Band discontinuity in GaAs/AlAs superlattices with InAs strained insertion-layers

We have theoretically investigated the valence-band discontinuity (ΔE_v) at the (100) GaAs/AlAs interface with the InAs strained insertion-layer. The theoretical calculation is carried out by the self-consistent tight-binding method with the sp³s* basis in the (GaAs)₅/(InAs)₁/(AlAs)₅/(InAs)₁ [100] superlattice. ΔEv at the GaAs/InAs(1ML)/AlAs interface is calculated to be 0.50 eV, which is practically equal to ΔEv = 0.51 eV at the GaAs/AlAs interface with no InAs layers. The insertion of the InAs monolayer changes the detail of valence charge density at the GaAs/AlAs interface but does not change ΔE_v. The result of calculation is in consistent with our experimental measurement by using the x-ray photoelectron spectroscopy.     

A “superlattice” model for a smooth GaAs/AlAs (001) heterointerface

The effect of a smooth interface potential on the electronic states in GaAs/AlAs (001) structures is investigated using the pseudopotential method. In this approach, the transition region between GaAs and AlAs is assumed to be a layer corresponding to a half-period of the (AlAs)₂(GaAs)₂ superlattice, with the potential of this layer being close to the real potential near the heterointerface. In this case, the intervalley mixing occurs at two boundaries and in the transition layer rather than at one boundary, as in the model with a sharply cut-off potential. It is shown that a smooth potential has an appreciable effect on electron tunneling in structures with thin layers. This effect is especially important in the case where short-wavelength X states are involved. For one GaAs/AlAs (001) boundary, the transition layer acts as a quantum well localizing the charge density of a mixed Γ-X state near the boundary. In structures with a layer thickness of less than 2 nm, the differences in the resonance energies obtained in the models with a smooth heterointerface and with a sharp heterointerface can be as high as ～0.1 eV. The envelopes of the wave functions associated with Γ ₁ ⁽¹⁾ , Γ ₁ ⁽²⁾ , and Γ ₃ ⁽¹⁾ superlattice valleys and with Γ₁, X ₁, and X ₃ valleys of GaAs and AlAs are analyzed. It is shown that the matching matrices for the envelope functions at the GaAs/(AlAs)₂(GaAs)₂ and (AlAs)₂(GaAs)₂/AlAs boundaries depend only weakly on the electron energy near the bottom of the conduction band and that the probability densities calculated using these functions agree with the results of many-band calculations. Therefore, these functions can be used to construct a model with a smooth interface potential in the framework of the effective-mass method.     

Electron tunneling in the GaAs/AlAs(111) structures with a smooth potential at heteroboundaries

The effect of smooth interface potential on the electron tunneling in the GaAs/AlAs(111) structures with thin layers is studied using the pseudopotential method. The transition region between the structure components is represented by a half-period of the hexagonal (GaAs) ₃(AlAs)₃ (111) superlattice. It is shown that the allowance for the smooth potential results in a decrease in the Γ-L-mixing, Fano-resonance narrowing, and disappearance of interface states at the GaAs/AlAs(111) interface as compared to the abrupt-interface model. The shifts of the lowest Γ-and L-resonances observed for the structures with the layer thickness <2 nm amount to ∼0.1 eV, which is in good agreement with the behavior of levels in quantum wells. The transmission coefficient of electrons with the energies 0–0.5 eV above the GaAs conduction-band bottom obtained by multivalley calculation is close to that calculated with allowance for the lowest conduction band states Γ ₁ ⁽¹⁾ and Γ ₁ ⁽²⁾ of superlattice and Γ ₁ and L ₁ of binary crystals. This indicates that a two-valley superlattice model of the smooth GaAs/AlAs(111) interface can be developed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 7–13, July, 2007.     

Modell einer Schraubenversetzung in piezoelektrischen Kristallen. I Allgemeine Theorie

Due to the piezoelectric effect, dislocations in piezoelectric crystals carry electrical fields and (or) charges, which can be quite large locally. The properties of a 〈001〉-screw dislocation in a cubic crystal are investigated in detail. In the present first part, two limiting cases are treated, using the following suppositions: 1. No free charge carriers are present; the occurring electric field and the proper potential field are studied; 2. The electric field is negligible small; the charge density of the free charge carriers is examined. Numerical values for ZnS and GaAs, which are of technical importance, are calculated and discussed. For the depths of the flat potential valleys around the 〈001〉-dislocations, we obtained the values:ΔE=0,082 eV (ZnS) andΔE=0,049 eV (GaAs).     

Characteristic electron energy loss structure for clean and oxidized chromium surfaces

AES and EELS spectra have been measured on clean and oxidized chromium surfaces. Auger peaks at 31.0 and 44.0 eV of the oxide are attributed to cross transitions between chromium and oxygen: {M_2,3(Cr)V(Cr)V(O)} and {M₁(Cr)L₁(O)V(Cr)} respectively. Core loss features are consistent with valence band structure with a newly observed loss peak at 41.1 eV for the oxidized surface being ascribed to a core exciton with binding energy E_b = 1.6 eV.     

应变层超晶格(InAs)n(InP)n(001)电子结构的LMTO计算

基于Linearized-Muffin-Tin Orbitals(LMTO)能带方法,采用内部求和计入空d轨道的处理,对(InAs)_n(InP)_n(001),(n=1,2,3)应变层超晶格的电子结构进行了第一性原理计算。得出了其能带结构,态密度分布(对n=1)。考察了In4d轨道对能带计算的影响,并采用冻结势方法求出了(InAs)_n(InP)_n(001),(n=1,2,3)的价带边不连续值△E_v。 关键词：     

Valence-band offset of p-NiO/n-ZnO heterojunction measured by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy was used to measure the valence-band offset (VBO) of the NiO/ZnO heterojunction grown on quartz substrate by radio frequency (RF) magnetron sputtering. Core levels of Ni 2p and Zn 2p were used to align the VBO of p-NiO/n-ZnO heterojunction. The valence-band offset (ΔEV) is determined to be 1.47 eV. According to the band gap of 3.7 eV for NiO and 3.37 eV for ZnO, the conduction-band offset (ΔEC) in the structure was calculated to be 1.8 eV, and it has a type-II band alignment.     

Electronic structure of strained Sin/Gen(001) superlattices

Using the empirical tight binding method we have investigated the electronic properties of the Si_n/Ge_n(001) strained superlattices as a function of the superlattice periodicity and the band misfit. For n ≥ 4 we have found that first and second conduction band states are localized in Si. The hole states localized in Ge appear for n ≥ 4. The difference between the direct and indirect band gaps is reduced from 2.01 eV for bulk Si to 0.01 eV for n=6 which can be considered to be quasi-direct. For the cases n=6 and n=8, the band gap might become direct for large values of band misfit.     

Optical characterization of the deep Cr level in GaAs

Comparison between the excitation spectra of luminescence (ESL) of the 0.57 eV band (typical of p or SI materials) and of the 0.62 eV band (typical of n samples) with the photoionization cross-sections σ⁰_n(hv) and σ⁰_p(hv) of the chronium level leads us to attribute unambiguously these luminescence transitions to the (Cr³⁺?Cr²⁺) center in chromium doped GaAs.     

A mixed bonding band structure calculation for GaAs and AlAs using the APW-k · p method

The Augmented Plane Wave (APW) method and the k.p expansion are used to obtain the band structure of GaAs in a mixed covalent and ionic bonding. The energy levels at the Γ point are also obtained for AlAs. The ionicity is introduced as a parameter into the crystalline potential. We explore the dependence of the energy levels and the momentum matrix elements on the values of the ionicity. The value of the ionicity which gave the direct gap Eg = 1.52eV for GaAs (the accepted experimental value) was found to be 0.1, and for AlAs Eg = 2.50 eV was obtained for an ionicity of 0.2.     

应力对(GaP)1/(InP)1(111)超晶格体内和表面电子结构的影响

考虑到应力对超薄层(GaP)₁/(InP)₁(111)结构中Ga-P和In-P键长的作用为均匀分布的情况,本文提出在紧束缚近似下,将应力的影响直接反映到Harrison的交迭积分项中,并利用Recursion方法全面计算了由Keating模型确定的稳定(GaP)₁/(InP)₁(111)超晶格体内和表面的电子结构,结果表明,这种材料的带隙为1.88eV,它比体材料GaP(2.91eV)和InP(1.48eV)的平均值小 关键词：     

Optical phonon damping in the ultrathin-layer GaAs/AlAs superlattices

We performed the precision measurements of the near-normal far-infrared reflection and transmission spectra of the MBE-grown (GaAs)_n/(AlAs)_n (n =  1, 2, 4) superlattices. The results obtained show a noticeable (almost twofold) increase in the AlAs-phonon damping when n is decreased from 4 to 1. We consider possible physical mechanisms of this increase and compare our results with those obtained by other authors. The main implication of the work is that the increase in the TO-phonon damping may be related to interface broadening whose role becomes more important when the superlattice period decreases.     

Time-resolved spectra and kinetics of the exciton photoluminescence in different types of GaAs/AlAs superlattices

The temperature dependence of the recombination dynamics of excitons is investigated by time-resolved photoluminescence spectroscopy in(GaAs)_n / (AlAs)_n superlattices, where n denotes the layer thickness in monolayers, for different types of band structures. In direct-gap superlattices with a layer thickness of the order or larger than the exciton Bohr radius, the carrier dynamics is dominated by the transfer from light-hole to heavy-hole excitons. When the layer thickness becomes smaller than the exciton radius, the dynamics of free excitons is controlled by localization. vb In quasi-direct and indirect-gap superlattices, the influence of lateral potential fluctuations due to interface roughness completely governs exciton recombination.     

Analysis of current–voltage and capacitance–voltage characteristics of perylene-monoimide/n-Si Schottky contacts

The electrical and interface state properties of Au/perylene-monoimide (PMI)/n-Si Schottky barrier diode have been investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements at room temperature. A good rectifying behavior was seen from the I–V characteristics. The series resistance (R_s) values were determined from I–V and C–V characteristics and were found to be 160 Ω and 53 Ω, respectively. The barrier height (Φ_b) of Au/PMI/n-Si Schottky diode was found to be 0.694 eV (I–V) and 0.826 eV (C–V). The ideality factor (n) was obtained to be 4.27 from the forward bias I–V characteristics. The energy distribution of interface state density (N_ss) of the PMI-based structure was determined, and the energy values of N_ss were found in the range from E_c ? 0.508 eV to E_c ? 0.569 eV with the exponential growth from midgap toward the bottom of the conduction band. The values of the N_ss without R_s are 2.11 × 10¹² eV^?1 cm^?2 at E_c ? 0.508 eV and 2.00 × 10¹² eV^?1 cm^?2 at E_c ? 0.569 eV. Based on the above results, it is clear that modification of the interfacial potential barrier for metal/n-Si structures has been achieved using a thin interlayer of the perylene-monomide.     

A study of the GaAs/AlAs(001) superlattice spectrum within the abrupt- and smooth-boundary models

The electron states and miniband spectra of (GaAs) _m (AlAs) _n (001) superlattices related to the Г- and X _z -valleys of conduction bands of bulk GaAs and AlAs crystals are considered within the framework of simplified abrupt- and smooth-heteroboundary models. The models are constructed on the basis of calculations made by the pseudopotential method. It is shown that the proposed models describe the results of the corresponding exact calculations in the approximation of the abrupt and smooth boundaries reasonably well. It is found that the difference between the smooth-boundary and abrupt-boundary models becomes pronounced in the case of superlattices with thin layers, where this difference results in a qualitatively different dispersion of lower minibands.     

Electron surface states in short-period superlattices: (GaAs)2/(AlAs)2(1 0 0)-c(4 × 4)

The electronic structure of (GaAs)₂/(AlAs)₂(1 0 0)-c(4 × 4) superlattice surfaces was studied by means of angular-resolved photoelectron spectroscopy (ARUPS) in the photon energy range 20-38 eV. Four samples with different surface termination layers were grown and As-capped by molecular beam epitaxy (MBE). ARUPS measurements were performed on decapped samples with perfect c(4 × 4) reconstructed surfaces. An intensive surface state was, for the first time, observed below the top of the valence band. This surface state was found to shift with superlattices’ different surface termination in agreement with theoretical predictions.     

Confined longitudinal and transverse phonons in GaAs/AlAs superlattices

The phonon spectrum of GaAs/AlAs superlattices along the (001) growth direction is calculated using a new approach based on a realistic unified treatment of the interactions in both bulk constituents, which naturally lends itself to dealing with superlattice geometries. Results for a (GaAs)₃(AlAs)₃ superlattice are shown with emphasis on different aspects of confinement.     

Mass spectrometry study of the kinetics of CdTe molecular-beam epitaxy: Part II. Cd, Te2, Te, and CdTe

The results of an in situ mass-spectrometric study of surface processes occurring during CdTe molecular beam epitaxy are presented. The measurements of kinetic parameters are performed with modulated Cd and Te₂ molecular beams with an intensity of 0.1–5.0 ML/s at a crystal temperature of 550–730 K. The experimental results are treated using a model in which condensation and evaporation proceed through adsorption and desorption steps. The desorption rates are 2–15 and 150 s^?1 for Te₂ and Cd, respectively. The activation energy of CdTe “evaporation” is found to be 1.2 eV; the desorption energies are E _d(Cd) = 1.0 eV and E _d(Te₂) = 0.3 eV. The adsobate coverage with cadmium atoms and tellurium molecules is estimated to be n(Cd) < 0.01, n(Te₂) = 0.02–0.20, and n(Te) = 0.2–1.0.     

Growth of (GaAs)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>(ZnSe)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> solid solution films and investigation of their structural and some photoelectric properties

Single-crystal films of the substitutional solid solution (GaAs)_{1 ? x} (ZnSe) _x (0 ≤ x ≤ 0.80) on GaAs substrates have been grown using liquid phase epitaxy. The X-ray diffraction patterns, photoluminescence spectra, and current-voltage characteristics of the n-(GaAs)-p-(GaAs)_{1 ? x} (ZnSe) _x (0 ≤ x ≤ 0.80) heterostructures prepared have been investigated. The lattice parameters of the film a _f = 5.6544 Å and the substrate a _s = 5.6465 Å have been determined, and the profile of the molecular distribution of the solid solution components has been obtained. The photoluminescence spectrum of the (GaAs)_{1 ? x} (ZnSe) _x (0 ≤ x ≤ 0.80) films exhibits a narrow peak (against the background of the broad luminescence band) with a maximum in the luminescence intensity at a photon energy of 2.67 eV due to the presence of Zn-Se bonds in the structure (ZnSe is covalently bonded to the tetrahedral lattice of the GaAs matrix). It has been shown that the direct branch of the current-voltage characteristics of the structures under investigation is described by an exponential dependence I = I ₀exp(qV/ckT) at low voltages (V > 0.3 V) and by a power-law dependence I ～ V ^α with exponents α = 4 at V = 0.4–0.8 V, α = 2 at V = 0.8–1.4 V, and α = 1.5 at V > 2 V. The experimental results have been explained in the framework of the double-injection model for the n-p-p ⁺ structure under the condition that the concentration distribution of nonequilibrium charge carriers has a minimum.     

Experimental determination of valence band offset at PbTe/Ge(1 0 0) interface by synchrotron radiation photoelectron spectroscopy

The band offset at the interface of PbTe/Ge (1 0 0) heterojunction was studied by the synchrotron radiation photoelectron spectroscopy. A valence band offset of ΔE_V = 0.07 ± 0.05 eV, and a conduction band offset of ΔE_C = 0.27 ± 0.05 eV are concluded. The experimental determination of the band offset for the PbTe/Ge interface should be beneficial for the heterojunction to be applied in new optoelectronic and electronic devices.