Atomic ordering in GaAsSb (0 0 1) grown by metalorganic vapor phase epitaxy

Epitaxial GaAsSb (0 0 1) semiconductor alloys grown by metalorganic vapor phase epitaxy exhibit several spontaneously ordered structures. A superlattice structure with three-fold ordering in the [1 1 0] direction has been previously observed by different groups. CuAu structures with (1 0 0) and (0 1 0) ordering planes have also been reported. The physical origin of CuAu ordering in III–V semiconductors has not yet been explained. In this work we report the effect of growth conditions on CuAu ordering in GaAsSb, including miscut from (0 0 1), growth rate, bismuth surfactant concentration, and growth temperature. These data point to a surface kinetic mechanism not based on dimer strain, but possibly due to one-dimensional ordering at step edges.     

Generation mechanism of CuAu-I type ordered structures in InGaAs crystals grown on (110) InP substrates by molecular beam epitaxy

The generation mechanism for CuAu-I type ordered structures in InGaAs grown on (110) InP substrates by molecular beam epitaxy is discussed. On the basis of previous work together with considerations of the atomic arrangement of the ordered structure and surface reconstruction on the (110) plane, we propose four possible models for the ordering. Through precise evaluation of these models, two models are selected as the most probable ones: these involve formation of the ordered structures on surfaces dominated by two monolayer steps. This model have been experimentally proven by the analyses of electron diffraction patterns from different crystals grown on different growth surfaces.     

Correlation of ordering formation and surface structure in (GaIn)P using modulated MOVPE

Modulated metalorganic vapour phase epitaxial growth (MOVPE) is used to clarify the role of the surface conditions on the ordering behaviour in ternary (GaIn)P layers. The alternating deposition of GaP and InP layers with individual thicknesses of up to one monolayer is successfully used for the growth of (GaIn)P bulk layers lattice matched to (100) GaAs substrates with various off-orientations. The layer quality and the degree of ordering are investigated using high-resolution X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence spectroscopy (PL), respectively. The application of modulated growth conditions for the deposition of (GaIn)P bulk layers has a strong influence on the degree of ordering achieved in the intermediate growth temperature regime where the highest degree of ordering occurs under continuous MOVPE. Beside a new boundary structure observed in layers grown under modulated flux conditions, the successful growth of highly ordered (GaIn)P layers grown using the modulated MOVPE technique support the model that up to 2 monolayers of the (GaIn)P growth surface are involved in the ordering formation process.     

无序激光晶体及其超快激光研究进展

激光是受激辐射的光放大,所辐射的波长取决于增益介质中关键电子的能级结构,特别是其最外层电子的状态决定了可能实现的激光特性.激光发展60年来,激光晶体作为激光的重要激活材料,推动了激光技术的进步和普及,是一个研究历史长而又异常活跃的研究领域.当前,超短超强脉冲激光在加工、医疗、国防等关系国计民生的领域有重要需求,适合超短...     

Microscopic modeling of the optical properties of semiconductor nanostructures

A brief overview of a consistent microscopic approach to model the optical and electronic properties of semiconductor nanostructures is presented. Coupled semiconductor Bloch and Maxwell equations are used to investigate the performance of semiconductor microcavity structures, photonic band gap systems, and lasers. The predictive potential of the microscopic theory is demonstrated for several examples of practical importance. Optical gain and output characteristics are computed for modern vertical external cavity surface emitting laser structures. It is shown how design flexibilities can be used to optimize the device performance. Nanostructures are proposed where semiconductor quantum wells are embedded in one-dimensional photonic crystals. For field modes spectrally below the photonic band edge it is shown that the optical gain and absorption can be enhanced by more than one order of magnitude over the value of the homogeneous medium. The increased gain can be used for laser action by placing quantum wells and a suitably designed photonic crystal structure inside a microcavity.     

Spatially and spectrally resolved measurement of optical loss in InGaN laser structures

The optical loss co-efficient in InGaN laser diodes, emitting at 410 nm, has been measured. The measurement technique is based on the transmission of internally generated spontaneous emission through varying lengths of the laser waveguide. It is unique in that it provides spectral and spatial information on the optical loss. The lasers studied are typical of InGaN structures showing a high degree of waveguide loss, _i=40cm⁻¹. The measurements also show clear evidence of higher order transverse modes in the direction perpendicular to the growth plane with resonant leakage of the optical field into the outer layers of the structure. This produces a modulation in the loss of these modes.     

Influence of the growth temperature and substrate orientation on the layer properties of MOVPE-growth (Ga,In)(As,P)/GaAs

The luminescence properties of In_1−xGa_xAs_yP_1−y layers and heterostructures grown lattice matched to GaAs by metalorganic vapour-phase epitaxy (MOVPE) were studied and correlated to the crystalline properties. For laser structures emitting around 800 nm a red-shift of the emission from the active layer (y = 0.72) grown at 680°C together with an anomalous temperature behaviour and excitation dependence of the bandgap is observed. Although some degree of ordering is observed for thick layers of this composition, polarization dependent photoluminescence does not indicate ordering of the quantum well to be the main reason for this excitation dependence. Instead, interfacial In-rich layers are found to be responsible. The thickness of these interfacial layers strongly depends on substrate misorientation and growth conditions.     

Combined atomic ordering in the A and B sublattices of perovskite structure

The simultaneous ordering of cations in the A and B sublattices of cubic perovskite structure has been investigated by the group-theoretical method. It is found that 147 ordered phases may exist. Among them, there may be 121 phases with simultaneous cation ordering in the 1(a)- and 1(c) positions of perovskite structure. It is shown that 53 phases are described by improper order parameters related to cation ordering, the formation of 56 phases is related to improper rotations of octahedra, and 15 phases are improper ferroelectrics. Calculated structures of some types of ordered low-symmetry perovskite modifications are presented.     

Group-theoretical study of cationic ordering in perovskite structure

Atomic ordering in the structure of cubic perovskite has been investigated by the group-theoretical method. The possibility of existence of 13 ordered phases in the 1(a) position (including four binary, two ternary, and five quaternary cationic superstructures) and 13 phases with different types of ordering in the 1(b) position (including four binary, two ternary, and five quaternary cationic superstructures) is established. Calculated structures of some types of ordered low-symmetry perovskite modifications are reported.     

Structure and properties of amorphous thin film for optical data storage

In this paper the magnetic and magneto-optical properties of amorphous rare earth-transition metal (RE-TM) alloys as well as the magnetic coupling in the multi-layer thin films for high density optical data storage are presented. Using magnetic effect in scanning tunneling microscopy the clusters structure of amorphous RE-TM thin films has been observed and the perpendicular magnetic anisotropy in amorphous RE-TM thin films has been interpreted. Experimental results of quick phase transformation under short pulse laser irradiation of amorphous semiconductor and metallic alloy thin films for phase change optical recording are reported. A step-by-step phase transformation process through metastable states has been observed. The waveform of crystallization propagation in micro-size spot during laser recording in amorphous semiconductor thin films is characterized and quick recording and erasing mechanism for optical data storage with high performance are discussed. The nonlinear optical effects in amorphous alloy thin films have been studied. By photo-thermal effect or third order optical nonlinearity, the optical self-focusing is observed in amorphous mask thin films. The application of amorphous thin films with super-resolution near field structure for high-density optical data storage is performed.     

Effect of inhomogeneity on the formation of regular domain structures in LiNbO3 crystal

The conditions necessary for the formation of regular domain structures in lithium niobate crystals used for quasi-synchronous non-linear optical transformations of laser radiation have been considered. The analysis of the mechanism and the kinetics of domain formation during thermoelectric repolarization showed that the true thickness of domain boundaries characterized by the combination of the faces of some simple forms of a crystal depends on both angles between the directions of the vectors of the temperature gradient and the spontaneous-polarization and on the homogeneity of the initial crystal.     

实现受激光发射探索Si基激光器

本文首先评论了近5年来各类Si基纳米材料在光增益和受激光发射特性研究方面所取得的最新进展.进而指出,晶粒有序的小尺寸和高密度纳米晶Si(nc-Si),具有载流子三维量子受限的局域化纳米结构和具有高激活浓度Er掺杂的nc-Si:Er/SiO2纳米薄膜,将是实现Si基激光器的主要有源区材料.最后,对这一领域的今后发展趋势进行了初步展望.预计在今后的3～5年内,实现Si基激光器的探索性研究高潮即将到来,并极有可能获得重大突破性进展,即在进一步提高发光效率的基础上,实现稳定可靠的光增益和受激光发射特性.而后再用3～5年的时间,通过优化结构形式与工艺技术,研制出具有器件实用化水平的Si基激光器.     

Strained AlGaInP quantum wire lasers

We report on the fabrication of strained separate-confinement heterostructure AlGaInP visible laser diodes with multiple quantum wire active regions formed in situ during gas source molecular beam epitaxy. Lateral composition modulation with a period on the order of 100 å perpendicular to the growth direction occurs spontaneously during the growth of the (GaP)₂/(InP)₂ short-period superlattice active region by the strain-induced lateral-layer ordering (SILO) process. Individual laser diodes fabricated from the same wafer but emitting parallel and normal to the wire axis exhibit highly anisotropic properties, including large difference in threshold current, emission wavelength, and opposite polarizations. These effects are explained in terms of the strained quantum wire potential. The additional degree of quantum confinement in quantum wire structure reduces the threshold current density by a factor of >2.7 when compared with an otherwise identical quantum well laser.     

Characteristics of GaAs?AlGaAs (SCH) lasers grown by low-temperature LPE technique

In this paper we report the performance and study of GaAs AlGaAs (SCH) laser. The structures were grown by a new variant of the LPE-technique with temperatures regimes lower than 600 °C. The Low Temperature LPE-technique, recently propoused by ANDREEV and coworkers at the IOFFE-Physico-Technical Institute exhibit growth rates of the growing layers as low as that corresponding to MBE or MOCVD techniques. In this way, it is possible to obtain and control layer thicknesses in the structure of the order of tens to hundreds of Ångströms. Laser heterostructures with active layer thickness of 30 nm, were performed and the lowest threshold current density of 437 A/cm² for a cavity length of 1.33 mm were obtained. A typical growth process is described and a SEM structure microphotograph is shown. The dependences of the threshold current density J_th and the differential quantum efficiency η_d with the cavity length L are reported. From these data, other laser parameters are evaluated such as internal optical losses α₁, internal quantum efficiency η_i and the dependence of the maximum of the gain factor against the nominal current density J_nom at threshold. This dependence was compared with the theoretical predictions and a good agreement was found.     

Effects of growth temperature and V/III ratio on surface structure and ordering in Ga0.5In0.5P

Surface photoabsorption (SPA) measurements were used to clarify the CuPt ordering mechanism in Ga_0.5In_0.5P layers grown by organometallic vapor phase epitaxy. The CuPt ordering is known to be strongly affected by the growth temperature and the input partial pressure of the phosphorus precursor, i.e. the V/III ratio. The SPA peak at 400 nm was found to be a measure of the concentration of [ 10]-oriented phosphorus dimers on the surface, which are characteristic of the (2 × 4) reconstruction. Both ordering, measured using the low temperature photoluminescence peak energy, and the SPA signal difference due to P dimers were studied versus the growth temperature and V/III ratio. The degree of order decreases markedly with increasing growth temperature above 620°C at a constant V/III ratio of 40. This corresponds directly to a decrease of the [ 10]-oriented P dimer concentration on the surface determined using SPA. Below 620°C, the degree of order decreases as the growth temperature decreases, even though the concentration of P dimers increases. The presence of an isotropic “excess P” phase observed in the SPA spectrum at 480 nm might be responsible for the decrease of CuPt ordering, although it has previously been attributed to the slow rearrangement of adatoms. The degree of order is found to decrease monotonically with decreasing V/III ratio in the range from 160 to 8 at 670°C and from 40 to 8 at 620°C. This also corresponds directly to the decrease of the P dimer concentration on the surface measured using SPA. At 620°C and a V/III ratio of 160, the degree of order decreased despite an increase of the P dimer concentration. This may also be due to the formation of the isotropic “excess P” phase on the surface. The direct correlation of the [ 10]-oriented P dimer concentration and the degree of order with changes in temperature ( ≥ 620°C) and V/III ratio (≤ 160 at 670°C and ≤ 40 at 620°C) suggests that, in this range of growth parameters, the (2 × 4) surface reconstruction is necessary to form the CuPt structure, in agreement with published theoretical studies.     

Domain structure and dielectric properties of TGS and DTGS single crystals depending on growth conditions

The influence of growth temperature and supersaturation of solution on domain structure and dielectric properties of TGS and DTGS single crystals was investigated. The qualitative and quantitative analysis of domain structures was carried out. The optimal growth conditions were found. It proves to be useful to arrange the measured values in table where lines correspond to growth temperatures and columns – to supersaturations for investigation of optimal growth conditions. The diagonal of extremal values exists in this table. The existence of optimal growth conditions explains in terms of interaction between domain and defect structures.     

Janus型二维磁电材料CrXX’(X/X’=S,Se,Te)第一性原理研究

自石墨烯被发现以来,各种具有新奇特性的二维材料受到了越来越多的关注。Janus型二维材料具有不对称的表面特性,这种特殊的结构往往具有独特的电学、磁学与光学性质,使其成为近年来材料科学领域研究的热点。本文搭建了Janus型结构CrXX’(X/X’=S,Se,Te)(CrSSe, CrSTe, CrSeTe),研究了体系的电学、磁学、光学性质,并探究了双轴应变对其电学、磁学、光学性质的影响。结果表明,CrSSe、CrSTe与CrSeTe均呈现金属性,都是电子的优良导体,三种体系的电子结构对外加应变具有很好的鲁棒性。CrXX’(X/X’=S,Se,Te)具有本征铁磁性,并且通过施加双轴应变可对其磁矩进行调控。此外,三种体系均具有较高的居里温度,特别是CrSTe的居里温度可达310 K。CrXX’(X/X’=S,Se,Te)还具有优异的可见光与紫外光吸收性能,应变可对其光吸收系数进行调控,并且压应变与拉应变可分别使其吸收谱线向短波与长波方向移动。本文的工作为进一步研究二维Janus单层CrXX’(X/X’=S,Se,Te)在新型室温自旋电子器件领域的应用提供了理论支持。     

Femtosecond laser-induced microfeatures in glasses and their applications

In the past decades, femtosecond laser has been widely used to study the interaction between light and matter, and dynamic processes of various reactions. In this article, we review femtosecond laser-induced microfeatures in glasses and their promising applications in micro-optics and optoelectronics. Various highly localized microstructures, e.g. color center defects, refractive index change, micro-void and micro-crack, have been observed in the glasses after the femtosecond laser irradiation. The mechanisms of the observed phenomena are discussed. We also demonstrate the fabrication of various micro-optical components, e.g. three-dimensional optical memory, optical waveguide, micro-grating, micro-lens and fiber attenuator, by using the femtosecond laser-induced structures.     

Synthesis of AlGaAs-based strained separately confined heterostructure laser diodes by low temperature liquid-phase epitaxy

AlGaAs-based strained separately confined heterostructure laser diodes operating in the 800–900 nm wavelength range have been fabricated from the structures grown using low temperature liquid-phase epitaxy (LPE). Addition of P/Sb to the active region introduces tensile/compressive strain in the structure and is found useful in tuning the emission wavelength. Details of the growth experiments, strain measurements, fabrication and characterization of the laser diodes are described.     

Structural study of levitated liquid Y2O3 using neutron scattering

The local structure of liquid Y₂O₃ at 2800 K has been investigated by combining aerodynamic levitation and laser heating with the neutron scattering technique. Despite relatively low counting statistics, we have been able to determine the short-range order in the liquid. In particular, we found Y–O and Y–Y coordination numbers around 6 and 12, confirming our previous study by anomalous X-ray scattering. The unusually sharp main diffraction peak observed with X-ray measurements, and characteristic of a high degree of chemical ordering, is strongly reduced in the neutron structure factor and thus indicates that it is mostly due to Y–Y correlations.