Consequences of interface corrugation on the lattice dynamics and Raman spectra in high-index AlAs/GaAs superlattices

We have studied the effect of the interface corrugation on the Raman response of (113), (112), (331) and (012)-oriented AlAs/GaAs superlattices. The phonon frequencies and displacements have been calculated using the bond charge model and a supercell approach, and the Raman spectra have been calculated using the bond polarizability model. In (113)-oriented superlattices, the corrugation would give rise to an additional mode coupling and an enhancement of the phonon confinement. The magnitude of these effects and the consequent modifications on the Raman spectra would be quite different for the two interface structures proposed in the literature. For the (112), (331) and (012) superlattices, the modifications induced by the in-plane periodicity would be weak, even in the case of ultrathin superlattices. This is due to the very small height of the interface corrugation and also because the proposed lateral periodicity would not induce any significant mode coupling.     

Mixed vibrational modes of PbTe nanocrystallites

Far-infrared reflectance (20–700 cm^?1) and Raman spectra of porous PbTe were studied. The spectral dependences found significantly differ from those observed in the case of bulk single-crystal PbTe. Aside from the transverse (TO) and longitudinal (LO) optical phonon modes, additional vibrational modes were detected in the spectra, whose frequencies are close to those in a bulk material. Their number and spectral positions are in satisfactory agreement with the results of calculations carried out to determine the vibrational mode frequencies for nanocrystallites taking into account boundary conditions at their surface. The macroscopic continuum model allows calculation of the frequencies of the mixed phonon modes in quantum dots by the simultaneous consideration of atomic displacements and the accompanying change in the electrostatic potential. It was found experimentally that the detected modes are mixed (LO-TO), as was predicted by theory. The data of atomic-force microscopy allowed us to estimate the average radius of quantum dots, which form the nanorelief of the porous PbTe layer.     

Raman scattering in (111) strained heterostructures

III–V based strained heterostructures grown along [hhk] directions are considered. The proportionality coefficients between the in-plane strain and the shift in the TO and LO phonon frequencies have been calculated from the elastic constants and phonon deformation potentials. GaAs/GaP (111) and GaAs/InP (111) systems, where the GaAs epitaxial layers are in compressive or tensile strain, respectively, have been grown by MOVPE at different times on A and B substrates and investigated by Raman scattering. The corresponding red or blue shifts of the frequencies of the LO and TO phonons are measured and the residual strain parallel to the interface is estimated. The Raman results are discussed on the basis of the morphology of the epilayer investigated by atomic force microscopy.     

ScAlN缓冲层厚度对Si(100)衬底上GaN外延层的影响

采用脉冲直流磁控溅射方法在Si(100)衬底上制备了ScAlN薄膜。以溅射的ScAlN作为缓冲层,在Si(100)衬底上用金属有机化学气相沉积(MOCVD)技术外延了GaN薄膜。使用高分辨X射线衍射、原子力显微镜和拉曼光谱研究了ScAlN缓冲层的厚度对ScAlN缓冲层和GaN外延层的影响。研究结果表明,ScAlN缓冲层的厚度是影响GaN薄膜晶体质量的重要因素。随着ScAlN厚度的增加,ScAlN的(002)面X射线衍射摇摆曲线半高宽持续减小,GaN的(002)面X射线衍射摇摆曲线半高宽先减小后增大。当ScAlN缓冲层厚度为500nm时,得到的GaN晶体质量最好,其中GaN(002)面的X射线衍射摇摆曲线半高宽为0.38°,由拉曼光谱计算得到的张应力为398.38MPa。     

(Si/Ge)n多层薄膜的设计制备及光吸收性能

采用射频磁控溅射技术,在石英玻璃衬底上沉积了具有不同层数和厚度的(Si/Ge)n多层薄膜。XRD、Raman光谱测试表明,溅射态薄膜为微晶结构,在溅射过程中层间扩散形成Si-Ge振动键,溅射时间和薄膜层数影响着薄膜层间的扩散和结晶率;FESEM结果表明,薄膜表面由颗粒团簇构成,层与层之间有明显界面。UV-vis光谱测试表明,(Si/Ge)n多层薄膜在可见光范围内具有较宽的吸收,增加薄膜层数可扩大太阳能光谱的响应范围,而增加Si单层膜厚度对光吸收范围的影响较小。     

Raman scattering in ZnGa2Se4 single crystals

The raman spectra of ZnGa₂Se₄ single crystals are studied at 300 K in various experimental configurations. The observed phonon frequencies are identified by their types of symmetry. The optical phonon frequencies are related to particular atomic displacements in the unit cell.     

Effects of annealing temperature and surface preparation on the formation of cobalt silicide interconnects

Cobalt silicide films have been prepared by rapid thermal annealing of cobalt layers sputter deposited on silicon substrates. We report on the evolution of silicide phases, surface and interface roughness as a function of the annealing temperature and silicon surface preparation. The characterizations are carried out by atomic force microscopy, X-ray diffraction, X-ray reflectivity, Raman spectroscopy, and transmission electron microscopy. The cleaning procedure of the silicon substrate affects the interface roughness and the silicide thickness, whereas little effects are found on the surface. On the other hand, surface roughness increases with annealing temperature.     

ReSe2薄膜的盐助生长及其光电特性研究

二硒化铼(ReSe2)因具有较好的红外光响应和各向异性特性而成为近年来的研究热点.采用盐辅助化学气相沉积技术,在SiO2/Si衬底上成功合成了大面积的单层ReSe2薄膜,其尺寸达到80μm.采用拉曼光谱(Raman)、光致发光光谱(PL)、原子力显微镜(AFM)和X射线光电子能谱(XPS)等手段对制备的ReSe2薄膜样品进行了形貌、光谱、厚度和元素成分的表征,结果表明所制备的ReSe2薄膜晶体质量高.基于单层ReSe2薄膜构筑了场效应晶体管,系统研究了其光电特性,结果显示器件的响应时间达毫秒级.     

多碱阴极XPS 分析研究

介绍了多碱阴极的特点及其在超二代像增强器中的应用。叙述了多碱阴极进行制作和封接,之后在XPS 分析仪器的预真空室中进行解封并送入仪器分析室进行分析的过程。对多碱阴极不同膜层厚度处各元素原子的百分比进行了XPS 分析。首先对多碱阴极的表面进行宽谱扫描,发现元素之后再针对各个元素进行窄谱扫描,之后进行氩离子刻蚀。刻蚀一定时间后再对多碱阴极表面进行宽谱或窄谱扫描,氩离子刻蚀一直进行到阴极玻璃窗的表面。通过氩离子刻蚀并结合XPS 能谱分析,得出了多碱阴极膜层内部不同厚度处Na 元素、K 元素和Sb 元素的原子百分比。结果表明:多碱阴极的结构是一种两层结构,即以Na2KSb 为基础层,表面再吸附Cs 原子层。两层膜层中,Na2KSb 基础层较厚,而Cs 原子层较薄,厚度仅为整个阴极膜层厚度的2.7%。另外,实测多碱阴极膜层中Na 原子、K 原子和Sb 原子数量的百分比并不完全遵循2:1:1 的化学计量比,并且在整个阴极膜层厚度范围内,三种元素的原子百分比并不保持恒定,未出现在某一位置处三种元素原子百分比达到2:1:1 的理想状态。原因是目前多碱阴极制作工艺对蒸发元素蒸发量的控制精度较低。理论和实践证明,只有获得具有严格化学计量比的Na2KSb 膜层才可能获得高的灵敏度,因此,如果采用分子束外延技术来制作多碱阴极,那么成分控制的精度将大大改善,2:1:1 的化学计量比才可能获得,阴极灵敏度才会得到进一步的提高。     

The absorption and coupling of an electromagnetic wave incident ona microstrip circuit with superstrate

The absorption of an electromagnetic wave incident on a microstrip circuit covered with a dielectric layer is studied. The exact analytical expression for the plane-wave reflection coefficient at the air-superstrate interface of a four-layer model for a superstrate-loaded microstrip circuit is given. Numerical results illustrating the power absorption by the microstrip circuit and the electric and magnetic fields on the surface of the circuit as a function of wave frequency, superstrate thickness, superstrate permittivity, and incident angle are presented. It is found that the power absorption by the circuit at resonant frequencies increases with increasing superstrate permittivity. Higher superstrate thickness and permittivity can both cause more resonant power absorption peaks in the frequency spectrum. Significant intensities of electric or magnetic fields can also exist on the surface of the circuit for almost all the frequencies, which may couple to the circuit and cause malfunctions or interference. Details of the numerical results are presented     

A novel technique for suppression of parasitic superfluorescence in backward Raman amplifiers

The major parasitic modes of backward Raman amplifiers are different forms of superfluorescent forward Raman scattering. It is shown that spatial, and/or temporal variation of the Raman frequency in the scattering medium can inhibit amplification of spontaneously scattered light by creating off-resonant conditions in the forward direction. If the frequencies of the optical pulses are properly varied in time (the pulses are chirped), the required resonance condition for amplification of a counterpropagating Stokes wave can be maintained throughout the medium. Raman transition frequencies can be varied in an atomic vapor by application of a magnetic field via the Zeeman effect. The use of thallium (T1) metal vapors for scattering XeCl or KrF excimer laser radiation is considered as a particular example.     

Visible light emitting Si/SiO2 superlattices

Six-period superlattices of Si/SiO₂ have been grown at room temperature using molecular beam epitaxy. With this mature technology, the ultra-thin (1–3 nm) Si layers were grown to atomic layer precision. These layers were separated by 1 nm thick SiO₂ layers whose thickness was also well controlled by using a rate-limited oxidation process. The chemical and physical structures of the multilayers were characterized by cross-sectional TEM, X-ray diffraction, Raman spectroscopy, Auger sputter-profile, and X-ray photoelectron spectroscopy. The analysis showed that the Si layer is free of impurities and is amorphous, and that the SiO₂/Si interface is sharp (0.5 nm). Photoluminescence (PL) measurements were made at room temperature using 457.9 nm excitation. The PL peak occurred at wavelengths across the visible range for these multilayers. The peak energy position E was found to be related to the Si layer thickness d by E (eV) = 1.60+0.72d⁻² in accordance with a quantum confinement mechanism and the bulk amorphous-Si band gap.     

Rapid Characterization of Ultrathin Layers of Chalcogenides on SiO2/Si Substrates

There has been emerging interest in exploring single‐sheet 2D layered structures other than graphene to explore potentially interesting properties and phenomena. The preparation, isolation and rapid unambiguous characterization of large size ultrathin layers of MoS₂, GaS, and GaSe deposited onto SiO₂/Si substrates is reported. Optical color contrast is identified using reflection optical microscopy for layers with various thicknesses. The optical contrast of these thin layers is correlated with atomic force microscopy (AFM) and Raman spectroscopy to determine the exact thickness and to calculate number of the atomic layers present in the thin flakes and sheets. Collectively, optical microscopy, AFM, and Raman spectroscopy combined with Raman imaging data are analyzed to determine the thickness (and thus, the number of unit layers) of the MoS₂, GaS, and GaSe ultrathin flakes in a fast, non‐destructive, and unambiguous manner. These findings may enable experimental access to and unambiguous determination of layered chalcogenides for scientific exploration and potential technological applications.     

Calculation of the Infrared Absorption for MX Compounds

In the theory calculation of lattice vibration,one acoustic and three optical branches were found to compose the phonon vibrating wpectrum.Some isolated modes with frequencies lying outside the continuum branches will arise under the defect states.These local model results in the sharp peaks in the infrared absorption and Raman spectra.From calculation of the infrared absorption,the mode with the infrared activity is obtained in the infrared absorption spectrum of MX compounds.     

Transverse optical phonon splitting in GaAs/AlAs superlattices grown on the GaAs(311) surface studied by the method of Raman light scattering

GaAs_n/AlAs_m superlattices grown on the GaAs (311)A and (311)B surfaces by molecular-beam epitaxy were studied by Raman light scattering. The form of the Raman scattering tensor allowed the TO _y and TO _x modes to be separately observed using various scattering geometries (the y and x axes correspond to atomic displacements along and across facets formed on the (311)A surface, respectively). The TO1_y and TO1_x modes exhibited splitting in superlattices grown on a faceted GaAs(311)A surface. The degree of splitting increased for superlattices with an average GaAs layer thickness of 6 monoatomic layers and less. No splitting was observed for superlattices grown under the same conditions on the (311)B surface, which indicates that the splitting effect is probably due to the formation of GaAs quantum wires on the faceted (311)A surface.     

Evidence for Chemicals Intermingling at Silicon/Titanium Oxide (TiOx) Interface and Existence of Multiple Bonding States in Monolithic TiOx

Metal oxides (MOs) are used in photovoltaics and microelectronics as surface passivating layers and gate dielectrics, respectively. The effectiveness of MOs predominantly depends on their structure and the nature of the semiconductor/MO (S/MO) interface. While some efforts are made to analyze interface behavior of a few MOs, greater fundamental understanding on the interface and structural behaviors of emerging MOs is yet to be established for enhanced scientific and technological developments. Here, the structure of atomic layer deposited titanium oxide (TiO_x) and the nature of the c‐Si/TiO_x interface on the atomic‐ to nanoscale are probed. A new breed of mixed oxide (SiO_x+TiO_x) interfacial layer with a thickness of ≈1.3 nm at the c‐Si/TiO_x interface is discovered, and its thickness further increases to ≈1.5 nm after postdeposition annealing. It is observed that both as‐deposited and annealed monolithic TiO_x films comprise multiple bonding states at varying film thickness, with an oxygen‐deficient TiO_x layer located close to the mixed oxide/TiO_x interface. The stoichiometry of this layer improves when reaching the middle and near surface regions of the TiO_x layer, respectively. This work uncovers several critical structural and interface aspects of TiO_x, and thus creates opportunities to control and design improved photovoltaic and electronic devices for future development.     

Comparative investigation of optical and structural properties of porous SiC

Paper presents results of the non-destructive characterization of porous SiC (PSiC) layers using atomic force microscope, Raman scattering, scanning electronic and X-ray diffraction spectroscopes. A comparative study of the Raman spectroscopy on the PSiC layers prepared at the different technological routine with the variation of the nanocrystallite sizes and the thickness of PSiC layers has shown a number of new features specific for nanocrystallite materials. The latter stimulates the modification of Raman scattering spectra, which have been discussed.     

微晶硅薄膜太阳电池中孵化层研究

对甚高频等离子体增强化学气相沉积技术制备的微晶硅薄膜太阳电池进行了研究.喇曼测试结果显示:微晶硅薄膜太阳电池在p/i界面存在着一定的非晶孵化层.孵化层的厚度随硅烷浓度的增加或辉光功率的降低而增大.可以通过适当的硅烷浓度或适当的辉光功率来降低孵化层的厚度.     

Electrical and interface properties of Au/DNA/n-Si organic-on-inorganic structures

The effect of the thickness and coverage rate of a DNA film on the electrical and interface properties of Au/DNA/n-Si organic-on-inorganic structures has been investigated. The thin film properties of the DNA deposited on n-Si wafer were characterized by atomic force microscopy. The effect of the thickness and coverage rate of the DNA layer was investigated by evaluating electrical parameters, such as the barrier height, ideality factor, series resistance, and interface state density. The thickness and coverage rate of the DNA layer significantly affects the electrical properties of the Au/DNA/n-Si organic-on-inorganic structures. The interface state density properties of the Au/DNA/n-Si diodes were determined by conductance technique. The results show that the interface state density decreases with decrease in both film thickness and coverage rate of the DNA in an acetate buffer, modifying the electronic parameters of the Au/DNA/n-Si diodes.     

Single-peak excitonic emission of CdSe ultra-thin quantum wells finished with fractional monolayers

Ultra-thin quantum wells (UTQWs) of CdSe grown by atomic layer epitaxy (ALE) present very interesting features, such as intense excitonic luminescence and relatively narrow width. Grown under adequate conditions only a single excitonic peak is exhibited in the photoluminescence (PL) spectrum, indicating the absence of thickness fluctuations. The PL peak of an UTQW grown with the same nominal thickness presents a blue shift if grown at a higher substrate temperature. This energy shift is attributed to changes in composition: instead of a pure CdSe UQTW we obtain a Zn_1?xCd_xSe UTQW with Cd content depending on growth temperature. The fact that even changing the growth temperature only a single excitonic peak is observed is interpreted in terms of a homogeneous mixing of the Cd and Zn atoms at the upper interface. The lack of homogeneity would produce islands and terraces, and then thickness fluctuations that would be clearly evident in the PL spectrum. In order to verify this assertion we have grown UTQWs finished with fractional monolayers, around 0.5 ML of CdSe. We produced UTQWs with 2.5, 3 and 3.5 ML and the result is that all the samples presented a single peak, red shifted with increasing CdSe coverage and monotonic increase in the full-width at half-maximum (FWHM) of the emission. This confirms the homogenization of the CdSe–ZnSe interface.