טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentElfimchev Sergey
SubjectIncorporation of Nitrogen and Phosphorus in Diamond Films
for Thermionic Energy Conversion
DepartmentDepartment of Chemistry
Supervisor Professor Alon Hoffman
Full Thesis textFull thesis text - English Version


Abstract

Thermionic energy convertor (TEC) transforms heat directly into electricity based on a thermionic effect and may have a lot of advantages relative to other engines. However, a TEC system requires electrodes with extremely low work functions, which is obviously difficult to attain.  Diamond is suggested to be a promising candidate for electron emission because of its low or even negative electron affinity (NEA). The work function values of less than 1eV were reported for a phosphorus-doped polycrystalline diamond (Poly-Di) films. In this work, we proposed a novel TEC based on nitrogen and phosphorus-doped Poly-Di films. High heat conversion theoretical efficiencies between 10% and 45% for a temperature range between 200-700°C was predicted.

The experimental part of the work related to the investigation of nitrogen and phosphorus-doped diamond films deposition by hot filament CVD (HF CVD) method in respect to TEC construction. In this work we investigated aspects of diamond growth from ammonia, methane and hydrogen precursors by hot filament CVD (HF CVD) method. The novel model for gas phase chemistry during the diamond deposition from ammonia, methane and hydrogen precursors is predicted in this work. The chemistry model is supported by the mass spectrometry measurements and fully explains the experimentally observed structural evolutions of diamond films. Optimization of the deposition parameters resulted in significant improvement of the thermionic electron emission properties of the diamond films. A deep understanding of the deposition process is very important to control the electrical, optical and mechanical properties of deposited diamond films.

In this work we investigated the possibility to deposit phosphorus-doped diamond using trimethylphosphine (TMP) gas in HF CVD system. In addition, a novel simple method for phosphorus-doped diamond films deposition by red phosphorus evaporation was developed. In this method red phosphorus is thermally evaporated and transferred inside the HF CVD chamber by hydrogen flow. The amount of phosphorus is controlled by the source temperature. The method described in this work for phosphorus-doped diamond deposition is controllable, safe and cheap. The novel method may be very useful for small diamond deposition systems, where the usage of the extremely toxic phosphine gas is impossible. 

In addition to the TEE, we also investigated the origin of visible sub-band gap photoelectron emission (PEE) from diamond films. We found that nanocrystalline diamond films have low electron emission yields, compared to microcrystalline diamond, due to the presence of high amount of defects in the former, which trap excited electrons before escaping into the vacuum. However, the low PEE yield of nanocrystalline diamond films was found to increase with temperature. The observed phenomenon of PEE increasing with temperature was explained by the novel trap assisted photon enhanced thermionic emission (ta-PETE) model. According to the ta-PETE model, photoelectrons are trapped by shallow traps, followed by thermal excitation at elevated temperatures and escape into the vacuum. The model may be useful for investigation of the diamond surface electronic states.