Browsing by Subject "Amorphous materials"

Now showing 1 - 11 of 11

Open Access
Atomistic structure simulation of silicon nanocrystals driven with suboxide penalty energies
(American Scientific Publishers, 2008) Yılmaz, Dündar E.; Bulutay, Ceyhun; Çağın, T.
The structural control of silicon nanocrystals embedded in amorphous oxide is currently an important technological problem. In this work, an approach is presented to simulate the structural behavior of silicon nanocrystals embedded in amorphous oxide matrix based on simple valence force fields as described by Keating-type potentials. After generating an amorphous silicon-rich-oxide, its evolution towards an embedded nanocrystal is driven by the oxygen diffusion process implemented in the form of a Metropolis algorithm based on the suboxide penalty energies. However, it is observed that such an approach cannot satisfactorily reproduce the shape of annealed nanocrystals. As a remedy, the asphericity and surface-to-volume minimization constraints are imposed. With the aid of such a multilevel approach, realistic-sized silicon nanocrystals can be simulated. Prediction for the nanocrystal size at a chosen oxygen molar fraction matches reasonably well with the experimental data when the interface region is also accounted. The necessity for additional shape constraints suggests the use of more involved force fields including long-range forces as well as accommodating different chemical environments such as the double bonds.
Open Access
Computational modeling of quantum-confined impact ionization in Si nanocrystals embedded in SiO2
(2007) Sevik, C.; Bulutay, C.
Injected carriers from the contacts to delocalized bulk states of the oxide matrix via Fowler-Nordheim tunneling can give rise to quantum-confined impact ionization (QCII) of the nanocrystal (NC) valence electrons. This process is responsible for the creation of confined excitons in NCs, which is a key luminescence mechanism. For a realistic modeling of QCII in Si NCs, a number of tools are combined: ensemble Monte Carlo (EMC) charge transport, ab initio modeling for oxide matrix, pseudopotential NC electronic states together with the closed-form analytical expression for the Coulomb matrix element of the QCII. To characterize the transport properties of the embedding amorphous SiO2, ab initio band structure and density of states of the α-quartz phase of SiO2 are employed. The confined states of the Si NC are obtained by solving the atomistic pseudopotential Hamiltonian. With these ingredients, realistic modeling of the QCII process involving a SiO2 bulk state hot carrier and the NC valence electrons is provided.
Open Access
Different behaviour of magnetic impurities in crystalline and amorphous states of superconductors
(Institute of Physics, 2002) Park, M.-A.; Savran, K.; Kim, Y.-J.
It has been observed that the effect of magnetic impurities in a superconductor is drastically different depending on whether the host superconductor is in the crystalline or the amorphous state. Based on the recent theory of Kim and Overhauser (KO), it is shown that as the system is getting disordered, the initial slope of the Tc depression is decreasing by a factor √ℓ/ξ0, when the mean free path ℓ becomes smaller than the BCS coherence length ξ0, which is in agreement with experimental findings. In addition, for a superconductor in a crystalline state in the presence of magnetic impurities the superconducting transition temperature Tc drops sharply from about 50% of Tc0 (for a pure system) to zero near the critical impurity concentration. This pure limit behaviour was indeed found by Roden and Zimmermeyer in crystalline Cd. Recently, Porto and Parpia have also found the same pure limit behaviour in superfluid He-3 in aerogel, which may be understood within the framework of the KO theory.
Open Access
Effect of milling time on the structure, micro-hardness, and thermal behavior of amorphous/nanocrystalline TiNiCu shape memory alloys developed by mechanical alloying
(Elsevier Ltd, 2014) Alijani F.; Amini, R.; Ghaffari, M.; Alizadeh, M.; Okyay, Ali Kemal
In the present paper, the effect of milling process on the chemical composition, structure, microhardness, and thermal behavior of Ti-41Ni-9Cu compounds developed by mechanical alloying was evaluated. The structural characteristic of the alloyed powders was evaluated by X-ray diffraction (XRD). The chemical composition homogeneity and the powder morphology and size were studied by scanning electron microscopy coupled with electron dispersive X-ray spectroscopy. Moreover, the Vickers micro-indentation hardness of the powders milled for different milling times was determined. Finally, the thermal behavior of the as-milled powders was studied by differential scanning calorimetery. According to the results, at the initial stages of milling (typically 0-12. h), the structure consisted of a Ni solid solution and amorphous phase, and by the milling evolution, nanocrystalline martensite (B19') and austenite (B2) phases were initially formed from the initial materials and then from the amorphous phase. It was found that by the milling development, the composition uniformity is increased, the inter-layer thickness is reduced, and the powders microhardness is initially increased, then reduced, and afterward re-increased. It was also realized that the thermal behavior of the alloyed powders and the structure of heat treated samples is considerably affected by the milling time.
Open Access
Photoluminescence from a VCSEL structure a-SiNx:H microcavity
(IEEE, 1999) Serpengüzel, A.; Darici, Y.
Microcavity effects on the photoluminescence (PL) of porous Si has already been reported. Recently, we have observed visible and near infrared (IR) PL from hydrogenated amorphous Si nitride (a-SiNx:H) grown by low temperature PECVD. We have also reported the enhancement and inhibition of PL in an a-SiNx:H microcavity formed with metallic mirrors. The a-SiNx:H used in the microcavity was grown both with and without ammonia (NH/sub 3/). For the Si rich a-SiNx:H grown without NH/sub 3/, the PL is in the red-near IR. For the N rich a-SiNx:H grown with NH/sub 3/, the PL is in the blue-green. In this paper, we report on the bright and spectrally pure PL of a-SiNx:H in a VCSEL structure microcavity.
Open Access
Postdeposition annealing on RF-sputtered SrTiO3 thin films
(AVS Science and Technology Society, 2017) Bayrak, T.; Kizir,S.; Kahveci, E.; Bıyıklı, N.; Goldenberg, E.
Understanding of structural, optical, and electrical properties of thin films are very important for a reliable device performance. In the present work, the effect of postdeposition annealing on stoichiometric SrTiO3 (STO) thin films grown by radio frequency magnetron sputtering at room temperature on p-type Si (100) and quartz substrates were studied. Highly transparent and well adhered thin films were obtained in visible and near infrared regions. As-deposited films were amorphous, while nanocrystalline and polycrystalline phases of the STO thin films formed as a function of annealing temperature. Films annealed at 300 ï¿½C showed nanocrystallinity with some amorphous phase. Crystallization started after 15 min annealing at 700 ï¿½C, and further improved for films annealed at 800 ï¿½C. However, crystallinity reduced for films which were annealed at 900 ï¿½C. The optical and electrical properties of STO thin films affected by postdeposition annealing at 800 ï¿½C: Eg values decreased from 4.50 to 4.18 eV, n(λ) values (at 550 nm) increased from 1.81 to 2.16. The surface roughness increased with the annealing temperature due to the increased crystallite size, densification and following void formation which can be seen from the scanning electron microscopy images. The highest dielectric constants (46 at 100 kHz) observed for films annealed at 800 ï¿½C; however, it was lower for 300 ï¿½C annealed (25 at 100 kHz) and as-deposited (7 at 100 kHz) STO films having ∼80 nm thickness.
Open Access
Strong enhancement of spontaneous emission in hydrogenated amorphous silicon nitride coupled-microcavity structures
(IEEE, 2001) Bayındır, Mehmet; Tanrıseven, Selim; Aydınlı, Atilla; Özbay, Ekmel
The modification of spontaneous emission from the hydrogenated amorphous silicon nitride layers in a coupled-microcavity (CMC) structure was investigated. The CMC structure was composed of alternating silicon-oxide and silicon-nitride multilayers. The results showed that the strong enhancement of spontaneous emisssion can be achieved throughout the cavity band.
Open Access
Structural, optical and electrical characteristics BaSrTiOx thin films: Effect of deposition pressure and annealing
(Elsevier BV * North-Holland, 2017) Bayrak, T.; Ozgit-Akgun, C.; Goldenberg, E.
Among perovskite oxide materials, BaSrTiOx (BST) has attracted great attention due to its potential applications in oxide-based electronics. However, reliability and efficiency of BST thin films strongly depend on the precise knowledge of the film microstructure, as well as optical and electrical properties. In the present work, BST films were deposited at room temperature using radio frequency magnetron sputtering technique. The impact of deposition pressure, partial oxygen flow, and post-deposition annealing treatment on film microstructure, surface morphology, refractive index, and dielectric constants were studied by X-ray diffraction, scanning electron microscopy, spectrophotometry, ellipsometry, photoluminescence, as well as capacitance-voltage measurements. Well-adhered and uniform amorphous films were obtained at room temperature. For all as-deposited films, the average optical transmission was ~ 85% in the VIS-NIR spectrum. The refractive indices of BST films were in the range of 1.90–2.07 (λ = 550 nm). Post-deposition annealing at 800 °C for 1 h resulted in polycrystalline thin films with increased refractive indices and dielectric constants, however reduced optical transmission values. Frequency dependent dielectric constants were found to be in the range of 46–72. However, the observed leakage current was relatively small, about 1 μA. The highest FOM values were obtained for films deposited at 0.67 Pa pressures, while charge storage capacity values increased with increased deposition pressure. Results show that room-temperature grown BST films have potential for device applications.
Open Access
Synthesis and size differentiation of Ge nanocrystals in amorphous SiO 2
(Springer, 2006) Aǧan, S.; Çelik-Aktaş, A.; Zuo, J. M.; Dana, A.; Aydınlı, Atilla
Germanosilicate layers were grown on Si substrates by plasma enhanced chemical vapor deposition (PECVD) and annealed at different temperatures ranging from 700-1010 °C for durations of 5 to 60 min. Transmission electron microscopy (TEM) was used to investigate Ge nanocrystal formation in SiO 2:Ge films. High-resolution cross section TEM images, electron energy-loss spectroscopy and energy dispersive X-ray analysis (EDX) data indicate that Ge nanocrystals are present in the amorphous silicon dioxide films. These nanocrystals are formed in two spatially separated layers with average sizes of 15 and 50 nm, respectively. EDX analysis indicates that Ge also diffuses into the Si substrate.
Open Access
TiO2 thin film transistor by atomic layer deposition
(SPIE, 2013) Okyay, Ali Kemal; Oruç, Feyza B.; Çimen, Furkan; Aygün, Levent E.
In this study, TiO2 films were deposited using thermal Atomic Layer Deposition (ALD) system. It is observed that asdeposited ALD TiO 2 films are amorphous and not suitable as TFT channel material. In order to use the film as channel material, a post-annealing process is needed. Annealed films transform into a polycrystalline form containing mixed anatase and rutile phases. For this purpose, devices are annealed at 475°C and observed that their threshold voltage value is 6.5V, subthreshold slope is 0.35 V/dec, Ion/Ioff ratios 2.5×106 and mobility value is 0.672 cm2/V.s. Optical response measurements showed that devices exhibits decent performance at ultraviolet region where TiO 2 has band to band absorption mechanism. © 2013 SPIE.
Open Access
Visible photoluminescence from low temperature deposited hydrogenated amorphous silicon nitride
(Pergamon Press, 1996) Aydınlı, A.; Serpengüzel, A.; Vardar, D.
Hydrogenated amorphous silicon nitride (a-SiNx:H) samples have been prepared by plasma enhanced chemical vapor deposition (PECVD) using a mixture of silane (SiH4), nitrogen and ammonia (NH3). Most films exhibit visible photoluminescence (PL) and some emit strong PL after annealing. While films grown without NH3 exhibit PL in the deep red, those grown with NH3 show PL in the green. The PL properties of these films with no oxygen (O) content are similar to those of silicon oxide (SiOx) films and porous Si. Using infrared and X-ray Photoelectron Spectroscopy, we suggest that PL from a-SiNx:H films originate from Si clusters which form during PECVD and crystallize upon annealing. We propose that the presence of O is not necessary for efficient PL.