Browsing by Subject "Silicon crystals."

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Open Access
Germanium alloys for optoelectronic devices
(2008) Erbil, Ayşe
Silicon has been the backbone of the mainstream electronics of the last fifty years. It is however, used in conjunction with other matierals, mainly with its oxides and nitrides. Germanium, on the other hand, is also a group IV element and has been used in the early stages of transistor and detector development. In addition to Si/Ge heterojunctions, bandgap engineering through SiGe alloys has also been used in photodetectors. Recent progress in light emitting devices utilizing Si nanocrystals suggest the use of Ge1-xNx layers as barriers due to its suitable band offsets [1]. Experiments have shown that Ge1-xNx is also a promising material for applications in photodiodes, amplifiers, optic fibers, protective coatings, etc [1]. Both Si and Ge are, however indirect bandgap semiconductors, lacking efficient light emission. On the other hand, strong light emission observed in Si nanocrystals has made the study of semiconductor nanocrystals an expanding field of interest due to potential applications in novel optoelectronic devices [2]. These nanocrystals exhibit strong luminescence and nonlinear optical properties that usually do not appear in the bulk materials [2]. SiGe nanocrystals attract attention due to the possibility of a tunable band gap with composition. In this study, formation of Ge1-xNx thin films and SiGe nanocrystals by plasma enhanced chemical vapor deposition (PECVD) reactor has been studied. We present the growth conditions and experimental characterization of the resulting thin films and nanocrystals. We used ellipsometry, Raman Spectrometry, Fourier Infrared Spectrometry (FTIR) and X-ray photoelectron Spectroscopy (XPS). For SiGe nanocrystals, 4 peaks in the Raman Spectra were observed around 295 cm-1, 400 cm -1, 485 cm-1 and 521 cm-1. These peaks are assigned to the Ge-Ge, Si-Ge, local Si-Si and crystalline Si-Si vibrational modes, respectively [3]. For the Ge1-xNx thin films FTIR spectrum showed the existence of the Ge-N bonds and its band offsets determined by XPS confirm its suitability for optoelectronic devices.
Open Access
Low dimensional structures for optical and electrical applications
(2008) Akça, İmran
Low dimensional structures such as quantum dots have been particularly attractive because of their fundamental physical properties and their potential applications in various devices in integrated optics and microelectronics. This thesis presents optical and electrical applications of low dimensional structures. For this purpose we have studied silicon and germanium nanocrystals for flash memory applications and InAs quantum dots for optical modulators. As a quantum dot, nanocrystals can be used as storage media for carriers in flash memories. Performance of a nanocrystal memory device can be expressed in terms of write/erase speed, carrier retention time and cycling durability. Charge and discharge dynamics of PECVD grown nanocrystals were studied. Electron and hole charge and discharge currents were observed to differ significantly and strongly depend on annealing conditions chosen for the formation of nanocrystals. Our experimental results revealed that, discharge currents were dominated by the interface layer acting as a quantum well for holes and route for direct tunneling for electrons. On the other hand, possibility of obtaining quantum dots with enhanced electro-optic and/or electro-absorption coefficients makes them attractive for use in light modulation. Therefore, waveguides of multilayer InAs quantum dots were studied. Electro-optic measurements were conducted at 1.5 µm and clear Fabry-Perot resonances were obtained. The voltage dependent Fabry-Perot measurements revealed that 6 V was sufficient for full on/off modulation. Electroabsorption measurements were conducted at both 1.3 and 1.5 µm. Since the structure lases at 1285 nm, high absorption values at 1309 nm were obtained. The absorption spectrum of the samples was also studied under applied electric field. Absorption spectra of all samples shift to lower photon energies with increasing electric field.
Open Access
On the strain in silicon nanocrystals
(2009) Yılmaz, Dündar
In this Thesis we present our achievements towards an understanding of atomistic strain mechanisms and interface chemistry in silicon nanocrystals. The structural control of silicon nanocrystals embedded in amorphous oxide is currently an important technological problem. First, our initial attempt is described to simulate the structural behavior of silicon nanocrystals embedded in amorphous oxide matrix based on simple valence force fields as described by Keatingtype potentials. Next, the interface chemistry of silicon nanocrystals (NCs) embedded in amorphous oxide matrix is studied through molecular dynamics simulations with the chemical environment being governed by the reactive force field model. Our results indicate that the Si NC-oxide interface is more involved than the previously proposed schemes which were based on solely simple bridge or double bonds. We identify different types of three-coordinated oxygen complexes, previously not noted. The abundance and the charge distribution of each oxygen complex is determined as a function of the NC size as well as the transitions among them. Strain has a crucial effect on the optical and electronic properties of nanostructures. We calculate the atomistic strain distribution in silicon NCsup to a diameter of 3.2 nm embedded in an amorphous silicon dioxide matrix. A seemingly conflicting picture arises when the strain field is expressed in terms of bond lengths versus volumetric strain. The strain profile in either case shows uniform behavior in the core, however it becomes nonuniform within 2- 3 ˚A distance to the NC surface: tensile for bond lengths whereas compressive for volumetric strain. We reconcile their coexistence by an atomistic strain analysis. Vibrational density of states (VDOS) affects the optical properties of Si-NCs. VDOS obtained by calculating velocity autocorrelation function (VACF) using velocities of the atoms is extracted from the molecular dynamics simulations. The information on bonding topology enables classification of atoms in the system with respect to their neighbor atoms. With help of this information we separate contributions of different type of atoms to the VDOS. Calculating VACF of different type of atoms such as surface atoms and core atoms of nanocrystal, to the system facilitates understanding of the effects of strain fields and interface chemistry to the VDOS.