Ionized Donor Density Research Articles

Degradation behavior of CIGS solar Cells: A parametric analysis

A parametric study of the effect of bulk and interface properties on device characteristics as a function of heat and light stress under open-circuit and short-circuit conditions is developed for Cu(In,Ga)(S,Se)2 solar cells using SCAPS-1D simulator. The variables and interdependencies modeled include: (1) conduction band offset (CBO) between buffer and absorber; (2) buffer ionized donor density; (3) CIGS shallow acceptor density; (4) CIGS deep acceptor density; (5) CIGS shallow donor density; (6) ionized acceptor concentration in the ordered vacancy compound (OVC) CIGS/buffer interface; and (7) back contact work-function. An increase in absorber shallow acceptor or a decrease in shallow donor density results in an increase in net carrier density and open-circuit voltage (VOC). A decrease in VOC is observed with an increase in the deep acceptors or neutral midgap defects in the CIGS due to higher charge carrier recombination. A change of CBO from spike to cliff condition is a dominant mechanism of decreasing VOC with an interface defect density of 1012 cm−2. The minimum depletion width is found to be specifically sensitive to CBO and interface defect density. An inflection in the capacitance–voltage curves (in reverse bias) is observed with simultaneous increase in bulk deep acceptor density and shallow donor density near the back contact.

Investigation of Trap States, Series Resistance and Diode Parameters in Al/Gelatin/n-Si Schottky Diode by Voltage and Frequency Dependent Capacitance and Conductance Analysis

Inorganic-organic Schottky contacts based on Gelatin on n-Si wafer have been prepared by a spin coating technique. The reverse and forward bias capacitance-voltage () and conductance-voltage () properties of the Al/Gelatin/n-Si Schottky diode at room temperature in the frequency range from 30 kHz to 1 MHz have been computed by taking into account the series resistance () and interface states () effects. The conductance and Hill-Coleman method were used to determine interfacial layer capacitance (), and The values of and were determined as 810 Ω and 1.52 x−1012 eV−1 cm−2 for 30 kHz and 38 Ω and 3.38 x 1011 eV−1 cm−2 for 1 MHz. Experimental results corroborated that the and are influential parameters which severely impact the basic electrical parameters of Al/Gelatin/n-Si Schottky diode. Both the measured capacitance () and conductance () were corrected in order to obtain the real diode capacitance () and conductance (). Schottky diode parameters such as barrier height (), ionized donor density (), Fermi level (), built-in voltage () were extracted from the frequency-dependent and relations. The values of was determined as 0.796 eV for 30 kHz and 1.090 eV for 1 MHz. The and values were observed to decrease, while the values increase as the frequency increases. Results reveal that the fabricated diode can potentially be used as a promising candidate material for electronics applications.

Self-consistently modeling the electronic behavior of “active” nanostructures: Scaling of ionized donor density at the nanoscale

We present a powerful and versatile method for calculating the electronic properties of real nanostructures. It is applied to a spherical quantum dot, self-consistently reconciling the non-linear Poisson equation with the Kohn-Sham equations, balancing the charge in the surface states and the “bulk” (dot interior). Excellent agreement is obtained between theory and experiment, with the model predicting a spatial dependency of donor ionization for dots below 10 nm radii. This has far reaching implications for nano-device design.

Photorefractive hologram writing with high modulation depth in photovoltaic media under different boundary conditions

Photorefractive recording response to sinusoidal intensity pattern in photovoltaic media under different boundary conditions, namely, open-circuit, crystal-resistor-circuit, short-circuit and crystal-voltage supply-circuit, are investigated by solving steady-state Kukhtarev equations numerically. In high modulation depth m limit and when photovoltaic field becomes sufficiently large, we find that both the magnitude and imaginary part of fundamental space-charge field E 1 as a function of m may transit from superlinear increment to sublinear increment for short-circuit condition. The role of photovoltaic field in holographic storage and two-wave mixing is also discussed in comparison with an earlier experimental literature [G. Cook et al., Opt. Commun. 192 (2001) 393]. We further generalize Gu et al.’s remark on photovoltaic field and dc space-charge field, which is for small m only [C. Gu et al., J. Appl. Phys. 69 (1991) 1167], to whole range of m in the limit donor density much larger than ionized donor density that the effect of photovoltaic field cannot be cancelled or considered as an externally applied field for an arbitrary non-zero m.

Space charge field enhancement in photorefractive materials by high frequency applied electric fields: analytical studies of resonances and enhancement by step-like wave forms

Analytical solutions are presented for the ionized donor density (related in a simple manner to the space charge field) when a high frequency electric field is applied to the photorefractive material. An approximate solution for an arbitrary profile of the applied field is found by generalizing the averaging technique of Stepanov and Petrov. An alternative approach for step-like wave forms is also presented. Examples are provided for three special cases when the periodic variation of applied electric field is in the form of a square wave, a symmetric pulse or an asymmetric pulse. Resonance conditions are discussed, and resonance curves are presented for the asymmetric pulse. For the symmetric pulse it is shown that the trap density limitations cannot be overcome.

Temporal evolution of the physical response during photorefractive grating formation and erasure for BSO

The time evolution of the charge carrier density, ionized donor density and space-charge field under a sinusoidal light pattern have been calculated by solving numerically the nonlinear differential equations for a photorefractive material (such as BSO). The time evolution for optical erasure starting from the steady-state solution has also been obtained. Arbitrary modulation depths have been considered for two relevant cases: pure diffussion and under an applied dc electric field of 5 kV/cm.

Electron density in AlxGa1-xAs/GaAs-based two-dimensional systems in the quantum Hall regime.

When a strong magnetic field B is applied perpendicular to the interface of a two-dimensional electron system (2DES), the total electron density in the structure may differ from that at B=0 because the density of states in the Landau quantized system differs from that at B=0. For the 2DES based on modulation-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs structures, the ionized donor density in the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As layer will vary with the magnetic field, which leads to differing total electron densities. In this paper we observe this effect by self-consistently calculating the electronic structure of selectively doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As-GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As single quantum wells. In the quantum Hall plateaus, the total electron density increases linearly with magnetic field. The numerical results are presented for the case with one and two occupied electronic subbands.

Higher harmonic gratings in photorefractive materials at large modulation with moving fringes

The nonlinear differential equations describing the response of photorefractive material to a moving sinusoidal intensity pattern that permits large modulation depth are solved by a numerical technique under steady-state condition. It is found that, at large modulation, the higher harmonics of the variables (space charge field, electron density, and ionized donor density) have appreciable amplitudes that can be further enhanced by moving the fringes. It is also shown that the optimized amplitudes of the higher harmonics of the space charge field can be strong and comparable with the fundamental and that the dependence of these higher harmonics of the space charge field on the spatial frequency is much weaker at large modulation.

Two-dimensional numerical simulation of Fermi-level pinning phenomena due to DX centers in AlGaAs/GaAs HEMTs

Fermi-level pinning phenomena due to DX centers in AlGaAs/GaAs HEMTs (high electron mobility transistors) are analyzed using two-dimensional numerical simulation based on a drift-diffusion model. A DX center model is introduced assuming Fermi-Dirac statistics for ionized donor density with the aluminum mole fraction dependence of the deep-donor energy level. The calculated results reveal that the decrease in transconductance of AlGaAs/GaAs HEMTs in a high-gate-bias region is caused by the existence of DX centers. This is because the Fermi level is pinned at deep-donor levels in the n-AlGaAs layer. Furthermore, the superiority of AlGaAs/InGaAs pseudomorphic HEMTs is discussed in terms of the Fermi-level pinning.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An equivalent circuit for spatial variations in photorefractive materials

It is shown that the spatial variations of physical quantities like space charge electric field, ionized donor density modulation and electron density modulation may be characterized by an equivalent circuit consisting of a capacitor in parallel with an inductor and a resistance in series.

N-Type SrTiO 3 thin films: Electronic processes and photoelectrochemical behavior

Amorphous thin n-SrTiO 3 films have been prepared using the RF sputtering technique in a reductive atmosphere. Their optical absorption and photoelectrochemical properties have been investigated. In the visible region, the films display an increase of the optical absorption coefficient with wavelength, which is correlated with some deep localized levels within the band-energy gap. The low flat-band potential ( V fb = −1.0 V in 1 M NaOH solution) and the high ionized donor density ( N D = 10 20 cm −3) characterize an n + degenerate semiconductor with a low electron affinity.

Negative TEO-Diode Conductance by Transient Measurement and Computer Simulation

A new method based on slow microwave transients due to steep bias-voltage steps gives a detailed negative-device-conductance function versus microwave-voltage amplitude V/sub ac/ for Gunn diodes. Measurements of GaAs and InP devices made by different fabrication processes as used by a variety of manufacturers show that basic differences in behavior exist. Some of these are representative of high switching speeds and others of good steady-state efficiencies. Computer simulation of Gunn devices with a range of mobility and ionized-donor density profiles oscillating in a suitable resonant structure leads to similar differences in negative-conductance functions. A first correlation between experimental and theoretical behavior is attempted, and it is possible to estimate the mobility and carrier-density profiles which could most likely be responsible for a certain device behavior. It is shown that an external locking signal affects the device's negative conductance only for small values of V/sub ac/, and experimental results confirm that this, in accordance with theoretical expectation, increases the switching speed only of certain types of diodes.

Post-breakdown conduction in thin film Al(Al 2O 3)CdSAl switching element

Thin film sandwich structures of the type Al(Al 2O 3)CdSAl, where the Al 2O 3 layer is thermally grown at room temperature and the CdS layer has a conductivity of the order of 5 × 10 -9 Ω -1 cm -1 , exhibit bistable switching in the post-breakdown stage. This stage, which consists of two distinct states of high and low conductivity, was studied with the help of conductivity and small signal capacitance measurements. It was found, as a result of these measurements, that the value of ionized donor density in the high conductivity state is about twice that in the low conductivity state, and further, that both the contacts are non-ohmic. In view of these results, the dominant mechanism of conduction in the post-breakdown stage was ascertained to be the one involving field ionization of traps.

Effect of density gradient of deep traps on Schottky barrier capacitance in thin films†

Abstract The effect of the density gradient of filled traps on the Schottky barrier capacitance in thin films has been ascertained. Different cases have been considered for various values of L, the profile parameter, and n0 (n = n0 exp (– x/L), n being the density of filled traps). It is found that for n0> N (N being the ionized donor density), the field distribution shows a minimum whereas a voltage maximum is obtained in the capacitance versus voltage curve. The effect of the density gradient of deep traps has been shown to modify the usual expression for the slope and the intercept of the 1/C2 versus V curve in the absence of such a gradient. Results of some experimental investigations have also been included to support the theoretical predictions. The experimental study of temperature dependence of capacitance yields an activation energy of 006 ev for coalescence of two deep lying traps to form a single new one.