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| 2. |
- Badel, Xavier
(författare)
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Development of macropore arrays in silicon and related technologies for X-ray imaging applications
- 2003
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Digital devices have started to replace photographic film inX-ray imaging applications. As compared to photographic films,these devices are more convenient to obtain images and tohandle, treat and store these images. The goal of the presentstudy is to develop macropore arrays and related silicontechnologies in order to fabricate X-ray imaging detectors formedical applications, and in particular for dentistry. Althougha few detectors are already available on the market, theirperformances, such as the X-ray sensitivity, can still beimproved. In addition, the image quality, defined by thespatial resolution and the signalto- noise ratio (SNR), shouldbe sufficiently high to enable diagnosis and, as regard to thepatient health, the X-ray dose should be reduced to aminimum.Three detector concepts were investigated. All of themrequire the formation of a macropore array in silicon as afirst step in the detector fabrication. Even though deepreactive ion etching was used to form these macropore arrays,silicon electrochemical etching in aqueous hydrofluoric acid(HF) solution has been more intensively studied. The porespacing was fixed to about 50 µm in order to achieve aspatial resolution of 10 lp/mm, as required in dentalapplication. Pore depths up to 420 µm with diameterranging from 10 to 40 µm, depending on the detectorconcept, have been achieved. Electrochemical etching of siliconis, indeed, a very promising technique to fabricate high aspectratio structures and damage-free macropore arrays. Thistechnique is based on a silicon dissolution reaction involvingthe species of the HF solution, silicon atoms and holes, thepositive charge carriers. As holes are the minority carriers inn-type silicon, they are usually photogenerated. However, wealso developed an alternative technique based on hole injectionfrom a forwardbiased p-n junction, and the possibility to formmacropore arrays and diverse threedimensional structures wasdemonstrated.The first detector concept investigated consists of asilicon charge-coupled device (CCD) in proximity with ascintillating guide screen. This screen is made of a siliconmacropore array filled with CsI(Tl), emitting photons at awavelength of 550 nm (green light) under X-ray exposure. Thevisible light is then reflected on the walls of the pores inorder to be detected by the CCD pixels. Both oxide and metalcan be used as a reflective layer. Such detectors were fullyfabricated and characterized, showing good spatial resolutionand comparable results with currently available detectorsconcerning the SNR and the X-ray dose. The second detectorstudied in this thesis uses photodiodes, instead of a CCD, inorder to detect the photons emitted from the scintillator. Thisconcept would lead to high charge collection efficiency sincethe diodes are formed in the silicon pore walls, making thedistance between the generation and detection points of thevisible photons short. However, this design implies two majordifficulties in the detector fabrication: formation of p-njunction in the pore walls and formation of contacts to thediodes. Thus, boron diffusion from a solid source andlow-pressure chemical vapor deposition of boron-dopedpoly-silicon were experimented. Both techniques were shown tobe successful. The last detector concept is based on thegeneration of electron/hole pairs in the semiconductor bulkunder X-ray exposure. The generated charges would then becollected by electrodes going through the bulk, requiringformation of deep and narrow pores. Siliconphoto-electrochemical etching was used and 425-µm deeppores with a diameter of 14 µm were formed, resulting inan aspect ratio of ~ 30 and an active area of 90 %.
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| 3. |
- Badel, Xavier, 1977-
(författare)
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Electrochemically etched pore arrays in silicon for X-ray imaging detectors
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
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| 5. |
- Christensen, Jens S.
(författare)
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Dopant diffusion in Si and SiGe
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Dopant diffusion in semiconductors is an interestingphenomenon from both technological and scientific points ofview. Firstly, dopant diffusion is taking place during most ofthe steps in electronic device fabrication and, secondly,diffusion is related to fundamental properties of thesemiconductor, often controlled by intrinsic point defects:self-interstitials and vacancies. This thesis investigates thediffusion of P, B and Sb in Si as well as in strained andrelaxed SiGe. Most of the measurements have been performedusing secondary ion mass spectrometry on high purityepitaxially grown samples, having in-situ incorporated dopantprofiles, fabricated by reduced pressure chemical vapordeposition or molecular beam epitaxy. The samples have beenheat treated both under close-to-equilibrium conditions (i. e.,long time annealings in an inert ambient) and conditions whichresulted in non-equilibrium diffusion (i. e., vacuum annealing,oxidation, short annealing duration, and protonirradiation).Equilibrium P and B diffusion coefficients in Si asdetermined in this thesis differ from a substantial part ofpreviously reported values. This deviation may be attributed toslow transients before equilibrium concentrations of pointdefects are established, which have normally not been takeninto account previously. Also an influence of extrinsic dopingconditions may account for the scattering of the diffusivityvalues reported in literature. B and Sb diffusion in Si underproton irradiation at elevated temperatures was found to obeythe so-called intermittent model. Parameters describing themicroscopic diffusion process were derived in terms of theintermittent diffusion mechanism, and it was found also thatthe presence of Sb strongly affected the B diffusion and viceversa.In relaxed Si1-xGex-alloys, which has the same lattice structure as Sibut a larger lattice constant, P diffusion is found to increasewith increasing Ge content (x≤ 0.2). In Si/SiGe/Si heterostructures, wherethe SiGe layer is biaxially strained in order to comply withthe smaller lattice parameter of Si, P diffusion in thestrained layer is retarded as compared with relaxed materialhaving the same Ge content. In addition, P is found tosegregate into the Si layer via the Si/SiGe interface and thesegregation coefficient increases with increasing Ge content inthe SiGe layer.
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| 6. |
- Doyle, James P.
(författare)
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Copper germanide schottky contacts to silicon and electrically active defects in n-type 6H-SiC and 4H-SiC epitaxial layers
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Metallization for contacts to silicon devices presents amajor challenge as the linewidths are further reduced into thesub-micron regime. Copper germanide due to its relatively lowroom temperature resistivity ( ~ 10 µΩ - cm) has beenexamined as a potential contact metallixation. In addition, dueto the growing interest in silicon carbide as a potentialwide-bandgap semiconductor candidate for high voltage, power,and frequency devices, a characterization of electricallyactive deep levels was conducted on as-grown and electron andion irradiated layers as such defects can influence theelectrical behaviour of active devices. N-type 6H- and 4H-SiChave been characterized by deep level transient spectroscopy(DLTS) in conjunction with MeV electron beam irradiation andkeV hydrogen and deuterium implantation. Intrinsic levels havebeen identified in both polytypes through an examination ofas-grown and irradiated samples.It is demonstrated that silicon outdiffusion into the coppergermanide contact causes an increase in the electricalresistivity. Additionally, deviation from ideal behaviour isobserved in current-voltage (IV) measurements an indicationthat this outdiffusion of silicon produces recombinationcenters primarily in the upper part of the bandgap near to thecontact-semiconductor interface. Additionally, copper germanideis found to be morphologically unstable during theamorphous-to-crystalline transformation. The high diffusivityof copper in amorphous germanium at temperatures as low as 200°C coupled with the independence of the barrier height onthe germanium fraction used during contact formation indicatesthat the interface between copper germanide and silicon issimilar to that of copper silicide and silicon.A number of electrically active deep levels have beencharacterized in both the 4H- and 6H-SiC polytypes. In 6H-SIC,many of the defects are found to be intrinsic as they areobserved to grow as a function of increasing irradiation dose.In contrast, as-grown 4H-SiC epitaxial layers, exhibit only asingle acceptor-like level at 0.70 eV below the conduction bandedge (Ec). After irradiation the level is not observed toincrease in concentration, but two new levels are found. Theselevels are found to be unstable at room temperature in contrastto the 6H-SiC defects. In 6H-SiC, the defect concentration isfound to limit the average carrier lifetime. Through the use ofthermal annealing, SIMS, and electron irradiation, a model oftheir Origin is proposed.Keywords:Deep level transient spectroscopy (DLTS),secondary ion mass spectrometry (SIMS), amorphous germanium,electron irradiation, ion irradiation, wide-bandgap
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| 7. |
- Fröjdh, Christer
(författare)
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Schottky barriers and Schottky barrier based devices on Si and SiC
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work is devoted to a study of the formation andcharacterisation of Schottky Barriers on differentsemiconductors with an extension to the development of devicesbased on Schottky barriers. The major part of the work has beendone on 6H-SiC, but Si and SiGe alloys have also been used.Different metals have been deposited on p-type and n-type6H-SiC and the properties of the created barriers have beeninvestigated by CV, IV and photoelectric techniques. Data fromother research groups have been collected in order toinvestigate the correlation between the barrier height and themetal work function. The conclusion is that for a number ofmetals there is a strong correlation between the barrier heightand the metal work function, while other metals showsignificant deviation from the Schottky-Mott theory. Largescatter in the data exists between different investigationsindicating that the method of sample preparation is veryimportant for the results. This is not surprising since theSchottky Barrier is mainly a surface device, which makes itvery sensitive to variations in the surface conditions.Extensive work has been done in order to explain thebehaviour of Schottky diodes fabricated on SiC and to qualifythe measurement techniques in presence of different anomaliesin the devices. A highly resistive interfacial layer presentbetween the bulk wafer and the epitaxial top layer on certainp-type 6H-SiC wafers has been found.A process for fabrication of buried Schottky and ohmiccontacts in Silicon by wafer bonding has been developed usingCo as a buried metal layer. During the heat treatment the metalreacts with the Silicon and forms CoSi2. The processed contacts exhibit the sameproperties as similar contacts produced by other techniques.Schottky contacts formed by deposition of W and Ti on differentSiGe alloys have been characterised. The pinning of the Fermilevel in these alloys is stronger than in pure Si.The device work included fabrication of Schottky diodes foruse as photodetectors and Permeable Base Transistors (PBT).Grid shaped Ti based Schottky diodes were fabricated in 6H-SiC.Optical characterisation shows that the diodes are sensitive inthe UV-range with a peak sensitivity around 300 nm. The diodesare insensitive to visible light. Diodes on p-type materialshow higher sensitivity than diodes on n-type material. This isexpected due to the higher barriers on p-type and the highermobility of electrons, increasing the contribution fromdiffusion.A process for the fabrication of PBT:s on Silicon using selfaligned CoSi2contacts was developed. Devices were fabricatedand DC-characterised. The first PBT on 6H-SiC was fabricatedusing a process based on etching of epitaxial layers. Ni wasused as etch mask and ohmic contact. Ti was used as gatemetal.
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| 8. |
- Grivickas, Paulius, 1974-
(författare)
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Optical studies of carrier transport and fundamental absorption in 4H-SiC and Si
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Fourier transient grating (FTG) technique and a novelspectroscopic technique, both based on free carrier absorption(FCA) probing, have been applied to study the carrierdiffusivity in 4H-SiC and the fundamental absorption edge in4H-SiC and Si, respectively. FTG is a unique technique capable of detecting diffusioncoefficient dependence over a broad injection interval rangingfrom minority carrier diffusion to the ambipolar case. In thiswork the technique is used for thin epitaxial 4H-SiC layers,increasing the time- and spatial-resolution of the experimentalsetup by factors of ~100 and ~10, respectively, in comparisonto the established Si measurements. It is found that thediffusion coefficient within the detected excitation range inn-type 4H-SiC appears to be lower than the analyticalprediction from Hall-mobility data. To explain this, it issuggested that the minority hole mobility is reduced withrespect to that of the majority one or that the hole mobilityvalue is in general lower than previously reported. Observeddifferences between the temperature dependency of the ambipolardiffusion and the Hall-prediction, on the other hand, areattributed to the unknown Hall factor for holes and theadditional carrier-carrier scattering mechanism in Hallmeasurements. Furthermore, at high excitations a substantialdecrease in the ambipolar diffusion is observed andadditionally confirmed by the holographic transient gratingtechnique. It is shown that at least half of the decrease canbe explained by incorporating into the theoretical fittingprocedure the calculated band-gap narrowing effect, taken fromthe literature. Finally, it is demonstrated that numerical datasimulation can remove miscalculations in the analytical Fourierdata analysis in the presence of Auger recombination. Measurements with variable excitation wavelength pump-probeare established in this work as a novel spectroscopic techniquefor detecting the fundamental band edge absorption in indirectband-gap semiconductors. It is shown that the techniqueprovides unique results at high carrier densities in doped orhighly excited material. In intrinsic epilayers of 4H-SiC,absorption data are obtained over a wide absorption range, atdifferent temperatures and at various polarizations withrespect to the c-axis. Experimental spectra are modeled usingthe indirect transition theory, subsequently extracting thedominat phonon energies, the approximate excitonic bindingenergy and the temperature induced band-gap narrowing (BGN)effect in the material. Measurements in highly dopedsubstrates, on the other hand, provide the first experimentalindication of the values of doping induced BGN in 4HSiC. Thefundamental absorption edge is also detected in highly dopedand excited Si at carrier concentrations exceeding theexcitonic Mott transition by several orders of magnitude. Incomparison to theoretical predictions representing the currentunderstanding of absorption behavior in dense carrier plasmas,a density dependent excess absorption is revealed at 75 K.Summarizing the mainfeatures of the subtracted absorption, itis concluded that an excitonic enhancement effect is present inSi.
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| 9. |
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| 10. |
- Janson, Martin, 1968-
(författare)
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Hydrogen diffusion and ion implantation in silicon carbide
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Secondary ion mass spectrometry (SIMS) has been employed tostudy the spatial distributions resulting from mass transportby diffusion and ion implantation in single crystal siliconcarbide (SiC). By a systematic analysis of this data,fundamental processes that govern these phenomena have beenderived.The acceptor atoms Al and B are known to be electricallypassivated by H in SiC. By studying the thermally stimulatedredistribution of implanted deuterium (2H) in various acceptordoped structures, it is found that hydrogen forms complexeswith the doping atoms, and also interacts strongly withimplantation induced defects. A comprehensive understanding ofthe formation and dissociation kinetics of these complexes hasbeen obtained. The extracted effective capture radius for theformation of 2H-B complexes is in good agreement with thatexpected for a coulomb force assisted trapping mechanism. Thelarge difference of 0.9 eV in the extracted dissociationenergies for the 2H-Al and 2H-B complexes suggests that theatomic configurations of the two complexes are significantlydifferent. Furthermore, by studying the migration behavior of Hin the presence of built-in electric fields, it is concludedthat all of the mobile H is in the positive charge state inp-type SiC.A large number of implantations have been performed withrespect to ion mass, energy, fluence, and crystal orientation.The electronic stopping cross sections in the low velocityregime for ions with atomic numbers 1 ≤ Z1 ≤ 15have been extracted from the ion range distributions. Theydisplay both Z1-oscillations and a smaller than velocityproportional stopping for ions with Z1 ≤ 8, in agreementwith previous reports for other materials. Furthermore, thedegree of ion channeling in various major axial and planarchannels of the 6H and 4H-SiC crystal has been explored. Twotypes of ion implantation simulators have been developed. Onebased on a statistical, data-base approach, and one atomisticsimulator, based on the binary collision approximation (BCA).By fitting BCA simulated profiles to the experimental profiles,detailed information about the electronic stopping andimplantation induced damage is extracted. In addition, thevacancy-related damage caused by the implantations has beeninvestigated by positron annihilation spectroscopy (PAS). Twotypes of implantation induced positron traps have been isolatedand are tentatively identified as a Si vacancy (VSi) and a Si-Cdivacancy (VSiVC). The extension of detected VSi is in goodagreement with that predicted by BCA simulations, and forimplantations with heavier ions VSi are revealed at far greaterdepths than the mean projected ion range due to deeplypenetrating channeled ions.
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