High intensity intensity ultraviolet (UV) and vacuum ultraviolet (VUV) radiation provide a singular dominant
narrow-band emission at various wavelengths(λ) between 108 - 351 nm. The use of dielectric-barrier discharges
in its embodiment of an excimer lamp as a photon-source provides a novel method to induce surface modification.
From its in relatively humble beginnings in ozone generation, the excimer lamp has found new applications in
the field of low-temperature processing of surfaces. Herein, a 15 year perspective of work done at the Materials
& Devices Group at University College London between 1992 and 2007 is presented. The excimer lamps'
application to the modification of surfaces for materials processing include: photo-induced formation of high-κ
dielectric thin films and more recently the UV-induced photo-doping of silicon substrates, amongst others. With
its robust yet inexpensive setup and flexibility of geometric configurations, they are easily coupled in parallel
resulting in the provision of high photon fluxes over large areas. These sources also have an incoherent and
almost monochromatic selectivity for application to process chemical pathway specific tasks by simple variation
of the discharge gas mixture. These sources are an interesting addition to and an alternative to lasers for scalable
industrial applications and have potential for a myriad of applications across different fields.
The use of high intensity ultraviolet (UV) and vacuum ultraviolet (VUV) radiation generated from decaying excimer complexes through dielectric barrier discharge (silent discharges) sources for the purposes of surface processing and modification is reviewed. Such sources provide a singular dominant narrow-band emission at various wavelengths(λ) between 126 - 354 nm. The remarkable simplicity of supplying these sources and flexibility of their geometric configurations allow them to be coupled in parallel thus providing high photon fluxes over large areas. The monochromatic selectivity allows for application to process and chemical pathway specific tasks by simple variation of the discharge gas mixture. These sources are an interesting addition to and as an alternative
to lasers for large scale industrial applications and their unique characterisitics have led to their use in a number of low-temperature material modification techniques, some of which are reviewed here. These include the photo-induced low-temperature formation of oxynitride layers, high-κ thin film layers and the post-deposition annealing of pulsed laser deposited (PLD) thin films.
We describe recent research into devices based on fibre Bragg gratings in polymer optical fibre. Firstly, we report on the inscription of gratings in a variety of microstructured polymer optical fibre: single mode, few moded and multimoded, as well as fibre doped with trans-4-stilbenmethanol. Secondly, we describe research into an electrically tuneable filter using a metallic coating on a polymer fibre Bragg grating. Finally we present initial results from attempts to produce more complex grating structures in polymer fibre: a Fabry-Perot cavity and a phase-shifted grating.
The optical properties of a set of high-k dielectrics HfO2 thin films obtained by two different modified metal organic chemical vapour deposition (MOCVD) techniques were studied using spectroscopic ellipsometry (SE). HfO2 thin films with thickness varying from 10-40 nm were formed over a range of temperatures (300-425°C). After deposition the sample were annealed by Rapid Thermal Annealing (RTP) at 800°C in an oxygen/argon ambient and UV annealing at 400°C in oxygen. The films were analysed physically using XRD and FTIR. The XRD results show that as-deposited HfO2 films microstructure strongly depends on deposition temperature. Both polycrystalline (T>365°C) and amorphous films (T<320oC) were formed. The polycrystalline structure is identified as monoclinic. The SE results demonstrate that as-deposited amorphous HfO2 thin films have a high degree of porosity. After annealing at 800oC in oxygen and in nitrogen ambient, due to the solid phase crystallisation, as-deposited amorphous HfO2 thin films become crystalline and the film porosities are strongly reduced. In addition, an increase of the refractive index and a decrease of the film thickness are also obtained. Optical properties of the as-deposited polycrystalline HfO2 are also improved after annealing and an increase of the refractive index and a decrease of the film thickness is also obtained.
Applications of photo-induced processes have over the years become essential technologies in several important industrial sectors. In this paper, the principles and properties of novel vacuum UV (VUV) and UV radiation generated by novel excimer sources are discussed. Compared with conventional sources, these excimers lamps offer narrow-band radiation at various wavelengths form 108-354 nm and over large areas. Since excimer complexes have no stable ground states self-absorption of the emitted radiation in the discharge is avoided. As a consequence, high efficiencies at high power densities can be achieved.
Thin films of hydroxyapatite (HAp) have been grown on Si, quartz, Ti, and Ge substrates by the pulsed laser deposition (PLD) method employing a KrF excimer laser (wavelength (lambda) equals 248 nm, pulsed duration (tau) FWHM equals 20 ns). The influence of the laser deposition parameters on the properties of the grown layers was investigated in order to optimize the Ca/P ratio and the crystalline structure. It was found that the optimum conditions for preserving the Ca/P ratio i.e. high oxygen pressures and low substrate temperatures do not coincide with those for obtaining adherent and crystalline layers i.e. low oxygen pressures and high substrate temperatures. For films deposited onto Ti substrates it was also found that high substrate temperatures promote the diffusion of Ti through the depositing film up to the surface where it gets oxidized. Further investigations are required before high quality HAp-coated Ti implants by PLD can be obtained.
The role of the sub-surface explosive boiling mechanism for droplet formation during the pulsed laser deposition of thin films has been examined. For photons with an energy lower than the target optical band-gap, hv<Eg, a highly perturbed target surface morphology exhibiting micrometer sized round- shaped cavities always formed, whereas for hv>Eg the surface was much less perturbed. Depth-profile estimations of the temperature profiles inside the laser irradiated targets were also quite different for these two cases. For low optical absorption coefficient (OAC) values, (alpha) < 5 X 104 cm-1, a thick layer of liquid formed whose maximum temperature was located at some point below the surface. Simultaneous to the cavity formation, the droplet density on the surface of the grown films was found to increase by orders of magnitude, indicating a clear connection between the target OAC, the temperature profile, the presence of the cavities and a high droplet density. The decrease of the OAC value during prolonged laser ablation could also explain the gradual increase of the droplet density observed when growing La0.5Sr0.5CoO3 films. Based on these results, we suggest that an important fraction of the droplets on the surface of films grown under laser irradiation conditions where the OAC is smaller than 5 X 104 cm-1 is caused by an explosive sub-surface boiling mechanism.
The principles and properties of vacuum ultraviolet (VUV) and ultraviolet (UV) light generated from a new type of excimer lamp are described. Compared with other lamps, these VUV and UV light sources can provide high photon fluxes over large-areas. These VUV and UV sources have been used to initiate the photo-deposition of dielectric and metallic thin films. The photo-deposited film properties, determined using ellipsometry, FTIR spectroscopy, UV spectrophotometer, and electrical measurements, showed that good quality films could be produced. Multilayered films of silicon oxide, silicon nitride, and silicon oxynitride can also be produced at low temperatures (below 300 degree(s)C). Very high deposition rates (500 angstrom/min) have been obtained by irradiating silane and oxygen gas mixture at low temperatures. This technique being relatively inexpensive in capital outlay, simple to apply, and readily scalable to large-areas provides interesting perspectives for optical and electronic applications.
Excimer lamp deposited ultra-thin (< 250 angstrom) silicon dioxide and silicon oxynitride films were characterized using spectroscopic ellipsometry (SE) and Fourier transform infrared (FTIR) spectroscopy. SE analysis of the photo-deposited SiO2 films revealed no variation in the refractive index (n) of the films for deposition temperatures of 200 degree(s)C and 300 degree(s)C. Using a Bruggeman effective medium approximation (EMA), SE analysis was employed to determine both the silicon oxynitride layer thicknesses and compositions as a function of deposition temperatures and gas ratio, defined as (N2O/(N2O + NH3)). From this analysis the optical properties of the silicon oxynitride thin films were extracted. It was observed that the refractive index for the 200 degree(s)C and 300 degree(s)C series of samples decreased from n equals 1.81 to 1.46 and n equals 1.72 to 1.46 respectively as a function of increasing gas flow ratio. FTIR analysis revealed spectral features characteristic of Si-O, Si-N, Si-H and N-H bonding. The most significant feature in all recorded spectra was a mixed spectral absorption band ranging from 800 cm-1 to 1300 cm-1. Both the integrated band area and peak wavenumber of this absorption band was found to be sensitive to the degree of nitridation and layer thickness of the thin films. The N-H stretching bond density was calculated from the N-H peak at 3360 cm-1 using appropriate calibration factors. A slight decrease in the N-H bond density with increasing gas flow rate was observed. This variation in bond density was significantly less than that observed for PECVD silicon oxynitride films.
We report the use of vacuum ultraviolet (VUV) light generated from a new type of excimer lamp to initiate the deposition of dielectric thin films in a photo-chemical vapor deposition process. Compared with other lamps, these pseudo-continuous light sources can provide high photon fluxes (more than a few watts) over large areas. The photo-deposited film properties were determined using the usual techniques of ellipsometry, FTIR spectroscopy, and electrical measurements. Good film quality was obtained making this technique highly attractive. A layered combination of silicon oxide, silicon nitride, and silicon oxynitride can be produced in the same reactor at temperatures below 300 degree(s)C. The technique also offers very good control of the stoichiometry in the case of Silicon oxynitride film deposition, and therefore provides interesting perspectives for optical applications.
As a `cold process' photochemical deposition enables us to obtain insulator and dielectric thin films on III-V compounds. To sustain the remarkable progress in the performance of electronic and optical devices may necessitate the utilization of this innocuous processing for novel applications. Possible areas of interest range from simply optically protective coating to layers for highly sophisticated optoelectronic integration of III-V and Si on the same chip.
To compensate for the loss of Pb during the preparation of BiSrCaCuO films we have deposited layers of PbO as part of a multilayer sandwich structure with BiPbSrCaCuO using the pulsed laser deposition technique. We have carefully optimized three parameters, namely the substrate temperature, post annealing temperature, and post annealing duration in order to produce the highest content of the 2223 phase in the films. By using a substrate temperature of 250 degree(s)C and ex-situ annealing at 854 degree(s)C for 15 hours in air we have produced highly c-axis oriented films which comprises some 95% 2223. These films typically exhibit Tc values of 105.5 K with associated critical current density of 2.5 X 104 A/Cm2 at 70 K.
The growth of large area and directly patterned oxides on silicon using a variety of photon wavelengths is described. In particular the drive towards low temperature and high quality ultrathin layers will be emphasised. Towards this goal we show that U. V. photons initiate more efficient growth than does visible radiation. 1.