Electrochemical etching of silicon on insulator

Cilj 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