Simple Effective Mass Research Articles

Interplay between structural changes, surface states and quantum confinement effects in semiconducting Mg2Si and Ca2Si thin films.

Ab initio techniques have been used to investigate structural changes in semiconducting Mg2Si and Ca2Si thin films (from 17 nm down to 0.2 nm corresponding to the 2D structure) along with band-gap variations due to quantum confinement. Cubic Mg2Si(111) thin films being dynamically stable at thicknesses (d) larger than 0.3 nm displayed an indirect band gap, the reduction of which with increasing d could be reasonably well described by the simple effective mass approximation. Only 2D Mg2Si has a unique structure because of the orthorhombic distortion and the direct band gap. Since the surface energy of cubic Ca2Si(111) films was lower with respect to any surface of the orthorhombic phase, which is the ground state for the Ca2Si bulk, the metastable in-bulk cubic phase in the form of thin films turned out to be preferable in total energy than any orthorhombic Ca2Si thin film for d < 3 nm. Sizable structural distortion and the appearance of surface states in the gap region of Ca2Si thin films with d < 3 nm could be the reason for an odd dependence of the band-gap variation on d.

Whisper Test – MasS Hearing Screening Programme

Aims and Objectives: To screen the school going mass for hearing loss and study its relationship with their academic performance and aptitude and compare it with pure tone audiometry and validate whisper test. Material and Methods: Procedure- Examiner stands arm distance (60cm) behind the patient. Mask the non-test ear by occluding the external auditory canal by continuously generating rubbing sound. The patient is explained to repeat the words. Test always started with consonants followed by vowels. If more than 80% were correct it was treated as passed. Results: In our study out of 86 students, 8 children failed in whisper test, but all passed in conversation test. In the present study performance was poor in failed students. The incidence of day dream, learning difficulty, inattentiveness was high with hearing impaired children. Conclusion: The whisper test is a simple effective diagnostic and mass hearing screening test which can predict an early hearing loss and if performed by a trained paramedical or social worker can differentiate in conductive, sensorineural, cochlear, and retrocochlear lesion. Since the whisper test does not require a qualified audiologist or otolaryngologist, it should be be a part of annual health check-up and national programme of prevention and control of deafness specifically hearing care in the elderly.

Electrostatic quantum dot confinement in phosphorene

We consider states localized by electrostatic potentials in phosphorene using an atomistic tight binding approach. From the results of the tight-binding calculations of the confined states we extract effective masses for the conduction band electrons in the armchair and zigzag directions. The masses derived in this way are used for a simple single-band effective mass model which, as we find, reproduces very well the tight-binding energy spectrum in external magnetic field, the probability densities and the interaction effects. Both methods produce Wigner crystallization for the ground-state of the electron pair with the single-electron islands separated in the armchair direction already for small quantum dots.

Thickness dependence of infrared lattice absorption and excitonic absorption in ZnO layers on Si and SiO2 grown by atomic layer deposition

Using spectroscopic ellipsometry from the midinfrared (0.03 eV) to the deep ultraviolet (6.5 eV), the authors determined the thickness dependence of the dielectric function for ZnO thin layers (5–50 nm) on Si and quartz in comparison to bulk ZnO. They observed a small blueshift of the band gap (∼80 meV) in thin ZnO layers due to quantum confinement, which is consistent with a simple effective mass theory in an infinite potential well. There is a drastic reduction in the excitonic effects near the bandgap, especially for thin ZnO on Si, which not only affects the excitonic absorption peak but also lowers the high-frequency dielectric constant by up to 40%. No significant change of the phonon parameters (except an increased broadening) in thin ZnO layers was found.

Electronic States in Cylindrical Core-Multi-Shell Nanowire

The recent advances in nanowire (NW) growth technology have made possible the growth of more complex structures such as core-multi-shell (CMS) NWs. We propose the approach for calculation of electron subbands in cylindrical CMS NWs within the simple effective mass approximation. Numerical results are presented for ${\rm GaAs/Al_{0.3}Ga_{0.7}As}$ radial heterostructure with AlGaAs-core and 4 alternate GaAs and AlGaAs shells. The influence of an effective mass difference in heterolayers is discussed.

Predictable spectroscopic properties of type-II ZnTe/CdSe nanocrystals and electron/hole quenching.

The spectroscopic properties of core/shell structured ZnTe/CdSe nanocrystals (NCs) have been systematically studied. By varying the ZnTe core diameter and the CdSe shell thickness, the absorption onset and the photoluminescence peak position of the ZnTe/CdSe NCs can be readily tuned over a wide range. The theoretical model based on an effective mass approximation demonstrates that the ZnTe/CdSe NCs have type II carrier localization in which the photoexcited electrons and holes are spatially separated and confined in the shell and core, respectively. The energetics of the conduction and valence bands and the bandgaps of the ZnTe/CdSe NCs are accurately predicted. The photoluminescent experiments show that electron quenchers having a large energy difference between their reduction potential and the lowest conduction band edge of the ZnTe/CdSe nanocrystals can completely quench the luminescence. Electron acceptors having a reduction potential only slightly below the conduction band edge partially quench the photoluminescence of the nanocrystals. In this case, the extent of quenching depends upon the thickness of the shell and the energy difference. Despite the confinement of photoexcited holes in the core, the photoluminescence could be still quenched by adsorbed hole quenchers. The extent of hole quenching depends upon the core size, the shell thickness and the oxidation potential of the quenchers. Basically, an increase in the core size and the shell thickness may lead to a decrease in the extent of hole quenching. The work presented here is of great interest since it can be extended to understand the spectroscopic properties and photoluminescence quenching behaviors of other core/shell semiconductor NCs.

Atomistic Tight-Binding Study of Contact Resistivity in Si/SiGe PMOS Schottky Contacts

The metal-semiconductor contact resistivity has started to play a critical role for the overall device performance as Si is reaching 10-nm size ranges. The International Technology Roadmap for Semiconductors (ITRS) target predicts a requirement of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> Ω·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> by 2023 which has been a challenging target to achieve. This paper explores the impact of doping concentration, Schottky barrier height, strain, and SiGe mole fraction on the resistivity of Si/SiGe p-type metal-oxide semiconductor (PMOS) contacts with 20-band atomistic tight binding quantum transport simulations. Commonly used simple effective mass approximation models are shown to overestimate the resistivity values. The predicted model results are compared with experimental data and the device parameters needed to achieve 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> Ω·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> are identified.

Tunneling current in HfO2 and Hf0.5Zr0.5O2-based ferroelectric tunnel junction

Ferroelectric tunnel junctions (FTJs) have been intensively explored for future low power data storage and information processing applications. Among various ferroelectric (FE) materials studied, HfO2 and H0.5Zr0.5O2 (HZO) have the advantage of CMOS process compatibility. The validity of the simple effective mass approximation, for describing the tunneling process in these materials, is examined by computing the complex band structure from ab initio simulations. The results show that the simple effective mass approximation is insufficient to describe the tunneling current in HfO2 and HZO materials, and quantitative accurate descriptions of the complex band structures are indispensable for calculation of the tunneling current. A compact k · p Hamiltonian is parameterized to and validated by ab initio complex band structures, which provides a method for efficiently and accurately computing the tunneling current in HfO2 and HZO. The device characteristics of a metal/FE/metal structure and a metal/FE/semiconductor (M-F-S) structure are investigated by using the non-equilibrium Green's function formalism with the parameterized effective Hamiltonian. The result shows that the M-F-S structure offers a larger resistance window due to an extra barrier in the semiconductor region at off-state. A FTJ utilizing M-F-S structure is beneficial for memory design.

Electronic structures of [001]- and [111]-oriented InSb and GaSb free-standing nanowires

We report on a theoretical study of the electronic structures of InSb and GaSb nanowires oriented along the [001] and [111] crystallographic directions. The nanowires are described by atomistic, tight-binding models, including spin-orbit interaction. The band structures and the wave functions of the nanowires are calculated by means of a Lanczos iteration algorithm. For the [001]-oriented InSb and GaSb nanowires, the systems with both square and rectangular cross sections are considered. Here, it is found that all the energy bands are doubly degenerate. Although the lowest conduction bands in these nanowires show good parabolic dispersions, the top valence bands show rich and complex structures. In particular, the topmost valence bands of the nanowires with a square cross section show a double maximum structure. In the nanowires with a rectangular cross section, this double maximum structure is suppressed, and the top valence bands gradually develop into parabolic bands as the aspect ratio of the cross section is increased. For the [111]-oriented InSb and GaSb nanowires, the systems with hexagonal cross sections are considered. It is found that all the bands at the Γ-point are again doubly degenerate. However, some of them will split into non-degenerate bands when the wave vector moves away from the Γ-point. Although the lowest conduction bands again show good parabolic dispersions, the topmost valence bands do not show the double maximum structure. Instead, they show a single maximum structure with its maximum at a wave vector slightly away from the Γ-point. The wave functions of the band states near the band gaps of the [001]- and [111]-oriented InSb and GaSb nanowires are also calculated and are presented in terms of probability distributions in the cross sections. It is found that although the probability distributions of the band states in the [001]-oriented nanowires with a rectangular cross section could be qualitatively described by one-band effective mass theory, the probability distributions of the band states in the [001]-oriented nanowires with a square cross section and the [111]-oriented nanowires with a hexagonal cross section show characteristic patterns with symmetries closely related to the irreducible representations of the relevant double point groups and, in general, go beyond the prediction of a simple one-band effective mass theory. We also investigate the effects of quantum confinement on the band structures of the [001]- and [111]-oriented InSb and GaSb nanowires and present an empirical formula for the description of quantization energies of the band edge states in the nanowires, which could be used to estimate the enhancement of the band gaps of the nanowires as a result of quantum confinement. The size dependencies of the electron and hole effective masses in these nanowires are also investigated and discussed.

Quantum dots with indirect band gap: power-law photoluminescence decay

In this work the experimental effect of a slow decay of the photoluminescence is studied theoretically in the case of quantum dots with an indirect energy band gap. The slow decay of the photoluminescence is considered as decay in time of the luminescence intensity, following the excitation of the quantum dot sample electronic system by a short optical pulse. In the presented theoretical treatment the process is studied as a single dot property. The inter-valley deformation potential interaction of the excited conduction band electrons with lattice vibrations is considered in the self-consistent Born approximation to the electronic self-energy. The theory is built on the non-equilibrium electronic quantum transport theory. The time dependence of the photoluminescence decay is estimated upon using a simple effective mass model. The numerical calculation of the considered model shows the power-law time characteristics of the photoluminescence decay in the long-time limit of the decay. We demonstrate that the nonadiabatic influence of the interaction of the conduction band electrons with the lattice vibrations provides a mechanism giving us the power-law time dependence of the photoluminescence intensity signal. This theoretical result emphasizes the role of the electron-phonon interaction in the nanostructures.

Excitonic wavefunction engineering based on type II quantum dots

We propose a semiconductor heterostructure that allows an effective control of the shape of the carriers wavefunctions by varying just one main structural parameter. The structure is formed by a type II quantum dot and a type I quantum well. We present the results of calculations for a particular system consisting of an InP/GaAs quantum dot and an InGaAs/GaAs quantum well using a simple effective mass model that provides a good insight on our structure. We show that the wavefunction of the carrier that remains outside the dot changes from a spheroidal to a ring‐like shape depending mainly on the separation between the well and the dot layers. This change has a significant impact on relevant excitonic properties such as its lifetime and electrical dipole, and it also determines the possibility of observing the optical Aharonov–Bohm effect.

Quantum field induced strains in nanostructures and prospects for optical actuation

We develop the mechanics theory of a phenomenon in which strain is induced in nanoscale structures in the absence of applied stress, due solely to the presence of quantum mechanical confinement of charge carriers. The direct effect of strain on electronic structure has been widely studied in recent years, but the “reverse coupling” effect that we investigate, which is only appreciable in the smallest structures, has been largely ignored even though its effects are present in first principles atomistic calculations. We develop a simple effective mass approach that can be used to model this universal physical phenomenon allowing a transparent scheme to identify its occurrence. We relate quantum field induced strain to acoustic polarons and identify the presence of this effect in density functional theory calculations of strain and quantum confinement in free-standing Si and GaAs quantum dots. Finally, we discuss the use of this quantum confinement induced strain as a mechanism for universal optical actuation in nanowire structures in the context of recent experimental results on carbon nanotubes.

Theoretical Characterization of Electronic Transitions in Co2+- and Mn2+-Doped ZnO Nanocrystals

Linear response time-dependent hybrid density functional theory has been applied for the first time to describe optical transitions characteristic of Co2+- and Mn2+-doped ZnO quantum dots (QDs) with sizes up to 300 atoms (∼1.8 nm diam) and to investigate QD size effects on the absorption spectra. Particular attention is given to charge-transfer (CT or “photoionization”) excited states. For both dopants, CT transitions are calculated to appear at sub-band-gap energies and extend into the ZnO excitonic region. CT transitions involving excitation of dopant d electrons to the ZnO conduction band occur lowest in energy, and additional CT transitions corresponding to promotion of ZnO valence band electrons to the dopant d orbitals are found at higher energies, consistent with experimental results. The CT energies are found to depend on the QD diameter. Analysis of excited-state electron and hole density distributions shows that, for both CT types, the electron and hole are localized to some extent around the impurity ion, which results in “heavier” photogenerated carriers than predicted from simple effective mass considerations. In addition to CT transitions, the Co2+-doped ZnO QDs also exhibit characteristic d−d excitations whose experimental energies are reproduced well and do not depend on the size of the QD.

Formalism of the transfer matrix and its application to Ⅲ/Ⅴ semicondutor quantum well systems

A formalism of transfer matrix method is presented and used to solve a one-dimensional time-independent Schrdinger equation based on a simple one-band effective mass model and the envelope function approximation. The accuracy of this method is proved by comparing the numerical solution and analytical solution for a GaAs-based type Ⅰ single quantum well system， and its applicability is demonstrated by experimental photoluminescence results of the InAs/GaSb-based type Ⅱ and broken-gap quantum well structures. The formalism is extended to calculating the subband energies and corresponding wavefunctions in the GaAs/GaAlAs-based type Ⅰ coupled multiple quantum well systems, showing that the formalism is universal and practical.

Theory of resonance energy transfer involving nanocrystals: The role of high multipoles

A theory for the fluorescence resonance energy transfer (FRET) between a pair of semiconducting nanocrystal quantum dots is developed. Two types of donor-acceptor couplings for the FRET rate are described: dipole-dipole (d-d) and the dipole-quadrupole (d-q) couplings. The theory builds on a simple effective mass model that is used to relate the FRET rate to measureable quantities such as the nanocrystal size, fundamental gap, effective mass, exciton radius, and optical permittivity. We discuss the relative contribution to the FRET rate of the different multipole terms, the role of strong to weak confinement limits, and the effects of nanocrystal sizes.

Pseudo-potential band structure calculation of InSb ultra-thin films and its application to assess the n-metal-oxide-semiconductor transistor performance

The band structure of InSb thin films with ⟨1 0 0⟩ surface orientation is calculated using the empirical pseudopotential method (EPM) to evaluate the performance of nanoscale devices using a InSb substrate. Contrary to the predictions by simple effective mass approximation methods (EMA), our calculation reveals that the Γ valley is still the lowest lying conduction valley. Based on EPM calculations, we obtained the important electronic structure and transport parameters, such as effective mass and valley energy minimum, of InSb thin film as a function of the film thickness. Our calculations reveal that the ‘effective mass’ of Γ-valley electrons increases with the scaling down of the film thickness. We also provide an assessment of nanoscale InSb thin film devices using a non-equilibrium Green's function under the effective mass framework in the ballistic regime.

Nominally forbidden transitions in the interband optical spectrum of quantum dots

We calculate the excitonic optical absorption spectra of (In,Ga)As/GaAs self-assembled quantum dots by adopting an atomistic pseudopotential approach to the single-particle problem followed by a configuration-interaction approach to the many-body problem. We find three types of allowed transitions that would be naively expected to be forbidden. (i) Transitions that are parity forbidden in simple effective mass models with infinite confining wells (e.g. 1S-2S, 1P-2P) but are possible by finite band-offsets and orbital-mixing effects; (ii) light-hole--to--conduction transitions, enabled by the confinement of light-hole states; and (iii) transitions that show and enhanced intensity due to electron-hole configuration mixing with allowed transitions. We compare these predictions with results of 8-band k.p calculations as well as recent spectroscopic data. Transitions in (i) and (ii) explain recently observed satellites of the allowed P-P transitions.

Band-edge diagrams for strained III–V semiconductor quantum wells, wires, and dots

We have calculated band-edge energies for most combinations of zincblende AlN, GaN, InN, GaP, GaAs, InP, InAs, GaSb and InSb in which one material is strained to the other. Calculations were done for three different geometries, quantum wells, wires, and dots, and mean effective masses were computed in order to estimate confinement energies. For quantum wells, we have also calculated band-edges for ternary alloys. Energy gaps, including confinement, may be easily and accurately estimated using band energies and a simple effective mass approximation, yielding excellent agreement with experimental results. By calculating all material combinations we have identified novel and interesting material combinations, such as artificial donors, that have not been experimentally realized. The calculations were perfomed using strain-dependent k-dot-p theory and provide a comprehensive overview of band structures for strained heterostructures.

Imaging the Photoionization of Individual CdSe/CdS Core−Shell Nanocrystals on n- and p-Type Silicon Substrates with Thin Oxides

The low-intensity photoionization of individual semiconductor nanocrystals, at 23 °C in dry nitrogen, is time-resolved over many hours for both S (532-nm excitation) and P (395-nm excitation) nanocrystal excited states using electrostatic force microscopy. Over 7000 calibrated charge measurements have been made on 14- and 21-Å-thick oxide layers. Photoexcited electrons tunnel across the oxide into the silicon, and multiple charges can build up on individual nanocrystals at intensities of only 0.1−0.01 W/cm2. The silicon dopant type influences the net nanocrystal charging via the interfacial band bending; P-type subtrates show a faster nanocrystal reneutralization rate due to their higher interfacial electron concentration. There is a huge range of photoionzation behavior for individual nanocrystals. This behavior is different for 395- and 532-nm excitation in the same nanocrystal. This individuality seems in part to reflect tunneling through spatially localized defect states in the oxide. The line widths of spatial charge images of individual nanocrystals and the semicontinuous rate of charge re-neutralization after excitation suggest that we observe trapped electron motion in the adjacent oxide and/or on the nanocrystal surface, in addition to the ionized nanocrystal. On average, tunneling of the excited P electron is faster by 1−2 orders of magnitude than that of the S electron; the data show direct photoionization from the excited P state. A kinetic model is developed, including the effect of charging energy on tunneling rate, and applied to ensemble average behavior. There is no quantitative agreement of the tunneling-rate dependence on oxide thickness and excitation energy with the simple 1D effective mass tunneling model. However, overall observed trends are rationalized in light of current thin-oxide tunneling literature.

Emission spectrum of a single quantum dot embedded in ap−njunction

We present theoretical studies of the spontaneous emission spectrum of an isolated self-assembled quantum dot (SAQD) embedded in a $p\ensuremath{-}n$ junction under bias. Keldysh's nonequilibrium Green's function approach is used to derive a set of nonlinear coupled rate equations, which are then solved to obtain the steady-state electron and hole occupation numbers. Combining this approach with a simple but realistic effective mass model, we calculate the emission spectrum of a SAQD and examine the relative peak strengths of exciton, positive and negative trions, and biexciton as functions of carrier tunneling rate. Our theoretical studies provide satisfactory explanation for the behavior of the emission spectrum observed in many recent experiments.