| Abstract | Otkrićem 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. |
| Abstract (english) | 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. |
