Visible Luminescence of SiGe/Si Quantum Wells Under an External Anisotropic Deformation

We realize for the first time an experimental investigation of the IR and visible luminescence of SiGe/Si heterostructures with quantum well subjected to an external anisotropic deformation. We show that tensile strain along the [100] direction enhances the absolute and relative intensities of visible luminescence by a factor of 7/3 at a temperature of 5 K. This effect is absent for a tensile strain along the [110] direction. We explain the phenomenon observed in view of the model of single-photon biexciton recombination for the system in which the bottom of the conduction band is formed by only two opposite electronic valleys.     

Strain-Induced Intrinsic Splitting of the Biexciton Ground State in SiGe/Si Quantum Wells

We investigate the possibility of achieving the coherence of a biexciton gas in SiGe/Si heterostructures when the degeneracy of electronic states is reduced by anisotropic deformation. We find that at a temperature of 2K, the increase in the visible luminescence upon application of the deformation amounted to 3.3–3.9. The effect turned out to be much stronger than at 5K (the gain in 2.3) and can indicate either the splitting of the ground state of biexcitons in a strainless structure or the appearance of coherence in a dense biexciton 2D gas.     

Temperature independence of the angle-resolved X-ray photoemission spectra of gold and platinum valence bands

Angle-resolved valence-band X-ray photoemission spectra were taken along the [111] and [100] directions of gold single crystals and along the [100] direction of a platinum single crystal at room temperature (293 K) and on a probe cooled nearly to liquid nitrogen temperature (77 K). No change was detected on cooling the samples, in contrast to expectations based on a simple direct-transition model. A simple “matrix-element” model appears to predict spectra well even at low temperatures, perhaps because the complexity of the high-energy final-state bands permits sampling effectively throughout the zone.     

Visible photoluminescence of nanometer-sized SiGe/Si heterostructure fabricated by ion implantation

An innovative fabrication technique for the nanometer-sized SiGe/Si heterostructure was developed in this study. Ge was induced in Si substrate by two-step ion implantation. The spherical SiGe nanoclusters are self-assembled in the Si substrate by subsequent rapid thermal annealing at 1,100 °C. The diameter of the spherical SiGe nanoclusters is 5–7 nm. Visible photoluminescence from this nanometer-sized SiGe/Si heterostructure at room temperature was investigated. We found three peak energies of visible luminescence spectra at 1.97, 2.13, and 2.16 eV, respectively. The luminescence intensity depends on the number of the nanoclusters and will be decreased because of the micro-defects around the heterostructure, which is discussed in detail.     

Optical and structural properties of ZnO nanorods grown by pulsed laser deposition without a catalyst

Pulsed laser deposition without a catalyst is used to grow ZnO nanorods less than 10 nm in diameter. The structure of the rods is studied by Raman scattering during excitation in the visible and UV regions. The temperature dependences of exciton spectra and the behavior of green luminescence are investigated in the temperature range 10–280 K. At room temperature, the luminescence intensity of the ZnO nanorods in the exciton region is higher than the green luminescence intensity by a factor of 7.8.     

Spontaneous and stimulated luminescence of CdSe

Luminescence of CdSe excited by various high density excitations at the temperature range from 77 to 300°K has been investigated. The luminescence temperature dependence and the photo-conductivity and luminescence dependences on intensity are discussed in terms of exciton-electron and exciton-exciton interaction. The “resonance” type model for latter interaction is proposed.     

CO adsorbed on MgO(100): a high resolution LEED study

The monolayer structure of CO molecules adsorbed on MgO(100) single crystal surfaces cleaved in situ has been analysed by LEED within the 30–56 K temperature range. At T ⩽ 40 K CO forms a 2 × 4 commensurate 2D solid phase. A sharp uniaxial transition occurs above this temperature, along the [10] surface direction which locks the monolayer into a new commensurate 2 × 3 phase stable over a temperature range of 8 K. Above 50 K, this second commensurate phase expands itself uniaxially in a sharp transition toward a solid with disorder increasing with temperature. This succession of transitions is an interesting illustration of the incomplete “devil's staircase”.     

Visible luminescence enhancement methods in SiGe/Si heterostructures

The possibility of increasing the photoluminescence signal of Si1?xGex/Si quantum wells in the visible spectral range due to a change in the conduction band structure and the interaction of many-body states with plasma oscillations of metal nanoparticles is studied. The sample band structure was controlled using a uniaxial strain of ～10?4. It is found that such an approach allows an increase in the emission intensity of biexcitons in the quantum well (x = 9%) by a factor of 2.4 at a temperature of 5 K. Metal nanoparticles deposited on the sample surface with a protective layer thickness of 20 nm allowed us to increase the luminescence intensity of quantum wells approximately by a factor of 2.7.     

Effect of uniaxial compression on traps of excitons and charge carriers in poly(9-vinylcarbazole) films

The effect of uniaxial pressure (1 × 10⁸ Pa) on the photoluminescence spectra and thermally stimulated luminescence curves of poly(9-vinylcarbazole) has been investigated in the temperature range of 5–295 K. The thermally stimulated luminescence curve of crystalline carbazole has been measured for comparison. The high-temperature wings of the thermally stimulated luminescence curves are approximated by a Gaussian function, the half-width of which characterizes the disorder of energy states of deep structural traps. It is concluded that the pressure inhibits conformational changes of polymer chains of poly(9-vinylcarbazole), which leads to the formation of sandwich-like excimers as well as to an ordering of the spatial arrangement of the side carbazolyl groups. As a result, the concentration of “excimer-forming” centers increases, whereas the degree of disorder of energy states of deep structural traps of charge carriers is reduced by almost half and remains unchanged after the depressurization.     

Angular-resolved secondary-electron-emission spectroscopy of clean and adsorbate covered tungsten single-crystal surfaces

Energy-distribution measurements are reported for secondary electrons back-scattered into a narrow angle about the normal direction to three low-index tungsten single-crystal surfaces, viz. (100), (110) and (111). Improved spectral resolution provides unambiguous evidence for scattering out of excited “final” states located above the vacuum level; the results for all three faces correlate closely with high-energy states of a calculated energy band structure, the intensity of emission being directly related to features in the one-dimensional density of unfilled states along the corresponding low-index symmetry directions. In the presence of ordered adsorbate monolayers, additional SEE spectral fine-structure is observed at energies which lie within finalstate band gaps of the crystal. Results are presented for the specific case of CO adsorption on W(110), which shows a distinct disorder-order structural transition after exposure of the clean surface to 10 L of gas at 300 K and subsequent annealing to temperatures ?1000 K. We interpret these adsorbate surface resonances to be due to two-dimensional Bloch-like surface states produced by the periodicity of the adsorbate layer, which manifest themselves as a direct consequence of the special circumstances associated with “band-gap emission”.     

高质量ZnO微米棒的制备及其光学性质

利用水热法制备ZnO微米棒。醋酸镁[Mg(CH3COO)2.4H2O]、醋酸锌[Zn(CH3COO)2.2H2O]和六次甲基四胺(C6H12N4)以一定比例配置成反应溶液,把反应溶液加热到90℃,反应时间为24h,能够在硅衬底上生长高质量的ZnO微米棒。用扫描电镜(SEM)和X射线衍射仪对ZnO微米棒的晶体结构和表面形貌进行了分析,结果表明,样品为细长条棒状结构,呈现六方纤锌矿结构,长径比可达10∶1,并且在[002]方向择优生长。在样品中并未发现镁离子,它有可能扮演着催化剂的角色。对ZnO微米棒的光致发光性能进行测量,由PL光谱分析可知,样品在384nm处有一个紫外发光峰,半峰全宽为13nm,紫外发光峰强度比可见发光峰强度大的多,样品的质量较好。     

Photoluminescence of gold, copper and niobium as a function of temperature

A specially constructed instrument for measuring the low intensity photoluminescence emission spectra of metals is described. It uses low luminescence optical components and dedicated sample mounting techniques. Room temperature measurements agree closely with literature spectra for high-purity gold and are found to be sensitive to 100 ppm impurities. Detailed spectra are presented, which are weakly temperature dependent, for gold, copper and unpolished niobium between room temperature and 100 K. We conclude that this work provides accurate luminescence data for Au from 300 K down to 100 K. Although the (variable temperature) luminescence data for Cu are consistent both with the room temperature experimental data in the literature and theory, we conclude the role of surface adsorbates and/or oxides cannot be ruled out. Theory suggests that Nb has a factor ∼50 lower luminescence intensity than Au and Cu because the real part of the refractive index is a factor ∼5 higher and the density of states ∼2 eV below the Fermi energy is a factor of ∼4 lower than Au and Cu. Measurements are presented for unpolished Nb, but given the lack of signal detection for polished Nb and that theory predicts very weak signals, we conclude that the luminescence signals from pure Nb still remain below the sensitivity of our instrument.     

Magnetostriction relative to pretransition in Ni50.5Mn24.5Ga25 single crystal

The strain behaviors as well as the structural and magnetic changes relative to the pretransition in the Ni_50.5Mn_24.5Ga₂₅ single crystals have been characterized by various methods, such as pretransition strain, magnetostriction, magnetization measurements, and TEM observations. A large magnetostriction up to 505 ppm measured in the [001] direction of the sample is obtained at the pretransition temperature with only a low magnetic field of about 1 kOe applied along the [010] direction. We found that not only the pretransition strain pronounces a more large change, but also the magnetostriction at a certain temperature exhibits a more large magnitude for field applied along the [010] direction than with field along the [001] direction. It is concluded that the magnetoelastic interaction is responsible for the premartensitic transition, and the magnetoelastic interaction in the [010] direction is stronger than that in the [001] direction.     

Anisotropic hysteretic Hall effect and magnetic control of chiral domains in the chiral spin states of Pr2Ir2O7

We uncover a strong anisotropy in both the anomalous Hall effect (AHE) and the magnetoresistance of the chiral spin states of Pr(2)Ir(2)O(7). The AHE appearing below 1.5 K at a zero magnetic field shows hysteresis which is most pronounced for fields cycled along the [111] direction. This hysteresis is compatible with the field-induced growth of domains composed by the 3-in 1-out spin states which remain coexisting with the 2-in 2-out spin ice manifold once the field is removed. Only for fields applied along the [111] direction, we observe a large positive magnetoresistance and Shubnikov-de Haas oscillations above a metamagnetic critical field. These observations suggest the reconstruction of the electronic structure of the conduction electrons by the field-induced spin texture.     

Inelastic neutron scattering study of commensurate-incommensurate phase transition in thiourea

This paper presents a study of the nature of the incommensurate lattice instability in deuterated thiourea by inelastic neutron scattering. It shows clearly a “soft mode” which condenses at the phase transition in a satellite reflexion. A study of related dispersion curves along the [O ξ O] direction shows an unusual spectral shape in a large temperature range above T_c.     

LaTiO3(110)薄膜分子束外延生长的精确控制和表面截止层的研究

LaTiO₃ 是一种典型的强关联电子材料, 其(110) 薄膜为通过晶格对称性、应变等的设计调控外延结构的物理性质提供了新的机会. 本文研究了SrTiO₃(110) 衬底表面金属La 和Ti 沉积所引起的微观结构变化, 进而利用电子衍射信号对分子束外延薄膜生长表面阳离子浓度的灵敏响应, 发展了原位、实时、精确控制金属蒸发源沉积速率的方法, 实现了高质量LaTiO₃(110) 薄膜的生长和对阳离子化学配比的精确控制. 由于LaTiO₃中Ti³⁺ 3d 电子的库仑排斥作用, 氧原子层截止的(110) 表面更容易实现极性补偿, 因此生长得到的薄膜表面暴露出单一类型的氧截止面.     

Magnetostriction of dilute rare-earth bismuthides: The effect of non-S-state ions versus “S-state” ions

Shape magnetostriction measurements along the [001] direction were carried out for dilute rare-earth bismuthides; LaBi: R (R = Ce, Sm, Gd, Tb, Er) and the antimonide LaSb: Gd. The magnetostriction of LaBi: Ce and LaBi: Sm reveal the “reverse magnetostriction” phenomenon, enabling us to extract an overall crystalline field splitting of 12 K for the former compound. The magnetostriction of LaBi: Gd and LaSb: Gd which are significantly larger than that of the “pure” hosts, can be explained by a modified version of a model previously suggested by us. The fitting procedure yields magnetoelastic coupling constants for LaSb: Gd and LaBi: Gd which has the correct sign and magnitude according to existing theories.     

Cathodoluminescence microscopic studies of α-HgI2 platelets and crystals

Cathodo Luminescence studies (CL) have been carried out on red mercuric-iodide (α-HgI₂) crystals and platelets grown by the vapor transport method. Panchromatic CL images revealed inhomogeneous distribution of growth-induced dislocations and terraces. The effect of prolonged KI (20%) treatment on the evolution of different CL bands has been investigated. CL spectra of the platelets at 80 K showed three luminescence bands at about 546, 567 and 624 nm. The intensity of the 567 nm band (band II) decreased after KI treatment, thus indicating the role of I vacancies on the evolution of this band. An additional band at about 555 nm is observed in as-grown crystals, but not revealed in platelets. Increase of the relative intensity of band III (624 nm) on prolonged KI treatment indicates the association of some surface states on this band along with the effect of impurities as reported earlier. Hg treatment causes an increase of total CL intensity and also the relative intensity of band II in platelets and crystals.     

Fabrication of ZnO nanostructures of various dimensions using patterned substrates

ZnO nanostructures were grown on silicon, porous silicon, ZnO/Si and AlN/Si substrates by low-temperature aqueous synthesis method. The shape of nanostructures greatly depends on the underlying surface. Scattered ZnO nanorods were observed on silicon substrate, whereas aligned ZnO nanowires were obtained by introducing sputtered ZnO film as a seed layer. Furthermore, both the combination of nanorods and the bunch of nanowires were found on porous silicon substrates, whereas platelet-like morphology was observed on AlN/Si substrates. XRD patterns suggest the crystalline nature of aqueous-grown ZnO nanostructures and high-resolution transmission electron microscopy images confirm the single-crystalline growth of the ZnO nanorods along [0 0 1] direction. Room-temperature photoluminescence characterization clearly shows a band-edge luminescence along with a visible luminescence in the yellow spectral range.     

Luminescence centers in porous silicon

Photoluminescence studies on porous silicon show that there are luminescence centers present in the surface states. By taking photoluminescence spectra of porous silicon with respect to temperature, a distinct peak can be observed in the temperature range 100–150 K. Both linear and nonlinear relationships were observed between excitation laser power and the photoluminescence intensity within this temperature range. In addition, there was a tendency for the photoluminescence peak to red shift at low temperature as well as at low excitation power. This is interpreted as indicating that the lower energy transition becomes dominant at low temperature and excitation power. The presence of these luminescence centers can be explained in terms of porous silicon as a mixture of silicon clusters and wires in which quantum confinement along with surface passivation would cause a mixing of andX band structure between the surface states and the bulk. This mixing would allow the formation of luminescence centers.