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Öğe Calculation of the changes in the absorption and refractive index for intersubband optical transitions in a quantum box(WILEY-V C H VERLAG GMBH, 2005) Karabulut, I; Unlu, S; Safak, HAnalytic forms of the changes of the absorption and refractive index associated with intersubband optical transitions in a quantum box are obtained by using the compact density matrix approach. Numerical results are presented for a typical AlGaAs/GaAs quantum box system. The linear, third-order nonlinear, and total refractive-index and absorption changes are examined as a function of the incident optical intensity. The results obtained show that the incident optical intensity has a great effect on the total absorption and refractive-index changes.Öğe Comment on "Electric field effect on the second-order nonlinear optical properties of parabolic and semiparabolic quantum wells"(AMER PHYSICAL SOC, 2005) Karabulut, I; Atav, U; Safak, HZhang and Xie [Phys. Rev. B 68, 235315 (2003)] presented their results for the second harmonic generation (SHG) susceptibility in electric-field-biased parabolic and semiparabolic quantum wells (QW's). In this Comment, we demonstrate that the results presented for the SHG coefficient in a parabolic QW with an applied electric field are not physically sound and they contradict with well-established properties of the Hermite polynomials which just happen to be the wave functions describing such a system. It is shown that in a parabolic QW with an applied electric field intersubband transitions contribute to neither second nor any higher order harmonic generation.Öğe Nonlinear optical rectification in asymmetrical semiparabolic quantum wells(PERGAMON-ELSEVIER SCIENCE LTD, 2005) Karabulut, I; Safak, H; Tomak, MThe optical rectification in a semiparabolic quantum well is theoretically investigated. The electronic states in a semiparabolic quantum well are calculated exactly, within the envelope function and the displaced harmonic oscillator approach. The nonlinearity resulting from the asymmetry of the confining potential is studied in the lowest order. The numerical results for the typical AlxGa1-xAs/GaAs material show that the large optical rectification coefficient decreases with increasing confining potential frequency of the semiparabolic quantum well. Moreover, the optical rectification coefficient also depends sensitively on the relaxation rate of semiparabolic quantum well system. (C) 2005 Elsevier Ltd. All rights reserved.Öğe Nonlinear optical rectification in semiparabolic quantum wells with an applied electric field(ELSEVIER SCIENCE BV, 2005) Karabulut, I; Safak, HThe optical rectification (OR) in a semiparabolic quantum well with an applied electric field has been theoretically investigated. The electronic states in a semiparabolic quantum well with an applied electric field are calculated exactly, within the envelope function and the displaced harmonic oscillator approach. Numerical results are presented for the typical AlxGa1-xAs/GaAs quantum well. These results show that the applied electric field and the confining potential frequency of the semiparabolic quantum well have a great influence on the OR coefficient. Moreover, the OR coefficient also depends sensitively on the relaxation rate of the semiparabolic quantum well system. (c) 2005 Elsevier B.V. All rights reserved.Öğe Optical constants of CuInSe2 thin films prepared by two-stage process(IOP PUBLISHING LTD, 2005) Yuksel, OF; Safak, H; Sahin, M; Basol, BMThin film CuInSe2 chalcopyrite semiconductors have been prepared on glass substrates by means of two-stage process. The structural properties and atomic compositions of films were determined by energy-dispersive analysis of Xrays (EDAX) and X-ray diffraction (XRD) measurements. Reflectance and transmittance measurements were performed on the films in the photon wavelength range of 300-2200nm. The samples used in the measurements have different Cu/In ratios. The reflectance and transmittance spectra were analyzed on the basis of multiple reflection model considering the absorbing film on a non-absorbing substrate and then complex refractive-index n*(E) n(E) + ik(E) and complex dielectric constant epsilon*(E) = epsilon(1) (E) + iepsilon(2)(E) were determined. It has been concluded that the films having higher Cu/In ratios show stronger absorption at low photon energy region than those having lower Cu/In ratios.Öğe Series resistance calculation for Ag contacts on single crystal layered p-SnS and p-SnSe compound semiconductors in the wide temperature range(ELSEVIER SCIENCE BV, 2005) Karadeniz, S; Tugluoglu, N; Sahin, M; Safak, HThis paper summarizes the first results of characteristics parameters obtained from current-voltage (I-V) measurements for Ag/p-SnS and Ag/p-SnSe structure. The reverse and forward bias current-voltage characteristics of Ag Schottky contacts on a Bridgman-Stockbarger grown p-SnS and p-SnSe layered semiconducting material have been measured at various temperatures. We have tried to determine contact properties such as apparent barrier heights Phi(BO), ideality factor n and series resistance R-s. The apparent barrier height and ideality factor calculated by using thermionic emission theory were found to be strongly temperature dependent. Evaluating forward I-V data reveals a decrease at the apparent barrier height, but an increase at the ideality factor with decrease in temperature. It is shown that the values of R-s estimated from Cheung's method were strongly temperature dependent and decreased with increasing temperature. It has been found that both contacts are of Schottky type. (c) 2005 Elsevier B.V. All rights reserved.Öğe Temperature dependence of current-voltage characteristics of Ag/p-SnS Schottky barrier diodes(ELSEVIER SCIENCE BV, 2005) Sahin, M; Safak, H; Tugluoglu, N; Karadeniz, SThe current-voltage (I-V)measurements on Ag/p-SnS Schottky barrier diodes in the temperature range 100-300 K were carried out. It has been found that all contacts are of Schottky type. The ideality factor and the apparent barrier height calculated by using thermionic emission (TE) theory were found to be strongly temperature dependent. The I-V curves is fitted by the equation based on thermionic emission theory, but the zero-bias barrier height (Phi(B0)) decreases and the ideality factor (n) increases with decreasing temperature. The conventional Richardson plot exhibits non-linearity below 200 K with the linear portion corresponding to activation energy of 0.32 eV. It is shown that the values of R, estimated from Cheung's method were strongly temperature dependent and decreased with increasing temperature. From the reverse-bias I-V graphs. it is found that the experimental carrier density (N-A) values increased with increasing temperature. (c) 2004 Elsevier B.V. All rights reserved.Öğe Temperature dependence of current-voltage characteristics of Ag/p-SnSe Schottky diodes(ELSEVIER SCIENCE BV, 2004) Tugluoglu, N; Karadeniz, S; Sahin, M; Safak, HThe current-voltage (I-V) characteristics of Ag/p-SnSe Schottky barrier diodes were measured in the temperature range 80-350 K. We have tried to determine some intrinsic and contact properties such as barrier heights, ideality factor, series resistance and carrier concentrations. The apparent barrier height and the ideality factor calculated by using thermionic emission (TE) theory were found to be strongly temperature dependent. Evaluating forward I-V data reveals a decrease at the zero-bias barrier height (Phi(BO)), but an increase at the ideality factor (n) with decrease in temperature, and these changes are more pronounced below 200 K. It is shown that the values of R-s estimated from Cheung's method were strongly temperature dependent and decreased with increasing temperature. From the reverse-bias I-V graphs, it is found that the experimental carrier density (N-A) values increased with increasing temperature. The conventional Richardson plot exhibits non-linearity below 200 K with the linear portion corresponding to activation energy of 0.35 eV. It has been found that all contacts are of Schottky type. (C) 2004 Elsevier B.V. All rights reserved.Öğe Temperature-dependent barrier characteristics of Ag/p-SnS Schottky barrier diodes(IOP PUBLISHING LTD, 2004) Karadeniz, S; Sahin, M; Tugluoglu, N; Safak, HThe current-voltage (I-V) characteristics of Ag/p-SnS Schottky barrier diodes were measured in the temperature range of 100-300 K and have been interpreted on the basis of the assumption of a Gaussian distribution of barrier heights (BHs) due to barrier height inhomogeneities that prevail at the interface. It is shown that the occurrence of a Gaussian distribution of the BHs is responsible for the decrease of the apparent barrier height Phi(B0), increase of the ideality factor n and nonlinearity in the activation energy plot at low temperatures. The inhomogeneities are considered to have a Gaussian distribution with a mean barrier height of (Phi) over bar (b0) = 0.649 eV and a standard deviation of sigma(s0) = 0.093 V at zero bias. Furthermore, the mean barrier height and the Richardson constant values were obtained as 0.642 eV and 12.89 A K-2 cm(-2), respectively, by means of the modified Richardson plot, In(I-0/ T-2) - (q(2)sigma(s0)(2)/2k(2)T(2)) versus 10(3)/T. Hence, it has been concluded that the temperature dependence of the I-V characteristics of the Schottky barrier on p-type SnS can be successfully explained on the basis of TE mechanism with a Gaussian distribution of the barrier heights. Furthermore, the value of the Richardson constant found is much closer than that obtained without considering the inhomogeneous barrier heights.Öğe Temperature-dependent barrier characteristics of Ag/p-SnSe Schottky diodes based on I-V-T measurements(IOP PUBLISHING LTD, 2004) Tugluoglu, N; Karadeniz, S; Sahin, M; Safak, HThe current-voltage (I-V) characteristics of Ag Schottky contacts on a Bridgman-Stockbarger grown p-type SnSe layered semiconducting material have been measured over the temperature range of 80-350 K. Their analysis based on the thermionic emission (TE) theory has revealed an abnormal decrease of zero-bias barrier height and increase of ideality factor at lower temperatures. This behaviour has been interpreted on the basis of the assumption of a Gaussian distribution of barrier heights due to barrier height inhomogeneities that prevail at the interface. The inhomogeneities are considered to have Gaussian distribution with a mean barrier height of (Phi) over bar (b0) 0.610 eV and standard deviation of sigma(s0) = 0.075 V at zero-bias. Furthermore, the mean barrier height and the Richardson constant values were obtained by means of the modified Richardson plot, In(I-0/T-2) - (q(2)sigma(s0)(2)/2k(2)T(2)) versus 1000/T, as 0.603 eV and 7.72 A K-2 cm(-2) respectively, of which latter is close to its theoretical value of 18 A K-2 cm(-2) used for the determination of the zero-bias barrier height. Hence, it has been concluded that the temperature dependence of the I-V characteristics of the Schottky barrier on p-type SnSe can be successfully explained on the basis of TE mechanism with Gaussian distribution of the barrier heights.
