含δ势垒的铁磁/半导体/铁磁异质结中的自旋输运和渡越时间

研究了含δ势垒的铁磁/半导体/铁磁异质结中自旋相关的透射概率和渡越时间,讨论了量子尺寸效应和Rashba自旋轨道耦合效应对隧穿特性的影响.研究结果表明：δ势垒的存在降低了自旋电子的透射概率,改变了透射概率的位相.Rashba自旋轨道耦合强度的增加加大了透射概率的振荡频率.不同自旋取向的电子隧穿异质结时,渡越时间随着半导体长度、Rashba自旋轨道耦合强度以及两铁磁电极中的磁化方向的夹角的变化而变化. 关键词： δ势垒')" href="#">δ势垒 铁磁/半导体/铁磁异质结 Rashba自旋轨道耦合效应 渡越时间     

Discrete tunneling of holes in porous silicon

It is pointed out that in the partial oxidation of porous silicon (PS) formed on heavily doped crystals, the topology of the pores can result in the formation of an anisotropic material with strings of nanometersized silicon granules embedded in insulating silicon dioxide SiO₂. In this range of granule sizes the correlation effects in the tunneling of electrons (holes) are strong on account of their Coulomb interaction. This should be manifested as discrete electron and hole tunneling at temperatures comparable to room temperature. The room-temperature current-voltage characteristics of n ⁺-PSp ⁺-p ⁺ diode structures with a PS interlayer on p ⁺-Si, which exhibit current steps on the forward and reverse branches, are presented. The current steps are attributed to discrete hole tunneling along the silicon strings in SiO₂. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 794–797 (25 May 1998)     

Electron-beam assisted growth of hexagonal boron-nitride layer

It has been known that a good quality h-BN layer can only be grown within a narrow temperature window of 1020–1100 K on a copper substrate. We found that the growth temperature window on Cu(111) surface could be lowered up to 100 K by ionizing and/or exciting borazine precursor gas with an electron-beam. The structures of a hexagonal boron nitride (h-BN) layers grown at various substrate temperatures on a Cu(111) were examined using scanning tunneling microscopy. We found that the grown h-BN film exhibits highly inert behavior with wide bandgap semiconductor characteristics.     

Properties of magnesium silicate doped with chromium in porous silicon

Forsterite doped with Cr⁴⁺ ions is prepared in silicon-based structures according to a simple technique. These structures are of interest due to the characteristic luminescence in the near-IR range. Forsterite is synthesized by impregnation of porous silicon layers on n⁺-Si and p⁺-Si substrates with subsequent annealing in air. A photoluminescence response at a wavelength of 1.15 μm is observed at room temperature in porous silicon layers doped with magnesium and chromium for which the optimum annealing temperature is close to 700°C. The photoluminescence spectrum of porous silicon on the p⁺-Si substrate contains a broad band at a wavelength of approximately 1.2 μm. This band does not depend on the annealing temperature and the magnesium and chromium content and is most likely associated with the presence of dislocations in silicon. The experimental EPR data and eletrical properties of the structures are discussed. It is found that layers of pure porous silicon and chromium-doped porous silicon on n⁺-Si subtructures exhibit indications of discrete electron tunneling.     

Theoretical aspects of electron transport in modulated structures

In the theory of transport in modulated structures we have studied both transport perpendicular and parallel to the heterojunction interfaces. In perpendicular transport we have investigated models for tunneling through double barriers and find that resonant tunneling and sequential tunneling lead to the same expression for the current as long as the width of the energy distribution of the injected electrons is larger than the width of the resonant level in the diode. We present results for phonon assisted tunneling between two wells in a model which remains valid even when the barrier shrinks and the tunneling probability becomes very high. In parallel transport we show that very satisfactory agreement with extensive measurements of the mobility in modulation doped structures in the whole temperature range from 4 K to 300 K can be obtained if one takes into account the complete quasi-two-dimensional subband structure and all the relevant scattering mechanisms. Having established this we apply this program to systems with more complicated double channel structures, and show how one can tailor the conductivity of a channel in which perpendicular resonant tunneling affects parallel transport.     

Electronic and optical properties of CuGaS2: First-principles calculations

Electronic structure and optical properties of CuGaS₂ are calculated using the full potential linearized augmented plane wave plus local orbitals method. The calculated equilibrium lattice is in reasonable agreement with the experimental data. The electronic structures indicate that CuGaS₂ is a semiconductor with a direct bandgap of 0.81802 eV. Furthermore, other experiments and theory also show that this material has a direct bandgap. It is noted that there is quite strong hybridization between Ga 3d and S 3s orbitals, which belongs to the (GaS₂)⁻. The complex dielectric functions are calculated, which are in good agreement with the available experimental results.     

Tunneling conductivity oscillations in a magnetic field in metal-insulator-narrow-gap-HgCdTe structures: The energy spectrum and spin-orbit splitting of 2D states

We study tunneling conductivity oscillations in a magnetic field in narrow-gap p-HgCdTe-oxide-metal (Yb, Al) structures. In tunnel structures with Yb we detect two types of tunneling conductivity oscillations. The first is related to the crossing of the Landau levels of two-dimensional (2D) states localized in the surface quantum well of the semiconductor, and has an energy E _F+eV, where E _F is the Fermi energy of the semiconductor and V is the bias voltage; the second has an energy E _F. We find that in such structures with an asymmetric quantum well there is strong spin-orbit splitting in the spectrum of the 2D states. In p-HgCdTe-oxide-Al tunnel structures the surface potential is much weaker and only oscillations of the first type are observed. We find that in such structures there is only one spin state of the 2D carriers, while the second is pushed into the continuous spectrum because of strong spin-orbit coupling. To analyze the experimental results we calculate the spectrum of 2D states localized in the surface quantum well in a semiconductor with a Kane dispersion law. We find that all the experimental results are in good agreement with the results of calculations. Finally, we discuss the features of “kinematically coupled” states in an asymmetric quantum well. Zh. éksp. Teor. Fiz. 112, 537–550 (August 1997)     

Vertical electronic transport in novel semiconductor heterojunction structures

The investigation of vertical transport in semiconductor heterojunction systems has recently undergone a renaissance due to improved epitaxial techniques in a number of material systems. By using resonant tunneling, we can perform electronic spectroscopy not only of the double barrier structure itself, but of any system (with quantized well states) suitably coupled to a resonant tunneling spectrometer. In designing such systems, an important degree of freedom is introduced by utilizing multi-component structures; for example, a GaAs contact — AlGaAs barrier — InGaAs quantum well. In this structure, the high electron affinity of the quantum well creates a “deep” quantum well, in which we demonstrate that quantum well states can be hidden from transport. Finally, we present results from microfabricated quantum well structures (“quantum dots”) which are sufficiently small in the lateral dimension to introduce size effects. Telegraph noise due to the lateral size of these structures has been observed, and the first indications of lateral quantization in all three dimensions in a semiconductor quantum well are presented.     

Resonant tunneling of electrons through virtual interference states formed as a result of reflection from the boundary of strongly doped region of GaAs

The paper is devoted to analysis of the electron transport through one-barrier GaAs/AlAs/GaAs heterostructures. The oscillating component of transport characteristics of symmetric one-barrier GaAs/AlAs/GaAs heterostructures with spacers, which is associated with resonance tunneling of electrons via virtual states formed in the spacer region of the structures due to reflection of electrons from the n^?-GaAs/n⁺-GaAs interface and their subsequent interference. It is shown that electrons are predominantly reflected coherently from the boundary of the strongly doped region as in the case of one-dimensional averaged potential of randomly arranged (beginning from this boundary) impurities. It is shown that low-energy virtual resonances are suppressed due to electron scattering as a result of their interaction with longitudinal optical (LO) phonons in the spacer region.     

Inelastic resonant tunneling

A general expression for the resonant contribution to a tunneling current has been obtained and analyzed in the tunneling Hamiltonian approximation. Two types of resonant tunneling structures are considered: structures with a random impurity distribution and double-barrier structures, where the resonant level results from size quantization. The effect of temperature on the current-voltage curves of tunneling structures is discussed. The study of the effect of potential barrier profile on the d ² I/dV ² line shape is of interest for experiments in inelastic tunneling spectroscopy. Various experimental situations where the inelastic component of the tunneling current can become comparable to the elastic one are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1151–1155 (June 1998)     

Scanning tunneling microscopy with atomic resolution on ReS2 single crystals grown by vapor phase transport

Atomic resolution images of layered transition metal-dichalcogenide ReS₂ single-crystals (n-type semiconductor) were obtained using a scanning tunneling microscope with a positive tip. In most cases only unresolved clusters of four rhenium atoms could be seen. Occasional images with higher resolution showed that these bright structures consist of four separated atoms. The symmetry of the imaged atoms is identical to that of the rhenium sublattice but not to that of the sulfur atoms. We conclude therefore that the main contribution to the tunneling current is due to the rhenium atoms, although the sulfur atoms are placed by about 0.15 nm closer to the tip. Thus for our positive bias of the tip the tunneling electrons originate from occupied rhenium states in the valence band of the semiconductor.     

On the motional states of dipolar impurities in ionic matrices

Abstract

The effect of lattice vibrations on the librational frequencies and tunneling frequencies is considered for the case of alkali halides doped with diatomic impurities. It has been shown that this effect is important for the tunneling frequency while the librational frequency changes only slightly. The implications of this have been discussed in light of certain anomalous results observed in KCl-OH^? and KCl-CN-.     

CdxHg(1−x)Te Alloy Colloidal Quantum Dots: Tuning Optical Properties from the Visible to Near‐Infrared by Ion Exchange

The energy gap between valence and conduction levels in colloidal semiconductor quantum dots can be tuned via the nanoparticle diameter when this is comparable to or less than the Bohr radius. In materials such as cadmium mercury telluride, which readily forms a single phase ternary alloy, this quantum confinement tuning can also be augmented by compositional tuning, which brings a further degree of freedom in the bandgap engineering. Here it is shown that compositional control of 2.3 nm diameter Cd_xHg_(1?x)Te nanocrystals by exchange of Hg²⁺ in place of Cd²⁺ ions can be used to tune their optical properties across a technologically useful range, from 500 nm to almost 1200 nm. Data on composition‐dependent changes in the optical properties are provided, including bandgap, extinction coefficient, emission energy and spectral shape, Stokes shift, quantum efficiency, and radiative lifetimes as the exchange process occurs, which are highly relevant for those seeking to use these technologically important QD materials.     

The C-V characteristics of TiO2/p-Si/Ag,GNR doped TiO2/p-Si/Ag and MWCNT doped TiO2/p-Si/Ag heterojunction devices

The TiO₂/p-Si/Ag, graphene (GNR) doped TiO₂/p-Si/Ag and multi-walled carbon nanotube (MWCNT) doped TiO₂/p-Si/Ag heterojunction devices were fabricated by electrospinning technique at same conditions. Their structural, morphological properties, thermal analyses (TGA), and capacitance voltage characteristics were studied and compared. The undoped, GNR and MWCNT doped TiO₂ structures obtained successfully according to XRD measurements. Morphological properties of the undoped, GNR and MWCNT doped TiO₂ composite structures have rod or ribbon like structures. The TGA result confirmed the GNR and MWCNT doped TiO₂ structures. The C-V and G-V measurements were employed for electrical characterization of the TiO₂/p-Si/Ag, GNR doped TiO₂/p-Si/Ag and MWCNT doped TiO₂/p-Si/Ag devices for various frequencies at room temperatures. The results imparted that the capacitance and conductance behaviors of all devices are strong functions of the frequency and voltage. The electrical parameters were calculated from C⁻²-V plots of the heterojunction devices and compared for three devices. The transient photocapacitance plots revealed that the devices can be employed for optical communication applications.     

Theoretical investigation on the passivation layer with linearly graded bandgap for the amorphous/crystalline silicon heterojunction solar cell

Passivation layer with linearly graded bandgap (LGB) was proposed to improve the performance of amorphous/crystalline silicon heterojunction (SHJ) solar cell by eliminating the large abrupt energy band uncontinuity at the a‐Si:H/c‐Si interface. Theoretical investigation on the a‐Si:H(p)/the LGB passivation layer(i)/c‐Si(n)/a‐Si:H(i)/a‐Si:H(n⁺) solar cell via AFORS‐HET simulation show that such LGB passivation layer could improve the solar cell efficiency (η) by enhancing the fill factor (FF) greatly, especially when the a‐Si:H(p) emitter was not efficiently doped and the passivation layer was relatively thick. But gap defects in the LGB passivation layer could make the improvement discounted due to the open‐circuit voltage (V_OC) decrease induced by recombination. To overcome this, it was quite effective to keep the gap defects away from the middle of the bandgap by widening the minimum bandgap of the LGB passivation layer to be a little larger than that of the c‐Si base. The underlying mechanisms were analysed in detail. How to achieve the LGB passivation layer experimentally was also discussed. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Optical spectroscopy of lanthanides doped in wide band-gap semiconductor nanocrystals

Currently, tripositive lanthanide (Ln³⁺) ions doped wide band-gap semiconductor nanocrystals (NCs) have been the focus of research interest due to their distinct optical properties and potential applications in optical devices and luminescent biolabels. Because of the low absorptions of parity-forbidden 4f-4f transitions for Ln³⁺, it is highly anticipated that the luminescence of Ln³⁺ ions embedded in wide band-gap NC lattices can be sensitized efficiently via exciton recombination in the host. For this purpose, the successful incorporation of Ln³⁺ into the lattices of semiconductor NCs is of utmost importance, which still remains intractable via conventional wet chemical methods. Here, the most recent progress in the optical spectroscopy of Ln³⁺ ions doped wide band-gap semiconductor NCs is discussed. Much attention was focused on the optical properties including electronic structures, luminescence dynamics, energy transfer as well as the up-conversion emissions of Ln³⁺ ions in ZnO, TiO₂, SnO₂ and In₂O₃ NCs that were synthesized in our laboratory using wet chemical methods.     

Near infrared transparent conducting CdO nanocrystallite films codoped with boron and hydrogen

Thin films of boron and hydrogen-codoped CdO (B&H-codoped CdO) oxide with different boron content have been prepared on glass and silicon substrates. The effects of codoping on the structural, electrical, and optical properties of the host CdO films were systematically studied. The structural study shows that doped boron ions occupied locations in interstitial positions and/or Cd²⁺-ion vacancies of CdO lattice. The bandgap of B-doped CdO shrinks by 25-38% compare to hydrogenated CdO. Such bandgap narrowing was phenomenological studied in the framework of the available models. The electrical behaviours show that all the prepared B&H-doped CdO films are degenerate semiconductors. However, the hydrogenated 7.9% boron doped CdO has resistivity of 1.52 × 10⁻⁴ Ω cm, mobility of 47.5 cm²/V s, and carrier concentration of 8.6 × 10²⁰ cm⁻³. The optoelectronic measurements in visible and NIR spectral range demonstrate the utility of the oxide/p-Si heterojunction in photodetection applications.     

铁磁/半导体/铁磁结构的双量子环自旋输运的特性

研究了与铁磁/半导体/铁磁结构相关的双量子环自旋输运的规律,研究结果表明：总磁通为零条件下,铁磁电极磁化方向反平行时,双量子环与单量子环相比提高了自旋电子透射概率的平均值.铁磁电极磁化方向平行时,双量子环对提高自旋向下电子平均透射概率的效果更明显;双量子环受到Rashba自旋轨道耦合作用影响时,自旋电子的平均透射概率明显高于单量子环,即使再加上外加磁场的影响,透射概率较高这一特征依然存在;双量子环所含的δ势垒具有阻碍自旋电子输运的作用,随δ势垒强度Z的增大透射概率 关键词： 双量子环 Rashba自旋轨道耦合 透射概率 δ势垒')" href="#">δ势垒     

Geometrical freedom for constructing variable size photonic bandgap structures

In order to study the design flexibility of photonic bandgap structures, we investigate different examples of 1D traditional Bragg layers and 2D photonic crystals. We have also considered a simple case of 3D woodpile structures. It turns out that in systems with large gaps, the evanescent waves penetrate into the bulk only distances comparable to one lattice constant. Therefore confinement of light can also be achieved without long range order, which leads to the introduction of novel photonic bandgap designs. Adhering to some constraints, the changes in the photonic bandgap in disordered structures are negligible. The important quantity to characterize the presence or absence of modes is the local photonic density of states, however bandgap phenomena in size and position disordered arrangements can also be verified with plane wave supercell calculations as well as finite difference time domain techniques.     

掺稀土元素(Y，La)的γ-Si3N4的电子结构和光学性质

采用基于密度泛函的平面赝势方法(PWP)和广义梯度近似(GGA)，计算了未掺杂和掺杂稀土(Y，La)的γ-Si₃N₄中N-Y(La)键的布居值和它们的键长、掺杂后能带结构和态密度.发现掺杂后的带隙要减小，并且可能形成新的半导体，这将为找到新的半导体提供一个方向.还进一步研究了掺杂稀土(Y，La)后的光学性质，掺杂后有更高的静态介电常数，可以作为新的介电材料和好的折射材料，这对于一定的光学元件有潜在的应用前景.