Undoped Diamond Films Research Articles

A positron annihilation study on the defect properties of doped diamond films

Doppler broadening measurements were performed on undoped, boron doped, and sulfur doped diamond films. The defect properties in these different diamond films were analyzed and the effect of boron concentration in the B-doped diamond films on these properties was studied. The Doppler broadening measurements were characterized with the shape parameter S and the wing parameter W. From these fitted characteristic S and W values of the diamond films and plots of S vs. position implantation energy, it was deduced that undoped and S-doped diamond films are rich of vacancy-like defects, while B-doped diamond films are poor of vacancy-like defects. This difference may originate from possible different charge state of the vacancy-like defects and from the incorporation of impurities in the different growth ambient of the films. By comparing the parameters obtained in the Doppler broadening measurements of diamond films with different boron concentration, we found that S values of B-doped diamond did not decreased with the increasing of boron concentration, which suggests that more damaged regions form in the higher boron concentration samples.

Evaluation of diamond gauge factor up to 500 °C

Gauge factors of boron-doped polycrystalline diamond films were investigated in the ranges from room temperature to 500 °C and from 0 to 0.07 MPa of applied pressure. A VCR fitting was used as a package for a measurement at high temperature. Undoped and boron-doped polycrystalline diamond films were deposited on a Si substrate by hot filament chemical vapor deposition (CVD) method. A chip was fabricated with bulk micro machining technique. The gauge factors were evaluated in the ranges from room temperature to 500 °C in furnace. The calculated values of the gauge factors were found to vary in a range of 5–241. The gauge factors decreased with increased temperature from room temperature to around 100–300 °C. However, with higher temperature up to 500 °C, it increased.

Ion-induced electron emission (IIEE) from undoped and B-doped diamond films induced by 1–10 KeV H + and Ar +

A newly developed experimental system enables measurements of IIEE at a very low ion flux (10 5 ions/s cm 2) avoiding the fast emission degradation caused by high ion fluxes (usually applied 10 10–10 13 ions/s cm 2). The method overcomes the difficulty of measurement of reliable ion-induced secondary electron emission (IIEE) yield, associated with fast degradation of the IIEE yield. We report on the investigation of the IIEE from hydrogenated undoped and B-doped diamond films as a function of (i) moderate heating in vacuum prior to the measurements, (ii) H + and Ar + energy in the range of 1–10 KeV, and (iii) film thickness and microstructure. An IIEE yield ( γ) enhancement was typically detected when the films were heated to 300 °C in vacuum. In the B-doped diamond film heated to 300 °C, γ rose nearly linearly from ∼ 20 to ∼ 100 electrons/ion, for 1 to 10 KeV Ar + ions, respectively. The values of γ obtained with H + showed a more moderate, nonlinear increase from ∼ 8 electrons/ion at 1 KeV up to ∼ 90 electrons/ion at 10 KeV. In heated undoped diamond films of different thicknesses the measured values of γ were similar for all the studied films and somewhat lower than in B-doped film: from ∼ 10–16 to 60–70 electrons per 1 to10 KeV Ar +, respectively, and from 14–26 to 50–60 electrons per 1 to10 KeV H +, respectively. The experimental results were interpreted using TRIM calculations.

UV Schottky photodiode on boron-doped CVD diamond films

We report on experimental study of photosensitivity and Q-DLTS spectra of polycrystalline CVD diamond UV photodetectors. The measured characteristics of Schottky photodiode on boron-doped diamond films are compared with those obtained for planar photoconductive structures (photoresistor type) based on undoped CVD diamond. The Schottky photodiode exhibited a sharp cut-off in photoresponse with spectral discrimination ratio (between wavelengths of 190 nm and 700 nm) as high as 5 · 10 5 at zero bias voltage (at zero dark current). The photodiode showed the maximum of photoresponse at wavelength < 190 nm, and a low density of trapping and recombination centers as evaluated with the Q-DLTS technique. The devices demonstrated the photoresponsivity at 190 nm from 0.03 to 0.1 A/W with quantum yield of 0.20 to 0.67 in closed circuit, while the photovoltage ≥ 1.6 V was measured in open circuit regime. Another type of UV detector, the planar photoconductive structures with interdigitizing ohmic electrodes fabricated on undoped diamond film and operated under a bias voltage, revealed a higher density of (surface) defect centers and the maximum photoresponse at ∼ 210 nm wavelength. A strong influence of UV light illumination on the Q-DLTS spectra of the planar photoconductive structures was observed. This effect can be used for development of new UV detectors and dosimeters based on the Q-DLTS signal measurements.

Influence of Positive Bias on Electrical Properties of UndopedNanocrystalline Diamond Films

By means of electron assisted hot filament chemical vapour depositiontechnology, nanocrystalline diamond films are deposited on polishedn-(100)Si wafer surface at 1 kPa gas pressure. The deposited films arecharacterized with a Raman spectrometer, atomic force microscope,semiconductor characterization system and Hall effect measurementsystem. The results show that, when bias current is larger than 2 A,sheet hole concentration can increase to a value greater than1013 cm−2 and undoped nanocrystalline diamond films with ap-type semiconducting characteristic form. Heterojunction between n-Sisubstrate and the nanocrystalline diamond films deposited with 2 A and6 A bias current has an evident junction effect. Hole formationmechanisms in the films are discussed.

An investigation of structural and electrical properties of boron doped and undoped nanocrystalline diamond films

AbstractNanocrystalline and microcrystalline undoped and boron doped diamond films are investigated by means of SEM, Raman and photocurrent techniques. SEM measurements on nanocrystalline diamond show the typical diamond columnar structure whereas for microcrystalline diamond such structure is not observed. Raman investigation shows a clear diamond peak at 1332 cm–1 for all investigated films and a Fano resonance is observed for the microcrystalline sample prepared with 2000 ppm of boron in the gas mixture indicating a transition into the semi‐metallic state. Photocurrent measurements also show a presence of boron in some films. It is found that when temperature is decreased lower than 80 K, the boron photocurrent threshold in the case of NCD material behaves differently compared to bulk diamond. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Sensitivity of Raman spectra excited at 325 nm to surface treatments of undoped polycrystalline diamond films

AbstractAbout 20 μm thick films were deposited in the same run by MPCVD at 900 °C on Si substrates and then hydrogenated in situ during 30 min with a hydrogen plasma at the same temperature. Their surfaces were kept as prepared or more or less strongly oxidized by annealing at 600 °C under ambient atmosphere or by sulphochromic acid or aqua regia treatments. Raman spectra were excited at 325 and 632.8 nm. Spectra of the as‐prepared sample exhibit structures around 2835 and 2895 cm–1 from monohydride carbon–hydrogen ascribed to the atomically flat (111) and (100) areas, respectively, on the facets of the sample surface crystallites. The decrease of these structures in the normalized spectra after the various oxidation treatments confirms these assignments. The decrease is smaller for the aqua regia treatment than for the two other treatments which give similar effects. Other Raman signals from sp2 C around 1589 cm–1 and CHx bonds around 2930, 2952, 3025 and 3050 cm–1 originate from species at the surface and within the films. Their variation with the oxidizing treatments indicates a significant contribution from the surface species. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Enhancement of electron emission from near-coalescent nanometer thick continuous diamond films

In this study we report on the dependence of electron emission properties of undoped diamond films on film thickness. It was found that regardless of primary excitation, the electron emission exhibits maximum values for near-coalescent nanometer thick continuous diamond films. The electron emission induced by photons, ions, electrons and electric fields was consistently investigated and compared for diamond film thicknesses from 70 nm up to 4.7 μm. The absolute quantum photoyield (QPY) value at photon energy of 8.85 eV increases with film deposition time from 2.5% (carbonized silicon substrate) to 14% for a continuous 70 nm thick diamond film, followed by a decrease to 12% and stabilization for films of thicknesses above ∼150–200 nm. The initial coefficient of ion induced electron emission (IIEE), by 140 keV Ar + ions, also reaches its maximum value of 116 at ∼100 nm diamond film thickness and decreases to 90–100 for thicker films. The secondary electron emission (SEE) shows maximum yield ( δ max) at primary electron energy of ∼1000 eV. The dependence of δ max on film thickness displays similar behavior as QPY value: it obtains its maximum value of δ max ∼10 at film thickness of ∼100 nm and drops to somewhat constant value of δ max ∼6.6 for thicker films. The threshold field, required for field electron emission (FEE) current of 0.3 μA/cm 2, reaches its minimum of ∼8.5 V/μm for ∼100 nm thick diamond film and monotonically increases for thicker diamond films. Possible reasons for the phenomenon of enhanced electron emission from the 70–100 nm thick diamond films have been discussed.

Surface functionalization of ultrananocrystalline diamond using atom transfer radical polymerization (ATRP) initiated by electro-grafted aryldiazonium salts

The integration of organic adlayers with diamond surfaces imparts many new properties to diamond materials, including lubrication, optical response, chemical sensing, and biocompatibility. The development of new immobilization chemistries is required in which organic layers are covalently attached to the diamond surface. Poly(methyl methacrylate) and polystyrene brushes (PMMA, PS, respectively) were grown from the surfaces of ultrananocrystalline diamond (UNCD) thin films that were modified by the electrochemical reduction of the diazonium salt BF 4 −, +N 2–C 6H 4–CH(CH 3)–Br. The grafted –C 6H 4–CH(CH 3)–Br species were effective in promoting surface initiated atom transfer radical polymerization (SI-ATRP) of the vinyl monomers. Attempts to perform SI-ATRP from undoped diamond films produced by microwave plasma assisted chemical vapour deposition (MPACVD) process was rather unsuccesful due to a very low extent of electrochemical modification by the brominated aryl diazonium salts. The formation of the polymer brushes was confirmed by XPS. The polymer brushes were further evaluated as adsorbents for human serum albumin (HSA) used as a model protein. Protein adsorption decreased in the order PS > PMMA, that is the decreasing trend of hydrophobic character.

Temperature evolution of photocurrent spectra in undoped and boron-doped homoepitaxial CVD diamond film

Photocurrent spectra in undoped and boron-doped homoepitaxial CVD diamond films were measured in the ultra-violet photon energy range 4 to 6 eV at different temperatures 250 to 450 K. A step-like structure starting at 4.6 eV was observed as a dominant structure on the spectra in both films. From the different temperature dependence of the step-like structure in between the undoped and boron-doped diamond films, it is suggested that the step-like structure is due to free holes generated by the optical transition of electrons from the valence band to localized absorption centers located at 0.8 to 0.9 eV below the bottom of the conduction band. The result of dual beam photocurrent measurement using a ultra-violet light and a He–Ne laser supports the existence of the absorption center near the conduction band.

Surface conductive layers on oxidized (111) diamonds

Surface conductive layers (SCL) on oxidized (111) diamonds with smooth surfaces after exposure to air were detected and characterized by Hall effect measurements. Hall effect measurements show that the conductivity is p type with sheet hole concentrations around of 1012cm−2 and Hall mobilities between 5 and 130cm2∕Vs. The SCL vanishes by thermal annealing at a temperature higher than 460K in He atmosphere, and recovers in air. These characteristics are similar to those generated by hydrogen termination. The experiments revealed that these SCLs are present on boron doped (111) and undoped (111) diamond films with smooth surfaces and natural IIa (111) diamonds, but not on (111) diamond films with rough surfaces and not on (100) diamonds.

Field electron emission from undoped, continuous, submicron-thick diamond films

The present work shows that the field electron emission (FEE) properties of polycrystalline diamond films can be enhanced by control over the film thickness. The FEE properties of undoped, continuous, and smooth submicron-thick diamond films with initial nucleation densities of ∼5×1010particles∕cm2 were investigated as a function of diamond film thickness. A set of films with thickness ranging from 70–100to830nm yielded turn-on field values of 6–8V∕μm and threshold field values of 8.5–17.5V∕μm (for 0.3μA∕cm2), respectively, without any conditioning. It was found that the films of thickness up to ∼370nm can sustain stable current density as high as 0.1A∕cm2 without morphological modification. The thicker films, however, suffer from a strong degradation of the film and breakdown. The best FEE (lower turn-on and threshold fields and morphological stability) was obtained for a thin (100nm) continuous diamond film. This result is suggested to be attributed mainly to the efficient electron conduction from the back contact to the surface.

Diamond Schottky barrier diodes with low specific on-resistance

We have fabricated sandwich-type diamond Schottky barrier diodes with high current density on an undoped homoepitaxial diamond film grown on a heavily boron-doped type Ib diamond substrate. The current–voltage measurements for the diodes show excellent rectification characteristics in a temperature range of 298–873 K. The current density flowing through the diodes and the specific on-resistance of the diodes were 310 A cm−2, 9.68 × 10−3 Ω cm2 and 637 A cm−2, 4.71 × 10−3 Ω cm2 at 3 V forward bias at 298 K and 873 K, respectively. The activation energies of the band-gap states calculated from reverse current were around 300 meV. The capacitance–voltage measurement showed the large difference between the space charge densities of the substrate and the CVD diamond film.

P-type doping by B ion implantation into diamond at elevated temperatures

We have studied B ion implantation at 400 °C into undoped homoepitaxial chemical vapor deposition diamond films and high-pressure and high-temperature (HPHT) synthetic IIa substrates. The highest Hall mobility at room temperature is 268 cm 2/Vs among B implanted homoepitaxial films, while it is 38 cm 2/Vs for the B implanted HPHT synthetic IIa substrate. The present result reveals that the quality of a doped layer is strongly dependent upon that of a diamond substrate employed for ion implantation.

High-quality homoepitaxial diamond (100) films grown under high-rate growth condition

We present advantages of high-power microwave plasma chemical vapor deposition (MPCVD) in homoepitaxial diamond film deposition. Diamond films grown at comparatively high growth rate of 3.5 μm/h showed intense free-exciton recombination emission at room temperature. The free-exciton decay time of the diamond film at room temperature, 22 ns, was much longer than that of type-IIa single crystal, indicating electronically high quality of the homoepitaxial films. Dislocation-related emissions were locally observed, a part of which created by mechanical polishing process was successfully removed by surface etching process using oxygen plasma. Another advantage of the high-power MPCVD is effective impurity doping; boron-doped diamond films with high carrier mobility and high carrier concentration were reproducibly deposited. An ultraviolet photodetector fabricated using the high-quality undoped diamond film showed lower noise equivalent power as well as higher photoresponsivity for ultraviolet light with better visible-blind property, compared to those of standard Si-based photodetectors. The high-power MPCVD is, thus, indispensable technique for depositing high quality diamond films for electronic devices.

Three-Dimensional Hydrogen Microscopy in Diamond.

For Abstract see ChemInform Abstract in Full Text.

Synthetic diamond electrodes: Photoelectrochemical behavior of vacuum-annealed undoped polycrystalline diamond films

The photocurrent and photopotential for undoped polycrystalline diamond film electrodes prepared by chemical vapor deposition and annealed in vacuum at 1500–1640°C are measured. The “metal-like” samples (annealed at ≥1630°C) have a negligible photosensitivity. Judging from the positive sign of the photopotential and the cathodic direction of the photocurrent, the material under study formally behaves as a p-type semiconductor. The photoeffects are presumably caused by structure defects, in particular, the dislocations in diamond crystallites formed close to intercrystalline boundaries during the high-temperature annealing.

Ultrathin continuous undoped diamond films: Investigation of nanoscale conduction properties

Using a specialized ultrasonic seeding technique to produce very high diamond nucleation density (>10 10 cm −2), ultrathin, continuous, polycrystalline films of thickness from 70 to 100 nm were deposited onto silicon substrates. Raman spectroscopy revealed these films to be of high quality, with a clear peak at 1332 cm −1 and also a peak at 1140 cm −1. These ultrathin films display excellent electron emission properties. The nanoscale electrical properties of the films were investigated as a function of film thickness and temperature by conducting probe atomic force microscopy (CP-AFM) to reveal the nature of the electrically conducting pathways in the films. Images of current distribution of the films show that the conductive properties of the films are not homogeneous on the nanoscale, with regions of varying electrical conductivity. The local and averaged I– V characteristics of the films shows a higher conductivity when electrons are being injected from the tip into the diamond sample than the reverse. The 100-nm-thick continuous diamond film, in which the crystallites just touch one another, is the most conductive due to the existence of conductive pathways along the diamond grains. For thicker films, the conductive pathways appear to be on the surface; but in this case, some facets of the diamond crystallites in the films appear to be more electrically active than others. The enhanced conductivity of the 100-nm-thick diamond film can explain its enhanced electron emission properties.

Micro-Raman scattering from undoped and phosphorous-doped (111) homoepitaxial diamond films: Stress imaging of cracks

We report postgrowth micro-Raman stress imaging of cracks in (111) homoepitaxial diamond films. Undoped and phosphorous-doped diamond thin films grown by microwave plasma-enhanced chemical-vapor deposition on Ib (111)-oriented diamond substrates have been studied by confocal micro-Raman spectroscopy. For comparison purposes, a film grown on a (100) Ib substrate was also examined. Thanks to the confocal optics, the Raman signal arising from the epilayer could be discriminated from that arising from the substrate. As was already reported, the (111) films exhibited substantial tensile stress, indicated by a downshift in the Raman peak and by spontaneous cracking in films grown thicker than 5μm. The sixfold symmetry of the cracks supported that the films were homoepitaxial. A high compressive stress was also detected at the substrate near surface, and a partial stress relaxation was observed to occur in the vicinity of the cracks. Possible origins of the high tensile stress observed in the (111) homoepitaxial diamond films are discussed.

Electrochemical characteristics of boron-doped, undoped and nitrogen-doped diamond films

Boron-doped, undoped and nitrogen-doped diamond films were synthesized by microwave plasma assisted chemical vapor deposition (MP-CVD). Raman spectroscopy, XPS, EPMA and UV–Vis were used to characterize the properties of the synthesized films. Electrochemical characteristics for several redox systems on the three kinds of diamond films were examined. For Li +/Li ( E 0=−3.05 V) and H +/H 2 ( E 0=0.00 V) redox couples, the marked differences in cyclic voltammetric (CV) behaviors were observed on the nitrogen-doped diamond films, whereas for Fe(CN) 3−/ 4− ( E 0=0.36), Au/AuCl 4 − ( E 0=1.00) and O 2/H 2O ( E 0=1.23 V) couples, the CV behaviors on the nitrogen-doped films were similar to those on the boron-doped or undoped diamond films. The significant differences of CV behaviors could be explained by hypothesizing that the electron transfers of the redox species in the solution on diamond electrodes happened at the top of valence band together with the surface doping model suggested by F. Maier and colleagues [F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85 (2000) 3472].