochemical applications. In this review, the difficulty of (ultra-thin) diamond film development and production, introduction of important stepping stones for thin diamond synthesis, and summarization of the main nucleation procedures for diamond film synthesis are elucidated. Thereafter, applications of thin diamond coatings are highlighted with a focus on applications relying on ultrathin diamond coatings, and the excellent properties of the diamond exploited in said applications are discussed, thus guiding the reader and enabling the reader to quickly get acquainted with the research field of ultrathin diamond coatings.

Nucleation of diamond films on heterogeneous substrates: a review

Diamond thin films are known to have properties similar to bulk diamond and have applications in both industry and fundamental studies in academia. The high surface energy of diamond makes it extremely difficult to grow diamond films on foreign substrates. Hence, to grow diamond films on non-diamond substrates, a nucleation step is needed. In this review various techniques used for diamond nucleation/seeding will be discussed. At present electrostatic seeding by diamond nanoparticles is the most commonly used seeding technique for nanocrystalline growth. In this technique the substrate is dipped in a nanodiamond solution to form a mono layer of diamond seeds. These seeds when exposed to appropriate conditions grow to form diamond layers. This technique is suitable for most substrates. For heteroepitaxial growth, bias enhanced nucleation is the primary technique. In this technique the substrate is biased to form diamond nuclei in the initial stages of growth. This technique can be used for any conducting flat surface. For growth on ceramics, polishing by diamond grit or electrostatic seeding can be used. Polishing the ceramics with diamond powder leaves small diamond particles embedded in the substrate. These small particles then act as seeds for subsequent diamond growth. Apart from these techniques, chemical nucleation, interlayer driven nucleation and mixed techniques have been discussed. The advantages and disadvantages of individual techniques have also been discussed.

Growth of diamond film on heterogeneous substrates assisted by nucleation/seeding.

Spectroscopic Ellipsometry of Nanocrystalline Diamond Film Growth

With the retention of many of the unrivaled properties of bulk diamond but in thin-film form, nanocrystalline diamond (NCD) has applications ranging from micro-/nano-electromechanical systems to tribological coatings. However, with Young's modulus, transparency, and thermal conductivity of films all dependent on the grain size and nondiamond content, compositional and structural analysis of the initial stages of diamond growth is required to optimize growth. Spectroscopic ellipsometry (SE) has therefore been applied to the characterization of 25-75 nm thick NCD samples atop nanodiamond-seeded silicon with a clear distinction between the nucleation and bulk growth regimes discernable. The resulting presence of an interfacial carbide and peak in nondiamond carbon content upon coalescence is correlated with Raman spectroscopy, whereas the surface roughness and microstructure are in accordance with values provided by atomic force microscopy. As such, SE is demonstrated to be a powerful technique for the characterization of the initial stages of growth and hence the optimization of seeding and nucleation within films to yield high-quality NCD.

Electronic Structure and Bonding at Interfaces Between cvd Diamond and Silicon

The interfacial structure of CVD diamond grown on silicon was studied using spatially resolved electron energy loss spectroscopy (EELS) in a UHV STEM with a subnanometer probe size. Both the plasmon and core excitations in the bulk appear to be localized on this scale. Spatial maps of the different bonding configurations of carbon were obtained by forming images from transmitted electrons that had undergone energy losses characteristic of threefold and fourfold coordinated carbon. Films grown on both prescratched silicon and intermediate amorphous carbon layers were examined. In the latter case, diamond nucleation on a narrow sp2 a-C occurred. For diamond grown directly on silicon, at some regions of the interface, threefold coordinated defect states smaller than 1 nm are observed on the diamond side of the interface while at other regions along the interface the presence of an intermediate 2nm thick SiC layer preserves the fourfold coordination of the carbon.

Surface Studies Relevant to the Initial Stages of Diamond Nucleation

Studies of diamond heteroepitaxy on silicon indicate that C-C surface species act as nucleation precursors. We have investigated the conversion of the Si(100) 2×1 surface to SiC using C2H4 to obtain an understanding of how C-C species may be formed and to determine the effect of an O-adlayer on enhancing or selecting the reaction channel which leads to these species. Under appropriate conditions, the interaction between C2H4 and the clean silicon surface yields both SiC and C-C species. The presence of an O-adlayer significantly reduces the activity of silicon and enhances the formation of sp2 and sp3 C-C species. These results provide key insights into diamond nucleation conditions in conventional growth processes.

Chemical vapour deposition of oriented diamond nanocrystallites by a bias-enhanced nucleation method

A microwave plasma chemical vapour deposition (MPCVD) system has been used to deposit nanometre-sized single-crystalline diamonds on 1 × 1 cm(2) Si(100) substrates. The distribution of deposited diamonds has good uniformity over the whole Si substrate surface by using a dome-shaped Mo anode which allows the application of bias-enhanced nucleation. The morphology and crystallinity of the deposits on Si were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with electron diffraction and lattice images. SEM and TEM observations show that oriented diamond nuclei as single crystals with facets can form on self-formed Si cones through epitaxial SiC within a short bias period. After a longer bias time, it has been observed that polycrystalline diamonds formed as a result of secondary nucleation.

Direct local epitaxy of diamond on Si(100) and surface-roughening-induced crystal misorientation

A direct diamond epitaxy on the silicon substrate is demonstrated not only at the interface formed during the growth process but also at the nucleation sites. The small (001) terraces with dimensions of several atomic distances at the site of nucleation are formed due to the roughening of silicon surface and lead to the grain misorientation. A model is presented which attempts to explain the initial stages of diamond growth. Predictions are made for methods of improving the nucleation of epitaxial diamond crystallites.

A nucleation site and mechanism leading to epitaxial growth of diamond films

A diamond nucleation site responsible for epitaxial growth of diamond on silicon by chemical vapor deposition (CVD) is identified in high-resolution transmission electron microscopic images. Other sites in the same sample leading to polycrystalline growth, but deleterious to epitaxial CVD growth, are also described. A mechanism for the heteroepitaxial growth of diamond is suggested, in which etching of the nondiamond carbon binder exposes and removes nonadherent nanodiamond nuclei, leaving intact only those directly nucleated on the silicon substrate. This work enhances our understanding of diamond nucleation and heteroepitaxial growth and its potential applications.

HEAT CONDUCTION IN NOVEL ELECTRONIC FILMS

▪ Abstract  Heat conduction in novel electronic films influences the performance and reliability of micromachined transistors, lasers, sensors, and actuators. This article reviews experimental and theoretical research on heat conduction in single-crystal semiconducting and superconducting films and superlattices, polycrystalline diamond films, and highly disordered organic and oxide films. The thermal properties of these films can differ dramatically from those of bulk samples owing to the dependence of the material structure and purity on film processing conditions and to the scattering of heat carriers at material boundaries. Predictions and data show that phonon scattering and transmission at boundaries strongly influence the thermal conductivities of single-crystal films and superlattices, although more work is needed to resolve the importance of strain-induced lattice defects. For polycrystalline films, phonon scattering on grain boundaries and associated defects causes the thermal conductivity to be strongly anisotropic and nonhomogeneous. For highly disordered films, preliminary studies have illustrated the influences of impurities on the volumetric heat capacity and, for the case of organic films, molecular orientation on the conductivity anisotropy. More work on disordered films needs to resolve the interplay among atomic-scale disorder, porosity, partial crystallinity, and molecular orientation.