The effect of nitrogen addition during high-rate homoepitaxial growth of diamond by microwave plasma CVD

The effect of nitrogen addition on growth rate, morphology and crystallinity during high-rate microwave plasma chemical vapor deposition (MPCVD) of diamond was investigated. Epitaxial diamond was grown on type Ib diamond (100) substrates using a 5-kW, 2.45-GHz microwave plasma CVD system with nitrogen addition in the methane and hydrogen source gases. In order to obtain high growth rates, we designed the substrate holders to generate high-density plasma. The growth rates ranged from 30 to 120 μm/h. The nitrogen addition enhanced the growth rate by a factor of 2 and was beneficial to create a macroscopic smooth (100) face avoiding the growth of hillocks. However, the (100) surfaces looked microscopically rough by bunched steps as the effect of nitrogen addition. The macroscopic smoothing during the growth enabled the long-term stable deposition required to obtain large crystals. The deposited diamond was characterized by optical microscope, Raman spectroscopy, cathodoluminescence spectroscopy and X-ray diffraction.     

Diamond growth by carbon ion implantation of diamond

Medium energy (5–25 keV) ¹³C⁺ ion implantation into diamond (100) to a fluence ranging from 10¹⁶ cm⁻² to 10¹⁸ cm⁻² was performed for the study of diamond growth via the approach of ion beam implantation. The samples were characterized with Rutherford backscattering/channelling spectroscopy, Raman spectroscopy, X-ray photoemission spectroscopy and Auger electron spectroscopy. Extended defects are formed in the cascade collision volume during bombardment at high temperatures. Carbon incorporation indeed induces a volume growth but the diamond (100) samples receiving a fluence of 4 × 10¹⁷ to 2 × 10¹⁸ at. cm⁻² (with a dose rate of 5 × 10¹⁵ at. cm⁻² s⁻¹ at 5 to 25 keV and 800 °C) showed no He-ion channelling. Common to these samples is that the top surface layer of a few nanometers has a substantial amount of graphite which can be removed by chemical etching. The rest of the grown layer is polycrystalline diamond with a very high density of extended defects.     

Improvements of crystallinity of single crystal diamond plates produced by lift-off process using ion implantation

A single crystal diamond substrate cut from a 9 mm thick ingot which was grown by chemical vapor deposition (CVD) was used to produce freestanding single crystal CVD diamond plates with improved crystallinity by the lift-off process using ion implantation. To reduce dislocations on the substrate surface, the ingot was sliced along the {100} plane parallel to the growth direction. In addition, the repeated lift-off processes reduced the surface damage on the substrate. These treatments were shown to improve the crystallinity of the CVD diamond plates produced by polarized light microscopy (PLM) and high-resolution X-ray.     

The effect of CO2 on the high-rate homoepitaxial growth of CVD single crystal diamonds

In this paper, we report the effect of gaseous carbon dioxide (CO₂) introduced in the typical reaction atmosphere of CH₄/H₂/N₂ (60/500/1.8 in sccm) on the growth rate, morphology and optical properties of homoepitaxy single crystal diamonds (SCDs) by microwave plasma chemical vapor deposition. The additional carbonaceous sources supplied by CO₂ are favorable to increase the growth rate, and meanwhile, the oxygen related species generated would enhance the etching effect not only to eliminate the non-diamond phase of SCD but also to decrease the growth rate. The appropriate addition of CO₂ can increase the high growth rate, decrease the surface roughness, and reduce the concentration of N-incorporation. It is demonstrated that adding CO₂ strongly affects the contents of various reaction species in plasma, which would determine the growth features of CVD SCDs.     

A consideration of ternary C-H-O diagram for diamond deposition using microwave in-liquid and gas phase plasma

The purpose of this study is to clarify the mechanism of diamond synthesis using an in-liquid plasma chemical vapor deposition (CVD) method. We investigated the chemical reactions from a liquid mixture of methanol and ethanol (in-liquid plasma CVD) and a gas mixture of methane and hydrogen (gas-phase CVD). Carbon monoxide (CO) is firstly synthesized and then a chemical reaction using the remaining carbon (C) and hydrogen (H) is induced to synthesize a carbon substance. Residual H radicals act as an etchant removing the incompletely binding carbon atom that hinders diamond crystal growth. From spectroscopic measurements, CO peaks were clearly observed when the oxygen component is contained in the raw materials. From the experimental results of carbon deposits using various liquid and gas mixtures as the raw materials, we found that the region of the remaining H and C after CO synthesis satisfying H/C > 20 is necessary to synthesize diamonds using in-liquid plasma CVD method. The region of H/C > 20 in the Bachmann C-H-O diagram nearly agrees with the experimental results of synthesizing diamonds.     

The influences of oxygen ion implantation on the structural and electrical properties of B-doped diamond films

The oxygen ion with a dose of 10¹⁴ (called CVDBO14) and 10¹⁵ cm^− 2 (called CVDBO15) was implanted into boron doped diamond films synthesized in chemical vapor deposition. The structural and electrical properties of different samples were characterized by XPS, Raman spectroscopy and 4-probe resistivity measurements. The results show that oxygen ion exists both in the diamond surface and the subsurface of the films. The FWHM values of CVDBO15 samples are higher than those of CVDBO14 samples, indicating that more damages existed in CVDBO15 samples. The resistivity of CVDBO15 sample series is smaller than those of CVDBO14 sample series, and the film with a larger FWHM value exhibits low resistivity. In the 1150 °C annealed sample, the activation energy decreases from 0.50 eV to 0.39 eV with the oxygen ion dose increasing from 10¹⁴ to 10¹⁵ cm^− 2. It is indicated that oxygen ion and the defects produced by ion implantation give contributions to the conductivity in diamond films. Some surface hydrogen is removed and pi-bonded carbon as well as C-H vibration is formed after annealing, which is also relative to the lower resistivity in the samples.     

Sharpening of CVD diamond coated tools by 0.5−10 keV Ar ion beam

CVD diamond coated tungsten carbide tools have been used for cutting and drilling of soft materials such as aluminum and copper alloys. However, it is very difficult to obtain a tool having a sharp tip of the order of sub-μm by mechanical abrasive polishing methods. Therefore, we applied ion beam processing for sharpening the cutting edge of diamond coated tungsten carbide tools. Result shows that it is possible to obtain a 20-80 nm order tip width of a CVD diamond coated knife processed by a 0.5-10 keV Ar⁺ ion beam, and the sharpening speed of a tip of the knife depends on the ion beam energy. Namely, a tip width of a knife processes by a 1.0 keV Ar⁺ ion beam at an ion dose of 2.7 × 10²⁰ ions/cm² becomes 20 nm, and a tip width of a knife processed by a 10 keV Ar⁺ ion beam at an ion dose of 5.4 × 10¹⁹ ions/cm² becomes 40 nm. However, a facet with an apex angle in the range of 60-100° was formed on the cutting edge of a knife with an initial apex angle of 55°, and we found that the facet angle can be controlled by choosing ion beam energy of 0.5-10 keV. Moreover, results show that the processed knife machined by a 0.5 keV Ar⁺ ion beam has very smooth rake and flank faces, and also has a small line edge roughness of the cutting edge compared to those of the sharpened knife by a 1.0-10 keV Ar⁺ ion beam.     

Synthesis of large single crystal diamond plates by high rate homoepitaxial growth using microwave plasma CVD and lift-off process

A process of making a large, thick single crystal CVD diamond plates has been developed. This process consists of high rate homoepitaxial growth of CVD diamond and subsequent lift-off process using ion implantation. By using this process, single crystal CVD diamond plates with the size of about 10 × 10 × 0.2–0.45 mm³ have been successfully fabricated. The crystallinity of the CVD diamond plates has been evaluated by X-ray topography, polarized light microscopy and high resolution X-ray diffraction. The results indicate the pretreatment of the seed substrate has strong effect on the crystallinity of the CVD diamond plates.     

Hall data analysis of heavily boron-doped CVD diamond films using a model considering an impurity band well separated from valence bands

We have performed a detailed analysis of Hall data taken in a temperature (T) region of 100-700 K for heavily boron-doped diamond thin films homoepitaxially grown using high power-density microwave plasma CVD method. Their substrates were composed of a high-quality undoped homoepitaxial CVD layer grown similarly on vicinal (001) high-pressure/high-temperature-synthesized Ib diamond. The atomic ratio of boron to carbon in the source gas was 8000 ppm. Hall data taken from each specimen at various T's demonstrated (1) a metallic behavior in the low T region well below room temperature and (2) a clear peak of the T dependence of positive Hall coefficients around 350 K. The latter suggests the presence of multi groups of holes having substantially different mobilities and energies. In fact, calculated curves based upon a multi-type-carrier transportation model can well reproduce the corresponding experimental data. The T dependences were analyzed through a fitting procedure for the various important parameters employed such as the activation energies from the valence bands to the energy-fluctuated acceptor levels and carrier mobilities averaged over the concerned bands. The present analysis strongly suggests as follows. (1) Almost T-independent density of holes with the lowest average mobility in each heavily boron-doped diamond film move in an impurity band, (2) light holes and heavy holes whose densities increase exponentially with increasing T run in the valence bands in the vicinities of their maxima at substantially higher mobilities than that in the impurity band, and (3) the impurity band is well separated in energy from the valence bands.     

A new regime for high rate growth of nanocrystalline diamond films using high power and CH4/H2/N2/O2 plasma

In this work, we report high growth rate of nanocrystalline diamond (NCD) films on silicon wafers of 2 inches in diameter using a new growth regime, which employs high power and CH₄/H₂/N₂/O₂ plasma using a 5 kW MPCVD system. This is distinct from the commonly used hydrogen-poor Ar/CH₄ chemistries for NCD growth. Upon rising microwave power from 2000 W to 3200 W, the growth rate of the NCD films increases from 0.3 to 3.4 μm/h, namely one order of magnitude enhancement on the growth rate was achieved at high microwave power. The morphology, grain size, microstructure, orientation or texture, and crystalline quality of the NCD samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction, and micro-Raman spectroscopy. The combined effect of nitrogen addition, microwave power, and temperature on NCD growth is discussed from the point view of gas phase chemistry and surface reactions.