Nanostrukture poluvodičkih materijala intenzivno se proučavaju dugi niz godina radi interesantnih svojstva koja se javljaju kada je dimenzionalnost sustava smanjena te se u poluvodički materijalima počinje javljati efekt kvantnog zatočenja. U nanostrukturiranim poluvodičkim materijalima Ge i Si uočena je fotoluminiscencija čiji mehanizmi nisu u potpunosti razjašnjeni. U tu svrhu napravljene su nanostrukture s germanijem tehnikom rasprašenja magnetronskog izvora čestica. Dobivene nanostrukture su istražene uz pomoć GIWAXS spektroskopije, FTIR spektroskopije, UV-Vis spektroskopije, fotoluminiscencije i vremenski razlučene fotoluminiscencije. Nanočestice nastale u 150 nm debelom sloju nastalom miješanjem SiO_2 i Ge u omjeru 50:50 pokazale su fotoluminiscentna svojstva koja su posljedica Si-NBOHC, Ge-NBOHC, Si-ODC defekata, te najintenzivniji doprinos koji je pripisan nakupinama germanija u SiO_2 matrici. Veličina nanočestica germanija je 5 nm te raste s temperaturom grijanja do 8 nm. U fotoluminiscentnom spektru nije uočen doprinos koji bi pripisali efektu kvantnog zatočenja. Stvaranjem višeslojnih uzoraka s debljinom sloja od 4 nm u kojem se nalazi poluvodič germanij ograničavamo veličinu nanočestice koja je između 4–5 nm, te se pojavljuje fotoluminiscentni doprinos koji bi mogao odgovarati efektu kvantnog zatočenja u nanočesticama germanija. Deformacija koju osjećaju kristaliti posljedica je istezanja, te je deformacija veća kod nanočestica većih dimenzija. U ovom radu uspješno su napravljene višeslojne poluvodičke nanostrukture Ge i Si koristeći se tehnikom naparavanja pomoću elektronskog snopa koja je nešto jednostavnija i manje zahtjevna. Fotoluminiscentni spektar pokazao je doprinos koji dolazi od Si-NBOHC i Si-ODC defekata, a najintenzivniji doprinos pripisan je nakupinama poluvodiča u amorfnoj SiO_2 matrici. U crvenom dijelu spektra nalaze se doprinosi koje pripisujemo nanočesticama poluvodiča. Naprezanje kristalita posljedica je istezanja kristalita germanija i silicija, jedino kod kristalita silicija na temperaturama ≤ 1000 °C posljedica deformacije je kompresija. Naprezanje je veće na većim kristalitima silicija i germanija.
|Abstract (english)|| |
Today, significant research efforts are being invested in the miniaturization of basic logic circuits and the integration of optics with electronics. By reducing the dimensions of structures to nanometer sizes and low dimensionality, unique properties and quantum effects become relevant. Varying the size and shape of the nanoparticles modifies the electronic band structure and precisely defined optical and electronic properties can be obtained. Silicon semiconductor nanostructures have an advantage due to compatibility with existing technology, while germanium provides a greater possibility of changing the electronic structure around the energy gap compared to silicon. Despite many years of research and a large number of published papers, it is still not possible to reliably and economically produce nanostructures of certain dimensions and with a small number of defects. All the mechanisms of photoluminescence in semiconductor nanostructures have not yet been identified and the question of how individual mechanisms work remains open. In this dissertation, an attempt was made to identify all observed photoluminescent contribution. These contributions originate from defects in nanostructures and are result from quantum confinement. An effort was made to determine the locations of photoluminescent defects in the nanostructure and the size of the nano crystallites that develop in the nanostructures. Moreover, here is shown how different sample production techniques affect the quality of nanostructures, i.e., the photoluminescent properties of nanostructures. In a theoretical introduction, it is explained when quantum confinement occurs and how it manifests itself in semiconductors with an indirect energy gap. Various examples of theoretical and experimental research on how quantum confinement affects the energy gap in silicon and germanium semiconductor nanoparticles are described. A brief overview of the possible recombination mechanisms that occur when we excite a sample with a laser is written. Then, an overview of possible defects that occur in the dielectric matrix of silicon dioxide is given. In addition to the dielectric, defects also occur on the surface of nanoparticles, and the number of defects increases with the size of the nanoparticle. Due to the different structural parameters between the nanoparticles and the surrounding dielectric matrix, stress felt by the silicon nanoparticles where the optical properties can be affected. The properties of the matrix adjacent to the surface of the nanoparticle can significantly affect defects and luminescent mechanisms. At the end of the chapter, the connection of optical parameters, such as the coefficient of reflectivity and absorption, with the refractive index and the dielectric function is given. Next, experimental setup and measurement methods is explained. Firstly, the technique of creating the sample is described: scattering from a magnetron source of particles and evaporation using an electron beam. The structural properties were determined by xii investigation of X-ray diffraction at a small angle of incidence and the Scherrer formula was used to determine the crystallite size. In addition to the size of the crystallite, the effect of deformation on crystallites is observed by this technique, which is determined by the Williams-Hall method and determines inhomogeneous deformation. From optical methods, infrared spectroscopy with Fourier transform, ultraviolet and visible spectroscopy in which reflectance was measured, and photoluminescent measurements were performed. Stationary and time-resolved photoluminescence was performed and described in more detail. The tested samples are then described. The technique of magnetron sputtering is a standard technique and it is used to make two types of nanostructures with germanium and silicon dioxide. The first structure with a thickness of 150 nm was obtained by simultaneous deposition of two targets SiO_2 and Ge in a ratio of 50:50. The second structure was made as a multilayer structure in which the layers of SiO_2 dielectrics and the layer in which the mixture of SiO_2 and Ge was in a ratio of 50:50 and the thickness of each layer was 4 nm. In the case of the obtained structures, the influence of the annealing temperature and the deposition temperature on the development of the structures obtained by heating the samples and their structural and photoluminescent properties were examined. Nanostructures obtained by electron beam evaporation consist of a Ge or Si semiconductor contained in a SiO_2 matrix. The particle size was controlled via a deposited semiconductor layer thickness of 2 or 4 nm. The results of this work show that the deposition temperature has an effect on the size of the semiconductor crystallite. The influence is greater in structures formed as a mixture of SiO_2 and Ge. Germanium crystals grow from 5 nm to 8 nm with increasing temperature, in addition, the dimensions of the crystallites depend a greatly on the plane in which they grow. In multilayer nanostructures, the size of the formed crystallites is determined primarily by the thickness of the layer. The temperature of crystallite formation is lower in multilayer samples. The stress of the crystallite was mainly due to the stretching of the germanium and silicon crystallites, only in the case of silicon crystallites at temperatures ≤ 1000 °C the deformation results in compression. The stress is higher on larger crystals. When the stress becomes large enough, the semiconductor crystals disappear, leaving voids behind. After the germanium crystals disappear, the photoluminescent signal increases, which leads to the conclusion that photoluminescence arises from defects. Photoluminescence is very similar in all semiconductor nanostructures of germanium. The greatest contribution to photoluminescence comes from defects, and only in the case of multilayer samples is observed the contribution that is a possible consequence of quantum confinement. In this paper, the mechanisms of photoluminescence arising from Si-NBOHC, Ge-NBOHC, Si-ODC defects were identified, and the most intense photoluminescent peak at 2.4 eV, which is attributed to defects in semiconductor clusters to the amorphous SiO_2 matrix, was better investigated. Ge-NBOHC defect is present only in samples obtained by sputtering technique from a magnetron source. In the structure formed as a mixture of Ge and SiO_2, Ge-NBOHC defect is found only at temperatures at which germanium crystals are observed, while in multilayer samples this defect occurs at crystallite formation temperatures xiii and survives at all heating temperatures. Other defects are observed in all samples. Nanostructures with silicon crystals that are produced with electron beam evaporation have a more pronounced contribution in the red part of the spectrum than germanium crystals and this part of the spectrum is attributed to quantum effects on Si or Ge semiconductor nanoparticles. Ge nanoparticles show only one photoluminescent contribution while Si nanoparticles show two components in the red region. When we look at the overall results, we come to the conclusion that Ge-NBOHC defects are formed in the shell around germanium nanoparticles and only in samples obtained by the sputtering technique from a magnetron source of particles. In samples obtained by electron beam evaporation, the Ge-NBOHC defect is not visible. However, the higher proportion of Si- NBOHC defects in the total spectrum at the temperatures at which nanoparticle formation begins are noticeable. This defect is present in all samples and at all temperatures, which indicates that the defect is trapped in the dielectric matrix, but also around nanoparticles. The Si-ODC defect shows that it is related to the Si-NBOHC defect and their ratio is inversely proportional. The defect that is visible at 2.4-2.3 eV and is attributed to defects in semiconductor clusters that remained in the amorphous SiO_2 matrix. This photoluminescent contribution is described by a stretched exponential curve with a dispersion coefficient varying 0.4-0.7 and a lifetime of 100-500 ns, which depends a lot on the individual nanostructure. Due to the complexity of this contribution, the mechanisms of this breakdown are still unknown and there is still much to research.