T. Badzian

Crystallization of diamond crystals and films by microwave assisted CVD (Part II)

Results of experiments on crystallization of diamond micro single crystals (∼10μm) and polycrystalline diamond films by microwave plasma assisted chemical vapor deposition are presented. Discussed are problems related to growth mechanisms from CH4H2 plasma: nucleation on different substrates (diamond, graphite, Si, βSiC, SiO2 and Ni), catalytic growth, Raman scattering from deposits and planar defects of diamond structure. Optimal conditions for diamond growth were found in relation to maximum growth rate and exceptional surface phenomena appearing at temperatures close to 1000°C.

The effect of surface treatment on the electrical properties of metal contacts to boron-doped homoepitaxial diamond film

Both doped and undoped homoepitaxial diamond films were fabricated using microwave plasma-enhanced chemical vapor deposition (CVD). The conductivity of the diamond film is strongly affected by the surface treatment. In particular, exposure of film surface to a hydrogen plasma results in the formation of a conductive layer which can be used to obtain linear (ohmic) I-V characteristics of the Au/diamond contacts, regardless of the doping level. It is shown how the proper chemical cleaning of the boron-doped homoepitaxial diamond surface allows the fabrication of Au-gate Schottky diodes with excellent rectifying characteristics at temperatures of at least 400 degrees C.<<ETX>>

Electrical characteristics of Schottky diodes fabricated using plasma assisted chemical vapor deposited diamond films

Schottky diodes were fabricated using gold and aluminum contacts to thin diamond films obtained by a microwave plasma assisted chemical vapor deposition process. The current‐voltage and capacitance‐voltage‐frequency characteristics of these devices are similar to those fabricated on a crystalline diamond base formed by traditional ultrahigh pressure process.

Silicon carbonitride: a rival to cubic boron nitride

Abstract It is common to divide hard materials into two groups: ultra-hard with diamond and cubic boron nitride and a second group of hard materials listed as SiC, TiC, etc. Between these two groups, a few ternary phases have a hardness overlapping that of cBN. We have perviously reported on the synthesis of B–C–Si, Si–O–C and Si–N–C crystalline phases. This paper reports on crystalline silicon carbonitride phase-synthesized using microwave plasmas with nitrogen or N2–CH4–H2 vapor etching silicon. The crystalline phase possesses the pseudo-α-Si3N4 structure. The microcrystals have a columnar form and an average size of 10xa0μm. The FTIR spectrum resembles that of α-Si3N4, but a set of new absorption peaks appears. This new material is much harder than the Si3N4 cutting tool insert, and it scratches single crystals of α-SiC, sapphire and even the (001) surface of diamond.

Silicon carbonitride, a new hard material and its relation to the confusion about ‘harder than diamond’ C3N4

Abstract The claim by Cohen (Science 261 (1993) 307) that powerful computational tools allow us to predict ‘properties of substances even before we have created them’ was made in conjunction with the claim of special properties for a hypothetical phase, C 3 N 4 . Among such properties was hardness, and it was asserted that, the covalent form of C 3 N 4 could be ‘harder than diamond.’ This assumption contradicted what chemists have known since 1816 in their experimentation with carbon nitrides. Never was there a single hint of the existence of a covalent, single bond C–N network. In the last decade some 400 papers have been written about this non-existent material of dubious significance (R.C. DeVries, Mater. Res. Innovat. 1 (1997) 161). No C 3 N 4 material has ever been made and the claims on both the chemical composition and crystal structure are clearly in error. The impact of such exaggerated claims on the scientific enterprise cannot be ignored. In contrast, we report herein on a related but real hard material, silicon carbonitride, with the α -Si 3 N 4 crystal structure modified by the introduction of carbon atoms. Synthesis of this Si–N–C crystalline material was possible by using a CH 4 /H 2 /N 2 microwave plasma etching of solid Si. Films on Si, SiC, Si 3 N 4 and diamond, as well as crystal agglomerates of a few mm 3 volume, have been prepared. This phase possesses a micro-hardness lower than cubic boron nitride and a band gap of 3.8 eV. The present experiments indicate that only 6 at.% of C have been incorporated into α -Si 3 N 4 . We might suggest that ‘first principles’ calculations be undertaken to explain the limited solubility of carbon in the α -Si 3 N 4 phase.

Synthesis of diamond from methane and nitrogen mixture

We have found that diamond can be synthesized from a mixture of CH4 and N2 without adding any H2. This new synthesis is sharply different from the common practice of diamond growth by chemical vapor deposition, which uses a hydrogen‐rich mixture of CH4 and H2. In this new approach, nitrogen becomes an active component of microwave plasma leading to diamond growth. Nitrogen participates in abstraction of hydrogen from the diamond surface. We hypothesize that formation of HCN is an indication of hydrogen abstraction that allows diamond to grow from CH4+N2 mixtures. As a consequence of surface processes, the crystal structure of the grown diamond is distorted. The sequence of tetrahedral layers is mixed (cubic and hexagonal) and it suffers from turbostatic disorder. Diamond films were characterized by x‐ray diffraction, Auger electron spectroscopy, x‐ray photoelectron spectroscopy, and Raman spectroscopy.

From diamond-like carbon to diamond coatings

Abstract Carbon films deposited by pyrolytic and plasma-activated deposition from hydrocarbon-containing gases encompass a virtual infinity of compositions (hydrogen concentrations) and structures (from amorphous to single crystal and with variable amounts of sp 1 , sp 2 and sp 3 bonding). Coatings which have a high degree of sp 3 bonding generally have properties, especially hardness, close to those of single- crystal diamond and are often referred to as diamond-like carbon. Recently, large grain size diamond crystals and continuous diamond coatings have been prepared by plasma chemical vapor deposition methods. Although such materials are different from the diamond-like carbon class of materials, there is clearly a continuum of materials which is expected to lead to vagueness and confusion in nomenclature. In this paper, such issues are dealt with and a working definition of “diamond” coatings is offered.

The barrier height of Schottky diodes with a chemical-vapor-deposited diamond base

A barrier height of 1.13±0.03 eV was measured for Al and Au rectifying contacts to p‐type chemical‐vapor‐deposited diamond thin films using the internal photoemission technique. The results are compared with experimental data reported for Schottky barriers on single‐crystal diamond.

Diamond Homoepitaxy by Chemical Vapor Deposition

Abstract Gem-quality single crystals of diamond were grown by homoepitaxy using microwave plasma assisted chemical vapor deposition. Raman spectra of the (001) films show very low background and the narrowest width at half maximum of the 1332 cm −1 peak was 1.7 cm −1 . A prism crystal 1.2 mm high was grown in the 〈110〉 direction at 1200°C and 1% CH 4 in H 2 . Its Raman peak has a width of 2.7 cm −1 . Homoepitaxy is a tool used to study growth processes by analyzing the geometrical details of surfaces morphologies. Growth steps on the (001) surface develop in the 〈110〉 directions while triangular plates appear on the (111) surface. The (110) atomic plane does not appear in the 〈110〉 growth direction with microfaceted (111) and (111) planes present instead. Crystal structure of diamond grown on the (001), (111), (110) substrates, as well as growth sectors developed in conjunction with this epitaxy, were studied by X-ray diffraction techniques: Laue, oscillation and rotation methods. Atomic force microscopy reached atomic resolution on the (001) surface. The presence of a 2.12 A reflection demonstrates distortion of cubic symmetry and deformation of carbon tetrahedra in the case of (111) epitaxy. In the 〈110〉 growth sector a polytype/superstructure network with periodicity three times larger than that of the cubic form was determined. In general it is possible to form tetrahedral carbons with the sequence of tetrahedra different from that of cubic and hexagonal diamonds.

High-temperature Schottky diodes with thin-film diamond base

High-temperature (500-580 degrees C) current-voltage (I-V) characteristics of gold contacts to boron-doped homoepitaxial diamond films prepared using a plasma-enhanced chemical vapor deposition (CVD) method are described. Schottky diodes were formed using gold contacts to chemically cleaned boron-doped homoepitaxial diamond films. These devices incorporate ohmic contacts formed by annealing Au(70 nm)/Ti(10 nm) layers in air at 580 degrees C. The experiments with homoepitaxial diamond films show that the leakage current density increases with the contact area. This implies that a nonuniform current distribution exists across the diode, presumably due to crystallographic defects in the diamond film. As a result, Au contacts with an area >1 mm/sup 2/ are essentially ohmic and can be used to form back contacts to Schottky diodes. Schottky diodes fabricated in this matter also show rectifying I-V characteristics in the 25-580 degrees C temperature range.<<ETX>>