121 research outputs found
Electrochemical etching of silicon on insulator
No full textCilj istraživanja u okviru ove disertacije je utvrditi strukturna, optička i kemijska svojstva nove vrste poroznog silicija izrađenog na pločicama s različito dopiranim epitaksijalnim slojem kao i na pločicama silicija na izolatoru. U tu svrhu izrađene su dvije komore za jetkanje te je napravljen eksperimentalni postav koji omogućava jetkanje pločica različitih dimenzija s istosmjernom i izmjeničnom strujom. Variranjem koncentracije etanolne otopine fluorovodične kiseline, gustoće struje jetkanja i vremena jetkanja dobile su se porozne strukture različitih morfologija i različitih optičkih svojstava. Uzorci su analizirani Ramanovom spektroskopijom, infracrvenom spektroskopijom s Fourierovom transformacijom, fotoluminiscentnom spektroskopijom i pretražnom elektronskom mikroskopijom. Jetkanjem monokristalnog silicija n-tipa u području jakih struja dobiven je fraktalni oblik površine velike efektivne površine po jedinici volumena, što može biti interesantno za primjene u biomedicini. Kod jetkanje epitaksijalnog n-tipa silicija različitih debljina od 5 i 20 μm na površini slojeva formiraju se veće pore dislokacijskog tipa koje vrše ulogu ulaznih kanal za F- ione u sloj. Kod produženih vremena jetkanja dolazi do kompletnog odvajanja epitaksijalnog sloja te se tako dobivaju dvije dramatično različite porozne strukture u epitaksijalnom sloju i supstratu. Ovaj postupak je ujedno i nova metoda za izradu samostojećeg sloja makroporoznog silicija koji se može koristiti u budućim istraživanjima razvoja biosenzora i termoelektričnih uređaja. Kod jetkanja supstrata n-tipa debljine 280 μm dobivaju se slojevi poroznog silicija s nano porama. Ovakvi filmovi pokazuju intenzivnu fotoluminescenciju. Najveća pažnja je posvećena jetkanju monokristalnog silicija p-tipa na izolatoru što predstavlja i znanstveni i tehnološki izazov zbog prirode samih proces jetkanja kao i zbog moguće primjene takvog silicija u razvoju novih senzora. Kod jetkanja istosmjernom strujom nastaju duboke pore većih dimenzija, a kod jetkanja izmjeničnom strujom nastaju vlaknaste strukture i otoci koji pokazuju jaku fotoluminiscenciju. Stajanjem na zraku ove strukture jako oksidiraju što dodatno ukazuje na njihovu nanometarsku poroznost. Izmjereni intenzitet fotoluminiscencije kod uzoraka silicija na izolatoru izuzetno je visok, te je u odnosu na sve uzorke predstavljene u okviru ove disertacije intenzivniji za faktor 100 i više puta. Na temelju istraženih svojstava različitih tipova proizvedenog poroznog silicija u budućim istraživanjima odredila bi se optimalna svojstva za razvoj biosenzora, termoelektričnih uređaja i podloga za površinski pojačano Ramanovo raspršenje (tzv. SERS).Nanostructured porous silicon (PS) is a novel material with distinguished structural, electrical and optical properties used in modern high technology devices, such as biological and chemical sensors, drug delivery systems, thermoelectric devices, etc. The aim of this study was to determine structural, optical and chemical properties of a novel type of porous silicon prepared on different silicon epitaxial wafers, as well as on silicon on insulator wafers. In the theoretical part of the dissertation the description of structural and optical properties of silicon is given. The electronic structure of crystalline silicon was described and the characteristics of the electronic structure which determine its optical properties were explained. The photon-electron interaction was described and the ways of photon emission and absorption were explained. The problem of enhancing the photon emission was also discussed, namely how to get photoluminescence from silicon, since it is an indirect semiconductor and accordingly very inefficient emitter. The third chapter gives a historical overview of the discovery of porous silicon and an overview of the previous research. The mechanisms of porous silicon formation were described together with the brief overview of silicon electrochemistry. The most important models for porous silicon formation and pore propagation were illustrated, whit the special emphasis on the quantum confinement model. In this chapter the chemical and physical properties of porous silicon were specified, together with the experimental techniques used for the determination of them. The effect of different parameters, such as current density, concentration of hydrofluoric acid (HF) solution and etching duration on pore formation was described too. Chemical properties of porous silicon were discussed in terms of infrared spectra where assignation of all vibrational bands was given. As far as physical properties are concerned, special emphasis was given to photoluminescence. The mechanisms that influence its efficiency and the models that describe its origin were given. The model of quantum confinement, as the most accepted model for the explanation of the origin of photoluminescence, was given special emphasis. Raman spectra of porous silicon were described too, together with the model of nanocrystal dimension calculation derived from Raman spectra. In the experimental part of this work two etching chambers were manufactured and the experimental set up was established which enabled etching of different size wafers with direct and alternating current. The method for porous silicon production from epitaxial wafers, polycrystalline wafers and silicon on insulator wafers was established. Porous structures with different morphology and optical properties were obtained by varying the concentration of HF ethanol solution, current density and etching duration. In this chapter the experimental procedures for structural and chemical investigations of produced PS were described. Structural properties were investigated by Raman spectroscopy and scanning electron microscopy (SEM). Chemical properties were investigated by Fourier Transform Infrared spectroscopy (FTIR), while photoluminescence was investigated by excitation in visible and infrared region. Several ways of porous silicon production were investigated: a) etching of n-type monocrystal silicon in high current regime; b) etching of n-type epitaxial silicon with 5 and 20 μm thick epitaxial layer; c) etching of 280 μm thick n-type silicon substrate on an epitaxial layer; d) etching of 40 and 460 μm thick p-type monocrystal silicon on insulator. In the first case, n-type monocrystal silicon was etched under the illumination from 250 W halogen lamp with currents which were over the critical electro polishing current. Although Raman spectroscopy of these samples confirmed the formation of nanometer structures, observed photoluminescence is of low intensity. This finding indicates that in these structures, some other centers which may cause the nonradiative recombination of excited electrons are generated. Observed fractal surface appearance and expected high specific surface area makes these systems interesting for the application in biomedicine where the porous silicon is used as an inert smart drug carrier. Interesting results were obtained when etching n-type epitaxial silicon with different thicknesses (5 and 20 mm) of an epitaxial layer. In the process of etching larger pores of dislocation type were formed on the surface of these layers. They serve as entrance channels for F- etching ions into the layer. So, under still smooth surface a branching network of interconnected micrometer size channels were formed. With prolonged etching the epitaxial layer was completely detached from the substrate and hence two dramatically different porous structures were obtained – thin epitaxial layer with micro-sized pores (macroporous silicon) and etched black layer in the substrate with nanometer size pores (mezzoporous silicon) which has very low reflectivity, so it is called black silicon. The size of pores was regulated by changing the etching parameters and hence the physical and chemical properties of the PS were changed too. This procedure is a novel method for macro porous free standing silicon production which can be used in further investigations and for development of biosensors and thermoelectric devices
Raman scattering enhancement using photonic nanojet of dielectric microspheres
No full textRamanova spektroskopija metoda je karakterizacije i identifikacije materijala bazirana na neelastičnom Ramanovom raspršenju fotona svjetlosti na materijalu. Primjenjuje se u raznim znanstvenim i industrijskim područjima, od fizike, kemije, biologije, medicine, znanosti o materijalima, do kontrola sigurnosti, detekcije nedozvoljenih tvari i proučavanja umjetničkih dijela. Zbog neelastičnosti procesa, Ramanovo raspršenje daje vrlo slab signal, zbog čega su razvijene različite metode pojačanja. Jedna od novih metoda pojačanja, čije su prednosti jednostavnost i niska cijena primjene, neinvazivnost, reproducibilnost i stabilnost, temelji se na fotonskom nanomlazu koji nastaje obasjavanjem mikroleće svjetlošću. Odabirom pogodnih parametara, fotonski nanomlaz može imati izrazito visok intenzitet, usku širinu, ili veliku duljinu, zbog čega se osim za Ramanovu spektroskopiju, istražuje i za primjene u nanolitografiji, super-rezoluciji, optičkim silama, pohrani podataka i sličnim poljima. Trenutno, njegova primjena u Ramanovoj spektroskopiji nedovoljno je istražena, a i sama svojstva i uvjeti za njegov nastanak nerazjašnjeni su. Istraživanjima u sklopu ovog doktorskog rada unaprijeđena je metoda pojačanja, te su dobivena i nova saznanja o fotonskom nanomlazu općenito. Napravljen je računalni program temeljen na Generaliziranoj-Lorenz Mie teoriji, kojim je izračunat širok raspon različitih konfiguracija za fotonski nanomlaz iz dielektrične mikrosfere. Dobiven je sistematičan uvid u svojstva i ovisnost fotonskog nanomlaza o parametrima te uvid u promjene pojedinih ovisnosti u različitim uvjetima. Pokazana je kritična važnost parametra pozicije upadne zrake. Odabirom određene kombinacije parametara dobiven je izrazito intenzivan, vrlo uzak ili izrazito dug fotonski nanomlaz daleko izvan mikrosfere. Istraživanjem područja visokog indeksa loma mikrosfere, modeliranjem je pokazano da fotonski nanomlaz može nastati izvan mikrosfere i kada je indeks loma viši od 2, što je u dosadašnjoj literaturi bilo označeno kao gornja granica. Varijacijom eksperimentalnih parametara optimizirano je pojačanje. Pomoću vertikalnog ramanskog mapiranja određena je optimalna pozicija upadne zrake za pojačanje te je pokazana njezina važnost. Izmjerena je ovisnost pojačanja o kolekcijskom vlaknu, mikroskopskom objektivu i veličini mikrosfere. Diskutiran je mehanizam pojačanja, koji smo podijelili u dva doprinosa - od fotonskog nanomlaza, te od kolekcijskog sustava. Dobiveno je kombinirano pojačanje mikrosfere i plazmonskog pojačanja. Dizajniran je i testiran novi sustav za mehaničku kontrolu mikrosfere pod mikroskopskim objektivom, kojim je moguće iskoristiti pojačanje za svaku točku ramanskog mapiranja. Dobiveno je pojačanje intenziteta i rezolucije mapiranja.Raman spectroscopy is a method for characterization and identification of materials, which is widely used in a broad range of scientific and industrial fields, like materials science, physics, chemistry, biology, medicine, security control, substance control and art examination. It is based on Raman scattering of light on molecules and crystals. As opposed to its elastic counterpart - Rayleigh scattering, Raman scattering is inelastic, which means that the scattered photon has a different energy than the incident one. Because of this, Raman scattering has low probability of occurrence and the scattered Raman intensity is very low. For this reason, many methods for the enhancement of Raman scattering have been developed through the years. One of the new methods of enhancement is based on photonic nanojet, which is a concentrated beam of light emerging from the shadow side of an illuminated microlens. This method of enhancement is characterized by a low cost and a simple principle of implementation. It is a non-invasive, reproducible and reliable way of enhancement. By careful choice of parameters, photonic nanojet can have very high intensity, very narrow width, or very long length. This makes it suitable not only for Raman scattering enhancement, but also for applications in nanolithography, super resolution, optical forces, data storage and similar fields. Although being a promising technique, the role and usage of photonic nanojet in Raman spectroscopy is currently underexplored. Moreover, the properties and conditions for emergence of the photonic nanojet generally are still not clear. This PhD dissertation is a result of four years of research on photonic nanojet and its usage for Raman spectroscopy. It is based on four published papers [1, 2, 3, 4], one still unpublished body of work, and a patent application [5, 6]. This research has resulted not only in the improvement of the method of Raman enhancement, but also with new findings in the general field of photonic nanojet. The research was performed from two angles, experimental and computational. The series of computer codes were written in order to model the photonic nanojet in various conditions. The codes, based on Generalized Lorenz-Mie theory, calculate the electric field intensity from scattering of a Gaussian beam on a dielectric microsphere, upon which a photonic nanoi Extended abstract jet emerges. A large amount of configurations was calculated which provided a systematic overview of photonic nanojet properties and its dependence on parameters. Also, the change of dependencies is detected and investigated by variation of other parameters. The parameters which are varied are the incident Gaussian beam wavelength, position and waist radius, and the microsphere radius. The microsphere refractive index was taken to correspond to SiO2 material. The investigated properties of a photonic nanojet are its maximum intensity, position, width and length. The incident Gaussian beam position is shown to be of critical importance for photonic nanojet properties. Two types of photonic nanojet are identified: Type 1 has lower intensity, its position is further away from the microsphere and has larger dimensions, while Type 2 has higher intensity, it is positioned close to the microsphere edge, and has smaller dimensions. The size matching between the incident beam waist radius and microsphere radius is shown to improve the intensity of the photonic nanojet, but it is not the main contribution. Proper positioning of the incident beam, small waist radius and short wavelength are shown to be important for high intensity. It is also shown that all parameters are important in their absolute value, and that size parameter from Lorenz-Mie theory cannot be applied. Furthermore, parameter combinations for the photonic nanojet of extremely high intensity, very narrow width, or extremely long length with long working distance are determined. In some regimes, intensity oscillations are also detected, and they are identified as whispering-gallery modes and Mie interferences. The occurrence of the photonic nanojet is also investigated when a high refractive index microsphere is used. The investigation followed three theoretical levels: geometrical optics, ray transfer matrix analysis, and Generalized Lorenz-Mie theory. Geometrical optics show that divergent incident light rays can be focused outside a high refractive index microsphere. Ray transfer matrix analysis show that divergent cone of a Gaussian beam produces output beam with a waist outside a high refractive index microsphere. The mathematical condition for that occurrence is derived. Finally, the Generalized Lorenz-Mie theory calculations show that a photonic nanojet can emerge outside the microsphere even when the refractive index of a microsphere is higher than two, which was up to now considered a limit in literature. The calculations also show the difference in focusing of the incident beam based on the refractive index of the microsphere, which is confirmed by the vertical Raman mapping. The Raman enhancement is optimized by variation of experimental parameters. Optimal position of the incident laser beam is determined by vertical Raman mapping, and explained with ray transfer matrix analysis. Laser beam profiles under the microscope objective are determined by a knife-edge method. Antenna effect of the microsphere for the enhancement is detected. The dependence of the enhancement on the collection fiber diameter, microscope objective and microsphere size is determined. Two microsphere materials were used: SiO2 and barium tiii Extended abstract tanate glass. The dependence on microsphere radius shows different behaviors depending on the objective used. The calculations of a photonic nanojet intensity are compared with experimental values of the Raman enhancement, which suggest that the photonic nanojet is not the only contribution to the enhancement. The enhancement strongly lowers by increasing the numerical aperture of the objective. The highest enhancement of the silicon substrate, of 19.29× is achieved in configuration of barium titanate glass microsphere of radius of 4.5 μm and 10× NA 0.25 microscope objective. The mechanism of the enhancement is discussed, which is separated into two contributions. The first contribution comes from the photonic nanojet, and the second contribution comes from the collection system. The model of the effective numerical aperture of the microsphere-objective system is presented, and compared with the experimental results. The usage of the microsphere for the enhancement was further improved by designing the new system for mechanical control of the microsphere. The system is called two-stemmed microsphere and allows positioning of the microsphere under the laser beam of the microscope objective independently of the substrate position. This way, Raman mapping can be performed in which each point is enhanced. The system is tested on a silicon substrate with domains separated by visible borders. Raman mappings are compared with atomic force microscope measurements. With two-stemmed microsphere, the intensity enhancement is 4× and the estimated resolution enhancement is 3×. Combined enhancement of SERS (surface-enhanced Raman scattering) and microsphere is achieved. The SERS substrates which were used were non-uniform and uniform silver nanoislands. The used analytes were 4-mercaptophenylboronic acid or 4-mercaptobenzoic acid. The non-uniform substrates combined with the microsphere show higher but less reproducible enhancement than the uniform substrates with the microspheres
Surface-enhanced Raman scattering: from the colloid to the stabile substrate
No full textSupstrati za površinski pojaĉano Ramanovo raspršenje (SERS) pripremljeni su na tri razliĉita naĉina: (i) sintezom koloidnih suspenzija srebra, (ii) depozicijom srebra na prethodno elektrokemijski jetkan silicij i (iii) naparavanjem srebra na monosloj polistirenskih mikrosfera. Koloidne suspenzije sintetizirane su redukcijom srebrove soli (AgNO3) s razliĉitim reducirajućim sredstvima. Prvi tip ĉvrstih podloga dobiven je depozicijom metala na porozni silicij metodom uranjanja u otopinu AgNO3, nanošenjem koloidne suspenzije srebra ili postupkom pulsne laserske ablacije. Drugi tip ĉvrstih SERS supstrata dobiven je depozicijom monodisperznih polistirenskih sfera (350 i 1000 nm) u monosloju na hidrofilnu staklenu podlogu te naparavanjem razliĉitih debljina metalnih filmova pomoću evaporatora. Sve pripremljene podloge testirane su na SERS aktivnost korištenjem testnih molekula (piridin, rodamin 6G, metilensko modrilo). Za koloidnu suspenziju gdje je veliĉina Ag ĉestica iznosila oko 40 nm i uz korištenje natrijevog borhidrida kao agregirajućeg sredstva postignut je faktor pojaĉanja za piridin reda veliĉine 106. Za podloge bazirane na poroznom Si najniţe dobivene granice detekcije su 10-10 mol L-1 za metilensko modrilo (uz pobudu 633 nm) i 10-9 mol L-1 za rodamin 6G (uz pobudu 514,5 nm). Za supstrate bazirane na polistirenskim sferama najbolje postignute granice detekcije su 10-9 mol L-1 za rodamin 6G i 1,2 ×10-3 mol L-1 za piridin. Sintetizirane koloidne SERS podloge primijenjene su za određivanje niskih koncentracija histamina i aflatoksina B1.The substrates for surface enhanced Raman scattering (SERS) were prepared in three different ways: (i) the synthesis of silver colloidal suspension, (ii) the deposition of silver on the previously electrochemically etched silicon and (iii) vapor deposition of silver on a monolayer of polystyrene microspheres. Colloidal suspensions were synthesized by reduction of silver salts (AgNO3) with various reducing agents. The first type of solid substrates was obtained by the deposition of metal on the porous silicon using immersion plating method, by applying Ag colloidal solution or by pulsed laser ablation. Another type of solid SERS substrates was prepared by deposition of monodisperse polystyrene spheres (350 and 1000 nm) in a monolayer on a hydrophilic glass surface. Different thicknesses of metal films were evaporated on the substrates using e-beam deposition. All prepared substrates were tested for their SERS activity using the test molecules (pyridine, rhodamine 6G, methylene blue). For the colloidal suspension where the size of Ag particles was about 40 nm and with the use of sodium borohydride as aggregating agent the enhancement factor for pyridine of the order of 106 was reached. For the substrates based on porous Si the lowest limits of detection obtained were 10-10 mol L-1 for methylene blue (with 633 nm excitation) and 10-9 mol L-1 for rhodamine 6G (with 514.5 nm excitation). For substrates based on polystyrene spheres best achieved detection limits were 10-9 mol L-1 for rhodamine 6G and 1.2 × 10-3 mol L-1 for pyridine. Synthesized colloidal SERS substrates have been applied for the detection of low concentrations of histamine and aflatoxin B1
Deposition and characterization of nanocrystalline silicon
No full textDeponirani su tanki filmovi silicija, silicijevog dioksida i silicijem bogatog oksida (SiOx) metodom LPVCD (Low Pressure Chemical Vapor Deposition). Depozicije su se odvijale pod različitim uvjetima u okviru kojih se varirao sastav i parcijalni tlak radnih plinova i temperatura depozicije. Termičkim oporavkom SiOx slojeva na temperaturama od 800 do 1200 oC dobile su se različite nanočestice silicija. Navedene nanočestice karakterizirane su pomoću Ramanove, infracrvene i apsorpcijske spektroskopije i pretražne elektronske mikroskopije. Razvijen je i teorijski model za Ramanovo raspršenje na optičkim i akustičkim vibracijama nanočestičnog silicija.Thin silicon, silicon-dioxide and silicon-rich oxide (SiOx) films were produced using the LPCVD (Low Pressure Chemical Vapor Deposition) method. The depositions were made under different conditions, where the composition and the partial pressures of the reactant gasses and the deposition temperature were varied. By thermal annealing of SiOx films under temperatures from 800 to 1200 oC nanoparticles of different sizes were formed. The nanoparticles were characterised using Raman, infrared and absorption spectroscopy and scanning electron microscopy. The theoretical model of Raman scattering on optical and acoustical vibrational modes of silicon nanoparticles was developed
Sensing properties of functionalized nanostructured silicon
No full textJedna od glavnih tema istraživanja u modernoj senzorici obuhvaća funkcionalizaciju nanostrukturiranih materijala u svrhu poboljšanja osjetljivosti i selektivnosti senzora. Eksperimentalni put prema unapređenju senzora uključuje izradu i karakterizaciju materijala velike specifične površine, njihovo tretiranje fizikalnim i kemijskim metodama, te ispitivanje njihovog odziva na pokusne agense. Doktorski rad će biti baziran na izradi nanostrukturiranog silicija, s naglaskom na porozni silicij dobiven elektrokemijskim jetkanjem. Fokus ovog rada je ispitati njegova senzorska svojstva, primarno spram para organskih otapala i plinova, te utjecaj funkcionalizacije kisikom na odziv takvog senzora.One of the main research topics in modern sensorics includes functionalisation of nanostructured materials with a goal of improving sensors’ sensitivity and selectivity. The experimental path leading to such improvement involves production and characterisation of materials with a large specific surface area, their physical and chemical treatment, and assessment of their response to test chemicals. The work of this thesis will be based on production of nanostructured silicon, with emphasis on porous silicon obtained by electrochemical etching. Focus of the thesis will be to explore the sensing properties of nanostructured silicon, primarily with respect to organic vapours and gases. Also, the effects of functionalisation and morphological characteristics on the sensor’s response will be studied
Hybrid junction of structured silicon and organic semiconductor for infrared light detection
No full textOtkrićem osjetljivosti hibridnih silicij/organskih fotodioda u bliskom infracrvenom području dodan je još jedan pretendent u utrci za nasljednika tehnologije InGaAs detektora u bliskom infracrvenom području. No, unatoč potencijalnim prednostima hibridne silicijske tehnologije, slabi odziv ovih fotodioda nije dozvolio potencijalne primjene, ali je izazvao akademski interes i potaknuo nastavak istraživanja. Tema ovoga rada je istraživanje mogućnosti poboljšanja odziva takvih fotodioda strukturiranjem silicijske podloge na nano i mikro skali prije nanošenja sloja organskog poluvodiča. Pretpostavka o mogućem poboljšanju svojstava strukturiranjem došla je iz tehnologije fotonaponskih ćelija, u kojima se strukturiranje površine koristi u svrhu zatočenja svjetlosti višestrukim refleksijama i povećanjem efektivne površine koja apsorbira svjetlost. U tu svrhu pripremljene su kemijskim metodama strukturirane površine silicija na nano i mikro skali, te hijerarhijski organizirane strukture. Na takvim podlogama deponirani su tanki slojevi organskog poluvodiča tirskog purpura metodom epitaksijalnog rasta u reaktoru s vrućim stjenkama. Na silicij i organski sloj nanešeni su ohmski aluminijski kontakti, čime je formirana fotodioda. Odziv fotodiode testiran je laserskom svjetlošću u bliskom infracrvenom području. Za sve metode strukturiranja ostvareno je višestruko pojačanje fotostruje u odnosu na hibridne fotodiode pripremljene na ravnim podlogama u jednakim uvjetima kao i strukturirane fotodiode. Najveće pojačanje, od do dva reda veličine, opaženo je na podlogama čija je površina strukturiranima u silicijske mikropiramide anizotropnim jetkanjem (100) orijentirane pločice silicija. Mjerenjem spektralnog odziva izmjeren je odziv od 4-5 mA/W u uvjetima reverzne polarizacije od - 1 V na telekomunikacijskoj valnoj duljini od 1550 nm. Mjerenjima apsorptancije u vidljivom i infracrvenom spektralnom području isključeno je zatočenje svjetlosti kao uzrok pojačanja fotostruje, a mjerenjem efektivne površine isključeno je povećanje efektivne površine. Predložen je model djelovanja hetero-fotodiode koji pretpostavlja zapinjanje Fermijeve i LUMO razine organskog poluvodiča unutar procjepa silicija. Kao razlog pojačanja fotostruje predložen je model pojačanja električnog polja na oštrim vrhovima strukturiranih podloga.Discovery of the sensitivity of the hybrid silicon/organic photodiodes in the near-infrared spectral range in 2009 by G. Matt in Linz has sparked not only purely academic interest, since the photo-sensitivity came from the sub-bandgap region of the both members that formed the hybrid heterojunction, but also has provoked interest with particular applications in mind. The telecom industry, using the spectral range of 1,3-1,6 μm for the long-range fiber telecommunications, is one of the possible users of su ch photodiodes if they could surpass the incumbent InGaAs photodetector technology ideally in both price and performance. However, the first photodiodes even after the optimization of the organic layer have remained far outside of the responsivity range necessary for the use in telecommunications. The topic of this thesis is the research of the possible improvement in responsivity of such photodiodes by structuring the silicon substrate on nano- and micro-scales prior to deposition of the thin organic layer. The assumption of improvement in photocurrent by the surface structuring came from research in the photovoltaics, where micro-structuring has been for a long time used for anti-reflection purposes, and as of recently nano and hierarchical structuring has successfully been used for light trapping and increase in cell effective surface area. To test the hypothesis differently structured silicon substrates were prepared by simple and affordable chemical etching methods. Surfaces with nano, micro or hierarchical micro-nano surface structuring were prepared, on top of which thin layers of organic semiconductor tyrian purple was deposited by hot-wall epitaxy, a vacuum deposition method known for producing high-quality films or small-molecule organic semiconductors. Ohmic contacts based on thin vacuum-evaporated aluminum films were placed on both silicon and organic semiconductor, forming a back-illuminated photodiode. Photoresponse of the diode was measured by J-V measurments under the excitation of nearinrfared laser light source. For ali differently structured photodiodes large improvements in short-circut photocurrent in comparison to photodiodes prepared in the same conditions on planar substrates were observed. The highest improvement, up to two orders of magnitude, was measured for silicon micropyramid structured substrates, prepared by anisotropic etching of ( IOO) oriented crystalline silicon substrates. Measurements of the spectral responsivity showed that the best performing photodiodes had the responsivity in the range of 4-5 mA/W under the reverse polarization of -I V, at the telecom-relevant wavelenth of 1550 nm. Light trapping as a possible method for improvement of the increase in photocurrent was ruled out by a series of measurements of transmittance and reflectance of the structured heterojunctions in the visible and near-infrared spectral range. By comparison ofthe increase in effective surface area of the junction between planar and best performing micropyramid structured samples, increase in junction area was also ruled out as a possible mechanism of the increase in photocurrent. Previously published models of the band-diagrams and mechanism of NIR sensitivity of the photodiodes of a type-II heterojunction with the Anderson rule-like band alignment bave predicted the dependence of the onset of the photocurrent on the LUMO level of the organic semiconductor. However, the complete lack of such behavior has brought fonvard an improved model containing the assumption of both the Fermi level and the organic HOMO level pinning to the near-midgap levels in silicon, which would explain the lack of variation of the barrier width measured as the energy of the photons causing the onset of the photocurrent. Electric-field assisted enhancement mechanism is proposed as a mechanism for increase of the photocurrent in the structured samples. It is demonstrated by numeric model that the increase in the electric field at the tip of the pyramid can be up to ten-fold in comparison to the electric field of an equipotential plane. Field-enhanced transport mechanisms such as the Fowler-Nordheim tunneling are strongly 11011-Iinearly dependent on the intensity of the electric field. If a mechanism of that type is responsible for the transport at the sharp points of the structured substrates, it would explain large improvements in the photocurrent
Analysis of structure, electronic and transport properties of heavily doped polycrystalline silicon thin films
No full textU radu su prikazana i analizirana svojstva visokodopiranih filmova polikristalnog silicija s primarnim ciljem razvoja grijaćeg, odnosno termoelektričnog elementa. Predstavljen je model rasta zrna iz amorfne faze koji za razliku od poznatih modela predviđa stagnaciju rasta prije nego se dosegne monokristalna faza. Potreba za preciznim predviđanjem ponašanja električnih karakteristika nameće i nužnost točnog određivanja koncentracije slobodnih nosilaca. Osim određivanja koncentracije nosilaca iz vrijednosti slojnog otpora, alternativno su predstavljeni modeli određivanja koncentracije nosilaca iz parametara Fano interakcije prema ponašanju O(_) vrpce Ramanovog spektra te iz linearne ovisnosti termostruje o temperaturi. Postignuta su zadovoljavajuća međusobna slaganja dobivenih vrijednosti. Postojeći modeli raspršenja nosilaca u obzir uzimaju elektron-elektron raspršenje kao korekciju ostalim dominantnim mehanizmima raspršenja. Mjerenjem ovisnosti električnog otpora o temperaturi, opaža se da je na niskim temperaturama elektron-elektron raspršenje dominantno do čak 80 K, a temperaturni raspon je funkcija koncentracije primjesa. Teorijski model iz literature koji opisuje vođenje električne struje u polikristalnom siliciju uklopljen je u proračun ovisnosti termoelektrične snage o koncentraciji. Prema predstavljenom izračunu, maksimalna vrijednost TE faktora snage nije samo funkcija koncentracije primjesa nego je i obrnuto proporcionalna veličini zrna polikristalnog silicija. Uzorak s najvišom koncentracijom atoma bora bio bi najbolji kandidat pri izradi grijaćeg elementa zbog gotovo linearne ovisnosti električnog otpora o temperaturi i pozitivnog temperaturnog koeficijenta te za termoelektrične uređaje zbog najvišeg iznosa faktora termoelektrične snage.Microelectronic industry as we know it today relies on the second most abundant material in the Earth's crust - silicon. In contrast to single-crystal silicon, polycrystalline silicon films contain grain boundaries which greatly influence the material properties, making them particularly sensitive to deposition conditions and doping levels. The subject of this thesis is an investigation of structural and electronic properties of heavily doped polysilicon thin films. Potential applications of these layers lie in silicon heating elements and thermoelectric devices. A silicon heating element is a structure consisting of a technical ceramic material (preferably AlN) onto which an active polysilicon layer is deposited. The advantage of this kind of heater is that the heat losses are relatively small compared to other resistive heaters as heat is transferred to the heated medium mainly by conduction. Until recently, silicon has not been considered a promising thermoelectric (TE) material due to its high thermal conductivity. However, recent publications have proven that nanostructuring could influence/decrease thermal conductivity, therewith potentially leading to competitive values of the TE figure of merit. Poly- or nanocrystalline silicon could be a great compromise between silicon nanowires that are cumbersome to use in volume applications and bulk monocrystalline silicon. After a general introduction, the second chapter gives an overview of the main properties of polycrystalline silicon films. Doped polycrystalline silicon has values of the thermal and electrical conductivity below those of a single-crystal layer with comparable carrier concentration at all temperatures. This means that the dominant phonon and electron scattering mechanism comes from the scattering on the grain boundaries. In the third chapter the thermoelectrical effects of Seebeck, Peltier and Thomson are introduced. Common thermoelectrics are put into perspective, while the main focus is set on silicon based systems and recent advances in the field related to this material. The deposition and experimental methods used in this work for the structural and electronic analysis are presented in chapter four. Two groups of samples were analysed, both deposited in the low pressure chemical vapour deposition (LPCVD) furnace. The first set of samples was deposited on oxidized (111) silicon wafers at 750°C and in-situ boron δ-doped. To achieve structural relaxation and additional dopant activation, samples were annealed at 1200°C for 1h. The second group consists of samples deposited onto (100) p-type Si wafers at 530 and 580°C and in-situ phosphorus doped. Subsequently, samples were subjected to rapid thermal annealing (RTA) at 950°C for 10, 20, 30 and 45s. The fifth chapter summarizes the main results of the investigation. Structure and morphology of the obtained films were determined with the help of scanning electron microscopy (SEM), x-ray diffraction (XRD) and reflectivity (XRR). The grain sizes of the polycrystalline films were calculated from the SEM micrographs by measuring the length of the small and the big axes of the ellipse covering the visible grain silhouette. For samples heavily doped with phosphorus, the grain size is a function of the annealing duration. Available models assume a final single-crystal state which is in contradiction with experimental results. On the basis of the obtained data, a new theoretical model which takes into account the grain growth stagnation is developed. Carrier concentration is one of the most important parameters when controlling electrical characteristics. The carrier concentration was determined from sheet resistance measurements and compared to the overall impurity concentration obtained from secondary ion mass spectroscopy. Two new models of the carrier concentration calculation are presented. The first one relies on the Fano interaction visible in the O(_) peak of Raman spectra of heavily doped silicon samples, and the second one takes into account the linear temperature dependence of the Seebeck coefficient. The thus obtained values are in good agreement with the ones obtained from sheet resistance measurements. Low temperature resistivity measurements revealed that at the lowest temperatures the dominating scattering mechanism is electron-electron scattering, as is characteristic for disordered metals. This kind of behaviour can also be found in some quasi 2D systems (at lower temperatures the electron mean free path becomes comparable to the film thickness). The characteristic T1/2 resistivity dependence even spans up to 80 K in the case of a boron concentration of p = 4,86・1019 cm-3 (the sample with the highest boron concentration investigated). As found in the literature, the dominant electron-electron interaction for heavily doped silicon thin films was not observed at temperatures higher than 1K. In this work, the obtained results also imply that the grain boundary scattering mechanism is temperature independent. A well established model for the carrier conduction in polycrystalline silicon was used to determine the power factor dependence on the grain size and doping effect. It was found that if the doping level increases, the grain size should decrease in order to keep the maximum TE power factor value constant. The sample with the highest boron concentration (p = 4,86・1019 cm-3) is the most promising candidate to be used as an active layer in heating elements due to its positive temperature coefficient which is rather constant in the whole temperature range. The same sample also has the highest TE power factor. However, the obtained TE power factor is still rather small when compared to today’s state-of-the-art thermoelectric materials. The final chapter summarizes the findings and conclusions of the presented work, and gives guidelines for future research and advances. Apart from increasing the figure of merit through nanostructuring, other ways to improve the thermoelectric performance could be through the formation of mixed structural phases which would additionally introduce scattering centres. Furthermore, another way to obtain a good theremoelectric material could be through electrochemical etching of heavily doped polycrystalline silicon with different doping levels. Following these avenues of research, and given the abundance of silicon as well as established production schemes, silicon based devices may soon boast a very competitive cost-benefit ratio/analysis (converted energy/production cost) in thermoelectric applications
SPECTROMETER FOR RAMAN AND PHOTOLUMINESCENCE SPECTROSCOPY WITH VARIABLE SPECTRAL RESOLUTION FOR MARINE APPLICATIONS
No full textThe maritime industry includes all stationary and floating infrastructure needed for maritime transport and trade. Therefore, it is directly related to the marine environment. However, it is necessary to follow the regulations issued by the International Maritime Organization (IMO) to reduce the damage to the marine environment and reduce the maintenance caused by the marine environment. One of the techniques used for this purpose is Raman and photoluminescence spectroscopy (PL). Raman and PL spectroscopy are important analytical tools in materials science that provide information on the vibrational and electronic properties of molecules and crystals. This dissertation presents the results of a novel approach to Raman and PL spectroscopy that takes advantage of variable spectral resolution by using zoom optics in a monochromator in front of the detector. The results show that the spectral intervals of interest can be acquired at different zoom factors, significantly reducing the acquisition time and changing the spectral resolution for different zoom factors. The smallest spectral intervals acquired at the maximum zoom factor yield higher spectral resolution suitable for the Raman spectroscopy. In contrast, larger spectral intervals acquired with the minimum zoom factor yield the lowest spectral resolution suitable for the PL spectroscopy. The proof-of-concept was demonstrated using a zoom lens with a zoom factor of 6. Then prototype spectrometer was demonstrated using a zoom lens with a zoom factor of 18. Special electronics were developed to allow automated Raman and PL spectra acquisition. The graphical user interface (GUI) was developed for spectra acquisition. The resulted spectra obtained were compared to those obtained with a high-quality commercial spectrometer. The comparison was performed on several different materials for Raman and PL and the statistical analysis of variance (ANOVA) was used to evaluate the spectra obtained. The results show that such spectrometer could be an efficient and fast tool for the search of Raman and PL bands of unknown materials and the subsequent spectral acquisition of the spectral interval of interest with a suitable spectral resolution. Therefore, it can be used to develop new materials used in the marine industry to reduce damage caused by the harsh marine environment.The maritime industry includes all stationary and floating infrastructure needed for maritime transport and trade. Therefore, it is directly related to the marine environment. However, it is necessary to follow the regulations issued by the International Maritime Organization (IMO) to reduce the damage to the marine environment and reduce the maintenance caused by the marine environment. One of the techniques used for this purpose is Raman and photoluminescence spectroscopy (PL). Raman and PL spectroscopy are important analytical tools in materials science that provide information on the vibrational and electronic properties of molecules and crystals. This dissertation presents the results of a novel approach to Raman and PL spectroscopy that takes advantage of variable spectral resolution by using zoom optics in a monochromator in front of the detector. The results show that the spectral intervals of interest can be acquired at different zoom factors, significantly reducing the acquisition time and changing the spectral resolution for different zoom factors. The smallest spectral intervals acquired at the maximum zoom factor yield higher spectral resolution suitable for the Raman spectroscopy. In contrast, larger spectral intervals acquired with the minimum zoom factor yield the lowest spectral resolution suitable for the PL spectroscopy. The proof-of-concept was demonstrated using a zoom lens with a zoom factor of 6. Then prototype spectrometer was demonstrated using a zoom lens with a zoom factor of 18. Special electronics were developed to allow automated Raman and PL spectra acquisition. The graphical user interface (GUI) was developed for spectra acquisition. The resulted spectra obtained were compared to those obtained with a high-quality commercial spectrometer. The comparison was performed on several different materials for Raman and PL and the statistical analysis of variance (ANOVA) was used to evaluate the spectra obtained. The results show that such spectrometer could be an efficient and fast tool for the search of Raman and PL bands of unknown materials and the subsequent spectral acquisition of the spectral interval of interest with a suitable spectral resolution. Therefore, it can be used to develop new materials used in the marine industry to reduce damage caused by the harsh marine environment
Synthesis and characterization of nanostructured silicon anodes for Li-ion cells
No full textLi-ionski galvanski članak je složen sustav pretvorbe i pohrane energije i kao takav predmet je mnogih istraživanja s ciljem upoznavanja mehanizma litijacije i unaprjeđenja elektrokemijskih značajki. U ovom radu istraživani su anodni materijali na bazi silicija za primjenu u Li-ionskim galvanskim člancima. Sintetizirani nanostrukturirani materijali karakterizirani su morfološki i strukturno. Isti materijali ispitani su kao elektrodni materijali za Li-ionske galvanske članke testirane u obliku dvoelektrodne ćelije posebno dizajnirane i izrađene u Laboratoriju za molekulsku fiziku i sinteze novih materijala. Uzorak „saćastog“ poroznog silicija dodatno unaprijeđenog srebrovim nanočesticama pokazao je superiorna elektrokemijska svojstva visokog kapaciteta od 3333 mAh g^−1 pri 0,067 A g^−1 i 2400 mAh g^−1 pri 0,2 A g^−1. U svrhu optimiranja pripravljene su elektrode u tri različite debljine: 20, 40 i 60 μm. Metodom elektrokemijske impedancijske spektroskopije (EIS) detaljno je ispitan mehanizam vezanja Li+ iona kojom se pokazalo da je u slučaju tanjih slojeva interkalacija olakšana. Nadalje, kako bi se povećao broj ciklusa punjenja i pražnjenja, elektrolit je unaprijeđen dodatkom fluoretilen karbonata (FEC) čime je postignuto da nakon 60 ciklusa inicijalni kapacitet padne na 50 % vrijednosti, dok bez dodatka elektrolitu kapacitet već nakon 10 ciklusa padne na 20 % inicijalnog kapaciteta. In situ Ramanovom spektroskopijom praćen je proces litijacije u ovisnosti o potencijalu i vremenu čime je potvrđena promjena u morfologiji kristalnog silicija kao i stvaranje krute elektrolitne faze (SEI). Ovaj rad daje detaljan pregled cijelog procesa od priprave do testiranja materijala galvanskih članaka i umnogome pridonosi razumijevanju mehanizama vezanja i otpuštanja litijevih iona u strukturu poroznog silicija.The lithium-ion (Li-ion) cell is a complex energy conversion and storage system that has been the subject of many studies investigating the lithiation mechanism and improving its electrochemical properties. In this work, silicon-based anode materials were investigated for use in Li-ion cells. Prepared electrodes were tested as electrode materials for Li-ion cells in the form of two-electrode cell specially designed and made in our laboratory. A sample of “honeycomb” porous silicon, enhanced with silver nanoparticles, showed excellent electrochemical properties with a capacity of 3333 mAh g^−1 at 0.067 A g^−1 and 2400 mAh g^−1 at 0.2 A g^−1. To optimize the electrodes, we prepared three different electrode thicknesses: 20, 40 and 60 μm. Using electrochemical impedance spectroscopy (EIS), we investigated the intercalation mechanism of Li+ ions in detail and found that diffusion is facilitated for thinner films. To increase the number of charge and discharge cycles, we improved the electrolyte by adding fluoroethylene carbonate (FEC), which resulted in the capacity fading to 50 % of the initial capacity after 60 cycles, while without the addition of FEC the capacity dropped to 20 % of the initial capacity after 10 cycles. Using in situ Raman spectroscopy, we studied the evolution lithiation as a function of potential and time, which confirmed the change in morphology of the crystalline silicon and formation of a solid electrolyte phase (SEI). This work provides a detailed overview of the entire process from preparation to testing of galvanic cell materials and contributes significantly to the understanding of the mechanisms of alloying and dealloying lithium ions into the structure of porous silicon
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