Haluk Şafak

Excitonic effects on the nonlinear optical properties of small quantum dots

The excitonic effects on the nonlinear optical properties of small quantum dots with a semiparabolic confining potential are studied under the density matrix formalism. First, within the framework of the strong confinement approximation, we present the excitonic states and then calculate the excitonic effects on the nonlinear optical properties, such as second harmonic generation, third harmonic generation, nonlinear absorption coefficient and refractive index changes. We find the explicit analytical expressions between the corresponding nonlinear optical properties with and without considering the excitonic effects. It is seen that these analytical expressions are related only to ratios of the effective masses of electron and hole. These explicit expressions indicate that the excitonic effects on the corresponding nonlinear optical properties become more important with increasing orders of the optical susceptibilities. In addition, we suggest a scaling rule for the nth-order susceptibility as γ(n+1)/2. The effect of the confining potential frequency on the corresponding nonlinear optical properties is also studied. Our results show a remarkable dependence of nonlinear optical properties on both the excitonic effects and the confining potential frequency.

A detailed analysis of current-voltage characteristics of Au/perylene-monoimide/n-Si Schottky barrier diodes over a wide temperature range

The current-voltage characteristics of Au/perylene-monoimide (PMI)/n-Si Schottky device have been investigated at a wide temperature range between 75 and 300 K in detail. The measured current-voltage (I-V) characteristics of the device show a good rectification behavior at all temperatures. The electronic parameters such as the ideality factor and the barrier height are determined from the experimental data using standard current-voltage analysis method and also temperature dependence of these parameters is analyzed. In addition to the standard analysis, using the Cheung and Cheung method, the series resistance and some other electrical properties are calculated for the device, and a good agreement is obtained between relevant diode parameters. It was observed that Au/PMI/n-Si Schottky diodes exhibit space charge limited (SCL) conduction at all temperatures. Therefore, we have analyzed this SCL current mechanism in more detail. From this analysis, several electronic parameters related with the SCL mechani...

Analysis of I–V measurements on Ag/p-SnS and Ag/p-SnSe Schottky barriers

Abstract In this study, we have performed current–voltage (I–V) measurements on single crystals p-SnS and p-SnSe at different temperatures in the vicinity of room temperature. These compound semiconductors belong to IV–VI layered material class. Hence, they show strong anisotropy for all properties. We realized all measurements on the easy cleavage plane perpendicular c-axis. From I–V characteristics, we have tried to determine some intrinsic and contact properties such as barrier heights, diode ideality factors and carrier concentrations. It has been found that both contacts are in Schottky type, while Ag/p-SnSe structures has showed better diode behavior.

The modification of Schottky barrier height of Au/p-Si Schottky devices by perylene-diimide

Perylene-diimide (PDI) thin film was fabricated by spin coating method on p-Si single-crystal substrate to prepare Au/PDI/p-Si Schottky device. The electrical properties of the Au/PDI/p-Si Schottky device were investigated by current-voltage (I–V) measurements in the temperature range 80–300 K and room temperature capacitance-voltage (C–V) measurement. Results showed a rectification behavior. Junction parameters such as ideality factor (n), barrier height (ϕB0), series resistance (Rs) interface state density (Nss), built-in potential (Vbi), carrier concentration (NA), and the width of the depletion layer (WD) were obtained from the I–V and C–V measurements. The values of ideality factor (n) and barrier height (BH) for the Au/PDI/p-Si structure from the I–V measurements were obtained as 1.77 and 0.584 eV at 300 K, 7.78 and 0.176 eV at 80 K, respectively. It was seen that the BH value of 0.584 eV calculated for the Au/PDI/p-Si structure was significantly larger than the value of 0.34 eV of conventional Au/p...

Intersubband resonant enhancement of the nonlinear optical properties in compositionally asymmetric and interdiffused quantum wells

We report the resonant enhancement of the second- and third-order optical nonlinearities in compositionally asymmetric quantum wells with finite confining potential and interdiffused quantum wells. The energy levels and the envelope wave functions in quantum wells are obtained by solving numerically the Schrodinger equation. The optical rectification, second and third harmonic generation coefficients are calculated within the framework of the density matrix formalism. The effect of the structure parameters such as the well width and the barrier height on the nonlinear optical properties is investigated in detail. The resulting nonlinear susceptibilities obtained in both quantum wells are considerably larger than those of bulk GaAs.

Photoionization cross section and refractive-index change of hydrogenic impurities in a CdS-SiO2 spherical quantum dot

In this study, we calculate the photoionization cross section and refractive-index change of an on-center hydrogenic impurity in a CdS-SiO2 spherical quantum dot. In numerical calculations, both the finite- and infinite-confinement cases are considered and a variational scheme is adopted to determine the energy eigenvalues for the impurity. The variations of the photoionization cross section with the dot radius, the refractive-index change, and the normalized photon energy are investigated, and the effect of the potential-barrier height on the cross section is discussed. The results obtained show that the photoionization cross section and the refractive-index change in CdS-SiO2 spherical quantum dots are sensitively dependent on the incident optical intensity and on the dot sizes.

PHOTOIONIZATION CROSS-SECTION AND OSCILLATOR STRENGTH OF HYDROGENIC IMPURITIES IN ZnS/SiO2 QUANTUM DOTS

In this study, the photoionization cross-section and oscillator strength for the intersubband electronic transitions associated with an on-center impurity in the ZnS/SiO2 spherical quantum dot have been calculated. The effects of dot radius, the normalized photon energy and the potential barrier height on the cross-section have been investigated. In the calculations, both the infinite and finite confinement cases have been considered.