Concentration Of Free Charge Carriers Research Articles

Statistics of charge carriers of quantum semiconductor film in the presence of strong lateral electrostatic field

The influence of an external strong electrostatic field on the statistical characteristics of equilibrium free charge carriers in a quasi-two-dimensional structure is considered using an example of an InAs semiconductor quantum film. The analysis is carried out for the case when a strong quantization regime for charge carriers is realized in the film and only the first and second film sub-bands of their quantized motion are occupied. The general analytic expressions for the energy of the quantized motion of charge carriers in a film in the presence of a strong electrostatic field are given. An explicit dependence of the width of the band gap of the sample on the value of the external field is obtained. It is shown that with the increase in the external field, the width of band gap of the sample increases. The calculations and corresponding analytical expressions for the chemical potential, concentration, energy and heat capacity of free charge carriers of an InAs quantum film in the presence of a strong electrostatic field are also presented. It is shown that with an increase in the external electrostatic field, the chemical potential of the system retains its negative sign, but increases by the magnitude. For a given value of the external field, the chemical potential of the system increases linearly with the increase in temperature. It is shown that at a given temperature the carriers’ density decreases with the increase in the external field. At a fixed value of the field, the concentration increases with the increase in temperature. The behavior of the energy and heat capacity of the charge carriers is also determined by the same dependence on the temperature of the system and the magnitude of the external field. It is also shown that the main contribution to the indicated characteristics of the sample is made namely by the first filled sub-band of quantized motion of charge carriers. If the second sub-band is also filled, its contribution to the determination of the statistical characteristics of the sample is exponentially smaller than the contribution of the first filled sub-band.

Specific features of the photoexcitation spectra of epitaxial InN layers grown by molecular-beam epitaxy with the plasma activation of nitrogen

The results of studies of the photoexcitation spectra of epitaxial InN layers formed by molecular-beam epitaxy with the plasma activation of nitrogen are reported. The concentration of free charge carriers in the layers is 1018–1019 cm–3. The photoconductivity, photoluminescence, and absorption spectra exhibit a shift of the long-wavelength threshold of interband transitions in accordance with the Burstein–Moss effect for n-InN with different concentrations of equilibrium electrons. In the samples, absolute negative photoconductivity with a nanosecond relaxation time is observed. The results of photoelectric, absorption, and luminescence spectroscopy experiments are correlated with the technological parameters and electron microscopy data.

Two-Dimensional Boron Polymorphs for Visible Range Plasmonics: A First-Principles Exploration.

Recently discovered two-dimensional (2D) boron polymorphs, collectively tagged borophene, are all metallic with high free charge carrier concentration, pointing toward the possibility of supporting plasmons. Ab initio linear response computations of the dielectric function allow one to calculate the plasmon frequencies (ω) in the selected example structures of boron layers. The results show that the electrons in these sheets indeed mimic a 2D electron gas, and their plasmon dispersion in the small wavevector (q) limit accurately follows the signature dependence ω ∝ √q. The plasmon frequencies that are not damped by single-particle excitations do reach the near-infrared and even visible regions, making borophene the first material with 2D plasmons at such high frequencies, notably with no necessity for doping. The existence of several phases (polymorphs), with varying degree of metallicity and anisotropy, can further permit the fine-tuning of plasmon behaviors in borophene, potentially a tantalizing material with utility in nanoplasmonics.

Interband and free charge carrier absorption in silicon at 800 nm: experiments and model calculations

The average transmitted power of a Ti:sapphire femtosecond (fs) laser beam through thin, oxidized silicon (Si) membranes in the 10–30 \(\, \upmu \text{m}\) thickness range at \(45{^\circ }\) external angle of incidence at first increases with the incident average laser power and then decreases considerably. This stationary state behavior is quantitatively reproduced by a recently derived model using an effective absorption coefficient \(\alpha _{\text {eff}} = \alpha _{1} + \alpha _{\text {FCA}}\), adding the coefficients of two independent linear absorption processes, \(\alpha _{1}\) of the indirect interband transition and \(\alpha _{\text {FCA}}\) of free charge carrier absorption (FCA). The relations between the empirical parameter \(\alpha _{\text {FCA}}\) and the model parameters \(n_{\text {eh}}\) (density of free charge carriers) and \(\sigma _{\text {eh}}\) (absorption cross section of free charge carriers) are found to depend on the Si membrane thickness and are briefly discussed. Using different sample thicknesses, experimental evidence has been found for a nonlinear intensity dependence of \(\alpha _{\text {FCA}}\)∼\(I_{0}^{n}\), where \(1\le n \le 3\) shows systematic variation. The FCA model in combination with its empirical fit parameters allowed to fully calculate the transmission behavior of the Si membranes within the investigated sample thickness and laser power regimes.

Strong Stimulated Mie Scattering From Plasmonic CuS Nanocrystals in Toluene or Pentane

We report here strong stimulated Mie scattering (SMS) from a grating formed by optically redistributed plasmonic CuS nanocrystals (NCs), which exhibit a strong near-infrared localized surface plasmon resonance. We demonstrate that CuS NCs dispersed in a solvent, such as toluene or pentane, interact strongly with light due to their large polarizability that arises from their high concentration of free charge carriers. The NCs redistribute in the intensity grating that is formed by the interference of forward pump light with backward Mie scattered light. This alignment generates a Bragg grating of CuS NCs, which produces efficient reflection to the plasmonically enhanced backward Mie scattering. We report SMS from CuS NCs suspended in toluene at varying concentrations, pumped by ~816-nm and ~10-ns laser pulses. At the optimum concentration and for a 2-cm cuvette length, the reflectivity (energy transfer efficiency from the input pump to the backward SMS) reached 29%, and the pump energy threshold was 4.8 times lower than that of stimulated Brillouin scattering in pure toluene. The time delay of the backward SMS pulse observed at lower pump energy levels is consistent with reflection from a grating formed by spatial redistribution due to drift mobility of nanoparticles. Changing the solvent from toluene to pentane that has lower viscosity, reduced the delay time, which is also consistent with the nanoparticle redistribution model. The temporal behavior, spectral line shape, and far-field distribution of the backward SMS are presented.

Effect of free charge carriers on birefringence and dichroism in anisotropic porous silicon layers

The infrared optical properties of anisotropic mesoporous silicon films containing free charge carriers (holes) are studied experimentally and theoretically. The results of simulation of the optical properties of the produced samples in the effective-medium approximation show a heavy dependence of the birefringence, reflection anisotropy, and dichroism on the concentration of free charge carriers. The unsteady behavior of the differential transmittance recorded for the samples at mutually perpendicular polarization directions of light are attributed to the effect of charge carriers with a concentration on the order of 1019 cm–3. The results of the study suggest that anisotropic silicon nanostructures are promising materials for infrared photonics and terahertz engineering.

Investigating the structural and optical properties of thermally evaporated 1,3,3-trimethylindolino-β-naphthopyrylospiran thin films

1,3,3-Trimethylindolino-β-naphthopyrylospiran (SO) thin films were synthesised using thermal evaporation technique. The crystal structure of the (SO) compound as powder and thin film forms was studied using X-ray diffraction (XRD), transmission electron microscope, and Fourier transformation infrared techniques. Subsequently, Miller’s indices, hkl, values were estimated for each individual peak in the XRD chart of powder form and given a polycrystalline with monoclinic structure. The optical properties of the thin films as-deposited were inspected using spectrophotometric measurements of reflectance and transmittance in the wavelength range from 200 to 2500 nm. The refractive index, n, and the absorption index, k, were calculated. The electronic transitions of as-deposited thin film were direct transitions with energy gap of ~2.3 eV. Regarding the obtained data in normal dispersion region, the optical parameters, such as oscillator energy, E o, dispersion energy, E d, lattice dielectric constant, e L, high frequency dielectric constant, e ∞, and the ratio of the free charge carrier concentration to the effective mass, N/m*, were estimated. Finally, the extinction molar coefficient, e molar, oscillator strength, the electric dipole strength, dielectric constant, and loss tangent are calculated for the films.

Thermoelectric power of Bi92Sb8 and Bi85Sb15 thin films

The results of studying the galvanomagnetic and thermoelectric properties of thin block Bi92Sb8 and Bi85Sb15 films on mica and polyimide substrates are presented. The method used for measuring the thermoelectric power allowed us to study the temperature dependence the thermoelectric power, without introducing additional deformations into the substrate–film system. A significant difference in the temperature dependences of the galvanomagnetic and thermoelectric properties of films on mica and polyimide is found. The free charge-carrier concentrations and mobilities in the films on mica and polyimide and levels of the chemical potential for electrons and holes are calculated within the two-band approximation. The difference in the charge-carrier parameters for films on mica and polyimide is associated with strains in the film–substrate system.

Photoactivation of an ionic p-type dopant used in a triarylamine based hole transporting material for enhancing conductivity of solution processed films

The ionic compound 4-isopropyl-4′-methyldiphenyliodonium tetrakis-(penta- fluorophenyl-borate) (DPI-TPFB) has been investigated as p-type dopant in a triarylamine (TAA) based hole transporting material (HTM). Charge transfer from the HTM to the iodonium salt has been verified and translates to an increase in conductivity of up to 5 orders of magnitude compared to non-doped HTM. UV photoactivation treatment of HTM:DPI-TPFB in solution was shown to increase conductivity by additional 2–3 orders of magnitude up to 2.6·10−2S/m. The concentration of free charge carriers and the related doping efficiency of DPI-TPFB was derived from current–voltage (I–V) measurements of hole only devices. It was possible to demonstrate that the activation treatment triggers a reaction on the molecular level of the iodonium salt. Possible causes for the increased doping efficiency of the system were explored using density functional theory calculations.

Universal experimental test for the role of free charge carriers in the thermal Casimir effect within a micrometer separation range

We propose a universal experiment to measure the differential Casimir force between a Au-coated sphere and two halves of a structured plate covered with a P-doped Si overlayer. The concentration of free charge carriers in the overlayer is chosen slightly below the critical one, f or which the phase transition from dielectric to metal occurs. One ha f of the structured plate is insulating, while its second half is made of gold. For the former we consider two different structures, one consisting of bulk high-resistivity Si and the other of a layer of silica followed by bulk high-resistivity Si. The differential Casimir force is computed within the Lifshitz theory using four approaches that have been proposed in the literature to account for the role of free charge carriers in metallic and dielectric materials interacting with quantum fluctuations. According to these approaches, Au at low frequencies is described by either the Drude or the plasma model, whereas the free charge carriers in dielectric materials at room temperature are either taken into account or disregarded. It is shown that the values of differential Casimir forces, computed in the micrometer separation range using these four approaches, are widely distinct from each other and can be easily discriminated experimentally. It is shown that for all approaches the thermal component of the differential Casimir force is sufficiently large for direct observation. The possible errors and uncertainties in the proposed experiment are estimated and its importance for the theory of quantum fluctuations is discussed.

Effect of Sn substitution for Se on dispersive optical constants of amorphous Se–Te–Sn thin films

ABSTRACTOptical constants of vacuum evaporated thin films in the Se90−xTe10Snx (x = 0, 2.5, 5 and 7.5 at %) system were calculated from reflectance and transmittance data in the wavelength range of 300–2500 nm using Murmann’s equations. The maximum value of refractive index increased and suffered a shift towards the short wavelength as the Sn content increases this behavior can be attributed to increasing the values of cross-linking chains density and decrease tailing. The variations in the real and the imaginary parts of the dielectric constant, the dissipation factor tan(δ), the optical conductivity, the volume, and surface energy loss functions with photon energy have also been reported. The normal dispersion of the refractive index is discussed regarding the single oscillator Wemple-DiDomenico model from which the dispersion parameters were determined. The single oscillator energy, the dispersion energy, the high-frequency dielectric constant, the ratio of free charge carrier concentration to the effective mass, plasma frequency, single oscillator strength and its position are then estimated as a function of alloy composition. It is interesting to note that the increase of Sn content on the parent Se90Te10 is connected to the decreasing covalent nature of the structure. Besides, the presence of such a sharp absorption edge in transmission spectra recommends Se90−xTe10Snx thin films as a good optical filter material.

Effect of annealing temperature on the evolution of structural, microstructural, and optical properties of spin coated ZnO thin films

Zinc oxide (ZnO) thin films were sol-gel spin coated on glass substrates and annealed at various temperatures from 300–500°C. Zinc acetate dihydrate (ZAD), monoethanolamine (MEA), and 2-methoxyethanol were used as the starting materials, stabilizer and solvent, respectively. The effect of annealing temperature on the structural and optical properties of the ZnO thin films was investigated by X-ray diffractometer (XRD), atomic force microscope (AFM), UV–VIS spectrophotometry and ellipsometry. The XRD results showed the films to have a preferential c-axis orientation, whereas the AFM results confirmed a columnar structure. The surface roughness increased with the increase in annealing temperature. Parameters such as ratio of free charge carrier concentration to effective mass (N/m*) and plasma frequency (ωp) were determined from the transmittance graph using the Wemple di Domenico model. Both N/m* and ωp were noticed to reduce with the increase in annealing temperature. Band gap decreased with the increase in the annealing temperature indicating absorption edge shift towards the red region.

Growth of arsenic-doped cadmium telluride epilayers by metalorganic chemical vapor deposition

We have studied arsenic incorporation from tris(dimethylamino)arsine into epitaxial CdTe layers in a metalorganic chemical vapor deposition (MOCVD) process. Arsenic incorporation into the layers has been shown to depend on the crystallographic orientation of their growth. Incorporation effectiveness increases in the order (111)B < (211)B < (100) < (310) < (211)A. Arsenic concentration in the CdTe layers is proportional to the tris(dimethylamino)arsine flow rate to the power 3/2. The dependence of arsenic incorporation on the ratio of the metalorganic tellurium and cadmium precursors in the vapor phase suggests that arsenic is incorporated predominantly into the anion sublattice. The maximum concentration of free charge carriers, p 295 = (1–2) × 1017 cm–3, was observed after annealing of the arsenic-doped CdTe layers in an argon atmosphere.

ECV investigation of ion-implanted photosensitive silicon structures for backside illuminated CCDs

Boron implanted Si structures used in back-side illuminated and electron bombarded charge coupled devices (CCD) were investigated by means of electrochemical capacitance- voltage (ECV) profiling. A set of test structures were specially fabricated using various energies and doses of boron implantation, as well as different materials were used as their coating layers: Al and silicon oxide. The concentration profiles of free charge carriers across the sample were experimentally obtained. Further, using Poisson equation and Fredholm equation of the first kind the distribution of free charge carrier concentration and electric field intensity in depth of the samples were calculated. By analyzing and comparison of simulated and experimentally obtained concentration profiles, the recommendations for optimization of sample parameters were proposed aiming at increase of sweeping field and decrease of surface potential impact on charge carrier transport.

Growth of hierarchical strontium incorporated cadmium sulphide for possible application in optical and electronic devices

In this work, we have incorporated strontium (Sr) into cadmium sulphide (CdS) complex to explore its effect on optical and electrical properties. In this regard Sr composited CdS and a bared CdS were prepared by hydrothermal technique. X-ray diffraction pattern of the materials were analyzed to obtain their crystalline structure, size and internal lattice strain. The compositional details were obtained from SEM-EDAX study. The optical properties of the films of synthesized materials were analyzed. The dependency of refractive index on incident photon energy was studied in detail for each material. The dispersion energy related parameters, dielectric relaxation constant and optical conductivity followed by dispersion of the refractive index were determined and discussed by employing the single electronic oscillator model. From wavelength dependent refractive index data, the plasma frequency, ratio of free charge carrier concentration to effective mass, lattice dielectric constant and high frequency dielectric constant were calculated to determine the discrepancy of the compounds. The third order optical susceptibility was analyzed to check their potential ability of performance in optical limiter and optical switching device application. The electrical characterization was performed by measuring the current–voltage characteristics of structure ITO/CdS:Sr/Al and ITO/CdS/Al. The rectifying I–V showed improved On/Off ratio under dark (25.03 and 19.98) and light (35.51 and 25.09) for ITO/CdS:Sr/Al compared to ITO/CdS/Al. The dark and photoconductivity of the film were also determined for both the films. The time dependent current response was analyzed at constant bias voltage ±1 V.

Direct comparison of atomic layer deposition and sputtering of In2O3:H used as transparent conductive oxide layer in CuIn1−xGaxSe2 thin film solar cells

In this study thin films of hydrogenated In2O3 (IOH) were fabricated by physical vapor deposition (PVD) with and without a post-annealing step, and by atomic layer deposition (ALD). The electro-optical properties on glass as well as the performance as a transparent conductive oxide (TCO) layer in CuIn1−xGaxSe2 (CIGSe)-based solar cells are compared and related to a ZnO:Al (AZO) baseline TCO. Corresponding TCO film thicknesses were adjusted to a resulting sheet resistance of about Rsh = 20 Ω/sq for all samples. Structural investigations were conducted by X-ray diffraction (XRD) and transmission electron microscopy (TEM), while Hall and optical absorption measurements were performed to analyze the electrical and optical quality of the window layers. It is shown that the fully crystallized IOH layers processed by ALD and PVD show similar microstructural and electro-optical properties, which are superior to the AZO baseline. The finalized solar cells were characterized by current-voltage and reflectance-corrected quantum efficiency measurements. While there is no significant gain in short circuit current density (Jsc) for as-deposited PVD In2O3 layers, the application of crystalline In2O3 TCOs leads to an improvement of more than 2mA/cm2 due to an increase in “optical” band gap energy and less free charge carrier absorption (FCA). The open circuit voltage (Voc) of the best cells is 10–15mV higher as compared to the AZO reference, independent of the crystallinity and process of the In2O3 films. The results indicate that the gain in Voc is due to inherent material properties of the IOH films and does not originate from less sputter damage or an affected i-ZnO/TCO interface. Device simulations show that the higher electron affinity χ of the IOH can explain an increased Voc if the Fermi level (EF) is pinned at the CIGSe/CdS interface and why it might not be possible to see the gain when alternative buffer layers are applied.

Thermodynamics of defect subsystem in zinc telluride crystals

Using method on the base of minimizing of thermodynamic potential in “crystal-vapor” system as a function of defect concentration the equilibrium concentration of point defects and free charge carriers in zinc telluride (ZnTe) crystals have been calculated depending on the technological factors of two-temperature annealing (annealing temperature [Formula: see text] and vapor pressure [Formula: see text] of zinc or [Formula: see text] of tellurium). It is shown that the dominant defects are zinc vacancies the charge state of which depends on the technological conditions of annealing.

Free charge carrier absorption in silicon at 800 nm

The transmission of a Ti:sapphire laser beam (c.w. and fs pulsed operation at 800 nm) through a 10-μm-thin oxidized silicon membrane at 45° angle of incidence at first increases with the incident laser power, then shows a maximum, and finally decreases considerably. This nonlinear transmission behavior is the same for c.w. and pulsed laser operation and mainly attributed to free charge carrier absorption (FCA) in Si. A simple FCA model is developed and tested.

Method for Determining the Doping Efficiency of Dispersed Semiconductor Metal Oxide Materials

In this paper, a method for determining the doping efficiency of dispersed semiconductor metal oxide materials is proposed proposing to use the dependences of the free charge carrier concentration, normalized to the concentration of the doping impurity (Ne spec.), on the content of this impurity. The possibilities of this method are demonstrated by the example of studying the effect of technological factors on the efficiency of doping of indium oxide with tin and doping of tin oxide with antimony. It is shown that it is impossible to achieve the concentration of free charge carriers in the ITO material, higher than that in ATO materials, due to the lower solubility of tin in the In2O3 lattice, as compared with the solubility of antimony in the SnO2 lattice.

Investigation of delta-doped pHEMT InGaAs/GaAs/AlGaAs structures by the electrochemical capacitance-voltage technique

In pHEMT devices, a two dimensional (2D) conducting channel representing a quantum well (QW) and adjacent layers of wide-gap semiconductors are fabricated on an undoped material which provides a high mobility of free carriers in the channel. During the development of pHEMT structures it is always very important to estimate and control the free charge carriers concentration, that is not a trivial task because of its redistribution which causes changing of the electric field distribution and, respectively, the energy spectrum of states in the 2DEG channel. By using the modern ECV technique, the pHEMT InGaAs/GaAs/AlGaAs structures were investigated. Free charge carriers concentration profiles were obtained, and the location of 5-layers and the carrier concentration in the QW were estimated.