On the effective electron mass in quantum well wires of ternary chalcopyrite semiconductors

Summary An attempt is made to study effective electron mass in quantum well wires of ternary chalcopyrite semiconductors by formulating a new 1D dispersion relation, within the framework of thek·p formalism considering the anisotropies in the band parameters. It is found, taking quantum well wires ofn-CdGeAs₂ as an example, that the effective Fermi level mass depends on the subband index due to the combined influence of crystal-field splitting parameter and the anisotropic spinorbit splitting parameters, respectively. The masses increase with increasing carrier degeneracy and decreasing film thickness, respectively. In addition, the well-known results for the corresponding parabolic energy bands have been derived as special cases of the generalized formulations.     

Theoretical study of the effective electron mass in ternary chalcopyrite semiconductors in the presence of crossed electric and magnetic fields

An attempt is made to investigate theoretically the effective electron mass in ternary chalcopyrite semiconductors at low temperatures on the basis of a newly derived dispersion relation of the conduction electrons under cross fields for the more generalized case which occurs from the consideration of the various types of anisotropies in the energy spectrum. It is found, taking degeneraten-CdGeAs₂ as an example, that the effective electron mass at the Fermi level along the direction of magnetic quantization depends on both the Fermi energy and the magnetic quantum number due to the combined influence of the crystal field splitting parameter and the anisotropic spin-orbit splitting parameter respectively, resulting in different effective masses at the Fermi level corresponding to different magnetic sub-bands. It is also observed that the same mass at the Fermi level in the direction normal to both magnetic and electric fields also varies both with Fermi energy and magnetic sub-band index, and the characteristic feature of cross-fields is to introduce the index-dependent oscillatory mass anisotropy. The theoretical results are in good agreement with the experimental observations as reported elsewhere.     

Influence of magnetic quantization on the effective electron mass in Kane-type semiconductors

Summary We study the effective electron mass at the Fermi level in Kane-type semiconductors on the basis of fourth order in effective mass theory and taking into account the interactions of the conduction electrons, heavy holes, light holes and split-off holes, respectively. The results obtained are then compared to those derived on the basis of the well-known three-band Kane model. It is found, takingn-Hg_1−x Cd _x Te as an example, that the effective electron mass at the Fermi level in accordance with fourth-order model depends on the Fermi energy, magnetic quantum number and the electron spin respectively due to the influence of band nonparabolicity only. The dependence of effective mass on electron spin is due to spin-orbit splitting parameter of the valence band in three-band Kane model and the Fermi energy due to band nonparabolicity in two-band Kane model. The same mass exhibits an oscillatory magnetic-field dependence for all the band models as expected since the origin of oscillations in the effective mass in nonparabolic compounds is the same as that of the Shubnikov-de Hass oscillations. In addition, the corresponding results for parabolic energy bands have been obtained from the generalized expressions under certain limiting conditions.     

Influence of magnetic quantization on the effective mass in semiconductor superlattices

An attempt is made to study the effect of a quantizing magnetic field on the effective electron mass in a semiconductor superlattice at low temperatures. It is found on the basis of the tight-binding approximation, taking GaAs-Ga_1–x Al _x As an example, that the effective mass at the Fermi level depends on the magnetic quantum number due to the cosine dependence of the wave-vector in the superlattice direction. The mass also exhibits oscillatory features in the presence of magnetic quantization because of its dependence on Fermi energy which oscillates with changing magnetic field.     

Photoluminescence spectra of doped GaAs films

We have studied the dependence of the photoluminescence (PL) spectrum on the doping level and the film thickness of n-GaAs thin films, both experimentally and theoretically. It has been shown theoretically that modification of the PL spectrum of p-type material by p-type doping is very small due to the large valence-band hole effective mass. The PL spectrum of n-type material is affected by two factors: (1) the electron concentration which determines the Fermi level in the material; (2) the thickness of the film due to re-absorption of the PL signal. For the n-type GaAs thin films under current investigation, the doping level as well as the film thickness can be very well calibrated by the PL spectrum when the doping level is less than 2×10¹⁸ cm^-3 and the film thickness is in the range of the penetration length of the PL excitation laser. PACS 78.20.-e; 78.55.Cr; 78.66.Fd     

de Haas–van Alphen effect in strongly correlated electron systems

We present the Fermi surface properties in strongly correlated electron systems of rare earth and uranium compounds via de Haas–van Alphen experiments. The conduction electrons with large cyclotron effective masses over 100m₀ (m₀: rest mass of an electron) are detected in CeRu₂Si₂, CeCoIn₅ and UPt₃. These electrons move slowly in the crystal. The topology of the Fermi surface and the cyclotron mass are compared to those of energy band calculations.     

Influence of quantization of band states on the effective electron mass in quaternary alloys

Summary An attempt is made to study the effective electron mass in quaternary alloys, taking a In_1−x Ga _x As _y P_1−y lattice matched to InP, by using the three-band Kane model under different physical conditions,e.g. bulk specimens, magnetic quantization, cross-field configuration, quantum well, electric-field-aided quantum well, magnetic-field-aided quantum well, quantum well under cross fields, quantum well wires, electric-field-aided quantum well wires, magnetic-field-aided quantum well wires and quantum well wires under cross fields by formulating the respective expressions. We have plotted the effective Fermi level mass with various physical variables under different conditions. In the presence of a quantizing magnetic field the effective mass depends on the spin splitting of Landau levels due to the spin-orbit splitting parameter of the valence bands. Under cross-field configuration and the various quantum confined low-dimensional systems, the effective masses depend on the respective quantum numbers in addition to the Fermi energies even for parabolic models because of the inherent features of such systems. In addition, the corresponding results for relatively wide-gap materials have also been obtained from our generalized formulations under certain limiting conditions.     

Effect of Quantizing Magnetic Field on Cyclotron Energy and Cyclotron Effective Mass in Size Quantized Films with Non-Parabolic Energy Band

The Fermi energy, cyclotron energy and cyclotron effective mass of degenerate electron gas in a sizeoquantized semiconductor thin film with non-parabolic energy bands are studied. The influences of quantizing magnetic field on these quantities in two-band approximation of the Kane model are investigated. It is shown that the Fermi energy oscillates in a magnetic field. The period and positions of these oscillations are found as a function of film thickness and concentration of electrons. Cyclotron energy and cyclotron effective mass are investigated as a function of film thickness in detail The results obtained here are compared with experimental data on GaAs quantum wells.     

Effects of oxygen content on the microstructures and optical properties of thermochromic vanadium oxide thin films

Reactive direct current magnetron sputtering and in situ thermal oxidation were used to prepare vanadium oxide (VO _X ) thin films with different oxygen contents. X-ray diffraction, Fourier transform infrared spectroscopy and a field emission scanning electron microscope were employed to characterize the films. The optical properties of the VO _X films at room temperature and 90 °C were investigated by applying an spectroscopic ellipsometer with a three-layer model of BEMA/Brendel–Bormann oscillator/substrate. It was demonstrated that the vanadium–oxygen bonds were strengthened, the film thickness and roughness decreased, while the grain size increased with increasing oxygen content. The increase in oxygen content had the effect of decreasing the near-infrared reflectance and free-electron concentration of the film at 90°C due to the decrease in the amount of VO₂.     

High-temperature superconductors as heterostructures

The wave-function envelope method is used to describe the electronic states of the cuprate high-T _c superconductors (HTSCs). In this method the 2D electronic states of the CuO₂ layers of a unit cell play the role of quantum wells, while the 2D states of the reservoir play the role of quantum barriers. Because of the different anisotropy of the 2D effective masses of the wells and barriers, some states on the Fermi surface (line) belong to CuO₂ layers and some states belong to the reservoir layers. This behavior of the electronic states explains characteristic features of HTSCs, such as the existence of regions on the Fermi surface with strongly different relaxation times, the weak suppression of d-type superconducting pairing by nonmagnetic scattering, and the coincidence of the angular dependence of the superconducting order parameter and the angular dependence of the electronic density of states (forward scattering predominating). The change in the signs of the components of the effective masses along the Fermi surface can result in the formation of hole pairs (biholes) or electron pairs (bielectrons) on account of the Coulomb interaction in the case of a negative reduced mass of the pairs. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 211–216 (10 August 1998)     

Oscillatory effective mass in degenerate narrow-gap semiconductors in a quantizing magnetic field

An attempt is made to study the effect of a quantizing magnetic field on the effective electron mass in degeneraten-type narrow-gap semiconductors at low temperatures. It is found, takingn-Hg_1−x Cd _x Te as an example, that the effective electron mass shows an oscillatory magnetic-field dependence as is expected because of the dependence of the effective mass in degenerate non-parabolic bands on Fermi energy which oscillates with changing magnetic field. The amplitude of oscillations is, however, found to be significantly influenced by the alloy composition whereas the period is found to be independent of the band non-parabolicity, i.e. of the compositional parameter in ternary semiconductors.     

Dynamical mean-field study of the Mott transition in thin films

The correlation-driven transition from a paramagnetic metal to a paramagnetic Mott-Hubbard insulator is studied within the half-filled Hubbard model for a thin-film geometry. We consider simple-cubic films with different low-index surfaces and film thickness d ranging from d=1 (two-dimensional) up to d=8. Using the dynamical mean-field theory, the lattice (film) problem is self-consistently mapped onto a set of d single-impurity Anderson models which are indirectly coupled via the respective baths of conduction electrons. The impurity models are solved at zero temperature using the exact-diagonalization algorithm. We investigate the layer and thickness dependence of the electronic structure in the low-energy regime. Effects due to the finite film thickness are found to be the more pronounced the lower is the film-surface coordination number. For the comparatively open sc(111) geometry we find a strong layer dependence of the quasi-particle weight while it is much less pronounced for the sc(110) and the sc(100) film geometries. For a given geometry and thickness d there is a unique critical interaction strength U _c2 (d) at which all effective masses diverge and there is a unique strength U _c1 (d) where the insulating solution disappears. U _c2 (d) and U _c1 (d) gradually increase with increasing thickness eventually approaching their bulk values. A simple analytical argument explains the complete geometry and thickness dependence of U_c2. U_c1 is found to scale linearly with U_c2. Received 19 August 1998     

Influence of the quantum size effect for grazing electrons on the electronic conductivity of metal films

The thickness dependence of the electronic conductivity of thin (5–150 nm) single-crystal (100) films of refractory metals is investigated at different temperatures ranging from 4.2 K to room temperature. Regions of square-root, quasilinear, and quadratic dependences are observed. The quasilinear thickness dependence is explained by the influence of quantum effects on the transverse motion of electrons in the case when electron scattering by the film surfaces dominates. For macroscopic film thicknesses 30–50 nm, much greater than the Fermi wavelength of an electron, quantum corrections to the electronic conductivity reach values of the order of 50%. This is a consequence of the quantum size effect for grazing electrons, which leads to an anomaly in electron scattering by the film surfaces. The region of the quadratic thickness dependence corresponds to the quantum limit, and the square-root region corresponds to the classical limit. The effect is explained in a quasiclassical two-parameter model (the effective angle α* for small-angle electrons and the parameter γ, equal to the ratio of this angle to the diffraction angle) that takes into account the diffraction angular limits for grazing electrons. The effect occurs for parameters α*≪1 and γ∼1 and differs from the “ordinary” quantum size effect. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 693–698 (10 December 1997)     

赝形InxGa1-xAs/In0.52Al0.48As异质结构中二维电子气的回旋共振光谱研究

采用回旋共振光谱方法，同时获得了具有较高电子气浓度的赝形In_0.80Ga_0.20As/In_0.53Ga_0.47As/In_0.52Al_0.48AsHEMT结构中最低两个子能带的费密面附近电子有效质量、 散射时间和迁移率.观察到该系统中能带非抛物性和波函数穿透所导致的电子回旋有效质量的显著增大效应，以及合金无序势和电离杂质散射所引起的电子散射时间和迁移率明显的子带依赖性. 关键词：     

Electronic properties of Chevrel phases by positron angular correlation

A comparative study of the Chevrel phases Mo₆S₈, Mo₆Se₈, PbMo₆S₈ and Cu_1.8Mo₆S₈ is made by the positron angular correlation technique at room temperature. Electronic properties like the number of conduction electrons per cluster, the density of states at the Fermi level, the electron effective masses and the Fermi velocities are obtained within the framework of the free electron model. The results are discussed in terms of available theoretical and other experimental data.     

Unoccupied electronic states and the interface formation between oligo(phenylene-vinylene) films and a Ge(111) surface

Thin films of tri-oligo(phenylene-vinylene) end-terminated by di-butyl-thiole (tOPV) were thermally deposited in UHV on Ge(111) substrates. The surface potential and the structure of unoccupied electron states (DOUS) located 5–20 eV above the Fermi level (E _F) were monitored during the film deposition using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. The electronic work function of the surface changed during the film deposition until it reached a stable value of 4.3±0.1 eV at a tOPV film thickness of 8–10 nm. Deposition of the tOPV under 3 nm led to the formation of intermediate DOUS structures that were replaced by another DOUS structure along with an increase in the tOPV deposit thickness up to 8–10 nm. The occurrence of the intermediate DOUS structure is indicative of a substantial reconfiguration of the electronic structure of the tOPV molecules due to the interaction with the Ge(111) surface. Analysis of the TCS data allowed us to assign the unoccupied electronic bands in tOPV located at 5.5–6.5 and 7.5–9.5 eV above the E _F as π* bands and at 11–14 and 16–19 eV above E _F as σ* bands.     

Electrons in thin films

The method of evaluation of broken-symmetry Green’s function is presented. This method was applied to electron subsystem of a thin film. It was found that electron concentrations are spatially dependent and their spatial distribution proves the existence of skin effect. The skin effect is most expressed in the films of minimal thickness. The internal energy of electrons lying in the film Fermi volume, decreases with the increase of temperature. It is the consequence of the Pauli principle, which does not allow change of electron velocities. Introduced heat increases electron effective masses and this leads to the decrease of internal energy.     

Temperature accelerated dielectric breakdown of PECVD low-k carbon doped silicon dioxide dielectric thin films

The dielectric breakdown strength of carbon doped silicon dioxide thin films with thickness d from 32 nm to 153 nm is determined at 25 °C, 50 °C, 100 °C, 150 °C and 200 °C, using I–V measurements with metal-insulator-semiconductor (MIS) structures. It is found that the dielectric breakdown strength, E_B, decreases with increasing temperature for a given film thickness. In addition, a film thickness dependence of breakdown is also observed, which is argued to show an inverse relation to thickness d in the form of E_B∝(d-d_c)^-n. The exponential parameter n and critical thickness limit d_c also exhibit temperature dependent behavior, suggesting a temperature accelerated electron trapping process. The activation energy for the temperature acceleration was shown to be thickness dependent, indicating a thickness dependent conduction mechanism. It is thereafter demonstrated that for relatively thick films (thickness >50 nm), the conduction mechanism is Schottky emission. For relatively thin films (thickness <50 nm), the Schottky conduction mechanism was obeyed at low field region while FN tunnelling was observed as a prevail one in the high field region. PACS 73.40.Qv     

The relationship between refractive index-energy gap and the film thickness effect on the characteristic parameters of CdSe thin films

CdSe thin films were deposited on glass substrates using Successive Ionic Layer Adsorption and Reaction (SILAR) method at room temperature and ambient pressure. The relationship between refractive index and energy bandgap was investigated. The film thickness effect on the structural, morphological, optical and electrical properties of CdSe thin films was investigated. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies showed that all the films exhibit polycrystalline nature with hexagonal structure and are covered well with glass substrates. The crystalline and surface properties of the films improved with increasing film thickness. The optical absorption studies revealed that the films are found to be a direct allowed transition. The energy bandgap values were changed from 1.93 to 1.87 eV depending on the film thickness. The electron effective mass (m_e^?/m_o), refractive index (n), optical static and high frequency dielectric constant (ε_o, ε_∞) values were calculated by using the energy bandgap values as a function of the film thickness. The resistivity of the films changed between 10⁶ and 10² Ω-cm with increasing film thickness at room temperature.     

Evolution of the electronic properties of transition metal nanoclusters on graphite surface

The electronic properties of nanoclusters of transition (Ni, Co, Cr) and noble (Au, Cu) metals deposited on the surface of highly oriented pyrolytic graphite (HOPG) are studied using the method of X-ray photoelectron spectroscopy. The laws of variation of a change ΔE _b in the binding energies of core-level electrons in the initial (ΔE _i) and final (ΔE _f) states of atoms in nanoclusters, the intrinsic widths γ of photoelectron lines, and their singularity indices α as functions of the metal cluster size d are determined. A qualitative difference in behavior of the ΔE _i(d) and α(d) values in metals of the two groups (Ni, Cr versus Co, Cu) is found. The values of the final-state energy (ΔE _f < 0) and the line width (Δγ > 0) in the clusters of all metals studied vary in a similar manner. It is shown that a significant contribution to E _i is due to a transfer of the valence-shell electrons at the cluster-substrate interface, which is caused by the contact potential difference. The value of an uncompensated charge per nanocluster is determined as a function of the cluster size and the number of atoms in the cluster. The behavior of ΔE _f(d) is controlled by the Coulomb energy of a charged cluster and by a decrease in the efficiency of electron screening, which is different in the metals studied. The broadening of photoelectron lines is determined by a spread of the cluster sizes and by lower electron screening in the final Fermi system. An asymmetry of the core-level electron spectra of nanoclusters can be explained using notions about the electron-hole pair excitation near the Fermi level. The effect of the structure of the density of electron states in the d band of transition metals on the asymmetry of photoelectron lines is considered and it is concluded that this structure near the Fermi level qualitatively changes with a decrease in the nanocluster size. The obtained results indicate that the behavior of the electron subsystem of clusters of the d-metals in a size range of 2–10 nm under consideration is close to the behavior of a normal Fermi system.