Olivier Gaudin

Improved operational lifetime of semiconducting polymer lasers by encapsulation

We report polymer distributed feedback lasers with dramatically extended operational lifetimes by using a simple encapsulation process. The lasers are configured as surface emitting, two-dimensional distributed feedback lasers based on the polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene]. The microstructure is transferred to the polymer surface through solvent assisted micromolding. Once encapsulated, a 2500-fold improvement in lifetime is demonstrated under ambient conditions, compared to the unencapsulated device. A blueshift of the emission wavelength observed during operation is characterized by absorption and ellipsometry measurements and attributed to a change in effective index due to a loss of conjugation in the polymer.

Diamond photodetectors for next generation 157-nm deep-UV photolithography tools

Abstract Next generation photolithography stepper tools will operate at 157 nm and require robust solid state photodetectors to ensure efficient operation and facilitate direct beam monitoring for photoresist exposure dosimetry. There is currently no commercial detector system able to fully meet all the demanding requirements of this application. Diamond, which is intrinsically visible blind and radiation hard, is an obvious candidate for consideration. In this paper we report the results of the first study to assess the viability of thin film polycrystalline diamond photodetectors for use in 157 nm F 2 –He based laser lithography tools. Co-planar inter-digitated gold photoconductor structures were fabricated on free standing thin film diamond and exposed to pulses from an industrial F 2 –He laser in the fluence range 0–1.4 mJ cm −2 . The electrical and optical characteristics of the devices have been measured and are compared to the response of a standard vacuum photodiode. The suitability of the diamond devices for use at 157 nm is discussed.

Diamond photoconductors: operational lifetime and radiation hardness under deep-UV excimer laser irradiation

Abstract The first study of long term pulse exposure and fluence level on the performance of CVD diamond photodetectors subjected to 193 nm excimer laser radiation has been performed. Whilst diamond is considered ‘radiation hard’ it is shown that damage to detector performance can be provoked at laser fluence levels considerably below that required for graphitisation or ablation. However, the application of defect passivation treatments prior to device use acts to considerably reduce the damaging effect of the radiation, such that devices suitable for stable laser monioring applications can be realised.

Thickness dependent absorption spectra in conjugated polymers: Morphology or interference?

The thickness dependence of the absorption spectrum of spin-coated films of poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] has been studied using reflectivity and variable angle spectroscopic ellipsometry measurements. It is found that, for films with thicknesses in the range of 18–178 nm, a single set of optical constants is sufficient to simulate accurately all the experimental data used, including the absorption spectra, independently of the film thickness or the processing conditions. Thus, the observed changes in the absorption spectrum with thickness can be fully accounted for by reflectivity and interference effects alone without the need to invoke morphology differences between films.

Imaging deep UV light with diamond-based systems

Diamond grown by chemical vapour deposition (CVD) techniques has shown great promise for the fabrication of high sensitivity, low dark current, fast and visible-blind deep UV photodetectors. In addition to the careful choice of substrate material, defect passivation treatments applied to the diamond after growth have been found to considerably enhance the detector characteristics achieved. In this paper, we report on the first purposefully designed one-dimensional CVD diamond imaging array for the detection of nanosecond 193-nm excimer laser pulses using this approach. It is shown to perform extremely well, giving less than 2% pixel-to-pixel variation in signal response, and is fast enough to avoid any sign of charge build-up during prolonged operation.

An impedance spectroscopic study of n-type phosphorus-doped diamond

An important development in the field of diamond electronics has been the production of n-type electrical characteristics following homoepitaxial diamond growth on (111) diamond in the presence of phosphorus-containing gases. Several studies have reported that a phosphorus donor level forms with an activation energy in the range of 0.43–0.6eV; the ground state for the donor level is considered to be at 0.6eV. Little is currently known about other electrically active defects that may be produced alongside the donor state when phosphorus is introduced. In this paper we report upon the use of impedance spectroscopy, which can isolate the differing components that contribute to the overall conductivity of the film. In Cole-Cole plots, two semicircular responses are observed for all temperatures above 75°C; a single semicircle being seen at temperatures below this. The results suggest the presence of two conduction paths with activation energies of 0.53 and 0.197eV. The former can be attributed to the phosphor...

Diamond deep UV photodetectors: reducing charge decay times for 1-kHz operation

Abstract Diamond grown by chemical vapour deposition (CVD) methods is thought to be ideal for the fabrication of visible blind, fast deep UV photodetectors. However, careful device design and selection of high-quality CVD thin film diamond is, in itself, insufficient for the realisation of high performance devices. Post-growth device treatments are capable of transforming the optoelectronic properties of the material such that commercially interesting devices result. In the present study we have shown that sequentially applied methane–air treatments continue to modify both the gain level and speed of the device. Three such treatments give an optimal gain level, whilst more treatments than this lead to an improved turn-off speed. For the first time we have demonstrated the successful operation of a CVD diamond photoconductive device at at least 1 kHz at 193 nm, a frequency that is required for state-of-the-art excimer laser applications at this wavelength.

Electrical properties of phenylene vinylene oligomer thin films

In this work, we report the results of electrical investigations on phenylene vinylene oligomer thin films with 5-phenyl, 4-vinyl compound (4PV). Synthesis of the oligomer powder was performed using a Wittig reaction of phosphonium salt with carbonyl group and thin films were deposited by conventional thermal evaporation under vacuum of the obtained powder. The conduction mechanisms of oligomer-based diodes were studied by measuring the current-voltage-temperature characteristics and the thermally stimulated current. In the high-temperature range, Poole-Frenkel emission is observed while, at lower temperatures, a hopping process probably occurs. Data from the thermally stimulated current measurements involve shallow traps (0.42 eV) which might originate from the 4PV/metal interface region.

High-speed diamond photoconductors: a solution for high rep-rate deep-UV laser applications

Beam monitoring of excimer lasers operating at high powers in the deep ultra-violet (DUV) is becoming increasingly important, due to the rapid proliferation of these systems in micromachining, photolithography, and other areas of industrial interest. This task requires radiation-hard detectors able to operate effectively for extended periods at high laser rep-rates. DUV-visible-blind photoconductors can be fabricated on polycrystalline CVD diamond, a material that is intrinsically radiation-hard and visible-blind. However, the performance of detectors fabricated on as-grown material is insufficient to meet the requirements of many excimer laser applications. In this paper, we show that sequentially applied post-growth treatments can progressively change both the gain and speed of these devices. Charge-sensitive deep-level transient spectroscopy (Q-DLTS) and transient photoconductivity (TPC) has been used to study the effect of these treatments on the defect structure of our thin-film diamond detector material. For the first time, we report the successful operation of a diamond photoconductive device with linear bias and fluence-response characteristics at more than 1 kHz at 193 nm.

Deep level transient spectroscopy of CVD diamond : the observation of defect states in hydrogenated films

Abstract Hydrogenated polycrystalline CVD diamond films support a number of defect states within the range 0.03–1.0 eV, as determined by charge-based deep level transient spectroscopy (Q-DLTS). The observation of a 30-meV state is direct evidence for such a shallow level in this material, and is most likely to be the acceptor state that gives rise to the p-type character of these films. Deeper levels, at 0.11 eV, 0.39 eV, 0.65 eV and 0.70 eV–1.0 eV can also be observed and again appear to be associated with the hydrogenation level within the near surface region of the CVD diamond film. The loss of the 0.11 eV level at temperatures greater than 417 K is most easily explained if adsorbates are being removed from the surface at this temperature.