Heavy phosphorus doping by epitaxial growth on the (111) diamond surface

ABSTRACTPhosphorus is incorporated into single crystal diamond during epitaxial growth at higher concentrations on the (111) crystallographic surface than on the (001) crystallographic surface. To form n+-type regions in diamond for semiconductor devices it is beneficial to deposit on the (111) surface. However, diamond deposition is faster and of higher quality on the (001) surface. A preferential etch method is described that forms inverted pyramids on the (001) surface of a substrate diamond crystal, which opens (111) faces for improved phosphorus incorporation. The preferential etching occurs on the surface in regions where a nickel film is deposited. The etching is performed in a microwave generated hydrogen plasma operating at 160 Torr with the substrate temperature in the range of 800-950 °C. The epitaxial growth of diamond with high phosphorus concentrations exceeding 1020 cm-3 is performed using a microwave plasma-assisted chemical vapor deposition process. Successful growth conditions were achieved with a feedgas mixture of 0.25% methane, 500 ppm phosphine and hydrogen at a pressure of 160 Torr and a substrate temperature of 950-1000°C. The room temperature resistivity of the phosphorus-doped diamond is 120-150 Ω-cm and the activation energy is 0.027 eV.

Boron doped diamond (BDD) has great potential in electrical, and electrochemical sensing applications. The growth parameters, substrates, and synthesis method play a vital role in the preparation of semiconducting BDD to metallic BDD. Doping of other elements along with boron (B) into diamond demonstrated improved efficacy of B doping and exceptional properties. In the present study, B and nitrogen (N) co-doped diamond has been synthesized on single crystalline diamond (SCD) IIa and SCD Ib substrates in a microwave plasma-assisted chemical vapor deposition process. The B/N co-doping into CVD diamond has been conducted at constant N flow of N/C ∼ 0.02 with three different B/C doping concentrations of B/C ∼ 2500 ppm, 5000 ppm, 7500 ppm. Atomic force microscopy topography depicted the flat and smooth surface with low surface roughness for low B doping, whereas surface features like hillock structures and un-epitaxial diamond crystals with high surface roughness were observed for high B doping concentrations. KPFM measurements revealed that the work function (4.74–4.94 eV) has not varied significantly for CVD diamond synthesized with different B/C concentrations. Raman spectroscopy measurements described the growth of high-quality diamond and photoluminescence studies revealed the formation of high-density nitrogen-vacancy centers in CVD diamond layers. X-ray photoelectron spectroscopy results confirmed the successful B doping and the increase in N doping with B doping concentration. The room temperature electrical resistance measurements of CVD diamond layers (B/C ∼ 7500 ppm) have shown the low resistance value ∼9.29 Ω for CVD diamond/SCD IIa, and the resistance value ∼16.55 Ω for CVD diamond/SCD Ib samples.

Plasma-assisted chemical vapor deposition process for depositing smooth diamond coatings on titanium alloys at moderate temperature

Introduction Since a p-type boron-doped diamond (BDD) has excellent electrochemical properties, such as wide potential window, low background current, and so on, it has been investigated for both of fundamentals and electrochemical applications1. On the other hand, few studies have reported on the electrochemical properties of n-type diamond. The electrons in the conduction band of the diamond have the quite high reducing ability because it is situated at much higher energy level than that of other semiconductors. Hence, the n-type diamond electrode may present new electrochemical properties. In this work, we study on the preparation and the characterization of polycrystalline phosphorus-doped diamond (PDD), which is the most promising candidate for n-type diamond, and investigate the electrochemical properties of PDD. Experimental The polycrystalline PDD films were deposited on Si wafer substrates by a microwave plasma-assisted chemical vapor deposition system in following steps. Firstly, undoped diamond layer was deposited in CH4/H2 plasma for 4 hours. Subsequently, red phosphorus sublimed at 400ºC was introduced into CH4/H2 plasma for 1 hour in order to deposit PDD layer. The surface morphology was characterized by scanning electron microscopy (SEM), and structure was investigated by Raman spectroscopy and X-ray diffraction (XRD). Phosphorus doping level was evaluated by secondary ion mass spectroscopy (SIMS). The hall effect measurement was performed using van der Pauw method. Electrical contacts were made by sputtering Ti and Au on the PDD surface, followed by annealing in vacuum at 450ºC for 45 min to form the Ti-C layer. Electrochemical properties were characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) in an aqueous solution of 0.1 M H2SO4. Mott-Schottky plots were obtained by EIS measurements. Moreover, these results were compared with electrochemical properties of BDD electrode. Results and discussion The SEM images showed a polycrystalline nature with grain size of ca. 2 μm. Raman spectrum showed a sharp peak from center zone optical phonon of diamond at 1332 cm−1 and a wide band from sp2-bonded carbon at around 1500 cm−1. Phosphorus concentration in the synthesized film was 1.36×1018 cm−3. From Hall effect measurement, it was confirmed that the PDD film was n-type semiconductor because hall coefficient was negative. From the CV measurement, the cathodic current was much larger than the anodic current, which is a characteristic of the interface between n-type semiconductor and solution (Fig. 1). Since p-type BDD showed the reverse behavior, this characteristic is ascribed to the influence of phosphorus doping. In order to estimate the position of band edges of PDD and BDD, the space charge capacitance C was measured by EIS measurement. The obtained C as a function of potential E was fitted with Mott-Schottky equation (1): 1/C 2 = (±2/eε0εSCN)(E−Efb −kT/e) (1) where the plus and minus signs represent the type of semiconductors, ε0 and εSC are the permittivities of free space and diamond, respectively, e is the electron charge, and N is the donor or acceptor density. The Mott-Schottky plots obtained by EIS measurement (Fig. 2) revealed that the synthesized PDD film is n-type semiconductor because a positive slope was obtained. Moreover, from the intercept of the potential axis, flat band potential (Efb ) of the PDD and BDD was estimated because kT/e is almost zero at room temperature (298 K). These obtained Efb values were used to determine the band edge positions. The difference between Efb and EVB of BDD was assumed to be ~0.4 eV from the acceptor level of boron. On the other hand, the difference between Efb and ECB of PDD was assumed to be ~0.6 eV from the donor level of phosphorus. From the result of comparing PDD and BDD, the band edges of PDD were found to agree well with the ones of BDD and also the single-crystalline PDD reported on the previous research2. References (1) Y. Einaga, J. Appl. Electrochem. 40. 1807 (2010). (2) Y. Mukuda, T. Watanabe, A. Ueda, Y. Nishibayashi, and Y. Einaga, Electrochim. Acta 179. 599 (2015). Figure 1

In this paper, the effect of boron doping on the electrical, morphological and structural properties of free-standing nanocrystalline diamond sheets (thickness ~ 1 μm) was investigated. For this purpose, we used diamond films delaminated from a mirror-polished tantalum substrate following a microwave plasma-assisted chemical vapor deposition process, each grown with a different [B]/[C] ratio (up to 20,000 ppm) in the gas phase. The developed boron-doped diamond (BDD) films are a promising semiconducting material for sensing and high-power electronic devices due to band gap engineering and thermal management feasibility. The increased boron concentration in the gas phase induces a decrease in the average grain size, consequently resulting in lower surface roughness. The BDD sheets grown with [B]/[C] of 20,000 ppm reveal the metallic conductivity while the lower doped samples show p-type semiconductor character. The charge transport at room temperature is dominated by the thermally activated nearest-neighbor hopping between boron acceptors through impurity band conduction. At low temperatures (<300 K), the Arrhenius plot shows a non-linear temperature dependence of the logarithmic conductance pointing towards a crossover towards variable range hopping. The activation energy at high temperatures obtained for lowly-doped sheets is smaller than for nanocrystalline diamond bonded to silicon, while for highly-doped material it is similar. Developed sheets were utilized to fabricate two types of diamond-on-graphene heterojunctions, where boron doping is the key factor for tuning the shape of the current-voltage characteristics. The graphene heterojunction with the low boron concentration diamond sheet resembles a Schottky junction behavior, while an almost Ohmic contact response is recorded with the highly doped BDD sheet of metallic conductivity. The free-standing diamond sheets allow for integration with temperature-sensitive interfaces (i.e. 2D materials or polymers) and pave the way towards flexible electronics devices.

Carrier compensation in (001) n-type diamond by phosphorus doping

Phosphorus (P) doped diamond thin films were grown by microwave plasma-assisted chemical vapor deposition (CVD) using phosphine (PH3) as a dopant source. The addition of PH3 in the gas ambient significantly influenced the optimal growth condition for diamond. The {111} surface of diamond was found to be the best surface as substrate for the growth of P-doped diamond. At a methane to hydrogen ratio of 0.15% and a substrate temperature of 950 °C, smooth {111} oriented homoepitaxial diamond thin films are obtained even with the addition of PH3 up to 1000 ppm to CH4. The epitaxial films grown with PH3/CH4 = 600 to 20000 ppm have shown clear n-type conduction as confirmed by Hall effect measurements over a wide temperature range. The activation energy of electrons was about 0.55 eV deduced from the temperature dependence of the carrier concentration. For the lightly doped sample, the Hall mobility of 28 cm2/Vs has been obtained in a temperature range of about 370 to 700 K.

Introduction Diamond, owing to its exceptional impact resistance, high thermal conductivity, broad-spectrum optical transparency, elevated breakdown field strength, and superior carrier mobility, finds widespread application in diverse fields such as high-power laser windows, efficient heat spreader, and high-performance semiconductor devices. Experimental 1. Heteroepitaxial Growth of Polycrystalline Diamond on Si, SiC, and Mo Substrates:In this part, substrates employed include double-side polished 4H-polytype SiC, (100) single-side polished single-crystal Si wafers, and Mo substrates. Ultrasonic grinding of substrates using diamond powder ethanol suspension to promote the dispersion of nucleation seeds, followed by sequential ultrasonic cleaning in acetone, ethanol, and deionized water.Subsequently, MPCVD was used for diamond growth, using hydrogen, methane, oxygen, and a mixture of hydrogen and nitrogen as reaction gases. Using different processes to produce efficient heat sinks and high-power laser windows. 2. Homoepitaxial Growth of Single-Crystal Diamond on Diamond Substrates:In the homoepitaxial growth process, (100) double-side polished single-crystal diamond substrates are utilized. No need for ultrasonic grinding steps, cleaning steps are the same as above. Then use MPCVD for homogeneous epitaxial growth of single-crystal diamond. 3. Investigation of Graphitized Surface Devices on Diamond:This study also investigated the graphitization devices on diamond surfaces. A high-temperature metal-catalyzed method is employed to promote surface graphitization of diamond. Firstly, polish and clean the diamond. Subsequently, a 300 nm thick nickel layer is deposited on the diamond surface. Rapid thermal annealing is then performed at 1300°C to form a highly conductive graphite layer. This process successfully prepares capacitor samples with a graphite-diamond-graphite three-layer structure. Results and Discussion 1. High-Power Laser Window Plates:The fabricated laser window plates utilize Si, SiC and Mo substrates, employing an ultra-low nitrogen growth process with a growth rate of approximately 3.7 µm/h. After double-sided polishing, the thickness reaches 1 mm. At room temperature, the transmittance at 10.6 µm wavelength approaches the theoretical maximum, reaching 67.9%. The overall thermal conductivity exceeds 1950 W/mK, approximating that of single-crystal diamond.Raman spectroscopy and XRD results indicate that the primary component is polycrystalline diamond with a (110) crystallographic orientation. Laser testing results demonstrate that these window plates possess a high laser-induced damage threshold with a peak energy of 60 J/cm2 and a peak power of 12 MW/mm2, capable of withstanding high-power CO2 laser output. 2. Heat Spreader:This study involves depositing diamond on Si and SiC substrates, forming diamond-based composite materials. This significantly improves the heat dissipation efficiency of the devices, bringing their performance closer to theoretical limits. The deposition process employs a low-nitrogen technique to balance growth rate and defect density, achieving a growth rate of approximately 5 µm/h.Raman spectroscopy and XRD results indicate that the primary component is polycrystalline diamond with a (110) crystallographic orientation. Although the increased nitrogen content results in lower optical transmittance compared to optical window plates, the grain size can be increased from 124 nm to 22 μm through production process adjustments, thereby enhancing thermal conductivity.Test results demonstrate that the thermal conductivity of the Si-diamond composite material reaches 450 W/mK,3 times that of single-crystal Si. The SiC-diamond composite material achieves a thermal conductivity of 500 W/mK,3.5 times that of SiC. 3. Single-Crystal Diamond:The transmittance and thermal conductivity of single-crystal diamond grown by homoepitaxial growth are superior to those of polycrystalline diamond. However, limitations in size and growth conditions restrict its large-scale application in heat spreaders and optical windows. Single-crystal diamond is more suitable for manufacturing semiconductor devices such as diamond capacitors, Schottky diodes, and hydrogen-terminated MOSFETs.The author has conducted research on doping of single-crystal diamond. Characterization through Raman spectroscopy, XRD, and TEM confirms that the primary component is single-crystal diamond with a (100) crystallographic orientation. The dislocation density is below 103/cm2, meeting the stringent requirements for semiconductor devices. 4. Graphite-Diamond-Graphite Capacitors:Utilizing prepared diamond, graphite-diamond-graphite capacitors were fabricated. Measurements using a semiconductor parameter analyzer revealed a high capacitance value of 4 nF. TCAD simulation indicated a breakdown voltage as high as 1100 V. This research explores a novel method for diamond capacitor fabrication. Conclusions Diamond heteroepitaxial grown on Si, SiC, and Mo substrates has prepared high-power laser windows with 67.9% optical transmittance at 10.6 μm wavelength, and composite materials with thermal conductivity enhanced to 3-3.5 times that of the original materials. Homoepitaxially grown diamond on single-crystal diamond substrates exhibits a dislocation density below 10³/cm², meeting the requirements for semiconductor device substrates. Furthermore, this research has developed a novel diamond capacitor fabrication technique, yielding capacitors with a capacitance of 4 nF and a breakdown voltage of 1100 V.

The n-type doping of diamond: Present status and pending questions

We demonstrate a new doping technique for chemical vapor deposition (CVD) growth of diamond. The method involves immersing a solid-state dopant source into the plasma during microwave plasma-assisted CVD. We applied this simple and versatile technique to the growth of boron-doped diamond. The grown films were characterized by X-ray diffraction (XRD), Raman microscopy, glow discharge optical emission spectroscopy (GDOES), and electrical conductivity measurements. The average concentration of boron was 0.5 atom % and the conductivity was 1.5 × 10−2 Ω cm, which showed irregular behavior at low temperature.

Brillouin light scattering has been used to investigate the elasticity properties of polycrystalline smooth fine-grained diamond films having various diamond qualities. They have been deposited on titanium alloy Ti-6Al-4V by a two-step microwave plasma-assisted chemical vapour deposition process at 600°C. Taking advantage from the detection of a number of different acoustic modes, a complete elasticity characterization of the films has been achieved.

Brillouin light scattering, Raman light scattering and x-ray diffraction were used to investigate the elastic and microstructural properties of polycrystalline and smooth fine-grained diamond films of varying diamond quality. They were deposited on a titanium alloy by a two-step microwave plasma-assisted chemical vapor deposition process at 600 °C. Their morphology and roughness were studied by scanning electron microscopy and atomic force microscopy. Their refractive indices were determined by the M-line spectroscopy technique. The diamond purity of all these coatings in terms of the sp3 bonding fraction was deduced from visible and UV Raman spectroscopy as a function of the deposition conditions. All the samples were found to be textured with a 〈011〉 crystallographic direction normal to the film plane, leading to essentially hexagonal symmetry of the elastic tensor. By taking advantage of the detection of a number of different acoustic modes, complete elastic characterization of the films was achieved. The elastic constants C11 and C66, respectively, were selectively determined from the frequency of the longitudinal and shear horizontal bulk modes traveling parallel to the film surface. The three remaining elastic constants, namely, C44, C33 and C13, were obtained from detection of the Rayleigh surface wave a bulk shear wave and the bulk longitudinal wave propagating at different angles from the normal to the surface. The values of the elastic constants depend on the deposition conditions and on the microstructural properties of the films, especially the diamond quality and the polycrystalline or smooth fine-grained nature of the diamond. For the polycrystalline diamond film with the best quality, the elastic constants are rather close to the Voigt or Reuss average estimate values using known bulk elastic constants of diamond, whereas those of the smooth fine-grained diamond films are reduced because of the poorer diamond quality leading to lower residual stress in the films.

Silicon-vacancy (SiV–) color center in diamond is of high interest for applications in nanophotonics and quantum information technologies, as a single photon emitter with excellent spectral properties. To obtain spectrally identical SiV– emitters, we doped homoepitaxial diamond films in situ with 28Si, 29Si, and 30Si isotopes using isotopically enriched (>99.9%) silane SiH4 gas added in H2–CH4 mixtures in the course of the microwave plasma-assisted chemical vapor deposition process. Zero-phonon line components as narrow as ∼4.8 GHz were measured in both absorption and luminescence spectra for the monoisotopic SiV– ensembles with a concentration of a few parts per billion. We determined with high accuracy the Si isotopic energy shift of SiV– zero-phonon line. The SiV– emission intensity is shown to be easily controlled by the doped epifilm thickness. Also, we identified and characterized the localized single photon SiV– sources. The developed doping process opens a way to produce the SiV– emitter ensembles...

In this work, the substrate holders of three principal geometries (flat, pocket, and pedestal) were designed based on E-field simulations. They were fabricated and then tested in microwave plasma-assisted chemical vapor deposition process with the purpose of the homogeneous growth of 100-μm-thick, low-stress polycrystalline diamond film over 2-inch Si substrates with a thickness of 0.35 mm. The effectiveness of each holder design was estimated by the criteria of the PCD film quality, its homogeneity, stress, and the curvature of the resulting “diamond-on-Si” plates. The structure and phase composition of the synthesized samples were studied with scanning electron microscopy and Raman spectroscopy, the curvature was measured using white light interferometry, and the thermal conductivity was measured using the laser flash technique. The proposed pedestal design of the substrate holder could reduce the stress of the thick PCD film down to 1.1–1.4 GPa, which resulted in an extremely low value of displacement for the resulting “diamond-on-Si” plate of Δh = 50 μm. The obtained results may be used for the improvement of already existing, and the design of the novel-type, MPCVD reactors aimed at the growth of large-area thick homogeneous PCD layers and plates for electronic applications.

Diamond microparticles were grown on etched tungsten wires using a microwave plasma-assisted chemical vapor deposition process. The apexes on cubo-octahedral particles bound by {100} and {111} facets were effectively used as tunneling tips for scanning tunneling microscopy. The atomically resolved surface image of highly oriented pyrolytic graphite was acquired. Tunneling characteristics revealed a higher electron emission from the diamond tips than that from the platinum–iridium tips. The same diamond tips were used to produce surface indentation and its image.