Undoped Polycrystalline Diamond Films Research Articles

The Hydrogenation Impact on Electronic Properties of p-Diamond/n-Si Heterojunctions.

The undoped polycrystalline diamond films (PDFs) have been deposited on n-type silicon (Si) by Hot Filament Chemical Vapor Deposition (HF CVD) technique. The reaction gases are a mixture of methane and hydrogen. The obtained PDFs were characterized by scanning electron microscopy (SEM) and Raman spectroscopy which, in addition to the diamond phase, also confirms the presence of sp hybridized carbon bonds. As-grown CVD diamond layers are hydrogen terminated and show p-type conductivity. The effect of the level of hydrogenation on the electrical properties of p-diamond/n-Si heterojunctions has been investigated by temperature dependent current–voltage (J-V/T) characteristics. The obtained results suggest that the energy distribution of interface states at the grain boundary (GB) subjected to hydrogenation becomes shallower, and the hole capture cross-section can be reduced. Hydrogenation can lead to a significant reduction of the GB potential barrier. These results can be interesting from the point of view of hydrogen passivation of GBs in microelectronics.

Electron affinity of undoped and boron-doped polycrystalline diamond films

The electron affinity of polycrystalline undoped and boron-doped diamond films was investigated by means of X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. It was demonstrated that both as-grown and hydrogenated polycrystalline diamond films exhibited true negative electron affinity (NEA). Positive electron affinity (PEA) was observed after diamond surface oxidation in plasma. NEA reduction was observed after hydrogenation of the polycrystalline film surface in plasma. This effect was related to depolarization of the surface dipoles, an increase in the density of surface defects, and an increase in contaminations on the surface. Boron doping of the diamond film had a negligible effect on electron affinity. Downward surface band bending was found for all the samples that were studied, and there was larger band bending for polycrystalline diamond surfaces modified in plasma than for as-grown polycrystalline diamond surfaces.

Charge-based deep level transient spectroscopy of B-doped and undoped polycrystalline diamond films

The undoped and B-doped polycrystalline diamond thin film was synthesized by hot filament chemical vapor deposition and microwave plasma, respectively. The structural characterization was performed by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The electrical properties of synthesized diamond layer were characterized by dc-conductivity method and charge deep level transient spectroscopy. The B-doped diamond layers show higher sp2/sp3 ratios in comparison with that of undoped layers what can have an essential influence on the localized density of states associated with shallow hydrogen acceptor states what is reflected in the values of activation energies which reached the values of 38 meV for B-doped and 55 meV for undoped diamond layers, respectively. The existence of deep level traps, as, for example, associated with B-related acceptors, was not observed.

Visible sub-band gap photoelectron emission from nitrogen doped and undoped polycrystalline diamond films

In this study the origin of visible sub-band gap photoelectron emission (PEE) from polycrystalline diamond films is investigated. The PEE yields as a function of temperature were studied in the wavelengths range of 360–520nm. Based on the comparison of electron emission yields from diamond films deposited on silicon and molybdenum substrates, with different thicknesses and nitrogen doping levels, we suggested that photoelectrons are generated from nitrogen related centers in diamond. Our results show that diamond film thickness and substrate material have no significant influence on the PEE yield. 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 phenomenon was explained by the 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. Activation energies of trap levels were estimated for undoped nanocrystalline, undoped microcrystalline and N-doped diamond films using the Richardson-Dushman equation, which gives 0.13, 0.39 and 0.04eV, respectively. Such low activation energy of trap levels makes the ta-PETE process very effective at elevated temperatures.

Superconductivity in heavily B-doped diamond layers deposited on highly oriented diamond films

We deposited a [100]-oriented B-doped diamond layer by three methods to clarify the effects of film morphology on the transition from metallic to superconducting diamond. Heavily B-doped [100]-oriented diamond layers were deposited on [first method] undoped polycrystalline diamond films with [111] faces, [second method] highly oriented undoped diamond (HOD) thin films with a pyramidal surface, and [third method] thick undoped HOD films with a pyramidal surface. We confirmed that the B-doped layer in the third method was oriented in the [100] direction by scanning electron microscopy (SEM). The highest transition temperatures were T c(onset) = 5.0 K and T c(zero) = 3.1 K for the B-doped layer deposited on a thick HOD film with a pyramidal surface under a zero magnetic field. By contrast, T c(onset) was 4.1 K for a heavily B-doped diamond layer deposited on a thin HOD film with a pyramidal surface, and was 3.9 K for a heavily B-doped diamond layer deposited on an undoped polycrystalline diamond film. These differences in T c for our samples are affected by disorder and effective hole-carrier doping in each sample. Using the third method, we successfully deposited a high-quality B-doped [100] layer in three steps: (first step) depositing a [100] HOD film on a Si [100] substrate, (second step) depositing an HOD film with a pyramidal surface, and (third step) depositing a [100]-oriented B-doped layer. The change in the electronic states due to the B-doping of diamond films and the film morphology were investigated by x-ray photoelectron spectroscopy (XPS) measurements and band calculations.

Electrical Properties of Hydrogen Terminated P-Type Diamond Film

Undoped high quality polycrystalline diamond films were grown by the microwave plasma chemical vapor deposition (MPCVD) method. The effects of hydrogen plasma treatment and vacuum annealing process on the p-type behavior of diamond films were investigated by the Hall effect method. The sheet carrier concentration increased and the sheet resistivity decreased with the treating time of hydrogen plasma and a stable value was achieved finally. After annealing the samples in vacuum at temperature above 600 °C, the sheet carrier concentration dropped dramatically. The origin of this hydrogen terminated p-type conductive layers is also discussed.

Dislocation electrical conductivity of synthetic diamond films

A relationship between the electric resistance of single-crystal homoepitaxial and polycrystalline diamond films and their internal structure has been investigated. It is established that the electrical conductivity of undoped homoepitaxial and polycrystalline diamond films is directly related to the dislocation density in them. A relation linking the resistivity ρ (∼1013–1015 Ω cm) with the dislocation density Γ (∼1014−4 × 1016 m−2) is obtained. The character of this correlation is similar for both groups of homoepitaxial and polycrystalline diamond films. Thin (∼1–8 μm) homoepitaxial and polycrystalline diamond films with small-angle dislocation boundaries between mosaic blocks exhibit dislocation conductivity. The activation energy of dislocation acceptor centers was calculated from the temperature dependence of the conductivity and was found to be ∼0.3 eV. The conduction of thick diamond films (h > 10 μm) with the resistivity ρ ≈ 108 Ω cm is determined by the conduction of intercrystallite boundaries, which have a nondiamond hydrogenated structure. The electronic properties of the diamond films are compared with those of natural semiconductor diamonds of types IIb and Ic, in which dislocation acceptor centers have activation energies in the range 0.2–0.35 eV and are responsible for hole conduction.

High-Performance Polycrystalline Diamond Micro- and Nanoresonators

Cantilever type MEMS resonators were fabricated using boron-doped (~5 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ) and undoped polycrystalline diamond (poly-C) films that were grown at 600degC or 780degC. The resonator dimensions ranged from 500 mum long, 10 mum wide, and 0.7 mum thick to 40 mum long, 100 nm wide, and 0.6 mum thick. Resonance frequencies and quality factors Qs were measured in vacuum, 10 5 torr, over the temperature range of 23degC-400degC. The measured values of the temperature coefficient of the resonance frequency were in the range of -7.2- -25.6 ppm. degC <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and seemed to be related to changes in the Young's modulus with temperature. Undoped poly-C cantilevers exhibit Qs as high as 116 000, the highest value reported for a cantilever resonator fabricated from a polycrystalline film. A thermally activated relaxation process seems to limit the measured Q -values for the highly doped poly-C 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.

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)

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.

Three Dimensional Hydrogen Microscopy in Diamond

AbstractWe introduce proton-proton scattering at a microprobe of 17 MeV protons to quantitatively image three dimensional hydrogen distributions in polycrystalline diamond at a lateral resolution better than 1 μm and high sensitivity. The images show that most of the hydrogen of a &lt;110&gt;-textured undoped polycrystalline diamond film is located at grain boundaries. The average amount of hydrogen is (8.1±1.5)·1014 atoms/cm2 along the grain boundaries which corresponds to about a third of a monolayer. The content within the grain is below the detection limit of 1.4·1016 atoms/cm2 (0.08 at-ppm).

Three-Dimensional Hydrogen Microscopy in Diamond

A microprobe of protons with an energy of 17 million electron volts is used to quantitatively image three-dimensional hydrogen distributions at a lateral resolution better than 1 micrometer with high sensitivity. Hydrogen images of a <110>-textured undoped polycrystalline diamond film show that most of the hydrogen is located at grain boundaries. The average amount of hydrogen atoms along the grain boundaries is (8.1 +/- 1.5) x 10(14) per square centimeter, corresponding to about a third of a monolayer. The hydrogen content within the grain is below the experimental sensitivity of 1.4 x 10(16) atoms per cubic centimeter (0.08 atomic parts per million). The data prove a low hydrogen content within chemical vapor deposition-grown diamond and the importance of hydrogen at grain boundaries, for example, with respect to electronic properties of polycrystalline diamond.

Electrochemical properties of undoped CVD diamond films vacuum-annealed at 1775–1915 K

Electrochemical behavior of vacuum-annealed undoped polycrystalline diamond films is studied. The film annealed at 1775 K appeared to be practically not conducting. With further increase in the annealing temperature above 1825 K, the film effective resistivity decreased from initial value of 10 11–10 12 Ω cm down to less than 0.1 Ω cm; the differential capacitance increased from ∼10 -3 to ∼50 μF per 1 cm 2 of geometrical surface; the transfer coefficients for electrochemical reactions in the [Fe(CN) 6] 3−/4− redox solution increased from ∼0.2 to 0.5; and the degree of reversibility of the electrochemical reaction increased. The observed changes of the electrode properties can be attributed to gradual change of the thickness and/or properties (first and foremost, conductivity) of the non-diamond carbon phase formed along the intercrystallite boundaries upon the annealing and outcropping at the film surface as ‘active sites’.

Nonuniform current distribution in metal/diamond/metal vertical structures

The role of grain boundaries (GBs) in undoped polycrystalline diamond films has been investigated by dc and ac electrical measurements in a wide temperature range. Hopping transport along GB percolating paths and field-assisted thermal ionization of trapped charges are observed at low and high electric field strength, respectively. The temperature dependence of the ac conductivity, which reduces to a universal curve according to a random free-energy barrier model, suggests that, at low field strengths, current flow is mainly confined into GB domains in a wide temperature range. Only in the high-temperature and high-field ranges, do crystalline diamond grains become involved in current transport.

Anomalous nonlinearity of IR photoconductivity of diamond polycrystalline films

Nonlinearity of the lux-ampere characteristics of undoped polycrystalline diamond films is revealed. The spectral dependences of this nonlinearity and photoconductivity are analyzed in the framework of the available models with a single impurity level or a single type of traps. It is shown that, for undoped polycrystalline diamond films in the range of impurity photoconductivity, the lux-ampere characteristics exhibit an anomalous nonlinearity when their slopes change from 1/2 to approximately unity with a decrease in the wavelength and an increase in the excitation level and the carrier lifetime. The results obtained are explained within the model of the coexistence of two different channels of conduction in diamond films. Two ranges of carrier photogeneration are established. For photon energies below 1.4 eV, the carrier generation with quadratic recombination is observed in a single channel. At higher photon energies, the carrier generation occurs in two channels simultaneously (in addition, the carriers are excited with linear recombination in the second channel). It is demonstrated that the equilibrium concentration of carriers in the second channel with linear recombination substantially exceeds the equilibrium concentration of carriers in the first channel with quadratic recombination, which hampers the detection of the first channel without photoexcitation.

Analysis of Impedance Spectra of Vacuum-Annealed Undoped Polycrystalline Diamond Electrodes

Impedance spectra for a new electrode material, undoped polycrystalline diamond films vacuum-annealed at 1775 to 1915 K, taken in indifferent electrolytes are analyzed. Regular changing of parameters of the equivalent circuit (constant-phase element, ohmic and faradaic resistances) with increasing anneal temperature is demonstrated. It is concluded that both the amount of a nondiamond conducting phase in the electrode bulk increased and its properties (first and foremost, the conductivity) changed with increasing anneal temperature.

Vacuum-Annealed Undoped Polycrystalline Diamond: a New Electrode Material

Electrochemical behavior of undoped polycrystalline diamond films annealed in a vacuum at 1775 to 1915 K is studied. The annealing at temperatures over 1825 K imparts conductance to initially insulating films, which permits using them as electrode material. With further increase in the annealing temperature to above 1915 K, the effective resistivity decreases from initial value of 1011 to 1012 Ω cm down to less than 0.1 Ω cm, the differential capacitance increases from ∼10–3 to ∼50 μF per cm2 of geometrical surface, the transfer coefficients for electrochemical reactions in the Fe(CN)63–/4– redox solution increase from ∼0.2 to 0.5, and the degree of reversibility of the electrochemical reaction increases. The observed changes in the electrode properties are caused by the formation, upon the annealing, of a nondiamond phase at the intercrystallite boundaries and defect areas in the crystallites; the outcroppings of the conducting phase play the role of active sites at the electrode surface. With increasing annealing temperature, both the amount of this phase and its conductivity increase.

Synchrotron radiation study of surface versus sub-surface deuterium in diamond films produced by exposure to deuterium activated by hot filament-high vacuum and ex situ microwave plasma

In the present study, the chemical state of adsorbed deuterium and its thermal stability in diamond films surface and sub-surface regions are investigated by photon stimulated ion desorption (PSID), near-edge X-ray absorption fine structure (NEXAFS), and secondary electron emission (SEE) measurements. To this effect, undoped polycrystalline diamond films were exposed to deuterium activated by ex situ microwave (MW) plasma or in situ by hot filament (HF) in ultrahigh vacuum conditions. D+ PSID measurements of polycrystalline diamond films were performed after annealing to temperatures in the range of 25–1000 °C. Even after moderate annealing to 350 °C of the MW-deuterated diamond film, PSID spectra showed a substantial decrease in total deuterium desorption yield, as well as change of the dynamics of desorption in favor of direct processes induced by core-hole relaxation. D+ PSID of diamond films in situ exposed to deuterium activated by the HF showed different behavior. The desorption yield of deuterium ions and the relation between direct and indirect desorption processes were nearly unchanged when heated up to ∼800 °C. The effect of annealing on D+ desorption yield and on the dynamics of the desorption process was associated with structural changes in the near-surface region as identified by NEXAFS spectroscopy, and changes in the electron emission properties, as measured by SEE. On the basis of these results it is suggested that in contrast to ex situ MW plasma deuteration, in situ exposure of the films to HF-produced activated deuterium does not lead to bulk diffusion of deuterium atoms.