As-grown Diamond Research Articles

Large deep-ultraviolet photocurrent in metal-semiconductor-metal structures fabricated on as-grown boron-doped diamond

Metal-semiconductor-metal planar devices have been fabricated on as-grown boron-doped homoepitaxial diamond thin films. They consist of two Schottky barriers connected back to back. The metal employed was a thin titanium (Ti) layer (5nm) followed by a gold (Au) cap layer (50nm), respectively. The structure shows a high ultraviolet photocurrent at 220nm, which is seven orders of magnitude higher than the reverse dark current (<1pA) for at least an applied voltage of ±0.4V. In addition, anomalous photoconductivity gain is observed. A plausible explanation could be the modification of the Schottky barrier under band-to-band illumination. The spectral photoresponse displays over six orders of magnitude discrimination between deep-ultraviolet (210nm) and visible light (630nm), and reveals a shoulder with an onset at 4.6eV.

Effect of metal coating on the formation of diamond whiskers in O 2 RF plasma

As-grown and metal-coated CVD diamond films were etched in an O 2 radio-frequency plasma. In this study, Molybdenum (oxidizable) and Nickel (non-oxidizable) were used as coating materials. The distribution of the whiskers was investigated and compared. In the case of as-grown diamond film, the whiskers were found preferentially formed at the diamond grain boundaries. The metal-coated diamond films were distinctly influenced by the oxidizability of the coating metal. Under the same etching condition, lower density whiskers (10 μm − 2 ) were obtained from the Mo-coated diamond film, and in this case the whiskers were about 50 nm in diameter and 200 nm in height. However, the peak intensities of the whiskers measured by cathodoluminescence remained the same as those of the non-etched diamond films. Compared to the Mo-coated film, whiskers obtained from the Ni-coated film were more densely distributed (40 μm − 2 ); also, their quality deteriorated with the etching, as did their cathodoluminescence peak intensities. In both cases, the properties of bonding structures were compared before and after etching. It was found that the whiskers kept the sp 3 diamond bonding structure even after etching.

An investigation of a thin metal film covering on HPHT as-grown diamond from Fe–Ni–C system

Scanning Raman spectroscopy, scanning auger microprobe, electron probe microanalysis, X-ray powder diffraction and transmission electron microscopy were used to investigate a thin metal film covering on HPHT as-grown diamond from Fe–Ni–C system. In different regions of the film, the fine structure of carbon and iron atoms are different. It is clear that the microstructure and composition in the inner part of the film near diamond are different from those in other regions of the film. The fine structures of carbon and iron atoms have been greatly transformed. Crystalline graphite and amorphous carbon were not found in the inner part near diamond. The diamond formation should be directly associated with carbide. There exist parallel relationships between the crystal faces of (Fe,Ni) and Fe 3C. We suggest that graphite could not be directly catalyzed into diamond structure in the molten film, and the carbon for diamond growth may be related to the decomposition of Fe 3C under the effect of (Fe,Ni) phases in the inner part, which can play an important role under diamond growth at high temperature and high pressure.

Luminescence study of defects in synthetic as-grown and HPHT diamonds compared to natural diamonds

The optically active defects in as-grown, high-pressure high-temperature-treated (HPHT), borondoped, and synthetic diamonds (SD) grown with a nitrogen-getter, as well as of natural diamonds (ND), were characterized by absorption and luminescence spectroscopies using different excitation sources. The laser-excited photoluminescence (PL) spectra of SDs show numerous sharp lines characteristic for nickel-related centers, whereas NDs yield mainly broad PL bands. The emission from the nickel-related defects in NIR range increases and the maxima of the bands shift to lower energies with increasing temperature. Under UV and electron beam excitation, the yellow synthetic diamonds display green luminescence patterns along octahedral directions. The UV-excited PL spectra of the yellow SDs show a green band associated with nickel-related optical species. NDs display broad bands centered at ca. 450 nm that are related to complex nitrogen-related aggregates formed in the mantle in a long period of geological time. The cathodoluminescence (CL) spectra of SDs reveal many nickel-related and simple nitrogen-vacancy defects. Differences among as-grown, HPHT, and boron-doped diamonds can be observed. The CL bands of NDs are partly formed or caused by complicated nitrogen aggregates, like N3 and N4 centers. The CL band of NDs shifted from blue to green with increasing irradiation time. The deep-UV (DUV) excited time resolved PL spectra of NDs showed fast-decaying donor-acceptor pair recombination (DAPR) bands at around 410 nm. With longer delay and gate times, the broad band shifted to ca. 435 nm. All synthetic diamonds display luminescence from nickel-related defects centered at ca. 480 and 530 nm at room temperature (RT) and 77 K, respectively, and all SDs had a sharp luminescence band at 694 nm due to a Cr3+ impurity in corundum inclusions.

Carbide Identification in Different Regions of a Thin Metal FilmCovering on an HPHT As-Grown Diamond Single Crystal from Ni–Mn–C System

Diamond single crystals were synthesized in the presence of Ni–Mncatalyst under high temperature and high pressure (HPHT). A thin metalfilm covering on as-grown diamond formed during diamond growth wasexamined using transmission electron microscopy. It was shownthat phase compositions of the region near the as-grown diamond aredifferent from those of other regions in the film. We found γ-(Ni,Mn)solid solution, diamond, Ni3C and Mn23C6 in the regionnear the as-grown diamond, while graphite, Mn7C3 andγ-(Ni,Mn) could be found in other regions of the film. Therelationship between the diamond growth and the carbides in the film wasanalysed briefly. It is suggested that the carbon source for diamondgrowth should be closely related to the decomposition of carbides in theregion near the diamond single crystal at HPHT, not being directly from thatof the graphite structure.

Quantitative analysis of hydrogen in chemical vapor deposited diamond films

In this investigation, diamond sheets have been deposited using chemical vapor deposition. The as-grown diamond sheets were characterized for hydrogen content using detailed infrared analysis. The deconvolution of the spectra in the three-phonon region (2700–3150 cm −1) showed a number of vibration modes corresponding to sp m CH n phase of carbon. The spectra representing (100) and (111) textured diamond sheets, grown under different conditions, was compared. The (100) textured sheets contained two dominant peaks centred at 2860 cm −1 and 2930 cm −1, which correspond to the symmetric and asymmetric stretch bands of CH 2 group. However, the sheet with (111) texture displayed multiple CH, CH 2 and CH 3 peaks on the spectra. Using a standard sample called polymethyl methacrylate with known concentration, H content associated with various modes was evaluated in the diamond sheets. IR active hydrogen in the CVD diamond sheets was also calculated by evaluating the area of the IR band. Using these measurements we suggest that the oscillator strength of the different IR modes varies depending upon the structure and H content of CVD diamond sheets.

Modelling of Electrical and Structural Properties of Thin Diamond Films

The objective of this article is to add to the experimental understanding of the relationship between the electrical properties of diamond and the growth input and output parameters. A wide selection of thin polycrystalline CVD diamond is investigated using the impedance spectroscopy (IS) modelling technique. The films are analyzed in the as-grown and annealed states. As part of this task, the as-grown dc resistivity of the films is correlated to four growth output parameters: average grain size, full width half maximum (FWHM), diamond to graphite signal ratio, and growth parameter, . In addition, the ac properties are analyzed in terms of a developed ac circuit model. Also investigated in this study are the annealing and temperature effects on electrical conductivity and ac circuit model parameters for as-grown and annealed diamond films.

Electrically active defects, conductivity, and millimeter wave dielectric loss in CVD diamonds

The native and impurity-induced defects, conductivity, and dielectric loss at frequencies 145–182 GHz in undoped and boron-doped CVD diamond films were investigated. The as-grown undoped diamond films displayed the acceptor point defects with a continuous energy spectrum and the activation energy of conductivity E a∼0.7 eV in the temperature range T=80–540K. The B-doped film showed two discrete boron-induced levels with activation energy E A∼0.37 and 0.25 eV, and the activation energy of conductivity E a∼0.3 and∼0.13 eV at the high and low temperatures, respectively. The activation energies E AL determined from the temperature dependencies (275–900 K) of the dielectric loss in millimeter wave range are in good agreement with the values E a and E A deduced from the electrical measurements. This indicates that the millimeter wave losses in CVD diamond are mainly due to excitation of electrically active native and impurity-induced defects. The electron irradiation (2 MeV) changes the parameters of boron-induced levels, reduces the concentration of the native defects with the continuous energy spectrum, and strongly, by four orders of magnitude, reduces electrical conductivity.

Change of cathodoluminescence spectra of diamond with continuous irradiation of low energy electron beam of 20 kV

Radiation hardness is one of superior properties of diamond. High energy electron beam in the range of MeV is used to produce radiation damages such as vacancy and interstitial atoms. However, remarkable change of cathodoluminescence (CL) spectra of type II synthetic diamond was observed with irradiation of 20 kV electron beam, when electron beam current is as high as 1 μA. The 420 and 540 nm lines are created with the irradiation of the beam, and grow followed by fading. A 410 nm broad band is found to grow with the irradiation. On the other hand, a 485 nm line goes down, while the free exciton line does not change in its intensity. Intensities of the bands depend on growth sector of the diamond, and remain after stopping the irradiation. The behavior of the luminescence bands suggests that as-grown diamond contains several defects changeable with electron beam with subthreshold energy.

Electrochemical behavior of Au nanoparticle deposited on as-grown and O-terminated diamond electrodes for oxygen reduction in alkaline solution

Au nanoparticle was electrochemically deposited on both as grown and oxygen-terminated (O-terminated) boron-doped diamond (BDD) films. The surface coverages of Au nanoparticle were 0.07 and 0.18 corresponding to the areas of Au 0.012 and 0.029 cm 2, respectively, as noted from linear sweep voltammetry. The SEM studies indicated different morphologies of Au deposition such as random distribution of small spherical particles at both the grain boundaries and the facets on the as grown diamond film and clusters principally on the cross edges of two facets on the O-terminated diamond. The electrochemical behavior for oxygen reduction was examined using differential pulse voltammetry (DPV), which confirmed the higher catalytic efficiencies of Au deposited as grown and O-terminated BDD electrodes when compared to a polycrystalline Au electrode. Moreover, the mechanism of Au nanoparticle deposited BDD films for the oxygen reduction was investigated by ac impedance and hydrodynamic voltammetric methods.

The preparation and properties of nanostructured diamond films deposited by a hot-filament chemical vapor deposition method via continuous ion bombardment

Nanostructured diamond films are deposited on mirror-polished silicon wafers under continuous ion bombardment by using a hot-filament chemical vapor deposition (HFCVD) chamber equipped with a substrate bias system. The morphology and phase purity of the diamond film, as related to the different deposition parameters, are evaluated by scanning electron microscopy (SEM) and Raman scattering spectroscopy. The results show that nanostructured diamond films with a large area, low stress and smooth surface can be synthesized on mirror-polished Si substrates by an economical HFCVD method, and the size of diamond clusters in the film can be controlled by a suitable selection of deposition parameters such as the bias current density and the ratio of CH 4 to H 2. Furthermore, the relationship between the film's stress and the diamond clusters' size for the films synthesized under various parameters is revealed. The measurement of field emission from an as-grown nanostructured diamond film shows properties of a low emission threshold and a stable high-emission current density.

Investigation of hydrogen incorporation in CVD diamond films using infrared reflection spectroscopy

In this work hydrogen incorporation in CVD diamond films of different quality has been assessed using infrared reflection and absorption spectroscopy. Infra red (IR) reflection spectra, taken from the rough growth surfaces of as-grown polycrystalline diamond films, show a well-defined two-phonon absorption intrinsic feature and different CH stretching bands depending on the quality of the film, and are similar to the corresponding transmission spectra. While IR reflection spectra taken from the smooth interfaces of the as-grown films show only weak absorption in the diamond two-phonon region and no visible CH stretching band, although the reflectance is much higher than that of the growth surfaces. This marked difference between the spectra taken from the growth surfaces and backsides of the films cannot be explained in terms of surface roughness, and is discussed on the base of the columnar growth of CVD diamond film and compared with SIMS and ERDA data in the literature. Our reflection results can not be explained using the model of hydrogen terminated at grain boundaries and evidence that the CH stretching band is associated with the intrinsic diamond two-phonon lattice absorption, which strongly indicates that infrared-active hydrogen is bonded to bulk diamond carbon atoms.

Fe and Ni impurities in synthetic diamond

Using synchrotron X-ray fluorescence (XRF) microanalysis, including XRF tomography, and Xray absorption near-edge structure (XANES) analyses, the distribution, and nature of incorporation of Fe and Ni impurities in as-grown diamond crystals, synthesized under high-pressure and hightemperature (HPHT) conditions, have been characterized. We find significantly different behavior for Fe and Ni as impurities in diamond. Nickel is dispersed and sector-zoned, with concentrations in {111} growth sectors at least 3 times those in {100} sectors, whereas Fe exists in the form of microaggregates or clusters with no observable sector correlation. Fe K-edge XANES shows that Fe is oxidized in diamond and has a valence of 2+. Comparison of XANES spectra from numerous standard compounds indicates that Fe is very likely bonded with oxygen as FeO.

Roughness control of polycrystalline diamond films grown by bias-enhanced microwave plasma-assisted CVD

A simple growth technique to control the surface roughness of polycrystalline diamond films is proposed. The films are grown using a microwave plasma-assisted chemical vapor deposition method, with varying the methane (CH 4) concentration at the stage of bias-enhanced nucleation. It is found from the field-emission scanning electron microscope spectra that nanocrystalline diamond nuclei are formed at a relatively high methane concentration, causing a secondary nucleation at the accompanying growth step. The RMS value of surface roughness for grown films, which is estimated from the atomic force microscope images, monotonically decreases from 165 to 53 nm with methane concentration. It is also observed that the surface roughness is closely related to the nucleation density. In addition, employing a two-step growth method, which consists of first-growth at 500 W and subsequent second-growth at 800 W, enables the as-grown polycrystalline diamond film to have a smaller roughness of approximately 46 nm. This is believed to be due to the secondary nucleation effect induced during the growth step.

Electron transfer kinetics on composite diamond (sp 3)–graphite (sp 2) electrodes

The concept of non-diamond sp 2 impurity states as charge transfer mediators on boron-doped diamond (BDD) surface was suggested as an explanation for the electrochemical behavior of synthetic diamond based electrodes. In order to verify this concept, graphite particles (sp 2) were deposited on diamond electrodes (sp 3) by mechanical abrasion. The behavior of the so prepared diamond–graphite composite electrodes were compared with those of as-grown (BDD ag) and those after mild anodic polarization (BDD mild). Outer-sphere electron transfer processes such as ferri/ferrocyanide (Fe(CN) 6III/II) and inner-sphere charge transfer reactions such as 1,4-benzoquinone/hydroquinone (Q/H 2Q) were chosen in order to investigate the electrochemical properties of these composite electrodes. Both redox systems became more reversible as the graphite (sp 2) loading increased. A strong analogy existed between as-grown diamond electrodes and diamond–graphite composite electrodes. Finally a model is proposed which describes the BDD electrode surface as a diamond matrix in which non-diamond (sp 2) impurity states are dispersed. These non-diamond sp 2 states on BDD surface acts as charge mediators for both inner-sphere and outer-sphere reactions.

A simple route to Ohmic contacts on low boron-doped CVD diamond

Formation and characterisation of titanium underlayer contacts (TiUL) for polycrystalline diamond grown on silicon substrates are described. The TiUL contacts are advantageous that they are easy to fabricate and permit direct probing of the electronic and electrochemical properties of the as-grown polycrystalline diamond surface. The TiUL contacts show nearly Ohmic behaviour over a wide potential range.

A new elegant technique for polishing CVD diamond films

It is well known that the columnar growth nature of CVD diamond results in a very rough growth surface and the surface roughness steeply increases with film thickness, especially for thick CVD diamond films. In this paper, we report the successful implementation of a new elegant technique for polishing thick polycrystalline CVD diamond films at high polishing rate of up to 10 μm/h. This technique involves polishing the as-grown polycrystalline diamond films with another thick as-grown polycrystalline diamond film, which acts as a polishing abrasive. Two types of diamond films were prepared using microwave plasma CVD and then polished for 2 h using the new polishing technique. A stylus profilometer, scanning electron microscopy and Fourier transform infrared spectroscopy were used to measure the surface roughness, characterize morphology and optical transmission of the samples before and after polishing, respectively. By polishing, thickness of 20–30 μm was removed from the top surface, and the mean surface roughness R a of the films reduced significantly, e.g. for one film R a reduced initially from 5.2 to 1.35 μm and the other from 3.2 to 0.55 μm. The principal advantages of this new polishing technique are simplicity, flexibility and time saving. This simple method can serve as ‘rough chipping’ to quickly remove the rough top surface and then combine with conventional polishing methods for precision machining to further reduce the surface roughness to a specific desired degree.

Differentiation of natural and synthetic gem-quality diamonds by luminescence properties

Abstract Laser-excited time-resolved and UV-excited static photoluminescence (PL) as well as cathodoluminescence (CL) techniques were applied to identify the origin of diamonds. Samples represented natural faced and rough diamonds from diamond market and different kimberlites as well as the most common high pressure–high temperature (HPHT) and as-grown synthetic diamonds. The time-resolved PL spectra of natural and synthetic diamonds display clear mutual differences. The static PL and CL spectra of natural diamonds revealed emission bands caused by complex nitrogen–vacancy (N–V)-aggregates whereas the bands of synthetic diamonds reflect simple N–V-aggregates and nickel-containing defects. The luminescence properties depended on the excitation method and also changed during the excitation. The study of PL colors revealed information about the emission distribution of the PL spectra giving possibilities for future applications. The results indicated the sensitivity of luminescence techniques and revealed interesting structural information about diamond materials.

Microstructure of high-pressure-synthesized diamonds under rapid quenching and electron irradiation

A type of synthetic diamond single crystal about 0.4–0.5 mm in dimension prepared under high pressure–high temperature (HPHT) in the presence of a FeNi molten catalyst was quenched from HPHT and irradiated with 300 keV electrons at room temperature. Transmission electron microscopy was employed to examine the microstructure of the diamond single crystal before and after electron irradiation. It was found that there exists a large amount of cellular interfaces in the quenched diamond sample, which indicates the growth condition of the diamond under HPHT. Hexagonal dislocation loops about several tens of nanometers in dimension were observed in the high-pressure-synthesized diamond single crystal before electron irradiation, which strongly suggests that a number of vacancies were quenched-in due to rapid quenching from high temperature at the end of diamond synthesis, and were aggregated in the synthetic diamond to form vacancy disks on the (111) plane, the collapse of such vacancy disks forming vacancy-type dislocation loops. After electron irradiation, it was found that defect clusters present as interstitial-character dislocation loops were formed in the electron-irradiated region of the diamond. The interstitial dislocation loops grow with increase of the irradiation time. The present study, in comparison to previous work on ion implantation on diamond, indicates that electron irradiation does not induce a phase transformation but produces interstitial dislocation loops due to the migration of interstitial atoms and vacancies. The result of the study directly indicates that interstitials and vacancies in diamond are mobile at room temperature under electron irradiation. Nitrogen, as the most important kind of impurity contained in the HPHT as-grown diamond, probably acts as nucleation of the interstitial loops.

A relation between a metallic film covering on diamond formed during growth and nanosized inclusions in HPHT as-grown diamond single crystals

One of the most important characteristics and basic phenomena during diamond growth from liquid metal catalyst solutions saturated with carbon at high temperature–high pressure (HPHT) is that there exists a thin metallic film covering on the growing diamond, through which carbon-atom clusters are delivered to the surface of the growing diamond by diffusion. A study of microstructures of such a metallic film and a relation between the thin metallic film and the inclusions trapped in HPHT as-grown diamond single crystals may be helpful to obtain high-purity diamond single crystals. It was found that both the metallic film and the HPHT as-grown diamond single crystals contain some nanostructured regions. Examination by transmission electron microscopy suggests that the microstructure of the thin metallic film is in accordance with nanosized particles contained in HPHT as-grown diamond single crystals. The nanosized particles with several to several tens of nanometers in dimension distribute homogeneously in the metallic film and in the diamond matrix. Generally, the size of the particles in the thin metallic film is relatively larger than that within the diamond matrix. Selected area electron diffraction patterns suggest that the nanosized particles in the metallic film and nanometer inclusions within the diamond are mainly composed of f.c.c. (FeNi)23C6, hexagonal graphite and cubic γ-(FeNi). The formation of the nanosized inclusions within the diamond single crystals is thought not only to relate to the growth process and rapid quenching from high temperature after diamond synthesis, but also to be associated with large amounts of defects in the diamond, because the free energy in these defect areas is very high. The critical size of carbide, γ-(FeNi)and graphite particles within the diamond matrix should decrease and not increase according to thermodynamic theory during quenching from HPHT to room temperature and ambient pressure.