Superconducting properties of homoepitaxial CVD diamond

Superconductivity was achieved above 10 K in heavily boron-doped diamond thin films deposited by the microwave plasma-assisted chemical vapor deposition (CVD) method. Advantages of the CVD method are the controllability of boron concentration in a wide range, and a high boron concentration, compared to those obtained using the high-pressure high-temperature method. The superconducting transition temperatures of homoepitaxial (111) films are determined to be 11.4 K for T_C onset and 8.4 K for zero resistance from transport measurements. In contrast, the superconducting transition temperatures of (100) films T_C onset = 6.3 K and T_C zero = 3.2 K were significantly suppressed.     

Electronic properties of CVD polycrystalline diamond films

Electronic properties of polycrystalline diamond films have been analysed using steady-state photoconductivity under UV (λ=190 nm) and red (λ=633 nm) illumination. We look at the effect of several post-deposition treatments: annealing in vacuum at 400°C, successive annealing under air and methane or the reverse, and X-ray exposure. These are shown to affect the photoresponses in different manners. Globally, the mobility–lifetime products of samples are found in the range 10⁻⁷–10⁻⁶ cm² V⁻¹ under UV illumination, and ratios of photoresponses in UV and red are in the range 10³–10⁵. First results obtained using the modulated photocurrent technique are also presented.     

Optical,thermal and mechanical properties of CVD diamond

The exceptional combination of mechanical, thermal, chemical and optical properties of diamond make this material an ideal choice for numerous applications in which materials are required to operate in adverse environments exposed to abrasion or chemical attack, or in situations requiring the dissipation of extreme levels of power, such as windows for high power lasers. Progress in the synthesis and processing of chemical vapour deposition (CVD) diamond achieved at De Beers¹ Industrial Diamond Division (Debid) has opened many of these applications by bringing to the market a diamond material this is available in large areas, in industrial quantities and with the high quality and reproducible properties needed to meet the demanding requirements of the industry. This paper will review the properties of the different CVD diamond material grades used in optical, thermal and mechanical applications.     

Elastic properties of single crystal diamond made by CVD

Brillouin light scattering has been used to investigate the elastic properties of high quality homoepitaxial diamond layers about 1 mm thick that have been elaborated by microwave plasma assisted chemical vapour deposition. Taking advantage of the detection of different acoustic modes, a complete elastic characterization of the crystal has been achieved. Three single crystal elastic constants, namely, c₁₁, (c₁₁ − c₁₂) / 2 and c₄₄ have been selectively determined, respectively, from the frequency of the longitudinal and of the shear horizontal bulk modes travelling parallel to the film surface. These determinations are in agreement with the frequency of the observed surface modes and of the bulk waves propagating at different angles from a normal single crystal film plane and consistent with the properties of natural diamond. By adding a low amount of nitrogen ranging from 2 to 50 ppm in the gas phase, the growth rates were increased from 6 to 33 μm/h whereas the mechanical properties of the resulting layers remained close to those of natural diamond.     

Electrical properties of thick boron and nitrogen contained CVD diamond films

The acceptor and donor defects of thick (approx. 0.4 mm) free-standing boron and nitrogen containing microwave plasma CVD polycrystalline diamond films were investigated. Charge-based deep level transient spectroscopy (Q-DLTS) was applied to study impurity-induced defects, their density and energy distribution in the energy range of 0.01 eV≤E−E_v≤1.1 eV above the valence band. It was shown, that differential capacitance–voltage, and Hall effect measurements combined with DLTS data can be used to determine the degree of compensation, and the concentration of compensating donors (mostly the positively charged single-substitutional nitrogen (N⁺)) in p-type CVD polycrystalline diamond films. It was found, that incorporated boron atoms induce three levels of electrically active defects. Two of them with concentration (2–3)×10¹⁶ cm⁻³ each have activation energies of 0.36 and 0.25 eV with capture cross-sections of 1.3×10⁻¹³ and 4.5×10⁻¹⁹ cm², respectively. The third type of defect has an activation energy of 0.02 eV, capture cross-section 3×10⁻²⁰ cm² and concentration 10¹⁵ cm⁻³, this shallow trap being a probable general caterer of holes in low-doped films. The total concentration of electrically active uncompensated acceptors in all p-type diamond samples was approximately 2×10¹⁷ cm⁻³ with hole concentration of approximately 1.5×10¹⁴ cm⁻³ and hole mobility in the range of 30–40 cm² V⁻¹ s⁻¹ at room temperature. If assumed that compensating donors are mostly nitrogen, the films contained no less than 3×10¹⁶ cm⁻³ of N⁺.     

Photoconductive properties of lightly N-doped single crystal CVD diamond films

In the preparation of high power diamond photoswitches, thick (more than 100 μm) lightly nitrogen-doped single crystals were grown at LIMHP, for which Differential Interference Contrast Microscopy, Raman spectroscopy, photoluminescence, and cathodoluminescence have confirmed good morphology and very low but well-controlled impurity doping level. In order to evaluate the effect of nitrogen incorporation on the electronic properties of these films, photoconductivity measurements have been carried out. In an initial study, I–V and transient photocurrent measurements were conducted on several films with N-doping from 0 to 20 ppm intentionally added to the gas phase during growth, resulting into nitrogen concentrations lower than 100 ppb in the film. The results of these measurements are presented showing typical semiconductor behavior in terms of gain versus settling time, relatively high external quantum efficiency (EQE) and corresponding derived μτ (mobility × lifetime) product. In particular, samples with no nitrogen showed EQEs of several hundreds while their settling time was quite long (tens of seconds). However, samples with small nitrogen addition were observed to have settling times decreasing below a few seconds while EQEs close to 10 showed that a compromise could be found between efficiency and response time.     

CVD diamond for spintronics

The ability to minimise, control and manipulate defects in CVD diamond has grown rapidly over the last ten years. The application which best illustrates this is probably that of quantum information processing (QIP) or ‘diamond spintronics’. QIP is a rapidly growing area of research, covering diverse activities from computing and code breaking to encrypted communication. All these applications need ‘quantum bits’ or qubits where the quantum information can be maintained and controlled. Controlled defects in an otherwise high perfection diamond lattice are rapidly becoming a leading contender for qubits, and offer many advantages over alternative solutions. The most promising defect is the NV⁻ defect whose unique properties allow the state of its electron spin to be optically written to and read from. Substantial developments in the synthesis of CVD diamond have produced diamond lattices with a high degree of perfection, such that the electron spin of this centre exhibits very long room temperature decoherence times (T₂) in excess of 1 ms. This paper gives a brief review of the advantages and challenges of using CVD diamond as a qubit host. Lastly the various qubit applications being considered for diamond are discussed, highlighting the current state of development including the recent development of high sensitivity magnetometers.     

CVD金刚石技术和应用(下)

通过大量文献资料和有关网页,叙述了近年来国内外化学气相沉积金刚石的制备、加工和应用技术研究的进展、商业化现状等.文章列举了大量图片和数据,试图展示近几年CVD金刚石有关方面的信息.文章显示,CVD金刚石技术研究和商业开发虽然都取得了长足的进步,但仍然有巨大的发展空间,特别是在传统的金刚石无法涉足的一些高技术应用领域.     

Transport properties of CVD diamond elucidated by DC and AC conductivity measurements

Detailed comprehensive study was carried out on diamond thin films to understand the charge carrier transport with respect to temperature and frequency domain of AC conductivity. Metal-Diamond-Metal structures were used to study the bulk transport and surface conductivity of freestanding films. Large differences were observed between the surface and bulk admittance values of as grown films. Bulk transport resulted in thermally activated behavior both in AC and DC measurements with activation energies of 0.65 eV for thermionic emission of trapped charge. Normalized Cole–Cole plots can be explained on the basis of a superposition of four Voigt elements representing an effective medium constituted by a distribution of grains, elongated in the growth direction, and temperature dependent leakage paths. A simple model of the grain boundary is introduced to discuss and justify the observed results.     

CVD金刚石技术和应用

通过大量文献资料和有关网页,叙述了近年来国内外化学气相沉积金刚石的制备、加工和应用技术研究的进展、商业化现状等.文章列举了大量图片和数据,试图展示近几年CVD金刚石有关方面的信息.文章显示,CVD金刚石技术研究和商业开发虽然都取得了长足的进步,但仍然有巨大的发展空间,特别是在传统的金刚石无法涉足的一些高技术应用领域.     

Electrical properties and defect analysis of neutron irradiated undoped CVD diamond films

High-quality detector-grade CVD diamond films have been irradiated with fast neutrons up to 2×10¹⁵ n/cm² (1 MeV neutron-equivalent). Thermally stimulated currents (TSC) and thermoluminescence (TL) analysis have been performed. After irradiation, a decrease in the TSC and TL signal intensity is observed, while no significant changes in the spectrum shape are found. The TSC analyses have been performed as a function of the electric field and the results have been discussed in terms of the Poole–Frenkel effect.     

Patterning of CVD diamond films by seeding and their field emission properties

Selective seeding for growing diamond on Si substrates was performed by conventional lithography using photoresist mixed with fine diamond particles. The selectivity was improved by filtering the diamond powder-photoresist mixture and carrying out reactive ion etching of patterned substrates. As a result, a selectivity up to 2.0 × 10² or higher was achieved. The resolution was of the order of 1 μm. Field emission from diamonds prepared using this selective growth method was observed without any postgrowth treatment. The measured current vs. voltage plot of a diode showed a rectifying characteristic. Under a forward bias, a current of about 15 μA was obtained at about 570 V, with a turn-on voltage of about 480 V. The emission current was comparable with that which had been observed for Si field emitter tips.     

Photoelectron spectroscopy of CVD diamond

We report a study of the electronic structure of thin diamond films by core state (X-ray photoelectron spectroscopy (XPS)) and valence band (UV photoelectron spectroscopy (UPS)) photoelectron spectroscopy. These techniques were used to investigate the different phases in the initial growth of polycrystalline diamond films on Si(100) substrates. The films were deposited by a standard microwave technique as well as by bias-enhanced microwave plasma chemical vapour deposition in a dilute mixture of methane in hydrogen. The influence of sample preparation (such as prebiasing or prescratching of the silicon surface with different sized diamond powder prior to deposition) on nucleation density and electronic structure was also investigated by scanning electron microscopy and photoelectron spectroscopy (PES) respectively. The surface composition was probed as a function of deposition time. The XPS data reveal the formation of an SiC phase at the early stage on nucleation, preceding the gradual growth of diamond. At intermediate stages a combination of different carbon phases was observed. The atomic structure of the interface phases is discussed and a growth model is proposed. Valence band spectra of the different samples show the extreme sensitivity of PES to impurities and to surface properties such as reconstruction. The obtained data were compared to valence band measurements of natural diamond and other forms of carbon and to some extent to data obtained with Raman spectroscopy.     

论化学气相沉积(CVD)金刚石技术最新发展

描述了国内外化学气相沉积(CVD)金刚石技术研究及产业最新进展,介绍了CVD金刚石的基本生产方法、加工手段、产品类型及应用领域。通过对国内外CVD金刚石技术及产品研究最新进展情况的介绍,指出了我国与世界先进的(CVD)金刚石技术存在的差距,阐述了CVD金刚石市场发展存在的问题并提出改进的建议。     

Thermal oxidation of CVD diamond

A thermal oxidation process of diamond films grown by chemical vapor deposition (CVD) has been studied. The oxidation was realized via heating of the CVD films in air. Pristine and oxidized CVD diamond films were analyzed with Raman spectroscopy and scanning electron microscopy (SEM) techniques. Raman spectroscopy revealed substantial changes in the polycrystalline diamond film composition induced by oxidation. A selective oxidation of disordered carbon and small size diamond crystallites was obtained at appropriate temperatures. A model explaining the formation and oxidation of the CVD diamond films containing the micrometer single diamond cores surrounded by the nanocrystalline diamond and disordered carbon has been proposed on the basis of the obtained results.     

Nitrogen incorporation in CVD diamond

Photoluminescence, optical absorption and electron spin resonance results are reported for diamond films homoepitaxially grown by chemical vapor deposition on 〈100〉-oriented natural IIa diamond. Measured dependencies of the corresponding signals on the growth temperature in the range 950–2100°C suggest that, while most of nitrogen is present in our films as single atoms, a small part (approx. 10⁻³ of all nitrogen) could be incorporated as N₂ molecules.     

Effect of non-homogeneity of Al/CVD diamond interfaces on AC properties

The capacitance and conductance of an Al/diamond interface was measured in the temperature range 231–334 K. The conductance and loss tangent curves presented two maxima. To explain these results, a simplified electrical equivalent circuit model was proposed that includes the electrical characteristics of the two phases of diamond. The equivalent circuit comprises the resistances and capacitances corresponding to depletion regions and the bulk for grains and grain boundaries, respectively. Simulation of the conductance and capacitance curves based on this model was used in theoretical analyses. Each curve maximum was associated with processes that occur in grains and grain boundary regions of polycrystalline diamond. Another more realistic model was also proposed, in which the capacitance and conductance of the equivalent circuit are described in terms of distribution functions.     

Persistent photocurrents in CVD diamond

Persistent photocurrents (PPCs) in undoped polycrystalline CVD diamond are generated by UV illumination with 5.5 eV photons. PPCs are several orders of magnitude larger than dark currents and can be annealed at T>550 K. The PPC annealing dynamic is stretched exponentially with an activation energy of 1.4 eV. Thermally stimulated currents (TSCs) are dominated by maxima around 550 K which are related to a trap level 1.4 eV deep in the energy gap of CVD diamond. UV illumination generates a photocurrent response with an onset at hν≥0.6 eV measured by spectrally resolved photocurrents. A model is discussed which accounts for the detected phenomena where electrons are captured in grain boundaries and holes in defects in the grains of CVD diamond.     

Laser plasma CVD diamond reactor

The results of multi-parametric investigations of laser plasmatron in application to CVD of diamond are presented. Different reaction chamber configurations were studied. CH₄/H₂/Xe(Ar) and CH₄/CO₂/H₂/Ar gas mixtures at total pressure 1.0–4.5 atm were used. Both convection and directed gas flows with velocity up to 50 cm/s delivered reactants to the substrate surface. Diamond film deposition rates up to 40–50 μm/h were reached. CW CO₂ laser beam power and intensity needed for a stationary plasma maintenance under various experimental conditions were found. Laser radiation absorption in plasma was measured. The possibility to combine laser plasma with an auxiliary electrical discharge is demonstrated. A comparison between laser and other plasma CVD techniques is made.     

Memory effects in CVD diamond

CVD diamond, as other wide energy gap semiconductors or insulators, is known to present polarization effects when used as a nuclear particle detector, which are due to the building up of an internal space-charge electric field and are responsible for “memory” effects if the polarization is not erased. Polarization effects are larger when the thickness of the sample is longer than the drift length of carriers and give rise to a shortening of the electric field region inside the sample. In this work, IBIC (Ion Beam Induced Charge) measurements with 1 and 2 MeV proton beams have been carried out on a relatively thin CVD diamond sample with the conclusion that no memory or hysteresis effects were present even by switching from one polarity to the opposite one in subsequent measurements. However, it resulted that the values of the charge collection efficiency (cce) did depend on counting rate and the lowering at large counting rates was attributed to a local polarization due to trapping. Finally, in some cases cce turned out to be larger than 1, a result which could be either due to local detrapping or to non-blocking contacts.