Optical-gap reduction in the ordered phases of GaInAs solid solution

The measured optical band gap of the (GaAs)₁(InAs)₁ (001) superlattice is smaller than that of the corresponding Ga_0.5In_0.5As random alloy. Ab-initio pseudopotential calculations show that the effect is caused by a repulsion between the lowest conduction bands, or, equivalently, by the onset of confinement of electrons to GaAs layers, and is accompanied by the decrease of the electron effective mass. Similar reductions are obtained for the indirect band gaps, and for cubic phases of GaInAs solid solution.     

GaAs/AIAs superlattices under pressure: steady-state and subpicosecond time-resolved measurements

Abstract

Low-temperature photoluminescence measurements under hydrostatic pressure were performed on [100]-, [311]-and [111]-grown GaAs/AlAs superlattices. The indirect optical transitions for all three growth directions were identified by their characteristic pressure dependences as originating from the X point of the AlAs conduction band. Subpicosecond-time-resolved measurements on a GaAs/AIAs superlattice show a decrease of electron transfer times from the GaAs layer into the A1As layer with pressure from 400 fs to 5Ofs and an intensity dependence of the pressure-induced crossover from type I to type II.     

(InAs)1/(GaSb)1超晶格原子链的第一原理研究

利用第一原理平面波赝势法, 对(InAs)₁/(GaSb)₁超晶格原子链的原子结构、力学特性、电子能带结构、 声子结构和光学特性进行研究, 并结合密度泛函理论数值原子轨道赝势法和非平衡格林函数法计算量子输运特性. 与二维层结构的(InAs)₁/(GaSb)₁超晶格相比, (InAs)₁/(GaSb)₁超晶格原子链的能带结构有明显不同, 在某些情况下表现为金属能带特性. 对理想条件下(InAs)₁/(GaSb)₁ 超晶格原子链的力学强度计算表明, 该结构可承受的应变高达 ε=0.19. 通过对声子结构的完整布里渊区分析, 研究了(InAs)₁/(GaSb)₁超晶格原子链的结构稳定性. 对两端接触电极为Al纳米线的InAs/GaSb超晶格原子链的电子输运特性计算表明, 电导随链长和应变的改变而发生非单调变化.光吸收谱的计算结果表现出在红外波段具有陡峭吸收边, 截止波长随超晶格原子链的结构而变化.预计InAs/GaSb超晶格原子链可应用于红外光电子纳米器件, 通过改变超晶格原子链的结构来调节光电响应波段.     

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：     

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.     

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.     

Delocalization of phonon-plasmon modes in GaAs/AlAs superlattices with tunnel-thin AlAs barriers

The technique of Raman spectroscopy has been used to investigate doped (n-type) and undoped GaAs/AlAs superlattices with AlAs barrier thicknesses from 17 to 1 monolayers. The peak corresponding to the scattering by a two-dimensional plasmon was found in the Raman spectrum of a doped superlattice with relatively thick barriers. The position of the experimental peak corresponded to the value calculated in the model of plasma oscillations in periodic planes of a two-dimensional electron gas. The electron tunneling effects played an increasingly prominent role as the AlAs barrier thickness decreased. The peaks corresponding to the scattering by coupled phonons with three-dimensional plasmons were found in the Raman spectra for a superlattice with an AlAs thickness of 2 monolayers; i.e., the delocalization of coupled modes was observed. In this case, the folding of acoustic phonons was observed in the superlattice under consideration, indicative of its good periodicity, while the localization of optical phonons in GaAs layers was observed in undoped superlattices with an AlAs thickness of 2 monolayers.     

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.     

Experimental discovery of the anisotropy of phonon-plasmon modes in GaAs/AlAs(100) superlattices

Doped (n-type) GaAs/AlAs superlattices with thicknesses of the GaAs and AlAs layers from 1.7 to 6.8 Å and 13.6 Å, respectively, have been studied by means of Raman spectroscopy. The use of a microattachment for Raman backscattering studies has allowed for the observation of the modes with the wave vector directed both across and along the superlattice layers (in the scattering from the side face of the superlattice). The theoretically predicted anisotropy of mixed phonon-plasmon modes caused by the anisotropy of the electron effective mass in the type II superlattices has been experimentally discovered.     

The electronic structure of InAs/GaSb(001) superlattices-two dimensional effects

Detailed calculations of the two dimensional effects in the electronic structure of InAs/GaSb(001)superlattices are presented for the first time. Comparison of the calculated thickness dependence of the superlattice band gap with optical absorption measurements shows that, at the Γ-point, the conduction band edge of InAs lies about 60 meV below the valence band edge of GaSb. Eigenfunctions of the highest light and heavy hole bands, and the lowest two conduction bands exhibit spatially confined nature in the GaSb and InAs regions respectively, thus establishing the two-dimensional nature of these bands. The calculated conduction band effective mass in the plane of the superlattice near the Γ-point is found to be enhanced by a factor of 2.5 over the bulk InAs value and compares very well with the appropriate mass extracted from recent magnetoresistance measurements.     

Optical properties and band structure of short-period GaAs/AlAs superlattices

We have studied six GaAs/AlAs superlattices with periods ranging from 18 to 60 Å and different average aluminum composition. Three of these samples are shown to be direct bandgap materials whose band structure differs strongly from that of the corresponding alloy, but is correctly described by an envelope function calculation. The three remaining samples are shown to be indirect both in real and reciprocal space. The lowest energy transitions are found to arise from an exciton involving a heavy hole state mostly confined in the GaAs layer and at the Brillouin zone center (Λ), and an electronic state of X character confined in the AlAs layers. Analysis of the time decay of the luminescence shows that this is a momentum-forbidden exciton made allowed by disorder scattering, which leads to a luminescence efficiency comparable to that of the direct bandgap samples. Partial lifting of the degeneracy of the three X orbitals by the superlattice potential is also observed. Finally, we take advantage of the strong dependence of these indirect transition energies on the band discontinuities to estimate the valence band offset to be about 550 meV in this system.     

Intervalley electron scattering by phonons in (AlAs)1(GaAs)3(001) superlattices

Intervalley electron scattering by phonons in (AlAs)₁(GaAs)₃(001) superlattices is studied using the pseudopotential method and a phenomenological model of the bonding forces. The deformation potentials between the conduction band extrema of the superlattice involving short-and long-wavelength phonons are calculated. It is shown that the mixing of states from the zinc-blende L valleys plays a greater role in intervalley scattering in a superlattice than the Γ-X mixing. In particular, due to L-L mixing, the Γ-X ₃ transitions, analogous to Γ-L transitions in zinc blende, have higher intensities than the analogues of Γ-X transitions (Γ₁-M ₅ and (Γ₁-Γ₃ transitions). The deformation potentials averaged over the scattering channels in the superlattice agree with the corresponding potentials in a solid solution, but all transitions in the superlattice have higher intensities for the lower states.     

Interface corrugation effect on the polarization anisotropy of photoluminescence from (311)A GaAs/AlAs short-period superlattices

Studying GaAs/AlAs superlattices containing a quantum-well-wire array revealed photoluminescence polarization anisotropy for samples with GaAs layers less than 21 Å thick. It was found that polarization for a thickness of more than 40 Å was mainly due to valence band anisotropy, whereas polarization for a thickness of less than 21 Å was equally attributable to both valence band anisotropy and anisotropy associated with interface corrugation. For a GaAs layer thickness of less than 21 Å, a blueshift of the Γ electron-Γ heavy hole transition was observed. In this transition, the position of the peak of photoluminescence from the GaAs/AlAs (311)A superlattices containing a quantum-well-wire array is shifted toward higher energies compared to the (311)B and (100) superlattices containing no quantum-well wire with the same GaAs layer thickness. The conclusion was made that a blueshift is observed in GaAs/AlAs superlattices with GaAs layers less than 21 Å thick and a red-shift is observed when the thickness is larger than 43 Å.     

Mechanical anisotropy and electronic properties of X2N2 (XH2 ): first-principles calculations*

The mechanical anisotropy, structural properties, electronic band structures and thermal properties of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are detailed and investigated in this work. The novel silicon nitride phase Si₂ N₂ (SiH₂ ) and germanium nitride phase Ge₂ N₂ (GeH₂ ) in the Cmc 2₁ structure are proposed in this work. The novel proposed Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are both mechanically and dynamically stable. The electronic band calculation of the HSE06 hybrid functional shows that C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are all wide band gap semiconductor materials, and C₂ N₂ (CH₂ ) and Si₂ N₂ (SiH₂ ) are direct band gap semiconductor materials, while Ge₂ N₂ (GeH₂ ) is a quasi-direct band gap semiconductor material, the band gap of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are 5.634 eV, 3.013 eV, and 2.377 eV, respectively. The three-dimensional and plane distributions of Young’s modulus, shear modulus and Poisson’s ratio of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) show that these materials have different degrees of mechanical anisotropy. The order of Young’s modulus of Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) in different directions is different from that of C₂ N₂ (CH₂ ). When the tensile axis is in a particular direction, the order of the Young’s modulus of Si₂ N₂ (SiH₂ ): E _[110] <E _[120] <E _[111] <E _[101] <E _[010] =E _[100] <E _[011] <E _[001], and the order of the Young’s modulus of Ge₂ N₂ (GeH₂ ): E _[110] <E _[111] <E _[101] <E _[120] <E _[100] <E _[010] <E _[011] <E _[001] .     

Ground state energy of the electron-hole liquid in type-II (GaAs)m/(AlAs)m quantum wells

The ground state energy of quasi-two-dimensional electron-hole liquid (EHL) at zero temperature is calculated for type-II (GaAs)_m/(AlAs)_m (5≤m≤10) quantum wells (QWs). The correlation effects of Coulomb interaction are taken into account by a random phase approximation of Hubbard. Our EHL ground state energy per electron-hole pair is lower than the exciton energy calculated recently for superlattices, so we expected that EHL is more stable state than excitons at high excitation density. It is also demonstrated that the equilibrium density of EHL in type-II GaAs/AlAs QWs is of one order of magnitude larger than that in type-I GaAs/AlAs QWs.     

First-principles study of stability of point defects and their effects on electronic properties of GaAs/AlGaAs superlattice

When the GaAs/AlGaAs superlattice-based devices are used under irradiation environments, point defects may be created and ultimately deteriorate their electronic and transport properties. Thus, understanding the properties of point defects like vacancies and interstitials is essential for the successful application of semiconductor materials. In the present study, first-principles calculations are carried out to explore the stability of point defects in GaAs/Al_0.5Ga_0.5As superlattice and their effects on electronic properties. The results show that the interstitial defects and Frenkel pair defects are relatively difficult to form, while the antisite defects are favorably created generally. Besides, the existence of point defects generally modifies the electronic structure of GaAs/Al_0.5Ga_0.5As superlattice significantly, and most of the defective SL structures possess metallic characteristics. Considering the stability of point defects and carrier mobility of defective states, we propose an effective strategy that Al_As, Ga_As, and Al_Ga antisite defects are introduced to improve the hole or electron mobility of GaAs/Al_0.5Ga_0.5As superlattice. The obtained results will contribute to the understanding of the radiation damage effects of the GaAs/AlGaAs superlattice, and provide a guidance for designing highly stable and durable semiconductor superlattice-based electronics and optoelectronics for extreme environment applications.     

First-principles Study on the Magnetic,Half-metal and Thermoelectric Transport Properties of Inorganic-Organic Hybrid Compounds [C4N2H12][Fe4Ⅲ(HPO3)2(C2O4)3]

The electronic structure, magnetic and half-metal properties of inorganic-organic hybrid compound [C₄N₂H₁₂][Fe₄^Ⅲ(HPO₃)₂(C₂O₄)₃] are investigated by using the full-potential linearized augmented plane wave (FPLAPW) method within density-functional theory (DFT) calculations. The density of states (DOS), the total energy of the cell and the spontaneous magnetic moment of [C₄N₂H₁₂][Fe₄^Ⅱ(HPO₃)₂(C₂O₄)₃] are calculated. The calculation results reveal that the low-temperature phase of [C₄N₂H₁₂][Fe₄^Ⅲ(HPO₃)₂(C₂O₄)₃] exhibits a stable ferromagnetic (FM) ground state, and we find that this organic compound is a half-metal in FM state. In addition, we have calculated antiferromagnetically coupled interactions, revealing the existence of antiferromagnetic (AFM), which is in agreement with the experiment. We have also found that [C₄N₂H₁₂][Fe₄^Ⅲ(HPO₃)₂(C₂O₄)₃] is a semiconductor in the AFM state with a band gap of about 0.40 eV. Subsequently, the transport properties for potential thermoelectric applications have been studied in detail based on the Boltzmann transport theory.     

The electronic structure of Si/GaP(110) interface and superlattice

It is argued that the (110) interface between group IV and III–V semiconductors are more likely to lend themselves to fabrication via MBE techniques. Results for the electronic structure of Si-GaP(110) superlattice are reported for the first time. Both, interface states and two dimensionally confined states are found. Sensitivity of the energy and charge distribution of these states to the interfacial geometry and the band edge discontinuity is investigated. The results are contrasted with the situation found for lattice matched III–V compound semiconductor systems such as GaAs/AlAs and GaSb/InAs.     

Anisotropy of long-wavelength optical phonons in GaAs/AlAs superlattices

The angular anisotropy of optical phonons in GaAs/AlAs (001) superlattices is investigated by Raman scattering spectroscopy. Scattering configurations allowed for phonons with wave vectors oriented along the superlattice layers and normally to them are used. For phonons localized in GaAs layers, the theoretically predicted mixing of the LO1 longitudinal modes with TO1 transverse modes in which atomic displacements occur along the normal to the superlattice is observed experimentally. These modes possess noticeable angular anisotropy. For transverse modes in which atoms move in the plane of the superlattice, the angular anisotropy is small.     

Photoluminescence studies of GaAs/AlAs short period superlattices

Low-temperature pressure-dependent photoluminescence measurements on short-period GaAs/AlAs superlattice structures are presented. Measurements show that the lowest energy conduction-band states are in the AlAs layers and the highest energy valence-band states are located in the GaAs layers. This result is supported by the following three experimental observations: (1) the observed pressure coefficient for the conduction-band to valence-band transition energy is negative, (2) the magnetic mass of this transition is “heavy”, and (3) the band-to-band absorption coefficient appears to be small. These experimental observations are in agreement with predictions of tight-binding calculations.