Impurity band conduction in CdHgTe crystals

Electrical conduction at 77 K in Cd_xHg_1−xTe, with the composition x ⩽ 0.2, is by electrons in the conduction band, by holes in the valence band and by holes in the impurity band. In samples with zero energy gap, x < 0.14, electrical conduction by holes in the valence band is comparable to electrical conduction by holes in the impurity band. In the open energy gap CdHgTe, electrical conduction by holes in the valence band is negligible in comparison to electrical conduction by holes in the impurity band. In CdHgTe samples, electrical conduction in the impurity band is described by the “Fermi Glass” model.     

Electron and hole spectra of silicon quantum dots

In the framework of perturbation theory, the first several one-particle energies and wave functions for electrons and holes (six for each) in spherical silicon quantum dots are obtained in the envelope function approximation (kp method). It is shown that the model of an isotropic dispersion relation with the mean reciprocal effective mass is applicable for the ground state of holes in the valence band. Anisotropy of the dispersion relation, which takes place for bulk semiconductors, becomes significant for the electron ground state in the conduction band as well as for all excited (both electron and hole) states.     

Si–Si bond as a deep trap for electrons and holes in silicon nitride

A two-stage model of the capture of electrons and holes in traps in amorphous silicon nitride Si₃N₄ has been proposed. The electronic structure of a “Si–Si bond” intrinsic defect in Si₃N₄ has been calculated in the tight-binding approximation without fitting parameters. The properties of the Si–Si bond such as a giant cross section for capture of electrons and holes and a giant lifetime of trapped carriers have been explained. It has been shown that the Si–Si bond in the neutral state gives shallow levels near the bottom of the conduction band and the top of the valence band, which have a large cross section for capture. The capture of an electron or a hole on this bond is accompanied by the shift of shallow levels by 1.4–1.5 eV to the band gap owing to the polaron effect and a change in the localization region of valence electrons of atoms of the Si–Si bond. The calculations have been proposed with a new method for parameterizing the matrix elements of the tightbinding Hamiltonian taking into account a change in the localization region of valence electrons of an isolated atom incorporated into a solid.     

Relaxation of femtosecond photoexcited electrons in a polar indirect band-gap semiconductor nanoparticle

A model calculation is given for the energy relaxation of a non-equilibrium distribution of hot electrons (holes) prepared in the conduction (valence) band of a polar indirect band-gap semiconductor, which has been subjected to homogeneous photoexcitation by a femtosecond laser pulse. The model assumes that the pulsed photoexcitation creates two distinct but spatially interpenetrating electron and hole non-equilibrium subsystems that initially relax non-radiatively through the electron (hole)-phonon processes towards the conduction (valence) band minimum (maximum), and finally radiatively through the phonon-assisted electron-hole recombination across the band-gap, which is a relatively slow process. This leads to an accumulation of electrons (holes) at the conduction (valence) band minimum (maximum). The resulting peaking of the carrier density and the entire evolution of the hot electron (hole) distribution has been calculated. The latter may be time resolved by a pump-probe study. The model is particularly applicable to a divided (nanometric) polar indirect band-gap semiconductor with a low carrier concentration and strong electron-phonon coupling, where the usual two-temperature model [1-4] may not be appropriate.     

Radiation-stimulated pulse conductivity of CsBr crystals

The radiation-stimulated pulse conductivity of CsBr crystals is investigated upon picosecond excitation with electron beams (0.2 MeV, 50 ps, 0.1–10 kA/cm²). The time resolution of the measuring technique is ～150 ps. It is shown that the lifetime of conduction band electrons is limited by their bimolecular recombination with autolocalized holes (V _k centers). A delay in the conduction current pulse build-up is revealed. This effect is explained within the proposed model, according to which the Auger recombination of valence band electrons and holes of the upper core band substantially contributes to the generation of conduction band electrons.     

Superfluid phase transition in two-dimensional excitonic systems

We study the superfluid phase transition in the two-dimensional (2D) excitonic system. Employing the extended Falicov–Kimball model (EFKM) and considering the local quantum correlations in the system composed of conduction band electrons and valence band holes we demonstrate the existence of the excitonic insulator (EI) state in the system. We show that at very low temperatures, the particle phase stiffness in the pure-2D excitonic system, governed by the non-local cross correlations, is responsible for the vortex–antivortex binding phase-field state, known as the Berezinskii–Kosterlitz–Thouless (BKT) superfluid state. We demonstrate that the existence of excitonic insulator phase is a necessary prerequisite, leading to quasi-long-range order in the 2D excitonic system.     

Prediction of an excitonic ground state in InAs/InSb quantum dots

Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict an excitonic ground state in the InAs/InSb quantum-dot system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the gap E(g) between InAs conduction states and InSb valence states vanishes. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wave functions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of discrete 0D confined energy levels. These correlation effects overcome E(g), leading to the formation of a biexcitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by approximately 15 meV) than the dot without excitons.     

Evidence of collective charge transport in the ohmic regime of o-TaS(3) in the charge-density-wave state by a photoconduction study

Using photoconduction techniques, we demonstrate that the low-temperature Ohmic conduction of o-TaS3 is not provided by band motion or hopping of single-particle excitations-electrons and holes excited over the Peierls gap. Instead, the low-temperature Ohmic conduction is mostly provided by collective excitations having an activation energy much less than the Peierls gap value and shunting the contribution of electrons and holes responsible for photoconduction.     

GaAs/AlGaAs超晶格的光致发光

在室温下测量了GaAs/A l0.3Ga0.7As超晶格的光致发光,发现在波长λ=761 nm处存在一较强的发光光峰,此发光峰目前尚未见报道。经理论分析表明,此峰是量子阱中的第一激发态电子与受主空穴复合发光。实验还观测到在λ=786 nm处,λ=798 nm处和λ=824 nm处分别存在一发光峰,分析表明λ=786 nm处的发光峰为量子阱阱中费米能级附近的电子与轻空穴复合发光;λ=798 nm处的发光峰为量子阱内的基态电子到轻空穴的复合发光;λ=824 nm处的发光峰为阱中激子复合复合发光。理论计算与实验结果符合的很好。     

A chemical-bond approach to doping,compensation and photo-induced degradation in amorphous silicon

We propose that during deposition of a-Si:H films a chemical equilibrium is established that relates the density of dangling-bond defects near mid-gap to the densities of electrons and holes in the conduction and valence band states. We develop the appropriate chemical reaction formalism and show that our model allows doping, compensation and photo-induced degradation to be treated within a single and unifying approach.     

Impurity-stabilized zinc-blende phase of wurtzite compounds

We propose here a new approach to stabilizing the cubic zinc blende phase of semiconductors that are usually more stable in the hexagonal wurtzite phase. We show that this can be done by taking advantage of the valence and conduction band offsets between the cubic and the hexagonal phases. Due to this band offset, it will cost less energy to insert electrons by shallow donors, or insert holes by 3d acceptors in the zinc blende structure, thus stabilizing the cubic phase.     

Conduction electron-exciton complexes in semiconductor single crystals

We consider the possibility of a bound state being formed from the pairing of an excited electron in the conduction band with an exciton in a semiconductor at low temperatures. The model consists of two levels (the valence and conduction bands) for a simple cubic lattice with periodic boundary conditions and the exciton is intermediate between the Wannier and Frenkel type excitons. The exciton which is discussed consistst of a tightly bound electron from the conduction band and a hole from the valence band on the same lattice site. Electrons and holes are, however, allowed to hop independently between nearest-neighbour lattice sites. The dispersion relations which determine the exciton and the electron-exciton modes are solved numerically. It is found that there are two branches for the coupled mode frequencies. This physical picture is analogous to that for polaritons and magnon-phonon modes in crystals.     

Optical spin injection and spin lifetime in Ge heterostructures

We demonstrate optical orientation in Ge/SiGe quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin polarizations and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct gap photoluminescence exceeds the theoretical bulk limit, yielding ～37% and ～85% for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is estimated to be ～0.5 ps and it is governed by transitions between light and heavy hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5?ns below 150?K. Theoretical analysis of the spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime of electrons.     

Spin Kinetics in Low-Dimensional Semiconductor Systems

Spin kinetics in low-dimensional semiconductor systems was investigated spectroscopically. In the structures, owing to the quantum confinement, the degeneracy of the heavy-hole (HH) and light-hole valence bands was removed. Semiconductor systems were pumped for governing transitions from the HH valence band to the conduction band for the generation of the conduction-band-electron spins, and a maximum ～80% initial spin polarization was obtained in the systems at liquid helium temperature. Distinct spin oscillations and polarization decay were also observed. Spin kinetics of the drifting electrons was studied as a function of the external magnetic field as well as that of the system temperature in the exact Voigt configuration.     

Electron and hole focusing in CoSi2/Si(111) observed by ballistic electron emission microscopy

In ballistic electron emission microscopy (BEEM) the propagation of hot carriers in thin metal films has long been treated using a free electron model. While the model explains many experimental findings, it cannot account for the lateral resolution observed for both electrons and holes on epitaxial CoSi(2)/Si(111), where interfacial point defects of atomic size appear as small as 1.3 nm, even below a 5.6 nm thick film. We present ab initio calculations explaining this high resolution in terms of conduction (valence) band structure focusing of electrons (holes), according to a recent Green's function approach to the BEEM process.     

New efficient mechanism of excitation of electroluminescence from erbium ions in crystalline silicon

In p–n junctions based on c-Si : Er we have realized highly efficient excitation of erbium electroluminescence at 1.54 μm with an efficiency close to unity. A possible mechanism is Auger recombination of electrons occupying the upper subband of the conduction band with free holes in the valence band whereas the energy of the recombination process is transferred by Coulomb interaction to 4f-electrons of an erbium ion transmitting it to the second excited state ⁴I_11/2 (excitation energy 1.26 eV). The observed three-level excitation of erbium ions is promising for development of a Si : Er laser.     

First-principles study on electronic structure of Sr2Bi2O5 crystal

The electronic structure of Sr₂Bi₂O₅ is calculated by the GGA approach. Both of the valence band maximum and the conduction band minimum are located at Γ-point. This means that Sr₂Bi₂O₅ is a direct band-gap material. The wide energy-band dispersions near the valence band maximum and the conduction band minimum predict that holes and electrons generated by band gap excitation have a high mobility. The conduction band is composed of Bi 6p, Sr 4d and O 2p energy states. On the other hand, the valence band can be divided into two energy regions ranging from −9.5 to −7.9 eV (lower valence band) and from −4.13 to 0 eV (upper valence band). The former mainly consists of Bi 6s states hybridizing with O 2s and O 2p states, and the latter is mainly constructed from O 2p states strongly interacting with Bi 6s and Bi 6p states.     

Solar cells as pseudo-Josephson junctions

We have identified pseudo-Josephson effects within currents in solar cells. Compared to the original Josephson junction of superconductors, we were able to obtain theoretical results in one-to-one correspondence with experimental observation of solar cell currents. We applied a pseudo-Josephson junction to the current along the axis of energy, that is, the momentum space. The theoretical form of DC-type currents before integration over frequency was also found to be of the tan-function. This pseudo-Josephson junction is, however, not in real space but in momentum space. Currents of solar cells were not formulated in terms of the unique semiconductor differences between electrons and holes but with our pseudo-Josephson junction in terms of conduction band electrons, energy gap, and valence band electrons in the solar cell materials. The mechanism of solar cell function is thus ascribed to a pseudo-Josephson effect in our scheme.     

Evidence of magnetic ordering in tellurium substituted FeSb2

We report on a⁵⁷Fe Mössbauer study of tellurium substituted FeSb₂, FeSb_2?x Tc _x (x=0.2, 0.4, 0.6), at temperatures between 4.2 K and 300 K. For all three alloys, the Mössbauer spectra at 4.2 K are characteristic of a magnetically ordered state. The hyperfine field at Fe site increases with increasing tellurium concentration. The magnetic character may be attributed to the existence of a very narrow band gap leading to fairly strong Coulomb and exchange interactions between holes in the valence band and electrons in the conduction band.