EPR study of hydrogen-related defects in boron-doped p-type CVD homoepitaxial diamond films

Boron-doped p-type single crystalline chemical vapor deposition (CVD) homoepitaxial diamond films were investigated by electron paramagnetic resonance (EPR). Carbon dangling bond defects, which were accompanied by a nearby hydrogen atom, were observed in boron-doped p-type CVD diamond films on a IIa substrate similar to those observed in undoped diamond. This result suggested that the energy level position of the defects is located below the Fermi energy of boron-doped diamond, at around 0.3 eV above the valence-band top. The reason why the Fermi energy could be changed by the incorporation of boron atoms at low density (10¹⁶–10¹⁷/cm³) in the film in spite of the existence of the large defect density of EPR centers (10¹⁸/cm³) is thought to be that the singly occupied electron states of defects are located near the band edge. As for the thermal annealing effect of the defects, it was revealed that the concentration of the defects and the mobility of the p-type film did not change after annealing up to 1200 °C which is much higher than the temperature of boron–hydrogen pair dissociation.     

The role of nitrogen in the deposition of polycrystalline diamond films

The role of nitrogen in the formation of polycrystalline diamond films prepared using a microwave plasma CVD system has been studied using micro-Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy (XPS). Although the nitrogen concentration in the films was too low to be detected by XPS, the Raman spectrum was found to be significantly affected by the nitrogen flow ratio. The intensity of the Raman peak at 1480 cm⁻¹ significantly decreases, whereas that of 1190 cm⁻¹ peak remains almost unchanged in comparison with the 1350 and 1550 cm⁻¹ peaks with increasing nitrogen flow ratio. In contrast, the preferentially (111)-orientated growth and the growth rate were little influenced by the nitrogen flow ratio. These results indicate that nitrogen plays a special role in the formation and structure of the polycrystalline diamond films studied in this report.     

Low temperature excitation spectrum of phosphorus in diamond

A set of n-type diamond thin films was investigated by infrared absorption spectroscopy from 4 K to 300 K. The films were grown by CVD on {111} synthetic diamond, and phosphorus-doped using from [P]/[C]=100 to 1000 ppm of phosphine in the gas phase during growth. The phosphorus concentration in the films ranges from approximately 5×10¹⁷ cm⁻³ to 5×10¹⁸ cm⁻³. At low temperature new absorption peaks were observed for the first time, and attributed to the 3P_+/− excited state of phosphorus and phonon-assisted electronic transitions. The energy of this new excited state is in accordance with the effective mass approximation, confirming the shallow level behaviour of phosphorus in diamond. Furthermore, this good agreement allowed us to propose accurate values of the electron effective masses (m_⊥ and m₆) and of the optical ionisation energy. A correlation between the electron mobility and the broadening of the phosphorus excited states was observed among the samples, giving new insights on the processes reducing the electron mobility.     

Growth of CVD heteroepitaxial diamond on silicon (001) and its electronic properties

Microwave CVD heteroepitaxial diamond film on a 4° off-axis Si(100) substrate is obtained by two stages. The first one is to grow oriented 3c-SiC layers on Si(100) using a non-toxic and non-inflammable (CH₃)₆Si₂NH organic compound carried by hydrogen. The following stage is to grow oriented diamond films on them under the atmosphere of CH₄ and H₂. In each stage there are bias and growth processions. The micro-Raman and micro-Auger analyses prove that there is a perfect orientation relationship between the film and substrate as following: diamond 〈001〉//3c-SiC〈001〉//Si〈001〉. The Hall effect indicates that the film is a P type, whose resistivity is 9.4×10⁻³ Ω cm, the Hall coefficient is 2.9 cm³/Q, the hole mobility is 309 cm²/V s and the carrier concentration reaches 2.2×10¹⁸ cm⁻³.     

Hydrogen diffusion and stability in polycrystalline CVD undoped diamond

Diffusion profiles and effusion experiments performed on post-hydrogenated (deuterated) CVD diamond layers (grain size 2 and 0.2 μm) are reported in order to study the configurations and stability of hydrogen bonding in polycrystalline undoped CVD diamond. Deuterium is used as a tracer to improve the hydrogen detection limit. The diamond layers are first annealed at 1200°C in order to out-diffuse hydrogen present in the as-grown sample. Then the samples are exposed either to a radiofrequency plasma or a microwave plasma and the deuterium diffusion profiles are analyzed by secondary ion mass spectrometry. For r.f. and microwave plasma, the diffusion profiles are explained in term of trapping on plasma-induced defects near the surface and/or on inter- and intra-granular defects. The mean free paths of deuterium and capture radius of traps are calculated by fitting the deuterium diffusion profiles and depend on the grain sizes. Some CVD diamond layers are deposited using a gas mixture (CH₄+D₂) and a deuterium concentration of 3×10¹⁹ cm⁻³, originating from the vector gas, is found in these as-grown samples. The stabilities of deuterium bonding in as-grown and post-deuterated samples are compared.     

n-type phosphorus-doped polycrystalline diamond on silicon substrates

The microwave plasma-assisted deposition of reproducible and homogeneously n-type phosphorus-doped polycrystalline (microcrystalline) diamond films on silicon substrates is described. The phosphorus incorporation is obtained by adding gaseous phosphine (PH₃) to the gas mixture during growth. The low CH₄/H₂ ratio (0.15%) and the use of the same growth parameters as for homoepitaxial {111} films, led to a good crystalline quality of the continuous polycrystalline diamond layers, confirmed by SEM images and Raman spectroscopy measurements.Secondary-ion mass spectrometry (SIMS) analysis measured a phosphorus concentration [P] of at least 7 × 10¹⁷ cm^− 3. Cathodoluminescence spectroscopy in our P-doped polycrystalline films shows a phosphorus bound exciton (BE^TO_P) peak between 5.142 and 5.181 eV. Cathodoluminescence and Raman-effect spectroscopy confirmed the improvement of the crystalline quality of our films as well as a decrease in the intensity of the internal strain when the grain size was decreased. Cathodoluminescence imaging and SIMS depth profile of phosphorus demonstrated a very good homogeneity of phosphorus incorporation in the films.     

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.     

High quality homoepitaxial CVD diamond for electronic devices

Recent achievements in homoepitaxial CVD diamond films for electronic devices have been discussed. We have successfully synthesized high-quality homoepitaxial diamond films with atomically flat surface by the microwave plasma chemical vapor deposition (CVD) using a low CH₄ concentration of CH₄/H₂ gas system less than 0.15% CH₄/H₂ ratio and Ib (001) substrates with low-misorientation angle less than 1.5°. These films are atomically flat over an area as large as 4×4 mm² and have shown a strong excitonic emission of 5.27 eV line, even at room temperature, with no essential emission lines in the visible light region in the cathodoluminescence (CL) spectra. Furthermore, high-quality Schottky junctions between Al and P type high-conductivity layers near the surface of these films have been obtained. Based on this growth method, we have also successfully synthesized B-doped diamond films using trimethylboron [B(CH₃)₃,TMB] gas as a B-doping source, whose Hall mobility is 1840 cm²/Vs at 290 K. Schottky junction fabricated by the B-doped diamond also shows excellent performances, indicating that the homoepitaxial diamond films presented here have a high potentiality for electronic devices.     

Nucleation and growth of diamond films on single crystal and polycrystalline tungsten substrates

Silicon has been the most widely studied substrate for the nucleation and growth of CVD diamond films. However, other substrates are of interest, and in this paper, we present the results of a study of the biased nucleation and growth of diamond films on bulk single and polycrystalline tungsten. Diamond films were nucleated and grown, using a range of bias and reactor conditions, and characterized by Raman spectroscopy and scanning electron microscopy (SEM). High-quality (100) textured films (Raman FWHM<4 cm⁻¹) could be grown on both single and polycrystalline forms of the tungsten substrate. On carefully prepared substrates, by varying the bias treatment, it was possible to determine the nucleation density over a 4–5 order range, up to ∼10⁹ cm⁻². Raman measurements indicated that the diamond films grown on bulk tungsten exhibited considerable thermal stress (∼1.1 GPa), which, together with a thin carbide layer, resulted in film delamination on cooling. The results of the study show that nucleation and growth conditions can be used to control the grain size, nucleation density, morphology and quality of CVD diamond films grown on tungsten.     

Influence of nucleation density on film quality,growth rate and morphology of thick CVD diamond films

The optimum growth parameters of our 5 kW microwave plasma CVD reactor were obtained using CH₄/H₂/O₂ plasma and high quality transparent films can be produced reproducibly. Among the films prepared in this system, the film of best quality has very smooth crystalline facets free of second nucleation and the full width at half maximum (FWHM) of the diamond Raman peak is 2.2 cm⁻¹, as narrow as that of IIa natural diamond. For this study, diamond films were grown on silicon substrates with low (10⁴–10⁵ cm⁻²) and high nucleation densities (>10¹⁰ cm⁻²), respectively. From the same growth run, a highly 〈110〉 textured 300 μm thick white diamond film with a growth rate of 2.4 μm/h was obtained from high nucleation densities (>10¹⁰ cm⁻²), and a white diamond film of 370 μm in thickness with a higher growth rate of 3 μm/h was obtained from low nucleation densities (5×10⁴–10⁵ cm⁻²) too. The effect of nucleation density on film quality, growth rate, texture and morphology was studied and the mechanism was discussed. Our results suggest that under suitable growth conditions, nucleation density has little effect on film quality and low nucleation density results in higher growth rate than high nucleation density due to less intense grain growth competition.     

Distribution of relaxation time in solution-processed polycrystalline CZTS thin films: Study of impedance spectroscopy

Here we report the complex impedance spectroscopic analysis of polycrystalline CZTS thin films synthesized by sol-gel spin coating technique without any post deposition sulphurization. The films are characterized by microstructural, compositional, optical and electrical studies to confirm the formation of kesterite phase of CZTS comprises of well distributed compact grains with the optical band gap 1.44?eV. Room temperature electrical characterizations of the CZTS thin films by four-probe and Hall effect technique revealed the p-type conductivity of the films with resistivity ~ 1.45?×?10^?2 Ω?cm, mobility ~ 3.7?×?10³ cm² V^?1 s^?1 and carrier concentration ~ 1.82?×?10¹⁷ cm^?3. The distribution of relaxation time (DRT) function with improved frequency resolution is reconstructed from the impedance spectra of CZTS film recorded in the frequency range 50?Hz to 5?MHz at room temperature to identify the number of electrical processes in the polycrystalline film. The Nyquist plot is fitted into electrical model consist of three parallel combinations of resistor (R) and capacitor (C) in series as three major peaks in DRT function indicates the presence of different relaxation processes with major contributions from core grains along with smaller contributions from grain boundary and interfaces. The room temperature frequency dependence of dielectric constant, loss tangent and ac conductivity is also studied for the CZTS films.     

Contribution of enhanced ionization to the optoelectronic properties of p-type NiO films deposited by high power impulse magnetron sputtering

Herein, intrinsic p-type conductivity of NiO films were enhanced by high power impulse magnetron sputtering (HiPIMS) technology, where more charged Ni³⁺ ions are created during the deposition process. The formation of Ni³⁺ ions are advantageous for strengthening the p-type conductivity of NiO films. As the pulse off-time increases from 0 μs to 3000 μs, Ni³⁺ concentration improves greatly, indicating the amount of Ni vacancies as well as the hole concentration significantly enhances. It confirms that HiPIMS is a preferential technology for preparing NiO films with high p-type conductivity. Especially, when pulse off-time reaches 3000 μs, a high carrier concentration of 2.86 × 10²¹ cm⁻³ and a relatively low electrical resistivity about 0.07 Ω·cm are achieved.     

Magnetoresistance of boron-doped chemical vapor deposition polycrystalline diamond films

The electrical properties and magnetoresistance of boron-doped polycrystalline diamond films grown on p-typed Si (100) by hot filament chemical vapor deposition have been investigated. As the atomic boron concentration increases from 3×10¹⁷ to 3×10¹⁹ cm⁻³, and grain size from 5 to 15 μm, the quality of diamond is improved, which causes the carrier mobility μ and longitudinal magnetoresistance change rate Δρ_///ρ₀ to increase. For a magnetic field (B) of 20 tesla and temperature 300 K, the longitudinal resistance change rate Δρ_///ρ₀ is up to 20%. Meanwhile, Δρ_///ρ₀ is proportional to μ²B² in a low field and proportional to μ^1.5B in a high field. It is the first time that a result is obtained in a high field.     

Light penetration depth dependence of photocarrier life time and the Hall effect in phosphorous-doped and boron-doped homoepitaxial CVD diamond films

Photocurrent in phosphorous-doped CVD diamond film of the bandgap of 5.5 eV with the density of 2 × 10¹⁸ cm^− 3 decreases with increasing photon energy in the energy range higher than 5.8 eV at room temperature (RT). The photocarrier life time is 0.3 ms at the excitation energy of 5.8 eV and decreases with increasing excitation energy. These show that the photocarriers, ascertained to be electrons by the Hall effect of the photocurrent, are trapped near the surface. The life time of photo-excited holes in Boron-doped CVD diamond film with the density of 9 × 10¹⁷ cm^− 3 is 35 ms at RT and decreases with decreasing Boron density, which is explained from the relation between the Fermi energy and the density.     

Surface morphology and electrical properties of boron-doped diamond films synthesized by microwave-assisted chemical vapor deposition using trimethylboron on diamond (100) substrate

Prime novelty: The smoothness of the synthesized boron-doped diamond was improved by the pre-treatment of a hydrogen plasma. Moreover, the Hall mobility also increased with this pre-treatment.Surface morphology and electrical properties, such as electrical conductivity, hole concentration and Hall mobility, were investigated for boron-doped diamond films, which were synthesized by microwave-assisted chemical vapor deposition (MPCVD) on a (100) diamond substrate. Trimethylboron (TMB) was used as a dopant source and methane (CH₄) was used as a carbon source. The morphology of the synthesized diamond surface depended on the MPCVD conditions such as TMB and CH₄ concentrations in the gas phase, and lower concentrations of TMB and CH₄ lead to a smoother surface. When the substrate was treated in a hydrogen plasma, the electrical properties of the boron-doped diamond films, as well as the smoothness of the surface, were improved. After optimizing the synthesis conditions, Hall mobility reached to 2020 cm² V⁻¹ s⁻¹ at 243 K for a diamond film with a hole concentration of 5×10¹² cm⁻³.     

Incorporation of lithium and nitrogen into CVD diamond thin films

High concentrations of lithium (~ 5 × 10¹⁹ cm^− 3) and nitrogen (~ 3 × 10²⁰ cm^− 3) have been simultaneously incorporated into single-crystal and microcrystalline diamond films using Li₃N and gaseous ammonia as the sources of Li and N, respectively. Using sequential deposition methods, well-defined localised layers of Li:N-doped diamond with a depth spread of less than ± 200 nm have been created within the diamond. The variation in Li:N content and amount of diffusion within the various types of diamond suggests a model whereby these atoms can migrate readily through the grain-boundary network, but do not migrate much within the grains themselves where the diffusion rate is much slower. However, the high electrical resistivity of the doped films, despite the high Li and N concentrations, suggests that much of the Li and N are trapped as electrically inactive species.     

Low resistivity p+ diamond (100) films fabricated by hot-filament chemical vapor deposition

By hot-filament (HF) chemical vapor deposition (CVD), heavily boron (B)-doped single-crystal diamond (100) films were fabricated and their structural and electrical properties were studied. We did not observe the soot formation, which is frequently observed and limits the performances in the case of microwave plasma (MWP) CVD. The B concentration was successfully controlled over the range from 10¹⁹ to 10²¹ cm^− 3. Hillock-free films were obtained, whose mean surface roughness measured by atomic force microscopy (AFM) was less than 0.1 nm. From the reciprocal space mapping (RSM) around 113 diamond reflection, it was revealed that the films possess the smaller lattice expansion than that expected from the Vegard's law. The room-temperature resistivity was decreased lower than 1 mΩ·cm for B concentration ~ 10²¹ cm^− 3. These results indicate that the HFCVD possesses large potential for fabricating the device-grade p⁺ diamond.     

Nucleation and growth of diamond films on aluminum nitride by hot filament chemical vapor deposition

Nucleation and growth of diamond films on aluminum nitride (ALN) coatings were investigated by scanning electron microscopy, Raman spectroscopy and scratch test. ALN films were grown in a magnetron sputtering deposition. The substrates were Si(111) and tungsten carbide (WC). Chemical vapor deposition (CVD) diamond films were deposited on ALN films by hot filament CVD. The nucleation density of diamond on ALN films was found to be approximately 10⁵ cm⁻², whereas over 10¹⁰ cm⁻² after negative bias pre-treatment for 35 min was −320 V, and 250 mA. The experimental studies have shown that the stresses were greatly minimized between diamond overlay and ALN films as compared with WC substrate. The results obtained have also confirmed that the ALN, as buffer layers, can notably enhance the adhesion force of diamond films on the WC.     

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.     

Neutron transmutation of 10B doped diamond

Free standing ¹⁰B isotope doped diamond films deposited by chemical vapor deposition in a microwave chamber were irradiated to thermal neutron fluence values of 0.32 × 10¹⁹, 0.65 × 10¹⁹, 1.3 × 10¹⁹, and 2.6 × 10¹⁹ n/cm². Cooling of the diamond films was maintained during irradiation. In a separate experiment, neutron irradiation to a total fluence of 2.4 × 10²⁰ n/cm² with equal fast and thermal neutrons was also performed on a diamond epilayer without cooling during irradiation. The formation of defects in the diamond films was characterized using Raman, FTIR, photoluminescence, electron paramagnetic resonance spectroscopy, and X-ray diffraction. It was found that defect configurations in diamond responsible for an increase in continuum background in the one-phonon region of Raman spectrum were absent in the films that have been cooled. The FTIR peak at 1530 cm^− 1 annealed in the sample irradiated to a fluence of 2.6 × 10¹⁹ n/cm² indicating that the sample reached a temperature of 300 °C during irradiation. Absence of characteristic infrared absorption peaks that were observed only upon annealing neutron irradiated diamond is used to conclude that the temperature of the sample during neutron irradiation to a fluence of 2.6 × 10¹⁹ n/cm² was well below 650 °C needed for mobility of defects and accumulation of stable unrecoverable damage. On the other hand, results from diamond epilayer subjected to equal thermal and fast neutron fluence of 2.4 × 10²⁰ n/cm² and without cooling showed that defects formed from displaced carbon atoms became mobile and formed complex configurations of irrecoverable damage. Electrical conductance of the unirradiated and irradiated diamond samples was measured as a function of temperature to determine the compensation of the p-type by the n-type charge carriers.