Microwave cyclotron resonance of hot electrons in p-type GaSb

A microwave photocreated cyclotron resonance signal is observed in p-type GaSb in the temperature range 1–30 K. Circular polarization and other measurements identify the carriers as electrons in the (000) conduction band. The problem of a surface effect on the measured peak position, reported for the first time is avoided by bulk carrier creation. An exponential loss of signal intensity at 27 K is explained by the theory of background plasma effects. The measured ωτ of 1·5–4 yields an electron collision time of τ ～ 10^?11 sec. The scattering mechanism at liquid helium temperatures is identified as being partly due to neutral defect acceptor scattering of hot electrons, with an unidentified residual scattering process. The electron polaron effective mass is measured to be (m *(polaron)/m₀) = 0·0412 ± 0·0012 for hot electrons with an average energy of ～ 14 MeV and is isotropic within 1 per cent. When corrections for conduction-band non-parabolicity and hot polaron effects are applied, the band-edge free electron mass is calculated to be (m₀*(free)/m₀) = 0·0396 ± 0·0021 (±5·2 per cent).     

Classical in-plane negative magnetoresistance and quantum positive magnetoresistance in undoped InSb thin films on GaAs (1 0 0) substrates

Magnetoresistance (MR) effects have been investigated in perpendicular and parallel magnetic fields at 300, 80 K and liquid He temperatures for undoped InSb thin films 0.1–2.3 μm thick grown on GaAs(1 0 0) substrates by MBE. At high temperatures, the intrinsic carriers show the parabolic negative MR observable only in magnetic fields parallel to the film. The skipping-orbit effect due to surface boundary scattering in the classical orbits in the plane vertical to the film has been argued to be responsible for the negative MR. At low temperatures (T=80 K), the transport is dominated by the two-dimensional (2D) electrons in the accumulation layers at the InSb/GaAs(1 0 0) hetero interface; MR is positive and shows a logarithmic increase with anisotropy between parallel and perpendicular field orientation, arising from the 2D weak anti-localization (WAL) that reflects the interplay between the spin-Zeeman effect and strong spin–orbit interaction caused by the asymmetric potential at the interface (Rashba term). The zero-field spin splitting energy of Δ₀13 meV, the electron effective mass of m^*0.10m₀ seven times of the band edge mass in bulk InSb and the effective g-factor of |g^*|15 in the accumulation layer have been inferred from fits of MR for the 0.1 μm thick film to the 2D WL theory.     

Submillimetre cyclotron resonance of electrons in GaP

The first observation of cyclotron resonance in n-type GaP is reported. The electrons were thermally excited at a temperature of 100 K and the resonance was observed at submillimetre wavelengths (337 μm) using a pulsed magnetic field of 0–300 kG. From experiments with B∥〈100〉, 〈110〉 and 〈111〉 it was found that the transverse effective mass for electrons is $\text{m}{}^1{}_{\mathrm{\perp }}\text{= 0.25 ± 0.01 m}{}_0$ and that the anisotropy factor for the conduction band ellipsoids is K = 20⁺¹⁰_-6.     

Fermi surface studies in the low- and high-T c phase of the organic superconductor β-(BEDT-TTF)2I3

We report de Haas-van Alphen (dHvA) measurements of the high-T _c modification (T _c = 7 - 8 K) and Shubnikov-de Haas results of the low-T _c modification (Tc ≈ 1:4 K) of the organic superconductor β-(BEDT-TTF)₂I₃. In the high-T _c phase we find a single closed two-dimensional orbit with a dHvA frequency F ₀ = (3815±10) T and a 1/cosθ dependence as expected for a quasi two-dimensional system (where θ is the angle between the magnetic field and the direction normal to the conducting planes). The effective cyclotron mass is m _c = (4:2±0:2)m _e. The dHvA signal shows a beating pattern caused by the three-dimensional warping of the Fermi cylinder. The inter-plane transfer integral estimated from the maximum beating frequency ΔF is t _⊥/εF ≈ 1/175. At the angles where the beating disappears, i.e., ΔF ≈ 0, the oscillations have a harmonic content which is much larger than expected from the Lifshitz-Kosevich formula. In the low-T _c phase weak SdH oscillations with a frequency of F _SdH ≈ 110 T and an effective cyclotron mass m _c ≈ 1:0m _e is found. These results suggest a possible reconstruction of the Fermi surface in the low-T _c phase.     

Experimental confirmation by galvanomagnetic methods of a complex transport model in In0.53Ga0.47As layers deposited by MBE on SI-InP

At low temperatures In_0.53Ga_0.47As samples show an increase of carrier concentration, which can be explained in terms of a two carriers transport model. This type of problem exists since the beginning of the semiconductor era, dating back to monocrystalline germanium.We propose that in all the investigated layers, there are X atoms or charged dislocations in the region of the first monolayers, which are built in during epitaxial growth. The layers were intentionally undoped. They form an impurity band in which low mobility carriers dominate over the localised electron scattering due to the s-d exchange interaction. These carriers do not freeze out at liquid helium temperature and give rise to two transport media for electrons; a conduction band at higher temperatures and an impurity band at lower temperatures. The electron which fall down onto the previously ionised X atoms, then move by thermally activated hopping. We show that the two carriers model for In_0.53Ga_0.47As epitaxial layers are confirmed by the carrier concentration-temperature, carrier concentration-magnetic field, resistivity-magnetic field behaviour, and also by YKA theory also. The differences between the two transport models are so distinctive that observed phenomena may exist. This paper presents experimental results, which constitute comprehensive evidence for the complicated structure of the semiconductor epitaxial layers on the sample of n-type In_0.53Ga_0.47As/InP layer with n=2.2×10¹⁵/cm³.     

Relaxation processes upon photoexcitation of semiconductor CdF<Subscript>2</Subscript> by laser pulses

The microwave-cavity-based technique is used to study the processes of photoionization of electrons from donor levels to the conduction band in semiconductor CdF₂ crystals doped with Y, In, or Ga. The samples were excited by periodic pulses of Nd-laser (λ = 1.06 μm, pulse width ～10 ns) in the temperature range 6–77 K. The transient processes were detected in the absorption and dispersion modes related to variation of the imaginary and real parts of the complex permittivity ?₁ ? i?₂ induced by the light pulses. The observed signals consisted of short peak at t ～ 0, approximately 40–70 ns in length, and a long tail with a duration of ～100 ms. The short peak is likely to be related to the stay of the photoexcited carriers in the conduction band, while the long tail is associated with the processes of excitation relaxation after the electrons coming back to the donor levels of the impurity band. The weak temperature dependence of the width of the peak at t ～ 0 is explained by the tunneling mechanism of relaxation of electrons through the energy (or, probably, spatial) barrier separating the bound and free states of the carriers in the semiconductor CdF₂.     

Emission of ballistic photoelectrons from <Emphasis Type="Italic">p</Emphasis>-GaN(Cs,O) with the effective negative electron affinity

This paper reports on a study of the emission of ballistic photoelectrons from p-GaN(Cs,O) with an effective negative electron affinity. At photon energies less than the GaN band gap width, where emission of electrons originates from photoexcitation of surface and near-surface states, an increment in the energy of ballistic electrons is equal to that of exciting photons, which is substantiated by the dispersionless character of the initial states. At photon energies exceeding the band gap width, the excess energy of light is partitioned among the kinetic energies of ballistic photoelectrons and holes in accordance with their effective masses. This relation was used to determine the effective hole mass along the c axis of the GaN lattice of the wurtzite structure, which turned out to be m* _h‖ = (0.60 ± 0.15)m ₀.     

Adsorbate band dispersions for C on Ru(0001)

We have studied carbon-induced two-dimensional energy bands on Ru(0001) using angleresolved photoelectron spectroscopy and have compared them with ab initio calculations. We find a nearly parabolic band (bottom at E_F ?9.8 eV at k_∥ = 0, effective mass ～ 1.5 m_e) which we assign to the C 2p_z valence states of a graphitic carbon overlayer. Compared to graphite, these states are bound more tightly by 2.3 eV.     

Determination of the g-factor of electrons in N-type silicon surface inversion layers

The g-factor of conduction electrons in the surface inversion layer on a silicon (100) surface has been determined using the tilted magnetic field method developed by Fang and Stiles.The value of (m¹/m₀)·g at the fixed magnetic field was independent of surface carrier density n_s, whereas it had a sharp peak at about 97 koe. At strong magnetic field limit the value was constant and 0.4. If we take the effective mass of conduction electrons in the inversion layer on the (100) surface as 0.2m₀, the g-factor is about two which is the same as that for conduction electrons in bulk silicon.     

Infrared spectroscopy of the intermediate-valence semiconductor YbB12

The dynamic conductivity and permittivity spectra of the intermediate-valence compound YbB₁₂ are measured in the frequency range (6–10⁴) cm^?1 (quantum energy 0.75 meV-1.24 eV) at temperatures of 5–300 K. Analysis of the spectral singularities associated with the response of free charge carriers has made it possible for the first time to determine the temperature dependences of their microscopic parameters, viz., concentration, effective mass, relaxation frequency and time, mobility, and plasma frequency. It is shown that the relaxation frequency decreases upon cooling from 300 K to the coherence temperature T ^* = 70 K for YbB₁₂, which is mainly associated with the phonon mechanism of scattering of charge carriers. For cooling below the coherence temperature T ^* = 70 K, the temperature dependence of the relaxation frequency for charge carriers of the Fermi-liquid type is found to be γ ～ γ₀ + T ², while their effective mass and relaxation time increase, respectively, to m ^*(20 K) = 34m ₀ (m ₀ is the free electron mass) and τ(20 K) = 4 × 10^?13 s, indicating the establishment of coherent scattering of carriers from localized magnetic moments of the f centers. At a temperature of T = 5 K, the conductivity spectrum contains an absorption line at a frequency of 22 cm^?1 (2.7 meV); the origin of this line can be associated with the exciton-polaron bound state. Since such a state was observed earlier in other intermediate-valence semiconductors (such as SmB₆, TmSe_1?x Te, and (Sm, Y)S), it is probably typical of this class of compounds.     

Self-trapped electrons in lithium tantalate

The electron spin resonance spectrum of Ta⁴⁺ ions has been investigated in LiTaO₃. These defects were produced either by irradiating as-grown crystals with X-rays or by optically bleaching crystals that had been previously reduced. In both cases, the Ta⁴⁺ ions were stable only at temperatures near or below 77 K. The g and hyperfine matrix parameters for the Ta⁴⁺ spectrum are g₆ = 1.503, g_⊥ = 1.172, A₆ = ∼0, and A_⊥ = 699 MHz. These parameters describe a 5d¹ electron on the tantalum ion, and we conclude that the defect represents a “self-trapped” electron at the normal Ta⁵⁺ site.     

High field effects in insulating CdF2: Gd crystals

CdF₂ crystals doped with Gd, before any conversion to semiconductor, exhibit at low temperatures electrical current and electroluminescence with d.c. voltage applied.The electrical conduction, attributed to electrons, is bulk limited and the i = i (V, T) characteristics are interpreted as a Poole-Frenkel conduction.The electroluminescence, due to intrinsic and impurity emission, is attributed to the impact process of field accelerated electrons, which give rise to electron-hole pair formation and impurity centre excitation.     

Galvanomagnetic properties of n-type Hg0.8Cd0.2Te

Galvanomagnetic properties of low and high mobility n-Hg_0.8Cd_0.2Te are reported. The experiments were carried out in magnetic fields up to 60 kG and between 1.8 and 77 K. The Hall coefficient does not show thermal and magnetic freeze-out of carriers. At 77 K the transversal magnetoresistance shows a proportionality ?⊥ ∝ B as was predicted by Gurevich and Firsov for the case of polar optical scattering in non-degenerated semiconductors. At 4 K where the mobility is governed by impurity scattering ?⊥ ∝ B^2.4 was observed in the extreme quantum limit. A negative longitudinal magnetoresistance was found at 77 K. The experimental results of high and low mobility samples show significant differences.     

On the structure of wave fields in the region of linear interaction between ordinary and extraordinary waves in two-dimensionally inhomogeneous magnetoactive plasmas

The spectra of the low-temperature photodissociation (photoionization) of Landau-Pekar polarons are calculated using the theory of quantum-coherent states and a new method of variation with respect to the parameters of phonon vacuum deformation. It is shown that the final polaron states upon photodissociation may have different numbers of phonons produced in a single dissociation event and different momenta of charge carriers. The spectrum of optical absorption related to the photodissociation of polarons exhibits a superposition of bands corresponding to various numbers of phonons formed as a result of dissociation of a single polaron. Due to a large width of the energy region corresponding to the final states of charge carriers, the halfwidth of each band is on the order of the energy of polaron coupling and is much greater than the phonon energy. For this reason, the individual phonon bands exhibit strong overlap. The very broad and, probably, structureless band formed as a result of the superposition of all these components begins at an energy equal to the sum of the polaron coupling energy (E _p) and the phonon energy. This band has a maximum at a frequency of about 5.6E _p/? and a halfwidth on the order of 5.6E _p/? at a unit effective mass (m* = m _e) of band electrons. For an effective charge carrier mass within m* = (1–3)m _e, the energy of the polaron band maximum can be estimated as 5E _p with an error of about 10%, and the halfwidth falls within 3.4E _p < ?Ω_1/2 < 5.6E _p. The multiphonon character of this band is related to a decay of the phonon condensate after the escape of charge carrier from a polaron. Such polarons are likely to be observed in the spectra of complex metal oxides, including high-temperature superconductors. Examples of such polaron bands in the reported absorption and photoconductivity spectra of nonstoichiometric cuprates, manganites, nickelates, and titanates are presented. A theory of the formation of Landau-Pekar polarons with the participation of branches of the polarization oscillations of the medium is developed. It is shown that, under certain conditions, such a multiphonon-dressed polaron can possess a coupling energy on the order of 0.2–0.3 eV, so that the maximum of the corresponding absorption band may occur at 1–1.5 eV.     

Study on the electronic structures of the reduced anatase TiO2 by the first-principle calculation

Employing the first-principle calculations based on the density functional theory (DFT) and the Molecule Orbital theory (MO), we have researched the electronic structures of the reduced anatase TiO₂ and its visible light photoactivity. The study is emphasized on the O vacancy, including the components of the defect states, the relationship with the bulk states and the way in which these electrons occupying the defect states are distributed in the real space. We find that the origin of the visible light photoactivity should be due to the transition of the excited electrons from the defect states σ_g orbital to the σ_u orbital in the upper conduction bands, rather than arising from the reduction of the band gap. The calculated results indicate that the localized defect states induced by the neutral and doubly ionized oxygen vacancies are all located in the band gap.     

Point-contact spectroscopy of electron relaxation mechanisms in semiconductors

The I–V characteristic of a tiny semiconducting channel connecting bulk electrodes is shown to have singularities arising due to phonon emission by hot electrons at energies eV = n?ω₀, where ω₀ is the optical phonon frequency and n = 1, 2, 3,…. The nonlinear part of the I–V curve provides direct information concerning the energy dependence of the elastic-scattering time of charge carriers.     

Electronic structure of α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>: Ab initio simulations and comparison with experiment

Al₂O₃ films 150 Å thick are deposited on silicon by the ALD technique, and their x-ray (XPS) and ultraviolet (UPS) photoelectron spectra of the valence band are investigated. The electronic band structure of corundum (α-Al₂O₃) is calculated by the ab initio density functional method and compared with experimental results. The α-Al₂O₃ valence band consists of two subbands separated with an ionic gap. The lower band is mainly formed by oxygen 2s states. The upper band is formed by oxygen 2p states with a contribution of aluminum 3s and 3p states. A strong anisotropy of the effective mass is observed for holes: m _h⊥ ^* ≈ 6.3m ₀ and m _h‖ ^* ≈ 0.36m ₀. The effective electron mass is independent of the direction m _e‖ ^* ≈ m _e⊥ ^* ≈ 0.4m ₀.     

First-principles investigation of impurity concentration influence on bonding behavior,electronic structure and visible light absorption for Mn-doped BiOCl photocatalyst

We performed first-principles calculation to investigate the bonding behavior, electronic structure and visible light absorption of Mn_xBi_1−xOCl (x=0, 0.0625, 0.09375 and 0.125) using density functional theory (DFT) within a plane-wave ultrasoft pseudopotential scheme. The relaxed structural parameters are consistent with the experimental results. The bonding behavior, bond orders, Mulliken charges and bond populations as well as formation energies are obtained. The calculated band structures and density of states show that Mn incorporation results in some impurity energy levels of Mn 3d states in forbidden band as well as valence band and conduction band, and that Mn 3d states, for the modest Mn doping concentration, not only can act as the capture center of excited electrons under longer wavelength light irradiation, but also may trap the photo-excited holes, improving the transfer of photo-excited carriers to the reactive sites. Our calculated optical absorption spectra exhibit that the spectral absorption edge is obviously red-shifted and extends to the visible, red and infrared light region due to the incorporation of Mn. Our calculated absorption spectra are in excellent agreement with the experimental results of Mn-doped BiOCl photocatalyst.     

低补偿度n-Hg1-xCdxTe的磁致金属-绝缘体相变和相变后的温度激活输运行为

报道了ｘ＝０．２１４组份、低补偿度（Ｋ《１）ｎ－Ｈｇ_1-xＣｄ_xＴｅ晶体在０．３─３０Ｋ温度范围，０─７Ｔ强磁场下的横向磁阻、电子霍耳迁移率、霍耳系数测量结果，观测到了磁致金属－绝缘体相变和相变后的温度激活输运行为。分析实验数据，提出：低补偿度、组份：ｘ＝０．２附近的ｎ－Ｈｇ_1-xＣｄ_xＴｅ，磁致金属－绝缘体相变（ＭＩＴ）发生的机理是载流子在浅施主杂质态上的磁冻结；发生磁冻结的前提是热冻出（ｔｈｅｒｍａｌ ｆｒｅｅｚｅ 关键词：     

The magnetooptical properties of InSe near the band gap

We have studied the magnetooptical properties of InSe near the band gap by wavelength modulation spectroscopy at 10 K with the electric vectorE perpendicular to theC-axis of the crystal. For the first time, Landau levels have been observed from which we obtain the reduced effective mass perpendicular to $\overrightarrow{C}$, μ_⊥ = (0.090 ± 0.010)m₀. The n = 1, 2 and 3 exciton lines along with a diamagnetic shift of the n = 1 level have been resolved from which we evaluate the Rydberg constant R = 15.0 ± 0.5 meV, the diamagnetic shift σ₁ = 14 ± 2 meV/(kG)² and ε_⊥ε_∥ = 81 ± 8. Evidence of the presence of the γ and ε polytypes in the sample due to stacking faults is given.