Controlled variation of the refractive index in ion-damaged diamond

A fine control of the variation of the refractive index as a function of structural damage is essential in the fabrication of diamond-based optical and photonic devices. We report here about the variation of the real part of the refractive index at λ = 632.8 nm in high-quality single-crystal diamond damaged with 2 and 3 MeV protons at low-medium fluences (10¹³–10¹⁷ ions cm^? 2). After implanting the samples in 125 × 125 μm² areas with a raster scanning ion microbeam, the variation of optical thickness of the implanted regions was measured with laser interferometric microscopy. The results were analyzed with a model based on the specific damage profile. The technique allows the direct fabrication of optical structures in bulk diamond based on the localized variation of the refractive index, which will be explored in future works.     

Fabrication of highly efficient TiO2/Ag/TiO2 multilayer transparent conducting electrode with N ion implantation for optoelectronic applications

Fabrication of highly conductive and transparent TiO₂/Ag/TiO₂ (referred hereafter as TAT) multilayer films with nitrogen implantation is reported. In the present work, TAT films were fabricated with a total thickness of 100 nm by sputtering on glass substrates at room temperature. The as-deposited films were implanted with 40 keV N ions for different fluences (1×10¹⁴, 5×10¹⁴, 1×10¹⁵, 5×10¹⁵ and 1×10¹⁶ ions/cm²). The objective of this study was to investigate the effect of N⁺ implantation on the optical and electrical properties of TAT multilayer films. X-ray diffraction of TAT films shows an amorphous TiO₂ film with a crystalline peak assigned to Ag (111) diffraction plane. The surface morphology studied by atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM) revealed smooth and uniform top layer of the sandwich structure. The surface roughness of pristine film was 1.7 nm which increases to 2.34 nm on implantation for 1×10¹⁴ ions/cm² fluence. Beyond this fluence, the roughness decreases. The oxide/metal/oxide structure exhibits an average transmittance ~80% for pristine and ~70% for the implanted film at fluence of 1×10¹⁶ ions/cm² in the visible region. The electrical resistivity of the pristine sample was obtained as 2.04×10⁻⁴ Ω cm which is minimized to 9.62×10⁻⁵ Ω cm at highest fluence. Sheet resistance of TAT films decreased from 20.4 to 9.62 Ω/□ with an increase in fluence. Electrical and optical parameters such as carrier concentration, carrier mobility, absorption coefficient, band gap, refractive index and extinction coefficient have been calculated for the pristine and implanted films to assess the performance of films. The TAT multilayer film with fluence of 1×10¹⁶ ions/cm² showed maximum Haacke figure of merit (FOM) of 5.7×10⁻³ Ω⁻¹. X-ray photoelectron spectroscopy (XPS) analysis of N 1s and Ti 2p spectra revealed that substitutional implantation of nitrogen into the TiO₂ lattice added new electronic states just above the valence band which is responsible for the narrowing of band gap resulting in the enhancement in electrical conductivity. This study reports that fabrication of multilayer transparent conducting electrode with nitrogen implantation that exhibits superior electrical and optical properties and hence can be an alternative to indium tin oxide (ITO) for futuristic TCE applications in optoelectronic devices.     

Surface Brillouin scattering on annealed ion-implanted CVD diamond

Single crystal <100> diamond samples were implanted with a total fluence of 1.5 × 10¹⁶  ions/cm² at single energy of 150 keV using carbon ions. This implantation fluence created a damage density that would not restore the diamond structure after annealing. Surface Brillouin scattering studies show that the elastic properties of the highly damaged diamond layer starts to transit from diamond-like to amorphous carbon state at an annealing temperature of 500 °C. The amorphous carbon layer is shown to have a sound velocity (elastic properties) similar to those reported for tetrahedral amorphous carbon (ta-C). Raman spectroscopy, EELS and HRTEM has been used in conjunction with the SBS data to monitor the changes in the carbon implanted diamond at different annealing temperatures.     

Neutron transmutation of 10B doped diamond

Free standing ¹⁰B isotope doped diamond films deposited by chemical vapor deposition in a microwave chamber were irradiated to thermal neutron fluence values of 0.32 × 10¹⁹, 0.65 × 10¹⁹, 1.3 × 10¹⁹, and 2.6 × 10¹⁹ n/cm². Cooling of the diamond films was maintained during irradiation. In a separate experiment, neutron irradiation to a total fluence of 2.4 × 10²⁰ n/cm² with equal fast and thermal neutrons was also performed on a diamond epilayer without cooling during irradiation. The formation of defects in the diamond films was characterized using Raman, FTIR, photoluminescence, electron paramagnetic resonance spectroscopy, and X-ray diffraction. It was found that defect configurations in diamond responsible for an increase in continuum background in the one-phonon region of Raman spectrum were absent in the films that have been cooled. The FTIR peak at 1530 cm^− 1 annealed in the sample irradiated to a fluence of 2.6 × 10¹⁹ n/cm² indicating that the sample reached a temperature of 300 °C during irradiation. Absence of characteristic infrared absorption peaks that were observed only upon annealing neutron irradiated diamond is used to conclude that the temperature of the sample during neutron irradiation to a fluence of 2.6 × 10¹⁹ n/cm² was well below 650 °C needed for mobility of defects and accumulation of stable unrecoverable damage. On the other hand, results from diamond epilayer subjected to equal thermal and fast neutron fluence of 2.4 × 10²⁰ n/cm² and without cooling showed that defects formed from displaced carbon atoms became mobile and formed complex configurations of irrecoverable damage. Electrical conductance of the unirradiated and irradiated diamond samples was measured as a function of temperature to determine the compensation of the p-type by the n-type charge carriers.     

Electrical and optical properties of diamond-like carbon films deposited by pulsed laser ablation

Pulsed laser ablation of a graphite target was carried out by ArF excimer laser deposition at a laser wavelength of 193 nm and fluences of 10 and 20 J/cm² to produce diamond-like carbon (DLC) films. DLC films were deposited on silicon and quartz substrates under 1 × 10^? 6 Torr pressure at different temperatures from room temperature to 250 °C. The effect of temperature on the electrical and optical properties of the DLC films was studied. Laser Raman Spectroscopy (LRS) showed that the DLC band showed a slight increase to higher frequency with increasing film deposition temperature. Spectroscopic ellipsometry (SE) and ultraviolet–visible absorption spectroscopy showed that the optical band gap of the DLC films was 0.8–2 eV and decreased with increasing substrate temperature. These results were consistent with the electrical resistivity results, which gave values for the films in the range 1.0 × 10⁴–2.8 × 10⁵ Ω cm and which also decreased with deposition temperature. We conclude that at higher substrate deposition temperatures, DLC films show increasing graphitic characteristics yielding lower electrical resistivity and a smaller optical band gap.     

D and D/He plasma interactions with diamond: Surface modification and D retention

Chemical vapour deposition (CVD) diamond is of interest as a plasma facing material in magnetically confined nuclear fusion devices. CVD diamond samples were exposed to ion fluences between 1.5 × 10²² ions m^− 2 and 1 × 10²⁴ ions m^− 2 in D, D/He and He plasmas. A significant surface coverage of 10 nm–100 nm sized hemispherical and conical surface features resulted from chemical erosion and subsequent redeposition on samples exposed to ≥ 10²³ ions m^− 2 when exposed to D containing plasmas. Near edge X-ray absorption fine structure (NEXAFS) spectra showed strong diamond features following plasma exposure, indicating that the diamond crystal structure on the surface was largely retained. A small increase in the sp² fraction (from 1.2% to 4–7%), was observed suggesting some amorphous carbon formation. Elastic recoil detection analysis (ERDA) revealed that D retention saturates at ~ 5.4% averaged over the depth resolution of 20 nm from the surface, for an ion fluence of ~ 10²³ ions m^− 2. The addition of He to the plasma resulted in a slower uptake of D, which was attributed to He ion induced desorption of D.     

Luminescence centers in proton irradiated single crystal CVD diamond

Diamond displays a large variety of luminescence centers which define its optical properties and can be either created or modified by irradiation. The main purpose of the present work is to study the radiation hardness of several of such centers in homoepitaxial single-crystal CVD diamond by following the evolution of photoluminescence and ionoluminescence upon 2 MeV proton irradiation. Luminescence decays were observed with values of the fluence at half of the starting luminescence (F_1/2) of the order of 10¹⁴ cm^? 2. The 3H center displayed a non-monotonic behavior, with a growing behavior and a subsequent decay with a rather high F_1/2 value (in the order of few a 10¹⁶ cm^? 2), maintaining at the highest fluences an intensity significantly higher than the blue A-band. A simple model based on a double-exponential trend was defined to fit with satisfactory accuracy the evolution of the 3H center. Several PL centers (namely: 3H, TR12, 491 nm and 494 nm) exhibited clear correlations and anti-correlations in their fluence dependences, which were considered in the attempt to acquire some insight into their possible alternative attributions.     

Effect of hydrogen implantation on the graphite used in high pressure diamond synthesis

The objective of the present work is to investigate the effect of hydrogen implantation on graphite in the high pressure diamond synthesis. A comparison of the graphite/diamond conversion for different fluences of hydrogen implantation revealed that the diamond nucleation and the total mass yield are always higher (up to 46%) than in experiments without implantation. The maximum nucleation for the studied cases occurred at a fluence of 1×10¹⁷ hydrogen ions/cm². This behavior is not observed when other ions, such as krypton and argon, are implanted on the graphite, or when hydrogen is present in the reaction cell but not implanted on the graphite. The results were interpreted as a consequence of the creation of additional tetrahedral sp³ bonded carbon atoms when the graphite is hydrogen implanted, which would act as effective diamond nucleation sites in the high pressure synthesis.     

Synthesis,characterization and third-order nonlinear optical properties of novel hyperbranched donor–acceptor polyfluorenes based on 1,3,6,8-tertsubstituted carbazole core

Two novel hyperbranched donor–acceptor polyfluorenes based on 1,3,6,8-tertsubstituted carbazole core (4BrCzP1 and 4BrCzP2) were synthesized and characterized. The third order nonlinear optical properties of the materials were studied using Z-scan technique with femtosecond Ti: sapphire laser with delivering pulses of 140 fs at 800 nm. The nonlinear optical refractive index was 9.29 × 10^?8 esu for 4BrCzP1 and 3.19 × 10^?7 esu for 4BrCzP2, respectively. While the third order nonlinear susceptibility was 1.29 × 10^?9 esu for 4BrCzP1 and 4.33 × 10^?9 esu for 4BrCzP2, respectively. The experimental results indicated that the hyperbranched polyfluorenes was a promising candidate in the application of third order nonlinear optical materials.     

Improved surface coverage of an optical fibre with nanocrystalline diamond by the application of dip-coating seeding

Growth processes of diamond thin films on the fused silica optical fibres (10 cm in length) were investigated at various temperatures. Fused silica pre-treatment by dip-coating in a dispersion consisting of detonation nanodiamond (DND) in dimethyl sulfoxide (DMSO) with polyvinyl alcohol (PVA) was applied. Nanocrystalline diamond (NCD) films were deposited on the fibres using the microwave plasma assisted chemical vapour deposition (MW PA CVD) method. The longitudinal variation of NCD morphology, structure and optical parameters were specifically investigated. The evolution of the film morphology and film thickness along the fibre length was studied using scanning electron microscopy (SEM). The chemical composition of the NCD film was examined with micro-Raman Spectroscopy. The sp³/sp² band ratio was calculated using the Raman spectra deconvolution method. An approximately 5 cm-long homogeneous diamond film has been obtained on the surface of the fibre sample. Thickness, roughness and optical properties of NCD films in the VIS–NIR range were investigated on the reference quartz slides using spectroscopic ellipsometry. The samples exhibited relatively low deviations of refractive index (2.3 ± 0.25) and extinction coefficient (0.05 ± 0.02) along the length of 5 cm, as estimated at a wavelength of 550 nm. In order to show the effectiveness of deposition process on optical fibres, diamond films were also grown on the fibre with induced long-period grating (LPG). The results of transmission measurements demonstrated that an LPG with diamond overlay exhibits the appropriate dependency on the optical properties of external medium. Thus, the deposition process has a negligible effect on the fibre transmission properties.     

Performance optimization of fluorine-doped tin oxide thin films by introducing ultrasonic vibration during laser annealing

Commercial fluorine-doped tin oxide (FTO) thin films were subjected to laser annealing coupled with ultrasonic vibration (48 kHz and 350 W). The effects of ultrasonic vibration, laser fluence and defocusing amount were systematically studied. Laser annealing could result in grain growth or damage of the FTO layer, and introducing ultrasonic vibration during laser annealing could effectively enhance the film compactness, decrease the film thickness and refine the grains in the film. As a result, the optical and electrical properties of the ultrasonic-vibration-assisted laser-annealed FTO films were significantly improved by using low laser fluences and high defocusing amounts, and were slightly deteriorated when high laser fluences and low defocusing amounts were adopted. The results indicated that the film obtained by ultrasonic-vibration-assisted laser annealing using a laser fluence of 0.6 J/cm² and a defocusing amount of 2.0 mm had the best overall photoelectric property with an average transmittance of 84.1%, a sheet resistance of 8.9 Ω/sq and a figure of merit of 1.99×10^–2 Ω^–1, outperforming that of the film obtained by pure laser annealing using the same experimental parameters. The present study confirms the efficacy of ultrasonic-vibration-assisted laser annealing in optimizing performance of FTO films.     

Graphene synthesis by C implantation into Cu foils

Cu foils of 2 × 2 cm² have been implanted with 70 keV C⁻ ions to nominal fluences of (2–10) × 10¹⁵ cm⁻² at room temperature (RT) and subsequently annealed at 900–1100 °C for 15 min, before being cooled to RT to form graphene layers on the Cu surfaces. Analyses with Raman spectroscopy and atomic force microscopy demonstrate that a continuous film of bi-layer graphene (BG) is produced for implant fluences as low as 2 × 10¹⁵ cm⁻², much less than the carbon content of the BG films. This suggests that the implanted carbon facilitates the nucleation and growth of graphene, with additional carbon supplied by the Cu substrate (0.515 ppm carbon content). No graphene was observed on unimplanted Cu foils subjected to the same thermal treatment. This implantation method provides a novel technique for the selective growth of graphene on Cu surfaces.     

The effect of fast neutron irradiation on the performance of synthetic single crystal diamond particle detectors

Diamond is known for its extreme hardness which may allow it to operate as a particle detector in high fluence environments even after absorption of large radiation doses. We present a study of the deterioration of the charge collection efficiency (CCE) due to neutrons produced by ²³⁵U fission, with irradiation fluences up to 1 × 10¹⁶ n cm^?2. The planar devices were fabricated by thermal evaporation of Au onto approx. 300 μm thick high purity chemical vapour deposited diamond produced by Element Six Ltd., UK. The detector performance was investigated as a function of bias voltage at room temperature using ²⁴¹Am α-particles and minimum ionising particles (MIPs) of a ⁹⁰Sr source. At low fluences up to 2 × 10¹³ n cm^? 2, the detectors reach the initial saturated signal amplitude after irradiation. However, the signal is less stable and deteriorates due to polarisation. This effect can be reduced by initial priming with X-rays. No peak could be distinguished in the detector response in the unprimed state after 10¹⁶ n cm^? 2 with bias voltages up to 1000 V (equivalent to 32 kV cm^?1). However, a peak at about 18% CCE could be recovered after priming.     

Degradation of single crystal diamond detectors in swift heavy ion beams

Single crystal diamond detectors were fabricated using Chemical Vapor Deposition (CVD) grown diamond plates for swift heavy ion beam detection. The detectors were then subjected to swift heavy ion beams in the energy range of 100–150 MeV/nucleon at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. The degradation mechanism of the detectors with respect to particle fluence is reported. After exposure to a particle fluence of ~ 4.0 × 10¹³/cm², the output signal amplitude of the detectors dropped to near 55% from its initial value.Post irradiation, the diamond samples were characterized by the transient current technique (TCT), to understand the effect of the beam induced damage in charge transport properties. A ²³²U alpha particle source was collimated through a 1 mm aperture to generate e–h pairs at one side of the diamond detectors. Charges drifted to the other side of the sample (provided minimum trapping occurred) on application of an external electric field. The current signal from the detectors was recorded by varying the external field between 0.1 V/μm to 1.0 V/μm in both polarities. The charge collection of the irradiated diamond samples were compared to the charge collection from a pristine non-irradiated sample. The charge carrier lifetime decreased in the heavily irradiated segment of the detector, as a consequence the charge collection reduced to ~ 55%–65%.     

Growth and properties of hydrogen-free DLC films deposited by surface-wave-sustained plasma

Hydrogen-free diamond-like carbon (DLC) films were deposited by a new surface-wave-sustained plasma physical vapor deposition (SWP-PVD) system in various conditions. Electron density was measured by a Langmuir probe; the film thickness and hardness were characterized using a surface profilometer and a nanoindenter, respectively. Surface morphology was investigated using an atomic force microscope (AFM). It is found that the electron density and deposition rate increase following the increase in microwave power, target voltage, or gas pressure. The typical electron density and deposition rate are about 1.87 × 10¹¹–2.04 × 10¹² cm^− 3 and 1.61–14.32 nm/min respectively. AFM images indicate that the grain sizes of the films change as the experimental parameters vary. The optical constants, refractive index n and extinction coefficient k, were obtained using an optical ellipsometry. With the increase in microwave power from 150 to 270 W, the extinction coefficient of DLC films increases from 0.05 to 0.27 while the refractive index decreases from 2.31 to 2.11.     

Effect of prior C,Si and Sn implantation on the etch rate of CVD diamond

Diamond films were implanted with C⁺, Si⁺ or Sn⁺ ions at multiple energies in order to generate a uniform layer of implantation-induced disorder. The implant energies of 60, 180, 330 and 525 keV for C⁺ ions, 200, 500 and 950 keV for Si⁺ ions and 750 and 2000 keV for Sn⁺ ions were selected to give an approximately constant vacancy concentration at depths over the range ∼ 0–0.5 μm. An analysis of the C⁺ implanted surfaces by Raman spectroscopy has shown an increase in non-diamond or sp²-bonded carbon at doses in the range 5 × 10¹³ to 5 × 10¹⁵ cm^− 2. In comparison, a completely non-diamond structure was evident after implantation with either Si⁺ ions at a dose of 5 × 10¹⁵ ions/cm² or Sn⁺ ions at ≥ 5 × 10¹⁴ cm^− 2. For a given dose, the etch rate of the diamond film was shown to increase with the mass of the implanted species in the order of C⁺, Si⁺ and Sn⁺. For a given implant species, the etch rate increased with the implant dose and the ion-induced vacancy concentration. The etch rate of the implanted diamond in various gases decreased in the order of O₂, CF₄/O₂ and CHF₃/O₂ plasmas.     

Improved inter-tube coupling in CNT bundles through carbon ion irradiation

Carbon ion irradiation of carbon nanotube (CNT) bundles to enhance mechanical performance is investigated using classical molecular dynamics. Strategies to achieve inter-tube cross-linking for improved shear response without a drastic reduction in tensile strength due to induced defects are considered. Deposition energies of 50–300 eV/ion, fluences of 4 × 10¹³ to 2 × 10¹⁴ cm⁻², and dosages of 2–60 MGy on 7-tube bundles are studied. Within 100–200 eV/ion, the level of cross-linking is directly proportional to dosage and therefore controllable. Lower energy irradiation produces smaller-sized defects so ∼100 eV/ion is the preferred energy. More than 10 different types of cross-link and a variety of defects are created. The defect level becomes excessive if either the energy or the fluence is set too high. Extension to larger bundles however is significantly more challenging. In 19-tube bundles, ∼500 eV/ion is required to form cross-links with the centre CNT, and at this energy careful control of fluence is required to avoid excessive damage. Thus ion irradiation for improving mechanical properties is best suited to small bundles. However, a scenario whereby small bundles are irradiated prior to twisting into ropes is suggested as a possible future method for producing macro-scale cross-linked CNT fibres.     

Ion implantation and anneal to produce low resistance metal–diamond contacts

Contacts to boron-doped, (100)-oriented diamond implanted with Si or with Si and B were formed and the effects of dose, implantation energy and anneal treatment on the specific contact resistance were examined. Ti/Au contacts on heavily implanted diamond (10¹⁶ Si ions cm⁻², E_i=30 keV or 10¹⁷ Si and B ions cm⁻², E_i=15 keV (Si) and E_i=10 keV (B)) had a specific contact resistance lower than the best contacts produced on unimplanted diamond. A specific contact resistance of (1.4±6.4)×10⁻⁷ Ω cm⁻² was achieved following a 450°C anneal. The results were consistent with a reduction in barrier height brought about by silicide formation. Light silicon implantation (10¹³ ions cm⁻²) or relatively light dual implantation (B, Si<10¹⁶ ions cm⁻²) did not reduce the specific contact resistance. Increasing the diamond conductivity by 4×10⁴ decreased the specific contact resistance by over three orders of magnitude, in agreement with the trend observed by Prins (J.F. Prins, J. Phys. D 22 (1989) 1562).     

Annealing-temperature-modulated optical,electrical properties,and leakage current transport mechanism of sol–gel-processed high-k HfAlOx gate dielectrics

Deposition of HfAlO_x gate dielectric films on n-type Si and quartz substrates by sol-gel technique has been performed and the optical, electrical characteristics of the as-deposited HfAlO_x thin films as a function of annealing temperature have been investigated. The optical properties of HfAlO_x thin films related to annealing temperature are investigated by ultraviolet-visible spectroscopy (UV–vis) and spectroscopy ellipsometry (SE). By measurement of UV–vis, average transmission of all the HfAlO_x samples are about 85% owing to their uniform composition. And the increase in band gap has been observed with the increase of annealing temperature. Moreover, the increase of refractive index (n) and density with the increase of annealing temperature are obtained by SE measurements. Additionally, the electrical properties based on Al/Si/HfAlO_x/Al capacitor are analyzed by means of the high frequency capacitance-voltage (C-V) and the leakage current density-voltage (J-V) characteristics. Results have shown that 400 °C-annealed sample demonstrates good electrical performance, including larger dielectric constant of 12.93 and lower leakage current density of 3.75×10⁻⁷ A/cm² at the gate voltage of 1 V. Additionally, the leakage current conduction mechanisms as functions of annealing temperatures are also discussed systematically.     

Formation of cBN nanocrystals by He+ implantation into hBN

The structural modifications of polycrystalline hexagonal boron nitride implanted with He⁺ beams at energies between 200 keV and 1.2 MeV to fluences of 1.0 × 10¹⁷ ions cm^? 2 were investigated using micro-Raman spectroscopy. The measured Raman spectra show evidence of implantation-induced structural transformations from the hexagonal phase to nanocrystalline cubic boron nitride, rhombohedral boron nitride and amorphous boron nitride phases. The first-order Longitudinal-Optical cubic boron nitride phonon was observed to be downshifted and asymmetrically broadened and this was explained using the spatial correlation model coupled with the high ion implantation-induced defect density.