Splitting Of Conduction Band Research Articles

Conduction-band spin splitting and negative magnetoresistance in A3B5 heterostructures.

The quantum interference corrections to the conductivity are calculated for an electron gas in asymmetric quantum wells in a magnetic field. The theory takes into account two different types of the spin splitting of the conduction band: the Dresselhaus terms, both linear and cubic in the wave vector, and the Rashba term, linear in wave vector. It is shown that the contributions of these terms into magnetoconductivity are not additive, as it was traditionally assumed. While the contributions of all terms of the conduction band splitting into the D'yakonov--Perel' spin relaxation rate are additive, in the conductivity the two linear terms cancel each other, and, when they are equal, in the absence of the cubic terms the conduction band spin splitting does not show up in the magnetoconductivity at all. The theory agrees very well with experimental results and enables one to determine experimentally parameters of the spin-orbit splitting of the conduction band.

Spin-superlattice formation in a type-II ZnSe/Zn0.96Fe0.04Se multiple-quantum-well structure.

We have investigated the carrier spin segregation process in a ZnSe/${\mathrm{Zn}}_{0.96}$${\mathrm{Fe}}_{0.04}$Se quantum-well structure by studying the energy and relative intensity of the ground-state exciton components as a function of applied magnetic field. The ${\mathit{e}}_{1}$${\mathit{h}}_{1}$ heavy-hole exciton initially tends towards type-II behavior as the field is applied but the trend reverses itself at higher fields as the conduction-band splitting of the magnetic barriers becomes significant. The interplay between the electron-hole Coulomb attraction and the tunable type-II confining potentials is described in the context of a variational model.

Band-spin-orbit-energy effects in conductivity of two-dimensional weakly disordered semiconductor systems

Effects of conduction-band splitting caused by the spin-orbit interaction on the magnetoresistance in strict two-dimensional disordered semiconductor systems are considered. The Dyakonov-Perel' process (the randomization of spin precession due to elastic scattering) is assumed to be the dominant spin-dephasing mechanism. Unlike results published previously, which turned out to be wrong, new expressions for the weak-localization corrections to the conductivity under an applied magnetic field are derived. The field dependence of the conductivity is proved to be of variable sign like that of metal films. The explicit forms of the correction are, however, markedly different from those known for the impurity spin-orbit scattering. Both cases-the perpendicular and parallel magnetic field, where an account of Zeeman splitting is necessary-are discussed.

Ballistic-electron-emission microscopy of strained Si1-xGex layers.

Ballistic-electron-emission microscopy (BEEM) has been used to investigate the effects of strain on ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ alloys. Lifting of the degeneracy of the conduction-band minimum of ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ due to lattice deformation has been directly measured by application of BEEM spectroscopy to Ag/Si structures. Experimental values for this conduction-band splitting agree well with calculations. In addition, an unexpected heterogeneity in the strain of the ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ layer is introduced by deposition of Au. This effect, not observed with Ag, is attributed to species interdiffusion and has important implications for metal-semiconductor devices based on pseudomorphic ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$/Si material systems.

Raman scattering on modulation-doped quantum wells: Intrinsic spin splitting of the GaAs conduction band

The spin splitting of the conduction band in GaAs is directly observed through the splitting of the intrasubband electronic Raman spectra. Its dependence on the magnitude and orientation of the wavevector is discussed in terms of a simple band structure model.

X1-X3 conduction-band splitting of AlxGa1-xAs observed in far-infrared photoinduced absorption related to the DX defect.

A detailed study of absorption processes related to the DX center in the far and midinfrared spectral ranges is presented. A correlation between the direct DX-center absorption and the photoinduced absorption bands is obtained, i.e,, between the IF1 band at 4500 cm -1 and the band due to photoionization of a shallow, hydrogenic donor. The IF1 band is identified as a transition from a shallow X-like donor to the higher minimum of the conduction band of X 3 symmetry. The X 1 -X 3 band splitting was estimated and showed good agreement with theory

Spin-filter effect of ferromagnetic europium sulfide tunnel barriers.

We have observed electron-spin polarization in the tunnel current in metal-EuS-metal junctions. The value of polarization can be as high as 85%. This polarization is attributed to the exchange splitting of the EuS conduction band as a result of the ferromagnetic transition occurring in EuS at about 15 K. This conduction-band splitting lowers the tunnel barrier height for spin-up electrons and raises it for spin-down electrons, giving rise to spin polarization in the tunnel current. Tunnel barrier lowering is also manifested in the observed decrease of junction resistance with temperature below 15 K. The exchange interaction between quasiparticles in Al and the magnetic ${\mathrm{Eu}}^{2+}$ ions in EuS greatly enhances the Zeeman splitting of the superconducting quasiparticle density of states; in fact, some junctions show Zeeman splitting corresponding to an internal field of ${\mathit{B}}_{\mathit{i}}^{\mathrm{*}}$=3 T before any external magnetic field is applied.

Electron-Spin Polarization in Tunnel Junctions with Ferromagnetic EuS Barriers

ABSTRACTSpin-resolved tunneling in the structure metal-EuS-metal has been obtained. The metal electrodes used in our experiments being non-magnetic, polarization of the tunnel current is attributed to the conduction band splitting in EuS below its ferromagnetic Curie temperature. Enhanced Zeeman splitting in the superconducting aluminum film is also observed.

Electric subbands in Si/SiGe strained layer superlattices

The subband structure for electrons in Si/SiGe strained layer superlattices along the (100) direction is calculated self-consistently including many-body effects in the local density approximation. The strain induced splitting of the six-fold degenerate conduction band results in different potential wells for different valleys. The charge distribution between the layers and valleys is calculated versus carrier concentration and conduction band offset for different strains. An estimate for the band offsets is obtained by comparison of experimental results with the calculations.

Theory of resonant tunneling through the insulator conduction band of a thin film metal-insulator-metal (MIM) heterojunction

An “atomic” model of an insulating barrier between two free-electron model metals is used to investigate resonant tunneling across the insulator in the presence of a medium to large, externally applied electric field (bias). The exact numerically calculated tunneling current exhibits a pronounced oscillatory bias dependence superposed on the dominant roughly exponential tunneling characteristic. The interpretation of these results in terms of an internal field emission or Fowler-Nordheim type tunneling subject to “periodic deviations” (or interferences) seems plausible and was suggested by Maserjian. To test this conjecture, a trapezoidal barrier model of our “atomic” model analyzed numerically. As expected, the trapezoidal barrier model could only qualitatively reproduce the oscillatory bias dependence of the barrier transmissivity and of the current. Furthermore this limited agreement depends on allowing the effective mass in the barrier to become a strictly adjustable parameter. This failure of the conventional model of the junction can be interpreted as follows: (i) For moderate external (bias) fields the trapezoidal barrier fails to account for the correct position dependence of the Blochwave vector in the insulator's conduction band, hence the correct interference conditions cannot be reproduced. (ii) For large external fields the band model itself begins to fail. An explanation of oscillatory bias dependence at the tunneling current in terms of splitting of the insulator's conduction band into a set of discrete Stark levels is suggested. It is demonstrated that a fit of the oscillatory tunneling characteristics in the “Fowler-Nordheim regime” is not a reliable technique to determine the effective mass in the thin insulating film of tunneling junctions over the energy interval containing the forbidden gap and the adjoining conduction-band.

Impurity scattering in magnetic semiconductors

The magnetoresistance of a degenarate magnetic semiconductor is calculated, using a theory in which charged impurities are screened by a spin dependent dielectric function. The screening function explicitly includes the coupling of the conduction electrons to the lattice of localized spins and thereby improves upon the work of Shapira and Kautz who consider only the effects of conduction-band splitting on the Thomas—Fermi screening length. Magnetic inhomogeneties induced by charged impurities in a magnetic semiconductor are also capable of scattering electrons. Resistivity resulting from this mechanism is also considered.

Positive magnetoresistance due to conduction-band splitting in EuSe and EuTe

A positive magnetoresistance observed in n-type EuSe and EuTe samples is explained by the effect of spin splitting of the conduction band on nonmagnetic scattering.

Recombination luminescence in irradiated silicon-effects of uniaxial stress and temperature variations†

Demonstration that luminescence in irradiated silicon consists of a spectral group between 0.80 and 1.0 eV which seems to be independent of impurities, while a lower energy group between 0.60 and 0.80 eV is seen only in pulled crystals. The small halfwidth and temperature dependence of the sharp zero-phonon lines observed in these spectra indicate that the luminescence arises from a bound-to-bound transition. A model is proposed for the transition mechanism. Stress data taken on the 0.79-eV zero-phonon line in pulled crystals can be fit by either a tetragonal 100 (in brackets) defect symmetry or by conduction-band splitting effects. It is suggested that the 0.79-eV zero-phonon line and the 0.60- to 0.80-eV spectral group arise from the EPR G-15 center. Stress data on a zero-phonon line at 0.97 eV associated with the 0.80- to 1.0-eV spectral group can be explained by a trigonal 111 (in brackets) defect. The divacancy is tentatively suggested as responsible for this luminescence spectra.