Electronic properties of InAs/GaAs nanowire superlattices

The electronic properties of strained InAs/GaAs nanowire superlattices are computed using a semi-empirical sp³d⁵s^* tight-binding model, taking strains, piezoelectric fields and image charge effects into account. Strain relaxation appears to be efficient in nanowire heterostructures, but is highly inhomogeneous in thin InAs layers. It digs a well in the conduction band that traps the electrons at the surface of the nanowires. This likely decreases the oscillator strength and might ease the capture of the electrons by nearby surface defects.     

Correlation between Si diffusion and Si-induced disordering in AlGaAs/GaAs superlattices

Compositional disordering of AlGaAs superlattices induced by Si ion implantation and subsequent annealing has been studied by secondary ion mass spectrometry (SIMS). Distinct correlation is found between the induced disordering and the rapid Si diffusion which occurs above the critical concentration of about 3×10¹⁸ cm⁻³. The annealing-condition dependence of the disordering suggests that AlGa intermixing is induced by the vacancy flow enhanced by the Si_IIISi_V pair movement which causes the rapid Si diffusion. SIMS depth profiles of the heat treated superlattices co-doped with Si and Be do not show any appreciable Si diffusion and induced disordering. This is well-explained by the formation of SiBe pairs which prevents that of Si_IIISi_V pairs.     

Optical properties of Si/CaF2 superlattices

We present a first-principle theoretical study of the dielectric functions of Si/CaF₂ superlattices. In particular, we investigate how the optical response depends on the thickness of the Si layers. Our results show that for very thin Si slabs (well width less than 20 Å) optical excitation peaks are present in the visible range. These peaks are related to strong transitions between localized states. Moreover, the static dielectric costant is considerably reduced. From the comparison made with recent experimental data on similar systems we conclude that the quantum confinement, a good surface passivation and the presence of localized states are the key ingredients in order to have photoluminescence in confined silicon based systems.     

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.     

Cyclotron masses in InGaAs/GaAs superlattices and InGaAs/AlAs superlattices

Cyclotron resonance (CR) measurements have been carried out to evaluate the effective mass of electron in (InGaAs)_n/(GaAs)_nsuperlattices (SLs) and (InGaAs)_n/(AlAs)_nSLs. To clarify the dependence of cyclotron mass on the monolayer numbern , we measured CR signals using pulsed high-magnetic fields up to 150 T and a far-infrared laser. We found clear cyclotron resonances in the transmission of 10.6 μ m at 75 T at room temperature in (InGaAs)_n/(GaAs)_nSLs and little dependence on the monolayer number n in the SLs. However, for (InGaAs)_n/(AlAs)_nSLs, a large dependence of cyclotron mass on the monolayer number n was observed. We consider that these dependencies are related to the difference between the barrier height in the SLs and the influence of nonparabolicity on the conduction subbands in the SLs.     

硅基超晶格Si1-xSnx/Si的能带结构

由于Si基发光材料能与现有的Si微电子工艺兼容,其应用前景被广泛看好. 设计具有直接带隙的Si基材料,备受实验和理论研究者的关注. 本文根据芯态效应、电负性差效应和对称性效应设计了Si基超晶格Si_1－xSn_x/Si. 其中Si_0.875Sn_0.125/Si为直接带隙材料. 在密度泛函框架内,采用平面波赝势法计算表明,Si_0.875Sn_{0.125相似文献}   

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)     

Phonon-plasmon interaction in tunneling GaAs/AlAs superlattices

The phonon-plasmon interaction in tunneling GaAs _n /AlAs _m superlattices (m=5and 6≥n≥0.6 monolayers) was studied by Raman scattering spectroscopy. The interaction of optical phonons localized in GaAs and AlAs layers with quasi-three-dimensional plasmons strengthens as the thickness of GaAs quantum wells decreases and the electronic states in the superlattices become delocalized due to tunneling. It is assumed that the plasmons also interact with the TO-like phonon modes localized in quantum islands or in thin ruffled layers.     

Managing disorder in random GaAs/AlGaAs superlattices

The artificial random Gaussian-type potential built in the GaAs/AlGaAs superlattices grown by molecular beam epitaxy was explored by various methods. The effect of the intentional disorder was shown to dominate intrinsic superlattice imperfections and its impact on the electronic properties was found to be in good agreement with the theoretical predictions. It was demonstrated that the modern state of the molecular beam epitaxy allows for a growth of the superstructured materials with well-defined disorder strength.