Browsing by Author "Goldenberg, E."

Now showing 1 - 9 of 9

Open Access
Effect of O2/Ar flow ratio and post-deposition annealing on the structural, optical and electrical characteristics of SrTiO3 thin films deposited by RF sputtering at room temperature
(Elsevier, 2015) Goldenberg, E.; Bayrak, T.; Ozgit Akgun, C.; Haider A.; Leghari, S.A.; Kumar, M.; Bıyıklı, Necmi
SrTiO3 (STO) thin films have been prepared by reactive RF magnetron sputtering on Si (100) and UV fused silica substrates at room temperature. The effect of oxygen flow on film characteristics was investigated at a total gas flow of 30 sccm, for various O2/O2 + Ar flow rate ratios. As-deposited films were annealed at 700 °C in oxygen atmosphere for 1 h. Post-deposition annealing improved both film crystallinity and spectral transmittance. Film microstructure, along with optical and electrical properties, was evaluated for both as-deposited and annealed films. Abroad photoluminescence emission was observed within the spectral range of 2.75–3.50 eV for all STO thin films irrespective of their deposition parameters. Upon annealing, the optical band gap of the film deposited with 0% O2 concentration slightly blue-shifted, while the other samples grown at higher oxygen partial pressure did not show any shift. Refractive indices (n) (at 550 nm) were in the range of 2.05 to 2.09, and 2.10 to 2.12 for as-deposited and annealed films, respectively. Dielectric constant values (at 100 kHz) within the range of 30–66 were obtained for film thicknesses less than 300 nm, which decreased to ~30–38 after postdeposition annealing.
Open Access
Fabrication of flexible polymer–GaN core–shell nanofibers by the combination of electrospinning and hollow cathode plasma-assisted atomic layer deposition
(Royal Society of Chemistry, 2015) Ozgit Akgun, C.; Kayaci, F.; Vempati S.; Haider A.; Celebioglu A.; Goldenberg, E.; Kizir S.; Uyar, Tamer; Bıyıklı, Necmi
Here we demonstrate the combination of electrospinning and hollow cathode plasma-assisted atomic layer deposition (HCPA-ALD) processes by fabricating flexible polymer-GaN organic-inorganic core-shell nanofibers at a processing temperature much lower than that needed for the preparation of conventional GaN ceramic nanofibers. Polymer-GaN organic-inorganic core-shell nanofibers fabricated by the HCPA-ALD of GaN on electrospun polymeric (nylon 6,6) nanofibers at 200 °C were characterized in detail using electron microscopy, energy dispersive X-ray analysis, selected area electron diffraction, X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence measurements, and dynamic mechanical analysis. Although transmission electron microscopy studies indicated that the process parameters should be further optimized for obtaining ultimate uniformity and conformality on these high surface area 3D substrates, the HCPA-ALD process resulted in a ∼28 nm thick polycrystalline wurtzite GaN layer on polymeric nanofibers of an average fiber diameter of ∼70 nm. Having a flexible polymeric core and low processing temperature, these core-shell semiconducting nanofibers might have the potential to substitute brittle ceramic GaN nanofibers, which have already been shown to be high performance materials for various electronic and optoelectronic applications.
Open Access
Hollow cathode plasma-assisted atomic layer deposition of crystalline AlN, GaN and AlxGa1−xN thin films at low temperatures
(Royal Society of Chemistry, 2014) Ozgit Akgun, C.; Goldenberg, E.; Okyay, Ali Kemal; Bıyıklı, Necmi
The authors report on the use of hollow cathode plasma for low-temperature plasma-assisted atomic layer deposition (PA-ALD) of crystalline AlN, GaN and AlxGa1 xN thin films with low impurity concentrations. Depositions were carried out at 200 C using trimethylmetal precursors and NH3 or N2/H2 plasma. X-ray photoelectron spectroscopy showed the presence of 2.5–3 at.% O in AlN and 1.5–1.7 at.% O in GaN films deposited using NH3 and N2/H2 plasma, respectively. No C impurities were detected within the films. Secondary ion mass spectroscopy analyses performed on the films deposited using NH3 plasma revealed the presence of O, C (both <1 at.%), and H impurities. GIXRD patterns indicated polycrystalline thin films with wurtzite crystal structure. Hollow cathode PA-ALD parameters were optimized for AlN and GaN thin films using N2/H2 plasma. Trimethylmetal and N2/H2 saturation curves evidenced the selflimiting growth of AlN and GaN at 200 C. AlN exhibited linear growth with a growth per cycle (GPC) of 1.0 A. For GaN, the GPC decreased with the increasing number of deposition cycles, indicating ˚ substrate-enhanced growth. The GPC calculated from a 900-cycle GaN deposition was 0.22 A. ˚ Ellipsometric spectra of the samples were modeled using the Cauchy dispersion function, from which the refractive indices of 59.2 nm thick AlN and 20.1 nm thick GaN thin films were determined to be 1.94 and 2.17 at 632 nm, respectively. Spectral transmission measurements of AlN, GaN and AlxGa1 xN thin films grown on double side polished sapphire substrates revealed near-ideal visible transparency with minimal absorption. Optical band edge values of the AlxGa1 xN films shifted to lower wavelengths with the increasing Al content, indicating the tunability of band edge values with the alloy composition.
Open Access
Low-Temperature Deposition of Hexagonal Boron Nitride via Sequential Injection of Triethylboron and N2/H2 Plasma
(Wiley-Blackwell Publishing, Inc., 2014) Haider A.; Ozgit Akgun, C.; Goldenberg, E.; Okyay, Ali Kemal; Bıyıklı, Necmi
Hexagonal boron nitride (hBN) thin films were deposited on silicon and quartz substrates using sequential exposures of triethylboron and N 2 /H 2 plasma in a hollow-cathode plasma- assisted atomic layer deposition reactor at low temperatures ( ≤ 450 ° C). A non-saturating film deposition rate was observed for substrate temperatures above 250 ° C. BN films were charac- terized for their chemical composition, crystallinity, surface morphology, and optical properties. X-ray photoelectron spec- troscopy (XPS) depicted the peaks of boron, nitrogen, carbon, and oxygen at the film surface. B 1s and N 1s high-resolution XPS spectra confirmed the presence of BN with peaks located at 190.8 and 398.3 eV, respectively. As deposited films were polycrystalline, single-phase hBN irrespective of the deposition temperature. Absorption spectra exhibited an optical band edge at ~ 5.25 eV and an optical transmittance greater than 90% in the visible region of the spectrum. Refractive index of the hBN film deposited at 450 ° C was 1.60 at 550 nm, which increased to 1.64 after postdeposition annealing at 800 ° C for 30 min. These results represent the first demonstration of hBN deposi- tion using low-temperature hollow-cathode plasma-assisted sequential deposition technique. © 2014 The American Ceramic Society.
Open Access
Low-temperature grown wurtzite InxGa1−xN thin films via hollow cathode plasma-assisted atomic layer deposition
(Royal Society of Chemistry, 2015-08) Haider A.; Kizir S.; Ozgit Akgun, C.; Goldenberg, E.; Leghari, S. A.; Okyay, Ali Kemal; Bıyıklı, Necmi
Herein, we report on atomic layer deposition of ternary InxGa1−xN alloys with different indium contents using a remotely integrated hollow cathode plasma source. Depositions were carried out at 200 °C using organometallic Ga and In precursors along with N2/H2 and N2 plasma, respectively. The effect of In content on structural, optical, and morphological properties of InxGa1−xN thin films was investigated. Grazing incidence X-ray diffraction showed that all InxGa1−xN thin films were polycrystalline with a hexagonal wurtzite structure. X-ray photoelectron spectroscopy depicted the peaks of In, Ga, and N in bulk of the film and revealed the presence of relatively low impurity contents. In contents of different InxGa1−xN thin films were determined by energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. Transmission electron microscopy also confirmed the polycrystalline structure of InxGa1−xN thin films, and elemental mapping further revealed the uniform distribution of In and Ga within the bulk of InxGa1−xN films. Higher In concentrations resulted in an increase of refractive indices of ternary alloys from 2.28 to 2.42 at a wavelength of 650 nm. The optical band edge of InxGa1−xN films red-shifted with increasing In content, confirming the tunability of the band edge with alloy composition. Photoluminescence measurements exhibited broad spectral features with an In concentration dependent wavelength shift and atomic force microscopy revealed low surface roughness of InxGa1−xN films with a slight increase proportional to In content.
Open Access
Low-temperature hollow cathode plasma-assisted atomic layer deposition of crystalline III-nitride thin films and nanostructures
(Wiley - V C H Verlag GmbH & Co. KGaA, 2015) Ozgit Akgun, C.; Goldenberg, E.; Bolat, S.; Tekcan, B.; Kayaci, F.; Uyar, Tamer; Okyay, Ali Kemal; Bıyıklı, Necmi
Hollow cathode plasma-assisted atomic layer deposition (HCPA-ALD) is a promising technique for obtaining III-nitride thin films with low impurity concentrations at low temperatures. Here we report our previous and current efforts on the development of HCPA-ALD processes for III-nitrides together with the properties of resulting thin films and nanostructures. The content further includes nylon 6,6-GaN core-shell nanofibers, proof-of-concept thin film transistors and UV photodetectors fabricated using HCPA-ALD-grown GaN layers, as well as InN thin films deposited by HCPA-ALD using cyclopentadienyl indium and trimethylindium precursors. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Optical characteristics of nanocrystalline AlxGa1-xN thin films deposited by hollow cathode plasma-assisted atomic layer deposition
(AVS Science and Technology Society, 2014) Goldenberg, E.; Ozgit-Akgun, C.; Bıyıklı, Necmi; Kemal Okyay, A.
Gallium nitride (GaN), aluminum nitride (AlN), and AlxGa 1-xN films have been deposited by hollow cathode plasma-assisted atomic layer deposition at 200 °C on c-plane sapphire and Si substrates. The dependence of film structure, absorption edge, and refractive index on postdeposition annealing were examined by x-ray diffraction, spectrophotometry, and spectroscopic ellipsometry measurements, respectively. Well-adhered, uniform, and polycrystalline wurtzite (hexagonal) GaN, AlN, and Al xGa1-xN films were prepared at low deposition temperature. As revealed by the x-ray diffraction analyses, crystallite sizes of the films were between 11.7 and 25.2 nm. The crystallite size of as-deposited GaN film increased from 11.7 to 12.1 and 14.4 nm when the annealing duration increased from 30 min to 2 h (800 °C). For all films, the average optical transmission was ∼ 85% in the visible (VIS) and near infrared spectrum. The refractive indices of AlN and AlxGa1-xN were lower compared to GaN thin films. The refractive index of as-deposited films decreased from 2.33 to 2.02 (λ = 550 nm) with the increased Al content x (0 ≤ x ≤ 1), while the extinction coefficients (k) were approximately zero in the VIS spectrum (>400 nm). Postdeposition annealing at 900 °C for 2 h considerably lowered the refractive index value of GaN films (2.33-1.92), indicating a significant phase change. The optical bandgap of as-deposited GaN film was found to be 3.95 eV, and it decreased to 3.90 eV for films annealed at 800 °C for 30 min and 2 h. On the other hand, this value increased to 4.1 eV for GaN films annealed at 900 °C for 2 h. This might be caused by Ga 2O3 formation and following phase change. The optical bandgap value of as-deposited AlxGa1-xN films decreased from 5.75 to 5.25 eV when the x values decreased from 1 to 0.68. Furthermore, postdeposition annealing did not affect the bandgap of Al-rich films. © 2014 American Vacuum Society.
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
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.