SwePub - search: WFRF:(Linnros Jan)

Enumeration	Reference	Cover	Find
1.	Bruhn, Benjamin, et al. (author) Blinking Statistics of Silicon Quantum Dots 2011 In: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 11:12, s. 5574-5580 Journal article (peer-reviewed)abstract The blinking statistics of numerous single silicon quantum dots fabricated by electron-beam lithography, plasma etching, and oxidation have been analyzed. Purely exponential on- and off-time distributions were found consistent with the absence of statistical aging. This is in contrast to blinking reports in the literature where power-law distributions prevail as well as observations of statistical aging in nanocrystal ensembles. A linear increase of the switching frequency with excitation power density indicates a domination of single-photon absorption processes, possibly through a direct transfer of charges to trap states without the need for a bimolecular Auger mechanism. Photoluminescence saturation with increasing excitation is not observed; however, there is a threshold in excitation (coinciding with a mean occupation of one exciton per nanocrystal) where a change from linear to square-root increase occurs. Finally, the statistics of blinking of single quantum dots in terms of average on-time, blinking frequency and blinking amplitude reveal large variations (several orders) without any significant correlation demonstrating the individual microscopic character of each quantum dot.
2.	Bruhn, Benjamin, et al. (author) Controlled fabrication of individual silicon quantum rods yielding high intensity, polarized light emission 2009 In: Nanotechnology. - : Institute of Physics Publishing (IOPP). - 0957-4484 .- 1361-6528. ; 20:50, s. 1-5 Journal article (peer-reviewed)abstract Elongated silicon quantum dots (also referred to as rods) were fabricated using a lithographic process which reliably yields sufficient numbers of emitters. These quantum rods are perfectly aligned and the vast majority are spatially separated well enough to enable single-dot spectroscopy. Not only do they exhibit extraordinarily high linear polarization with respect to both absorption and emission, but the silicon rods also appear to luminesce much more brightly than their spherical counterparts. Significantly increased quantum efficiency and almost unity degree of linear polarization render these quantum rods perfect candidates for numerous applications.
3.	Bruhn, Benjamin, et al. (author) Transition from silicon nanowires to isolated quantum dots : Optical and structural evolution 2013 In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87:4, s. 045404- Journal article (peer-reviewed)abstract The evolution of the structural and optical properties of a silicon core in oxidized nanowalls is investigated as a function of oxidation time. The same individual nanostructures are characterized after every oxidation step in a scanning electron microscope and by low-temperature photoluminescence, while a representative sample is also imaged in a transmission electron microscope. Analysis of a large number of recorded single-dot spectra and micrographs allows to identify delocalized and localized exciton emission from a nanowire as well as confined exciton emission of a nanocrystal. It is shown how structural transitions from one-to zero-dimensional confinement affect single-nanostructure optical fingerprints.
4.	Bruhn, Benjamin, et al. (author) Transition fromsilicon nanowires to isolated quantum dots : Optical and structural evolution 2009 Other publication (pop. science, debate, etc.)
5.	Janda, Petr, et al. (author) Modified spontaneous emission of silicon nanocrystals embedded in artificial opals 2007 In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 40:19, s. 5847-5853 Journal article (peer-reviewed)abstract Si nanocrystals (NCs) were embedded in synthetic silica opals by means of Si-ion implantation or opal impregnation with porous-Si suspensions. In both types of sample photoluminescence (PL) is strongly Bragg-reflection attenuated (up to 75%) at the frequency of the opal stop-band in a direction perpendicular to the ( 1 1 1) face of the perfect hcp opal structure. Time-resolved PL shows a rich distribution of decay rates, which contains both shorter and longer decay components compared with the ordinary stretched exponential decay of Si NCs. This effect reflects changes in the spontaneous emission rate of Si NCs due to variations in the local density of states of real opal containing defects.
6.	Sychugov, Ilya, et al. (author) Light emission from silicon nanocrystals: probing a single quantum do 2006 In: Applied Surface Science. - : Elsevier. - 0169-4332 .- 1873-5584. ; 252:15, s. 5249-5253 Journal article (peer-reviewed)abstract Analysis of low-temperature photoluminescence measurements performed on single silicon nanocrystals is presented. The luminescence emission linewidth of Si nanocrystals is found to be less than thermal broadening at low temperature, confirming the atomic-like nature of their energetic states. Beside the main peak the low-temperature spectra reveal a similar to 6 meV replica, the origin of which is discussed. For some of the investigated dots, we also observe a similar to 60 meV transverse optical (TO) phonon replica. The regular arrangement of individual nanocrystals used in this work enables combined high-resolution transmission electron microscopy (TEM) and low-temperature photoluminescence characterization of the same single quantum dot.
7.	Sychugov, Ilya, et al. (author) Luminescence blinking of a Si quantum dot in a SiO2 shell 2005 In: Physical Review B. Condensed Matter and Materials Physics. - : The American Physical Society. - 1098-0121 .- 1550-235X. ; 71:11, s. 115331-1-115331-5 Journal article (peer-reviewed)abstract The phenomenon of on-off luminescence intermittency - blinking - in silicon nanocrystals was studied using a single-dot microphotoluminescence technique. From recordings of the luminescence intensity trace, on- and off-time distributions were extracted revealing exponential behavior, as expected for systems with blinking of a purely random nature. The corresponding switching rates for on-off and off-on processes exhibit different dependence on the excitation intensity. While the on-off switching rate grows quadratically with the excitation, the inverse process is nearly pumping power independent. Experimental findings are interpreted in terms of a dot "charging" model, where a carrier may become trapped in the surrounding matrix due to thermal and Auger-assisted processes. Observed blinking kinetics appear to be different from that of porous silicon particles.
8.	Sychugov, Ilya, et al. (author) Narrow luminescence linewidth of a silicon quantum dot 2005 In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 94:8, s. 087405 (1)-087405 (4) Journal article (peer-reviewed)abstract Single-dot luminescence spectroscopy was used to study the emission linewidth of individual silicon nanocrystals from low temperatures up to room temperature. The results show a continuous line narrowing towards lower temperatures with a linewidth as sharp as 2 meV at 35 K. This value, clearly below the thermal broadening at this temperature, proves the atomiclike emission from silicon quantum dots subject to quantum confinement. The low temperature measurements further reveal a similar to6 meV replica, whose origin is discussed. In addition, an similar to60 meV TO-phonon replica was detected, which is only present in a fraction of the dots.
9.	Sychugov, Ilya, et al. (author) Single dot optical spectroscopy of silicon nanocrystals: Low temperature measurements 2005 In: Optical materials (Amsterdam). - : Elsevier BV. - 0925-3467 .- 1873-1252. ; 27:5, s. 973-976 Journal article (peer-reviewed)abstract Single dot spectroscopy allows studying properties of a single nanocrystal avoiding inhomogeneous broadening of the emission band. Here, data obtained by this technique for Si nanocrystals fabricated by electron beam lithography, plasma etching and subsequent size-reduction by oxidation are presented. First, blinking (on–off intermittence) of the luminescence was observed for most individual nanocrystals, although some exhibited relatively stable luminescence. As a result of the quantum confinement effect spectra with different emission wavelengths for different nanocrystals were recorded. While at room temperature the full width at half-maximum of the nanocrystal emission peaks was measured to be 100–150 meV, at 80 K the linewidth for some dots appeared to be about 25 meV only. The observed temperature dependence of the homogeneous linewidth may lead to an understanding of the exciton–phonon interaction in indirect band-gap quantum dots.
10.	Valenta, Jan, et al. (author) Coexistence of 1D and Quasi-0D Photoluminescence from Single Silicon Nanowires 2011 In: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 11:7, s. 3003-3009 Journal article (peer-reviewed)abstract Single silicon nanowires (Si-NWs) prepared by electron-beam lithography and reactive-ion etching are investigated by imaging optical spectroscopy under variable temperatures and laser pumping intensities. Spectral images of individual Si-NWs reveal a large variability of photoluminescence (PL) along a single Si-NW. The weaker broad emission band asymmetrically extended to the high-energy side is interpreted to be due to recombination of quasi-free 1D excitons while the brighter localized emission features (with significantly variable peak position, width, and shape) are due to localization of electron hole pairs in surface protrusions acting like quasi-0D centers or quantum dots (QDs). Correlated PL and scanning electron microscopy images indicate that the efficiently emitting QDs are located at the Si-NW interface with completely oxidized neck of the initial Si wall. Theoretical fitting of the delocalized PL emission band explains its broad asymmetrical band to be due to the Gaussian size distribution of the Si-NW diameter and reveals also the presence of recombination from the Si-NW excited state which can facilitate a fast capture of excitons into QD centers.
11.	Valenta, Jan, et al. (author) Electroluminescence of single silicon nanocrystals 2004 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 84:9, s. 1459-1461 Journal article (peer-reviewed)abstract We report on measurements of room-temperature electroluminescence from single silicon nanocrystals. The electrically driven emission reveals typical characteristics of single-nanocrystal luminescence: the peak wavelength variations, narrowing of spectral bands, a high degree of linear polarization, and intensity fluctuations (blinking) observed on a scale of minutes. From the count rate statistics of individual nanocrystals, we conclude that the yield of radiative emission is as high as 19%. These findings may open a route to highly efficient all-silicon light emitters.
12.	Valenta, Jan, et al. (author) Light-Emission Performance of Silicon Nanocrystals Deduced from Single Quantum Dot Spectroscopy 2008 In: Advanced Functional Materials. - : Wiley. - 1616-301X .- 1616-3028. ; 18:18, s. 2666-2672 Journal article (peer-reviewed)abstract Spectra of individual silicon nanocrystals within porous Si grains are studied by the wide-field imaging microspectroscopy and their ON-OFF, blinking is detected by the confocal single-photon-counting microscopy. Observed spectral and blinking properties comprise all features reported before in differently prepared single Si nanocrystals (SiNCs). Former apparently contradictory results are shown to be due to different experimental conditions. When the effect of dark periods (OFF switching) is removed the common ultimate photoluminescence properties Of SiO2 passivated SiNCs are found, namely the quantum efficiency (QE) of about 10-20% up to the pumping rate corresponding to one exciton average excitation per quantum dot. At higher pump rates the QE is slowly decreasing as the 0.7th power of excitation. This is most likely due to Auger recombination which, however, seems to be weakened compared with measurements of nanocrystal assemblies. We conclude that SiNCs may be pumped above one exciton occupancy to yield a higher light emission, being advantageous for applications.
13.	Valenta, Jan, et al. (author) Photoluminescence spectroscopy of single silicon quantum dots 2002 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 80:6, s. 1070-1072 Journal article (peer-reviewed)abstract Photoluminescence (PL) from single silicon quantum dots have been recorded and spectrally resolved at room temperature. The Si nanocrystals (NCs) were fabricated using electron-beam lithography and reactive ion etching resulting in Si nanopillars that were subsequently oxidized to produce luminescent silicon cores. The NCs are organized in a regular matrix which enables repeated observation of a specific single NC. By reflection and PL imaging, the emission is shown to originate from the Si nanopillars. The single-NC PL spectrum has a single band with a width of similar to130 meV. The emission is polarized in arbitrary directions suggestive of geometrical differences in the shape of the nanocrystals. The quantum efficiency of the PL has been found to reach as much as 35% for some nanocrystals. Our experiments support the quantum-confinement model for the PL emission of Si nanocrystals and elucidate the critical role of defect passivation.
14.	Valenta, Jan, et al. (author) Polarization of photoluminescence excitation and emission spectra of silicon nanorods within single Si/SiO2 nanowires 2011 In: Physica Status Solidi. C, Current topics in solid state physics. - : Wiley-VCH Verlagsgesellschaft. - 1610-1634 .- 1610-1642. ; 8:3, s. 1017-1020 Journal article (peer-reviewed)abstract Polarization properties of individual silicon nanowires are studied using an optical micro-spectroscopy setup equipped with a Fresnel rhomb to rotate the polarization of the exciting laser and the analyzer to characterize the polarization of emitted photoluminescence. The Si nanowire samples are prepared by electron-beam lithography, plasma etching and oxidation. The fabricated wires are embedded in SiO2 and oriented parallel to the Si substrate. Due to the fluctuating wire diameter (around 5 nm) the very long wires (several tens of μm) are effectively divided into an array of quantum rods (prolate ellipsoids). These structures have strong photoluminescence under UV-blue excitation at room temperature. The degree of photoluminescence linear polarization of both excitation and emission is very high, between 0.9-1, and reveals relatively low fluctuations at different spots of the wires. Experimental results are compared with available theoretical models leading to the conclusion that the high polarization degree is mostly due to surface charges (dielectric confinement) with smaller contribution of quantum confinement effects.
15.	Afrasiabi, Roodabeh, et al. (author) Effect of microwave-assisted silanization on sensing properties of silicon nanoribbon FETs 2015 In: Sensors and actuators. B, Chemical. - : Elsevier B.V.. - 0925-4005 .- 1873-3077. ; 209, s. 586-595 Journal article (peer-reviewed)abstract An important concern with using silicon nanoribbon field-effect transistors (SiNR FET) for ion-sensing is the pH-response of the gate oxide surface. Depending on the application of the FET sensor, this response has to be chemically manipulated. Thus in silicon oxide-gated pH-sensors with integrated sensor and reference FETS, a surface with high pH-sensitivity, compared to the bare gate oxide, is required in the sensor FETs (SEFET), whereas in the reference FETs (REFET) the surface has to be relatively pH-insensitive. In order to control the sensitivity and chemistry of the oxide surface of the nanoribbons, a silanization reagent with a functional group is often self-assembled on the SiNR surface. Choice of a silanization reaction that results in a self-assembled layer on a silicon oxide surface has been studied extensively over the past decades. However, the effect of various self-assembled layers such as monolayers or mixed layers on the electrical response of SiNR FETs in aqueous solution needs to be exploited further, especially for future integrated SEFET/REFET systems. In this work, we have performed a comprehensive study on 3-aminopropyltriethoxysilane (APTES) silanization of silicon oxide surfaces using microwave (MW) heating as a new biocompatible route to conventional methods. A set of complementary surface characterization techniques (ellipsometry, AFM and ATR-FTIR) was used to analyze the properties of the APTES layer deposited on the silicon surface. We have found that a uniform monolayer can be achieved within 10 min by heating the silanization solution to 75 °C using MW heating. Furthermore, electrical measurements suggest that little change in device performance is observed after exposure to MW irradiation. Real-time pH measurements indicate that a uniform APTES monolayer not only reduces the pH sensitivity of SiNR FET by passivating the surface silanol groups, but also makes the device less sensitive to cation concentration in the background electrolyte. Our silanization route proves promising for future chemical surface modification of on-chip REFETs.
16.	Afrasiabi, Roodabeh, et al. (author) Integration of a droplet-based microfluidic system and silicon nanoribbon FET sensor 2016 In: Micromachines. - : MDPI AG. - 2072-666X. ; 7:8 Journal article (peer-reviewed)abstract We present a novel microfluidic system that integrates droplet microfluidics with a silicon nanoribbon field-effect transistor (SiNR FET), and utilize this integrated system to sense differences in pH. The device allows for selective droplet transfer to a continuous water phase, actuated by dielectrophoresis, and subsequent detection of the pH level in the retrieved droplets by SiNR FETs on an electrical sensor chip. The integrated microfluidic system demonstrates a label-free detection method for droplet microfluidics, presenting an alternative to optical fluorescence detection. In this work, we were able to differentiate between droplet trains of one pH-unit difference. The pH-based detection method in our integrated system has the potential to be utilized in the detection of biochemical reactions that induce a pH-shift in the droplets.
17.	Afrasiabi, Roodabeh, et al. (author) Microwave-assisted silanization of SiNW-FET : characterization and effect on sensing properties Other publication (other academic/artistic)
18.	Afrasiabi, Roodabeh (author) Silicon Nanoribbon FET Sensors : Fabrication, Surface Modification and Microfluidic Integration 2016 Doctoral thesis (other academic/artistic)abstract Over the past decade, the field of medical diagnostics has seen an incredible amount of research towards the integration of one-dimensional nanostructures such as carbon nanotubes, metallic and semiconducting nanowires and nanoribbons for a variety of bio-applications. Among the mentioned one-dimensional structures, silicon nanoribbon (SiNR) field-effect transistors (FET) as electro-chemical nanosensors hold particular promise for label-free, real-time and sensitive detection of biomolecules using affinity-based detection. In SiNR FET sensors, electrical transport is primarily along the nanoribbon axis in a thin sheet (< 30 nm) serving as the channel. High sensitivity is achieved because of the large surface-to-volume ratio which allows analytes to bind anywhere along the NR affecting the entire conductivity by their surface charge. Unfortunately, sensitivity without selectivity is still an ongoing issue and this thesis aims at addressing the detection challenges and further proposing effective developments, such as parallel and multiple detection through using individually functionalized SiNRs.We present here a comprehensive study on design, fabrication, operation and device performance parameters for the next generation of SiNR FET sensors towards multiplexed, label-free detection of biomolecules using an on-chip microfluidic layer which is based on a highly cross-linked epoxy. We first study the sensitivity of different NR dimensions followed by analysis of the drift and hysteresis effects. We have also addressed two types of gate oxides (namely SiO2 and Al2O3) which are commonly used in standard CMOS fabrication of ISFETs (Ion sensitive FET). Not only have we studied and compared the hysteresis and response-time effects in the mentioned two types of oxides but we have also suggested a new integrated on-chip reference nanoribbon/microfluidics combination to monitor the long-term drift in the SiNR FET nanosensors. Our results show that compared to Al2O3, silicon-oxide gated SiNR FET sensors show high hysteresis and slow-response which limit their performance only to background electrolytes with low ionic strength. Al2O3 on the other hand proves more promising as the gate-oxide of choice for use in nanosensors. We have also illustrated that the new integrated sensor NR/Reference NR can be utilized for real-time monitoring of the above studied sources of error during pH-sensing. Furthermore, we have introduced a new surface silanization (using 3-aminopropyltriethoxysilane) method utilizing microwave-assisted heating which compared to conventional heating, yields an amino-terminated monolayer with high surface coverage on the oxide surface of the nanoribbons. A highly uniform and dense monolayer not only reduces the pH sensitivity of the bare-silicon oxide surface in a physiological media but also allows for more receptors to be immobilized on the surface. Protocols for surface functionalization and biomolecule immobilization were evaluated using model systems. Selective spotting of receptor molecules can be used to achieve localized functionalization of individual SiNRs, opening up opportunities for multiplexed detection of analytes.Additionally, we present here a novel approach by integrating droplet-based microfluidics with the SiNR FET sensors. Using the new system we are able to successfully detect trains of droplets with various pH values. The integrated system enables a wide range of label-free biochemical and macromolecule sensing applications based on detection of biological events such as enzyme-substrate interactions within the droplets.
19.	Andersson, Henrik, 1975- (author) Position Sensitive Detectors : Device Technology and Applications in Spectroscopy 2008 Doctoral thesis (other academic/artistic)abstract This thesis deals with the development, processing and characterization of position sensitive detectors and, in addition, to the development of compact and cost effective spectrometers. Position sensitive detectors are used to measure, with great accuracy and speed, the position of a light spot incident on the surface. Their main use is for triangulation, displacement and vibration measurements. A type of position sensitive detector based on the MOS principle and using optically transparent indium tin oxide as a gate contact has been developed. This type of detector utilizes the MOS principle where an induced channel forms beneath the gate oxide in the surface of the Silicon substrate. One and two dimensional detectors have both been fabricated and characterized. The first measurements showed that the linearity did not fulfil expectations and it was suspected that stress induced by the gate contact could be the reason for the seemingly high nonlinearity. Further investigations into both the p-n junction and the MOS type position sensitive detectors lead to the conclusion that the indium tin oxide gate is responsible for inducing a substantial stress in the surface of the detector, thus giving rise to increased position nonlinearity. The heat treatment step which was conducted was determined to be critical as either a too short or too long heat treatment resulted in stress in the gate and channel leading to position nonlinearity. If a correctly timed heat treatment is performed then the detector’s linearity is in parity with the best commercial position sensitive detectors. In addition, the development of very small, compact and cost effective spectrometers has been performed with the aim of constructing devices for use in the process industry. The development of a wedge shaped array of Fabry-Perot interferometers that can be mounted directly on top of a detector makes it possible to construct a very compact spectrometer using the minimum amount of optics. This wedge interferometer has been evaluated by means of array pixel detectors and position sensitive detectors for both the infrared and the visible wavelength ranges. When used with a position sensitive detector it is necessary to use a slit to record the intensity of the interferogram for many points over the detector, equivalent to pixels on an array detector. Usually the use of moving parts in a spectrometer will impose the use of high precision scanning mechanisms and calibration. By using a position sensitive detector for the interferogram readout both the position and the intensity are known for every measurement point and thus the demands placed on the scanning system are minimized.
20.	Badel, Xavier, et al. (author) Doping of electrochemically etched pore arrays in n-type silicon : processing and electrical characterization 2005 In: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 152:4, s. G252-G258 Journal article (peer-reviewed)abstract Silicon p-n diodes formed in the walls of deep pores have been electrically characterized. The pores were electrochemically etched in low-doped n-type silicon substrates, and the entire pore array was doped p(+) by boron diffusion at 1050 degrees C. Two different process flows were investigated to disconnect the p(+) layers from one pore to another. The first consists of removing a few micrometers of silicon at the top of the sample using reactive ion etching after diffusion while the second enables the prevention of doping at the top of the pore walls with an oxide, acting as a barrier during diffusion. Current-voltage and capacitance-voltage characteristics of p-n junctions are presented and related parameters, such as the serial resistance and the ideality factor are discussed. The results show good rectifying behavior of the diodes with a reverse current about four to five decades smaller than the forward current. Measurements with two pores connected in a transistor-like configuration (p(+)/n(-)/p(+)), were also performed. Device simulations were used to examine the device behavior. Finally, our results demonstrate that pores could work as individual detector pixels for moderate reverse voltages, suitable for radiation imaging applications.
21.	Badel, Xavier, et al. (author) Electrochemical etching of n-type silicon based on carrier injection from a back side p-n junction 2003 In: Electrochemical and solid-state letters. - : The Electrochemical Society. - 1099-0062 .- 1944-8775. ; 6:6, s. C79-C81 Journal article (peer-reviewed)abstract A technique for electrochemical etching of n-type silicon in aqueous hydrofluoric acid is presented. This technique differs from photoelectrochemical etching because the holes (positive carriers) needed for the dissolution reaction to occur, are not photogenerated. The principle developed here is to inject these positive carriers using a p-n junction under forward bias formed at the back side of the sample. Drift-diffusion of holes through the wafer thickness allows a chemical dissolution reaction at the interface with the electrolyte. To enable holes diffusing through the wafer the minority carrier lifetime must be sufficiently high making the technique well adapted for high resistivity silicon. However, extension to low resistivity wafers has been achieved. Results show the possibility of forming pore arrays and diverse 3D structures.
22.	Badel, Xavier, 1977- (author) Electrochemically etched pore arrays in silicon for X-ray imaging detectors 2005 Doctoral thesis (other academic/artistic)abstract Digital devices have now been introduced in many X-ray imaging applications, replacing slowly traditional photographic films. These devices are preferred as they offer real time imaging, easy handling and fast treatment of the images. However, the performance of the detectors still have to be improved in order to increase the image quality, and possibly reduce the X-ray dose, a vital parameter for medical use. In this thesis, three different new detector concepts have been investigated. All designs use pore arrays, which are ideal starting structures to form pixellated detectors. Electrochemical etching of n-type silicon in aqueous hydrofluoric acid solution (HF) has been studied to form these pore arrays. A broad range of pores have been fabricated with diameters varying from 200 nm to 40 µm and with depths reaching almost the wafer thickness, thus leading to very high aspect ratios. The technique was also found to be suitable for the formation of other types of structures such as pillars and tubes on the sub micrometer scale. The etching is based on the dissolution of silicon in HF under anodic bias and a supply of positive electrical carriers (holes). As holes are the minority carriers in n-type silicon, they are usually photo-generated. In this work an alternative technique, based on hole injection from a forward-biased pn junction, has been successfully pioneered. The first X-ray imaging detector concept presented in the thesis consists of a silicon charge coupled device (CCD) in proximity with a scintillating screen. The screen is made from a pore array having reflective pore walls and filled with CsI(Tl), emitting photons at a wavelength of 550 nm under X-ray exposure. The secondary emitted photons are light-guided by the pore walls and then detected by the CCD pixels. Detectors were fully fabricated and characterized. This concept provides good spatial resolution with negligible cross talk between adjacent pixels. The dependences of the detector efficiency on pore depth and on the coating of the pore walls are presented. Although most of the produced detectors had a detective quantum efficiency of about 25%, some detectors indicate that efficient scintillating screens can be achieved approaching the theoretical limit as set by poissonian statistics of the X-ray photons. The two other detector designs require the formation of vertical pn junctions, i.e. in the pore walls. In one concept the secondary emitted photons are detected by photodiodes located in the pore walls. This would lead to high charge collection efficiency as the photons do not have to be guided to one end of the pore. However, high noise due to the direct detection of X-rays in the diodes is expected. The other concept is based on generation of electron-hole pairs in a semiconductor and the ‘3D’ detector, where an array of vertical electrodes is used to separate the charges via an electric field. To uniformly dope the inside of deep pores, both boron diffusion and low-pressure chemical vapor diffusion of boron-doped poly-silicon were shown to be successful techniques. This was confirmed by SIMS profiles taken through the pore wall thickness. Finally, the possibility to form individual junction in each pore was shown. The diodes were electrically characterized, demonstrating good rectifying behavior and sensitivity to light.
23.	Badel, Xavier, et al. (author) Formation of ordered pore arrays at the nanoscale by electrochemical etching of n-type silicon 2004 In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 36:1/3, s. 245-254 Journal article (peer-reviewed)abstract Electrochemical etching has been studied to structure n-type silicon substrates at the nanoscale. In this work, well-ordered pore arrays with diameters in the range of 150-500 nm and depths up to 50 mum have been fabricated. The pores were successfully formed by anodic etching in (100)oriented n-type silicon wafers of low-resistivity, typically 1 Omegacm, using aqueous hydrofluoric acid solutions. The lithographic step was performed in a thermally grown oxide using a stepper and dry oxide etching technique. Two types of oxide openings and pitch sizes were tested. The smallest oxide opening realised at this stage was 0.5 mum for a pitch of 1 mum. Stable pore formation was obtained and the smallest pore size obtained was about 200 nm with an aspect ratio close to 100.
24.	Badel, Xavier, et al. (author) Formation of pn junctions in deep silicon pores for X-ray imaging detector applications 2003 In: Nuclear Instruments and Methods in Physics Research Section A. - 0168-9002 .- 1872-9576. ; 509:1-3, s. 96-101 Journal article (peer-reviewed)abstract The formation of pn junctions in deep silicon pores has been studied for a new concept of X-ray imaging detectors. The sensitive part of the device is an array of CsI(Tl) columns formed by filling a silicon matrix of pores having pn junctions in their walls. Under X-ray illumination, the CsI(TI) scintillator emits photons that are collected by the pn junctions. Relatively high signal collection efficiency is expected. However, the formation of pn junctions inside pore walls represents a challenging step in the detector fabrication. In this work pore matrices were fabricated in n-type silicon by deep reactive ion etching and by photo-electrochemical etching. The pn junctions were formed either by boron diffusion or deposition of boron doped poly-silicon. Various techniques were used to analyze the transverse depth profiles of boron atoms at different pore depths. The study shows successful results for pn-junctions formed both by diffusion and by poly-silicon deposition.
25.	Badel, Xavier, et al. (author) Improvement of an X-ray imaging detector based on a scintillating guides screen 2002 In: Nuclear Instruments and Methods in Physics Research Section A. - 0168-9002 .- 1872-9576. ; 487:1-2, s. 129-135 Journal article (peer-reviewed)abstract An X-ray imaging detector has been developed for dental applications. The principle of this detector is based on application of a silicon charge coupled device covered by a scintillating wave-guide screen. Previous studies of such a detector showed promising results concerning the spatial resolution but low performance in terms of signal to noise ratio (SNR) and sensitivity. Recent results confirm the wave-guiding properties of the matrix and show improvement of the detector in terms of response uniformity, sensitivity and SNR. The present study is focussed on the fabrication of the scintillating screen where the principal idea is to fill a matrix of Si pores with a CsI scintillator. The photoluminescence technique was used to prove the wave-guiding property of the matrix and to inspect the filling uniformity of the pores. The final detector was characterized by X-ray evaluation in terms of spatial resolution, light output and SNR. A sensor with a spatial resolution of 9 LP/mm and a SNR over 50 has been achieved using a standard dental X-ray source and doses in the order of those used at the moment by dentists (around 25 mR).

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