band-A Emission Research Articles

Comparison of surface and subsurface damage of mosaic single-crystal diamond substrate processed by mechanical and plasma-assisted polishing

The surface and subsurface damage of a mosaic single-crystal diamond (SCD) (100) substrate processed by mechanical polishing (MP) and plasma-assisted polishing (PAP) were studied. Scanning white-light interferometer (SWLI) (84-μm square) observation for different local positions showed that the surface damage such as scratches and deep pits were observed on the mosaic SCD substrate after MP, while such damage was completely removed after PAP. Scanning cathodoluminescence (CL) measurement of band-A emission for different local positions showed that scratches with high luminescence intensity attributed to subsurface damage were observed on the mosaic SCD substrate after MP, while such specific luminescence was not observed after PAP. Micro-Raman spectroscopy measurement showed that peak position shift of diamond Raman lines attributed to residual stress and full width at half maximum (FWHM) broadening attributed to destruction of the crystalline structure were observed on the mosaic SCD substrate after MP, while such a peak position shift and FWHM broadening were not observed after PAP.

Characterization of crystallographic defects in homoepitaxial diamond films by synchrotron X-ray topography and cathodoluminescence

Crystallographic defects in a p-type homoepitaxial diamond film grown by microwave plasma-assisted chemical vapor deposition on a synthetic high-pressure high-temperature type-Ib (001) substrate were characterized by synchrotron radiation X-ray diffraction topography (XRT) and cathodoluminescence (CL). CL mapping indicated typical luminescent spots corresponding to the band-A emission around 420 nm. The band-A spots correspond to the spots observed by XRT for both diffraction vectors g = [044] and [404], and are considered to be mixed dislocations with a dislocation direction t = [001]. Typical dislocations in the film, such as edge and perfect 60° dislocations were determined by utilizing the relationship between the diffraction vector g, Burgers vector b, and the dislocation line vector t.

Mapping of extended defects in B-doped (001) homoepitaxial diamond films by electron-beam-induced current (EBIC) and cathodoluminescence (CL) combination technique

We have studied on a spatial distribution of extended defects with shell-shape pits in B-doped (001) homoepitaxial diamond films using electron-beam-induced current (EBIC) and cathodoluminescence (CL) combination technique. The EBIC image corresponded to the comprehensive one of band-A emission (430 nm) in CL measurements. In addition, both EBIC and CL images also corresponded to optical microscope image (Nomarski) with shell-shape pits. These experimental results indicate that the shell-shape pits are accompanied with localized electrically-active-defects originating from extended defects such as dislocation with band-A CL emission. It is suggested that extended defects such as dislocations result in the formation of shell-shape pits.

High-pressure and high-temperature annealing effects on CVD homoepitaxial diamond films

High-pressure and high-temperature (HPHT) annealing effects on the chemical vapor-deposited (CVD) homoepitaxial diamond films were investigated. By the HPHT annealing, the intensity of free-exciton (FE)-related emission was increased by ∼ 2 times and the luminescence bands from 270 to 320 nm, which originate from 5RL and 2BD bands, were almost completely eliminated in the cathodoluminescence (CL) spectrum. The CL intensity of band-A emission, which is related to crystal defects in diamond, was also decreased. The hole mobility at room temperature was increased from 826 to 1030 cm 2/Vs by HPHT annealing. These results suggest that HPHT annealing decreases the crystalline defects and improves the optical and electronic properties of homoepitaxial diamond films.

Electroluminescence of diamond films induced by a scanning tunneling microscope

We report the scanning tunneling microscope induced band-A emission from boron-doped polycrystalline diamond films fabricated by chemical-vapor deposition (CVD). The broad blue emission occurs at a bias above ±3.4 V with double peaks at 410 and 450 nm and is attributed to the dislocation-related defect centers. Greatly enhanced green emissions around 530 nm are observed at high positive bias. This, together with strongly bias- and polarity-dependent emission intensities and spectra, leads us to propose that the boost in the green emission at high bias is probably related to the minority electron injection into the boron-related acceptor states in the subsurface.

Near-field cathodoluminescence of nanoscopic diamond properties

Near-field detection cathodoluminescence (NF-CL) analyses on MWCVD diamond has been carried out in a scanning near-field optical microscope (SNOM)/scanning electron microscope hybrid system to ensure that even the smallest structures with modified material properties can be evaluated. The CL signal is detected directly with the SNOM tip above the recombination center while the sample is irradiated simultaneously as well as homogeneously with an electron beam. The achievable resolution of this NF-CL analyses has been improved to approximately 50 nm. In addition, it was possible to determine electronic properties such as the local diffusion lengths by keeping the position of the SNOM probe constant and scanning the electron beam. Using this technique, distributions of defects within small diamond grains became visible without any sample preparation. Dislocations, hydrogen plasma induced defects, and nitrogen impurities were separated on a H post-treated MW-CVD diamond single crystal. Clusters of the 575-nm luminescence system were detected. A shift from blue to green of the Band-A emission depicted a dislocation.

Origin of band-A emission in homoepitaxial diamond films

Origin of the band-A emission of diamond was investigated to use homoepitaxial diamond films by means of scanning cathodoluminescence (CL) using field emission scanning electron microscopy and high-resolution transmission electron microscopy (HRTEM). It was revealed that the band-A emission localized only inside of the non-epitaxial crystallites (unepitaxial crystallites or UCs), and grain boundaries and dislocations around the emission site of the band-A were also localized in the UC sites; cross-sectional CL revealed in sub-micron scale that some parts in an UC showed the band-A emission, and HRTEM revealed that there were the same structures of grain boundaries as those had π* states determined by electron energy loss spectroscopy (EELS), where π* states correspond to sp2 states of defects in diamond. These experimental results indicate that one of the origins of the band-A emission is attributed to sp2 states of defects in diamond located in grain boundaries and/or dislocations.

Surfaces of undoped and boron doped polycrystalline diamond films influenced by negative DC bias voltage

The hydrogen ion bombardment is performed by applying a negative bias voltage to the substrate during microwave plasma chemical vapor deposition process, using only hydrogen as reactant gas. The size of (001) faces increases after hydrogen ion etching while other grains are etched off. The surfaces of [001]-oriented films after doping boron are investigated by scanning electron microscopy (SEM) and cathodoluminescent (CL) spectra. The absence of the band-A emission in the CL spectra means a low density of dislocation in the films. It is the first time that the peak at 741.5 nm and the broad peak at approximately 575 and 625 nm in the CL spectra were reduced efficiently after boron doping in (001) polycrystalline diamond films and to propose that these phenomena should be explained in simple terms with penetration of the lattice nets of the [001]-oriented faces model.

Effects of hydrogen ion bombardment and boron doping on (0 0 1) polycrystalline diamond films

Hydrogen ion bombardment was carried out by applying a negative bias voltage to the substrate during a microwave plasma chemical vapor deposition process, using only hydrogen as reactant gas. The size of (0 0 1) faces increases after hydrogen ion etching while other grains are etched off. The surfaces of [0 0 1] directionally oriented films after boron doping were investigated by scanning electron microscopy (SEM) and cathodoluminescent (CL) spectra. The absence of the band-A emission in the CL spectra indicates a low density of dislocations in the films. It is the first indication that the peak at 741.5 nm and the broad peak at around 575 and 625 nm in the CL spectra are reduced efficiently after boron doping in (0 0 1) polycrystalline diamond films. We propose that these phenomena could be explained in simple terms by penetration or adsorption through the lattice nets of the [0 0 1] directionally oriented surfaces model.

Defects in Device Grade Homoepitaxial Diamond Thin Films Grown with Ultra-Low CH4/H2 Conditions by Microwave-Plasma Chemical Vapor Deposition

Recent progress in our study of high-quality homoepitaxial diamond thin film growth was achieved thanks to a low CH4/H2 ratio used during microwave-plasma chemical vapor deposition (CVD). Deposited films showed atomically flat surfaces and excitonic emission (5.27 eV) at room temperature, while band-A emission (2.95 eV) originating from defect-related centers was decreased. Using these films, we have also realized excellent properties of Schottky junctions between Al and the high-conductivity layer near the hydrogenated surface, indicating that these films are device grade ones. On the other hand, when the CH4 concentration was increased, homoepitaxial diamond films contained unepitaxial crystallites (UC), which degraded the Schottky junction properties. Other parts of the top flat surface showed excitonic emission at room temperature, indicating that this region is still of the device grade quality. We found that the band-A emission only appeared at UC sites. High-resolution transmission electron microscopy (HRTEM) revealed that many amounts of (111) Σ3 twin boundaries and some defects around them existed.

Comparative study of band-A cathodoluminescence and Raman spectroscopy in CVD diamond films

Abstract A set of diamond films was grown by microwave plasma enhanced chemical vapour deposition using a CO 2 –CH 4 gas mixture. Film morphology, preferential orientation and crystal quality were systematically changed by varying the CH 4 concentration and substrate temperature in the ranges 47–52% and 750–850 °C, respectively. The resulting films were characterised by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and cathodoluminescence (CL). The crystalline quality of the films, as assessed by Raman spectroscopy, increases at lower substrate temperatures ( T s =750 °C) and when moving from (110) towards (100) texturing. Independently of the substrate temperature, a strong decrease of the band-A cathodoluminescence at 435 nm is found as the film preferential orientation goes from (110) to (100). A clear correlation between the width of the diamond Raman line and the band-A emission is observed, giving insight into the nature of this band. In particular, this result is consistent with the attribution of band-A CL to the presence of dislocations.

A Study of the Origin of Band-A Emission in Homoepitaxial Diamond Thin Films

AbstractThe band-A emission (around 2.8 eV) observed in high quality (device-grade) homoepitaxial diamond films grown by microwave-plasma chemical vapor deposition (CVD) was studied by means of scanning cathodoluminescence spectroscopy and high-resolution transmission electron microscopy. Recent progress in our study on homoepitaxial diamond films was obtained through the low CH4/H2 conditions by CVD. These showed atomically flat surfaces and the excitonic emission at room temperature, while the band-A emission (2.95 eV) decreased. Using these samples, we found that the band-A emission only appeared at unepitaxial crystallites (UC) sites, while other flat surface parts still showed the excitonic emission. High-resolution transmission electron microscopy revealed that there were grain boundaries which contained π-bonds in UC. This indicates that one of the origin of the band-A emission in diamond films is attributed to π bonds of grain boundaries.

Nature of band-A cathodoluminescence inCVD diamond films

Several sets of diamond films have been grown by Microwave Plasma-Enhanced Chemical Vapor Deposition (MWPECVD) using CH4-H2 and CO2-CH4 gas mixtures. The growth conditions were systematically changed in order to obtain different film morphologies, preferential orientations and crystal qualities. In particular, the CH4 concentration in the gas mixture was varied in a wide range to obtain different preferential orientations. Substrate growth temperatures as high as 950 °C were used to intentionally obtain highly defective films. The resulting films were characterized by X-Ray Diffraction, Raman spectroscopy and Scanning Electron Microscopy. Cathodoluminescence has been investigated in the energy range 1.55-6.20 eV (200-800 nm). A strong dependence of the 2.85 eV (435 nm) band-A emission on the CH4 concentration is found. Moreover, this dependence does not substantially change with the growth conditions (substrate temperature, nucleation pretreatment, gas mixture etc.). The position of the diamond Raman peak is also correlated with the band-A emission. These results are discussed in terms of crystal defects and diamond film texturing induced by the growth process.

Diamond films epitaxially grown by step-flow mode

Homoepitaxial diamond films were grown by the step-flow mode. The obtained films grown by this mode were characterized using atomic force microscopy, reflection high-energy electron diffraction (RHEED), cathodoluminescence (CL), and junction properties of Al-Schottky contacts. It is found that the surfaces of the films are covered with macroscopic steps running parallel to the [1 1 0] direction. The narrow and streaky RHEED patterns show the surfaces of the films are flat with 2 × 1 and 1 × 2 double-domain structure. From the CL spectra, strong excitonic luminescence is clearly observed in the temperature range between 120 and 300 K. The absence of the band-A emission in the CL spectra suggests a low density of dislocation in the films. The current-voltage characteristics of Al-Schottky contacts on the as-deposited diamond films show excellent junction properties with the ideality factor of 1.1. These results indicate immense potential for diamond as a semiconducting material.

Cathodoluminescence spectroscopy of synthetic diamond films

A set of diamond films was grown by microwave chemical vapor deposition (MWCVD) using a CH4-H-2 gas mixture. Structural and crystallographic defects were induced in the samples either by choosing a relatively high substrate temperature, T-s = 950 degrees C, or by intentional contamination. In addition, different preferential orientations were obtained by appropriate changes to the CH4 concentration in the gas mixture. The resulting films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. Room temperature and liquid nitrogen temperature cathodoluminescence (CL) were investigated in the 200-800 nm wavelength range. A clear correlation with the growth conditions of the so-called band-A emission (435 nm) and of some sharp spectral features was observed. An explanation of this effect is given in terms of crystal defects which are induced by the growth process. (C) 1997 Elsevier Science S.A.

Band-A emission in synthetic diamond films: A systematic investigation

Several sets of diamond films were grown by microwave plasma enhanced chemical vapor deposition (CVD) using a CH4–H2 gas mixture. The growth conditions were systematically changed in order to obtain different film morphologies, preferential orientations, and crystal qualities. In particular, the substrate temperature Ts and the CH4 concentration in the gas mixture were varied in the ranges 850–950 °C and 0.4%–2.0%, respectively. The resulting films were characterized by x-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Room-temperature cathodoluminescence has been investigated in the energy range 1.55–6.20 eV (200–800 nm). A clear correlation of the 2.85 eV (435 nm) emission band, the so-called band A, with both the substrate temperature and CH4 content in the gas mixture during deposition, was observed. An explanation is given in terms of crystal defects and diamond film texturing induced by the growth process.

Donor-acceptor pair recombination in synthetic type-IIb semiconducting diamond.

Steady-state- and time-resolved-photoluminescence (PL) and cathodoluminescence (CL) studies have been carried out on several synthetic type-IIb boron-doped semiconducting diamond samples. The behavior of the 1.84-eV red donor-acceptor pair band is compared directly with that of the well-known green band-A emission near 2.3 eV. Time-dependent CL studies show conclusively that the green band is also a donor-acceptor pair emission, thus confirming at least a part of the model introduced by Dean 30 years ago. Time-dependent PL measurements also indicate that the green and red emissions are in direct competition, thus suggesting a model in which pair emission occurs from two different donors to the neutral boron acceptor.