Quantum-size shift in PbTe/BaF2MQW structure photoluminescence spectra

PbTe/BaF₂multi-quantum-well (MQW) structures were grown on cleaved (111) BaF₂substrates by the MBE method. The substrate temperature was 330 ^°C. The MQWs consist of 10 pairs of 10 nm thickness BaF₂barriers and PbTe wells with widths ranging from 5 to 20 nm. For this highly mismatched system we have measured photoluminescence spectra at 77 K and have demonstrated the quantum-size confinement consistent with the model of wells with infinite barriers. A remarkable deformation shift due to the thermal expansion coefficient difference is observed at cryogenic temperatures.     

Ballistic transport in PbTe-based nanostructures

We describe our study of ballistic transport in nanostructures of lead telluride, PbTe. Submicron devices have been fabricated by electron beam lithography and chemical etching of 50 nm wide PbTe single quantum wells embedded between Pb_0.92Eu_0.08Te barriers grown by MBE on BaF₂. The electron concentration in the devices was tuned by the gate voltage applied across an interfacial p–n junction. The most important observation was zero-magnetic field conductance quantization (in multiplies of 2e²/h) in narrow constrictions of dimensions comparable to electron mean free path calculated from transport mobility. This indicates considerable relaxation of requirements for quantum ballistic transport in comparison with other materials. We argue that the huge static dielectric constant of PbTe (₀=1350 at 4.2 K) leads to suppression of the long-range Coulomb potentials of charged impurities and, thus, provides favorable conditions for the conductance quantization.     

分子束外延PbTe单晶薄膜的反常拉曼光谱研究

采用分子束外延(MBE)方法在BaF₂(111)衬底上生长了高质量的PbTe单晶薄膜， 拉曼光谱测量观察到了表面氧化物的振动模、布里渊区中心(q≈0)纵光学(LO)声子振动模以 及声子-等离子激元耦合模振动.随着显微拉曼光谱仪激光光斑聚焦深度的改变，各拉曼散射 峰的峰位、积分强度、半高宽等都表现出不同的变化趋势. 随着激光光斑聚焦位置从样品表 面上方3μm处变化到表面下方3μm处，PbTe外延薄膜的LO声子频率从119cm^-1移 动到124cm^{-1关键词： PbTe外延薄膜 拉曼散射 纵光学声子}     

Morphology of nanostructures formed on the van der Waals surface of GaSe layered crystals annealed in sulfur vapor

The morphology of nanostructures, which were grown on the (0001) van der Waals surface of layered GaSe crystals and annealed under thermodynamically equilibrium conditions at high pressures of sulfur vapor, has been investigated using atomic force microscopy. The morphology and phase composition of the nanostructures are determined by the deformation and chemical interactions between the vapor phase and the crystal surface and by the thermodynamic conditions of annealing. Elements of nanostructures are formed as small Ga₂S₃ nanocrystallites, pyramidal quantum dots, and quantum rings on the defective (0001) van der Waals surface that contains nanosized indentations (nanocavities) and liquid gallium.     

PbTe/CdTe量子点的光学增益

PbTe/CdTe量子点是一类新型异系低维结构材料,实验发现具有强的室温中红外光致发光现象.为研究这一材料体系的发光特性,建立了理论模型,计算了PbTe/CdTe量子点的光学跃迁和增益.模型基于k·p包络波函数方法并考虑了PbTe能带结构的各向异性.分析了量子点光学增益与量子点尺寸、注入载流子浓度的关系.结果表明,当注入载流子浓度在(0.3—3)×10¹⁸cm^-3范围时,尺寸为15—20nm的量子点可以产生 关键词： PbTe/CdTe量子点 光学增益 铅盐矿半导体     

125Te NMR chemical-shift trends in PbTe–GeTe and PbTe–SnTe alloys

Complex tellurides, such as doped PbTe, GeTe, and their alloys, are among the best thermoelectric materials. Knowledge of the change in ¹²⁵Te NMR chemical shift due to bonding to dopant or “solute” atoms is useful for determination of phase composition, peak assignment, and analysis of local bonding. We have measured the ¹²⁵Te NMR chemical shifts in PbTe-based alloys, Pb_1−xGe_xTe and Pb_1−xSn_xTe, which have a rocksalt-like structure, and analyzed their trends. For low x, several peaks are resolved in the 22-kHz MAS ¹²⁵Te NMR spectra. A simple linear trend in chemical shifts with the number of Pb neighbors is observed. No evidence of a proposed ferroelectric displacement of Ge atoms in a cubic PbTe matrix is detected at low Ge concentrations. The observed chemical shift trends are compared with the results of DFT calculations, which confirm the linear dependence on the composition of the first-neighbor shell. The data enable determination of the composition of various phases in multiphase telluride materials. They also provide estimates of the ¹²⁵Te chemical shifts of GeTe and SnTe (+970 and +400±150 ppm, respectively, from PbTe), which are otherwise difficult to access due to Knight shifts of many hundreds of ppm in neat GeTe and SnTe.     

Conductance anomalies in strained quantum wires: the case of PbSe and PbTe

We show that conducting edge channels are formed in free standing wires of PbSe/BaF₂and PbTe/BaF₂as the temperature is lowered. We interpret the effect as a result of spatially inhomogeneous strain caused by a difference between the thermal expansion coefficients of the epilayer and the substrate. The presence of these edge channels can explain anomalous mesoscopic effects observed previously in these wires. Theoretical calculations based on the stress maps obtained by Jainet al.for narrow epitaxial stripes seem to support the experimental observations.     

InSb quantum dots and quantum rings on InAs-rich surface

We report a study of InSb nanoobjects (quantum dots and quantum rings) grown on InAs-rich surface by liquid phase epitaxy. Characterization of the sample surface was performed using atomic force microscopy (AFM). The bimodal formation of the uncapped InSb quantum dots (QDs) was observed for the growing on a binary InAs substrate. Uniform high-density (1 × 10¹⁰ cm⁻²) quantum dots with a height of 3 nm were obtained at T = 420-430 °C, whereas low-density (5 × 10⁸ cm⁻²) big quantum dots were 9 nm in height. As a buffer layer, lattice-matched InAsSb_0.12P_0.25 solid solution was deposed on InAs substrate using metal-organic vapour phase epitaxy. Deposition from the InSb melt on the buffer layer resulted in the formation of InSb nanoobjects with density as high as 3 × 10¹⁰ cm⁻².     

Midinfrared photoluminescence from SnTe/PbTe/CdTe double quantum wells grown by molecular beam epitaxy

Molecular beam epitaxial growth and photoluminescence (PL) properties of SnTe/PbTe/CdTe double quantum wells (DQWs) on (1 0 0)-oriented GaAs substrates are reported. These DQWs were consisted of a very thin SnTe/PbTe QW nested in a 10-nm-thick PbTe/CdTe QW. Efficient midinfrared PL was observed from the DQWs at 300 K in agreement with the coherent SnTe/PbTe growth on the thick CdTe barrier layer. The PL peak wavelength of the DQWs was found to increase with the SnTe thickness d by covering a wide range of the 3–5 μm atmospheric window with d≤2.5 monolayer.     

Ab initio-based approach to the reconstruction on InAs(111)A wetting layer grown on GaAs substrate

The surface reconstructions on InAs(111)A wetting layer grown on GaAs substrate are investigated by our ab initio-based approach incorporating the chemical potentials of In atom and As molecules in the vapor phase as functions of temperature and beam equivalent pressure. The calculated results imply that the most stable surface structure of InAs with/without lattice constraint from the substrate is the In-vacancy surface under conventional growth conditions. The In-vacancy surface is dramatically stabilized on the wetting layer, since the atoms around the In-vacancy are easily displaced to effectively lower the strain energy due to the lattice constraint from the GaAs substrate. Distinctive feature between InAs(111)A surfaces with and without lattice constraint is found in the stable adsorption sites. In adatoms favor the In-vacancy site on the InAs without lattice constraint in contrast to the interstitial sites on the InAs wetting layer. These results suggest that the surface structure and adsorption-desorption behavior on the wetting layer are crucial for investigating the growth processes of nanostructures such as quantum dots and stacking fault tetrahedrons.     

X-ray characterization of precipitates in cerium-doped PbTe and SnTe crystals

The homogeneity, solid solubility, and chemical bonds in the new materials PbTe, SnTe doped with Ce were investigated. Scanning electron microscope observation and electron probe microanalysis carried out on PbTe crystals doped with Ce, revealed three types of Ce-rich precipitates with following compositions: CeTe₂, Ce₃Te₇, Ce₂Te₅ and small admixture of PbTe in precipitates. The solubility of Ce in PbTe matrix was estimated as 0.5±0.1 at. %. The solubility of PbTe in CeTe₂ and Ce₃Te₇ was found to be 3±0.5 at. %, but 7±0.5 at. % in the case of Ce₂Te₅. In SnTe crystal doped with Ce only one kind of precipitate with composition Ce₂SnTe₅ was found. Cerium solubility in SnTe matrix was estimated to be 1±0.25 at. %. According to our knowledge this is the first report of the identification of Ce₂SnTe₅ compound. The similar compounds Ce₂SnS₅ and Ce₂SnSe₅ are known. Received: 9 December 1998 / Accepted: 9 February 1999 / Published online: 28 April 1999     

Local instability in BaF2:Mn2+: Analysis of lattice relaxation

Abstract

The distortion undergone by BaF₂ : Mn²⁺ below 50 K is examined here through the analysis of the experimental isotropic (A_s) and anisotropic (A_p) superhyperfne constants. This quantitative analysis support that the “short” Mn²⁺-F^? distance experiences a decrement of 8(4) pm with respect to the distance R= 232(2) existing in the O_h MnF₈ ^6- complex in BaF₂ at room temperature. By contrast the “long” Mn²⁺-F^? distance becomes 250(3) pm.     

溅射沉积自诱导混晶界面与Ge量子点的生长研究

在不同的沉积温度下采用离子束溅射技术,在Si基底上生长得到分布密度高、尺寸单模分布的圆顶形Ge量子点.研究发现:随沉积温度的升高Ge量子点的分布密度增大,尺寸减小,当沉积温度升高到750 ℃时,溅射沉积15个单原子层厚的Ge原子层,生长得到高度和底宽分别为14.5和52.7 nm的Ge量子点,其分布密度高达1.68×10¹⁰ cm^-2;Ge量子点的形貌、尺寸和分布密度随沉积温度的演变规律与热平衡状态下气相凝聚的量子点不同,具有稳定形状特征和尺寸分布的Ge量子点是 关键词： Ge量子点 离子束溅射沉积 表面原子行为 混晶界面     

Photocatalytic Properties of Hybrid Nanostructures Based on Nanoparticles of TiO<Subscript>2</Subscript> and Semiconductor Quantum Dots

Multilayer hybrid nanostructures based on nanoparticles of TiO₂ and CdSe/ZnS semiconductor quantum dots were formed. Generation of active forms of oxygen by hybrid nanostructures under visible-light irradiation is demonstrated to take place, which is indicative of efficient photoinduced electron transfer from quantum dots to titanium-oxide nanoparticles contained in the hybrid nanostructures.     

Corrugated single layer templates for molecules: From <Emphasis Type="Italic">h</Emphasis>-BN nanomesh to graphene based quantum dot arrays

Functional nano-templates enable self-assembly of otherwise impossible arrangements of molecules. A particular class of such templates is that of sp ² hybridized single layers of hexagonal boron nitride or carbon (graphene) on metal supports. If the substrate and the single layer have a lattice mismatch, superstructures are formed. On substrates like rhodium or ruthenium these superstructures have unit cells with ～3-nm lattice constant. They are corrugated and contain sub-units, which behave like traps for molecules or quantum dots, which are small enough to become operational at room temperature. For graphene on Rh(111) we emphasize a new structural element of small extra hills within the corrugation landscape. For the case of molecules like water it is shown that new phases assemble on such templates, and that they can be used as “nano-laboratories” where many individual processes are studied in parallel. Furthermore, it is shown that the h-BN/Rh(111) nanomesh displays a strong scanning tunneling microscopy-induced luminescence contrast within the 3 nm unit cell which is a way to address trapped molecules and/or quantum dots.     

Light Trapping and Down‐Shifting Effect of Periodically Nanopatterned Si‐Quantum‐Dot‐Based Structures for Enhanced Photovoltaic Properties

Periodically nanopatterned Si structures have been prepared by using a nanosphere lithography technique. The formed nanopatterned structures exhibit good anti‐reflection and enhanced optical absorption characteristics. The mean surface reflectance weighted by AM1.5 solar spectrum (300–1200 nm) is as low as 5%. By depositing Si quantum dot/SiO₂ multilayers (MLs) on the nanopatterned Si substrate, the optical absorption is higher than 90%, which is significantly improved compared with the same multilayers deposited on flat Si substrate. Furthermore, the prototype n‐Si/Si quantum dot/SiO₂ MLs/p‐Si heterojunction solar cells has been fabricated, and it is found that the external quantum efficiency is obviously enhanced for nanopatterned cell in a wide spectral range compared with the flat cell. The corresponding short‐circuit current density is increased from 25.5 mA cm^?2 for flat cell to 29.0 mA cm^?2 for nano‐patterned one. The improvement of cell performance can be attributed both to the reduced light loss and the down‐shifting effect of Si quantum dots/SiO₂ MLs by forming periodically nanopatterned structures.     

Helium scattering and work function investigation of co adsorption on Pt(111) and vicinal surfaces

CO adsorption on Pt(111) and vicinal Pt(111) surfaces has been studied by means of work function variation and He scattering measurements. AES and LEED were used mainly for correlations with other work. Special attention has been paid to the low coverage regime (θ_co < 0.1) with emphasis on surface structural dependencies. The minimum of the work function versus CO exposure curve occurs at a coverage less than 11% on “kink-free” surfaces. This is much lower than the hitherto commonly accepted value of 33%, and does not relate to any observed LEED superstructure. The value of Δφ_min depends strongly on the surface structure. For an “ideal” Pt(111) surface with a step density less than 10^?3 at a temperature of 300 K, Δφ_min = ?240 meV. The scattering cross section Σ of CO adsorbed on Pt(111) for 63 meV He is typically > 250 Ų, i.e. much larger than expected from the Van der Waals radii of He and CO. For two nominal Pt(111) surfaces with step densities of 10^?2 and less than 10^?3, respectively, the measured Σ values varied by a factor of three. This can be explained by preferential CO occupation of defect sites, which are already not “seen” by thermal helium. By comparing results on a stepped (997) and a kinked (12 11 9) Pt surface with similar defect densities, the kinks are proven to play a decisive role. They probably form saddles in the recently proposed activation barrier for migration between terrace and step sites.     

Cu adsorption on Pt(111) and its effects on Chemisorption: A comparison with electrochemistry

The growth modes and interaction of vapor-deposited Cu on a clean Pt(111) surface have been monitored by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and work function measurements. The LEED data indicate that below 475 K Cu grows in p(1 × 1) islands in the first monolayer with the interatomic Cu spacing the same as the Pt(111) substrate. The second monolayer of Cu grows in epitaxial, rotationally commensurate Cu(111) planes with the CuCu distance the same as bulk Cu. For substrate temperatures below ～ 475 K, the variation of work function and “cross-over beam voltage” with Cu coverage show characteristic features at one monolayer that are quite useful for calibration of θ_Cu. Above 525 K, Cu-Pt alloy formation was observed in AES and LEED data. Thermal desorption spectroscopy of H₂ and CO has demonstrated that simple site blocking of the Pt(111) surface by vapor-deposited Cu occurs linearly with chemisorption being essentially eliminated at θ_Cu = 1.0–1.15. Conclusions drawn from this work correlate very favorably with the well-known effects of under potentially deposited copper on the electrochemistry of the H₂2H⁺ couple at platinum electrodes.     

Synchrotron investigations of the specific features in the electron energy spectrum of silicon nanostructures

The Si L _{2, 3} x-ray absorption near-edge structure (XANES) spectra of porous silicon nanomaterials and nanostructures with epitaxial silicon layers doped by erbium or containing germanium quantum dots are measured using synchrotron radiation for the first time. A model of photoluminescence in porous silicon is proposed on the basis of the results obtained. According to this model, the photoluminescence is caused by interband transitions between the energy levels of the crystalline phase and oxide phases covering silicon nanocrystals. The stresses generated in surface silicon nanolayers by Ge quantum dots or clusters with incorporated Er atoms are responsible for the fine structure of the spectra in the energy range of the conduction band edge and can stimulate luminescence in these nanostructures.     

Surface structure of cubic ionic crystals studied by optical second-harmonic generation

Polarization and azimuthal dependencies of optical second-harmonic generation at the surface of cubic ionic crystals have been measured on polished surfaces of BaF₂ (111) and (100), CaF₂ (111) and NaCl (111), using a fundamental wavelength of 532 nm. The results are interpreted in terms of available theory, which shows that for transparent cubic media only the dipolar surface contribution depends on the crystal orientation. For BaF₂ (111) and NaCl (111) the crystallineC _3vgeometry could be identified but not for CaF₂ (111). Although the nature of the electronic surface structure giving rise to a dipolar polarizability remains undetermined, the experimental result suggests that it may be due to impurity or defect states in the band gap at the surface.