GaN-based heterostructures: electric--static equilibrium and boundary conditions

In the GaN-based heterostructures, this paper reports that the strong electric fields induced by polarization effects at the structure boundaries complicate the electric--static equilibrium and the boundary conditions. The basic requirements of electric--static equilibrium for the heterostructure systems are discussed first, and it is deduced that in the application of the coupled Schr\"{o}dinger--Poisson model to the heterostructures of electric--static equilibrium state, zero external electric field guarantees the overall electric neutrality, and there is no need to introduce the charge balance equation. Then the relation between the screening of the polar charges in GaN-based heterostructures and the possible boundary conditions of the Poisson equation is analysed, it is shown that the various boundary conditions are equivalent to each other, and the surface charge, which can be used in studying the screening of the polar charges, can be precisely solved even if only the conduction band energy is correctly known at the surface. Finally, through the calculations on an AlGaN/GaN heterostructure with typical structure parameters by the coupled Schr\"{o}dinger--Poisson model under the various boundary conditions, the correctness of the above analyses are validated.     

The asymmetry of the tunneling time in type II semiconductor heterostructures

The tunneling time asymmetry in type II semiconductor heterostructures is related to the phase difference of the reflection coefficients for the two tunneling directions. Analytical expressions and numerical simulations are given for the difference between the left-to-right and right-to-left tunneling times in asymmetric, single and multiple barrier type II heterostructures.     

Structural characterisation of GaAlN/GaN HEMT heterostructures

(GaN/GaAlN/GaN)//Al₂O₃(00.1) HEMT heterostructures have been studied by X-ray scattering techniques, transmission electron microscopy and atomic force microscopy. X-ray reflectometry has been used to determine with a high accuracy both the individual layer thicknesses and the interfacial roughness, in spite of the weak electronic density contrast between layers. From the Fourier inversion method and using a simulation software, the roughness of the interface corresponding to the two-dimensional electron gas location has been determined equal to 0.5 nm. Both high resolution X-ray diffraction and transmission electron microscopy experiments have shown the excellent crystallinity of the heterostructures. Finally, the surface morphology has been inferred using atomic force microscopy experiments.     

Epitaxial growth and characterization of layered magnetic nanostructures

We describe the construction and operation of an ultrahigh-vacuum system devoted to the study of layered magnetic nanostructures. The apparatus includes two growth chambers, where specimens nanostructured along the direction of growth (heterostructures, nanometric and subnanometric thin films and multilayers) are deposited either by molecular beam epitaxy or pulsed laser deposition, and a measurement chamber, where they are analyzed in situ by a variety of electron spectroscopies. Magnetic characterization is obtained by spin resolved inverse photoemission spectroscopy and magneto optical Kerr effect technique. Vacuum transfer towards other experimental facilities is also available. As examples of application, results from half metallic magnetic oxides, such as magnetite (Fe₃O₄) and manganite (La_2/3Sr_1/3MnO₃) thin films, and ferromagnet/semiconductor interfaces (Fe/Ge(0 0 1)) are also reported.     

Magnetotransport properties of two-dimensional electron gas in AlGaN/AlN/GaN heterostructures

Magnetotransport measurements are carried out on the AlGaN/AlN/GaN in an SiC heterostructure, which demonstrates the existence of the high-quality two-dimensional electron gas (2DGE) at the AlN/GaN interface. While the carrier concentration reaches 1.32 × 10¹³ cm ^{- 2} and stays relatively unchanged with the decreasing temperature, the mobility of the 2DEG increases to 1.21 × 10⁴ cm²/(V·s) at 2 K. The Shubnikov—de Haas (SdH) oscillations are observed in a magnetic field as low as 2.5 T at 2 K. By the measurements and the analyses of the temperature-dependent SdH oscillations, the effective mass of the 2DEG is determined. The ratio of the transport lifetime to the quantum scattering time is 9 in our sample, indicating that small-angle scattering is predominant.     

钙钛矿氧化物异质结的研究进展

在过去几年中,为了研究钙钛矿氧化物异质结的物性和潜在的应用前景,我们进行了许多探索。在本文中,我们总结了对钙钛矿氧化物异质结一些物理性质的理论描述,揭示了La0.9Sr0.1MnO3/SrNb0.01Ti0.99O3异质结中发现的正磁电阻效应和横向光电效应的物理起源。我们提出的理论模型将有助于理解界面对钙钛矿氧化物异质结各种性质的影响。     

The photonic analogue of the graded heterostructure: Analysis using the envelope approximation

We develop and deploy an envelope function formalism in order to study graded photonic crystals – periodic photonic media whose features vary continuously with position. We use the method to reduce the properties of the uniform crystals to the spectral position of their band edges and an effective mass-like term. These are employed within the Schrödinger-like equation derived herein to determine the behaviour of light inside a specific graded photonic crystal created using depth-dependent infiltration of an artificial opal.     

Tailoring the spin density waves in Fe/Cr multilayers by selective inclusion of Sn, V and Mn

The possibility of tailoring the spin density waves in Fe/Cr(0 0 1) multilayers through the selective inclusion of Sn, V and Mn monolayers is investigated with the density functional tight-binding linear muffin-tin orbital method in the generalized gradient approximation of the exchange and correlation potential. Despite the non-magnetic character of Sn and V when substituting Cr atoms located at the nodes, the modifications induced on the spin density waves are important due to the strong hybridization. In general we find that V modifies drastically the global features of the spin density waves leading to the onset of a magnetically dead region in the Cr spacer whereas Mn inserted at the nodes rather destroys the density wave working in favor of stabilizing the layered antiferromagnetic structure. The trends obtained are consistent with experimental data when available. Since both the magnetic profile and the position of the nodes at the spacer can be modified, the present results are relevant in the context of the spin-dependent transport through magnetic multilayers in which the magnetoresistance will vary if the scattering regions across the transport direction are modified.