ICMCTF2000 Session D3/G6: Electronic, Optical and Thermal Applications of Diamond and Related Materials

The CVD diamond films are one of promising materials for a cold emitter, because the as-grown (hydrogen-terminated) diamond surface has negative electron affinity (NEA). The current density and threshold electric field of the electron emission strongly depend on the impurity atoms and defect structures in the diamond film.

In this presentation, we will characterize the defect structures in the nitrogen (N) doped diamond film by using electron spin resonance (ESR) method. Moreover, we will discuss the Influence of defects in the diamond film on the electron emission.

The diamond films were deposited by a hot filament CVD method on n- and p-type Si substrates from CH₄, N₂ and H₂ gases. The ratio of N atom to C atom (N/C ratio) in the source gas was varied from 0 (undoped) to 10.

The undoped diamond film emitted electrons with the current of 66 µmA/cm² at 20 V/µmm. The current density increased to 250 µmA/cm² with an increase in the N /C up to 10. The N-doped diamond films exhibit the P_dia- center (g=2.003 and ΔH_PP=3 Oe) and the P_ac- center (g=2.003 and ΔH_PP =8 Oe) in addition to the hyperfine signal due to the substitutional nitrogen atoms in the diamond film. These ESR centers originate from carbon dangling bonds in the diamond film and carbon dangling bonds in the non- diamond phase carbon region, respectively. The spin densities in the N-doped diamond film increased to in order of 10¹⁸ spins/cm² with introducing the N/C up to 10. These results indicate that the high-density defects, which were introduced by N doping, work for the electron transportation mechanism in the diamond film. The diamond films with the high- density defects emit electrons with high current density, because the electrons are effectively supplied from the back of the diamond film to the NEA surface by the hopping conduction. This message includes an attached Microsoft Word 6.0/95 file with the formatted text of your abstract. It shows the text exactly as it would appear in the proceedings. If you cannot read the file in this format, please ignore it. We cannot provide any other file format. The following abstract was received. Please review this information CAREFULLY, and inform us of any ERRORS. Include your abstract number (674) in any correspondence. Please IGNORE the line-breaks shown in this e-mail message and reply ONLY if text is missing or wrong. When asking for corrections please be VERY SPECIFIC as to the location of the correction. If there are problems with Greek letters or other symbols please specify the location. If your text included special format codes for: accented letters, the Greek beta or mu characters, Angstrom, degree or plus/minus, the format codes have been replaced with the corresponding characters. These special characters may not be correctly visible in this email, particularly if you are using a text-only email reader. Please do NOT worry about these characters. They will appear correctly in all publications. Abstract Number: 674 Title: Electron Emission from Nitrogen Doped Diamond Film: Influence of Defects Introduced by the Nitrogen Doping Present Session: UNASSIGNED, Unassigned Abstracts Invited Paper: No Submitted for publication: No Authors: 1. (Presenter) (Correspondent) Y. Show ; Tokai University, Japan 2. T. Matsukawa ; Tokai University, Japan 3. H. Ito ; Tokai University, Japan 4. M. Iwase ; Tokai University, Japan 5. T. Izumi ; Tokai University, Japan Abstract: What session do you want your abstract in? Do you want an oral or a poster? If you want to publish in the Proceedings I will need the name and address of 3 referees. Thank you, Mary Gray The CVD diamond films are one of promising materials for a cold emitter, because the as-grown (hydrogen-terminated) diamond surface has negative electron affinity (NEA). The current density and threshold electric field of the electron emission strongly depend on the impurity atoms and defect structures in the diamond film. In this presentation, we will characterize the defect structures in the nitrogen (N) doped diamond film by using electron spin resonance (ESR) method. Moreover, we will discuss the Influence of defects in the diamond film on the electron emission. The diamond films were deposited by a hot filament CVD method on n- and p-type Si substrates from CH 4, N2 and H2 gases. The ratio of N atom to C atom (N/C ratio) in the source gas was varied from 0 (undoped) to 10. The undoped diamond film emitted electrons with the current of 66 ?A/cm2 at 20 V/?m. The current density increased to 250 ?A/cm2 with an increase in the N/C up to 10. The N-doped diamond films exhibit the Pdia-center (g=2.003 and ?HPP=3 Oe) and the Pac-center (g=2.003 and ?HPP=8 Oe) in addition to the hyperfine signal due to the substitutional nitrogen atoms in the diamond film. These ESR centers originate from carbon dangling bonds in the diamond film and carbon dangling bonds in the non-diamond phase carbon region, respectively. The spin densities in the N-doped diamond film increased to in order of 1018 spins/cm2 with introducing the N/C up to 10. These results indicate that the high-density defects, which were introduced by N doping, work for the electron transportation mechanism in the diamond film. The diamond films with the high-density defects emit electrons with high current density, because the electrons are effectively supplied from the back of the diamond film to the NEA surface by the hopping conduction.

Amorphous hydrogenated carbon films with different nitrogen content (a-C:H:N) were deposited (thickness 2.65-3.01 microm) by chemical vapour deposition of acetylene + nitrogen (0-62 vol.%) onto glass/Si substrates at a deposition temperature ~523 K and a negative bias voltage ~500 V. The fundamental optical absorption edge was studied by optical transmittance measurements. With increasing nitrogen content (0 to 20 at.%) in the films the band gap and Urbach tail width decreased. Thick films deposited without nitrogen had high compressive stress (~3 GPa). Introduction of nitrogen in the precursor gas was effective in reducing the stress to less than 1 GPa with improved adhesion of the film on the substrate. The stresses in the films were determined by a non-destructive optical technique (from optical absorption band tail) so that substrate effect on the stress measurement prevalent in the indentation technique could be eliminated. Raman, photoluminescence and FTIR spectra were analysed to ascertain the quality of the films. The sp ³ / sp ² ratio in the films were estimated from the FTIR spectra by considering the C-H absorption around ~2900 cm ^-1. With increase of nitrogen content in the DLC films the ratio decreased and the absorption peak due to N-H appeared.

Studies on the temperature dependence of the electrical conductivity (σ) of the films indicated variable range hopping conduction to be predominant at low temperature with high density of states. From the slope of the plot of ln σ versus T ^-1/4 the density of states N(E _F) was computed which indicated N(E _F) =3.2x10²³ eV^-1 cm^-3.

In the Raman spectrum there were two bands around 1376 (D-peak) and 1595 cm ^-1 (G peak) for the film deposited on Si, whereas, for the film simultaneously deposited on glass substrate the bands were shifted to around 1360 cm ^-1 (D peak) and 1584 cm ^-1 (G peak). The positions of these lines in the Raman spectrum of the DLC film could be correlated with the amount of sp³ C-C bonds and stress. It was observed that even for identical deposition environment, the films on glass had less sp² C-C than those on Si substrates although the intensity ratio of the D and G peaks did not show intense variation. For ~17 at.% nitrogen content in the film, the intensity ratios (I _D / I _G) of D to G peak were ~0.78 and 0.79 on Si and glass substrates respectively.

Photoluminescence of a-C:H:N films was studied with different excitation energies (E _ex) below and above the band gap. The optical parameters (absorption coefficient, band gap and refractive index) of a-C:H:N films were determined from the transmittance and ellipsometric measurements. Optical properties of the films could be explained by a two phase model which considers the existence of s ² clusters (lower band gap region) embedded within a sp ³ matrix (higher band gap region). Carrier confinement within the sp ² clusters gives rise to radiative recombination producing strong photoluminescence (PL) at room temperature. The sp ³ / sp ² ratio in the films were estimated from the FTIR spectrum by considering C-H absorption around ~2900 cm ^-1.

Diamond films of 5-40 µm thickness were synthesized by combustion flame method and by Microwave Plasma-assisted Chemical Vapour Deposition (MPCVD). Examinations of diamond coatings by Scanning Electron Microscopy (SEM), Electron Dispercive Spectroscopy (EDS) and by Raman Spectroscopy (RS) have shown that the morphology of diamond coatings obtained by combustion flame is similar to that obtained by MPCVD and the quality of it is close to that of natural diamond.

Surface resistivity of as-deposited, annealed samples and those treated in hydrogen plasma have been measured as a function of temperature. The as-deposited and treated samples obtained by MPCVD show low resistivity at room temperatures and their surface-resistance exhibits a peak at 200^°C during the first heating phase which can be attributed to the surface dehydrogenation. This phenomena was not observed on the diamond coatings obtained by combustion flame method. The difference in behaviour is mainly due to hydrogen terminated or oxygen terminated surface dangling bonds.

The purpose of this paper is to present the electrical characteristics obtained from diamond coatings grown by different deposition methods and to discuss the influence of hydrogen and oxygen terminated diamond surface on the electrical properties.