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.     

Exchange-mediated anisotropy of (ga,mn)as valence-band probed by resonant tunneling spectroscopy

We report on experiments and theory of resonant tunneling anisotropic magnetoresistance (TAMR) in AlAs/GaAs/AlAs quantum wells (QW) contacted by a (Ga,Mn)As ferromagnetic electrode. Such resonance effects manifest themselves by bias-dependent oscillations of the TAMR signal correlated to the successive positions of heavy (HH) and light (LH) quantized hole energy levels in GaAs QW. We have modeled the experimental data by calculating the spin-dependent resonant tunneling transmission in the frame of the 6 x 6 valence-band k.p theory. The calculations emphasize the opposite contributions of the (Ga,Mn)As HH and LH subbands near the Gamma point, unraveling the anatomy of the diluted magnetic semiconductor valence band.     

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.     

不同晶向、厚度的超晶格界面的平均键能行为

对三种不同晶面（（１００），（１１０）和（１１１））的超晶格（ＡｌＡｓ）₁（ＧａＡｓ）₁，（ＡｌＡｓ）₃（ＧａＡｓ）₃的电子结构进行了第一性原理的计算，采用冻结势万法系统地分析了在不同的晶向和不同的周期层厚度的情况下，超晶格界面处的电荷转移、平均键能的对齐行为和价带边的不连续性。进一步从第一性原理的数值计算上检验了以平均键能为能量参考的异质界面价带边不连续性的理论计算方法。 关键词：     

Zone–edge optical transitions in GaAs/AlAs lateral superlattices grown on vicinal surfaces by molecular beam epitaxy

We report on new features in the photoluminescence excitation (PLE) spectra and PLE linear polarization spectra of GaAs/AlAs lateral superlattices grown by molecular beam epitaxy (MBE). These lines appear systematically as the tilt angle of the lateral superlattice is varied. They are identified as zone–edge excitonic transitions by comparison between experimental data and detailed numerical calculations of optical transitions including valence-band mixing and tilt effects.     

A theoretical study on the electronic structure of GaAs/AlAs and AlAs/GaAs core/shell nanoparticles

Results of theoretical investigation on the structural and electronic properties of GaAs/AlAs and AlAs/GaAs core/shell nanoparticles are presented. We have considered relaxed structures of essentially spherical parts of the zinc-blende crystal structure. The electronic properties and the total energy were calculated using density-functional tight-binding method. Our results include the charge distribution, density of states (DOSs), electronic energy levels (in particular HOMO and LUMO), HOMO-LUMO gap, excitation spectra and their variation with shell thickness for both GaAs/AlAs and AlAs/GaAs core/shell systems.     

Enhanced electron tunneling and Γ–X interlayer intervalley electron transfer in a triple-barrier heterostructure

The enhanced electron tunneling effect and the electron Γ–X intervalley interlayer transfer in the AlAs/GaAs (001) triple-barrier heterostructure have been investigated both experimentally and theoretically. The effects of the external bias on the electronic structure, Γ–X state mixing and higher lying excited energy states are studied. The experimental observations show good agreement with the theoretical predictions based on the scattering theoretical approach of Green's function theory, which can handle electron interlayer intervalley propagation through the layered aperiodic heterostructure under the external bias.     

Structure and photoluminescence study of type-II GaAs quantum wires and dots grown on nano-faceted (3 1 1)A surface

(3 1 1)A GaAs/AlAs corrugated superlattices (CSLs) and satellite (3 1 1)B and (1 0 0) SLs were studied using Raman spectroscopy, high-resolution transmittance electron microscopy (HRTEM) and photoluminescence (PL). The thickness of GaAs layers was varied from 1 monolayer (ML) to 10 ML, the thickness of AlAs barriers was 10 ML in (3 1 1) direction. The strongest modification of the Raman spectra is found for the case of partial (<1 nm) GaAs filling of the AlAs surface. The calculated and experimental Raman spectra demonstrated a good agreement for both complete (1 nm) and partial (<1 nm) GaAs filling of the AlAs surface. According to Raman and HRTEM data, in the case of partial filling of (3 1 1)A AlAs surface, GaAs forms quantum well wires of finite length (quantum dots). A drastic difference of PL from grown side-by-side (3 1 1)A and (3 1 1)B SLs was observed. A strong room temperature PL in the green–yellow spectral region was observed in GaAs/AlAs (3 1 1)A CSLs containing GaAs type-II quantum dots.     

DX centers in selectively Si-doped GaAs---AlAs superlattices

DX center has been characterized in four GaAs---AlAs superlattices grown by MBE at 580°C. The structures are uniformly Si-doped or selectively Si-doped in the AlAs layers or in the middle of the GaAs layers or on both sides of the interfaces. Deep level transient spectroscopy measurements (DLTS) put in evidence one dominant electron trap, with an activation energy for thermal emission of about 0.42eV for all the superlattices. This defect shows a thermally activated capture cross section and a large concentration except for the case where the only GaAs layers are doped. For the first time, a study of the capture reveals a capture activation energy of 0.36-0.37 eV, which allows us to locate the DX level nearly 60 meV below the conduction miniband. From these results, we show that the observed DX center is related to silicon in the AlAs layers. For the case when the AlAs barriers are not doped, the DX level is due to the Si diffusion from the middle of the wells towards the barriers, the Si atoms having diffused during the growth.     

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.     

Oxide states spectroscopy of real Ge surfaces

The electronic structure of surface oxide has been investigated by measurements of internal photoemission and photoconductivity measurements for p-type single crystalline Ge specimens with real surfaces. The photo-response curves show bulk and surface effects which are separated by differential methods. Emission from the bulk into oxide states or from oxide states into the bulk result in positive or negative changes of surface conductivity. The oxide states are distributed in energy with maxima near the conduction-band edge and the valence-band edge of the oxide. Changes of the electronic structure are observed by X-irradiation and storage in laboratory air.     

Donor-acceptor recombination in type-II GaAs/AlAs superlattices

A study is reported of steady-and nonsteady-state photoluminescence of intentionally undoped and uniformly silicon-doped type-II (GaAs)₇(AlAs)₉ superlattices grown by MBE simultaneously on (311)A-and (100)-oriented GaAs substrates. It has been established that at elevated temperatures (160>T>30 K) the superlattice spectra are dominated by the line due to the donor-acceptor recombination between donors in the AlAs layers and acceptors located in the GaAs layers. The total carrier binding energy to the donor and acceptor in a pair has been determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1734–1739 (September 1998)     

Exciton Recombination and Spin Dynamics in Indirect-Gap Quantum Wells and Quantum Dots

The behavior of excitons in heterostructures with indirect-gap GaAs/AlAs quantum wells and (In, Al)As/AlAs quantum dots is discussed. The possibilities of controlled change of the exciton radiative recombination time in the range from dozens of nanoseconds to dozens of microseconds, experimental study of the spin dynamics of long-lived localized excitons, and use of the optical resonant methods for exciting the indirect-band exciton states are demonstrated.     

FIRST-PRINCIPLE SELF-CONSISTENT PSEUDOPOTENTIAL CALCULATION OF THE ELECTRONIC STRUCTURES OF SHORT-PERIOD (GaAs)m(AlAs)n SUPERLATT1CES

With the local density approximation, the band structares of the short-period (GaAs)₁(AlAs)₁ and (GaAs)₂(AlAs)₁ superlattices are calculated by using the first-principle self-consistent pseudopotential method. The results show that the (GaAs)₁(AlAs)₁ superlattice is an indirect semiconductor, and the lowest conduction band state is at point R in the Brillouin zone; the (GaAs)₂(AlAs)₁ superlattice is a direct semiconductor and the lowest conduction band state is at point Γ. The squared matrix elements of transition are calculated. The pressure coefficients of energy gaps of the (GaAs)₁(AlAs)₁ and (GaAs)₂(AlAs)₁ superlattices are calculated and compared with those obtained by hydrostatic pressure experiments.     

Electron and hole states in a quantum ring grown by droplet epitaxy: Influence of the layer inside the ring opening

The electronic structure of the conduction and valence bands of a quantum ring containing a layer inside the ring opening is modeled. This structure (nanocup) consists of a GaAs nanodisk (the cup’s bottom) and a GaAs nanoring (the cup’s rim) which encircles the disk. The whole system is embedded in an (Al,Ga)As matrix, and its shape resembles realistic ring structures grown by the droplet epitaxy technique. The conduction-band states in the structure are modeled by the single-band effective-mass theory, while the 4-band Luttinger–Kohn model is adopted to compute the valence-band states. We analyze how the electronic structure of the nanocup evolves from the one of a quantum ring when the size of either the nanodisk or the nanoring is changed. For that purpose, (1) the width of the ring, (2) the disk radius, and (3) the disk height are separately varied. For dimensions typical for experimentally realized structures, we find that the electron wavefunctions are mainly localized inside the ring, even when the thickness of the inner layer is 90% of the ring thickness. These calculations indicate that topological phenomena, like the excitonic Aharonov–Bohm effect, are negligibly affected by the presence of the layer inside the ring.     

Multilayers as defect barriers

A multilayer structure consisting of undoped GaAs and AlAs layers was incorporated into a buffer layer on GaAs substrates. This reduces the spreading of impurities from the substrate (e.g. tellurium) into layers deposited on top of the multilayer system as observed by SIMS and C/V profiling. Incorporation coefficients for tellurium in GaAs, AlAs and multilayers were calculated.     

Formation of InAs quantum dots and wetting layers in GaAs and AlAs analyzed by cross-sectional scanning tunneling microscopy

We have used cross-sectional scanning-tunneling microscopy (X-STM) to compare the formation of self-assembled InAs quantum dots (QDs) and wetting layers on AlAs (1 0 0) and GaAs (1 0 0) surfaces. On AlAs we find a larger QD density and smaller QD size than for QDs grown on GaAs under the same growth conditions (500 °C substrate temperature and 1.9 ML indium deposition). The QDs grown on GaAs show both a normal and a lateral gradient in the indium distribution whereas the QDs grown on AlAs show only a normal gradient. The wetting layers on GaAs and AlAs do not show significant differences in their composition profiles. We suggest that the segregation of the wetting layer is mainly strain-driven, whereas the formation of the QDs is also determined by growth kinetics. We have determined the indium composition of the QDs by fitting it to the measured outward relaxation and lattice constant profile of the cleaved surface using a three-dimensional finite element calculation based on elasticity theory.     

FTIR-spectroscopy of (GaAS)n/(AlAs)m superlattices

The optical properties of (GaAs)_n/(AlAs)_m superlattices in the infra-red spectral region have been studied. The confinement of optical phonons has been observed in both GaAs and AlAs layers of superlattices under investigation. The superlattice modes caused by the coupling between LO phonons and collective intersubband excitations have been found in doped superlattices. Macroscopic and microscopic calculations have been used for the analysis of experimental results. Good agreement with experiment has been obtained.     

FeS2(pyrite)电子结构与光学性质的密度泛函计算

采用局域密度近似的自洽密度泛函理论计算了FeS2的电子结构与光学性质。费米能级附近区域的能带与态密度计算表明价带极大值在X（100）点和导带极小值在G（000）点，直接带隙和音接带隙分别为0.74eV、0.6eV。并用电子结构信息精确计算了介质极化矩阵元，从而给出了FeS2的介电函数虚部及相关光学参量，理论结果与实验符合甚佳。