Effective Mass Approximation Theory Research Articles

Spin-polarized transport properties in diluted-magnetic-semiconductor/semiconductor superlattices under light-field assisted

Based on the single electron effective mass approximation theory and the transfer-matrix method, the spin polarized transport properties of electrons in a diluted-magnetic-semiconductor/semiconductor superlattice are studied. The influence of a light-field and a magnetic-field on spin polarized transport and the tunneling time in the superlattice structure are discussed in more detail. The results show that, due to the sp-d electron interaction between conduction band electrons and doped Mn ions, giant Zeeman splitting occurs. It is shown that a significant spin-dependent transmission and the position and width of the resonant-transmission-band of spin-dependent electron can be manipulated by adjusting the magnetic- and light-field. Considering the light field irradiation, the resonance band of electron is deformed and broadened with the increase of the light field intensity. For the case of a strong magnetic field, the transmission coefficient (TC) in the low-energy region is almost zero when the light field is not added, but with the increase of light intensity, the TC increased significantly in the zone increases significantly, that is, a quasi-bound band appears. These features are due to the energy exchange between electrons and the light field when electrons tunnel through the superlattice structure under light irradiation. In addition, light and magnetic fields can significantly change the spin polarization of electrons. Under a certain magnetic field intensity (<i>B</i> = 2 T), the light field significantly changes the spin polarization of electrons, the main effect is that the width of the spin polarization platform narrows and oscillatory peaks are accompanied on both sides of the platform. This effect is strengthened with the increase of the light field intensity. However, when the magnetic field is stronger (<i>B</i> = 5 T), the opposite is true. These show that the spin polarization can be modulated by the light field. Finally, the tunneling time of spin-up and spin-down electrons is studied by the evolution of Gaussian wave packets in the structure. The results show that the tunneling time depends on a spin of electrons, and it can be seen that the tunneling time of the spin-down electron is shorter than that of the spin-up electron in the superlattice structure. These remarkable properties of spin polarized transport may be beneficial for the devising tunable spin filtering devices based on diluted magnetic semiconductor/semiconductor superlattice structure.

First principles calculation of electronic properties and effective mass of zinc-blende GaN

Based on the first principle pseudopotential plane wave method, the electronic structure of zinc-blende semiconductor GaN is calculated. Using the relativistic treatment of valence states, the spin orbit splitting energy of valence band top near the center of Brillouin region is calculated. Based on the effective mass approximation theory, the effective mass of electrons near the bottom of the conduction band and the effective mass of light and heavy holes near the Γ point along the directions of [100], [110] and [111] are calculated. These parameters are valuable and important parameters of optoelectronic materials.

Adjustment of Terahertz Properties Assigned to the First Lowest Transition of (D+, X) Excitonic Complex in a Single Spherical Quantum Dot Using Temperature and Pressure

This theoretical study is devoted to the effects of pressure and temperature on the optoelectronic properties assigned to the first lowest transition of the (D+,X) excitonic complex (exciton-ionized donor) inside a single AlAs/GaAs/AlAs spherical quantum dot. Calculations are performed within the effective mass approximation theory using the variational method. Optical absorption and refractive index as function of the degree of confinement, pressure, and temperature are investigated. Numerical calculation shows that the pressure favors the electron-hole and electron-ionized donor attractions which leads to an enhancement of the binding energy, while an increasing of the temperature tends to reduce it. Our investigations show also that the resonant peaks of the absorption coefficient and the refractive index are located in the terahertz region and they undergo a shift to higher (lower) therahertz frequencies when the pressure (temperature) increases. The opposite effects caused by temperature and pressure have great practical importance because they offer an alternative approach for the adjustment and the control of the optical frequencies resulting from the transition between the fundamental and the first excited state of exciton bound to an ionized dopant. The comparison of the optical properties of exciton, impurity and (D+,X) facilitates the experimental identification of these transitions which are often close. Our investigation shows that the optical responses of (D+,X) are located between the exciton (high energy region) and donor impurity (low energy region) peaks. The whole of these conclusions may lead to the novel light detector or source of terahertz range.

Limitation of bulk GeSn alloy in the application of a high-performance laser due to the high threshold

In this work, the gain characteristics of the GeSn alloy are systematically investigated with effective mass approximation theory, and the potential of bulk GeSn in the application of a high-performance laser is discussed. The gain could not be enhanced persistently as the mole fraction of Sn continuously increases and becomes negative as the Sn fraction beyond 18%. An Sn fraction dependent doping scheme is proposed to effectively reduce the threshold injected carrier density to the lowest of 1.10×1018 cm-3. The optimum doping type varies from n-type to p-type as the Sn fraction increases with the conversion fraction of 10%. With doping optimization, the lowest threshold current density of bulk GeSn based laser is predicted to be 1.225 kA/cm2 for a designed n-Si0.157Ge0.643Sn0.200/p-Ge0.84Sn0.16/p-Si0.157Ge0.643Sn0.200 double heterostructure laser, indicating the inadequacy of bulk GeSn alloy of being the gain material for a high-performance laser.

Growth dynamics of zinc selenide quantum dots: the role of oleic acid concentration and synthesis temperature on driving optical properties

The growth dynamics of zinc selenide quantum dots (ZnSe QDs) using oleic acid as stabilizer was addressed in this work. The QDs were synthesized by hot-injection method, using 2:1 and 10:1 oleic acid/zinc acetate ratio at 170, 190, and 210 °C. The syntheses were carried out for 30 min, 1, 2, and 3 h and the samples were characterized by transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, and UV-Vis spectroscopy. The QDs diameter was evaluated by Effective Mass Approximation theory from the optical band gap. Lower oleic acid/zinc acetate molar ratio resulted in smaller particles and narrower size distribution. These results, evaluated under the light of Classical Nucleation and Growth Theory, were attributed to a smaller solubility of the monomers in the oleic acid solution. This effect leads to an increased supersaturation, promoting faster nucleation and, eventually, diffusion controlled growth, resulting in particles with narrower size distribution.

Photon-Assisted Spin-Dependent Tunneling in a Magnetic-Field Tunable Diluted Magnetic Semiconductor Structure with a Nonmagnetic Barrier

Using the Floquet theory and effective-mass approximation theory, we have investigated the terahertz (THz) photon-assisted transport through a magnetic-field tunable diluted magnetic semiconductor (DMS) structure with a nonmagnetic barrier under multi-physical fields, including an external magnetic field, an electric field and a THz field. It is found that a THz field and an electric field can greatly affect the transmission coefficient and current density as well as the corresponding polarizations. As a result, some asymmetric ‘Fano’ type transmission resonance appear at some specific energy. With the applied bias increasing, some quasi platforms of the polarization began to take shape. Furthermore, the transmission coefficient suppression or augmentation can be clearly seen from the variation tendency of current density. It is noteworthy that the amplitude of current–density polarization becomes larger under a THz field and a small magnetic field. These remarkable properties of spin polarization might be beneficial to the design of spin filtering devices.

Study on characteristics of CdS quantum dot-sensitized solar cells prepared by successive ionic layer adsorption and reaction with different adsorption times

Cadmium sulfide (CdS) quantum dots (QDs) were adsorbed on a titanium dioxide (TiO2) nanoporous film by successive ionic layer adsorption and reaction (SILAR) method with different adsorption times to study the influences of different SILAR adsorption times on CdS quantum dot-sensitized solar cells (QDSCs). The optical properties of CdS sensitized TiO2 films were studied by scanning electron microscopy and UV-Vis absorbance spectroscopy. The particle size of the CdS QDs was approximated using the effective mass approximation theory from the absorbance spectra. The photovoltaic characteristics of the CdS QDSCs were analyzed by I–V characteristics and electrochemical impedance spectroscopy under air mass 1.5 illumination. As a result, the particle size of the CdS QDs became larger and light harvesting was enhanced with increasing SILAR adsorption time. The maximum photovoltaic conversion efficiency of the CdS QDSCs (1.86%) was obtained at the SILAR adsorption time of 30 min with the highest short circuit current density and lowest charge transfer resistance.

Study of the confined states in AlxGa1−xAs/GaAs/vacuum surface quantum well

In this paper, we investigate the optical properties of confined electronic states in ultra-thin Al0.27Ga0.73As/GaAs/vacuum surface quantum wells by using photoreflectance spectroscopy at room temperature. Well-resolved doublet structures were found in the spectra. The energy of the features increases with decreasing well width in agreement with the predictions of a model of the transition energy between confined electron and hole states in a surface quantum well. Both the transition broadening and intensity behaviors are also well explained by the effective mass approximation theory. The offset between the un-perturbed theoretical transition energy and the experimental data has been explained by surface-state interaction effects. Moreover, the fact that the light hole ground state in the surface quantum well can be pushed out from the surface quantum well has been directly observed experimentally.

Role of surface passivating ligand and growth temperature on the size quantization effects of colloidal hybrid (MEH-PPV/P3HT:PbSe) nanocomposites

In this work, it has been demonstrated that a systematic combination of the reaction and growth conditions allows the possibility to produce high quality PbSe nanocrystals over a broad range of diameters with narrow size distributions without resorting to any size selective precipitation techniques. Effect of capping agents and critical growth parameters such as injection and growth temperature on the morphology of PbSe nanocrystallites is analyzed and discussed in terms of Efros'effective mass approximation theory and Wang's hyperbolic band model respectively. It is found that a hyperbolic Wang's band model is sufficiently accurate to explain the size quantization effects of PbSe nanocrystals. The capping agent/ligand (tri-n-octylphosphine and oleic acid mixture) helps to control the grain growth of the nanocrystals during its formation and increases the dispersibility of the nanocrystals in conducting polymer matrices viz. poly[2-methoxy, 5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly(3-hexylthiophene) (P3HT) for its application in photovoltaics.

Donor Binding Energy in GaAs/Ga1−x AlxAs Quantum Well: the Laser Field and Temperature Effects

Based on the effective-mass approximation theory and variational method, the laser field and temperature effects on the ground-state donor binding energy in the GaAs/Ga1−xAlxAs quantum well (QW) are investigated. Numerical results show that the donor binding energy depends on the impurity position, laser parameter, temperature, Al composition, and well width. The donor binding energy is decreased when the laser field and temperature are increased in the QW for any impurity position and QW parameter case. Moreover, the laser field has an obvious influence on the donor binding energy of impurity located at the vicinity of the QW center. In addition, our results also show that the donor binding energy decreases (or increases) as the well width (or Al composition x) increases in the QW.

Growth and characterization of optically confined particulate films of anatase TiO2

TiO2 particulate films with optically confined anatase phase were deposited on glass substrates using an aqueous precursor by dip-coating method. The precursor solution was prepared by mixing appropriate amounts of titanium tetraisopropoxide, isopropyl alcohol, and Triton X-100 in an airtight vessel. Without further treatment, the solution was used for the coating process. After deposition, the films were baked at 300°C, 450°C, and 600°C for 15 min in air atmosphere using a box furnace. Physical properties of such films were investigated by X-ray diffraction, scanning electron microscopy, and UV-visible spectrophotometry. Effective mass approximation theory was applied to explain the optical confinement effect which is realized in those films because of its crystallite size. The energy bandgap values for the confined anatase phase of TiO2 particulates were predicted by the effective mass approximation model and are in good agreement with our experimental results. These observed results were discussed in detail.

Quantum confinement observed in the x-ray absorption spectrum of size distributed meteoritic nanodiamonds

We present a near edge x-ray absorption fine structure study of artificial chemical vapor deposition diamonds with sizes in the micron range and of size distributed nanodiamonds extracted from the Murchison meteorite. The modified peak shape in the case of the nanodiamond sample is explained by the interplay between the size distribution of the nanodiamonds and quantum confinement effects within a quantitative analysis based on the effective mass approximation theory.

Microstructural and optical investigations of sol–gel derived ferroelectric BaTiO 3 nanocrystalline films determined by spectroscopic ellipsometry

Ferroelectric BaTiO 3 nanocrystalline films (BTNFs) with the crystalline sizes of about 30 nm were grown on Pt/Ti/SiO 2/Si substrates by a modified sol–gel method. Spectroscopic ellipsometry (SE) was used to characterize the films in the photon energy range of 1.5–5.0 eV with a five-phase layered model (air/surface rough layer/BaTiO 3/interface layer/Pt). The optical properties in the transparent and absorption regions have been investigated with the Forouhi–Bloomer dispersion relation. With the aid of the structural and dielectric function models, the microstructure and electronic structure of the BTNFs can be readily obtained. It was found that the refractive index reaches the value of 2.20 in the transparent region. Based on the Sellmeier dispersion analysis, the single-oscillator energy is about 4.7 eV for the BTNFs. The long wavelength refractive index n ( 0 ) can be estimated to about 2.00 at zero point. The direct optical band gap energy approaches approximately 4.2 eV and Urbach band tail energy is 262 ± 2 and 268 ± 1 meV respectively with increasing crystalline size. A higher band gap observed can be owing to the known quantum confinement effect in the nanocrystalline formation and different fraction of amorphous and crystalline components. The theoretical analysis based on the effective mass approximation theory is well used to explain these experimental data.

Electronic transport properties of thin, channel regions from SOI through GOI: A first-principles study

To investigate transport properties of the channel region in silicon on insulator (SOI) and germanium on insulator (GOI), we studied the electronic structure of hydrogen-terminated Si(100), (111) and (110), and Ge(100) and (111) slab models by the density functional theory. It is found that the electronic properties around 30 nm thickness are almost the same for those of the bulk and they deviate from the naive effective mass approximation theory in the range thinner than 4 and 10 nm for Si and Ge, respectively. The effective masses in the Si <001 > and Ge < 111 > confined models are almost constant and equal to the bulk ones throughout the calculated range. However, the masses of the other models strongly depend on layer thickness. These behaviors are related to the off-diagonal terms in the effective mass tensors. In the Ge < 100 > model, we found that the confinement causes the exchange of the conduction band bottom between the Γ and the Brillouin zone boundary, which may cause the enhancement of the electron mobility.

The effect of ultraviolet irradiation on the photothermal, photoluminescence and photoluminescence excitation spectra of Mn-doped ZnS nanoparticles

Research involving Mn doped nanocrystalline ZnS (ZnS:Mn) has grown in recent years, partly due to the high quantum luminescence efficiencies that have been reported. We measured the photoacoustic (PA), the photoluminescence (PL) and the photoluminescence excitation (PLE) spectra of surface-passivated and unpassivated ZnS:Mn. The effects of UV irradiation on the PL and PLE spectra were also studied. A decrease in the PA intensity after UV exposure was observed for the ZnS:Mn, indicating a decrease in the nonradiative relaxation probability. The observed increase in PL intensity indicates a corresponding increase in the radiative transition probability. For the PLE spectra, possible aggregation of the primary particles could have resulted in the lower measured energy of the PLE peak compared to the value predicted by the effective mass approximation theory.

Two-photon-absorption-induced nonlinear photoresponse in GaAs∕AlGaAs quantum-well infrared photodetectors

Using a free-electron laser(FEL) source, we have studied the two-photon-absorption (TPA) effect in GaAs∕AlGaAs quantum-well infrared photodetector (QWIP). The TPA-induced photoresponse in QWIPs has been measured under different FEL excitation power by the photoconductivity method. The effective-mass approximation theory is used for the QWIP structure to explain the photoresponse behavior. It is demonstrated that the TPA-induced photocarrier density is proportional to the square of the excitation power. Based on the experimental results, the TPA coefficients of QWIPs were obtained to be 0.0045, 0.0030, 0.0103, and 0.0061cm∕MW for the excitation lines of 10.6, 10.7, 11.9 and 13.2μm, respectively. The dependence the TPA coefficients on the excitation wavelength is explained by our theoretical model.

Study on the confined states in single surface quantum wells

We have studied the optical properties of surface quantum wells with different width of wells (5 nm and 10 nm) by in-situ photo-modulated reflectance spectroscopy on molecular beam epitaxy (MBE) system. The single surface quantum well (SQW) is confined by the vacuum on one side and by Al 0.3Ga 0.7As barrier on the other side. In experiments, we have observed clearly the transitions from the confined hole states to the confined electron states. The transitions between those excited states in 10 nm surface quantum well are observed for the first time. The effects of the surface on the confined states have been well studied by combination of the PR spectra and the effective mass approximation theory.

The Binding Energy of the $Sb$ Donor State in Ge_{1-x}Si_{x} Grystals

The singlet ground-state binding energy \varepsilon_1 of antimony impurity in Ge_{1-x}Si_x(0\leq x\leq, 3) has been determined on the basis of Hall measurements. It is shown that \varepsilon_1 increases linearly with Si concentration in Ge-like (0\leq x\leq 0, 12) and Si-like (x\geq,15) crystals. Experimental results have been analysed within effective mass approximation theory for shallow donors. Central core effect influence in the ground-state binding energy of antimony in Ge-like crystals was estimated.

Direct measurement of near-band-gap electrorefraction in Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As thin-film structures

We report the first direct measurement of near-band-gap Franz–Keldysh electrorefraction in Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As p-i-n single-crystal thin-film Fabry–Perot structures with semitransparent metallic mirror contacts. These measurements are performed for various reverse biases and at photon energies ranging from 9 to less than 1 meV from the GaAs band edge. The measured refractive index variation is several times larger than that predicted by the effective mass approximation theory.

Franz–Keldysh electrorefraction and electroabsorption in bulk InP and GaAs

Franz–Keldysh electrorefraction and electroabsorption were measured for semi-insulating InP and GaAs at several electric fields and several wavelengths near the absorption edge. For InP, the experimental results are described well by an effective mass approximation theory with a correction that accounts for the exponential absorption tail. It is shown that, at least in InP, Franz–Keldysh electrorefraction is under some conditions much stronger than the Pockels effect.