Subband Effective Masses Research Articles

Single electron tunneling in double and triple quantum wells

Electron localization and tunneling in laterally distributed double quantum well (DQW) and triple quantum well (TQW) are studied. Triangular configuration for the TQWs as well as various quantum well (QW) shapes and asymmetry are considered. The effect of adding a third well to a DQW is investigated as a weakly coupled system. InAs/GaAs DQWs and TQWs were modeled using single subband effective mass approach with effective potential simulating the strain effect. Electron localization dynamics in DQW and TQW over the whole spectrum is studied by varying the inter-dot distances. The electron tunneling appeared highly sensitive to small violations of the DQW mirror symmetry. We show that the presence of a third dot increases the tunneling in the DQW. The dependence of the tunneling in quantum dot (QD) arrays on inter-dot distances is also discussed.

Electron transfer between weakly coupled concentric quantum rings

The electronic structure of the semiconductor double concentric quantum nano-ring (DCQR) is studied under the single sub-band effective mass approach. We show that in the weakly coupled DCQR, that has been placed in transverse magnetic field, the electron spatial transition between the rings can occur due to electron level anti-crossing. The anti-crossing of the levels with different radial quantum numbers provides the conditions when the electron tunneling between rings becomes possible. Results of numerical simulation for the electron transition are presented for DCQRs of different geometry. In particular, the system of a QR with a QD located at center of this QR is considered.

Electron position: jumping in double concentric quantum rings

ABSTRACTSemiconductor heterostructures as quantum dots or quantum rings (QR) demonstrate discrete atom-like energy level structure. In an atom the position of an electron can be changed by electromagnetic field influence with accompaniment of quantum number change. In present work we show that in the weak coupled Double Concentric Quantum Ring (DCQR), the electron jumping is possible due to tunneling accompanied with level anti-crossing which has a place in a magnetic field. We study DCQR composed of GaAs in an Al0.70Ga0.30As substrate under influence of a magnetic field. In our model the DCQR is considered within three dimensional single sub-band effective mass approach. When a magnetic field is applied in the z direction, perpendicular to the DCQR plane. The results of the numerical calculations for DCQR are presented for DCQRs of different geometry.

Band Structure Dependence of Electron Mobility in Modulation-Doped Lattice-Matched InAlAs/InGaAs/InAlAs Heterostructures

Variable-magnetic-field Hall measurements were performed on modulation-doped In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As double heterostructures to investigate the influence of channel layer thickness on the two dimensional electron gas mobility. Quantitative mobility spectrum analysis revealed that the first and second subband mobilities increased with the channel thickness. This enhancement is attributed to the decreased subband effective mass, subband carrier density, and alloy disorder scattering potential.

Bilayer oscillation of subband effective masses in Pb/Ge(111) thin-film quantum wells

Subband dispersions of quantum-well states in Pb thin films on Ge(111) have been measured with angle-resolved photoemission spectroscopy. The effective masses at the surface zone center exhibit a bilayer oscillation with thickness, in both magnitude and sign. This behavior is attributed to a strong interaction between Pb quantum-well subbands and the Ge valence maximum near the Fermi level, which occurs about every two monolayers.

Electronic and level statistics properties of Si/SiO2 quantum dots

Spherical shaped Si quantum dots (QDs) embedded into the SiO2 substrate are considered in the single sub-band effective mass approach. Nonparabolicity of the Si conduction band is described by the energy dependence of electron effective mass. Calculations of low-lying single electron and hole energy levels are performed. For small sizes QD (diameter D<6nm) there is a strong confinement regime when the number of energy levels is restricted to several levels. The first order of the perturbation theory is used to calculate neutral exciton recombination energy taking into account the Coulomb force between electron and heavy hole. The PL exciton data are reproduced well by our model calculations. For weak confinement regime (size D>10 nm), when the number of confinement levels is limited by several hundred, we considered the statistical properties of the electron confinement. Distribution function for the electron energy levels is calculated and results are discussed.

Modeling of modern MOSFETs with strain

We review modeling techniques used to compute strain induced performance enhancement of modern MOSFETs. While p-channel MOSFETs were intensively studied, electron transport in strained structures received surprisingly little attention. A rigorous analysis of the subband structure in thin silicon films under stress is performed. Calculated subband effective masses are shown to strongly depend on shear strain and film thickness. A decrease of the transport effective mass under tensile stress in [110] direction and an additional splitting between the unprimed subbands with the same quantum number guarantees a mobility enhancement even in ultra-thin (001) silicon films. This increase of mobility and drive current combined with the improved channel control makes multi-gate MOSFETs based on thin films or silicon fins preeminent candidates for the 22 nm technology node and beyond.

Modeling Techniques for Strained CMOS Technology

We review modeling techniques used to compute strain induced performance enhancement of modern MOSFETs. While p-channel MOSFETs were intensively studied, electron transport in strained structures received surprisingly little attention. A rigorous analysis of the subband structure in thin silicon films under stress is performed. Calculated subband effective masses are shown to strongly depend on shear strain and film thickness. A decrease of the transport effective mass under tensile stress in [110] direction and an additional splitting between the unprimed subbands with the same quantum number guarantees a mobility enhancement even in ultra-thin (001) silicon films. This increase of mobility and drive current combined with the improved channel control makes multi-gate MOSFETs based on thin films or silicon fins preeminent candidates for the 22nm technology node and beyond.

Impact of Confinement of Semiconductor and Band Engineering on Future Device Performance

A rigorous analysis of the subband structure in thin silicon films under stress is performed. Calculated subband effective masses are shown to depend on shear strain and thickness simultaneously. The effective masses and the subband splitting determine transport in silicon films. Decrease of the transport effective mass controlled by the shear strain component guarantees mobility enhancement even in ultra-thin silicon films. This increase of mobility and drive current combined with the improved channel control makes multi-gate MOSFETs based on thin films or silicon fins preeminent candidates for the 22nm technology node and beyond.

Si/SiO2 Quantum Dots: Electronic Properties

AbstractSpherical shaped Si quantum dots (QDs) embedded into the SiO2 substrate are considered in the single sub-band effective mass approach. The electron and heavy hole sub-bands are taken into account. The energy dependence of electron effective mass is applied especially for small size QD. Calculations of low-lying single electron and hole energy levels are performed. For QD of small sizes (diameter D≤6 nm) there is a strong confinement regime when the number of energy levels is restricted to several levels. The first order of the perturbation theory is used to calculate neutral exciton recombination energy taking into account the Coulomb force between electron and heavy hole. The PL exciton data are reproduced well by our model calculations. We also compare the results with those obtained within model [1]. For weak confinement regime (size D≥10 nm), when the number of confinement levels is limited by several hundred, we considered the statistical properties of the electron confinement. Distribution function for the electron levels is calculated and results are discussed.

Enhancement of subband effective mass in Ag/Ge(111) thin film quantum wells

Subband dispersions of quantum-well states in Ag films on Ge(111) have been determined by angle-resolved photoemission. The effective masses of the subbands at the zone center increase substantially for decreasing film thicknesses. This peculiar behavior is attributed to a kinetic constraint for standing-wave formation governed by a momentum-dependent phase-shift function. No evidence is found for in-plane electron localization within the confined geometry.

Effects of collective excitations on the quantum well intersubband absorption

The dependence of the intersubband absorption spectra on the Coulomb interaction and quantum well (QW) width is studied. Rather than following the Fermi–Dirac distribution, we have solved the intersubband equations of motion to determine the subband population self-consistently. We have gone beyond the linear absorption theory to show the effect of various many-body interactions on the absorption spectra. It is found that the redistribution of electrons in excited states reduces the absorption. Our results indicate that the line shape and peak position are determined by the interplay of different collective excitations, such as the Fermi edge singularity and the intersubband plasmon. The dependence of the absorption spectrum on the QW width and the subband effective masses is also discussed.

A microscopic study of Landau level states in quantum wires

The electronic states of GaAs/AlGaAs quantum well wires under an applied magnetic field are studied by using the empirical pseudopotential method. The energy spectra are calculated as a function of the magnetic field strength. The results show a strong non-parabolicity and field-dependent subband effective mass. The formation of bulk-like Landau levels and edge (skipping) states results in a rapid increasing of density of states at the subband bottom reflecting the change of the 1D subband structures to 2D-like Landau levels. The asymptotic behaviours of the Landau levels in the weak and strong magnetic field limits are also obtained by a simple effective mass analysis and used to interpret the results of the microscopic calculation.

Subband effective mass and mobility of two-dimensional electrons in uniformly Si-doped Ga0.47In0.53As/Al0.48In0.52As multiple quantum wells

Subband effective mass and mobility of a two-dimensional electron gas in uniformly Si-doped Ga0.47In0.53As/Al0.48In0.52As multiple quantum wells are estimated. In three samples with well widths of 130, 200, and 300 Å and a fixed barrier width of 68 Å, up to three two-dimensional subbands are found. Maximum enhancement of electron effective mass is 25% over the bulk value at a Fermi energy of 108 meV. These estimated effective masses are slightly smaller than previously reported values, and this is explained by recent theories which assume only nonparabolic correction. When the temperature ratio, (Dingle temperature)/(mobility temperature), is assumed to be identical in different subbands in each sample, the electron mobility of the first excited subband is at most twice that of the ground subband in the sample with the widest well. The effects of intersubband scattering and of screening on the subband mobilities are also discussed.

Resonant 2D magnetopolarons in accumulation layers on n-Hg 0.8Cd 0.2Te

Cyclotron Resonance measurements on accumulation-layer electrons at anodic oxide films on n-(Hg,Cd)Te are reported. Up to six subband cyclotron resonances are observed and discontinuities are seen in the subband effective masses, due to resonant polaron effects. The coupling is strongest for the higher subbands, where it is enhanced over that for the bulk, and occurs at the “HgTe-like” LO phonon frequency. The behaviour of the lowest (i=0,1) subbands is more complex, with the coupling being weaker and the dominant interaction seeming to occur at the TO frequency for low values of N S.