Donor Binding Research Articles

Barrier Thickness and Hydrostatic Pressure Effects on Hydrogenic Impurity States in Wurtzite GaN/AlxGa1−xN Strained Quantum Dots

Within the framework of the effective mass approximation, barrier thickness and hydrostatic pressure effects on the ground‐state binding energy of hydrogenic impurity are investigated in wurtzite (WZ) GaN/AlxGa1−xN strained quantum dots (QDs) by means of a variational approach. The hydrostatic pressure dependence of physical parameters such as electron effective mass, energy band gaps, lattice constants, and dielectric constants is considered in the calculations. Numerical results show that the donor binding energy for any impurity position increases when the hydrostatic pressure increases. The donor binding energy for the impurity located at the central of the QD increases firstly and then begins to drop quickly with the decrease of QD radius (height) in strong built‐in electric fields. Moreover, the influence of barrier thickness along the QD growth direction and Al concentration on donor binding energy is also investigated. In addition, we also found that impurity positions have great influence on the donor binding energy.

Hydrogenic Donor Binding Energy in a Strained BxGa1-xN/BN Quantum well

Group III nitride materials are well known for their superior thermal, mechanical and optical properties. They can be used for short-wavelength light-emitting diodes, laser diodes and optical detectors and high electron mobility transistors and they have a high melting point, high thermal conductivity, a stronger resistance to oxidation at high temperatures and transparency to a large spectrum. Group III-Nitride alloys can be easily integrated in integrated electronic systems including current amplifiers. In the present work, the binding energy of a hydrogenic donor is studied taking into account the geometrical confinement and the various Boron alloy content. GaN semiconducting material acts as the inner quantum well material whereas BN material acts as the outer barrier material. The energy eigen values and binding energy of a hydrogenic donor are obtained using variational technique within a single band effective mass approximation.

Hyperfine Stark effect of shallow donors in silicon

We present a complete theoretical treatment of Stark effects in doped silicon, whose predictions are supported by experimental measurements. A multi-valley effective mass theory, dealing non-perturbatively with valley-orbit interactions induced by a donor-dependent central cell potential, allows us to obtain a very reliable picture of the donor wave function within a relatively simple framework. Variational optimization of the 1s donor binding energies calculated with a new trial wave function, in a pseudopotential with two fitting parameters, allows an accurate match of the experimentally determined donor energy levels, while the correct limiting behavior for the electronic density, both close to and far from each impurity nucleus, is captured by fitting the measured contact hyperfine coupling between the donor nuclear and electron spin. We go on to include an external uniform electric field in order to model Stark physics: With no extra ad hoc parameters, variational minimization of the complete donor ground energy allows a quantitative description of the field-induced reduction of electronic density at each impurity nucleus. Detailed comparisons with experimental values for the shifts of the contact hyperfine coupling reveal very close agreement for all the donors measured (P, As, Sb and Bi). Finally, we estimate field ionization thresholds for the donor ground states, thus setting upper limits to the gate manipulation times for single qubit operations in Kane-like architectures: the Si:Bi system is shown to allow for A gates as fast as around 10 MHz.

Effects of temperature on binding energy and the nonlinear optical properties of a hydrogenic donor in a GaAs/Ga0.8Al0.2As pillbox immersed in a Ga0.6Al0.4As material

Effect of temperature on the hydrogenic donor binding energy is found in a GaAs/Ga0.8Al0.2As pillbox surrounded by Ga0.6Al0.4As for various lengths of the pillbox and the barrier thicknesses. Photoionization cross section as a function of normalized photon energy, for various values of temperatures, is computed. Some nonlinear optical properties as a function of photon energy are dealt for a fixed length of the pillbox. Optical rectification coefficients, second order and the third order harmonic generation coefficients are found as a function of photon energy for various values of temperatures. It is shown that the optical rectification and second order and the third order harmonic generation coefficients are quite sensitive to the effect of temperature. It is also observed that the electronic properties and the nonlinear optical properties have more influence on the temperature and the geometrical confinement. The results are in good agreement with the recent investigations.

Effect of external fields on the energies of hydrogenic donor with the anharmonic confinement potential

The impurity binding energy in the GaAs−Ga1−xAlxAs system is studied with an anharmonic type confinement potential by taking into account the influence of the external electric and magnetic fields within the framework of the effective mass approximation and asymptotic iteration method (AIM). The influence of the external electromagnetic fields and anharmonicity on a donor binding energy is examined systematically. It is shown that the donor binding energy is highly dependent on the external electric and magnetic fields and the confinement potential shapes. Both the electric and magnetic fields are increased, the binding energies increase for each of them. However, the behaviors of increase in the weak and strong fields’ regimes have different character a bit. Furthermore, when the more anharmonicity is considered, the binding energy of donor slightly increases as well.

Donor states in inverse opals

We calculate the binding energy of an electron bound to a donor in a semiconductor inverse opal. Inverse opals have two kinds of cavities, which we call octahedral and tetrahedral, according to their group symmetry. We put the donor in the center of each of these two cavities and obtain the binding energy. The binding energies become very large when the inverse opal is made from templates with small spheres. For spheres less than 50 nm in diameter, the donor binding can increase to several times its unconfined value. Then electrons become tightly bound to the donor and are unlikely to be thermally activated to the semiconductor conduction band. This conclusion suggests that inverse opals will be poor conductors.

A silicon donor layer in heavily doped GaN

The properties of GaN layers heavily doped with silicon are investigated via photoluminescence spectroscopy. It is shown that excitons cease localizing on donors at a silicon atom concentration of 1.6 × 1019 cm−3; at higher dopant concentrations, the excitons decay. It is established that the donor binding energy falls as the silicon concentration rises.

Carbon related donor bound exciton transitions in ZnO nanowires

Several shallow donor bound exciton photoluminescence (PL) transitions are reported in ZnO nanowires doped with carbon. The emission energies are in the range of 3360.8–3361.9 meV, close to previously reported emission lines due to excitons bound to donor point defects, such as Ga, Al, In, and H. The addition of small amounts of hydrogen during growth results in a strong enhancement of the PL of these carbon related emission lines, yet PL and annealing measurements indicate no appreciable bulk hydrogen. The observation of two electron satellites for these emission lines enables the determination of the donor binding energies. The dependence of exciton localization energy on donor binding energy departs somewhat from the usual linear relationship observed for group III donors, indicating a qualitatively different central cell potential, as one would expect for a complex. Emission lines due to excitons bound to ionized donors associated with these defects are also observed. The dependence of the PL emission intensities on temperature and growth conditions demonstrates that the lines are due to distinct complexes and not merely excited states of each other.

Study of impurity position effect in pyramid and cone like quantum dots

In the present work, we have studied the effect of impurity position on energy levels, the donor binding energy and third harmonic generation of a pyramid and a cone like quantum dot. For this goal, we have calculated the energy levels, wave functions and binding energy using finite element method for various impurity positions. Our results show that the impurity location plays an important and considerable role in the electronic and optical properties in a pyramid and a cone like quantum dot. We found that the third harmonic generation and binding energy of both quantum dots have a maximum value at a special impurity position. The special impurity location is different for the two quantum dots. Also, we have deduced that the third harmonic generation of a cone like quantum dot is larger than a pyramid quantum dot for same volumes and heights.

Vibrational spectroscopy of the water-nitrate complex in the O-H stretching region.

The vibrational spectroscopy of the nitrate-water isotopologues is studied in the O-H and O-D stretching regions using temperature-dependent infrared multiple photon dissociation spectroscopy combined with calculations of the anharmonic spectra. At a temperature of 15 K a series of discrete peaks is observed in the IRMPD spectra of NO3(-)·H2O, NO3(-)·HDO, and NO3(-)·D2O. This structure is considerably more complex than predicted by harmonic calculations. A signal is only observed in the hydrogen-bonded O-H (O-D) stretching region, characteristic of a double hydrogen-bond donor binding motif. With increasing temperature, the peaks broaden, leading to a quasi-continuous absorption from 3150 to 3600 cm(-1) (2300-2700 cm(-1)) for NO3(-)·H2O (NO3(-)·D2O) and, above 100 K, an additional band in the free O-H (O-D) stretching region, suggesting the population of complexes containing only a single hydrogen bond at higher internal energies. Vibrational configuration interaction calculations confirm that much of the structure observed in the IRMPD spectra derives from progressions in the water rocking mode resulting from strong cubic coupling between the O-H (O-D) stretch and water rock degrees of freedom. The spectra of both NO3(-)·H2O and NO3(-)·D2O display a strong peak that does not derive from the water rock progression but results instead from a Fermi resonance between the O-H (O-D) stretch and H-O-H (D-O-D) bend overtone. Additional insight into the nature of the O-H (O-D) stretch and water rocking coupling in these complexes is provided by an effective Hamiltonian that allows for the cubic coupling between the O-H stretch and water rock degrees of freedom.

Resonant Raman scattering and photoluminescent properties of nonpolar a-plane ZnO thin film on LiGaO2 substrate

Nonpolar a-plane ZnO thin films are grown on LiGaO2 substrates and their optical and vibronic characteristics are investigated. Raman spectroscopy under nonresonant and resonant excitations shows the phase purity and resonant multiphonon process. From multiphonon Raman peaks up to the 6th order in the resonant spectra, the LO phonon frequencies for wave vectors away from the Γ point are estimated using the cascade model. Polar LO phonons with E1 symmetry play an important role in the indirect transitions of hot excitons. The optical anisotropy has been studied on the basis of polarization-dependent photoluminescence. The origin of photoluminescence is studied and a donor binding energy of 74.6 meV is found.

Energy Spectra of CdS/Cd1-xZnx S Nano Dot Under The Influence of Magnetic Field

We present a theoretical study on shallow donor binding energies of CdS/ CdZnS nano dot as a strength of applied magnetic field along Z direction for various Zn concentration. Calculations are carried out by using the technique of variational ansatz within the frame work of effective mass approximation. Our results show that the binding energies are drastically affected by the dot radius, the strength of magnetic field and concentrations.

Donor binding energy under magnetic field in cylindrical nanotube with two GaAs/GaAlAs quantum wells

A systematic study of the binding energy of the ground state of a hydrogenic off-axis donor in a cylindrical quantum wire containing two quantum wells in a section of the tube layer is calculated in the presence of a uniform magnetic field applied parallel to the wire axis with different potential shape. We express the wave function as a product of combinations of s and p subband wave functions and an envelope function that depends only on the electron-ion separation . By using the variational principle we derive a differential equation for the envelope function, which we solve numerically. Two peaks in the curves for the dependence of the ground-state binding energies on the donor distance from the axis are presented and it is shown that the increasing the magnetic field increasing the binding energy while the impurity is located in the QW1, whereas the opposite occurs when the impurity is located in the QW2.

Competition Effects of Electric and Magnetic Fields on Impurity Binding Energy in a Disc-Shaped Quantum Dot in the Presence of Pressure and Temperature

We have theoretically studied the combination effects of the electric and magnetic fields on the binding energy of an on-center donor impurity in disc-shaped GaAs/Al0.3Ga0.7As quantum dots (QDs) with emphasis on the competition effects between the two fields under externally applied pressure and temperature. The electric field is applied along the radial direction of the QD, while the magnetic field is applied along the growth direction. The numerical method we employed in the present calculations is the potential morphing method in the framework of the effective mass approximation. Our results show that the two fields exhibit a competition effect on the donor binding energy, leading to an invariant binding energy as in the zero field case at a critical line. This line separates the region corresponding to the “red shift” of the donor binding energy from the one referring to the “blue shift” of the binding energy. Comparing to the magnetic field effect, increasing in the QD sizes or applied pressure is found to favour more its electric counterpart, enlarging the region satisfied by the “red shift” of the donor binding energy. The temperature effect appears to decrease the donor binding energy. However, the competition effects between the two fields retain the same when the temperature varies.

Epitaxial growth of ZnO on (1 1 1) Si free of an amorphous interlayer

Single crystalline (0 0 0 1) ZnO films were grown by pulsed-laser deposition on (1 1 1) Si substrates containing thin Sc2O3 buffer layers (1 and 5 nm) which were prepared by molecular-beam epitaxy at 700 °C. Both x-ray diffraction and transmission electron microscopy reveal that the ZnO films grown at 240 and 400 °C are highly crystalline with good epitaxy. The commonly seen amorphous SiOx layer has been successfully eliminated from the interface between the ZnO/Sc2O3 film and (1 1 1) Si substrate, resulting in improved electrical properties. Rectifying effects are observed in the heterojunction exhibiting a turn-on voltage of 1.08 V and an ideality factor of 17.7. The room temperature mobility is 65 cm2 V−1 s−1. A donor binding energy of 46.7 meV is determined by temperature-dependent photoluminescence measurements.

The combined effect of pressure and temperature on the impurity binding energy in a cubic quantum dot using the FEM simulation

Using the finite element method, the binding energies of a hydrogenic shallow donor impurity are investigated in a cubic GaAs/Ga1−xAlxAs quantum dot structure with realistic potential barrier height under the combined effect of hydrostatic pressure and temperature in the framework of the effective mass approximation. In the calculations, we have taken into account the electronic effective mass, dielectric constant, and conduction band offset between the dot and barriers varying with pressure and temperature. The results we have obtained show that the donor binding energy varies with the dot size, the confining potential, the position of impurity, the hydrostatic pressure and temperature. It is also found that the donor binding energy increases linearly with the pressure in direct gap regime and its variation is larger for narrower dots only and drops slightly with the temperature. A good agreement is obtained with the existing literature values.

Structural analysis of human 2′-O-ribose methyltransferases involved in mRNA cap structure formation

The 5′ cap of human messenger RNA contains 2′-O-methylation of the first and often second transcribed nucleotide that is important for its processing, translation and stability. Human enzymes that methylate these nucleotides, termed CMTr1 and CMTr2, respectively, have recently been identified. However, the structures of these enzymes and their mechanisms of action remain unknown. In the present study, we solve the crystal structures of the active CMTr1 catalytic domain in complex with a methyl group donor and a capped oligoribonucleotide, thereby revealing the mechanism of specific recognition of capped RNA. This mechanism differs significantly from viral enzymes, thus providing a framework for their specific targeting. Based on the crystal structure of CMTr1, a comparative model of the CMTr2 catalytic domain is generated. This model, together with mutational analysis, leads to the identification of residues involved in RNA and methyl group donor binding.

Effects of built-in internal fields and Al alloy content on donor binding energy of a hydrogenic impurity in a wurtzite GaN/AlGaN quantum dot

Built-in internal field induced donor hydrogenic binding energy of an impurity in a wurtzite GaN/AlGaN quantum dot is investigated. The built-in internal electric field has the contribution from spontaneous and piezo-electric polarisation. The computations are carried out with the inclusion of conduction band non-parabolicity through the energy dependent effective mass. It is calculated with a variational approach within the framework of single band effective-mass approximation. A two-parametric trial wave function is employed in order to improve the results. The effects of quantum confinement and the strength of internal electric fields on the donor binding energy are discussed. The results show that the strength of the internal field is of the order MV/cm and it has more influence on the geometrical confinement and the composition of Al alloy content in the GaN/AlxGa1-xN quantum dot. These results are in good agreement with the other investigators.

Spin-orbit interaction in a quantum pseudodot: pressure effect

In the present work, the Schrodinger equation for a pseudo-dot is solved analytically by using the Laplace transformation. Then, by using the variational procedure, the donor binding energy is calculated with a hydrogenic donor impurity at the center. Finally, the hydrostatic pressure effect on the spin-orbit interaction and the binding energy is studied. According to the obtained results, it is found that the binding energy increases with increasing pressure. The level splitting due to the spin-orbit interaction increases by increasing pressure. The splitting decreases with increasing pseudodot size. The change of spin-orbit interaction energy for a GaAs quantum pseudo-dot is very small due to the pressure even at very low temperature.

An effective screened Coulomb interaction in a quasi-one-dimensional system

Abstract In this paper, we derive an analytical expression for the screened Coulomb potential between charge carriers in quasi-one-dimensional (Q1D) semiconductor structures. As an application, this potential has been used to investigate the screening effect on the binding energy of a neutral donor ( D 0 ) in quantum wires (QWRs). It is found that the screening effect decreases the neutral donor binding energy, and the screening effects are more obvious in wide QWRs than that in narrow ones. Dependence of screening length on temperature and carrier concentration has also been discussed.