Hasan Efeoglu

Interpreting the nonideal reverse bias C-V characteristics and importance of the dependence of Schottky barrier height on applied voltage

This work presents an attempt related to the charging behaviour of interface states to the nonideal forward bias current-voltage (I-V) and the reverse bias capacitance-voltage (C-V) characteristics of AlnSi Schottky barrier diodes. The diode showed nonideal I-V behaviour with an ideality factor of 1.50 and was thought to have a metal-interface layer-semiconductor configuration. Considering that the interface states localized at the interfacial layer-semiconductor interface are in equilibrium with the semiconductor, the energy distribution of the interface states was exactly determined from the forward bias I-V characteristics by taking into account the bias dependence of the effective barrier height, θe. The determination of the intercept voltage and interface state density was made by means of a simple interface charge model which has been developed in detail. The I-V characteristics were used for determining the voltage dependence of the barrier height. Although the change in barrier height with applied biasis small, it is important for exactly determining the shape of the interface state density distribution curve. At a frequency of 500 kHz, the nonlinear reverse bias C−2−V plot with the curvature concave downward has been only thought of to be due to the contribution of the capacitance of the interface state charges. It is concluded that the nonlinear nature of C−2−V plots in the frequency range 50–200 kHz has been caused by the interface state charges as well as inversion layer and inversion layer charges. It has been understood by means of the interface state charge model that the C−2−V plots cannot only be interpreted in terms of the contribution of the interface state charges to the device capacitance.

Effect of series resistance on the forward current-voltage characteristics of Schottky diodes in the presence of interfacial layer

Abstract In order to make an accurate determination of Schottky diode parameters such as the ideality factor, the barrier height and the series resistance [using forward current-voltage ( I - V ) characteristics in the presence of an interfacial layer], a novel calculation method has been developed by taking into account the applied voltage drop across the interfacial layer ( V i ). The parameters obtained by accounting for the voltage drop V i have been compared with those obtained without considering the above voltage drop. To examine the consistency of this approach, the comparison has been made by means of Schottky diodes fabricated on a n -type semiconductor substrate with different bulk thickness. It is shown that the voltage drop across the interfacial layer will increase the ideality factor and the voltage dependence of the I - V characteristics. In addition, it is shown that the series resistance value increases as the semiconductor bulk thickness has been increased.

Temperature dependence of reverse bias capacitance–voltage characteristics of Sn/p-GaTe Schottky diodes

A Schottky barrier diode on unintentionally doped p-type GaTe grown by the directional freezing method was obtained and characterized by the capacitance–voltage technique as a function of temperature (100–300 K). Using vacuum evaporated Sn as the Schottky barrier contact and In for the ohmic contact, high-quality diodes were produced. The discrepancy between Schottky barrier heights (BHs) obtained from current–voltage–temperature and capacitance–voltage–temperature measurements is explained by the introduction of a spatial distribution of BHs due to barrier height inhomogeneities that prevail at the metal/GaTe interface. The deviations of apparent BHs were investigated by considering the microstructure of the metal/GaTe interface. It was found that the dispersion of this distribution across the contact area grew increasingly larger at lower temperatures and was responsible for the increasing difference between apparent BHs obtained from the two techniques.

Recombination processes in erbium-doped MBE silicon

The incorporation of erbium from a solid source into molecular beam epitaxy (MBE) Si and Si/Ge alloys grown at substrate temperatures of 500 degrees C and 700 degrees C has been studied by photoluminescence, electrical measurements, secondary-ion mass spectrometry (SIMS), Rutherford backscattering (RBS) and transmission electron microscopy (TEM). Erbium concentrations between 1018 and 1022 cm-3 were obtained but the maximum photoluminescence intensity was from samples with an erbium concentration of 2*1018 cm-3. Above this concentration the onset of erbium precipitation could just be observed by TEM. The authors found no shallow donors or acceptors attributable to erbium but they observed a high concentration of deep acceptors with an activation energy of 360 meV; these may be due to impurities in the erbium source rather than being directly related to the rare earth. Implantation with oxygen is found to enhance the Er3+-related photoluminescence signal when measured at temperatures greater than 77 K but to have little effect on the low-temperature luminescence. A detailed study of the temperature dependence of the luminescence reveals tree quenching mechanisms with average activation energies of approximately 5, 20 and 130 meV. The authors attribute the first two to de-excitation effects in the matrix, and the last to processes competing with the internal 4f transition.

Barrier characteristics of Cd/p-GaTe Schottky diodes based on I–V–T measurements

The current–voltage (I–V) characteristics of Cd/p-GaTe Schottky barrier diodes were measured in the temperature range 90–330 K. The apparent barrier height and the ideality factor derived by using thermionic emission (TE) theory were found to be strongly temperature dependent. Evaluating forward I–V data reveals a decrease of zero-bias barrier height (Φb0) but an increase of ideality factor (n) with decrease in temperature, and these changes are more pronounced below 150 K. The conventional Richardson plot exhibits nonlinearity below 150 K with the linear portion corresponding to an activation energy of 0.52 eV. The value of effective Richardson constant (A*) turns out to be 6.74 × 10−2 A K−2 cm−2 against the theoretical value of 119.4 A K−2 cm−2. It is demonstrated that the findings cannot be explained on the basis of tunnelling and image force lowering effects. Also, the concept of the flat-band barrier height (Φfb) fails to account for the temperature dependence of the diode parameters. Finally, it is demonstrated that these anomalies result due to the barrier height inhomogeneities prevailing at the metal–semiconductor interface. The inhomogeneities are considered to have Gaussian distribution with a mean barrier height of = 0.886 eV and a standard deviation of σs0 = 0.091 eV at zero bias. Furthermore, the mean barrier height and the Richardson constant values were obtained as 0.875 eV and 62.2 A K−2 cm−2, respectively, by means of the modified Richardson plot, ln(J0/T2) − (q2σ2s0/2k2T2) versus 1000/T. Hence, it has been concluded that the temperature dependence of the I–V characteristics of the Schottky barrier on p-type GaTe can be successfully explained on the basis of TE mechanism with Gaussian distribution of the barrier heights.

Aging of porous silicon and the origin of blue shift

Aging effects of porous silicon (PS) and the origin of blue shift are investigated. Photoluminescence (PL) measurements of the PS prepared with HF-EtOH solution showed a 210 meV blue shift after 1.5 months. It is found from deconvolution of the PL spectra that this shift is not fully related to the quantum confinement (QC) effect. For stable PS formation, a HFEtOH-H2O2 solution is used. A stable luminescence at 2.01 eV with a Gaussian distribution is obtained when the samples are kept in H2O2 for 45 min after the anodization.

Novel Design of Porous Silicon Based Sensor for Reliable and Feasible Chemical Gas Vapor Detection

In this study, an innovative design for porous silicon (PSi) based sensors is proposed to eliminate some problems of conventional PSi sensors such as undesired reflections from imprecise positioning of the fiber optic cable and the PSi structure. Our design has three stages as hole milling for fiber socket, fabrication of filter structure, and integration of the fiber optic cable to the bulk Si. We have carried out reflectivity measurements of alcohols with close refractive index values by the proposed and conventional sensors. From the measurements, we note that both sensors have equal sensitivity in identifying alcohols, that repeatability of our proposed sensor is relatively higher, and that our sensor allows easy positioning and flexibility in sensing applications. Nonetheless, our proposed sensor necessitates a thorough fabrication process and a methodological preparation.

The effect of silicon loss and fabrication tolerance on spectral properties of porous silicon Fabry-Perot cavities in sensing applications

In this paper, we investigate the effect of non-uniformities (enlargement of current passage, non-equal surface current densities, etc.) in axial as well as transverse directions of a porous silicon Fabry-Perot (FP) cavity as well as loss nature of bulk silicon on spectral properties of this cavity, even that cavity is created with an anisotropic etching process. Without correct and comprehensive characterization of such cavities by incorporating these non-uniformities and inherent lossy nature of a cavity, detection and identification of biological and chemical molecules by that cavity may yield unpredictable and misleading results. From our simulations, we note the following two key points. First, effects of the refractive index and the thickness of microcavity region of a lossless or lossy FP cavity on resonance wavelength is more prevailing than those of first and last layers. Second, the effect of some small loss inside the FP cavity is not detectable by the measurement of resonance wavelength whereas the same influence is noticeable by the measurement of reflectivity. We carried out some measurements from two different regions on the fabricated cavities to validate our simulation results. From a practical point of view in correct detection and/or identification of lossy biological or chemical vapor by FP cavities, we conclude that not only the measurement of resonance wavelength as well as its shift but also the reflectivity value at the resonance wavelength or some specific wavelengths should be utilized.

Current-voltage and capacitance-voltage characteristics of metallic polymer/InSe(:Er) Schottky contacts

Abstract An investigation of metallic polypyrrole polymer (MPP)/ n -InSe(:Er) (by an anodization process) Schottky barrier diodes (SBDs) fabricated on a cleaved n -type InSe(:Er) substrate, which is a layered semiconductor, has been made. The metallic polypyrrole film provides a good rectifying contact to the n- InSe(:Er) semiconductor. The current–voltage ( I – V ) and capacitance–voltage ( C – V ) characteristics of the diode have been determined at room temperature. The diode shows nonideal I – V behavior with an ideality factor greater than one. In addition, the I – V characteristics of the (MPP)/ n -InSe(:Er) device shows an improvement with an increased Φ b0 and a decreased ideality factor after the polymer melt processing step. The reverse bias C −2 – V characteristics of the diode shows a non-linear behavior.

Excitonic absorption and Urbach-Martienssen's tails in Er-doped and undoped n-type InSe

Optical absorption spectra of InSe and InSe:Er single crystals were investigated just below and in the excitonic resonance energy region. The temperature dependence of the free exciton transition associated with the direct gap of InSe and InSe:Er were measured in the temperature range 10<T<340 K. The parameters describing the temperature variation of both the spectral position and the broadening function of the excitonic resonance confirm the dominating role of the average energy of crystal phonons. The Lorenzian lineshape was used to fit the excitonic structures. The increased absorption intensity and the narrowed lineshape of the excitonic resonances in InSe:Er crystals were attributed to the [Er] = 0.03 at% dopant atoms. The exponentially increasing absorption tail was explained as an Urbach-Martienssens (U-Ms) tail for both InSe and InSe:Er samples in the 100-340 K temperature range. The characteristic tail width, Urbachs energy EU, was obtained as a function of temperature. The temperature dependence of EU was interpreted based on the general models of this rule. The Urbachs energy decreased as a function of temperature in the temperature region investigated for the Er-doped sample. Such a decrease of the Urbachs energy can be explained to be due to the reduction of the electronic distortion caused by the structural disorders associated with the planar defects in the crystal lattice of InSe by the Er-doping procedure.