Diamond-based thermionic emission devices could provide a means to produce clean and renewable energy through direct heat-to-electrical energy conversion. Hindering progress of the technology are the thermionic output current and threshold temperature of the emitter cathode. In this report, we study the effects on thermionic emission caused by in-situ exposure of the diamond cathode to beta radiation. Nitrogen-doped diamond thin films were grown by microwave plasma chemical vapour deposition on molybdenum substrates. The hydrogen-terminated nano-crystalline diamond was studied using a vacuum diode setup with a 63Ni beta radiation source-embedded anode, which produced a 2.7-fold increase in emission current compared to a 59Ni-embedded control. The emission threshold temperature was also examined to further assess the enhancement of thermionic emission, with 63Ni lowering the threshold temperature by an average of 58 ± 11 oC compared to the 59Ni control. Various mechanisms for the enhancement are discussed, with a satisfactory explanation remaining elusive. Nevertheless, one possibility is discussed involving excitation of pre-existing conduction band electrons that may skew their energy distribution toward higher energies.
Effective control of device geometry is key to mitigating high localized electric fields in next-generation SiC power devices. Advanced trench processing allows for highly tunable trench-gate architectures in trench MOSFETs. By utilizing a two-step inductively coupled plasma reactive ion etch (ICP-RIE) process, a high degree of trench base corner rounding can be achieved, irrespective of trench opening corner geometry prior to post etch treatments. Sentaurus TCAD device modelling highlights the importance of effective electric field dispersion at the gate oxide using rounded trench corners, while I-V characterization of fabricated trench MOS-capacitor devices demonstrate the influence of trench base corner rounding on gate oxide breakdown.
Thermionic emission relies on the low work function and negative electron affinity of the, often functionalized, surface of the emitting material. However, there is little understanding of the interplay between thermionic emission and temperature-driven dynamic surface transformation processes as these are not represented on the traditional Richardson–Dushman equation for thermionic emission. Here, we show a new model for thermionic emission that can reproduce the effect of dynamic surface changes on the electron emission and correlate the components of the thermionic emission with specific surface reconstruction phases on the surface of the emitter. We use hydrogenated <100> single-crystal and polycrystalline diamonds as thermionic emitters to validate our model, which shows excellent agreement with the experimental data and could be applicable to other emitting materials. Furthermore, we find that tailoring the coverage of specific structures of the C(100)-(2 × 1):H surface reconstruction could increase the thermionic emission of diamond by several orders of magnitude.
Abstract Material band structures of occupied electronic states are obtainable using conventional angle‐resolved photoemission experiments, leaving the unoccupied states far less explored. Here, an alternative approach is built on and expanded to investigate thermalized photoelectrons emitted from crystal surfaces. A model for electron emission is constructed and reveals the material unoccupied state band structure. Potentially applicable to any material and independent from the secondary electron generation mechanism, it is demonstrated on diamond and copper using different light sources. Moreover, the diamond indirect band gap is directly observed and the transverse effective mass at the conduction band minimum can be experimentally obtained, m t = (0.21 ± 0.015) m e . This offers a convenient path for angle‐resolved photoemission data interpretation and empty‐state information extraction.
Density functional theory calculations were performed to model the adsorption of up to 1 monolayer (ML) of aluminium on the bare and O-terminated (1 0 0) diamond surface. Large adsorption energies of up to -6.36 eV per atom are observed for the Al-adsorbed O-terminated diamond surface. Most adsorption sites give a negative electron affinity (NEA), with the largest NEAs -1.47 eV on the bare surface (1 ML coverage) and -1.36 eV on the O-terminated surface (0.25 ML coverage). The associated adsorption energies per Al atom for these sites are -4.11 eV and -5.24 eV, respectively. Thus, with suitably controlled coverage, Al on diamond shows promise as a thermally-stable surface for electron emission applications.
Thermionic emitting materials are relevant for several technological applications like electron guns, X-ray sources, or thermionic energy converters. As new materials and surface functionalisations that enable thermionic emission are developed, it is essential to be able to test them in a repeatable and reliable manner. Here, we present a CO2 laser-heated system for thermionic tests that can be used to test the thermionic emission current of different materials regardless of the optical properties or form factor. Our system can reach sample temperatures of T ≈ 1000 °C and can follow pre-programmed heating profiles. Additionally, a double thermo-electrical decoupling provides a very low electrical noise environment while keeping the sample heat loss to a minimum. Experimental data on sample temperature and thermionic current from a hydrogen terminated single crystal diamond are presented and discussed.
Substitutional clusters of multiple light element dopants are a promising route to the elusive shallow donor in diamond. To understand the behaviour of co-dopants, this report presents an extensive first principles study of possible clusters of boron and nitrogen. We use periodic hybrid density functional calculations to predict the geometry, stability and electronic excitation energies of a range of clusters containing up to five N and/or B atoms. Excitation energies from hybrid calculations are compared to those from the empirical marker method, and are in good agreement. When a boron-rich or nitrogen-rich cluster consists of three to five atoms, the minority dopant element-a nitrogen or boron atom respectively-can be in either a central or peripheral position. We find B-rich clusters are most stable when N sits centrally, whereas N-rich clusters are most stable with B in a peripheral position. In the former case, excitation energies mimic those of the single boron acceptor, while the latter produce deep levels in the band-gap. Implications for probable clusters that would arise in high-pressure high-temperature co-doped diamond and their properties are discussed.
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