B.E. Williams

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.

Growth and characterization of diamond films on nondiamond substrates for electronic applications

Recent advances in the chemical vapor deposition (CVD) and characterization of diamond films on nondiamond substrates are reviewed. Major growth techniques, including hot filament CVD; microwave, RF, or DC plasma enhanced CVD; and combustion flame growth; as well as a number of hybrid and novel approaches, are described and analyzed. Results from the major categories of diamond film characterization, including diamond phase identification, nucleation and interfacial phenomena, morphology, and defects, as well as their correlations with electrical properties, are examined and discussed. Although most of the information presented is equally applicable to protective and wear-resistant coating application, emphasis is placed in the areas most pertinent to microelectronics. >

In situ growth rate measurement and nucleation enhancement for microwave plasma CVD of diamond

Laser reflection interferometry (LRI) has been shown to be a useful in situ technique for measuring growth rate of diamond during microwave plasma chemical vapor deposition (MPCVD). Current alternatives to LRI usually involve ex situ analysis such as cross-sectional SEM or profilometry. The ability to measure the growth rate in ‘real-time’ has allowed the variation of processing parameters during a single deposition and thus the extraction of much more information in a fraction of the time. In situ monitoring of growth processes also makes it possible to perform closed loop process control with better reproducibility and quality control. Unfortunately, LRI requires a relatively smooth surface to avoid surface scattering and the commensurate drop in reflected intensity. This problem was remedied by greatly enhancing the diamond particle nucleation via the deposition of an intermediate carbon layer using substrate biasing. When an unscratched silicon wafer is pretreated by biasing negatively relative to ground while in a methane-hydrogen plasma, nucleation densities much higher than those achieved on scratched silicon wafers are obtained. The enhanced nucleation allows a complete film composed of small grains to form in a relatively short time, resulting in a much smoother surface than is obtained from a film grown at lower nucleation densities.

The analysis of defect structures and substrate/film interfaces of diamond thin films

Diamond is an excellent candidate material for use in selected electronic and wear resistant coating applications due to its superior hardness, strength and thermal conductivity as well as its high electron drift velocity, chemical and thermal stability, radiation hardness and optical transmission. Electronic devices of particular interest include those having high-power, -frequency and -temperature applications, as well as those for chemically harsh and/or high radiation flux environments. The recent development of techniques for growth of crystalline diamond films at low pressures using common hydrocarbon and H2 gases has created the potential for growing thin films for such devices or wear resistant coatings and a host of related applications. In this research, diamond thin films grown from a low pressure methane-hydrogen gas mixture by microwave plasma enhanced chemical vapor deposition (CVD) have been examined by various transmission electron microscopy (TEM) techniques including bright and dark field, high resolution (HREM), selected area diffraction (SAD) and electron energy loss spectroscopy (EELS). Columnar growth of polycrystalline grain structure, twins, stacking faults, dislocations and intermediate layers were characteristic of the diamond films. No sp2 bonding character in the grains, defects or grain boundaries was detected by EELS.

Analysis via transmission electron microscopy of the quality of diamond films deposited from the vapor phase

Abstract The quality of diamond films deposited from the vapor phase was analyzed via transmission electron microscopy. Diamond films grown by different deposition processes and under various conditions were examined and the processing microstructure relationships were established. Defect structures are first reviewed here. Twinning was the predominant defect observed in all of the diamond samples, but stacking faults and dislocations were also found. It was found that a lower methane concentration resulted in a lower defect density (higher quality) in diamond films grown by microwave-plasma-enhanced chemical vapor deposition. The defect density in diamond films was also reduced if reverse bias was applied in a bias-controlled hot-filament chemical vapor deposition system, in contrast with the high defect density which occurred under the forward bias condition. Finally, the imperfection density was substantially reduced if diamond films were grown at higher substrate temperatures in an oxyacetylene torch.

Surface Morphology And Defect Structures In Microwave CVD Diamond Films

Polycrystalline diamond films were deposited by the microwave-plasma chemical-vapor-deposition (CVD) on Si substrates using a mixture of methane and hydrogen for the source gas. In the morphology study of diamond films using a scanning electron microscope (SEM), it was found that upon increasing the methane concentration (hereafter denoted by c in units of vol%), the surface texture changed discontinuously from (111) to (100) at around c=0.4%, and gradually from (100) to microcrys-talline above c=1.2%. The diamond-Si interfaces and the defect struc-tures in the films were investigated by transmission electron micro-scopy (TEM). The film growth process was investigated by SEM, and it was found that the appearance of small grains and the formation of well-defined diamond faces took place repeatedly with time during the CVD synthesis. The film morphology of boron-doped diamond films on Si substrates and on non-doped diamond films were also presented.

Chemical Vapor Deposition And Characterization Of Diamond Films Grown Via Microwave Plasma Enhanced CVD

Diamond is an excellent candidate material for use in electronic and wear resistant coating applications due to its hardness, strength, thermal conductivity, high electron drift velocity, chemical and thermal stability, radiation hardness and optical transmission. Electronic devices of particular interest include high power/high frequency devices and devices to be utilized in high temperature, chemically harsh and/or high radiation flux environments. The recent development of techniques for growth of crystalline diamond films using low pressure gases has created the potential for growing thin films for such electronic devices or wear resistant coatings. In this research, diamond thin films grown on silicon by microwave plasma enhanced chemical vapor deposition were characterized by a variety of materials analysis techniques including secondary ion mass spectroscopy (SIMS), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and infrared spectroscopy (IR). This paper reports the characterization of these polycrystalline diamond films and discusses the impurities, bonding, and structure of the as-grown diamond films.

In-Vacuo Surface Analysis of Diamond Nucleation and Growth on SI (111) and Polycrystalline Tantalum

The growth of diamond thin films at low temperature and low pressure has made it an excellent candidate material for use in electronic and wear resistant coating applications. However, for diamond to reach its true potential in electronic applications, high quality monocrystalline diamond films must be grown on economically viable non-diamond substrates.