Franz A.M. Koeck

Combined visible light photo-emission and low temperature thermionic emission from nitrogen doped diamond films

This study reports a photoemission threshold of ∼1.5 eV from nitrogen-doped nanocrystalline diamond, which ranks among the lowest photo-threshold of any non-cesiated material. Diamond films on molybdenum substrates have been illuminated with light from 340 to 550 nm, and the electron emission spectrum has been recorded from ambient to ∼320 °C. The results display combined thermionic and photo-electron emission limited by the same low work function and indicate that the two emission processes are spatially separated. These results indicate the potential for a solar energy conversion structure that takes advantage of both photoemission and thermionic emission.

Demonstration of Diamond-Based Schottky p-i-n Diode With Blocking Voltage > 500 V

Diamond is considered to be the ultimate semiconductor for power devices due to its high breakdown electric field, high carrier mobility, and superior thermal properties. The success of diamond-based electronic devices has been difficult due to critical challenges involved with poor doping efficiency and achievement of ohmic contacts. Achieving n-type diamond has proved to be more difficult over p-type so far. In this letter, we report the achievement of n-type doping in diamond, verified using Hall measurements, which was then used to fabricate Schottky p-i-n diodes measuring a forward current density greater than 300 A/cm2 at 4 V and breakdown voltage of over 500 V with a 3.5-μm -thick drift layer. A Silvaco simulation was performed which agreed well with the experimental data showing turn-ON voltage of 1 V and an ideality factor of 1.04, consistent with the model of a p-i-n diode with a fully depleted n-type contact.

Speckle Suppression by Decoherence in Fluctuation Electron Microscopy

We compare experimental fluctuation electron microscopy (FEM) speckle data with electron diffraction simulations for thin amorphous carbon and silicon samples. We find that the experimental speckle intensity variance is generally more than an order of magnitude lower than kinematical scattering theory predicts for spatially coherent illumination. We hypothesize that decoherence, which randomizes the phase relationship between scattered waves, is responsible for the anomaly. Specifically, displacement decoherence can contribute strongly to speckle suppression, particularly at higher beam energies. Displacement decoherence arises when the local structure is rearranged significantly by interactions with the beam during the exposure. Such motions cause diffraction speckle to twinkle, some of it at observable time scales. We also find that the continuous random network model of amorphous silicon can explain the experimental variance data if displacement decoherence and multiple scattering is included in the modeling. This may resolve the longstanding discrepancy between X-ray and electron diffraction studies of radial distribution functions, and conclusions reached from previous FEM studies. Decoherence likely affects all quantitative electron imaging and diffraction studies. It likely contributes to the so-called Stobbs factor, where high-resolution atomic-column image intensities are anomalously lower than predicted by a similar factor to that observed here.

Analysis of the reverse I-V characteristics of diamond-based PIN diodes

Diamond is one of the most promising candidates for high power and high temperature applications, due to its large bandgap and high thermal conductivity. As a result of the growth and fabrication process of diamond-based devices, structural defects such as threading dislocations (TDs) may degrade the electrical properties of such devices. Understanding and control of such defects are important for improving device technology, particularly the reverse breakdown characteristics. Here, we show that the reverse bias current-voltage characteristics in diamond PIN diodes can be described by hopping conduction and Poole-Frenkel emission through TDs over the temperature (T) range of 323 K < T < 423 K, for typical values of the TD density found in epitaxially grown materials.

Temperature dependent simulation of diamond depleted Schottky PIN diodes

Diamond is considered as an ideal material for high field and high power devices due to its high breakdown field, high lightly doped carrier mobility, and high thermal conductivity. The modeling and simulation of diamond devices are therefore important to predict the performances of diamond based devices. In this context, we use Silvaco® Atlas, a drift-diffusion based commercial software, to model diamond based power devices. The models used in Atlas were modified to account for both variable range and nearest neighbor hopping transport in the impurity bands associated with high activation energies for boron doped and phosphorus doped diamond. The models were fit to experimentally reported resistivity data over a wide range of doping concentrations and temperatures. We compare to recent data on depleted diamond Schottky PIN diodes demonstrating low turn-on voltages and high reverse breakdown voltages, which could be useful for high power rectifying applications due to the low turn-on voltage enabling high forward current densities. Three dimensional simulations of the depleted Schottky PIN diamond devices were performed and the results are verified with experimental data at different operating temperatures

P-i-n diodes enabled by homoepitaxially grown phosphorus doped diamond with breakdown electric field >1.25 MV/cm

Owing to its rich material properties, such as high critical electric field, superior thermal conductivity and high electron and hole mobility, diamond has the potential of becoming the material of choice for high power electronic applications. In spite of superior bulk electrical and thermal properties, the only well-known use of diamond in power electronics has been as a heat sink. This is because of the lack of good quality homoepitaxially grown n-type diamond and also the difficulty involved in achieving ohmic contacts due to its very low-work function ( 0.9 eV). Although several approaches have been explored to obtain highly doped n-type diamond very few have been successful. In this presentation we report the successful fabrication and characterization of a p-i-n diode enabled by the development of low-resistance contacts to n-type diamond using Ti/Pt/Au metal contacts. This was made possible by a novel growth scheme where highly P-doped homoepitaxial diamond could be grown consistently.

Thermionic and Photon-Enhanced Emission from CVD Diamond: Influence of Nanostructure, Doping, and Substrate

Thermionic electron emitters based on doped diamond films have shown significant emission at less than 500°C. Results have established that it is necessary to control the electron affinity, doping levels and concentration, and band bending, and these properties have been achieved with engineered multilayered structures with controlled morphology, doping and substrate. Recently, visible light photo-electron emission has been demonstrated using the same diamond film emitters. This report presents a spectroscopic and surface electron microscopy study of photo-and thermionic emission from nitrogen doped diamond films with controlled morphology on metal substrates. Electron emission spectra were recorded to 500°C, while illuminated with sub diamond band gap light. Significant photo-induced emission was observed with an efficiency greater than metal photo cathodes.

Al2O3 dielectric layers on H-terminated diamond: Controlling surface conductivity

This study investigates how the surface conductivity of H-terminated diamond can be preserved and stabilized by using a dielectric layer with an in situ post-deposition treatment. Thin layers of Al2O3 were grown by plasma enhanced atomic layer deposition (PEALD) on H-terminated undoped diamond (100) surfaces. The changes of the hole accumulation layer were monitored by correlating the binding energy of the diamond C 1s core level with electrical measurements. The initial PEALD of 1 nm Al2O3 resulted in an increase of the C 1s core level binding energy consistent with a reduction of the surface hole accumulation and a reduction of the surface conductivity. A hydrogen plasma step restored the C 1s binding energy to the value of the conductive surface, and the resistance of the diamond surface was found to be within the range for surface transfer doping. Further, the PEALD growth did not appear to degrade the surface conductive layer according to the position of the C 1s core level and electrical measurement...

Band offsets of epitaxial cubic boron nitride deposited on polycrystalline diamond via plasma-enhanced chemical vapor deposition

Cubic boron nitride (c-BN) has been deposited on nitrogen-doped polycrystalline diamond films via plasma-enhanced chemical vapor deposition employing fluorine chemistry. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were utilized to characterize the c-BN/diamond interface. TEM observations indicated local epitaxy of c-BN on diamond, while h-BN was also observed at the interface. XPS measurements indicated that c-BN growth continued after nucleation. The band offsets between c-BN and diamond were deduced from XPS measurements. The c-BN valence band maximum (VBM) was 0.8 ± 0.1 eV above the diamond VBM, which corresponded to the c-BN conduction band minimum (CBM) of 1.7 ± 0.1 eV above the diamond CBM. Comparison with offsets predicted by theoretical calculations suggests that a C-N interface was obtained.

Doped diamond thin film electron sources for thermionic energy conversion

Thermionic energy conversion is a process that allows direct conversion of heat into electrical energy without mechanically moving components. In a thermionic converter electrons from the emitter traverse a small gap, are collected by a counter-electrode, the collector, and a self generated voltage develops across the gap. We have prepared prepared an ultra-nanocrystalline diamond (UNCD) based thermionic electron emitter that exhibits a low effective work function of typically 1.4 eV. This was attributed in part to reduced band bending and to the negative electron affinity (NEA) surface. A thermionic energy converter comprised of 2 diamond electrodes were positioned to establish a 25 micron gap and the emitter which was operated at temperatures up to 700 Celsius with a self generated open circuit voltage of 0.35 V. The reduced power output of the device was in part attributed to space charge effects and diamond film resistivity. Utilizing surface ionization effects at the emitter by introducing atomic hydrogen into the converter gap resulted in significant power output increase. With atomic hydrogen in the gap, the converter was operated up to 750 Celsius indicative of efficient surface ionization for charge transfer as well as a stable NEA diamond surface.