Jeffrey T. Glass

Raman scattering characterization of carbon bonding in diamond and diamondlike thin films

The atomic bonding configurations of carbon bonding in diamond and diamondlike thin films are explored using Raman scattering. The general aspects of Raman scattering from composites are presented. Effects are discussed due to crystalline or amorphous structures, large versus microcrystalline domains, and strong optical absorption and transparent regions. The Raman scattering from diamondlike films shows several features which are attributed to microcrystalline graphitelike structures which all originate from the same region in the sample. In contrast, the spectra of diamond films show features attributed to different components of a composite film. Components identified are crystalline diamond, and disordered and microcrystalline graphitic structures. The presence of precursor microcrystalline or amorphous diamond structures is also suggested.

Textured diamond growth on (100) β‐SiC via microwave plasma chemical vapor deposition

Textured diamond films have been deposited on β‐SiC via microwave plasma chemical vapor deposition preceded by an in situ bias pretreatment that enhances nucleation. Approximately 50% of the initial diamond nuclei appear to be aligned with the C(001) planes parallel to the SiC(001), and C[110] directions parallel to the SiC[110] within 3°. The diamond was characterized by Raman spectroscopy and scanning electron microscopy.

Textured growth of diamond on silicon via in situ carburization and bias‐enhanced nucleation

Ordered diamond films have been deposited on single‐crystal silicon substrates via an in situ carburization followed by bias‐enhanced nucleation. Textured diamond films with greater than 50% of the grains oriented D(100)//Si(100) and D〈110〉//Si〈110〉 were grown in both a horizontal and vertical microwave plasma chemical vapor deposition reactor. Separate diamond films from each of the two reactors were analyzed both by scanning electron microscopy and Raman spectroscopy. The in situ carburization is speculated to form an epitaxial SiC conversion layer, thus providing an economical alternative to obtaining epitaxial diamond films on single‐crystal SiC.

Polyethylenimine-enhanced electrocatalytic reduction of CO2 to formate at nitrogen-doped carbon nanomaterials

Nitrogen-doped carbon nanotubes are selective and robust electrocatalysts for CO2 reduction to formate in aqueous media without the use of a metal catalyst. Polyethylenimine (PEI) functions as a co-catalyst by significantly reducing catalytic overpotential and increasing current density and efficiency. The co-catalysis appears to help in stabilizing the singly reduced intermediate CO2(•-) and concentrating CO2 in the PEI overlayer.

Chemical vapor deposition and characterization of 6H‐SiC thin films on off‐axis 6H‐SiC substrates

High‐quality, monocrystalline 6H‐SiC thin films have been epitaxially grown on 6H‐SiC {0001} substrates which were prepared 3° off‐axis from 〈0001〉 towards 〈1120〉 at 1773 K via chemical vapor deposition (CVD). Essentially, no defects were generated from the epilayer/substrate interface as determined by cross‐sectional transmission electron microscopy (XTEM). Double positioning boundaries which were observed in β‐SiC grown on 6H‐SiC substrates were eliminated as confirmed by plan‐view TEM. A strong dependence of the surface morphology of the as‐grown thin films on the tilting orientation of the substrates was observed and reasons for this phenomenon are discussed. The unintentionally doped 6H‐SiC thin films always exhibit n‐type conduction with a carrier concentration on the order of 1016 cm−3. Au‐6H‐SiC Schottky barrier diodes were fabricated on the CVD 6H‐SiC thin films and it was found that the leakage current at a reverse bias of 55 V was only 3.2×10−5 A/cm2. This is compared to SiC films grown on oth...

Critical evaluation of the status of the areas for future research regarding the wide band gap semiconductors diamond, gallium nitride and silicon carbide

Abstract The extreme thermal and electronic properties of diamond and of silicon carbide, and the direct band gap of gallium nitride, provide multiplicative combinations of attributes which lead to the highest figures of merit for any semiconductor materials for possible use in high power, high speed, high temperature and high frequency applications. The deposition of monocrystalline diamond, at or below 1 atm total pressure and at a temperature T , has been achieved on diamond substrates; the deposited film has been polycrystalline on all other substrates but the achievement is no less significant. For electronic applications, heteroepitaxy of single-crystal films of diamond, an understanding of mechanisms of nucleation and growth, methods of impurity introduction and activation, and further device development must be achieved. Stoichiometric gallium nitride free of nitrogen vacancies has apparently not been obtained. Thus, knowledge of the defect chemistry of this material, the growth of semiconducting films on foreign substrates, and the development of insulating layers and of their low temperature deposition as well as device fabrication procedures must be achieved. By contrast, all of these problems have already been solved for silicon carbide, including the operation of a MOSFET at 923 K — the highest operating temperature ever reported for a field-effect device. However, considerable research remains to be done regarding the development of large silicon carbide substrates, of ohmic and rectifying contacts, of new types of devices, and of low temperature techniques for the deposition of insulating layers. Fugitive donor and acceptor species in unintentionally doped samples must also be identified and controlled.

Material and electrical characterization of polycrystalline boron‐doped diamond films grown by microwave plasma chemical vapor deposition

Crystal/material quality and electrical properties of B‐doped diamond films synthesized by microwave plasma chemical vapor deposition were investigated. Raman spectroscopy verified the presence of diamond and indicated that the crystal quality increased with B doping. Secondary‐ion mass spectroscopy showed that the B/C ratios in the films were larger than the B/C ratios in the gas phase, possibly due to differences in B and C sticking coefficients. Electrode patterns of Pt were fabricated on the films and electrical properties were investigated. On undoped diamond films with a residual B concentration of ∼5×1017 cm−3, these contacts were rectifying with small reverse leakage currents and on B‐doped diamond films with a B concentration of 200 and 400 ppm, they yielded ohmic behavior. The temperature dependence of the resistivity showed that these doped films had activation energies, an order of magnitude smaller than that associated with the B impurity level in diamonds. The small activation energies assoc...

The origin of the broadband luminescence and the effect of nitrogen doping on the optical properties of diamond films

Raman and various photoluminescence (PL) techniques were employed to investigate the role of nitrogen doping on the optical spectra of chemical‐vapor‐deposited (CVD) diamond films and to determine the origin of the characteristic broadband luminescence which is observed from approximately 1.5 to 2.5 eV and centered at ∼2 eV. The PL transitions attributed to the zero‐phonon lines (ZPL) of nitrogen centers are observed at 1.945 and 2.154 eV. A new possible nitrogen center at 1.967 eV is also observed as well as the band A luminescence centered at ∼2.46 eV. The experimental results preclude the possibility of the broadband PL being due to electron‐lattice interaction of the nitrogen ZPL centers. We establish the presence of an in‐gap state distribution in CVD diamond films attributed to the sp2 disordered phase and show that its optical transitions are the likely cause of the broadband luminescence. A model of the in‐gap state distribution is presented which is similar to models previously developed for amor...

Oriented diamond films grown on nickel substrates

A previously reported multistep hot filament chemical vapor deposition process for nucleating diamond directly on nickel substrates has been further refined to increase the nucleation density and improve the orientation of diamond films. The process employed heavy seeding of both 〈100〉 and 〈111〉 oriented single crystal Ni surfaces with diamond powders to enhance the nucleation density of diamond films. The deposition conditions were adjusted to allow for 〈100〉 and 〈111〉 orientations to grow on similarly oriented substrates to form nearly complete films with grain boundaries being eliminated. Thus, the technique holds promise for developing heteroepitaxial diamond films for microelectronics applications.

Epitaxial nucleation of diamond on β-SiC via bias-enhanced microwave plasma chemical vapor deposition

Abstract Diamond has been successfully nucleated on mirror finish single-crystal β-SiC films via bias-enhanced microwave plasma chemical vapor deposition. Initial scanning electron microscopy indicated that approximately 50% of the diamond grains were oriented relative to the SiC substrate. Further, high resolution cross-sectional transmission electron microscopy (TEM) and electron diffraction confirmed that the diamond was in epitaxial alignment with the silicon carbide, with the D(100)//SiC(100) and D〈110〉//SiC〈110〉. The high resolution TEM also revealed an approximate 5° tilt about 〈110〉 towards 〈110〉. This tilting is believed to be the result of the high density of misfit dislocations at the interface. Speculations on the role of biasing in the promotion of epitaxial diamond nucleation on a foreign substrate are also discussed.