Comparison of Shubnikov-de Haas effect and cyclotron resonance on Si(100) MOS transistors under uniaxial stress

The stress induced population of the non-equivalent subbands E₀ to E′₀ is investigated by measurements of the magnetoconductivity — the Shubnikov-de Haas oscillations — and the cyclotron resonance taken on the same MOS transistor under identical conditions. In cyclotron resonance with increasing stress, a gradual transfer of carriers from E₀ to E′₀ is observed, whereas the Shubnikov-de Haas periodicity is unaffected and always yields a valley degeneracy of two. A domain model is required to resolve this apparent discrepancy.     

A plasma resonance study of valley transfer in (001) Si inversion layers

We observed the two-dimensional plasmons of the two-component electron plasma in the (001) Si-inversion layer resulting from simultaneous population of the [001] valley, E₀, and the [010] valley, E₀′, subbands under a compressional uniaxial stress along [010]. Our data show an onset of electron transfer from E₀ to E₀′ at X = (1.4 ± 0.1) kbar for n = 1.67 × 10¹² cm^?2 and X = (1.2 ± 0.2) kbar for n = 2.60 × 10¹² cm^?2, consistent with the theory of Takada and Ando that includes the electron-electron correlation effects.     

Hartree calculations for n-inversion layers on stressed silicon surfaces

The energy splittings and occupation number densities in n-type inversion layers are estimated in a Hartree calculation with a parametrized exponential potential. Good agreement with experiment is obtained for low temperatures and for electron concentrations 1×10¹²cm^?2 ?N_s?1×10¹³cm^?2. On the basis of these calculations the influence of uniaxial stress is considered.For a 001-stress along a (100)-surface one finds that (i) the stress induced subband splitting is a function of the electron density, and (ii) the splitting E₁-E₀ within a subband system depends on the applied stress.     

Optical and intervalley scattering in quantized inversion layers in semiconductors

The momentum relaxation time for scattering of electrons in quantized levels of an inversion layer on a semiconductor surface is calculated for interactions via optical and intervalley phonons. A selection rule is found which prohibits transitions between subbands belonging to the same valley or set of valleys, at least in the zero order to which these scattering processes may occur. Relaxation times for the zero-order interaction and the first-order interaction are obtained for intervalley phonons. The results are applied to the case of a (100)-silicon surface, with electrons in the three lowest subbands (with energy levels E₀, E₁, E'₀) of the two sets of valleys. Agreement with the experimental data of Fang and Fowler is good when the combined effects of intervalley and acoustic scattering are considered.     

Energy levels of n-channel accumulation layer on InP surface

Electronic structure of the low lying quantized subbands is calculated for the electron accumulation layer on InP (100) system in a metal-insulator-semiconductor (MIS) structure. Hartree self-consistent technique at an arbitrary temperature has been used, neglecting many-body effects. In contrast to Si MIS system, a second subband is found to be populated even at low temperatures and moderate densities. Excitation energy, E₁₀, is found to be about 30–40 meV in the temperature range 0–300 K at an electron density of 10¹² cm^?2.     

Spectroscopy of the electron subbands on (1 1 1)-Si

We present measurements of infrared inter-subband absorption for electron subbands on (1 1 1)-Si for densities N_s up to ～ 10¹³ cm^?2 at 4.2 K for both parallel- and perpendicular-excitation geometries. Contrary to previously published work the depolarization shift is identified as a sizeable splitting of the resonance energies E^⊥₀₁ and E^z.dfnc;₀₁. The comparison with a recent calculations is given.     

AlxGa1-x N/GaN调制掺杂异质结构的子带性质研究

通过低温和强磁场下的磁输运测量研究了Al_0.22Ga_0.78N/GaN调制掺杂异质结构中2DEG的子带占据性质和子带输运性质.在该异质结构的磁阻振荡中观察到了双子带占据现象，并发现2DEG的总浓度随第二子带浓度的变化呈线性关系.得到了该异质结构中第二子带被2DEG占据的阈值电子浓度为7.3×10¹²cm^-2.采用迁移率谱技术得到了不同样品的分别对应于第一和第二子带的输运迁移率.发现当样品产生应变弛豫时第一子带的电子迁移 关键词： AlGaN/GaN异质结 二维电子气 子带占据 输运迁移率     

Electronic parameters and electronic structures in modulation-doped highly strained InxGa1−xAs/InyAl1−yAs coupled double quantum wells

Electronic parameters of a two-dimensional electron gas (2DEG) in modulation-doped highly strained In_xGa_1−xAs/In_yAl_1−yAs coupled double quantum wells were investigated by performing Shubnikov-de Haas (S-dH), Van der Pauw Hall-effect, and cyclotron resonance measurements. The S-dH measurements and the fast Fourier transformation results for the S-dH at 1.5 K indicated the electron occupation of two subbands in the quantum well. The electron effective masses of the 2DEG were determined from the cyclotron resonance measurements, and satisfied qualitatively the nonparabolicity effects in the quantum wells. The electronic subband structures were calculated by using a self-consistent method.     

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation-δ-doped GaAs/Al_0.3Ga_0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E_LO at B>27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E_LO+(E₂−E₁), where E₂, and E₁ are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm⁻¹ around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LL of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process.     

两个子带占据的In0.53Ga0.47As/In0.52Al0.48As量子阱中填充因子的变化规律

研究了低温(15K)和强磁场(0—13T)条件下, InP基In_053Ga_047As/In_052Al_048As量子阱中电子占据两个子带时填充因子随磁场的变化规律.结果表明,在电子自旋分裂能远小于朗道能级展宽的情况下,如果两个子带分裂能是朗道分裂能的整数倍时,即ΔE₂₁=kω_c(其中k为整数),填充因子为偶数;当两个子带分裂能为朗道分裂能的半奇数倍时,即ΔE₂₁=(2k+1)ω_c/2,填充因子出现奇数. 关键词： 053Ga_047As/In_052Al_048As量子阱')" href="#">In_053Ga_047As/In_052Al_048As量子阱 填充因子 磁输运     

Photon avalanche in a doped quantum well

The possibility of a photon avalanche in a doped quantum well irradiated by IR light is predicted. The proposed model includes the three lowest size-quantization subbands. The exciting IR light frequency is assumed to be in resonance with the transition between the second and third subbands. Probabilities of the Auger transitions responsible for the avalanche-like multiplication of electrons in excited states are calculated for the above-threshold light intensities (j>j _th). By numerically solving the rate equations for electron populations in the three subbands, it is shown that the values j _th in quantum wells with the free-carrier densities n ₀～10¹² cm^?2 are of the order of hundreds of kilowatt per square centimeter and do not depend on the rates of phototransition between the first and second subbands. Characteristic times of establishing the quasi-equilibrium distributions of electrons over the subbands lie in the picosecond range and steeply increase at near-threshold intensities.     

Intersubband resonance polarons in Al/δ-GaAs tunneling junctions

Tunneling spectra of the A1/δ-GaAs junctions fabricated by molecular-beam epitaxy in the regime of “intimate” contact of A1 with GaAs (100) were studied at 1.6 K in a magnetic field B parallel to the two-dimensional electron-gas layer. The concentration of 2D electrons in the δ-layer grown at a distance of 20 nm from the A1/GaAs interface was 1.1 × 10¹² cm^?2 and corresponded to a partial filling of only the lowest subband E ₀. The tunneling spectra exhibited many-particle features, viz., a zero-bias anomaly, lines of longitudinal optical (LO) phonons, and characteristic “dips” corresponding to the energies E _i of the 2D subbands. In the B fields below the critical value B _c ?11 T, the levels of 2D subbands underwent the usual diamagnetic shifts. At B≥B _c , the E ₁ (B) term pinning and the anticrossing of the E ₁(B) and E ₀(B)+2?ω_LO terms were observed, where ?ω_LO is the LO-phonon energy in GaAs. The observed effects are interpreted as manifestations of resonance intersubband polarons arising in the δ-layer upon reaching the E ₁(B _c )?E ₀(B _c )=2?ω_LO resonance.     

Detection limits in Auger electron spectroscopy

The sources of background noise in Auger electron microanalysis are analyzed in order to evaluate the minimum detectable concentration x_m and the minimum number of detectable atoms y_m that can be reached. The best choices of operating conditions (the energy E₀, intensity I₀, and spot size d₀ of the incident electron beam, and the duration of the experiment t_e) are deduced for bulk and thin film analysis. The main results are: (i) The choice of E₀ is not very stringent, at least when E₀ ? 5E_i(A), where E_i(A) is the ionization energy, (ii) For a given electron dose received by the sample, x_m is improved by the use of the largest incident spot size while y_m is improved by the use of the finest spot size. The results also hold for other microanalytical techniques such as electron energy loss spectroscopy or electron probe microanalysis. (iii) Chemical identification of a single atom will be possible on samples able to tolerate very large electron doses by using incident electron beams 10 nm or less in diameter. The expected performance of a coincidence technique first suggested by Wittry is also discussed.     

Cyclotron resonance of electrons and holes in electric surface subbands of tellurium

We present first measurements of the submillimeter-cyclotron resonance of electrons and holes in electric surface subbands of tellurium. From the resonance in the inversion layer we have obtained for the magnetic field paralled to the trigonal axis, the cyclotron mass of the surface electron m_ce = (0.117 ± 0.002)m₀. The resonance of the accumulation layer splits and suggests the contribution of different nonparabolic subbands.     

The role of hybridization on valence transitions in the spinless Falicov-Kimball model

The spinless Falicov-Kimball model with hybridization is studied in one dimension using small-cluster exact-diagonalization calculations. The resultant exact solutions are used to examine the f-state occupation n ^f as a function of f-level energy E _f, hybridization V and d-f interaction U. In addition, the phase diagram based on the finite-size scaling analysis of the single particle excitation energy is discussed both for the symmetric (E _f = 0) and unsymmetric (E _f ≠ 0) case. A number of remarkable results are found. (i) The f-electron occupation number as a function of E _f changes continuously, confirming renormalization-group results. No discontinuous transitions are observed at finite V. (ii) In the symmetric case the insulating state persists as the ground state for all nonzero V. (iii) Outside the symmetric point the hybridization stabilizes the metallic state for lower V and the insulating state for higher V. (iv) The metal-insulator transition takes place at V = V _c > 0. The values of the critical hybridization strength V _c are of order 10^?2.     

Fourier transform spectroscopy of the CO-stretching band of O-18 methanol

The high-resolution Fourier transform spectrum of the ν₈ CO-stretching band of CH₃¹⁸OH between 900 and 1100 cm⁻¹ has been recorded at the Canadian Light Source (CLS) synchrotron facility in Saskatoon, and the majority of the torsion-rotation structure has been analyzed. For the ν_t = 0 torsional ground state, subbands have been identified for K values from 0 to 11 for A and E torsional symmetries up to J values typically well over 30. For ν_t = 1, A and E subbands have been assigned up to K = 7, and several ν_t = 2 subbands have also been identified. Upper-state term values determined from the assigned transitions using the Ritz program have been fitted to J(J + 1) power-series expansions to obtain substate origins and sets of state-specific parameters giving a compact representation of the substate J-dependence. The ν_t = 0 subband origins have been fitted to effective molecular constants for the excited CO-stretching state and a torsional barrier of 377.49(32) cm⁻¹ is found, representing a 0.89% increase over the ground-state value. The vibrational energy for the CO-stretch state was found to be 1007.49(7) cm⁻¹. A number of subband-wide and J-localized perturbations have been seen in the spectrum, arising both from anharmonic and Coriolis interactions, and several of the interacting states have been identified.     

Multiple wavelength electroluminescence and laser generation in p-i-n resonant tunneling heterostructures

We report a design and electroluminescence (EL) investigation of a p-i-n resonant tunneling device based on an Al_0.4Ga_0.6As/GaAs graded-index waveguide heterostructure. The intrinsic region of the structure consists of a quantum well (QW) surrounded by multiple barrier energy filters providing simultaneous resonant occupation of electron and heavy-hole second excited subbands in the QW. Several peaks are observed in the EL spectra, confirming occupation of the excited subbands. The EL efficiency displays a resonant behavior accompanied by an S-shaped negative differential resistance region in the voltage–current characteristic. Current bistability is demonstrated, leading to bistability in the EL and laser generation spectra.     

Shubnikov-de Haas effect of n-inversion layers in InSb grain boundaries

Hall effect and Shubnikov-de Haas effect have been investigated in a n-inversion layer adjacent to the grain boundary in p-InSb bicrystals. The observed quantum oscillations of the magnetoconductivity result from a superposition of the Shubnikov-de Haas effect in two occupied subbands. The electron densities of the subbands n_si(n_s1=3.5 × 10¹¹cm^-2;n_s0= 1.1 × 10¹²cm^-2) and the effective cyclotron mass of the lower subband m_c0=(0.023 ± 0.002)m₀ have been evaluated.     

Zener tunneling between the landau levels in quasi-two-dimensional electronic Corbino disks at large filling factors

The magnetotransport characteristics of quasi-two-dimensional electronic Corbino disks based on a wide GaAs quantum well with two filled subbands of size quantization are studied. At low temperatures and high magnetic fields, the Corbino disks exhibit the oscillations of differential conductivity, which are periodic with respect to dc electric field E _dc. It is shown that the observed oscillations are related to the Zener tunneling induced by E _dc. The tunneling occurs between the filled and empty Landau levels under conditions corresponding to the absence of the Hall field.     

Magnetic Bloch states and Hall conductivity of a two-dimensional electron gas in a periodic potential without inversion center

Quantum states of 2D electrons are studied in a periodic potential without inversion center in the presence of a magnetic field. It is shown that the energy spectrum in magnetic subbands is not symmetric about the center of magnetic Brillouin zone E(k)≠E(?k). Singularities (phase branching points) of the electron wave function, which determine the quantization law of Hall conductivity σ_xy, are studied in the k space. It is found that a sharp change takes place in the number of points in the magnetic Brillouin zone and in the corresponding values of topological invariants determining the Hall conductivity of filled subbands. It is noted that the longitudinal conductivity of a lattice without inversion center placed in a magnetic field is not invariant with respect to a change in sign of the electric field, and a photovoltaic effect must arise in an ac electromagnetic field.