Thermoluminescent behaviour of diamond films subjected to UV irradiation

The thermoluminescent behaviour of diamond films subjected to UV irradiation was studied by using an UV lamp of 254 nm wavelength. The UV irradiation was achieved by placing the samples 15 cm away from an UV source for different periods. The thermoluminescent signal was integrated from 0 to 350°C at a linear heating rate of 10°C/s in a N₂ atmosphere. The corresponding luminescence spectra show an excitation band centered at 450 nm while the emission band is centered around 500 nm at room temperature. The diamond films were synthesized on molybdenum substrates by the combustion flame technique and characterized by Raman spectroscopy and scanning electron microscopy. Received: 31 May 1999 / Reviewed and accepted: 22 July 1999     

A study of the phosphorescence mechanism of polycrystalline diamond films

A study of the phosphorescence mechanisms in polycrystalline diamond films was carried out through their thermoluminescent (TL) vanishing glow response. The polycrystalline diamond films phosphoresced when kept at room or higher temperatures after being excited with a UV light source. The observed behaviour of shallow and deep traps during the phosphorescence process can be explained with a simple time-dependent model. The diamond film phosphorescence was induced by exciting with a UV light source of 4 W and 254 nm wavelength. The TL vanishing glow curves were integrated from room temperature to 350°C at a linear heating rate of 10°C s^-1 in a N₂ atmosphere. The optical response of the diamond films was studied by means of its luminescence spectra, showing a broad emission band centered around 500 nm.     

Diamond Crystallization in the System B4C–C

Superhard polycrystalline diamond material consisting of crystallites less than 20 m in size and containing less than 5 wt % B₄C is synthesized in the graphite–B₄C system at 2600–2800 K and 8–9 GPa. In the Raman spectrum of this material, the main band (1332 cm^–1) is shifted to lower frequencies by 40 cm^–1, typical of heavily boron-doped diamond films. Based on experimental data, a mechanism is proposed for the transformation of graphite into polycrystalline diamond at temperatures between the melting points of the B₄C–diamond and B₄C–graphite eutectics.     

Broad band photoluminescence studies of diamond layers grown by hot-filament CVD

Photoluminescence and Raman spectroscopy were employed to investigate the broad band luminescence in thin diamond films grown on a silicon substrate by the HF CVD technique. The broad band luminescence with a maximum emission at 1.8–2 eV observed for CVD diamonds is characteristic for amorphous carbon with sp²-hybridized carbon bonds. As was shown by the Raman spectroscopy our diamond layer contained certain amounts of amorphous carbon phase and diamond nanocrystals which were the source of an additional energy state within the diamond energy gap. The experimental results precluded the possibility of broad band luminescence being due to the electron–lattice interaction. The amorphous carbon and diamond nanocrystals admixture in polycrystalline diamond layer introduced a defect state in the energy gap not in the form of point defects but rather in the form of a line or extended defects. In consequence these extended defects were responsible for the broad PL spectrum in the CVD diamond films.     

Evidence of hexagonal diamond in plasma-deposited carbon films

Hexagonal diamond grains of 30 nm diameter together with graphite and SiC are seen in predominantly amorphous carbon films deposited at low temperature on Si substrates from a CH₄ plasma vapour source. The different crystalline phases are identified by grazing-angle X-ray diffraction which allows for substrate rotation and tilting to enable the 2 peaks to be correlated with the angular displacements of specific planes. Electron energy-loss spectroscopy shows the chemical composition of the films to be predominantly carbon with traces of oxygen. Raman spectroscopy shows the peaks to be associated with amorphous carbon and graphite, together with a peak at 1170 cm^–1 which is attributed to microcrystalline hexagonal diamond.     

Ramanspektroskopische Charakterisierung von ultra‐dünnen Kohlenstoff‐Schutzschichten für die Magnetspeichertechnologie

Raman‐spectroscopy is a standard tool for structural characterization of ultra‐thin (<10 nm) amorphous carbon films which are used as protective overcoats in the magnetic storage industry. It provides powerful information on the bonding structure of the films. The Raman‐spectra of amorphous carbons are dominated by the D‐ and G‐bands at around 1350 cm^‐1 – 1600 cm^‐1 whose position and intensity are used for interpreting the carbon bonding. Several carbon films have been investigated using green (λ = 514.5 nm) and ultraviolet (λ = 244 nm) laser‐light. The dispersion of the G‐peak is the most crucial of parameters to describe the internal structure of the films since it distinguishes between graphite‐ and diamond‐like carbon. A high G‐peak‐dispersion corresponds to a high sp³‐fraction. These information are not available by single wavelength investigations due to the so called hysteresis effect causing the Raman‐spectra of different samples accidentally to look similar albeit having a different internal structure. The dual‐method we are here introducing avoids the hysteresis effect and provides good estimations on the sp³‐content and the mass density of different carbon systems. Furthermore, UV‐Raman analysis leads to quantification of the nitrogen content of nitrogen‐doped carbon layers by using the relative intensity of the 2200 cm^‐1 band in the UV‐spectrum. The great advantage of ramanspectroscopic investigations is its celerity. Acquisition times are seldom higher than 1.5 min. Additionally, Raman‐spectroscopy is a non‐destructive tool which leaves the investigated samples undamaged for further processing and makes it an attractive method for insitu‐analysis in the magnetic storage industry.     

Thermal conductivity of CVD diamond films

Diamond films 60 and 170 µm in thickness were grown by PACVD (plasma-assisted chemical vapor deposition) under similar conditions. The thermal diffusivity of these freestanding films was measured between 100 and 300 K using AC calorimetry. Radiation heat loss from the surface was estimated by analyzing both the amplitude and the phase shift of a lock-in amplifier signal. Thermal conductivity was calculated using the specific heat data of natural diamond. At room temperature, the thermal conductivity of the 60 and 170 m films is 9 and 16 W-cm^–1. K^–1 respectively, which is 40–70% that of natural diamond, The temperature dependence of thermal conductivity of the CVD diamond films is similar to that of natural diamond, Phonon scattering processes are considered using the Debye model, The microsize of the grain boundary has a significant effect on the mean free path of phonons at low temperatures. The grain in CVD diamond film is grown as a columnar structure, Thus, the thicker film has the larger mean grain size and the higher thermal conductivity. Scanning electron microscopy (SEM) and Raman spectroscopy were used to study the microstructure of the CVD diamond films. In this experiment, we evaluated the quality of CVD diamond film of the whole sample by measuring the thermal conductivity.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Field emission from diamond and diamond-like carbon films

Field emission from diamond and diamond-like carbon thin films deposited on silicon substrates has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. The diamond-like carbon films were deposited using the radio frequency chemical vapor deposition method. Field emission studies were carried out using a sphere-to-plane electrode configuration. The results of field emission were analyzed using the Fowler-Nordheim model. It was found that the diamond nucleation density affected the field emission properties. The films were characterized using standard scanning electron microscopy, Raman spectroscopy, and electron spin resonance techniques. Raman spectra of both diamond and diamond-like films exhibit spectral features characteristic of these structures. Raman spectrum for diamond films exhibit a well-defined peak at 1333cm^?1. Asymmetric broad peak formed in diamond-like carbon films consists of D-band and G-band around 1550 cm^?1 showing the existence of both diamond (sp³ phase) and graphite (sp² phase) in diamond-like carbon films.     

The effects of oxygen on diamond synthesis by hot-filament chemical vapor deposition

The effects of oxygen addition on the synthesis of diamond are extensively studied by using the hot-filament chemical vapor deposition (HFCVD) method, in which it is simple and easy to control the deposition parameters independently. Diamond films are deposited on silicon wafers under the conditions of substrate temperature 530–950 C; total reaction pressure 700–8000 Pa; and methane concentration 0.4–2.4% in both CH₄–H₂ and CH₄–H₂–O₂ systems.At deposition conditions of low substrate temperature, high CH₄ concentration or high total pressure, soot-like carbon and/or graphite are deposited without oxygen addition. When even a small amount of oxygen (about 0.6%) is added, well-faceted diamond films are observed in scanning electron microscopy micrographs and a sharp diamond peak in the Raman spectra appears. The range of deposition parameters for high-quality diamond syntheses are extended by oxygen addition (low substrate temperature, high methane concentration and high reaction pressure).     

Structure and properties of highly crystallized graphite films based on polyimide Kapton

Highly crystallized graphite films were prepared by heat treatment of carbonized polyimide films (Kapton) at temperatures of 2700 and 3050° C. Interlayer spacing d ₀₀₂ at room temperature, and electrical resistivity, magnetoresistance and Hall coefficient at room and liquid nitrogen temperatures were measured. All of these data indicate high crystallinity of the graphitized Kapton films obtained. For the graphite films heat treated at 3050° C mean-square mobilities were estimated from the magnetoresistance data at 1 T to be 0.91 m² V^–1 sec^–1 at room temperature and 2.3 m² V^–1 sec^–1 at liquid nitrogen temperature; the value at liquid nitrogen temperature corresponds to that for a pyrolytic graphite heat treated at 3200° C for 1 h (PG 3200). Magnetic field dependence of Hall coefficient at liquid nitrogen temperature for this sample also agrees well with that for PG 3200. Scanning electron micrographs on the surfaces show that the present graphite films consist of grains of large crystallites, and grain size increases as the crystallinity of the material improves.     

Persistent conductivity in post-growth doped ZnO films following pulsed UV laser irradiation

Solution and rf sputter deposited doped ZnO films were subjected to cumulative 4-ns pulses of 355 nm light from a pulsed Nd:YAG laser at fluences between 5 and 150 mJ/cm². Film densification, change in refractive index, and an increase in conductivity were observed following room temperature irradiation in air, a carbon monoxide reducing environment, or under vacuum. At fluences between 20 and 80 mJ/cm², the films did not damage catastrophically under irradiation and high visible transparency persisted. The increase in conductivity is attributed to creation of oxygen vacancies and subsequent promotion of free carriers into the conduction band. Effects were most pronounced for films treated in vacuum. All treated films became insulating again upon equilibration in air at room temperature after several days. Films were characterized by means of UV-VIS-NIR transmission spectroscopy, Raman spectroscopy, and Hall measurements. Analysis of interference fringes in measured transmission spectra allowed evaluation of optical properties. Raman measurements showed an increase of LO mode intensity with respect to TO mode intensity as the films became more conducting in accord with previous work. Results of this study are not only important for continued development of transparent conducting oxides, but also provide compelling evidence for the role of free carriers as initiators of laser damage in wide bandgap metal oxide films.     

Nanosecond pulsed excimer laser machining of chemical vapour deposited diamond and highly oriented pyrolytic graphite: Part I An experimental investigation

A laser beam offers the benefits of high precision, contamination-free, high speed, and low bulk temperature for machining of chemically vapour deposited (CVD) diamond thin films that in turn enable ultrafine finishing of diamond coated cutting tool inserts and drills, and for finishing and drilling of diamond coated multichip module applications. In this work, laser hole drilling and polishing of CVD diamond (free-standing diamond and coated tool inserts) and HOPG (highly oriented pyrolytic graphite) using a 248 nm wavelength, 23 ns pulsed excimer laser were conducted. The threshold energy fluence required for ablation of diamond and graphite was nearly the same but the material removal rate rapidly increases with the energy fluence for the graphite compared to diamond. At an energy fluence of 10 J cm^-2, the depth removed per pulse was 0.05 μm and 0.30 μm for diamond and graphite respectively. Raman microprobe analysis indicates that the laser machining induced the transformation of diamond to disordered forms of carbon in CVD diamond and some transformation of graphite to diamond in HOPG. The experimental data indicates that the transformation of diamond to graphite requires an energy input of 1.44 × 10⁷ J per mole. For a given set of laser parameters, the depth per pulse was substantially higher for diamond coated tool inserts compared to the free-standing diamond. The surface roughness of CVD diamond was reduced by 0.25 μm per pulse at an energy fluence of 16 J cm^-2 This revised version was published online in November 2006 with corrections to the Cover Date.     

Laser annealing of neutron irradiated boron-10 isotope doped diamond

¹⁰B isotope doped p-type diamond epilayer grown by chemical vapor deposition on (110) oriented type IIa diamond single crystal substrate was subjected to neutron transmutation at a fluence of 2.4 × 10²⁰ thermal and 2.4 × 10²⁰ fast neutrons. After neutron irradiation, the epilayer and the diamond substrate were laser annealed using Nd–YAG laser irradiation with wave length, 266 nm and energy, 150 mJ per pulse. The neutron irradiated diamond epilayer and the substrate were characterized before and after laser annealing using different techniques. The characterization techniques include optical microscopy, secondary ion mass spectrometry, X-ray diffraction, Raman, photoluminescence and Fourier Transform Infrared spectroscopy, and electrical sheet conductance measurement. The results indicate that the structure of the irradiation induced amorphous epilayer changes to disordered graphite upon laser annealing. The irradiated substrate retains the (110) crystalline structure with neutron irradiation induced defects.     

Surface morphology of polycrystalline diamond films etched by Ar+ beam bombardment

Polycrystalline diamond films etched by Ar⁺ beam bombardment were investigated by scanning electron microscopy and Raman spectroscopy. In an ion sputtering apparatus, an etching rate of 14 m C^–1 was obtained when 10 kV-accelerated Ar⁺ ions penetrated with an angle of 15–30° from the normal. A number of cavities were created on the surface treated at low incidence angle. In contrast, micro-prominence was seen under the condition of high incidence angle. The degree of surface roughness on etched films was also changed with the incidence angle of the beam. A relatively smooth surface appeared after the treatment with an incidence angle of 15°. Raman spectroscopy revealed that the physical etching of diamond is effective in obtaining high quality surface of polycrystalline diamond films.     

Oxidation kinetics of hexagonal boron nitride powder

The isothermal oxidation of hexagonal boron nitride powders was carried out at 900–1050°C in dry oxygen and air. The oxidation kinetics were found to obey linear rate law and were described by the surface chemical reaction-controlled shrinking cylindrical model. The apparent activation energies were found to be 298 and 330 kJ mol^–1in dry oxygen and air, respectively. The oxygen partial pressure dependence of the oxidation rate constant at 1000 °C is well represented by a Langmuir-type equation. Microscopic examination of the oxidized sample after removal of the oxide scale with water suggested that the rate of attack by oxygen was determined by an anisotropy due to the crystallographic direction, similar to oxidation in graphite. The volatilization of B₂O₃ was observed only in dry oxygen and obeyed a linear rate law, and was found not to affect the oxidation reaction.     

Diamond synthesis from CO-H2 mixed gas plasma

Particulate or film-like diamond was prepared on silicon substrates from CO-H₂ mixed gas using a microwave plasma technique. The growth rate of diamond without graphite and amorphous carbon, as measured by Raman spectroscopy, was 9m h^–1 for particles and 4mh^–1 for flims. These values were larger than those in other source gas systems, such as CH₄-H₂, CH₄-H₂-H₂O and CH₃OH-H₂. The good formation rate and high quality of diamond in the CO-H₂ system was attributed to acceleration of methyl radical formation by the reaction of excited CO and H₂ molecules and removal of by-product graphite by OH radicals in the plasma.     

Thermal oxidation of amorphous GaSe thin films

In this work the results of the thermal oxidation of GaSe thin films in air at different temperatures are presented. The structural and morphological characteristics of the thermally annealed products were studied by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The as-deposited GaSe films were amorphous and they transformed into polycrystalline GaSe films with a hexagonal crystal structure at a temperature around 400 °C. Thermal oxidation at 650 °C resulted in the formation of mixed Ga₂Se₃ and Ga₂O₃ compounds both in the monoclinic phase. At higher temperatures, Ga₂Se₃ disappeared and complete oxidation of the initial compound occurred. The optical energy gaps of products were determined at room temperature by transmittance measurements using UV–vis–NIR spectroscopy.     

Spectroscopic studies of hydrogen related defects in CVD diamond

Thin diamond films prepared by the hot filament chemical vapour deposition (HFCVD) method at various deposition pressures have been characterized using a variety of spectroscopic techniques. Interpretation of the spectral details have provided useful information about the nature of the films. Deposition pressure appears to affect the quality of the diamond films which is reflected in terms of the position and width of the characteristic Raman peak of diamond. Raman spectra of the films prepared at low deposition pressures showed the presence of a sharp peak at ∼1332 cm⁻¹ characteristic of theT _2g mode of diamond. The study of the effect of deposition pressure on the diamond growth, shows that in the range between 20 torr and 60 torr, there is little effect on the width and the shift of the 1332 cm⁻¹ Raman peak. However, at higher pressures the peak showed a blue shift and was considerably broadened. These studies indicate the development of strain in the lattice due to the introduction of unetched hydride layer, at higher deposition pressures, as well as distortions in the lattice leading to partial lifting of the degeneracy of theT _2g mode. A broad band corresponding to the non-diamond phase (which exists at the grain boundaries, interface or as inclusions inside the grain), which can be attributed to the effect of hydrogen impurity creeping into the lattice at higher deposition pressures is also observed. SEM and XRD patterns have confirmed the dominance of diamond phase in these films.     

Field emission studies of CVD diamond thin films: effect of acid treatment

Field emission from CVD diamond thin films deposited on silicon substrate has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. Field emission studies of as-deposited and acid-treated films were carried out using ‘diode’ configuration in an all metal UHV chamber. Upon acid treatment, the field emission current is found to decrease by two orders of magnitude with increase in the turn-on voltage by 30%. This has been attributed to the removal of sp² content present in the film due to acid etching. Raman spectra of both the as-deposited and acid-treated films exhibit identical spectral features, a well-defined peak at 1333 cm⁻¹ and a broad hump around 1550 cm⁻¹, signatures of diamond (sp³ phase) and graphite (sp² phase), respectively. However upon acid treatment, the ratio (I_d/I_g) is observed to decrease which supports the speculation of removal of sp² content from the film. The surface roughness was studied using atomic force microscopy (AFM). The AFM images indicate increase in the number of protrusions with slight enhancement in overall surface roughness after acid etching. The degradation of field emission current despite an increase in film surface roughness upon acid treatment implies that the sp² content plays significant role in field emission characteristics of CVD diamond films.     

Influence of annealing temperature on the structural, mechanical and wetting property of TiO2 films deposited by RF magnetron sputtering

TiO₂ films have been deposited on silicon substrates by radio frequency magnetron sputtering of a pure Ti target in Ar/O₂ plasma. The TiO₂ films deposited at room temperature were annealed for 1 h at different temperatures ranging from 400 °C to 800 °C. The structural, morphological, mechanical properties and the wetting behavior of the as deposited and annealed films were obtained using Raman spectroscopy, atomic force microscopy, transmission electron microscopy, nanoindentation and water contact angle (CA) measurements. The as deposited films were amorphous, and the Raman results showed that anatase phase crystallization was initiated at annealing temperature close to 400 °C. The film annealed at 400 °C showed higher hardness than the film annealed at 600 °C. In addition, the wettability of film surface was enhanced with an increase in annealing temperature from 400 °C to 800 °C, as revealed by a decrease in water CA from 87° to 50°. Moreover, the water CA of the films obtained before and after UV light irradiation revealed that the annealed films remained more hydrophilic than the as deposited film after irradiation.