G. Lavareda

THIN FILM POSITION SENSITIVE DETECTOR BASED ON AMORPHOUS SILICON P-I-N DIODE

The application of hydrogenated amorphous silicon (a–Si:H) to optoelectronic devices are now well established as a viable low cost technology and is presently receiving much interest. Taking advantage of the properties of a–Si:H based devices, single and dual axis large area (up to 80×80 mm2) thin film position sensitive detectors (TFPSD) based on a–Si:H p–i–n diodes have been developed, produced by plasma enhanced chemical vapor deposition. In this study, the main optoelectronic properties presented by the TFPSD as well as their behavior under operation conditions, concerning its linearity and signal to noise ratio, are reported.

LARGE-AREA 1D THIN-FILM POSITION-SENSITIVE DETECTOR WITH HIGH DETECTION RESOLUTION

Abstract The aim of this work is to present the main optoelectronic characteristics of large-area one-dimensional position-sensitive detectors (1D TFPSDs) based on amorphous silicon (a-Si) p-i-n diodes. From that, the device resolution, response time and detectivity (defined as being the reciprocal of the noise equivalent power pattern) are derived and discussed concerning the field of applications of the 1D TFPSDs.

Material properties, project design rules and performances of single and dual-axis a-Si:H large area position sensitive detectors

Abstract We have developed large area (up to 80mm×80mm) Thin Film Position Sensitive Detectors (TFPSD) based on hydrogenated amorphous silicon (a-Si:H). Although crystalline silicon PSDs have been realized and applied to optical systems, their detection area is small (less than 10mm×10mm), which implies the need of optical magnification systems for supporting their field of applications towards large area inspection systems, which does not happen by using a-Si:H devices. The key factors for the TFPSDs resolution are the thickness uniformity of the constituting layers, the geometry and the position of the contacts. In this paper we present data on single and dual-axis rectangular TFPSDs correlating, their performances with the different underlying lateral effects. For the single axis-detector, with two opposite extended contacts, the output photocurrent difference to sum ratio is a linear function of the position of a narrow incident light beam, even for low illumination levels (below 20 lux). For the dual-axis detector with extended contacts, at all four sides (except for small gaps at the vertices due to edge effects) an almost linear relation has been found between the incident light spot position along both axis and the corresponding output photocurrents.

Effect of rf power on the properties of ITO thin films deposited by plasma enhanced reactive thermal evaporation on unheated polymer substrates

Abstract The influence of the radiofrequency (rf) deposition power on the properties of indium tin oxide (ITO) thin films deposited onto unheated polymer substrates is studied. The deposition technique used is the radiofrequency plasma enhanced reactive thermal evaporation (rf-PERTE). Results show that (1) the total transmittance at 500 nm is always around 80% and (2) that the electrical resistivity is 10−3 Ω cm for ITO films deposited in the rf power range 45–50 W. X-ray diffraction analysis showed the presence of a crystalline phase. Scanning electron microscopy (SEM) revealed that the surface morphology of the ITO films does not vary significantly with rf power.

Large Area Position Sensitive Detector Based on Amorphous Silicon Technology

We have developed a rectangular dual-axis large area Position Sensitive Detector (PSD), with 5 cm × 5 cm detection area, based on PIN hydrogenated amorphous silicon (a-Si:H) technology, produced by Plasma Enhanced Chemical Vapor Deposition (PECVD). The metal contacts are located in the four edges of the detected area, two of them located on the back side of the ITO/PIN/A1 structure and the others two located in the front side. The key factors of the detectors resolution and linearity are the thickness uniformity of the different layers, the geometry and the contacts location. Besides that, edge effects on the sensors corner disturb the linearity of the detector. In this paper we present results concerning the linearity of the detector as well as its optoelectronic characteristics and the role of the i-layer thickness on the final sensor performances.

Ultrasensitive microchip sensor based on boron-containing polyfluorene nanofilms

A fluorene-based π-conjugated copolymer with on-chain dibenzoborole units was used in the development of a nanocoated gold interdigitated microelectrode array device which successfully detects fluoride in a broad range of concentrations (10(-11)-10(-4) M) in aqueous solution, upon impedance spectroscopy measurements. A calibration curve obtained over this range of concentrations and a new analytical method based on impedance spectroscopy measurements in aqueous solution is proposed. The sensor nanofilm was produced by spin-coating and diagnosed via spectroscopic ellipsometry, AFM, and electrically conductivity techniques. Changes in the conductivity due to the boron-fluoride complex formation seem to be the major mechanism behind the dependence of impedimetric results on the fluoride concentration.

Dependence of TFT performance on the dielectric characteristics

The study of thin film transistors (TFTs) and MIS structures using a variety of amorphous silicon nitride thin films (a-SiN x :H) as the gate dielectric is presented. These nitride films were deposited with N/Si ratios between 0.64 and 1.74, in order to evaluate the dependence of the TFT performance near the stoichiometric a-SiN x :H composition (1.33). These films were produced in a magnetically confined PECVD system. A set of samples was prepared for capacitance measurements (e r determination), FTIR measurements (Si-H and N-H bonds relative abundance), Rutherford backscattering spectrometry and elastic recoil detection measurements (elemental concentration and density calculation). TFT results show a close relationship between the field effect mobility (μ FE and the hydrogen concentration in the nitride films. Also, nitride films with the highest densities lead to superior electrical performance. These films are over-stoichiometric (N-rich), showing that, unlike CVD thermal nitride, the best a-SiN x :H are not necessarily stoichiometric. TFTs with N-poor a-SiN , ( : H films present a high degree of 1(V) hysteresis due to charge trapping on the a-Si:H/a-SiN x :H interface. This effect is also confirmed by C(V) measurements performed on MIS structures.

Effect of thickness on the properties of ITO thin films deposited by RF- PERTE on unheated, flexible, transparent substrates

Abstract The influence of thickness on the main properties of indium tin oxide (ITO) thin films deposited onto flexible, transparent polymer substrates at room temperature was studied. The deposition technique used was radio-frequency (RF) plasma-enhanced reactive thermal evaporation (RF-PERTE) of a 90% In–10% Sn alloy in the presence of oxygen. Results show that (1) the total transmittance is always approximately 80% at a wavelength of 500 nm when the thickness of the ITO films exceeds 70 nm and (2) ITO thin films with electrical resistivity of 6.2×10 −3 Ω cm, free carrier mobility of approximately 1.2 cm 2 V −1 s −1 and free carrier concentration of approximately 8.6×10 20 cm −3 are obtained, for films 100 nm thick. SEM investigations revealed that the surface morphology of the growing ITO thin film is dominated by the surface features of the flexible, transparent polymer substrate in the micron and submicron ranges, and does not vary significantly with thickness.

Influence of the initial layers on the optical and electrical properties of ITO films

Abstract The influence of early stage growth structure of indium tin oxide (ITO) thin films, deposited by reactive thermal evaporation (RTE) on optical and electrical properties, was studied by atomic force microscopy (AFM) measurements. ITO thin films were deposited at T s =440 K from an In:Sn alloy in the presence of added oxygen with (series A ), and without (series B ) a shutter between the substrate and the crucible. In series A , the presence of crystallisation seeds smaller than 20 nm was observed in the initial stages of growth. The mean grain size value and the number of grains per aggregate increased with film thickness reaching, respectively, 50 nm and 8 for t =100 nm. In series B , the initial stages show the presence of individual features with characteristics dimensions of up to 100 nm scattered on the surface. With increasing thickness ITO thin films of series B appear to grow similarly to series A . In series A , both transmittance and sheet-resistance were improved.

Properties of high growth rate amorphous silicon deposited by MC-RF-PECVD

Hydrogenated amorphous silicon (a-Si : H) thin films have been deposited on glass and crystalline silicon substrates by magnetically confined RF-PECVD (MC-RF-PECVD) at different RF power densities in order to verify the influence of this deposition parameter on the density of states (DOS) and growth rate (RG). It was found that the highest growth rate, 7.8 ( A is obtained for a-Si : H films deposited with an RF power density of 14.3 mW/cm 3 . For the DOS calculation, constant photocurrent method (CPM) data have been used. The lowest value of DOS is approximately 8 � 10 15 /eV/cm 3 and was obtained for a-Si : H films produced with an RF power density in the range of 10–20 mW/cm 3 . Infrared spectroscopy shows that when the RF power density increases, the concentration of SiH3 groups decreases and the concentration of SiH groups increases. At 8 mW/cm 3 , a maximum of the SiH2 concentration is obtained. At this point, a maximum of the optical gap (1.9 eV) is observed and a minimum of the dark conductivity is verified. We conclude that the best films are achieved in an RF power density range (7.1–21.4 mW/cm 3 ) for which an increase of SiH and a decrease of both SiH2 and SiH3 are simultaneously obtained. Thereafter, for higher power densities, an inversion of DOS and growth rate behaviour are observed due to ion bombardment. r 2002 Elsevier Science Ltd. All rights reserved.