Electrical Properties of B-doped homoepitaxial diamond films grown from UHP gas sources

It is confirmed that a small amount of nitrogen incorporated into chemical vapor deposited diamond films dramatically affects their electrical properties. Nitrogen can be incorporated into diamond films through the leak of vacuum system and/or from the impurity in source gases. Because a nitrogen atom can be a deep donor in diamond crystal, the p-type semiconducting properties of boron doped diamond films can be degraded even by the small amount of nitrogen. The small amount of nitrogen in chemical vapor deposited diamond films was measured by cathodoluminescence spectroscopy. For the detection of nitrogen, the N–V center was intentionally induced by defect formation through ion beam irradiation and subsequent annealing. The luminescence intensity of the N–V center was decreased by reducing the leak of the vacuum system and by upgrading the purity of the source gases. Both the carrier density and the Hall mobility of the boron doped diamond films were successfully improved by the control of nitrogen contamination. Using extremely high pure CH₄, H₂ and B₂H₆ in a tightly sealed vacuum system, the total amount of nitrogen impurity in the source gas was controlled to <80 ppm in the N/C atomic ratio resulting in a Hall mobility of 1600 cm²/Vs with a hole concentration of >10¹⁴ cm⁻³ at the room temperature in a 10-ppm-boron doped homoepitaxial diamond film.     

High temperature thermal conductivity of free-standing diamond films prepared by DC arc plasma jet CVD

Free-standing diamond films with 1.68 mm in polished thickness have been prepared by DC arc plasma jet CVD. By means of simply changing the placing orientation of diamond films along the laser transmission direction while testing, the through-thickness thermal conductivity (κ_⊥) together with the in-plane (κ_//) thermal conductivity of free-standing diamond films were measured by laser flash technique over a wide temperature range. Results show that the thermal conductivity κ_⊥ and κ_// of free-standing diamond films are up to 1916 and 1739 Wm^− 1 K^− 1 at room temperature, respectively, showing small anisotropy (9%), and following the relationship κ ~ T^− n as temperature rises. The conductivity exhibits similar value compared to that of high-quality single crystal diamond above 500 K for both through-thickness and in-plane directions of CVD diamond films. The effects of impurities and grain boundaries on thermal conductivity of diamond films with increasing temperature were discussed.     

Preparation and characterization of transparent indium- doped TiO2 films deposited by MOCVD

Titanium dioxide (TiO₂) films doped with different indium (In) concentrations have been prepared on SrTiO₃ (STO) substrates by high vacuum metalorganic chemical vapor deposition (MOCVD). X-ray diffraction (XRD) analyses revealed the TiO₂ films doped with low In concentrations to be [001] oriented anatase phase and the films with high In concentrations to present polycrystalline structures. The 1.8% In-doped TiO₂ film exhibited the best electrical conductivity properties with the lowest resistivity of 8.68×10⁻² Ω cm, a Hall mobility of 10.9 cm² V⁻¹ s⁻¹ and a carrier concentration of 6.5×10¹⁸ cm⁻³. The films showed excellent transparency with average transmittances of over 85% in the visible range.     

Properties of boron-doped epitaxial diamond layers grown on (110) oriented single crystal substrates

Boron doped diamond layers have been grown on (110) single crystal diamond substrates with B/C ratios up to 20 ppm in the gas phase. The surface of the diamond layers observed by scanning electron microscopy consists of (100) and (113) micro-facets. Fourier Transform Photocurrent Spectroscopy indicates substitutional boron incorporation. Electrical properties were measured using Hall effect from 150 to 1000 K. Secondary ion mass spectrometry analyses are consistent with the high incorporation of boron determined by electrical measurements. A maximum mobility of 528 cm² V^− 1 s^− 1 was measured at room temperature for a charge carrier concentration of 1.1 10¹³ cm^− 3. Finally, properties of boron doped (110) diamond layers are compared with layers on (100) and (111) orientated substrates.     

Quality of diamond wafers grown by microwave plasma CVD: effects of gas flow rate

We found a strong impact of gas flow rate on diamond growth process in a 5 kW microwave plasma chemical vapour deposition reactor operated on CH₄-H₂ gas mixtures. Diamond films of 0.1–1.2 mm thickness and 2.25 in. in diameter were produced at H₂ flow rates varied systematically from 60 sccm to 1000 sccm at 2.5% CH₄. The highest growth rate, 5 μm h⁻¹, was observed at intermediate F values (≈300 sccm). Carbon conversion coefficient (the number of C atoms going from gas to diamond) increases monotonically up to 57% with flow rate decrease, however, this is accompanied with a degradation of diamond quality revealed from Raman spectra, thermal properties and surface morphology. High flow rates were necessary to produce uniform films with thermal conductivity >18 W cm⁻¹ K⁻¹. Diamond disks with very low optical absorption (loss tangent tgδ<10⁻⁵) in millimetre wave range (170 GHz) have been grown at optimized deposition conditions for use as windows for high-power gyrotrons.     

Deposition and characterization of nanocrystalline diamond films on Co-cemented tungsten carbide inserts

Nanocrystalline diamond films were deposited on Co-cemented tungsten carbides using bias-enhanced hot filament CVD system with a mixture of acetone, H₂ and Ar as the reactant gas. The effect of Ar concentration on the grain size of diamond films and diamond orientation was investigated. Nanocrystalline diamond films were characterized with field emission scan electron microscopy (FE-SEM), Atomic force microscopy (AFM), Raman spectroscopy and X-ray diffraction spectroscopy (XRD). Rockwell C indentation tests were conducted to evaluate the adhesion between diamond films and the substrates. The results demonstrated that when the Ar concentration was 90%, the diamond films exhibited rounded fine grains with an average grain size of approximately 60–80 nm. The Raman spectra showed broadened carbon peaks at 1350 cm^− 1 and 1580 cm^− 1 assigned to D and G bands and an intense broad Raman band near 1140 cm^− 1 attributed to trans-polyacetylene, which confirmed the presence of the nanocrystalline diamond phase. The full width at half maximum of the <111> diamond peak (0.8°) was far broader than that of conventional diamond film (0.28°–0.3°). The Ra and RMS surface roughness of the nanocrystalline diamond film were measured to be approximately 202 nm and 280 nm with 4 mm scanning length, respectively. The Ar concentration in the reactant gases played an important role in the control of grain size and surface roughness of the diamond films. Nanocrystalline diamond-coated cemented tungsten carbides with very smooth surface have excellent characteristics, which made them a promising material for the development of high performance cutting tools and wear resistance components.     

Electrical contacts to nitrogen incorporated nanocrystalline diamond films

The effect of surface plasma treatment on the nature of the electrical contact to the nitrogen incorporated nanocrystalline diamond (n-NCD) films is reported. Nitrogen incorporated NCD films were grown in a microwave plasma enhanced chemical vapor deposition (MPECVD) reactor using CH₄ (1%)/N₂ (20%)/Ar (79%) gas chemistry. Raman spectra of the films showed features at ∼ 1140 cm^− 1, 1350 cm^− 1(D-band) and 1560 cm^− 1(G-band) respectively with changes in the bonding configuration of G-band after the plasma treatment. Electrical contacts to both untreated and surface plasma treated films are formed by sputtering and patterning Ti/Au metal electrodes. Ohmic nature of these contacts on the untreated films has changed to non-ohmic type after the hydrogen plasma treatment. The linear current–voltage characteristics could not be obtained even after annealing the contacts. The nature of the electrical contacts to these films depends on the surface conditions and the presence of defects and sp² carbon.     

Raman scattering by defect-induced excitations in boron-doped diamond single crystals

Polarized Raman spectra of the oriented boron-doped diamond with a different content of boron (≤ 200 ppm) were obtained with 514.5 and 1064 nm excitations. The additional bands were found in the region below 1200 cm^− 1. Their intensity increased with doping. It was shown that in polarized spectra these bands were in agreement with the singularities of density of phonon states (DOS) of diamond for the A_1g, E_g and F_2g symmetries. It was assumed that the ~ 900 cm^− 1 band which does not coincide with any DOS peak and has the highest resonance character may be attributed to the localized mode of boron in a diamond lattice. The spectra were accompanied by continuum that had the same symmetry F_2g as optical phonon at 1333 cm^− 1.     

Influence of SiC particle addition on the nucleation density and adhesion strength of MPCVD diamond coatings on Si3N4 substrates

It is generally accepted that SiC layers are often involved in the adhesion efficiency of chemical vapour deposition (CVD) diamond films on Si-containing substrates. Si₃N₄–SiC composite substrates with different amounts of SiC particles (0–50 wt%) were then used for diamond deposition. Samples were produced by pressureless sintering (1750°C, N₂ atmosphere, 2–4 h). The diamond films were grown on a commercial MPCVD reactor using H₂/CH₄ mixtures. Despite there being no special substrate pre-treatment, the films were densely nucleated when SiC was added (N_d≈1×10¹⁰ cm⁻²) with primary nanosized (∼100 nm) particles, followed by a less dense (N_d≈1×10⁶ cm⁻²) secondary nucleation. Indentation experiments with a Brale tip of up to 588 N applied load corroborated the benefit of SiC inclusion for a strong adhesion. The low thermal expansion coefficient mismatch between Si₃N₄ and diamond resulted in very low compressive stresses in the film, as proved by micro-Raman spectroscopy.     

Silicon carbide films from polycarbosilane and their usage as buffer layers for diamond deposition

Thin films of polycarbosilane (PCS) were coated on a Si (100) wafer and converted to silicon carbide (SiC) by pyrolyzing them between 800 and 1150 °C. Granular SiC films were derived between 900 and 1100 °C whereas smooth SiC films were developed at 800 and 1150 °C. Enhancement of diamond nucleation was exhibited on the Si (100) wafer with the smooth SiC layer generated at 1150 °C, and a nucleation density of 2 × 10¹¹ cm^− 2 was obtained. Nucleation density reduced to 3 × 10¹⁰ cm^− 2 when a bias voltage of − 100 V was applied on the SiC-coated Si substrate. A uniform diamond film with random orientations was deposited to the PCS-derived SiC layer. Selective growth of diamond film on top of the SiC buffer layer was demonstrated.     

Diamond based field-effect transistors with SiNx and ZrO2 double dielectric layers

A diamond-based field-effect transistor (FET) with SiN_x and ZrO₂ double dielectric layer has been demonstrated. The SiN_x and ZrO₂ gate dielectric are deposited by plasma-enhanced chemical vapor deposition (PECVD) and radio frequency (RF) sputter methods, respectively. SiN_x layer is found to have the ability to preserve the conduction channel at the surface of hydrogen-terminated diamond film. The leakage current density (J) of SiN_x/ZrO₂ diamond metal-insulator-semiconductor FET (MISFET) keeps lower than 3.88 × 10^− 5 A·cm^− 2 when the gate bias was changed from 2 V to − 8 V. The double dielectric layer FET operates in a p-type depletion mode, whose maximum drain-source current, threshold voltage, maximum transconductance, effective mobility and sheet hole density are determined to be − 28.5 mA·mm^− 1, 2.2 V, 4.53 mS·mm^− 1, 38.9 cm²·V^− 1·s^− 1, and 2.14 × 10¹³ cm^− 2, respectively.     

Semiconducting carbon films from a natural source: camphor

Thin films of carbon have been grown on alumina substrates by the pyrolysis of camphor at 900 °C for 2 h in an argon atmosphere, followed by sintering for various time periods. The effect of sintering time on the surface morphology, conductivity, carrier concentration, mobility and bandgap of camphor-pyrolyzed films is discussed. Structural characterizations are performed on the basis of XRD and SEM analyses. Electrical conductivity measurements of these films, as a function of temperature, suggest them to be semiconductors. A Hall-effect study of the as-grown films shows their carrier concentrations to be of the order of 10¹⁷ cm⁻³. The Hall mobilities of these films are found to vary from 1702 to 10263 cm² V⁻¹ s⁻¹. The thermal bandgaps of these films are found to decrease with increasing of sintering time. Thus, by controlled sintering of camphor-pyrolyzed carbon films, it is possible to obtained a semiconductor with the desired bandgap. Therefore, camphor-pyrolyzed semiconducting carbon films seem to be a promising material with which to develop a photovoltaic solar cell.     

Growth and properties of Cu3SbS4 thin films prepared by a two-stage process for solar cell applications

Cu₃SbS₄ is a promising material for thin film heterojunction solar cells owing to its suitable optical and electrical properties. In this paper, we report the preparation of Cu₃SbS₄ thin films by annealing the Sb₂S₃/CuS stacks, produced by chemical bath deposition, in a graphite box held at different temperatures. The influence of annealing temperature on the growth and properties of these films is investigated. These films are systematically analyzed by evaluating their structural, microstructural, optical and electrical properties using suitable characterization techniques. X-ray diffraction analysis showed that these films exhibit tetragonal crystal structure with the lattice parameters a=0.537 nm and b=1.087 nm. Their crystallite size increases with increasing annealing temperature of the stacks. Raman spectroscopy analysis of these films exhibited modes at 132, 247, 273, 317, 344, 358 and 635 cm⁻¹ due to Cu₃SbS₄ phase. X-ray photoelectron spectroscopy analysis revealed that the films prepared by annealing the stack at 350 °C exhibit a Cu-poor and Sb-rich composition with +1, +5 and −2 oxidation states of Cu, Sb and S, respectively. Morphological studies showed an improvement in the grain size of the films on increasing the annealing temperature. The direct optical band gap of these films was in the range of 0.82–0.85 eV. Hall measurements showed that the films are p-type in nature and their electrical resistivity, hole mobility and hole concentration are in the ranges of 0.14–1.20 Ω-cm, 0.05–2.11 cm² V⁻¹ s⁻¹ and 9.4×10²⁰–1.4×10¹⁹ cm⁻³, respectively. These structural, morphological, optical and electrical properties suggest that Cu₃SbS₄ could be used as an absorber layer for bottom cell in multi-junction solar cells.     

CIGS absorbing layers prepared by RF magnetron sputtering from a single quaternary target

Cu(In_1−xGa_x)Se₂ (CIGS) thin films were prepared by RF magnetron sputtering from a single quaternary target at multiple processing parameters. The structural, compositional, and electrical properties of the as-deposited films were systematically investigated by XRD, Raman, SEM, and Hall effects analysis. The results demonstrate that by adjusting the processing parameters, the CIGS thin films with a preferential orientation along the (112) direction which exhibited single chalcopyrite phase were obtained. The films deposited at relatively higher substrate temperature, sputtering power, and Ar pressure exhibited favorable stoichiometric ratio (Cu/(In+Ga):0.8–0.9 and Ga/(In+Ga):0.25–0.36) with grain size of about 1–1.5 µm, and desirable electrical properties with p-type carrier concentration of 10¹⁶−10¹⁷ cm⁻³ and carrier mobility of 10–60 cm²/Vs. The CIGS layers are expected to fabricate high efficiency thin film solar cells.     

Semiconducting amorphous carbon thin films for transparent conducting electrodes

Semiconducting amorphous carbon thin films were directly grown on SiO₂ substrate by using chemical vapor deposition. Raman spectra and transmission electron microscopy image showed that the a-C films have a short-range ordered amorphous structure. The electrical and optical properties of the a-C thin films were investigated. The films have sheet resistance of 3.7 kΩ/□ and high transmittance of 82%. They exhibit metal-oxide-semiconductor field effect transistor mobility of 10–12 cm² V⁻¹ s⁻¹ at room temperature, which is comparable to previous reported mobility of amorphous carbon. The optical band gap was calculated by Tauc’s relationship and photoluminescence spectra showed that the films are semiconductor with an optical band gap of 1.8 eV. These good physical properties make the a-C films a candidate for the application of transparent conducting electrodes.     

From micro to nanocrystalline transition in the diamond formation on porous pure titanium

A novel composite material of nanocrystalline (NCD) and/or microcrystalline (MCD) diamond films grown on porous titanium (Ti) substrate was obtained by hot filament chemical vapor deposition technique. Diamond films were grown using 1.5 vol.% CH₄ in a balanced mixture of Ar/H₂. The grain size control was obtained by varying the argon concentration from 0 up to 90 vol.% at substrate temperature of 870 K. Porous Ti substrates were obtained by powder metallurgy and presented an inter-connected open porosity. Scanning electron microscopy images of diamond/Ti exposed the substrate covered by a continuous textured coating which changed from MCD to NCD morphology; depending on the amount of Ar concentration in the feed gas. Micro-Raman spectra showed the characteristic t-polyacethylene peaks around 1150 cm^− 1 and 1470 cm^− 1, associated to NCD formation for samples grown with Ar concentration higher than 40 vol.%. X-ray diffraction patterns identified the diamond and TiC peaks, where the crystallinity of (111) TiC phase decreased as the Ar amount increased. This behavior was associated to diamond (220) peak increase for films grown with Ar concentration higher than 70 vol.%. Diamond crystallite size was also evaluated from Sherrer's formula in the range of 11 up to 20 nm.     

Effects of plasma treatments on the nitrogen incorporated nanocrystalline diamond films

The nitrogen incorporated nanocrystalline diamond (NCD) films were grown on n-silicon (100) substrates by microwave plasma enhanced chemical vapor deposition (MPECVD) using CH₄/Ar/N₂ gas chemistry. The effect of surface passivation on the properties of NCD films was investigated by hydrogen and nitrogen-plasma treatments. The crystallinity of the NCD films reduced due to the damage induced by the plasma treatments. From the crystallographic data, it was observed that the intensity of (111) peak of the diamond lattice reduced after the films were exposed to the nitrogen plasma. From Raman spectra, it was observed that the relative intensity of the features associated with the transpolyacetylene (TPA) states decreased after hydrogen-plasma treatment, while such change was not observed after nitrogen-plasma treatment. The hydrogen-plasma treatment has reduced the sp²/sp³ ratio due to preferential etching of the graphitic carbon, while this ratio remained same in both as-grown and nitrogen-plasma treated films. The electrical contacts of the as-grown films changed from ohmic to near Schottky after the plasma treatment. The electrical conductivity reduced from ~ 84 ohm^– 1 cm^− 1 (as-grown) to ~ 10 ohm^– 1 cm^− 1 after hydrogen-plasma treatment, while the change in the conductivity was insignificant after nitrogen-plasma treatment.     

Room-temperature UV-ozone assisted solution process for zirconium oxide films with high dielectric properties

A facile, low-cost, and room-temperature UV-ozone (UVO) assisted solution process was employed to prepare zirconium oxide (ZrO_x) films with high dielectric properties. ZrO_x films were deposited by a simple spin-coating of zirconium acetylacetonate (ZrAcAc) precursor in the environment-friendly solvent of ethanol. The smooth and amorphous ZrO_x films by UVO exhibit average visible transmittances over 90% and energy bandgap of 5.7 eV. Low leakage current of 6.0 × 10⁻⁸ A/cm² at 3 MV/cm and high dielectric constant of 13 (100 Hz) were achieved for ZrO_x dielectrics at the nearly room temperature. Moreover, a fully room-temperature solution-processed oxide thin films transistor (TFT) with UVO assisted ZrO_x dielectric films achieved acceptable performances, such as a low operating voltage of 3 V, high carrier mobility of 1.65 cm² V⁻¹ s⁻¹, and on/off current ratio about 10⁴–10⁵. Our work indicates that simple room-temperature UVO is highly potential for low-temperature, solution-processed and high-performance oxide films and devices.     

Thermal analysis of submicron nanocrystalline diamond films

The thermal properties of sub-μm nanocrystalline diamond films in the range of 0.37–1.1 μm grown by hot filament CVD, initiated by bias enhanced nucleation on a nm-thin Si-nucleation layer on various substrates, have been characterized by scanning thermal microscopy. After coalescence, the films have been outgrown with a columnar grain structure. The results indicate that even in the sub-μm range, the average thermal conductivity of these NCD films approaches 400 W m^− 1 K^− 1. By patterning the films into membranes and step-like mesas, the lateral component and the vertical component of the thermal conductivity, k_lateral and k_vertical, have been isolated showing an anisotropy between vertical conduction along the columns, with k_vertical ≈ 1000 W m^− 1 K^− 1, and a weaker lateral conduction across the columns, with k_lateral ≈ 300 W m^− 1 K^− 1.     

Formation of a heavily B doped diamond layer using an ion implantation technique

In this report, we present a study on lattice and electronic structures of B doped layers formed using B implantation into diamond. Boron layers were produced using the multiple-energy B ion implantation (total dose: 2.1 × 10¹⁵ to 1.7 × 10¹⁷ cm^− 2) into type IIa diamond at ~ 400 °C. Optical absorption and Hall effects were measured in the range of 80−1000 K for investigating the change of the lattice and electronic structures with the B concentration in diamond. The p-type carrier conduction was observed at 80−1000 K in all the samples. While a lightly B doped sample displays typical semiconductive, temperature-dependent valence-band conduction, heavily B doped samples have the very weak or almost zero temperature dependence of the carrier concentrations, resistivity and Hall mobility in this temperature region, suggesting characteristics of a p-type degenerate semiconductor. In such heavily doped samples, broad optical absorption bands, most likely corresponding to Drude absorption originating from free holes, were observed. The minimum resistivity and the sheet resistance at room temperature among the samples were 1.4 mΩcm and 56 Ω/□, respectively. These results indicate that very low-resistive p-type degenerate semiconducting layers were produced, preserving diamond lattice (preventing graphitization), despite high-dose ion irradiation.