Effective Mass Calculations Research Articles

First-principle calculations of effective mass of silicon crystal with vacancy defects

AbstractThe energy band structures and electron (hole) effective masses of perfect crystalline silicon and silicon with various vacancy defects are investigated by using the plane-wave pseudopotential method based on density functional theory. Our results show that the effect of monovacancy and divacancy on the energy band structure of crystalline silicon is primarily reflected in producing the gap states and the local states in valence band maximum. It also causes breaking the symmetry of energy bands resulting from the Jahn-Teller effect, while only producing the gap states for the crystalline silicon with hexavacancy ring. However, vacancy point defects could not essentially affect the effective masses that are derived from the native energy bands of crystalline silicon, except for the production of defect states. Simultaneously, the Jahn-Teller distortions only affect the gap states and the local states in valence band maximum, but do not change the symmetry of conduction band minimum and the nonlocal states in valence band maximum, thus the symmetry of the effective masses. In addition, we study the electron (hole) effective masses for the gap states and the local states in valence band maximum.

Effective Mass Calculations for Two-dimensional Gas of Dipolar Fermions

We consider a two-dimensional system of ultracold dipolar fermions with dipole moments aligned in the perpendicular direction. We use the static structure factor information from Fermi-Hypernetted-Chain calculations to obtain the effective many-body dipole–dipole interaction and calculate the many-body effective mass of the system within the $$G_{0}W$$ approximation to the self-energy. A large cancellation between different contributions to the self-energy results in a weak dependence of the effective mass on the interaction strength over a large range of coupling constants.

Donors in Ge as qubits —Establishing physical attributes

Quantum electronic devices at the single-impurity level demand the understanding of the physical attributes of dopants with an unprecedented accuracy. Germanium-based technologies have been developed recently, creating the necessity to adapt the latest theoretical tools to the unique electronic structure of this material. We investigate basic properties of donors in Ge which are not known experimentally, but are indispensable for qubit implementations. Our approach provides a description of the wave function at multiscale, associating microscopic information from density functional theory and envelope functions from state-of-the-art multivalley effective mass calculations, including a central-cell correction designed to reproduce the energetics of all group-V donor species (P, As, Sb and Bi). With this formalism, we predict the binding energies of negatively ionized donors ( state). Furthermore, we investigate the signatures of buried donors to be expected from scanning tunneling microscopy (STM). The naive assumption that attributes of donor electrons in other semiconductors may be extrapolated to Ge is shown to fail, similarly to earlier attempts to recreate in Si qubits designed for GaAs. Our results suggest that the mature techniques available for qubit realizations may be adapted to germanium to some extent, but the peculiarities of the Ge band structure will demand new ideas for fabrication and control.

PbS/CdS Core/Shell Quantum Dots by Additive, Layer-by-Layer Shell Growth

We demonstrate additive shelling of PbS core quantum dots by CdS using room-temperature colloidal atomic layer deposition. Combining in-depth electron microscopy, absorption spectroscopy and elemental analysis, we present a structural model of the PbS/CdS core/shell quantum dots after each of the successive colloidal ALD cycles. In particular, we find that such core/shell quantum dots are cation-rich and we argue that this nonstoichiometry should be attributed to a Cd-excess at the outer surface. Analyzing various PbS/CdS core/shell QDs, we demonstrate that CdS shells have no effect on the energy of the PbS band-edge transition for PbS core QDs larger than ≈4 nm, a finding we support by effective mass calculations. In spite of this, CdS shell growth generally decreases the photoluminescence quantum yield of PbS/CdS core/shell QDs as compared to PbS core QDs. This suggests that photoluminescence quenching is mainly caused by defects at the PbS/CdS interface.

Understanding the photoelectrochemical properties of nanostructured CeO2/Cu2O heterojunction photoanode for efficient photoelectrochemical water splitting

Nanostructured thin films of CeO2 sensitized by overlayering thin layers of Cu2O with varying thickness have been studied for the first time as photoelectrode in photoelectrochemical (PEC) water splitting. Effective mass calculations for electrons and holes in bulk CeO2 and Cu2O using first principles based on Density Functional Theory (DFT) have also been attempted to explain the enhanced charge separation at CeO2/Cu2O heterojunction interface. All samples were characterized by X-ray diffractometer (XRD), Scanning Electron Microscope (SEM), Atomic Force Microscopy (AFM) and UV–Visible spectrophotometer. The photoelectrochemical activity of the samples was investigated in a three electrode quartz cell system and maximum photocurrent density of 2.89 mA cm−2 at 0.7 V/SCE was obtained for the CeO2/Cu2O heterojunction with overall thickness of 397 nm. Improved conductivity and better separation of the photogenerated charge carriers at the CeO2/Cu2O heterojunction as compared to individual components may be responsible for the higher photocurrent density. The possible mechanism for the enhanced photocurrent density has been explained using heterojunction model based on density functional theory calculations.

Statistical nanoscale study of localised radiative transitions in GaN/AlGaN quantum wells and AlGaN epitaxial layers

Compositional disorder has important consequences on the optical properties of III-nitride ternary alloys. In AlGaN epilayers and AlGaN-based quantum heterostructures, the potential fluctuations induced by such disorder lead to the localisation of carriers at low temperature, which affects their transition energies. Using the correlations between micro-photoluminescence, scanning transmission electron microscopy and atom probe tomography we have analysed the optical behaviour of Al0.25Ga0.75N epilayers and that of GaN/AlGaN quantum wells, and reconstructed in three dimensions the distribution of chemical species with sub-nanometre spatial resolution. These composition maps served as the basis for the effective mass calculation of electrons and holes involved in radiative transitions. Good statistical predictions were subsequently obtained for the above-mentioned transition and localisation energies by establishing a link with their microstructural properties.

Inhomogeneities and Effective Mass in Doped Mg2Si

Magnesium silicide (Mg2Si)-based materials are promising candidates as thermoelectric components for mid-temperature range (500–900 K) energy conversion. Many different approaches for determining the parabolicity of the conduction band have been suggested in the literature, while the values of the effective mass m* dL reported, lie between 0.46 and 1.1 m0. The aim of this work is to contribute in elucidating the discrepancy observed in the effective mass values of the lower conduction band of highly doped Mg2Si and examine whether this discrepancy could be attributed to the method of determination or to the sample’s characteristics. We present the results of effective mass calculations at room temperature (RT) by applying different experimental methods and models (parabolic and non-parabolic) in two different groups of samples; one yielding profound inhomogeneities (Sb-doped) and one yielding homogeneous (Bi-doped) samples. Concluding this analysis, it seems that the lower conduction band of Mg2Si is more likely described as non-parabolic. Comparing the two groups of samples, our analysis indicated that the effective mass may be significantly underestimated for samples with dopant and content-modulated composition.

Illustration of high-active Ag2CrO4 photocatalyst from the first-principle calculation of electronic structures and carrier effective mass

Although Ag2CrO4 has been frequently studied as a highly active photocatalytic material under visible light irradiation in recent years, we are still less-known on its detailed mechanism. Also, it is difficult to illustrate this issue just from the experimental discussion. Contrarily, theoretical investigation can deepen our understanding on its photocatalytic mechanism from the electronic level. In this work, the crystal structures, band structures, density of states, and chemical bonding for Ag2CrO4 were studied by the first-principles calculation based on the density functional theory. The calculation results indicate that Ag2CrO4 has an indirect band gap of ca. 1.42eV, a deep position of valence band edge and a strong optical absorption coefficient, implying that Ag2CrO4 has strong oxidation ability and high photocatalytic activity for decomposing organic pollutant under visible light irradiation. Moreover, our calculation also indicates that Ag2CrO4 has small effective mass of electrons and holes, and great effective mass difference between hole and electron, which can respectively facilitate the migration and separation of electrons and holes, and finally improve the photocatalytic performance.

Phonon bottleneck in GaAs/AlxGa1−xAs quantum dots

We report low-temperature photoluminescence measurements on highly-uniform GaAs/AlxGa1−xAs quantum dots grown by droplet epitaxy. Recombination between confined electrons and holes bound to carbon acceptors in the dots allow us to determine the energies of the confined states in the system, as confirmed by effective mass calculations. The presence of acceptor-bound holes in the quantum dots gives rise to a striking observation of the phonon-bottleneck effect.

Diamagnetic and paramagnetic shifts in self-assembled InAs lateral quantum dot molecules

We uncover the underlying physics that explains the energy shifts of discrete states of individual InAs lateral quantum dot molecules (LQDMs) as a function of magnetic fields applied in the Faraday geometry. We observe that ground states of the LQDM exhibit a diamagnetic shift while excited states exhibit a paramagnetic shift. We explain the physical origin of the transition between these two behaviors by analyzing the molecular exciton states with effective mass calculations. We find that charge carriers in delocalized molecular states can become localized in single QDs with increasing magnetic field. We further show that the net effects of broken symmetry of the molecule and Coulomb correlation lead to the paramagnetic response.

Characterization of High Mobility InAs/InGaAs/InAlAs Composite Channels by THz Magneto-Photoresponse Spectroscopy

Inserted narrow InAs quantum wells in InAs/InGaAs/InAlAs heterostructures have been used to achieve higher mobility for high-electron-mobility transistors (HEMTs) with ultra-low-power and low-noise amplification characteristics and for spin-based devices. Due to the large nonparabolicity of the conduction band of InAs and the penetration of the confined electronic envelope function into the adjacent layer(s), accurate calculations of effective mass and g-factor of charge carriers can be problematic. Methods of making precise determinations of the mass and other electronic parameters are thus of interest. We have applied magneto-photoresponse and -transmissions measurements at several THz laser frequencies in concert with dc magnetotransport measurements at low temperature (T = 1.6 K) to determine various electronic parameters (effective mass, carrier density, g-factor, mobility and the quantum scattering time) of the 2DEG in an InAs/In0.75Ga0.25As/In0.75Al0.25As inserted channel structure. This characterization method can also be used to probe the effect of strain, Rashba field, etc on the properties of charge carriers in such structures.

Impact of strain on hole mobility in the inversion layer of PMOS device with SiGe alloy thin film

Advanced MOSFET devices formed from Si-based materials, such as silicon-germanium alloys, are simple and low cost to manufacture. This work focuses on hole mobility in the inversion layer of PMOSFETs using alloy channel materials. The primary topic of this work is the theoretical calculation of effective mass and hole mobility in the silicon-germanium alloy p-type metal-oxide-semiconductor field-effect transistor (PMOSFET) inversion layer. The strain conditions considered in the calculations are intrinsic strain resulting from growing the silicon-germanium alloy thin films on the three orientation Si substrates. The hole mobility of silicon-germanium alloy inversion layer for PMOSFET under substrate strain and various germanium mole fractions are all investigated. The impact of wafer orientation and channel direction on the hole mobility is analyzed using the Kubo–Greenwood formalism.

Two-photon absorption in CdSe colloidal quantum dots compared to organic molecules.

We discuss fundamental differences in electronic structure as reflected in one- and two-photon absorption spectra of semiconductor quantum dots and organic molecules by performing systematic experimental and theoretical studies of the size-dependent spectra of colloidal quantum dots. Quantum-chemical and effective-mass calculations are used to model the one- and two-photon absorption spectra and compare them with the experimental results. Currently, quantum-chemical calculations are limited to only small-sized quantum dots (nanoclusters) but allow one to study various environmental effects on the optical spectra such as solvation and various surface functionalizations. The effective-mass calculations, on the other hand, are applicable to the larger-sized quantum dots and can, in general, explain the observed trends but are insensitive to solvent and ligand effects. Careful comparison of the experimental and theoretical results allows for quantifying the range of applicability of theoretical methods used in this work. Our study shows that the small clusters can be in principle described in a manner similar to that used for organic molecules. In addition, there are several important factors (quality of passivation, nature of the ligands, and intraband/interband transitions) affecting optical properties of the nanoclusters. The larger-size quantum dots, on the other hand, behave similarly to bulk semiconductors, and can be well described in terms of the effective-mass models.

Spin-Pumping Efficiency in Room-Temperature CdSe Nanocrystal Quantum Dots

To understand and optimize optical spin initialization in room-temperature CdSe nanocrystal quantum dots (NCQDs), we studied the dependence of the time-resolved Faraday rotation signal on pump energy (Ep) in different sizes of NCQDs. The resulting spin-pumping efficiency (SPE) spectrum differs significantly from the absorption spectrum. In larger NCQDs, we observe two peaks in the SPE spectrum, whereas in smaller NCQDs, there is only one peak before a low, broad plateau. On the basis of our effective mass calculations, the Ep dependence of SPE arises from the competition between the heavy hole (hh), light hole (lh), and split-off (s.o.) band contributions to the optical transitions. At lower Ep, hh transitions dominate and give rise to the low-energy peaks. Because of the increasing contributions from the lh and s.o. bands which produce spin polarization in the opposite direction, SPE is reduced at higher Ep. The different number of peaks in larger and smaller NCQDs is attributed to size-dependence of the...

Improved Photoelectrochemical Water Splitting Performance of Cu2O/SrTiO3 Heterojunction Photoelectrode

Nanostructured thin films of Cu2O modified by overlayering SrTiO3 with varying thickness have been studied for the first time as photoelectrode in photoelectrochemical (PEC) water splitting. Effective mass calculations for electrons and holes in bulk SrTiO3 and Cu2O using DFT first-principles have also been attempted to explain the enhanced charge separation at Cu2O/SrTiO3 interface. All samples were characterized using XRD, SEM, and UV–vis spectrometry. The influence of surface modification of Cu2O with varying thickness of SrTiO3 on PEC performance has been investigated. Photocurrent density for Cu2O/SrTiO3 heterojunction with overall thickness of 343 nm at 0.8 V/SCE was found to be 2.52 mA cm–2 which is 25 times higher than that of pristine Cu2O (0.10 mA cm–2 at 0.8 V/SCE). Theoretical studies showed that the electrons in SrTiO3 had large effective masses as compared to electrons in Cu2O at conduction band minima indicating weak mobility of photogenerated electrons in SrTiO3 and strong mobility in Cu2O leading to improved separation of charge carriers resulting in the enhancement of photocurrent densities at the Cu2O/SrTiO3 heterojunction.

L-asparagine crystals with wide gap semiconductor features: Optical absorption measurements and density functional theory computations

Results of optical absorption measurements are presented together with calculated structural, electronic, and optical properties for the anhydrous monoclinic L-asparagine crystal. Density functional theory (DFT) within the generalized gradient approximation (GGA) including dispersion effects (TS, Grimme) was employed to perform the calculations. The optical absorption measurements revealed that the anhydrous monoclinic L-asparagine crystal is a wide band gap material with 4.95 eV main gap energy. DFT-GGA+TS simulations, on the other hand, produced structural parameters in very good agreement with X-ray data. The lattice parameter differences Δa, Δb, Δc between theory and experiment were as small as 0.020, 0.051, and 0.022 Å, respectively. The calculated band gap energy is smaller than the experimental data by about 15%, with a 4.23 eV indirect band gap corresponding to Z → Γ and Z → β transitions. Three other indirect band gaps of 4.30 eV, 4.32 eV, and 4.36 eV are assigned to α3 → Γ, α1 → Γ, and α2 → Γ transitions, respectively. Δ-sol computations, on the other hand, predict a main band gap of 5.00 eV, just 50 meV above the experimental value. Electronic wavefunctions mainly originating from O 2p-carboxyl, C 2p-side chain, and C 2p-carboxyl orbitals contribute most significantly to the highest valence and lowest conduction energy bands, respectively. By varying the lattice parameters from their converged equilibrium values, we show that the unit cell is less stiff along the b direction than for the a and c directions. Effective mass calculations suggest that hole transport behavior is more anisotropic than electron transport, but the mass values allow for some charge mobility except along a direction perpendicular to the molecular layers of L-asparagine which form the crystal, so anhydrous monoclinic L-asparagine crystals could behave as wide gap semiconductors. Finally, the calculations point to a high degree of optical anisotropy for the absorption and complex dielectric function, with more structured curves for incident light polarized along the 100 and 101 directions.

Size-Dependent Photoionization in Single CdSe/ZnS Nanocrystals

Fluorescence intermittency in single semiconductor nanocrystals has been shown to follow power law statistics over many decades in time and in probability. Recently, several studies have shown that, while "off" dwell times are insensitive to almost all experimental parameters, "on" dwell times exhibit a pump-power dependent exponential truncation at long times, suggestive of enhanced biexciton photoionization probabilities at high excitation powers. Here we report the dependence of this on-time truncation on nanocrystal radius. We observe a decrease in the per-pulse photoionization probability from 1.8(2) × 10(-4) to 2.0(7) × 10(-6) as the CdSe core radius increases from 1.3 to 3.5 nm, with a radius scaling for the probability for charge ejection arising from biexciton formation P(ionize)(r) is proportional to 1/r(3.5(5)). Effective mass calculations of the exciton wave functions show that the product of fractional electron and hole probabilities in the trap-rich ZnS shell scale similarly with nanocrystal radius. Possible charge ejection mechanisms from such a surface-localized state are discussed.

Simulation of the electronic properties of InxGa1−xAs quantum dots and their wetting layer under the influence of indium segregation

We present anisotropic nonparabolic position-dependent effective-mass calculations of the bound energy levels of electrons confined in lens-shaped InxGa1−xAs quantum dots embedded in a GaAs matrix. The strain and In gradient inside the quantum dots and their wetting layer (due to the strong In segregation effect present in the InxGa1−xAs/GaAs system) were taken into account. The bound eigenstates and eigenenergies of electrons in a finite 3D confinement potential were determined by the full numerical diagonalization of the Hamiltonian. The quantum dots and their wetting layer were sliced into a finite number of monolayers parallel to the substrate surface, each one with a specific In concentration, in order to be able to reproduce any composition profile along the growth direction. A comparison between the eigenenergies of the “pure” InAs quantum dots and the quantum dots with an inhomogeneous In content indicates that In segregation dramatically affects their electronic structure and must be taken into account if one wishes to accurately simulate the real optoelectronic properties of such nanostructures.

On the existence of localized states in quantum pillars

We have investigated the localization of electrons in quantum pillars, i.e. in nanometric structures in which electrons are confined by 5 sides, but are free to communicate with a bulk substrate of the same material through the 6th side and hence can be considered as open from this side. All nanopillars produced by an etching top-down process conform to this physical model. At first sight, electrons in such structures should have an equal probability of being everywhere, so confinement effects and consequently band-gap changes should not be present. However, experimentally, confinement effects are observed. We show, by a proper effective mass equation analysis of the combined pillars and substrate system, that eigenstates with a high degree of localization exist in the pillars – irrespective of the height of the pillar. Moreover, the eigenenergies of these states are extremely close to the energies of the quantum boxes that would be produced if the pillars were closed by all six sides. This analysis provides the justification for the previous application of effective mass calculations on the pillars alone.

Diffusion quantum Monte Carlo calculation of the quasiparticle effective mass of the two-dimensional homogeneous electron gas

The quasiparticle effective mass is a key quantity in the physics of electron gases, describing the renormalization of the electron mass due to electron-electron interactions. Two-dimensional electron gases are of fundamental importance in semiconductor physics, and there have been numerous experimental and theoretical attempts to determine the quasiparticle effective mass in these systems. In this work we report quantum Monte Carlo results for the quasiparticle effective mass of a two-dimensional homogeneous electron gas. Our calculations differ from previous quantum Monte Carlo work in that much smaller statistical error bars have been achieved, allowing for an improved treatment of finite-size effects. In some cases we have also been able to use larger system sizes than previous calculations.