Subband Energy Levels Research Articles

Physics-Based Compact Model for III–V Digital Logic FETs Including Gate Tunneling Leakage and Parasitic Capacitance

A physics-based compact model is developed for III-V field-effect transistors for digital logic applications. Quasi-ballistic ratios, trapezoidal quantum-well subband energy levels, and 2-D source/drain influence on both electrostatics and capacitance are considered. Furthermore, gate tunneling leakage current and parasitic capacitance models are included. These latter effects are important in future technology logic applications, particularly in circuits such as high-density cache arrays. In this paper, we describe the III-V compact model including the gate leakage current and parasitic capacitance analytical models. The efficacy of the compact model in a practical circuit environment is demonstrated using transient simulations of a 6T-static random access memory cell. In addition, we provide design guidelines for optimization of the intrinsic and the extrinsic structure with regard to the parasitic effects.

Hall Factor in Ultrathin-Body Silicon-on-Insulator n-Type Metal–Oxide–Semiconductor Field-Effect Transistors

Hall factor (γH) is investigated experimentally in the ultrathin-body (UTB) silicon-on-insulator (SOI) n-type metal–oxide–semiconductor field-effect transistors (MOSFETs) with the SOI thicknesses (TSOI) of less than 10 nm. It is demonstrated experimentally for the first time that when TSOI is thicker than 4.2 nm, γH decreases slightly with a reduction of TSOI, whereas when TSOI is thinner than 4.2 nm, γH decreases drastically and becomes almost unity in TSOI of 2.5 nm. The γH is calculated considering only phonon scattering. Then, calculated γH is compared with γH obtained experimentally. In addition, it is found that in phonon scattering γH is determined dominantly by the intravalley scatterings of the twofold valleys. The dominance of the intravalley scatterings of the twofold valleys in the γH determination is attributed to the energy levels of subbands whereby most of the scatterings are the intravalley scatterings of the twofold valleys.

Magnetic control of Rashba splittings in symmetric InAs quantum wells

We propose a mechanism to control the Rashba-induced subband splitting by a magnetic field using a symmetric double quantum well (QW) system, where the lowest two subbands are coupled by a position-dependent Rashba parameter α ( z ) . In such a system, all subbands are spin degenerate due to the time reversal symmetry and the spatial inversion symmetry at zero magnetic field, despite the presence of the Rashba spin–orbit interaction. Applying an external magnetic field parallel to the QW plane ( B ∥ y ^ ) lifts this spin degeneracy breaking the time reversal symmetry, where the spin splitting energies are controllable in the range between zero and 2.9 meV, the latter being on the same order of magnitude as a typical Rashba splitting in a narrow asymmetric QW. We find that the first and second subband energy levels for a selected spin state with k ∥ = ( k F , 0 , 0 ) anticross each other, and that the energy of the subband splitting Δ 0 , equivalent to the Rashba splitting for the case of single QWs, can be determined from the value of the anticrossing magnetic field B ac. These results suggest that the investigation in the symmetric double QWs would provide useful approaches for quantitative understanding of the Rashba spin–orbit interaction.

Intense THz laser effects on off-axis donor impurities in GaAs–AlGaAs coaxial quantum well wires

The binding energy of an off-axis shallow-donor impurity in a GaAs coaxial cylindrical quantum well-wire under the action of an intense, high-frequency laser field is calculated using a variational procedure within the effective-mass approximation. We take into account the laser dressing effects on both the impurity Coulomb potential and the confinement potential. Numerical calculations of the ground state subband energy levels based on a finite element method are performed for different barrier thicknesses and laser field parameters. Our model indicates a possible tuning of the well potential shape along the laser polarization direction. Strong distortions of the electron probability density under intense laser field conditions are also predicted. The study proves that the presence of the laser field partially breaks down the degeneracy of the states for donors symmetrically positioned within the structure. The results obtained show that the impurity energy levels in coaxial quantum wires can be significantly modified and controlled by intense laser fields.

Simultaneous effects of laser field and hydrostatic pressure on the intersubband transitions in square and parabolic quantum wells

The intersubband transitions in square and parabolic quantum wells under simultaneous action of the hydrostatic pressure and high-frequency laser field have been investigated. We found that the laser-induced blueshift effect on the subband energy levels may be tuned by the pressure action. Our calculations revealed that the oscillator strength of the transition between the ground and the first excited levels depends on the quantum well width and shape, laser field intensity and hydrostatic pressure. This combined effect of pressure and laser field offers a new degree of freedom in the optoelectronic devices applications.

Softening of the tunneling gap in modulation-doped GaAs/AlGaAs asymmetric coupled double quantum wells in magnetic fields

Magnetophotoluminescence emissions were measured from modulation doped GaAs/AlGaAs asymmetric double quantum wells, wherein a thin barrier (25 Å) was sandwiched between a single quantum well (SQW) and a single heterojuction (SHJ). In the SQW, Landau level mixing is observed at the quantum Hall states. At ν<2, the lowest Landau level transition undergoes an exciton transition. For the SHJ region, the free carrier transitions become excitonic at the crossing point of the GaAs free exciton and the tunneling band gap shows a marked softening. An exciton-exciton interaction is shown to be responsible for the behavior of the subband energy levels in magnetic fields.

Analytical modeling of tunneling current through SiO2–HfO2 stacks in metal oxide semiconductor structures

This work presents an original approach to model direct tunneling current through high-κ dielectrics including SiO2 interfacial oxide from electron inversion layers. Quantum confinement is taken into account by means of an improved triangular well approximation including physically-based analytical corrections of subband energy levels. An efficient way to compute tunnel transmission probability is also proposed, taking into account the reflections on discontinuous dielectrics interfaces. Finally, this model has been successfully validated by comparison to both numerical simulations and experimental results.

Theoretical Investigation of Excited States of Oligothiophene Anions

Electron-hole symmetry upon p- and n-doping of conducting organic polymers is rationalized with Hückel theory by the presence of symmetrically located intragap states. Since density functional theory (DFT) predicts very different geometries and energy level diagrams for conjugated pi-systems than semiempirical methods, it is an interesting question whether DFT confirms the existence of electron-hole symmetry predicted at the Hückel level. To answer this question, geometries of oligothiophene anions with 5-19 rings were optimized and their UV/vis spectra were calculated with time-dependent DFT. Although DFT does not produce symmetrically placed sub-band energy levels, spectra of cations and anions are almost identical. The similarity in transition energies and oscillator strengths of anions and cations can be explained by the fact that the single sub-band energy level of cations lies above the valence band by the same amount of energy as the single sub-band level of anions lies below the conduction band. This and the resemblance of the energy level spacings in valence bands of cations to those in conduction bands of anions give rise to peaks with equal energies and oscillator strengths.

Plasma dispersion effect in heavily doped antimony-based passive optical waveguides

Theoretical analysis showed that the refractive-index change induced by plasma dispersion effect in heavily doped antimony-based optical waveguides can be pushed beyond the saturated value attainable with high absorption by shifting the absorption peak to the lower wavelength side of the pump wavelength. The efficiency of the resulting cross-phase modulation is dependent on the offset of the subband energy levels from the conduction band edge. For a given subband structure, the cross-phase modulation efficiency increases as the subband structure is placed closer to the conduction band edge.

ARPES measurements on Si(1 1 1) hole subband induced by Pb and Ga adsorption

The subband dispersions in the Si(1 1 1) p-type inversion layers induced by Pb and Ga adsorbed surface structures were measured by angle-resolved photoemission spectroscopy (ARPES). The surface structures used here were Si ( 1 1 1 ) 3 3 × 3 3 − PbGa and Si(1 1 1)6.3 × 6.3–Ga. Si ( 1 1 1 ) 3 3 × 3 3 − PbGa is a new surface phase found in this study. Because it is significant in our study to investigate potential effects of surface superstructures on the hole subband dispersion, we investigated the subband energy levels quantitatively comparing them with those calculated using the triangular approximation. It was found that the energy separation of the adjacent subband quantum levels in the inversion layers induced by gallium adsorption does not follow the triangular approximation. The possible band bending shape was proposed to explain the quantum level spacing of the subbands in Ga-induced inversion layers.

Design of a novel periodic asymmetric intra-step-barrier coupled double strained quantum well electroabsorption modulator at 1.55 μm

A novel structure for electroabsorption modulator at 1.55μm with In(1−x−y)Ga(x)Al(y)As periodic asymmetric intra-step-barrier coupled double strained quantum well (AICD-SQW) is proposed. The intra-step-barrier in the structure provides a high optical saturation power due to delayed red shift. The asymmetric strained well layers reduce the oscillator strength at zero field, and as a result a lower insertion loss is obtained. This is due to the deep separation of electron and heavy hole wave functions. This structure shows that electroabsorption modulator (EAM) properties such as large change in absorption, high extinction ratio, large Stark shift, very low insertion loss, zero chirp, and higher figures of merit are possible to be achieved simultaneously as compared with intra-step quantum well (IQW). In numerical analysis, the exciton equation in momentum space is solved numerically, using Gaussian quadrature method (GQM), to obtain the exciton binding energy and oscillator strength. The asymmetric quantum well structure Hamiltonian is numerically solved by transfer matrix method (TMM), to obtain the electron and hole subband energy levels, taking the strain into account. The electroabsorption coefficient is calculated for different applied electric field for TE input light polarization. The EAM parameters such as extinction ratio, insertion loss, chirp and the figures of merit are calculated and the performances of AICD-SQW are compared with IQW.

Effect of In-segregation on subbands in GaInNAs/GaAs quantum wells emission around 1.3 and 1.55 micron

The effect of In-segregation on optical properties in 7.5-nm GaInNAs/GaAs single quantum well (QW) is studied theoretically. The nominal (In, N) contents in the QW are chosen to be (0.35, 0.015) and (0.39, 0.03) for the emission wavelengths around 1.3 and 1.55 μm, respectively. Muraki’s model is used to model the composition profiles in the QWs. In-plane strain, confinement potential, and subband energy levels of the QW are calculated using multi-band effective mass theory. We show a space-indirect transition between light holes localized in indium deficient region and electrons localized in indium rich region of the quantum well. Our results show that the optical transition energies are approximately constant for the segregation efficiencies smaller than 0.7 in both QWs.

비 중심 Si δ-doping 층을 갖는 GaAs-AlxGa1-x양자우물에서 전계에 따른 전자 분포

The electric property in the <TEX>$GaAs-Al_{x}Ga_{1-x}$</TEX> quantum well with the Si <TEX>${\delta}-doping$</TEX> layer in a non-central position is studied through the effect of the electric field intensity on the electron distribution. The finite difference method is used for the calculation of the subband energy level and its wavefunction. In order to account for the change of the potential energy due to the charged particles, the self consistent method is employed. As the Si <TEX>${\delta}-doping$</TEX> layer becomes closer to the heterojunction interface, the electrons less affected by Coulomb scattering are greatly increased under the external electric field. Therefore, the high speed device is suggested due to the fact that the high mobility electrons can be increased by positioning the <TEX>${\delta}-doping$</TEX> layer in the quantum well and by applying the electric field intensity.

An Analytic Model to Account for Quantum&amp;#8211;Mechanical Effects of MOSFETs Using a Parabolic Potential Well Approximation

An analytic model to account for the quantum-mechanical effects (QMEs) of the MOSFETs using a parabolic potential well approximation is presented in this paper. Based on the solution of the coupled Schroumldinger and Poisson equations following the Wentzel-Kramer-Brillouin method, a transcendental equation of the subband energy level has been rigorously derived to obtain an approximate analytic solution for the subband energy levels and the inversion charge centroid. Calculated results from the obtained analytical solution are compared with the previous approximate solutions reported in the literature and the numerically simulated data. A good agreement between the analytical and numerical is obtained, proving the validity of the analytic modeling of QMEs

Monte Carlo study of surface roughness scattering in Si inversion layer with improved matrix element

The electron mobility in the inversion layer of metal oxide semiconductor field-effect transistors was calculated by using a Monte Carlo method. We utilized an improved matrix element for accurately calculating the surface roughness scattering among the many factors determining the electron mobility. The improved matrix element employed different effective electric fields (Eeff) for each subband energy level. From the simulation results, we demonstrated that the relative proportion of surface roughness scattering was about three times greater than that of acoustic phonon scattering at an electric field of 1MV∕cm. In particular, the electron mobility curve calculated with the improved matrix element showed better consistency with universal experimental data as compared to a curve calculated with a conventional matrix element.

Control of subband energy levels of quantum dots using InGaAs gradient composition strain-reducing layer

We propose a method to control the subband energy levels of quantum dots (QDs) using an InGaAs gradient composition strain-reducing layer (GC-SRL). A large band shift of 70meV was realized using a GC-SRL at the fourth-order energy level in both the samples. In addition, the QDs with and without a GC-SRL exhibited an exponential and constant increase in the subband space, respectively. These results indicate square-well-shaped and crucible-shaped potential band structures. The GC-SRL enabled the control of not only the subband energy but also the confinement energy of these potential structures.

A compact model to predict quantized sub-band energy levels and inversion layer centroids of MOSFETs with a parabolic potential well approximation

A compact model to predict sub-band energy levels and inversion charge centroids in the MOSFET surface inversion layer has been presented in this paper for parabolic potential well approximation. Based on a coupled solution of the Schrödinger equation and the Poisson equation following the WKB method, one transcendental equation of the sub-band energy level has been rigorously derived and then the approximate analytical solutions for the sub-band energy levels and the inversion charge centroids have been obtained. The analytical results are compared with the numerical data and a good agreement between the analytical and numerical is found.

Al2Gax-1A3-GaAs양자우물에서 시도함수에 따른 결합에너지

The binding energy in the n-type <TEX>$GaAs/Al_xGa_{1-x}As$</TEX> quantum well is calculated. The shooting method, modified from the finite difference method, is used for the calculation of the subband energy level and its wave function. In order to account tot the change of the potential energy due to the charged particles, impurities and electrons, the self consistent method is employed. The wave function used for the calculation of the binding energy is assumed to be composed of the envelope function and hydrogenic 1s function. Then, the binding energies calculated by taking into account lot two different types of the hydrogenic 1s function are compared.

Theoretical investigation of intersubband transition in Al xGa 1−xN/GaN/Al yGa 1−yN step quantum well

A modified self-consistent method is introduced for the design of Al x Ga 1− x N/GaN step quantum well (SQW) with the position and energy-dependent effective mass. The effects of nonparabolicity are included. It is shown that the nonparabolicity effect is minute for the lowest subband energy level and grows in size for the higher subband states. The effects of nonparabolicicty have significant influence on the transition energies and the oscillator strengths and should be taken into account in the investigation of the optical transitions. The strong asymmetric property introduced by the step quantum well magnifies the weak intersubband transition from the ground state to the third state (1→3). It is shown that in an appropriate scope, the intersubband transition (1→3) has the comparable oscillator strength with transition from the ground state to the second one (1→2), which suggests the possible application of the two-color photodetectors. The results of this work should provide useful guidance for the design of optically pumped asymmetric quantum well lasers and quantum well infrared photodetectors (QWIPs).

The self-consistent calculation of a spherical quantum dot: A quantum genetic algorithm study

In this study, we have calculated the subband energy level, potential profile, and the corresponding wavefunction and chemical potential for different temperatures and donor concentrations in a spherical quantum dot self-consistently. We have also investigated the effect of exchange-correlation potential on the energy levels. In addition, we have checked the applicability of quantum genetic algorithm to a realistic self-consistent quantum dot problem. In all computations, the penetration of wavefunction to the barrier region is taken into account.