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Resonant cavity LEDs (RCLEDs) are a viable and low-cost alternative light source to lasers for optical communication systems in the 1.3 µm O-band. Most work in this area has been conducted on InP-based material, which is inherently costly, devices often require cooling and the refractive index contrast for constructing mirrors is low. Here, we demonstrate a high-performance GaAs-based RCLED using a dilute nitride GaInNAs active layer emitting in the 1.3 μm wavelength window. While previous 1.3 µm RCLEDs have used metallic mirrors on the back of the device, we exploit the high refractive index contrast of the GaAs/AlAs system to place Distributed Bragg mirrors on both sides of the active layer and achieve superior performance. The external quantum efficiency of the devices is 20% and the full width at half maximum of the emission spectrum is 5.2 nm at room temperature, into a narrow angular cone. The emission power from an 88 μm diameter aperture is 0.5 mW, which, together with the narrow spectral linewidth, makes the device suitable for deployment in a coarse Wavelength Division Multiplexing (WDM) communications system.

Simulated and experimental properties of a Ga(1-x)In(x)As(1-y)N(y)nipi solar cell involving V-grooves for contact formation are reported. In particular, using a drift-diffusion model, we simulate the conversion efficiency, the short-circuit current density (J(SC)), and the open-circuit voltage (V-OC) as a function of the number of nipi junctions. Based on the modelling results, optimized nipi solar cell incorporating five n-p junction pairs was grown on a p-type GaAs (100) substrate using molecular beam epitaxy (MBE). The bandgap of the nipi structure was determined to be 1eV. The metal contacts of the nipi solar cell structure were processed in the form of mesa and V-groove. These shapes enable both vertical and horizontal carrier transport within the solar cell. The effect of thermal annealing on J-V characteristics of both type of devices is finally assessed. The results point out that the V-groove sample has better photovoltaic characteristics than the mesa structure sample.

We report on a novel dilute nitride-based resonant cavity enhanced photodetector (RCEPD) operating at 1.286 μm. The RCEPD was fabricated using 21 pairs top and 24 pairs bottom GaAs/AlGaAs distributed Bragg reflectors for mirrors and 7 nm thick nine GaAs/Ga 0.65 In 0.35 N 0.02 As 0.98 quantum wells as the absorption region. For a 15 μm diameter window, the photocurrent at 1.286 μm is 27 μA and 42 μA, at V = 0 and −1 V, respectively, whereas the dark current is as low as 1.7 nA at −1 V. At the operating wavelength, an excellent wavelength selectivity with a full width at half maximum (FWHM) of 5 nm, and a high quantum efficiency of 43% are demonstrated. The device exhibits significant internal gain at very small reverse bias voltages of V ⩾ −2 V with an overall quantum efficiency of 67%. These are the best ever recorded values for a dilute nitride RCEPD.

In this work, the electronic bandstructure of GaAs 1-x Bi x /GaAs single quantum well (QW) samples grown by molecular beam epitaxy is investigated by photomodulated reflectance (PR) measurements as a function of Bi content (0.0065 ? x ? 0.0215) and substrate orientation. The Bi composition is determined via simulation of high-resolution x-ray diffraction measurement and is found to be maximized in the 2.15%Bi and 2.1%Bi samples grown on (100) and (311)B GaAs substrates. However, the simulations indicate that the Bi composition is not only limited in the GaAsBi QW layer but extends out of the GaAsBi QW towards the GaAs barrier and forms a GaAsBi epilayer. PR spectra are fitted with the third derivative function form (TDFF) to identify the optical transition energies. We analyze the TDFF results by considering strain-induced modification on the conduction band (CB) and splitting of the valence band (VB) due to its interaction with the localized Bi level and VB interaction. The PR measurements confirm the existence of a GaAsBi epilayer via observed optical transitions that belong to GaAsBi layers with various Bi compositions. It is found that both Bi composition and substrate orientation have strong effects on the PR signal. Comparison between TDFF and calculated optical transition energies provides a bandgap reduction of 92 meV/%Bi and 36 meV/%Bi and an interaction strength of the isolated Bi atoms with host GaAs valence band (C BiM ) of 1.7 eV and 0.9 eV for (100) and (311)B GaAs substrates, respectively. © 2018 IOP Publishing Ltd. Fundação de Amparo à Pesquisa do Estado de São Paulo: 14/ 50513-7, 16/10668-7 Firat University Scientific Research Projects Management Unit: FYD-2016-20128, ONAP-52321 115F063 Academy of Finland: 259111 695116 European Research Council This work was partially supported by the Scientific Research Projects Coordination Unit of Istanbul University (ONAP-52321 and FYD-2016-20128) and supported by The Scientific and Technical Research Council of Turkey (TUBITAK) under Grant No. 115F063. YGG acknowledges the financial support from FAPESP ( grants numbers 16/10668-7 and 14/ 50513-7). JP, JH and MC acknowledge the financial support from European Research Council (ERC AdG AMETIST, #695116) and the Academy of Finland (TransPhoton, #259111).

We investigate electronic transport properties of as-grown and annealed n-type modulation-doped Al0.15Ga0.85As/GaAs1-xBix (x = 0 and 0.04) quantum well (QW) structures using magnetotransport measurements in the temperature range 4.2 K and 60 K and at magnetic fields up to 18 T. Thermal annealing process was applied at two different temperatures, 700 degrees C and 350 degrees C during 60 s and 180 s, respectively. We find that electron effective mass and 2D electron density in as-grown Bi-containing sample are slightly lower than that in Bi-free one. Furthermore, quantum electron mobility and quantum scattering time are observed to be decreased in Bi-containing samples. The annealing process at 700 degrees C causes a slight increase in electron effective mass and 2D electron density. A negligible decrease in electron effective mass and an increase in 2D electron density are determined following annealing at 350 degrees C. The observed change in electron effective mass following thermal annealing process is attributed to changing 2D electron density in the samples. No improvement on quantum electron mobility and quantum scattering time are observed following thermal annealing at both process temperatures. We determine that one electron subband (e1) for as-grown and annealed (at 700 degrees C for 60 s) Bi-containing QWs and two electron subbands (e1 and e2) for the annealed (at 350 degrees C for 180 s) GaAsBi QW sample and the Bi-free QW sample contribute to electronic transport. Our results reveal that there is no significant direct effect of Bi on effective electron mass, but an indirect effect, in which Bi can provoke changes in 2D electron density and hence causes not to observe actual band-edge electron mass but a deviation from its band-edge value. Therefore, it can be concluded that dispersion curve of conduction band does not change as an effect of Bi incorporation in GaAs.

In this work, magnetoresistance of as-grown and annealed n- and p-type modulation-doped Ga0.68In0.32NyAs1-y/GaAs single quantum well structures with various nitrogen concentrations has been studied. At low temperatures and low magnetic fields, in n-type samples negative and in p-type samples positive, magnetoresistance has been observed. The observed negative magnetoresistance in n-type samples is an indication of enhanced backscattering of electrons due to the weak localization of the electrons as an effect of the N-induced defects. Nitrogen concentration and thermal annealing dependence of the magnetoresistance have been studied for both n- and p-type samples. The observed decrease in the negative magnetoresistance in n-type and enhanced positive magnetoresistance in p-type samples following thermal annealing have been explained by considering thermal annealing-induced improvement of mobility and the crystal quality in N-containing samples. After thermal annealing, the magnitude of negative magnetoresistance decreases and the breaking of the weak localization is achieved at lower magnetic fields in n-type samples. It is observed that as the mobility of the sample increases, critical magnetic field of negative to positive magnetoresistance transition becomes lower.

9th Nanoscience and Nanotechnology Conference (NANOTR) -- JUN 24-28, 2013 -- Erzurum, TURKEY, WOS: 000344380500018, In this work, observed zero-bias offsets in I-V characteristics and differences in J-V characteristics of staircase quantum well infrared photodetectors were investigated. Temperature and voltage sweep rate dependence of the zero-bias offsets were studied on mesa structures shaped in different diameters. Furthermore, effect of mesa diameter on J-V characteristics was investigated. The temperature, initial bias voltage and voltage sweep rate dependence of the zero-bias offsets were explained by a qualitative model, which is based on a RC equivalent circuit of the quantum well infrared photodetector, Scientific and Technological Research Council of Turkey (TUBITAK) [108T721]; COST Action [MP0805]; Scientific Research Projects Coordination Unit of Istanbul University [3587], This study was partially supported by The Scientific and Technological Research Council of Turkey (TUBITAK) Project Number 108T721, partially supported by COST Action MP0805, partially supported by Scientific Research Projects Coordination Unit of Istanbul University, Project Number 3587. We are also grateful to University of Sheffield for growing the QWIP structure.

The structural and photovoltaic properties of the GaAs/GaAsBi pin solar cell with GaAs0.983Bi0.017 active layer are investigated by optical and electrical measurement techniques. The bandgap of GaAsBi active layer is determined to be 1.3 eV at room temperature. Current density-voltage (J-V) under AM 1.5G spectrum and spectral response measurements are carried out to determine photovoltaic properties of the solar cell. The presence of a midgap trap levels in GaAsBi active layer is identified by deep level transient spectroscopy (DLTS). J-V characteristics is analysed by using Sah-Noyce-Shockley (SNS) theory which includes the midgap trap found in DLTS measurement. The observed deviation between experimental and calculated J-V results is ascribed for metallic cluster formation at the interface between GaAs emitter and GaAsBi intrinsic active layer. Interface metallic clusters create local Schottky junction between emitter and active layers. The best fit to the experimental J-V characteristic of the solar cell is obtained by considering the presence of GaBi metallic cluster at the interface between GaAs emitter and GaAsBi active layer of the solar cell. We showed that work function of interface clusters have a significant effect on the open circuit voltage and filling factor.

In this work electronic hand structure of dilute bismide GaAs/GaAs1-xBix quantum well structures with 1.8% and 3.75% bismuth compositions have been investigated both experimentally and theoretically. Photoluminescence (PL) measurements reveal that effective bandgap of the samples decreases approximately 65 meV per bismuth concentration. Temperature dependence of the effective bandgap is obtained to be higher for the sample with higher bismuth concentration. Moreover, both asymmetric characteristic at the low energy tail of the PL, and full width at half maximum (FWHM) of PL, peak increase with increasing bismuth composition as a result of increased Bi related defects located above valence band (VB). In order to explain composition dependence of the effective bandgap quantitatively, valence hand anti-crossing (VBAC) model is used. Bismuth composition and temperature dependence of effective bandgap in a quantum well structure is modeled by solving Schrodinger equation and compared with experimental PL data.

International audience; We report on an investigation of optical and structural properties of as-grown and thermal annealed GaBixAs1−x (x = 0.012, 0.018 and 0.03) epilayers by photomodulated reflectance (PR) spectroscopy, atomic force microscopy (AFM) and micro-Raman spectroscopy. PR spectra are analyzed by third derivative functional form (TDFF) lineshape. Optical transition energies are calculated using strain-included valence band anticrossing (VBAC) model. According to the results, optical transitions are from conduction bandedge to heavy hole bandedge and to light hole bandedge, which are affected by compressive strain and effect of strain is clearly seen when Bi concentration exceeds 2%. We also determine the band-anticrossing interaction parameter CBiM as 1.33 eV. TDFF analysis of as-grown and thermal annealed samples reveal that thermal annealing causes an unexpectedly large redshift of ∼22 meV/Bi% in the bandgap that corresponds to the increase of Bi composition by ∼25% in the GaAs lattice. The origin of this redshift is analyzed by AFM and Raman spectroscopy. Surface morphology of the GaBiAs samples show that droplets appear on the GaBiAs surface and their sizes increase with increasing Bi concentration, but tend to decrease following thermal annealing process and some surface droplets leave pits in their locations. Raman spectroscopy and AFM results give hints of diffusion of some Bi atoms from surface into the epilayer that causes an increase in Bi concentration of the alloy. This increase explains the observed annealing-induced redshift of the bandgap in PR measurement.

Flavopiridol is a synthetically produced flavonoid that potently inhibits the proliferation of human tumor cell lines. Flavopiridol exerts strong antitumor activity via several mechanisms, including the induction of cell cycle arrest and apoptosis, and the modulation of transcriptional regulation. The aim of the present study was to determine the effect of flavopiridol on a subpopulation of cluster of differentiation (CD)44+/CD24− human breast cancer MCF7 stem cells. The CD44+/CD24− cells were isolated from the MCF7 cell line by fluorescence-activated cell sorting and treated with 100, 300, 500, 750 and 1,000 nM flavopiridol for 24, 48 and 72 h. Cell viability and proliferation assays were performed to determine the inhibitory effect of flavopiridol. Gene expression profiling was analyzed using Illumina Human HT-12 v4 Expression BeadChip microarray. According to the results, the half maximal inhibitory concentration (IC50) value of flavopiridol was 500 nM in monolayer cells. Flavopiridol induced growth inhibition and cytotoxicity in breast cancer stem cells (BCSCs) at the IC50 dose. The present study revealed several differentially regulated genes between flavopiridol-treated and untreated cells. The result of the pathway analysis revealed that flavopiridol serves an important role in translation, the ribosome biogenesis pathway, oxidative phosphorylation, the electron transport chain pathway, carbon metabolism and cell cycle. A notable result from the present study is that ribosome-associated gene expression is significantly affected by flavopiridol treatment. The data of the present study indicate that flavopiridol exhibits antitumor activity against CD44+/CD24− MCF7 BCSCs through different mechanisms, mainly by inhibiting translation and the ribosome biogenesis pathway, and could be an effective chemotherapeutic molecule to target and kill BCSCs.

DAG (Eastern Anatolia Observatory in Turkish) will be the newest and largest (4m) observatory of Turkey in both optical (VIS) and near-infrared (NIR) Owith its robust observing site infrastructure. The telescope is designedOto house 2 Nasmyth platformes which will be dedicated to NIR and VIS observations. A collaboration has recently been established among four Turkish universities including FMV Isik University (for adaptive optics systems), Middle East Technical University (fort measurement, test and calibration purposes), Istanbul University (for new technology instruments, e.g. MKIDs) and as the coordinator Ataturk University (for obtaining NIR and VIS instruments). In this paper the status of the recently approved FPI project and its aims are presented and possible collaboration opportunities are emphasized. Publisher's Version

As-grown and annealed Ga0.6In0.4N0.005As0.995/GaAs with 1, 3, and 7 quantum well structures grown by Molecular Beam Epitaxy (MBE) are investigated using Photoluminescence (PL) and Photo-Induced Current Transient Spectroscopy (PICTS) measurements in order to determine the effects of the number of quantum wells and thermal annealing on optical properties and qualities of the structures. PICTS results reveal that all the samples with different quantum well numbers have two dominate trap levels which related to GaInNAs layer. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The humidity-sensing properties of ZnO nanowires synthesized by carbothermal catalyst-free vapor solid (VS) technique were studied. The morphology and the crystal structure were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The humidity adsorption and desorption kinetics of the synthesized ZnO nanowires were investigated via quartz crystal microbalance (QCM) measurements. The observed positive frequency shift of ZnO nanowires when loaded on the QCM crystal under varying relative humidity conditions can be explained in terms of visco-elastic variations in their mechanical stiffness., Istanbul University (2526 and UDP 6728); State Planning Organization of Turkey (DPT2003K120390); Scientific and Technological Research Council of Turkey (TBAG 109T240)

We present the results of longitudinal carrier transport under a high electrical field in n- and p-type modulation-doped Ga0.68In0.32NyAs1−y/GaAs (y = 0.009, 0.017) quantum well (QW) structures. Nitrogen composition-dependent drift velocities of electrons are observed to be saturated at and at 77 K for the samples with y = 0.009 and y = 0.017, respectively, while the drift velocities of holes do not saturate but slightly increase at the applied electric field in the range of interest. The hole drift velocity is observed to be higher than the electron drift velocity. The electron mobility exhibits an almost temperature-independent characteristic. On the other hand, the hole mobility exhibits a conventional temperature dependence of modulation-doped QW structures. As the temperature increases, the drift velocity of the electrons exhibits an almost an temperature-insensitive characteristic, but, on the other hand, for holes, drift velocity decreases approximately from 107–106 cm s−1. It is observed that the drift velocities of electrons and holes are N-dependent and suppressed at higher electric fields. Furthermore, experimental results show that there is no evidence of negative differential velocity (NDV) behaviour for both n- and p-type samples. To explore the observed electron and hole drift velocity characteristic at high electric fields, we use a simple theoretical model for carrier transport, which takes into account the effect of non-drifting hot phonons. The mobility mapping technique (comparison method) is used to extract hot hole temperature in order to employ it in the non-drifted phonon distribution and to obtain the drift velocity–electric field curves. Then hot electron temperatures are obtained from the drift velocity–electric field curves as a fit parameter using non-drifted hot phonon dynamics. The analytical model is well-matched to the experimental –E curves, indicating that carrier-hot phonon scattering is the main reason for suppressing the NDV mechanism in GaInNAs/GaAs QW structures with a carrier density higher than 1017 cm−3.

We present the details of a proposed microwave kinetic inductance detector (MKID) for the DAG (Eastern Anatolia Observatory in Turkish) telescope, DAG-MKID. The observatory will have a modern 4m size telescope that is currently under construction. Current plan to obtain the first light with the telescope is late 2019. The proposed MKID based instrument will enable astronomers to simultaneously detect photons in the relatively wide wavelength range of 4000-13500 Å with a timing accuracy of μs and spectral resolution R = /▵ =10-25. With a planned field of view of approximately an arcminute, DAG-MKID will mostly be used for follow-up observations of transient or variable objects as well as a robust tool to measure photometric redshifts of a large number of galaxies or other extra-galactic objects. Publisher's Version

We present a comprehensive study of spectral photoluminescence (PL), photoconductivity and Hall mobility in undoped, n and p-type modulation-doped quantum wells of Ga1-x In x N y As1-y /GaAs with varying nitrogen concentration. We show that the increasing nitrogen composition red shifts the energy gap and this red shift is accompanied with a reduction of the 2D electron mobility in the quantum wells. True temperature dependence of the band gap, free from errors associated with nitrogen induced exciton trapping effects, is observed because in the modulation doped QW samples PL emission is dominated by band-to-band recombination and the S-shape temperature dependence is eliminated. Excellent fit to semi-experimental Varshni equation is obtained and the temperature dependence of the band gap in the linear regime (dE/dT) is tabulated as a function of nitrogen concentration and the type of dopant.

Nano-scaled zinc oxide combs/ribbons, nanorods, nanorods with sharp needle-like tips and hexagonal-shaped nanoparticles have been synthesized by using vapor liquid solid (VLS) growth technique under different growth conditions. Morphologies of the obtained nanostructures were examined by using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS) techniques. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

JAK/STAT is an evolutionarily conserved pathway and very important for second messenger system. This pathway is important in malignant transformation and accumulated evidence indicates that this pathway is involved in tumorigenesis and progression of several cancers. It was possible to assume that activation of JAK/STAT pathway is associated with increase in the expressions of ICAM/1 and VCAM-1. In this study we hypothesized that when cells were maintained as spheroids or monolayers, the structure of cancer stem cells (CSCs) could show differentiation when compared with non-CSCs.DU-145 human prostate cancer cells were cultured using the Ege University molecular embryology laboratory medium supplemented wıth 10% fetal bovine serum. Clusters of differentiation 133 (CD133)(+high)/CD44(+high) prostate CSCs were isolated from the DU145 cell line by using BD FACSAria. CD133//CD44+ CSCs were cultured until confluent with 3% noble agar. The expression of these proteins in CSCs and non-CSCs was analyzed by immunohistochemistry.Different expression profiles were observed in the conventional two-dimensional (2D) and three-dimensional (3D) experimental model system when CSCs and non-CSCs were compared. Human prostate CSCs exhibited intense ICAM-1 and VCAM-1 immunoreaction when compared with non-CSCs. These findings were supported by the fact that VCAM-1 on the surface of cancer cells binds to its counterreceptor, the α4β1 integrin (also known as very-late antigen, VLA-4), on metastasis-associated macrophages, triggering VCAM-1-mediated activation of the phosphoinositide 3-kinase growth and survival pathway in cancer cells.The results of this study showed that changes in JAK/STAT pathway are related with adhesion molecules and could affect cancer progression.

International audience; In this study, molecular beam epitaxial-grown GaAs/GaBiAs single quantum well systems with two different Bi contents were investigated. Spectral dependence of room temperature photomodulated reflectance (PR) and photoluminescence (PL) measurements in the temperature range of 35–300 K were employed. PR spectra indicate that increasing Bi concentration promotes a tendency to approach quantized higher energy levels in the heavy and light holes' bands due to the different effects of compressive strain, which depends on Bi concentrations. In addition, a defect level is identified at 0.71 eV at room temperature PR spectra and is attributed to a AsGa antisite defect in GaAs barrier layers caused by the low temperature growth process. From the analysis of the temperature dependence of emission energy and amplitude in the PL spectra, localized states are determined in the range of 8 to 45 meV and attributed to the different bonding configuration of Bi clusters. Low temperature PL results imply that Bi cluster states tend to move into the valance band when Bi content increases from 2.4 to 7.0% in the GaBiAs system.

AIM: The aim of this study was to provide retrospective information about the prescribed drugs among pregnant woman who referred to Ege University Department of Pharmacology and Clinical Pharmacology. MATERIAL-METHOD: A retrospective study was conducted among pregnant woman who referred to our department between January 1, 2005 and December 31, 2005. Initially, all women were evaluated by an obstetrician and and the effects of the drugs during pregnancy was determined according to the United States Food and Drug Administration (FDA) risk classification system database RESULTS: 345 pregnant women were evaluated during the study. 43 % of the women were referred from Ege University Medical Faculty Department of Obstetrics and Gynaecology and 24 % from public hospitals, whereas 33% were consigned from doctor offices. 30% of the women were between 25-29 years of age. The most commonly prescribed drugs were anti-infectives (26.6 %), opioid and non-opioid analgesics (18.3%), antidepressants (8.4%) and flu medications (8.2 %). According to the FDA classification 1.4% of the prescribed drugs were in category A, 42.9% were in category B, 42.4% were in category C, 6.7% were in category D and 3.1% were in category X. 83.6% of the pregnant women had medications during first trimester of their pregnancies. Only 10% of the women received a single drug. CONCLUSIONS: Our study reveals that during their pregnancies an important proportion of women encounter drugs which might have a teratogenic potential. This proportion. can be reduced by interrogation of the patient about pregnancy before prescribing drugs.

An investigation is presented of thermal annealing effects on spectral photoconductivity and photoluminescence in sequentially grown GaInNAs/GaAs and GaInAs/GaAs quantum well structures. Experiments have been carried out at temperatures between 30 K and 300 K. The results indicate that thermal annealing improves the optical quality of GaInNAs, but may cause either a blue shift, as commonly observed by other groups, or a red shift depending on the growth technique. The anneal-induced blue-shift behaviour can be explained in terms of two competing mechanisms involving the redistribution of nearest-neighbour configuration and the change of quantum well profile. The red shift is explained in terms of hydrogen-induced chemical effects.

An investigation of the Photoconductivity (PC) and Inplane Photovoltage (IPV) response of an undoped GaInNAs quantum well (QW) was carried out in the transient and steadystate modes using the 1.064µm line of a Nd:YAG laser as the excitation source. In this paper we focus on the behaviour of the nonlinear response of the transient PC (TPC) signal to excitation intensity. All PC measurements were performed at room temperature. Temperature and excitation intensity were considered instead in the study of the IPV signal.

The operation of the hot electron light emitting and lasing in semiconductor heterostructure – vertical cavity surface emitting laser (HELLISH–VCSEL) devices is based on hot carrier transport parallel to the layers of Ga1-x Alx As p–n junction. It is therefore a field-effect device and the light emission from the device is independent of the polarity of the applied voltage. The authors present a study of the temperature dependence of the operational characteristics of the device. Experimental studies comprise the measurements of the I–V characteristics, electroluminescence, reflectivity, and temperature-dependent light-applied electric field (L–F) characteristics. To obtain the optimum operation temperature, the gain calculations, reflectivity spectra, and band gap calculations were carried out at different temperatures. The temperature dependence of the threshold electric field is compared with the model calculations, where radiative transitions without momentum (k) selection are considered.

We report the results of our studies of optical and electro-optic properties of GaInNAs/GaAs single quantum wells grown by chemical beam epitaxy. The quantum wells have been characterised by scanning transmission electron microscopy and energy dispersive X-ray analysis. Photoluminescence measurements from sequentially grown GaInAs and GaInNAs quantum wells were carried out between 4 K and room temperature. A significant difference in the temperature dependence of GaInNAs band gap compared to nitrogen-free GaInAs is observed. Photoluminescence results are used to determine the interband transition energies. The results are compared with the theoretical values obtained using the band-anticrossing model. When the device is illuminated with monochromatic light, a finite photovoltage develops in the plane of the quantum wells due to Fermi level fluctuations.

We have performed magnetoresistance measurements on n- and p-type modulation doped GaInNAs/GaAs quantum well (QW) structures in both the weak (B < 0.08 T) and the high magnetic field (up to 18 T) at 75 mK and 6 K. We observe that the quantum oscillations in rho(xx) and quantum Hall effect (QHE) plateaus in rho(xy) are affected from the presence of the nitrogen in the III-V lattice. The enhancement of n- related scatterings and electron effective mass with increasing nitrogen causes lower electron mobility and higher two-dimensional (2D) electron density, leading to suppressed QHE plateaus in rho(xy) up to 7 T at 6 K. The Shubnikov de Haas (SdH) oscillations develop at lower magnetic fields for higher mobility samples at 6 K and the amplitude of SdH oscillations decreases with increasing nitrogen composition. The well-pronounced QHE plateaus are observed at 75 mK and at higher magnetic fields up to 18 T, for the p-type sample. For n- type samples, the observed anomalies in the characteristic of QHE is attributed the nitrogen-related disorders and overlapping of fluctuating Landau levels. The low magnetic field measurements at 75 mK reveal that the n-type samples exhibit weak antilocalization, whereas weak localization is observed for the p-type sample. The observation of weak antilocalization is an indication of strong electron spin-orbit interactions. The low field magnetoresistance traces are used to extract the spin coherence, phase coherence and elastic scattering times as well Rashba parameters and spin-splitting energy. The calculated Rashba parameters for nitrogen containing samples reveal that the nitrogen composition is a significant parameter to determine the degree of the spin-orbit interactions. Consequently, GaInNAs-based QW structures with various nitrogen compositions can be beneficial to adjust the spin-orbit coupling strength and may be used as a candidate for spintronics applications.

The electron Lande g factor (g∗) is investigated both experimentally and theoretically in a series of GaBixAs1−x/GaAs strained epitaxial layers, for bismuth compositions up to x=3.8%. We measure g∗ via time-resolved photoluminescence spectroscopy, which we use to analyze the spin quantum beats in the polarization of the photoluminescence in the presence of an externally applied magnetic field. The experimental measurements are compared directly to atomistic tight-binding calculations on large supercells, which allows us to explicitly account for alloy disorder effects. We demonstrate that the magnitude of g∗ increases strongly with increasing Bi composition x and, based on the agreement between the theoretical calculations and experimental measurements, elucidate the underlying causes of the observed variation of g∗. By performing measurements in which the orientation of the applied magnetic field is changed, we further demonstrate that g∗ is strongly anisotropic. We quantify the observed variation of g∗ with x, and its anisotropy, in terms of a combination of epitaxial strain and Bi-induced hybridization of valence states due to alloy disorder, which strongly perturbs the electronic structure.

We present electronic transport in n- and p-type modulation-doped GaInNAs/GaAs quantum well structures. The Hall mobility of electrons in the n-type material decreases dramatically with increasing nitrogen composition. The mobility of 2D holes in p-modulation-doped quantum wells is significantly higher than that of 2D electrons in n-modulation-doped material with similar nitrogen concentration. The mobility of 2D electrons is discussed using a S-matrix model for N-related alloy scattering. The results indicate that the electron mobility is intrinsically limited by scattering from nitrogen complexes. The high mobility of 2D holes is explained in terms of negligible effect of nitrogen on valance band and the absence of scattering with localized nitrogen complexes.

Temperature and nitrogen dependence of 2D carrier mobility in as-grown and annealed Ga1-xInxNyAs1-y/GaAs quantum well (QW) structures (x = 0.32; y = 0, 0.009, and 0.012) are investigated. An analytical model that accounts for the most prominent scattering mechanisms is used to explain the characteristic of temperature dependence of the carrier mobility. An expression for alloy scattering-limited mobility in N-related alloys is developed to explain the behavior of hole mobility for N-containing p-type samples. Analytical modeling of temperature dependence of the electron mobility indicates that N-related alloy scattering and interface roughness scattering are the dominant mechanism at the entire temperature range of interest. The temperature insensitivity of the electron mobility is explained in terms of the overriding effect of N-related alloy scattering and high 2D electron density. A deviation between theoretical and experimental electron mobility at low temperatures is observed not to have any dependency on N concentration. We, therefore, suggest that C-NM interaction parameter of the band anti-crossing (BAC) model must be defined as temperature dependent in order to explain the observed low temperature characteristics of electron mobility. The hole mobility is mainly restricted by interface roughness and alloy scatterings at temperatures lower than 100 K, whilst high temperature hole mobility is drastically affected from optical phonon scattering. Moreover, the hole mobility at high temperatures exhibits an N-independent characteristic and hole density starts to increase at temperatures above 70 K, which is explained using the concept of parallel conduction. Extraction of the hole density in each transport channel (QW and barrier) by using a simple parallel conduction extraction method (SPCEM) shows that, in p-type samples, low temperature hole mobility takes place in quantum well, while as temperature increases barrier channel also contribute to the hole mobility and becomes dominant at high temperatures. The experimental and calculated Hall mobility results reveal that thermal annealing has decreased interface roughness and alloy scatterings.

WOS: 000313393300034, The effect of indium and nitrogen mole concentrations on the nonlinear optical properties in a Ga1-xInxNyAs1-y/GaAs single quantum well under the intense laser field is theoretically studied within the effective mass approximation and the envelope function approach. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. The numerical results show that the linear, third-order nonlinear and total absorption and refractive index changes depend both on the intense laser field and on the indium and nitrogen concentrations. From the findings of this study, it has been concluded that the linear and nonlinear optical properties in a Ga1-xInxNyAs1-y/GaAs single quantum well under the intense laser field can be tuned by changing the indium and nitrogen mole fraction. (c) 2012 Elsevier B.V. All rights reserved., Scientific and Technological Research Council of Turkey (TUBITAK) [COST 109T650]; Scientific Research Project Fund of Cumhuriyet University [F-360], This research was supported by The Scientific and Technological Research Council of Turkey (TUBITAK) for a research Grant COST 109T650 and also was partially supported by Scientific Research Project Fund of Cumhuriyet University under the project number F-360.

Nitrogen dependence of hole effective mass and hole mobility in p-type modulation doped Ga0.68In0.32NyAs1-y/GaAs quantum well structures with y = 0, 0.009, 0.012, 0.017 are investigated using magnetotransport and Hall effect measurements. Observed N-dependent reduction of the hole effective mass is explained by stronger confinement of holes. Hole effective mass is also found to have hole density dependence due to the strain-induced valance band non-parabolicity. A tendency to decrease in hole effective mass upon annealing can be attributed to the reduction of well width and/or decrease in hole density. A significant improvement in low temperature hole mobility is observed after annealing. (C) 2013 AIP Publishing LLC.

We have investigated the temperature dependence of the band gap energy in GaInNAs, GaNAs and InGaAs quantum wells. In the structures containing nitrogen the well-known S-shaped characteristic was observed at low temperatures. We explain this anomalous temperature behaviour by strong carrier localization in potential fluctuations at low temperatures. In the nitrogen free samples, there was no S-shaped behaviour and the empirical Varshni dependence was followed.

In this study, we investigate the effect of annealing and nitrogen amount on electronic transport properties in n- and p-type-doped Ga0.68In0.32N y As1 − y /GaAs quantum well (QW) structures with y = 0%, 0.9%, 1.2%, 1.7%. The samples are thermal annealed at 700°C for 60 and 600 s, and Hall effect measurements have been performed between 10 and 300 K. Drastic decrease is observed in the electron mobility of n-type N-containing samples due to the possible N-induced scattering mechanisms and increasing effect mass of the alloy. The temperature dependence of electron mobility has an almost temperature insensitive characteristic, whereas for p-type samples hole mobility is decreased drastically at T > 120 K. As N concentration is increased, the hole mobility also increased as a reason of decreasing lattice mismatch. Screening effect of N-related alloy scattering over phonon scattering in n-type samples may be the reason of the temperature-insensitive electron mobility. At low temperature regime, hole mobility is higher than electron mobility by a factor of 3 to 4. However, at high temperatures (T > 120 K), the mobility of p-type samples is restricted by the scattering of the optical phonons. Because the valance band discontinuity is smaller compared to the conduction band, thermionic transport of holes from QW to the barrier material, GaAs, also contributes to the mobility at high temperatures that results in a decrease in mobility. The hole mobility results of as-grown samples do not show a systematic behavior, while annealed samples do, depending on N concentration. Thermal annealing does not show a significant improvement of electron mobility.

WOS: 000312586500001, PubMed ID: 23075073, We report on the Mg-doped, indium-rich Ga (x) In1-x N (x < 30). In the undoped material, the intrinsic electron density is very high and as a result there is no detectable photoconductivity (PC) signal within the range of temperatures of 30 < T < 300 K. In the Mg-doped material however, where the conductivity is reduced, there is a strong PC spectrum with two prominent low-energy peaks at 0.65 and 1.0 eV and one broad high-energy peak at around 1.35 eV. The temperature dependence of the spectral photoconductivity under constant illumination intensity, at T > 150 K, is determined by the longitudinal-optical phonon scattering together with the thermal regeneration of non-equilibrium minority carriers from traps with an average depth of 103 +/- 15 meV. This value is close to the Mg binding energy in GaInN. The complementary measurements of transient photoluminescence at liquid He temperatures give the e-A(0) binding energy of approximately 100 meV. Furthermore, Hall measurements in the Mg-doped material also indicate an activated behaviour with an acceptor binding energy of 108 +/- 20 meV., TUBITAK Ankara [EEEAG-105E076]; Istanbul University [9571]; COST Action [MP 0805], We are grateful to WJ Schaff for the samples, to TUBITAK Ankara for the funding (project number EEEAG-105E076) and to Istanbul University for the funding (project number 9571). We also acknowledge the COST Action MP 0805 for funding the international scientific collaboration.

WOS: 000308271400022, I-V characterization of an n-type quantum well infrared photodetector which consists of stepped and graded barriers has been clone under dark at temperatures between 20-300 K. Different current transport mechanisms and transition between them have been observed at temperature around 47 K. Activation energies of the electrons at various bias voltages have been obtainer, from the temperature dependent I-V measurements. Activation energy at zero bias has been calculated by extrapolating the bias dependence of the activation energies. Ground state energies and barrier heights of the four different quantum wells have been calculated by using an iterative technique, which depends on experimentally obtained activation energy. Ground state energies also have been calculated with transfer matrix technique and compared. with iteration results. Incorporating the effect of high electron density induced electron exchange interaction on ground state energies; more consistent results with theoretical transfer matrix calculations have been obtained, Scientific and Technological Research Council of Turkey (TUBITAK) [108T721]; COST Action [MP0805]; Scientific Research Projects Coordination Unit of Istanbul University [3587], This work was partially supported by The Scientific and Technological Research Council of Turkey (TUBITAK) Project Number 108T721, partially supported by COST Action MP0805, partially supported by Scientific Research Projects Coordination Unit of Istanbul University, Project Number 3587.

WOS: 000315516800001, PubMed ID: 23146126, In this study, photomodulated reflectance (PR) technique was employed on two different quantum well infrared photodetector (QWIP) structures, which consist of n-doped GaAs quantum wells (QWs) between undoped Al (x) Ga1-x As barriers with three different x compositions. Therefore, the barrier profile is in the form of a staircase-like barrier. The main difference between the two structures is the doping profile and the doping concentration of the QWs. PR spectra were taken at room temperature using a He-Ne laser as a modulation source and a broadband tungsten halogen lamp as a probe light. The PR spectra were analyzed using Aspnes' third derivative functional form. Since the barriers are staircase-like, the structure has different ground state energies; therefore, several optical transitions take place in the spectrum which cannot be resolved in a conventional photoluminescence technique at room temperature. To analyze the experimental results, all energy levels in the conduction and in the valance band were calculated using transfer matrix technique, taking into account the effective mass and the parabolic band approximations. A comparison of the PR results with the calculated optical transition energies showed an excellent agreement. Several optical transition energies of the QWIP structures were resolved from PR measurements. It is concluded that PR spectroscopy is a very useful experimental tool to characterize complicated structures with a high accuracy at room temperature., Scientific and Technological Research Council of Turkey (TUBITAK) [108T721]; COST Action [MP0805]; Scientific Research Projects Coordination Unit of Istanbul University [3587, UDP 16607]; Ministry of Development of Turkey [2010K121050], We are grateful to Dr. Bulent Aslan and Dr. Ugur Serincan from Anadolu University for growing the ANA samples with MBE. This work was partially supported by The Scientific and Technological Research Council of Turkey (TUBITAK; project number 108T721), COST Action MP0805, Scientific Research Projects Coordination Unit of Istanbul University (project numbers 3587 and UDP 16607), and the Ministry of Development of Turkey (project number 2010K121050).

3rd International Symposium on Growth of III-Nitrides (ISGN) -- JUL 04-07, 2010 -- Montpellier, FRANCE, WOS: 000301569300046, In this work, a quantum well infrared photodetector structure which consists of three different well thicknesses with three different barrier compositions producing a staircase-like conduction band profile with the reputation of 30 periods has been investigated. Dark current measurements have been done at the range from 6 K to 290 K temperature. Activation energies of the carriers have been obtained from the temperature dependence of the I-V measurements. The change of the activation energy with bias voltage has also been obtained. From the activation energy at zero bias and calculated quasi Fermi energy, barrier heights of the quantum wells and ground state energies were obtained. All obtained ground state energies have been found to be consistent with the results obtained from calculations with the transfer matrix method, Univ Montpellier, Natl Centre Sci Res, Inst Phys, Conseil Regional Languedoc Roussillon, Montpellier Agglomerat, AIXTRON, SAFC Hitech, Scientific and Technological Research Council of Turkey (TUBITAK) [108T721]; Cost Action [MP0805]; Scientific Research Projects Coordination Unit of Istanbul University [3587], This study was partially supported by The Scientific and Technological Research Council of Turkey (TUBITAK) Project Number 108T721, partially supported by Cost Action MP0805 and partially supported by Scientific Research Projects Coordination Unit of Istanbul University, Project Number 3587.

Effects of high electron concentration on the band gap energy of InN films having different layer thicknesses as 600 and 800 nm are investigated experimentally and theoretically. Electron concentrations of the samples are obtained through the Hall measurements accomplished between 77K and room temperature. Optical characterization of the samples is carried out using the photoluminescence (PL) measurements and the observed PL spectra are explained considering the high electron concentration related effects, i.e. Burstein-Moss shift, band renormalization and band tailing in non-parabolic k.p model. Extracted PL results indicate that the samples have approximately 0.685 eV band gap energy at 77 K. Effective mass of the carriers, which is calculated as 0.097m(0) for electron concentration of similar to 10(19) cm(-3), are also observed to be influenced by the high carrier concentration. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

WOS: 000275763100028, ZnO nanoparticles have been synthesized by the sol-gel method with approximately 10 nm diameter and the humidity adsorption and desorption kinetics of ZnO nanoparticles were investigated by quartz crystal microbalance (QCM) technique. The morphology and crystal structure of the ZnO nanoparticles have been characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The roughness of the surface has been investigated using atomic force microscope (AFM). The dynamic Langmuir adsorption model was used to determine the kinetic parameters such as adsorption and desorption rates and Gibbs free energy under relative humidity between 45% and 88%. The relative sensitivity of the ZnO nanoparticles-based humidity sensor was determined by electrical resistance measurements. Our reproducible experimental results show that ZnO nanoparticles have a great potential for humidity sensing applications at room temperature operations. (C) 2009 Elsevier B.V. All rights reserved., Research Fund of the Istanbul UniversityIstanbul University [2526, UDP 3670], This work was supported by Research Fund of the Istanbul University. Project number: 2526 and Project number: UDP 3670.

The operation of the Hot Electron Light Emitting and Lasing in Semiconductor Heterostructure -- Vertical Cavity Surface Emitting Laser (HELLISH-VCSEL) devices is based on hot carrier transport parallel to the layers of Ga1-xAlxAs p-n junction. It is therefore a field - effect device and the light emission from the device is independent of the polarity of the applied voltage. In this study, we present the temperature dependence of the operational characteristics of the device. Experimental studies comprising of the measurements of the I-V characteristics, electroluminescence, reflectivity, and temperature dependent light-applied electric field (L-F) characteristics are conducted to find the optimum operating temperature of the device.

Teke, Ali (Balikesir Author), We have investigated in-plane photovoltage (IPV) and photoluminescence (PL) in sequentially grown Ga0.8In0.2As/GaAs and Ga0.8In0.2N0.015As0.985/GaAs-quantum wells. Temperature, excitation intensity, spectral and time dependent study of the IPV, arising from Fermi level fluctuations along the layers of the double quantum well structure, gives valuable information about the nonradiative centers and hence about the optical quality of the GaInNAs quantum well. It also provides information about the radiative transition energies in all the layers. In order to obtain either the trap activation energies and the detrapping rates of photogenerated carriers in the GaInNAs the IPV results are analyzed,in terms of a theoretical model based on random doping fluctuations in nominally undoped multilayer structures. The PL results are analyzed in terms of the band anticrossing model to obtain the electron effective mass from the coupling parameter C-NM

Teke, Ali (Balikesir Author), We have studied the optical quality of sequentially grown undoped Ga0.8In0.2As and Ga0.8In0.2N0.015As0.985 quantum wells (QWs). Spectral and time-resolved in-plane photovoltage (IPV) and photo-induced transient spectroscopy (PITS) techniques were used in this investigation. Two clear peaks have been observed and analysed in the PITS experiment. Spectral and transient IPV in the same samples has been investigated and a selective light was used as the excitation source to separate the GaInNAs IPV from the other layers. IPV can be explained in terms of random fluctuations of the Fermi level in undoped QWs. Spectral and time-resolved IPV measurements can therefore be used to obtain qualitative and quantitative information about interband transitions and trap activation energies.

We have developed a model for the rapid calculation of interband transitions in dilute nitride quantum well structures. The model assumes parabolic bands with the conduction band modified using the band anticrossing approach. The model is compared to results from k · p models, which while being more accurate and flexible than our model, require extensive computational time. The model predictions are compared to photoluminescence and photomodulated reflectance measurements of interband transitions in a number of GaNAs quantum well samples.

Electronic transport in unintentionally doped GaxIn1-xN alloys with various Ga concentrations (x = 0.06, 0.32 and 0.52) is studied. Hall effect measurements are performed at temperatures between 77 and 300 K. Temperature dependence of carrier mobility is analysed by an analytical formula based on two-dimensional degenerate statistics by taking into account all major scattering mechanisms for a two-dimensional electron gas confined in a triangular quantum well between GaxIn1-xN epilayer and GaN buffer. Experimental results show that as the Ga concentration increases, mobility not only decreases drastically but also becomes less temperature dependent. Carrier density is almost temperature independent and tends to increase with increasing Ga concentration. The weak temperature dependence of the mobility may be attributed to screening of polar optical phonon scattering at high temperatures by the high free carrier concentration, which is at the order of 1014 cm−2. In our analytical model, the dislocation density is used as an adjustable parameter for the best fit to the experimental results. Our results reveal that in the samples with lower Ga compositions and carrier concentrations, alloy and interface roughness scattering are the dominant scattering mechanisms at low temperatures, while at high temperatures, optical phonon scattering is the dominant mechanism. In the samples with higher Ga compositions and carrier concentrations, however, dislocation scattering becomes more significant and suppresses the effect of longitudinal optical phonon scattering at high temperatures, leading to an almost temperature-independent behaviour.