Effect of annealing on the density of defects in epitaxial CdTe (211)/GaAs

CdTe thin films were grown on GaAs (211) wafers by molecular beam epitaxy as the buffer layer for HgCdTe infrared detector applications. We studied the effect of annealing on the density of dislocation of these CdTe thin films under varying annealing parameters such as annealing temperature, annealing duration, and number of cycles. Annealings were carried out using a homemade annealing reactor possessing a special heater element made of a Si wafer for rapid heating. The density of dislocations, which were made observable with a scanning electron microscope after etching with an Everson solution, were calculated by counting the number of dislocations per unit surface area, hence the term etch pit density (EPD). We were able to decrease EPD values by one order of magnitude after annealing. For example, the best EPD value after a 20-min annealing at 400°C was ∼ 2 × 107 cm−2 for a 1.63-μm CdTe thin film which was about 9.5 × 107 cm−2 before annealing. We also employed Raman scattering measurements to see the changes in the structural quality of the samples. From the Raman measurements, we were able to see improvements in the quality of our samples from the annealing by studying the ratio of 2LO/LO phonon mode Raman intensities. We also observed a clear decrease in the intensity of Te precipitations-related modes, indicating a decrease in the size and number of these precipitations.SSM (Undersecretariat for Defence Industries of Turkey) and ASELSA

The structural, optical and morphological properties of CaF2 thin films by using Thermionic Vacuum Arc (TVA)

In this study, calcium fluoride (CaF2) thin films have been prepared by Thermionic Vacuum Arc (TVA) technique on glass substrates. In this technique CaF2 thin films are produced by condensing the plasma of anode material generated in the TVA setup under high vacuum conditions on glass substrates. Crystal structures as well as optical and surface properties of CaF2 antireflective (AR) coated thin films were investigated. X-ray diffraction (XRD) measurements showed that amorphous CaF2 thin films were formed. Optical and surface properties of CaF2 films have been studied based on optical transmittance, reflectance, refractive index and atomic force microscopy imaging (AFM). Our results also show that CaF2 coated samples exhibit lower reflectance (R). From our optical studies, we have observed that CaF2 thin films have high AR properties.TUBITAK (108M608); ESOGU Scientific Research Commission (200819045

Fourier transform infrared spectroscopy of III-V compound semiconductors

The 3d-transition metal impurities (Mn, Co, and Cu) incorporated into III-V zinc-blende hosts (GaAs, GaP, or InP) exhibit well resolved excitation lines followed by a photoionization continuum in their infrared absorption spectra. They are as sociated with transitions from a “1s-like” ground state to various “p-like” excited states characteristic of a hole bound to a Coulomb center. Their spacings agree remarkably well with those predicted in the effective mass theory for single acceptors (A. Baldereschi and N. O. Lipari, Phys. Rev. B 9, 1525 (1974)) as expected for 3d-transition metal ions substitutionally replacing the group III cations of the host. The shape of the photoionization spectra: the occurrence of the simultaneous excitation of the Lyman transitions in combination with the zone center longitudinal optical (LO) phonon and hence lying in the photoionization continuum and displaying Fano-like asymmetries; the additional continuum excitations to and beyond the p1/2 valence band maximum; …, all these features are described and interpreted. Cu acceptors in GaAs show Lyman doublets, indicative of two independent centers; it is speculated that they could arise from He-like 3d104 s24p1, Cu2− , and H-like 3d94s 24p1, Cu−, Coulomb centers. The infrared transmission spectrum of Si doped MBE-grown GaAs epilayers, 2–2.5 μm thick, measured in the oblique (Berreman) geometry, revealed distinct minima in p-polarization. Given epilayer thickness \u3c\u3c reststrahlen wavelength, the minima are identified as the zone center transverse optic phonon (ωTO) and the high frequency LO phonon-plasmon coupled mode (ω+). Analysis of the experimental data yielded free carrier concentrations, ranging from 2.5 × 1017 to 1.4 × 1018 cm−3. The same technique with MBE-grown Si doped In0.53Ga0.47As epilayers (0.5 to 1 μm thick) yielded ω+ modes corresponding to free carrier concentrations 8.2 × 1016 to 2.7 × 10 19 cm−3. The observations of the transmission minima in the Berreman geometry and their interpretation demonstrate a direct and simple method for deducing free carrier concentrations over a wide range

Investigation of the structure of alpha-lactalbumin protein nanotubes using optical spectroscopy

Alpha-lactalbumin (α-la) is one of the major proteins in whey. When partially hydrolysed with Bacillus licheniformis protease, it produces nanotubular structures in the presence of calcium ions by a self-assembly process. This study presents investigation of α-la protein structure during hydrolysis and nanotube formation using optical spectroscopy. Before spectroscopic measurements, nanotubes were examined with microscopy. The observed α-la nanotubes (α-LaNTs) were in the form of regular hollo strands with a diameter of about 20 nm and the average length of 1 μm. Amide and backbone vibration bands of the Raman spectra displayed remarkable conformational changes in α and β domains in the protein structure during nanotube growth. This was confirmed by the Fourier-transform infrared (FTIR) spectroscopy data. Also, FTIR analysis revealed certain bands at calcium (Ca++) binding sites of COO- groups in hydrolysed protein. These sites might be critical in nanotube elongation.Scientific and Technological Research Council of Turkey (TOVAG-109O866); İzmir Institute of Technology, Scientific Research Projects fund İYTE03, 2009-201

The growth of silver nanostructures on porous silicon for enhanced photoluminescence: The role of AgNO3 concentration and deposition time

###EgeUn###Silver nanostructures were obtained by using the electrodeposition method on n-type porous silicon (PSi) under different deposition times and concentrations of AgNO3 solutions. The analyses of the structural and photoluminescence properties of PSi/Ag were studied by SEM, XRD and photoluminescence spectroscopy. SEM analysis showed that the shape and size of Ag nanostructures significantly depend on the deposition time and concentration. It was found that spherical nanoparticles and thin Ag dendrites were obtained in short deposition times at 1 and 5 mM AgNO3 concentrations, whereas, Ag complex dendrite nanostructures formed in long deposition times. It was also found that only micro-sized Ag particles were formed at 10 mM. XRD results revealed that the degree of crystallization increases with increasing concentration. Photoluminescence analysis showed that the deposition time and concentration of AgNO3 remarkably affect the PL intensity of PSi/Ag samples. We determined a PL enhancement of similar to 2.7 for the PSi/Ag deposited at 120 s for 1 mM AgNO3. The improved PL intensity of PSi/Ag nanostructures can be explained by the combination of quantum confinement and surface states. PL analyses also indicated that with increasing deposition time and AgNO3 concentrations, the PL intensity of PSi/Ag structures significantly decreases due to the auto-extinction phenomenon.Ege University, Research Project FoundationEge University [2013FEN058]This work was supported by the Ege University, Research Project Foundation (Project no: 2013FEN058)

The growth of silver nanostructures on porous silicon for enhanced photoluminescence: The role of AgNO

Silver nanostructures were obtained by using the electrodeposition method on n-type porous silicon (PSi) under different deposition times and concentrations of AgNO3 solutions. The analyses of the structural and photoluminescence properties of PSi/Ag were studied by SEM, XRD and photoluminescence spectroscopy. SEM analysis showed that the shape and size of Ag nanostructures significantly depend on the deposition time and concentration. It was found that spherical nanoparticles and thin Ag dendrites were obtained in short deposition times at 1 and 5 mM AgNO3 concentrations, whereas, Ag complex dendrite nanostructures formed in long deposition times. It was also found that only micro-sized Ag particles were formed at 10 mM. XRD results revealed that the degree of crystallization increases with increasing concentration. Photoluminescence analysis showed that the deposition time and concentration of AgNO3 remarkably affect the PL intensity of PSi/Ag samples. We determined a PL enhancement of ∼2.7 for the PSi/Ag deposited at 120 s for 1 mM AgNO3. The improved PL intensity of PSi/Ag nanostructures can be explained by the combination of quantum confinement and surface states. PL analyses also indicated that with increasing deposition time and AgNO3 concentrations, the PL intensity of PSi/Ag structures significantly decreases due to the auto-extinction phenomenon

Origin of a localized vibrational mode in a GaSb substrate with a MBE-grown ZnTe epilayer

A localized vibrational mode (LVM) with a remarkable fine structure is observed in the infrared transmission spectrum of a ZnTe epilayer grown with molecular beam epitaxy (MBE) on a GaSb substrate. On the basis of the Zn and Te deposited on the GaSb substrate during the MBE growth of ZnTe, and assuming diffusion of Zn and Te into GaSb, the LVM is attributed to Zn, substitutionally replacing either the cation, Ga (ZnGa), or the anion, Sb (Zn Sb). The frequency of the LVM and its fine structure can then be interpreted in terms of the infrared active modes of 64Zn substituting for Sb as an anti-site impurity and treating the centre as an XY4 quasimolecule. With X≡64Zn and Y≡ 69Ga and 71Ga, occupying the nearest-neighbour sites reflecting all the possible combinations and permutations as well as the natural isotopic abundance of Ga, the fine structure of the LVM can be accounted for quantitatively.International Cooperation Research Program of Korean Ministry of Science and Technology (M6-0403-0079); 2003 Special Research Fund from Sogang University and US National Science Foundation (DMR0405082