Çoklu ölçekte kendiliğinden oluşan üç boyutlu eşyösüz rastgele silisyum kuvantum nokta ağı.

The most important problem limiting the impact of nanotechnology is probably the difficulty in effectively linking nanoscale materials and processes to the macroscopic world. Topology and material properties are intricately coupled and conditions that pertain to atomic, microscopic and macroscopic scales are often seemingly mutually exclusive. This thesis introduces a state-of-the-art nanostructure that hierarchically builds itself from the atomic to the microscopic scales, which can connect to the macroscopic world without detracting from its nanoscale properties. The three dimensional anisotropic random network of silicon quantum dots is largely isotropic in the atomic scale but it grows to become anisotropic in the microscopic scale. We show that quantum confinement is preserved and the current flows through the network without relying on inefficient tunnelling currents. Former pertains to the atomic scale and latter manifesting at the microscale; these two scale-dependent features were thought to be mutually exclusive prior to this thesis. The structure is self-assembled from a silicon-rich silicon oxide thin film. Microscale self-assembly is kinetically driven under nonequilibrium conditions established by magnetron sputter deposition and relies on control of surface diffusion through a surface temperature gradient. Atomic scale self-assembly is chemically driven under local nonequilibrium conditions provided by fast stochastic deposition and relies on control of phase separation by stabilizing nominally unstable suboxides. We show that our fabrication methodology is inherently modular, material-independent, and is not affected substantially by the initial conditions, as self-assembly under nonequilibrium conditions and nonlinear dynamics sweeps aside a large number of factors that influence the details of thin-film growth, but provides simple a couple of “rules” with clearly identifiable corresponding experimental conditions to determine the final morphology.Ph.D. - Doctoral Progra

Gözenekli si ve ge nanoyapılarının ileri teknoloji güneş gözelerine uygulanması

TÜBİTAK MFAG01.04.2014Uluslararası ikili işbirliği projesi olan bu çalışmada amaca uygun olarak gözenekli silisyum ve germanyum nanoyapılar silisyum oksit matris içerisinde büyütülmüş ve optoleektronik aygıt üretiminde kullanılmıştır. Söz konusu yapıları elde edebilmek için silisyum ve germanyum-zengin silisyum oksit ince filmler fiziksel yolla buhar depolama yöntemlerinden yararlanarak üretilmiş ve kristal oluşumunu sağlamak için de yüksek sıcaklık fırını, hızlı termal tavlama ve lazer tavlama yöntemleri gibi farklı teknikler kullanılarak tavlanmıştır. Bu sayede farklı boyutlarda nanokristaller oluşturulmuş ve elde edilen yapıların optik band aralığının değişken olması sağlanmıştır. Böylece bu proje ile güneş spektrumundan daha fazla ve daha verimli yararlanmak amaçlı optik band aralığı ayarlanabilir malzemeler üretmiş bulunmaktayız. Öte yandan ürtetilen filmler, aygıt üretiminde gerekli olan elektrik akım geçişini sağlamk açısından da istenen şartları sağlamıştır. Böylece proejnin en önemli hedefi olan kuantum etkileri koruyarak, iletken tabakaların elde edilmesi hedefine ulaşılmıştır. Daha sonra bu yapılar, heteroeklem türü p-n eklemlei üretilmiş ve bu eklemlerin elektro- optik özellikleri belirlenmiştir. Si nanoyapılı örnekler güneş gözesi özellikleri sergilerken, Ge nanoyapılı örnekler de daha çok fotoalgılama yeteneği öne çıkmıştır. Bu çalışma ile ilk defa üretilen ve umut verici sonuçlar veren bu tür aygıtların daha da geliştirilmesi için yeni çalışmaların yürütülmesi gerekmektedir. Proje önerisi 24 ay gibi nispeten kısa bir sürede çok sayıda iş yapmayı hedefleyen bir proje olarak sunulmuştur. Uluslararası bir organizasonu da içeren zorlu bir programı olanproje hedeflerine büyük ölçüde ulaşmıştır. Çok sayıda deney ve çalışma yürütülmüştür. Bu çalışmaların bir özeti bu sonuç raporunda sunulmuştur.In this international bilateral cooperation project, silicon and germanium nanostructures embedded in silicon oxide matrix have been produced, optimized and used in some optoelectronic devices. In order to obtained these new material types, Si and Ge materials were co-sputtered together with SiO2 on a substrate, and annealed using conventional and rapid thermal annealing systems as well as laser annealing systems. In this way, nanostructures embedded in a dielectric matrix have been obtained. Such nanostructures are expected to yield materials with tunable band gap which depends on this size of the nanoparticles. A device having different band gap is expected to provide a more efficient light harvesting from the solar radiation. On the other hand, when these nanostructure are somehow interconnected, electrical transport becomes more feasible, which is contrary to the case where isolated nanocrystals are used. We have reached this goal through a series of experiments during this project. We have shown that both electrical transport and quantum confinement can be obtained in the same material system. After having optimized the material system, we have applied them to heterojunction type p-n diodes for demenstration. Devices with Si nanostructure have exhibited solar cell properties with weak efficiency, while those with Ge nanostructures have shown photodiode features only. These type of device have been produced for he first time in this project. They have shown some promising features. However, they need to be studied and optimized for an actual appliction. This porject was submitted as an ambitious study to be performed in a relatively short time. In addition to its scientific and tecjhnical program, it also included international oragnization tasks, which sometimes slows down the prject execution In spite of laa these difficulties, and the heavy content, we have achived most of goals through extensive experiments and studies. Below, we present a summary of our project acitivities and the results obtained from these studies

In-chip microstructures and photonic devices fabricated by nonlinear laser lithography deep inside silicon

Silicon is an excellent material for microelectronics and integrated photonics1–3 with untapped potential for mid-IR optics4. Despite broad recognition of the importance of the third dimension5,6, current lithography methods do not allow fabrication of photonic devices and functional microelements directly inside silicon chips. Even relatively simple curved geometries cannot be realised with techniques like reactive ion etching. Embedded optical elements, like in glass7, electronic devices, and better electronic-photonic integration are lacking8. Here, we demonstrate laser-based fabrication of complex 3D structures deep inside silicon using 1 µm-sized dots and rod-like structures of adjustable length as basic building blocks. The laser-modified Si has a different optical index than unmodified parts, which enables numerous photonic devices. Optionally, these parts are chemically etched to produce desired 3D shapes. We exemplify a plethora of subsurface, i.e., “in-chip” microstructures for microfluidic cooling of chips, vias, MEMS, photovoltaic applications and photonic devices that match or surpass the corresponding state-of-the-art device performances

Spectroscopic ellipsometry study of Si nanocrystals embedded in a SiOx matrix: Modeling and optical characterization

In this work, we report on a spectroscopic ellipsometry study of thin films of silicon nanocrystals embedded in a SiOx matrix that is performed to better understand how substoichiometric SiOx species affect the optical properties of these systems. The silicon nanocrystals are fabricated in a SiOx matrix by thermal annealing of magnetron-sputtered thin films in a wide range of x values (0< x< 2) in order to produce every possible substoichiometric SiOx species in various fractions. A generic optical model to describe the thin film content with various SiOx volumetric fractions is developed and used to analyze the data obtained from the spectroscopic ellipsometry. Tauc-Lorentz and Bruggeman effective medium approximation models are employed to determine the ellipsometric angles (psi and Delta) and the complex dielectric function. We show that the optical properties of the nc-Si embedded in a SiOx matrix are highly dependent on the value of x and on the volumetric fractions of the substoichiometric SiOx species present in the system

Influence of Ge content and annealing conditions on the PL properties of nc-Si1-xGex embedded in SiO2 matrix in weak quantum confined regime

Ilday, Serim/0000-0002-1620-6367WOS: 000341466300027Fabrication of Si (nc-Si), Ge (nc-Ge), and Si1-xGex (nc-Si1-xGex) nanocrystals embedded in SiO2 matrix is achieved by thermal annealing of magnetron-sputtered thin films. Effects of annealing conditions, namely duration and temperature, as well as Ge content on the photoluminescence properties are investigated. Origin and evolution of the photoluminescence signal in the weak quantum confinement regime are discussed. It is found that photoluminescence signals can be decomposed into four Gaussian peaks originating from Ge-related radiative defects located at the sub-oxide (GeOx), either inside the matrix or at the interface region (peak M), nc-Si1-xGex/SiO2 interface-related localized states (peak I), localized states in the amorphous Si1-xGex bandgap (peak A) and quantum confinement of excitons in small nanocrystals (peak Q). the role of small and large nanocrystals in the photoluminescence mechanism is investigated by varying the mean nanocrystal size from 3 nm to 23 nm (from strong to weak quantum confined regime). Our results demonstrate that the quantum confinement effect in Ge nanocrystals manifests though spectral blueshift due to increase in Ge content. We also propose that the decreasing photoluminescence signal intensity with an increase in Ge content may originate from Ge-related nonradiative P-b centers. (C) 2014 Elsevier B.V. All rights reserved.Turkish Ministry of EducationTurkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [KHK-649/61 md]; TUBITAK (The Scientific & Technological Research Council of Turkey) ProjectTurkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [106M549]The authors acknowledge the Turkish Ministry of Education (KHK-649/61 md) and TUBITAK (The Scientific & Technological Research Council of Turkey) Project Contract no. 106M549 for financial support. the authors also acknowledge the efforts of S. Foss and I. Yildiz in the early stages of this study

Rich complex behaviour of self-assembled nanoparticles far from equilibrium

A profoundly fundamental question at the interface between physics and biology remains open: what are the minimum requirements for emergence of complex behaviour from non-living systems? Here, we address this question and report complex behaviour of tens to thousands of colloidal nanoparticles in a system designed to be as plain as possible: the system is driven far from equilibrium by ultrafast laser pulses that create spatiotemporal temperature gradients, inducing Marangoni flow that drags particles towards aggregation; strong Brownian motion, used as source of fluctuations, opposes aggregation. Nonlinear feedback mechanisms naturally arise between flow, aggregate and Brownian motion, allowing fast external control with minimal intervention. Consequently, complex behaviour, analogous to those seen in living organisms, emerges, whereby aggregates can self-sustain, self-regulate, self-replicate, self-heal and can be transferred from one location to another, all within seconds. Aggregates can comprise only one pattern or bifurcated patterns can coexist, compete, endure or perish

Nano patterning of AISI 316L stainless steel with Nonlinear Laser Lithography: Sliding under dry and oil-lubricated conditions

Femtosecond laser-based Nonlinear Laser Lithography (NLL) was applied to AISI 316L stainless steel, which requires surface modification to achieve satisfactory tribological behaviour. NLL advances over the well-known Laser Induced Periodic Surface Structures (LIPSS) in terms of uniformity and long-range order of high speeds, over large areas. A galvanometric scanner head was used for an high production rate. Dry and lubricated sliding tests, considering different orientations of the nanotexture showed that COF values after NLL treatment are significantly lower. In lubricated tests, COF values of NLL-treated surfaces are nearly half the values of untreated surfaces, whereas the difference further increases when measured in dry conditions, where the orientation of the surface texturing influences the results

Nonlinear laser lithography for indefinitely large-area nanostructuring with femtosecond pulses

Dynamical systems based on the interplay of nonlinear feedback mechanisms are ubiquitous in nature(1-5) . Well-understood examples from photonics include mode locking(6) and a broad class of fractal optics(7), including self-similarity(8). In addition to the fundamental interest in such systems, fascinating technical functionalities that are difficult or even impossible to achieve with linear systems can emerge naturally from them(7) if the right control tools can be applied. Here, we demonstrate a method that exploits positive nonlocal feedback to initiate, and negative local feedback to regulate, the growth of ultrafast laser- induced metal-oxide nanostructures with unprecedented uniformity, at high speed, low cost and on non-planar or flexible surfaces. The nonlocal nature of the feedback allows us to stitch the nanostructures seamlessly, enabling coverage of indefinitely large areas with subnanometre uniformity in periodicity. We demonstrate our approach through the fabrication of titanium dioxide and tungsten oxide nanostructures, but it can also be extended to a large variety of other materials

Laser-slicing of silicon with 3D nonlinear laser lithography

Recently, we have showed a direct laser writing method that exploits nonlinear interactions to form subsurface modifications in silicon. Here, we use the technique to demonstrate laser-slicing of silicon and its applications