Franz Koeck

R&D of diamond films in the Frontier Carbon Technology Project and related topics

R&D activities on diamond chemical vapor deposition (CVD) and field emission in the Frontier Carbon Technology Project are presented. The topics are (1) morphology control of diamond films grown by a 60-kW, 915-MHz microwave plasma CVD reactor, (2) growth technology of large single crystal diamond with a low density of defects, (3) heteroepitaxial growth technology of diamond films on Pt, (4) fabrication of sharp emitter tips on single crystal diamond, (5) field emission study from diamond particles, and (6) intense field emission from ion implanted homoepitaxial diamond layer. Research results of field emission obtained by Kyoto University and North Carolina State University are also described.

Electrical and photoelectrical characterization of undoped and S-doped nanocrystalline diamond films

Nanocrystalline diamond (NCD) films are being intensively researched for a variety of potential applications, such as optical windows, electrochemical electrodes, and electron emitting surfaces for field emission displays. In this study Zr, Ti, Cu, and Pt on intrinsic and lightly sulfur-doped (n-type) NCD films were electrically and photoelectrically characterized. Intrinsic and sulfur-doped NCD films were synthesized on 1in. diameter quartz and silicon substrates by microwave plasma assisted chemical vapor deposition. All metals showed linear (Ohmic) current-voltage characteristics in the as-deposited state. The Schottky barrier heights (ΦB) at the metal-film interface were investigated using x-ray and ultraviolet photoelectron spectroscopies. The undoped NCD films exhibited a negative electron affinity and a band gap of 5.0±0.4eV. The ΦB were calculated based on this band gap measurement and the consistent indication from Hall measurements that the films are n-type. The ΦB values were calculated from sh...

Thermionic Converters Based on Nanostructured Carbon Materials

Thermionic energy converters are based on electron emission through thermal excitation and collection where the thermal energy is directly converted into electrical power. Conventional thermionic energy converters based on emission from planar metal emitters have been limited due to space charge. This paper presents a novel approach to thermionic energy conversion by focusing on nanostructured carbon materials, sulfur doped nanocrystalline diamond and carbon nanotube films as emitters. These materials exhibit intrinsic field enhancement which can be exploited in lowering the emission barrier, i.e. the effective work function. Moreover, emission from these materials is described in terms of emission sites as a result of a non‐uniform spatial distribution of the field enhancement factor. This phenomenon can prove advantageous in a converter configuration to mitigate space charge effects by reducing the transit time of electrons in the gap due to an accelerated charge carrier transport.

Thermionic and field electron emission devices from diamond and carbon nanostructures

Electron emission from carbon materials has been based on two effects: field enhancement from conducting nanostructures and barrier lowering due to the negative electron affinity of diamond surfaces. Moreover, n-type doping with P and N can enable a low work function. This presentation details the significant scientific issues related to thermionic and field electron emission and describes potential applications ranging from energy conversion, sensors, high power electronics, and nano electronics and photonics.

Vacuum Thermionic Energy Conversion Based on Nanocrystalline Diamond Films

Vacuum thermionic energy conversion achieves direct conversion of heat into electrical energy. The process involves thermionic electron emission from a hot surface and collection of the electrons on a cold surface where the two surfaces are separated by a small vacuum gap. Results are presented which indicate that nanocrystalline diamond films could lead to highly efficient thermionic energy conversion at temperatures less that 700°C. A critical element of the process is obtaining a stable, low work function surface for thermionic emission. Results are presented which establish that N-doped diamond films with a negative electron affinity can exhibit a barrier to emission of less than 1.6 eV. Films can be deposited onto field enhancing structures to achieve an even lower effective work function. Alternatively, nanocrystalline diamond films prepared with S doping exhibit field enhanced thermionic emission and an effective work function of ~1.9 eV. The field enhanced structures can reduce the effect of space charge and allow a larger vacuum gap. The possibility of a low temperature nanocrystalline diamond based thermionic energy conversion system is presented.

Thermionic and field electron emission from nanostructured carbon materials for energy conversion and vacuum electronics

While initial interest in using diamond materials for electron emission was derived from the observation of a negative electron affinity of the material, the nanoscale structure of the material has proven to be critical to obtaining field emission at an applied field of less than 2 V//spl mu/m. This study presents topographic and emission site images of nanocrystalline diamond films. The results suggest that morphology variations are insufficient to explain the observed emission patterns. The thermionic properties of sulphur doped nanocrystalline diamond films and carbon nanotubes were measured and analyzed in terms of Schottky barrier lowering. The results indicated a general consistency of the field emission and thermionic emission from the same films. The potential for thermionic energy conversion based on these films is presented.