Homoepitaxial single crystal diamond growth at different gas pressures and MPACVD reactor configurations

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in a 2.45 GHz reactor was investigated at high microwave power density varied from 80 W/cm³ to 200 W/cm³. Two methods of achieving high microwave power densities were used (1) working at relatively high gas pressures without local increase of electric field and (2) using local increase of electric field by changing the reactor geometry (substrate holder configuration) at moderate gas pressures. The CVD diamond layers with thickness of 100–300µm were deposited in H₂–CH₄ gas mixture varying methane concentration, gas pressure and substrate temperature. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. The high microwave power density conditions allowed the achievement of the growth rate of high quality single crystal diamond up to 20 µm/h. Differences in single crystal diamond growth at the same microwave power density 200 W/cm³ for two process conditions—gas pressure 210 Torr (flat holder) and 145 Torr (trapezoid holder)—were studied. For understanding of growth process measurements of the gas temperature and the concentration of atomic hydrogen in plasma were made.     

Improved microwave plasma cavity reactor for diamond synthesis at high-pressure and high power density

Microwave plasma assisted synthesis of diamond is experimentally investigated using high purity, 2–5% CH₄/H₂ input gas chemistries and operating at high pressures of 180–240 Torr. A microwave cavity plasma reactor (MCPR) was specifically modified to be experimentally adjustable and to enable operation with high input microwave plasma absorbed power densities within the high-pressure regime. The modified reactor produced intense microwave discharges with variable absorbed power densities of 150–475 W/cm³ and allowed the control of the discharge position, size, and shape thereby enabling process optimization. Uniform polycrystalline diamond films were synthesized on 2.54 cm diameter silicon substrates at substrate temperatures of 950–1150 °C. Thick, freestanding diamond films were synthesized and optical measurements indicated that high, optical-quality diamond films were produced. The deposition rates varied between 3 and 21 μm/h and increased as the operating pressure and the methane concentrations increased and were two to three times higher than deposition rates achieved with the MCPR operating with equivalent input methane concentrations and at lower pressures (≤ 140 Torr) and power densities.     

Investigation of diamond deposition uniformity and quality for freestanding film and substrate applications

In this paper, we report on microwave CVD deposition of high quality polycrystalline diamond and on related post-processing steps to produce smooth, flat and uniformly thick films or diamond substrates. The deposition reactor is a 2.45 GHz microwave cavity applicator with the plasma confined inside a 12 cm diameter fused silica bell jar. The deposition substrates utilized are up to 75 mm diameter silicon wafers. The substrate holder is actively cooled with a water-cooled substrate holder to achieve a substrate surface temperature of 600–1150 C. The pressure utilized is 60–180 Torr and the microwave incident power is 2–4.5 kW. Important parameters for the deposition of thick films with uniform quality and thickness include substrate temperature uniformity as well as plasma discharge size and shape. As deposited thickness uniformities of ± 5% across 75 mm diameters are achieved with simultaneous growth rates of 1.9 μm/h. The addition of argon to the deposition gases improves film deposition uniformity without decreasing growth rate or film quality, over the range of parameters investigated. Post-processing includes laser cutting of the diamond to a desired shape, etching, lapping and polishing steps.     

Recent advances in high-growth rate single-crystal CVD diamond

There have been important advances in microwave plasma chemical vapor deposition (MPCVD) of large single-crystal CVD diamond at high growth rates and applications of this diamond. The types of gas chemistry and growth conditions, including microwave power, pressure, and substrate surface temperatures, have been varied to optimize diamond quality and growth rates. The diamond has been characterized by a variety of spectroscopic and diffraction techniques. We have grown single-crystal CVD diamond over ten carats and above 1 cm in thickness at growth rates of 50–100 μm/h. Colorless and near colorless single crystals up to two carats have been produced by further optimizing the process. The nominal Vickers fracture toughness of this high-growth rate diamond can be tuned to exceed 20 MPa m^1/2 in comparison to 5–10 MPa m^1/2 for conventional natural and CVD diamond. Post-growth high-pressure/high-temperature (HPHT) and low-pressure/high-temperature (LPHT) annealing have been carried out to alter the optical, mechanical, and electronic properties. Most recently, single-crystal CVD diamond has been successfully annealed by LPHT methods without graphitization up to 2200 °C and < 300 Torr for periods of time ranging from a fraction of minute to a few hours. Significant changes observed in UV, visible, infrared, and photoluminescence spectra are attributed to changes in various vacancy centers and extended defects.     

Design of a new TM021 mode cavity type MPCVD reactor for diamond film deposition

A new TM₀₂₁ mode cavity type microwave plasma chemical vapor deposition (MPCVD) reactor for diamond film deposition was derived by analyzing the TM₀₂₁ resonant pattern of microwave electric field in an idealized TM₀₂₁ mode reactor. Important characteristics of the reactor, including microwave electric field, electron density, gas temperature as well as absorbed microwave power density were obtained by using a phenomenological model of hydrogen plasma. On the basis of the simulation, a new TM₀₂₁ mode cavity type MPCVD reactor was built and 2-inch diameter freestanding diamond films were synthesized at 6 kW input microwave power. Raman and optical transmission spectroscopy analyses indicate that the diamond films prepared by using the new TM₀₂₁ mode cavity type reactor are of high quality.     

Experimentally defining the safe and efficient,high pressure microwave plasma assisted CVD operating regime for single crystal diamond synthesis

The detailed experimental behavior of a microwave plasma assisted chemical vapor deposition (MPACVD) reactor operating within the high, 180–300 torr, pressure regime is presented. An experimental methodology is described that first defines the reactor operating field map and then enables, while operating at these high pressures, the determination of the efficient, safe and discharge stable diamond synthesis process window. Within this operating window discharge absorbed power densities of 300–1000 W/cm³ are achieved and high quality, single crystal diamond (SCD) synthesis rates of 20–75 μm/h are demonstrated. The influence of several input experimental variables including pressure, N₂ concentration, CH₄ percentage and substrate temperature on SCD deposition is explored. At a constant pressure of 240 torr, a high quality, high growth rate SCD synthesis window versus substrate temperature is experimentally identified between 1030 and 1250 °C. When the input nitrogen impurity level is reduced below 10 ppm in the gas phase the quality of the synthesized diamond is of type IIa or better.     

A novel microwave plasma reactor with a unique structure for chemical vapor deposition of diamond films

With the aid of numerical simulation, a novel microwave plasma reactor for diamond films deposition has been designed. The new reactor possesses a unique structure, neither purely cylindrical nor purely ellipsoidal, but a combination of the both. In this paper, the design strategy of the new reactor together with a simple but reliable phenomenological simulation method will be described. Preliminary experiments show that uniform diamond films of high quality could be deposited using the new reactor, and the deposition rate of diamond films is typically about 3 μm/h at 6 kW input power level on a 2 inch diameter silicon substrate.     

Rapid deposition of polycrystalline diamond film by DC arc plasma jet technique and its RF MESFETs

Diamond is a promising semiconductor material for high power, high frequency and high temperature electronic devices. High-purity polycrystalline diamond with large grain size has showed prominent RF properties. In this work, polycrystalline free-standing diamond film with grain size of 150 μm was grown by DC arc plasma jet technique with a growth speed as high as 20 μm/h. The prepared diamond sample showed high-purity with a (220) preferred orientation by the XRD and Raman spectra measurements. By a self-aligned process, hydrogen terminated p-type diamond MESFETs with gate length of 100 nm were fabricated on the 15 mm × 15 mm diamond film and showed good DC and RF performances with drain saturation current 225.7 mA/mm and maximum oscillation frequency (f_max) 46.8 GHz.     

Hybrid diamond/graphite films: Morphological evolution,microstructure and tribological properties

Diamond films were deposited on silicon substrate by microwave plasma enhanced chemical vapor deposition (MPCVD) using H₂ and CH₄ gas mixture. The morphological evolution process was characterized systematically by means of field-emission scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. Special attention has been paid to the influence of the methane concentrations on the microstructures of diamond films, which shows a gradual transition from nanocrystalline to microcrystalline films, and finally displays a hybrid diamond-graphite nanostructure with the length of a few micrometers. Finally, the friction behaviour of the hybrid films was studied. The value of the friction coefficient of the hybrid films is 0.10 and the corresponding wear resistance is below 1.9 × 10^− 7 mm³/N·m in diamond composites/Al₂O₃ sliding system in ambient atmosphere under dry sliding conditions. It is a convenient path to synthesize hybrid diamond/graphite nanostructures by MPCVD depending on higher methane concentrations in the absence of nitrogen or argon. The structure is appropriate for the potential applications as high efficient mechanical tools.     

Substrate temperature effects on the electron field emission properties of nitrogen doped ultra-nanocrystalline diamond

For the purpose of improving the electron field emission properties of ultra-nanocrystalline diamond (UNCD) films, nitrogen species were doped into UNCD films by microwave plasma chemical vapor deposition (MPCVD) process at high substrate temperature ranging from 600° to 830 °C, using 10% N₂ in Ar/CH₄ plasma. Secondary ion mass spectrometer (SIMS) analysis indicates that the specimens contain almost the same amount of nitrogen, regardless of the substrate temperature. But the electrical conductivity increased nearly 2 orders of magnitude, from 1 to 90 cm^− 1 Ω^− 1, when the substrate temperature increased from 600° to 830 °C. The electron field emission properties of the films were also pronouncedly improved, that is, the turn-on field decreased from 20 V/μm to 10 V/μm and the electron field emission current density increased from less than 0.05 mA/cm² to 15 mA/cm². The possible mechanism is presumed to be that the nitrogen incorporated in UNCD films are residing at grain boundary regions, converting sp³-bonded carbons into sp²-bonded ones. The nitrogen ions inject electrons into the grain boundary carbons, increasing the electrical conductivity of the grain boundary regions, which improves the efficiency for electron transport from the substrate to the emission sites, the diamond grains.     

Nano- and micro-crystalline diamond growth by MPCVD in extremely poor hydrogen uniform plasmas

It is well established that argon rich plasmas (> 90% Ar) in Ar/CH₄/H₂ gas mixtures lead to (ultra)nanodiamond nucleation and growth by microwave plasma chemical vapour deposition (MPCVD). Nonetheless, in the present work, both microcrystalline and nanocrystalline diamond deposits developed under typical conditions for ultrananocrystalline (UNCD) growth by MPCVD. Silicon substrates were pretreated by abrasion using two different diamond powder types, one micrometric (< 0.5 μm) and the other nanometric (∼ 4 nm), the latter obtained by detonation methods. Samples characterization was performed by SEM (morphology), AFM (roughness and morphology) and micro-Raman (structure).For all samples, Raman analysis revealed good crystalline diamond quality with an evident ∼ 1332 cm^− 1 peak. The Raman feature observed at ∼ 1210 cm^− 1 is reported to correlate with two other common bands at ∼ 1140 cm^− 1 and ∼ 1490 cm^− 1 characteristic of nano- and ultra-nanocrystalline diamond.A new growth process is proposed to explain the observed morphology evolution from nano- to microcrystalline diamond. Based on this, the microcrystalline morphology is in fact a crystallographically aligned construction of nanoparticles.     

Electron density in moderate pressure diamond deposition discharges

The electron density is measured in a microwave plasma-assisted chemical vapor deposition diamond reactor at moderate pressures (5–60 Torr) using a millimeter-wave open resonator technique. The discharge plasma is generated using a resonant cavity excited at the frequency 2.45 GHz. The absorbed input power of 400 W generates a microwave discharge above a substrate holder located in a quartz dome. The discharge, shaped as a plasma ball or hemisphere, is positioned in the resonant path of the millimeter-wave open resonator operating at the frequency 32.2 GHz. The millimeter-wave open resonator operates with a quality factor of approximately 3500. The shift in the resonant frequency of the resonator when the discharge plasma is present allows the determination of the electron density. The electron densities measured were in the range of (1–6)×10¹¹ cm⁻³ for the pressure range of 5–60 Torr. Electron density measurements for various reactor parameter settings are presented for both pure hydrogen and hydrogen–methane mixture discharges.     

Grain size dependent physical and chemical properties of thick CVD diamond films for high energy density physics experiments

We report on the grain size dependent morphological, physical and chemical properties of thick microwave-plasma assisted chemical vapor deposited (MPCVD) diamond films that are used as target materials for high energy density physics experiments at the Lawrence Livermore National Laboratory. Control over the grain size, ranging from several μm to a few nm, was achieved by adjusting the CH₄ content of the CH₄/H₂ feed gas. The effect of grain size on surface roughness, morphology, texture, density, hydrogen and graphitic carbon content was systematically studied by a variety of techniques. For depositions performed at 35 to 45 mbar and 3000 W microwave power (power density ~ 10 W cm^− 3), an abrupt transition from micro-crystalline diamond to nanocrystalline diamond was observed at 3% CH₄. This transition is accompanied by a dramatic decrease in surface roughness, a six percent drop in density and an increasing content in hydrogen and graphitic carbon impurities. Guided by these results, layered nano-microhybrid diamond samples were prepared by periodically changing the growth conditions from nano- to microcrystalline.     

Fabrication of diamond nanowires for quantum information processing applications

We present a design and a top-down fabrication method for realizing diamond nanowires in both bulk single crystal and polycrystalline diamond. Numerical modeling was used to study coupling between a Nitrogen Vacancy (NV) color center and optical modes of a nanowire, and to find an optimal range of nanowire diameters that allows for large collection efficiency of emitted photons. Inductively coupled plasma (ICP) reactive ion etching (RIE) with oxygen is used to fabricate the nanowires. Drop-casted nanoparticles (including Au, SiO₂ and Al₂O₃) as well as electron beam lithography defined spin-on glass and evaporated Au have been used as an etch mask. We found Al₂O₃ nanoparticles to be the most etch resistant. At the same time FOx e-beam resist (spin-on glass) proved to be a suitable etch mask for fabrication of ordered arrays of diamond nanowires. We were able to obtain nanowires with near-vertical sidewalls in both polycrystalline and single crystal diamond. The heights and diameters of the polycrystalline nanowires presented in this paper are ≈ 1 μm and 120–340 nm, respectively, having a 200 nm/min etch rate. In the case of single crystal diamond (types Ib and IIa) nanowires the height and diameter for different diamonds and masks shown in this paper were 1–2.4 μm and 120–490 nm with etch rates between 190 and 240 nm/min.     

High-rate homoepitaxial growth of CVD single crystal diamond by dc arc plasma jet at blow-down (open cycle) mode

Growth and applications of large size high quality single crystal diamond is one of the most significant progresses in the field of CVD diamond film research ever made in the past 15 years. However, up to now most of the works were done by microwave plasma CVD operating at high pressures. In the present investigation it is demonstrated that relatively high quality single crystal diamond layer with the FWHM of the diamond Raman peak of less than 2 cm^− 1 and the FWHM of the diamond (400) reflection X-ray Rocking Curve of 0.028° can be obtained by the 20 kW dc arc plasma jet operating at blow down (open cycle) mode at a growth rate as high as 36 μm/h. The reason why dc arc plasma jet is also suitable for high quality epitaxial growth of single crystal diamond is that very high concentration of atomic hydrogen can be easily provided by the extremely high gas temperature due to the arc discharging. Detailed results on the effects of the ratio of H₂/Ar, the distance between the substrate to the anode nozzle of the arc plasma torch, the concentration of methane, and the substrate temperature on the growth of single crystal diamond are presented, and discussed comprehensively, and compared to that with the MWCVD, and with our previous work on the 30 kW dc arc plasma jet operating at arc root rotation and gas recycling mode.     

Effect of total reaction pressure on electrical properties of boron doped homoepitaxial (100) diamond films formed by microwave plasma-assisted chemical vapor deposition using trimethylboron

Boron was doped into diamond films which were synthesized homoepitaxially on polished (100) diamond substrates by means of microwave plasma-assisted chemical vapor deposition (MPCVD) using trimethylboron as the dopant at a constant substrate temperature of 1073 K. The morphologies and electrical properties of the synthesized diamond films were dependent on the total reaction pressure. A maximum Hall mobility, 760 cm² V⁻¹ s⁻¹, was obtained for the film synthesized at 10.7 kPa. The values of Hall mobility were comparable with those obtained for B₂H₆-doped films at corresponding hole concentrations.     

Effect of H2/Ar plasma on growth behavior of ultra-nanocrystalline diamond films: The TEM study

Incorporation of H₂ species into Ar plasma was observed to markedly alter the microstructure of diamond films. TEM examinations indicate that, while the Ar/CH₄ plasma produced the ultrananocrystalline diamond films with equi-axed grains (~ 5 nm), the addition of 20% H₂ in Ar resulted in grains with dendrite geometry and the incorporation of 80% H₂ in Ar led to micro-crystalline diamond with faceted grains (~ 800 nm). Optical emission spectroscopy suggests that small percentage of H₂-species (< 20%) in the plasma leads to partially etching of hydrocarbons adhered onto the diamond clusters, such that the C₂-species attach to diamond surface anisotropically, forming diamond flakes, which evolve into dendrite geometry. In contrast, high percentage of H₂-species in the plasma (80%) can efficiently etch away the hydrocarbons adhered onto the diamond clusters, such that the C₂-species can attach to diamond surface isotropically, resulting in large diamond grains with faceted geometry. The field needed to turn on the electron field emission for diamond films increases from E₀ = 22.1 V/μm (J_e = 0.48 mA/cm² at 50 V/μm applied field) for 0% H₂ samples to E₀ = 78.2 V/μm (J_e < 0.01 mA/cm² at 210 V/μm applied field) for 80% H₂ samples, as the grains grow, decreasing the proportion of grain boundaries.     

Improved DC arc-jet diamond deposition with a secondary downstream discharge

The efficiency of a polycrystalline diamond coating in a DC arc-jet setup with a power of 5–10 kW was enhanced by extension of a high temperature jet core. Two approaches were investigated. First, an expansion of the plasma jet anode channel was employed. It allowed us to increase the process efficiency from 5.5 to 10 mg/h/kW by optimizing values of pressure and CH₄ content in a reactionary Ar/H₂/CH₄ mixture. In the second approach, the extension of the hot jet core was achieved by an additional downstream discharge with a power of 2.5 kW. The secondary discharge was initiated between a positively biased auxiliary electrode and the arc-jet nozzle. This method allowed us to obtain a growth rate of 40 μm/h on an area of 12 cm², which corresponds to an efficiency of 16 mg/h/ kW.     

Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD

High-quality polycrystalline diamond film has been extremely attractive to many researchers, since the maximum transition frequency (f_T) and the maximum frequency of oscillation (f_max) of polycrystalline diamond electronic devices are comparable to those of single crystalline diamond devices. Besides large deposition area, DC arc jet CVD diamond films with high deposition rate and high quality are one choice for electronic device industrialization. Four inch free-standing diamond films were obtained by DC arc jet CVD using gas recycling mode with deposition rate of 14 μm/h. After treatment in hydrogen plasma under the same conditions for both the nucleation and growth sides, the conductivity difference between them was analyzed and clarified by characterizing the grain size, surface profile, crystalline quality and impurity content. The roughness of growth surface with the grain size about 400 nm increased from 0.869 nm to 8.406 nm after hydrogen plasma etching. As for the nucleation surface, the grain size was about 100 nm and the roughness increased from 0.31 nm to 3.739 nm. The XPS results showed that H-termination had been formed and energy band bent upwards. The nucleation and growth surfaces displayed the same magnitude of square resistance (R_s). The mobility and the sheet carrier concentration of the nucleation surface were 0.898 cm/V s and 10¹³/cm² order of magnitude, respectively; while for growth surface, they were 20.2 cm/V s and 9.97 × 10¹¹/cm², respectively. The small grain size and much non-diamond carbon at grain boundary resulted in lower carrier mobility on the nucleation surface. The high concentration of impurity nitrogen may explain the low sheet carrier concentration on the growth surface. The maximum drain current density and the maximum transconductance (g_m) for MESFET with gate length L_G of 2 μm on H-terminated diamond growth surface was 22.5 mA/mm and 4 mS/mm, respectively. The device performance can be further improved by using diamond films with larger grains and optimizing device fabrication techniques.     

Reactive ion etching of nanocrystalline diamond for the fabrication of one-dimensional nanopillars

One-dimensional diamond nanostructures (diamond nanopillars) have been fabricated using nanocrystalline diamond films (NCD) as a starting material, and electron beam lithography (EBL) and reactive ion etching in an inductively coupled O₂ plasma (ICP-RIE) as processing techniques. In a first step, the etch rates have been determined as a function of four major plasma parameters, namely the ICP power, the rf power applied to the substrate holder, the pressure, and the oxygen flow rate. These parameters have been varied in wide ranges. In order to get insight into the mechanisms of the etching process, etching experiments have been performed with unpatterned NCD films by varying the process times using rather short intervals. Finally, EBL has been applied prior to the etching to obtain one-dimensional pillars with diameters from 200 nm to 1 μm. Scanning electron microscopy has been employed to characterize the pillars. First results showed the process developed to be successful, and first examples will be presented.