L. Pereira

Electron field emission from patterned nanocrystalline diamond coated a-SiO2 micrometer-tip arrays

We report the fabrication of patterned nanocrystalline diamond (NCD) submicrometer-tip arrays. This includes synthesis of silica (a-SiO2) templates by conventional vapor-liquid-solid method and conformal coating of the a-SiO2 nanowires with 5–10nm sized nanodiamond grains by microwave plasma chemical vapor deposition. Detailed structural investigations were carried out by high resolution transmission electron microscopy. Electron field emission of nanodiamond emitter arrays was observed with a threshold field of 5.5V∕μm. A high emission current density of 10mAcm−2 at 11V∕μm has been obtained. This value is comparable to those of high quality NCD films deposited on silicon substrates.We report the fabrication of patterned nanocrystalline diamond (NCD) submicrometer-tip arrays. This includes synthesis of silica (a-SiO2) templates by conventional vapor-liquid-solid method and conformal coating of the a-SiO2 nanowires with 5–10nm sized nanodiamond grains by microwave plasma chemical vapor deposition. Detailed structural investigations were carried out by high resolution transmission electron microscopy. Electron field emission of nanodiamond emitter arrays was observed with a threshold field of 5.5V∕μm. A high emission current density of 10mAcm−2 at 11V∕μm has been obtained. This value is comparable to those of high quality NCD films deposited on silicon substrates.

Electrical characterization of CVD diamond-n+ silicon junctions

The electrical characteristics of CVD-diamondn-Si heterojunction devices are reported. Below 250 K the diodes show an unusual inversion of their rectification properties. This behavior is attributed to an enhanced tunneling component due to interface states, which change their occupation with the applied bias. The temperature dependence of the loss tangent shows two relaxation processes with different activation energies. These processes are likely related with two parallel charge transport mechanisms, one through the diamond grain, and the other through the grain boundary. 2001 Elsevier Science B.V. All rights reserved.

Time resolved photoluminescence and cathodoluminescence of CVD diamond films

Abstract Photoluminescence and cathodoluminescence of chemically vapordeposited grown diamond thin films have been studied both in undoped and nitrogen doped samples. Time resolved spectroscopy allowed for the identification of different overlapping photoluminescence bands in the blue, with lifetime ranging from 0.13 to 50 ms. The nature of the weak photoluminescence band with maximum at 2.37 eV is discussed. Cathodoluminescence investigations on the samples revealed a much faster luminescence decay in the lower ns-range for the blue band, while decay times of a few tens of nanoseconds are observed for the nitrogen-induced red band. Interestingly, the decay time of the blue band assumes a maximum at about 300 K, while the intensity decreases in the entire investigated range between 80 K and 390 K. Possible mechanisms will be discussed.

Interface Properties and Capacitance–Voltage Behaviour of Diamond Devices Prepared by Microwave-Assisted CVD

Free standing diamond films were used to study the effect of diamond surface morphology and microstructure on the electrical properties of Schottky barrier diodes. By using free standing films both the rough top diamond surface and the very smooth bottom surface are available for post-metal deposition. Rectifying electrical contacts were then established either with the smooth or the rough surface. The estimate of doping density from the capacitance–voltage plots shows that the smooth surface has a lower doping density when compared with the top layers of the same film. The results also show that surface roughness does not contribute significantly to the frequency dispersion of the small signal capacitance. The electrical properties of an abrupt asymmetric n+(silicon)–p(diamond) junction have also been measured. The I–V curves exhibit at low temperatures a plateau near zero bias, and show inversion of rectification. Capacitance–voltage characteristics show a capacitance minimum with forward bias, which is dependent on the environment conditions. It is proposed that this anomalous effect arises from high level injection of minority carriers into the bulk.

Correlation between crystal structure, quality and luminescence in torch flame grown diamond films

Flame grown diamond films on a molibdenium substrate are characterised by cathodoluminescence (CL), photoluminescence (PL), micro-Raman and scanning electron microscopy in the emissive mode. Different crystal habit regions grow with an annular symmetry. A correlation between morphology, CL, PL and Raman measurements is obtained for each region. The influence of the growth parameters is discussed.

Electrical AC behaviour of MPCVD diamond Schottky diodes

Abstract The present work reports some experimental results on the electrical AC behaviour of metal–undoped diamond Schottky diodes fabricated with a free-standing MPCVD diamond film (5 μm thick). The metals are gold for the ohmic contact and aluminium for the rectifier. The capacitance and loss tangent vs. frequency shows that capacitance presents a relaxation maximum at frequencies near 10 kHz at room temperature. Although the simple model (small equivalent circuit) can justify the values for the relaxation, it cannot justify the departure from the Debye model, also verified in the Cole–Cole plot. Taking into account the existence of traps in the depletion region, a best fit to the experimental results was obtained. The difference between the Fermi level and the band edge of 0.2–0.3 eV is in agreement with the activation energy found from the loss tangent analysis. The capacitance with applied voltage (Mott–Schottky plots) gives a defect density of 1016 cm−3 with contact potentials near 0.5 V and the profile of defect density obtained shows a major density (approx. 1017 cm−3) in a layer with a thickness less than 50 nm from the junction, decreasing by one order of magnitude with increasing distance. Finally a structural model is proposed to explain the AC behaviour found.

Use of Electroless Plating Copper Thin Films for Catalysis

Recently, it was demonstrated that copper thin films show good adsorption characteristics for organic polar and non-polar compounds. Also, these films when used in small cavities can favor preconcentration of these organic compounds. It is also known that copper oxide can provide catalysis of organic compounds. Therefore, the aim of this work is the study of copper thin film catalysis when used in small cavities. Copper thin films, 25 nm thick, were deposited on silicon and/or rough silicon. These films do not show oxide on the surface when analyzed by Rutherford backscattering. Also, Raman analysis of these films showed only silicon bands, due to the substrate, however infrared spectroscopy shows oxide bands for films exposed to organic compound aqueous solutions. Cavities with copper films deposited inside were tested with a continuous flow of n-hexane, acetone or 2-propanol admitted in the system. The effluent was analyzed by Quartz Crystal Microbalance. It was shown that n-hexane or acetone can be trapped. The system also shows good reproducibility. Tests of catalysis were carried out using Raman spectroscopy and heating the films up to 300°C during 3 minutes after exposure to n-hexane, 2- propanol and acetophenone – pure or saturated aqueous solution. After the exposure, Raman spectra present intense bands only for 2-propanol, indicating that adsorption easily occurs. However, after heating with all solutions it was not found only silicon bands. Raman microscopy after heating also showed copper oxide cluster formation and, eventually, graphite formation. Although the heating provides oxide copper formation, this reaction does not produce a high amount of residues, which means that catalysis is possible in this condition. Thus, a simple device using copper thin films can be useful as sample pretreatment on microTAS development.

Characterisation of different habits in torch-flame-grown diamond and diamond-like films

Abstract Diamond and diamond-like films grown by the torch flame method have been analysed by different techniques, namely scanning electron microscopy, Raman spectroscopy, electron paramagnetic resonance and catholuminescence microscopy, in order to investigate the main structural features of the different habits present in the films. A correlation between the observed crystal habits, the defect structure and the formation of diamond and/or diamond-like phases has been established. This enables a general quality assessment of the deposits and their correlation with some features usually observed in carbon films.

Photoconductivity and electrical properties of diamond films grown by MPCVD

Abstract The electrical and photoconductive features of as-grown microwave-plasma-assisted chemical-vapour deposition (MPCVD) diamond films are studied in correlation with magnetic results obtained from electron paramagnetic resonance (EPR). Also, the morphology is analysed by atomic force microscopy (AFM) showing 〈111〉 crystals with a good uniformity of the deposit. The photoresponse as well the current-voltage features observed show an efficient photogeneration of carriers while the optoelectronic characteristics of the metal-diamond junction have an ideality factor of 1.6 together with a rectification ratio of about 10 4 at ±2.5 V. The nature of the mechanisms responsible for the conduction is discussed.

Low cost microstructures for preconcentration of polar and non-polar organic compounds

Films produced by plasma polymerization of ethyl ether and methyl or ethyl acetate show good adsorption characteristic for polar and non-polar organic compounds. These films when used in microchannels machined in a 3D-structure present some preconcentration of organic compounds. Therefore, the aim of this work is to investigate the physical-chemical preconcentration mechanisms on this structure. The test molecules used were n-hexane and 2-propanol. Quartz crystal microbalance and mass spectrometry were used to measure preconcentration. Two different procedures for reactant injection on the structure were used: a continuous flow during several minutes or a small amount injected on a single pulse and in a few seconds. The microchannels were also modified by the introduction of small ceramic particles for enhancement of the flow dispersion. It was possible to notice for all films a similar kinetic of retention. The main removal mechanism is adsorption. Although all films can provide the removal of the adsorbents molecules, the most important characteristic for the adsorption and/or retention is the surface condition. Thus, the retention of polar compound can be troubled if a non-polar compound was used previously. The most promising films for retention are ethyl ether and ethyl acetate when n-hexane and 2-propanol are used as test molecules. The results using n-hexane or 2-propanol point out the use of low-cost microchannels for preconcentration development.