M. A. Tamor

Raman ‘‘fingerprinting’’ of amorphous carbon films

We compare the Raman spectra and other macroscopic properties of nearly one hundred amorphous carbon films deposited at five research laboratories by a total of five different methods in search of correlations useful for both process control and basic understanding of the structure of these materials. For the full range of carbon‐hydrogen alloys, including so‐called ‘‘amorphous diamond,’’ hydrogenated ‘‘diamondlike’’ carbon, and plasma‐polymers, a simple parametrization of the Raman spectrum in the usual 1000 cm−1 to 2000 cm−1 range can be used as a reliable predictor of hydrogenation and other properties (e.g., optical gap, hardness). Raman features in the 200 cm−1 to 1000 cm−1 range, a spectral region not usually reported for carbon films, may also be used as an indicator of hydrogenation. These growth method independent correlations greatly enhance the utility of Raman spectroscopy as a non‐destructive characterization and process control tool.

Piezoresistivity in vapor‐deposited diamond films

We report the observation of a very large piezoresistive effect in both polycrystalline and homoepitaxial chemical‐vapor‐deposited diamond films. The gauge factor for polycrystalline p‐type diamond at 500 microstrains was found to be only 6 at room ambient, but increased rapidly with temperature, exceeding that of polycrystalline silicon (30) at 35 °C, and that of single‐crystal Si (120) at 50 °C. For strain and current flow in the [100] direction, the gauge factor of a (100)‐oriented homoepitaxial diamond film was found to be at least 550 at room temperature. Although the origins and unexpected temperature dependence of piezoresistive effect in diamond are not yet understood, these findings may suggest diamond‐based sensors with performance significantly superior to that of their Si counterparts.

A study of gas chemistry during hot‐filament vapor deposition of diamond films using methane/hydrogen and acetylene/hydrogen gas mixtures

The composition of the reaction gases in a hot‐filament reactor for chemical vapor deposition of diamond films was analyzed using a gas chromatograph coupled with a quartz microprobe. Concentrations of several hydrocarbons were determined as functions of filament temperature (FT) and the position of the probe relative to the filament for two feed gases, methane/hydrogen and acetylene/hydrogen. The diamond growth rate was measured as a function of FT in both feed gases. The major chemical process in these reaction systems is found to be conversion between methane and acetylene with ethane and ethylene as reaction intermediates. For FT≤1800 °C, the chemical reactivity is low, and no diamond deposition is observed. For FT≥1900 °C, nearly identical chemical composition near the filament is obtained from both feed gases (indicating possible attainment of thermodynamic equilibrium in the gas mixtures), and the measured diamond growth rates are similar. A substantial depletion of carbon in the reaction gases nea...

Piezoresistive microsensors using p-type CVD diamond films

Abstract Chemical-vapor deposited semiconducting diamond is an excellent sensor material for harsh environments and high temperatures. The piezoresistive gauge factors measured at 300 K are in the ranges 200–550 and 6–25 for homoepitaxial and polycrystalline p-type diamond films, respectively. The gauge factor for polycrystalline films decreases with decreasing resistivity, but increases with increasing temperature. A multi-sensor microchip, with a number of diamond test structures and a minimum feature size of 5 μm, has been fabricated using a six-mask process. The chip, employing diamond both as an electronic and a mechanical material, is expected to help commercialize integrated diamond sensors in the near term.

Optical study of silicon-containing amorphous hydrogenated carbon

Silicon-containing amorphous hydrogenated carbon films deposited by a plasma-enhanced chemical vapor deposition process were studied using both Raman and ellipsometry spectroscopies. Analyses of the experimental data from both these techniques yielded valuable information about the microstructure of the films. The silicon incorporation in amorphous hydrogenated carbon breaks down large size sp2 carbon clusters and enhances sp3 bonding. The reduction of large sp2 graphitic defects, the enhancement of sp3 bonding, and the associated microstructure changes are responsible for the desired properties of silicon-containing amorphous hydrogenated carbon.

Synthesis and electrical characterization of boron‐doped thin diamond films

Patterned semiconducting polycrystalline diamond films have been synthesized by hot‐filament CVD using in situ doping by pure boron powder. P‐type conduction was confirmed by both Hall and Seebeck effects. The quality of deposited films, as determined by SEM and Raman spectroscopy, was unaffected by the doping. The resistivity and Hall mobility measured by the Van der Pauw method were in the range of 20–100 Ω cm and 2–32 cm2 V−1 s−1, respectively. The dopant activation energies, as computed from the resistivity versus temperature curves (up to 300 °C), were in the range of 0.38–0.30 eV corresponding to Hall concentration in the range of 9×1015–2×1017 cm−3 and boron concentration in the range of 1017–1021 cm−3. The estimated impurity concentration is consistent with SIMS results.

Laser reflective interferometry for in situ monitoring of diamond film growth by chemical vapor deposition

A simple laser reflective interferometer has been employed for in situ monitoring of diamond film growth in a hot‐filament chemical vapor deposition reactor. This method uses a low power HeNe laser beam reflected at normal incident from the substrate. The high refractive index of the diamond film and the relatively high reflectivity of the Si substrate result in pronounced and easily detected interference oscillations in the reflected beam intensity. The oscillation period provides an accurate and immediate measure of the growth rate. In addition, the variations of the extrema of the oscillations provide an estimate of the quality and surface texture of the diamond films. Significant improvement in research productivity has been realized by using this technique.

Germanium/gallium arsenide alloys grown by molecular-beam epitaxy

Metastable germanium/gallium arsenide alloys have been grown by molecular‐beam epitaxy on (100)‐oriented GaAs substrates. Double‐crystal diffractometry shows that the layers have crystalline perfection comparable to that of good crystals of GaAs. Electron mobilities in the range 500–1500 cm2 V−1 s−1 are observed from epilayers with electron concentrations in the range 1017–1019 cm−3. Hole mobilities were relatively small at <40 cm2 V−1 s−1 with concentrations in the range 3×1017–3×1020 cm−3. We find substantial nonstoichiometry of the GaAs component with As/Ga atom ratios ranging from 0.94 to 1.17. As‐deficient through moderately As‐rich alloys exhibit a positive deviation from Vegard behavior that changes to a negative deviation with further increase in the As content. The closeness of lattice match with GaAs substrates allows the epilayer/substrate combination to retain a coherent interface with epilayer thicknesses up to at least 1.5 μm. This is demonstrated by measurements of tetragonal distortion and...

Thin film diamond temperature sensor array for harsh aerospace environment

The feasibility of using polycrystalline CVD diamond films as temperature sensors in harsh aerospace environment associated with hypersonic flights was tested using patterned diamond resistors, fabricated on flat or curved oxidized Si surfaces, as temperature sensors at temperatures between 20 and 1000 C. In this temperature range, the measured resistance was found to vary over 3 orders of magnitude and the temperature coefficient of resistance to change from 0.017/K to 0.003/K. After an annealing treatment, the resistance change was reproducible within 1 percent on the entire temperature range for short measuring times.

CVD diamond film temperature sensors

Due to a unique combination of its mechanical, electrical, thermal, and chemical properties, diamond is an excellent material for temperature and heat flux sensors. Although natural diamond and synthetic diamond thermistors were demonstrated for temperatures below 600 K already in the 1960s, they were never commercialized mainly due to high cost. Recently, there has been a renewed interest in diamond thermistors because of rapid progress in diamond film fabrication using the chemical vapor deposition (CVD) process, which can produce diamond films on nondiamond substrates such as Si at a cost comparable to any other material fabricated routinely in the IC industry. Due to these developments, there is a tremendous potential of diamond temperature sensors for high-speed and high-temperature applications, especially in harsh environments. In fact, diamond sensors may be the first application of diamond in the area of passive diamond electronic devices, because sensor structures do not require single crystal diamond and n-type doping which is difficult to achieve. In the present work, we demonstrate that p- type polycrystalline diamond thermistors show temperature and response-time ranges of 80-1373 K and 290ns-25microsecond(s) , respectively. The fabrication technology of a multisensor diamond microchip is discussed for commercialization of diamond sensors in the near term.