Angle-resolved photoemission study of CoSi2 nanofilms grown on Si(111) substrates

We have carried out an angle-resolved photoemission study for CoSi₂ nanofilms grown on the Si(111)-7×7 substrates. The surface of CoSi₂(111) nanofilm changes from the bulk-truncated surface to the surface with additional Si-bilayer by annealing at higher temperature above 825 K. The angle-resolved photoemission spectra of the CoSi₂ nanofilm annealed at 853 K show the spectral features originated from the surface resonance state on the CoSi₂ surface terminated by Si-bilayer. From the detailed photoemission study, we discuss the surface electronic structure in CoSi₂(111) nanofilms grown on Si(111) substrates.     

Characterization of silicon nanowires grown on silicon, stainless steel and indium tin oxide substrates

Silicon nanowires (SiNWs) have been grown on crystalline silicon (Si), indium tin oxide (ITO) and stainless steel (SS) substrates using a gold catalyst coating with a thickness of 200 nm via pulsed plasma-enhanced chemical vapor deposition (PPECVD). Their morphological, mineralogical and surface characteristics have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman analysis. SiNWs growth is accompanied by oxidation, thus yielding partially (SiO _x ) and fully oxidized (SiO₂) Si sheaths. The mean diameters of these SiNWs range from 140 to 185 nm. Si with (111) and (220) planes exists in SiNWs grown on all three substrates while Si with a (311) plane is detected only for Si and ITO substrates. Computational simulation using density functional theory (DFT) has also been conducted to supplement the experimental Raman analyses for crystalline Si and SiO₂. XPS results reveal that ca. 30 % of the SiNWs have been oxidized for all substrates. The results presented in this paper can be used to aid selection of appropriate substrates for SiNW growth, depending on specific applications.     

On the characteristics of AlGaN films grown on (111) and (001) Si substrates

High Al-content Al_xGa_1−xN films were deposited on (001) and (111) Si substrates at 1000 °C using high temperature AlN buffer layers. Experimental results show that Al_xGa_1−xN films grown on (111) Si substrates exhibit better crystalline quality than that in the films deposited on (001) Si substrates. Cracks were found in the high Al-content Al_xGa_1−xN/(111) Si samples but they were not observed in the Al_xGa_1−xN films grown on (001) Si substrates having the same film thicknesses and Al compositions. Based upon the results of X-ray diffraction (XRD) and transmission electron microscopy (TEM), it appears that mono-crystalline Al_xGa_1−xN films were achieved on (111) Si substrates while columnar structure was observed in the Al_xGa_1−xN/(001) Si samples. According to the depth profiles of Al_xGa_1−xN/Si samples using secondary ion mass spectroscopic (SIMS) analyses, enhanced Al inter-diffusion in the Al_xGa_1−xN/(001) Si samples was identified. Room temperature (RT) photoluminescence (PL) measurements of the Al_xGa_1−xN (x≦0.10)/(111) Si samples exhibit strong near band edge luminescence. The PL emission linewidth was found to decrease with the decrement of Al-content.     

Growth of silicon carbide films via C60 precursors

Epitaxial silicon carbide films are grown on Si(100) and Si(111) substrates at surface temperatures between 950 and 1250 K via c₆₀ precursors. Films have been grown up to thicknesses greater than 1 μm. The growth rate of the SiC film is not limited by the surface reaction rate of C₆₀ with silicon at these temperatures, rather by the arrival rate of the reactants Si (by diffusion) or C₆₀. This results in rapid film growth. Films have been characterized by low energy electron diffraction, X-ray diffraction, and Auger depth profiling. X-ray diffraction suggests the growth of β-SiC in the temperature range investigated. Auger depth profiling shows the film is stoichiometric. Selective crystalline silicon carbide growth is achieved on patterned silicon-silicon oxide samples.     

The effect of silicon-substrate orientation on the local piezoelectric characteristics of LiNbO<Subscript>3</Subscript> films

The domain structure of lithium-niobate thin films grown on Si(111) and Si(100) substrates coated with a native oxide layer with a thickness of no less than 2 nm is investigated by X-ray diffraction, scanning electron microscopy and piezoresponse force microscopy. The films are synthesized by the rf magnetron sputtering of a single-crystal lithium-niobate target. A high degree of grain orientation in the polycrystalline films is demonstrated. The piezoelectric coefficients d _zz of the lithium-niobate films on Si(111) and Si(100) substrates are calculated from the measured dependences of the amplitude of the piezoresponse signal on the ac voltage applied between the cantilever tip and the substrate. Piezoelectric hysteresis loops are obtained in the remanent piezoelectric response regime     

Temperature-dependent photoluminescence spectra of GaN epitaxial layer grown on Si(111) substrate

In this paper, the temperature-dependent photoluminescence(PL) properties of Ga N grown on Si(111) substrate are studied. The main emission peaks of Ga N films grown on Si(111) are investigated and compared with those grown on sapphire substrates. The positions of free and bound exciton luminescence peaks, i.e., FX A and D0 X peaks, of Ga N films grown on Si(111) substrates undergo red shifts compared with those grown on sapphire. This is attributed to the fact that the Ga N films grown on sapphire are under the action of compressive stress, while those grown on Si(111) substrate are subjected to tensile stress. Furthermore, the positions of these peaks may be additionally shifted due to different stress conditions in the real sample growth. The emission peaks due to stacking faults are found in Ga N films grown on Si(111) and an S-shaped temperature dependence of PL spectra can be observed, owing to the influence of the quantum well(QW) emission by the localized states near the conduction band gap edge and the temperature-dependent distribution of the photo-generated carriers.     

IR absorption enhancement for ionized p -nitrobenzoic acid on silver island films epitaxially grown on the 7 × 7 reconstructed Si(111) surface

An enhanced infrared absorption is reported for p-nitrobenzoic acid (PNBA) adsorbed on molecular-beam-epitaxially grown silver films at room temperature on 7×7 reconstructed Si(111) substrates. Reflection high-energy electron diffraction reveals the formation of Ag(111) islands even for 82-monolayer-thick silver deposition on the substrate. The NO₂ symmetric stretch of the surface-ionized species (PNBA^-) is a maximum in intensity at a thickness of 64 monolayers and is one order of magnitude stronger than the same mode of PNBA deposited at a thickness of 5 nm on a Si wafer. Based on the change in film morphology and in light transmittance with silver film thickness, this absorption enhancement is ascribed to the excitation of transverse collective electron resonance of the silver islands. The thickness dependence of the enhancement and of the film morphology will be compared with those observed for silver films deposited on oxidized Si(111) substrates. PACS 61.14.Hg; 68.37.Hk; 78.30.-J     

The comparison between CdS thin films grown on Si(111) substrate and quartz substrate by femtosecond pulsed laser deposition

Two kinds of cadmium sulfate (CdS) thin films have been grown at 600 °C onto Si(111) and quartz substrates using femtosecond pulsed laser deposition (PLD). The influence of substrates on the structural and optical properties of the CdS thin films grown by femtosecond pulsed laser deposition have been studied. The CdS thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL) and Raman spectroscopy. Although CdS thin films deposited both on Si(111) and quartz substrates were polycrystalline and hexagonal as shown by the XRD , SEM and AFM results, the crystalline quality and optical properties were found to be different. The size of the grains for the CdS thin film grown on Si(111) substrate were observed to be larger than that of the CdS thin film grown on quartz substrate, and there is more microcrystalline perpendicularity of c-axis for the film deposited on the quartz substrate than that for the films deposited on the Si substrate. In addition, in the PL spectra, the excitonic peak is more intense and resolved for CdS film deposited on quartz than that for the CdS film deposited on Si(111) substrate. The LO and TO Raman peaks in the CdS films grown on Si(111) substrate and quartz substrate are different, which is due to higher stress and bigger grain size in the CdS film grown on Si(111) substrate, than that of the CdS film grown on the amorphous quartz substrate. All this suggests that the substrates have a significant effect on the structural and optical properties of thin CdS films. PACS 81.15.Fg; 81.05.Ea; 78.20.-e; 78.67.-n; 42.62.-b     

Specific features and mechanisms of photoluminescence of nanostructured silicon carbide films grown on silicon in vacuum

The light-emitting properties of cubic silicon carbide films grown by vacuum vapor phase epitaxy on Si(100) and Si(111) substrates under conditions of decreased growth temperatures (T _gr ∼ 900–700°C) have been discussed. Structural investigations have revealed a nanocrystalline structure and, simultaneously, a homogeneity of the phase composition of the grown 3C-SiC films. Photoluminescence spectra of these structures under excitation of the electronic subsystem by a helium-cadmium laser (λ_excit = 325 nm) are characterized by a rather intense luminescence band with the maximum shifted toward the ultraviolet (∼3 eV) region of the spectral range. It has been found that the integral curve of photoluminescence at low temperatures of measurements is split into a set of Lorentzian components. The correlation between these components and the specific features of the crystal structure of the grown silicon carbide layers has been analyzed.     

Photoluminescence of nanostructured cubic-lattice silicon carbide films grown on a silicon wafer

The light-emitting properties of cubic-lattice silicon carbide SiC films grown on Si(100) and Si(111) substrates with VPE at low temperatures (T _gr ∼ 700°C) are discussed. Investigations of the grown films reveal a homogeneous nanocrystalline structure involving only the 3C-SiC phase. When the electron subsystem of the structure is excited by a He-Cd laser emitting at λ_exit = 325 nm, the photoluminescence (PL) spectra contain a rather strong emission band shifted by about 3 eV toward a short-wave spectral region. At low temperatures, the PL integral curve is split into a set of Lorentz components. The relation between these components and the peculiarities of the energy spectrum of electrons in the nanocrystalline grains of the silicon carbide layers is discussed.     

Si3N4/Si表面Si生长过程的扫描隧道显微镜研究

利用原位扫描隧道显微镜和低能电子衍射分析了Si的纳米颗粒在Si₃N₄/Si(111)和Si₃N₄/Si(100)表面生长过程的结构演变.在生长早期T为350—1075K范围内,Si在两种衬底表面上都形成高密度的三维纳米团簇,这些团簇的大小均在几个纳米范围内,并且在高温退火时保持相当稳定的形状而不相互融合.当生长继续时,Si的晶体小面开始显现.在晶态的Si₃N₄(0001)/S 关键词： 氮化硅 扫描隧道显微镜 纳米颗粒     

SrTiO3(110) thin films grown directly on different oriented silicon substrates

We have grown (110)-oriented SrTiO₃ (STO) thin films on silicon without any buffer layer, by means of pulsed laser deposition technique. The crystal structures of the grown films were examined by X-ray diffraction analysis including θ–2θ scan and rocking curve as well as Laue diffraction methods. STO films with single (110) out-of-plane orientation were formed on all (100), (110) and (111)-oriented Si substrates. The in-plane alignments for the epitaxial STO films grown directly on Si (100) were found as STO[001]//Si[001] and STO[11̄0]//Si[010]. The results should be of interest for better understanding of the growth of perovskite oxide thin films on silicon wafers. PACS 77.55.+f; 68.55.JK; 81.15Fg     

Effect of an electron beam on CaF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript> epitaxial layers on Si

Atomically smooth CaF₂ and BaF₂ layers have been sequentially grown on Si(111) substrates by molecular beam epitaxy. Pore macrodefects have been revealed at the points of the action of an electron beam from a diffractometer when analyzing the crystal structure of the surface during the growth with the subsequent observation using atomic force microscopy. The formation of these macrodefects is associated with the decomposition of fluorides by high-energy electrons, which is accompanied by the desorption of fluorine and the drift current of positive ions from the electron charge drains.     

Influence of substrate temperature and ion-beam energy on the syntheses of aluminium nitride thin films by nitrogen-ion-assisted pulsed-laser deposition

Aluminium nitride (AlN) thin films have been grown on Si(100), Si(111) and Sapphire Al₂O₃(001) substrates by pulsed KrF excimer laser (wavelength 248 nm, duration 30 ns) ablation of an AlN target with the assistance of nitrogen-ion-beam bombardment. The influence of process parameters such as substrate temperature and ion-beam energy has been investigated in order to obtain high-quality AlN films. The AlN films deposited by pulsed-laser deposition (PLD) have been characterized by X-ray diffraction (XRD) to determine the crystalline quality, grain size and growth orientation with respect to the substrate. The XRD spectra of AlN films on Si(100), Si(111) and Sapphire substrates yield full-width-half-maximum (FWHM) values of approximately 1.6. The bonding characteristics in the films have been evaluated by Raman spectroscopy. The chemical composition of the films has been characterized by X-ray photoelectron spectroscopy (XPS). The surface morphology of the films has been measured by atomic force microscopy (AFM). At a substrate temperature of at least 600 °C, polycrystalline AlN films with orientations of AlN(100) and AlN(101) have been synthesized. PACS 68.55.-a; 81.15.Fg; 77.84.Bw     

Optical Analysis of Nanocrystalline ZnO Films Coated on Porous Silicon by Radio Frequency (RF) Magnetron Sputtering

Zinc oxide thin films have been deposited onto porous silicon (PSi) substrates at high growth rates by radio frequency (RF) sputtering using a ZnO target. The advantages of the porous Si template are economical and it provides a rigid structural material. Porous silicon is applied as an intermediate layer between silicon and ZnO films and it contributed a large area composed of an array of voids. The nanoporous silicon samples were adapted by photo electrochemical (PEC) etching technique on n-type silicon wafer with (111) and (100) orientation. Micro-Raman and photoluminescence (PL) spectroscopy are powerful and non-destructive optical tools to study vibrational and optical properties of ZnO nanostructures. Both the Raman and PL measurements were also operated at room temperature. Micro-Raman results showed that the A₁(LO) of hexagonal ZnO/Si(111) and ZnO/Si(100) have been observed at around 522 and 530 cm^–1, re- spectively. PL spectra peaks are distinctly apparent at 366 and 368 cm^–1 for ZnO film grown on porous Si(111) and Si(100) substrates, respectively. The peak luminescence energy in nanocrystalline ZnO on porous silicon is blue-shifted with regard to that in bulk ZnO (381 nm). The Raman and PL spectra pointed to oxygen vacancies or Zn interstitials which are responsible for the green emission in the nanocrystalline ZnO.     

Morphological properties of AlN and GaN grown by MOVPE on porous Si(111) and Si(111) substrates

Metal Organic Vapour Phase Epitaxy (MOVPE) of AlN and GaN layers at a temperature of 1080 C were performed on porous Si(111) and Si(111) substrates. The thermal stability of porous silicon (PS) is studied versus growth time under AlN and GaN growth conditions. The surface morphology evolution of the annealed PS is revealed by scanning electron microscopy (SEM). Porous Si(111) with low porosity (40%) is more thermally stable than porous Si(100) with relatively high porosity (60%).AlN layers with various thicknesses were grown under the same conditions on the two substrates. Morphological properties of AlN were studied by atomic force microscopy (AFM) and compared taking into account the two different surfaces of the substrates. The two growth kinetics of AlN were found to be different due to the initial surface roughness of the PS substrate. The effect of AlN buffer morphology on the qualities of subsequent GaN layers is discussed. Morphological qualities of GaN layers grown on PS are improved compared to those obtained on porous Si(100) but are still less than those grown on Si substrate.     

Si衬底上InP纳米线的晶体结构和光学性质

采用金属有机化学气相沉积技术,利用自催化法,在Si(100)、(111)衬底上成功生长了InP纳米线。利用扫描电镜观察样品表面,在Si(100)、(111)衬底上生长的纳米线形貌相似,纳米线面密度不同。利用X射线衍射和透射电镜研究纳米线的生长取向和晶体结构,结果显示纳米线具有闪锌矿结构,生长方向〈111〉,并且具有层状孪晶结构。与InP体材料相比,纳米线的光致发光峰位蓝移,半峰全宽增大,拉曼散射TO和LO峰向低波数频移,频移随激发光功率减弱而减小。     

Electron diffraction on GaAs nanowhiskers grown on Si(100) and Si(111) substrates by molecular-beam epitaxy

The crystal structure of GaAs nanowhiskers grown by molecular-beam epitaxy on Si(111) and Si(100) substrates is investigated using reflection high-energy electron diffraction (RHEED). It is revealed that, in both cases, the electron diffraction images contain a combination (superposition) of systems of reflections characteristic of the hexagonal (wurtzite and/or 4H polytype) and cubic (sphalerite) phases of the GaAs compound. The growth on the Si(111) substrates leads to the formation of nanowhiskers with hexagonal (wurtzite and/or 4H polytype) and cubic (sphalerite) structures with one and two orientations, respectively. In the case of the Si(100) substrates, the grown array contains GaAs nanowhiskers that have a cubic structure with five different orientations and a hexagonal structure with eight orientations in the (110) planes of the substrate. The formation of the two-phase crystal structure in nanowhiskers is explained by the wurtzite—sphalerite phase transitions and/or twinning of crystallites.     

Comparative AES and RHEED study of the formation of Pd silicide on clean and oxide covered Si(100) and (111) surfaces

The formation and epitaxial orientation of Pd silicide on clean and native oxide covered Si(100) and (111) surfaces was studied by Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). Pd was vapor deposited in UHV on to the substrates up to thicknesses of about 6 nm. On clean Si substrates, ultra-thin Pd deposits reacted to form Pd₂Si already at room temperature, as detected by a characteristic splitting of the Si LVV Auger peak. However, a polycrystalline structure with very small crystallite sizes was indicated by diffuse ring patterns in RHEED. When the initial thickness of the Pd deposit exceeded about 3 nm, the diffraction ring pattern of unreacted metal developed. During annealing of room temperature deposits of Pd, the (100) and (111) substrates behaved differently. Larger crystallites formed on Si(100), but the films remained polycrystalline, though textured. On Si(111), virtually perfect epitaxial re-orientation of the silicide was found. When the substrates were initially covered with native oxide of about 2 nm thickness, silicide formation started at about 200°C, resulting in polycrystalline, but strongly textured Pd₂Si. Upon further annealing at temperatures up to 600°C, an additional phase of epitaxially oriented Pd₂Si developed on Si(111), similar to that on clean Si(100). In all experiments, extended annealing at temperatures above 250°C caused segregation of Si to the surface. This was accompanied by the development of an additional peak in the Auger electron spectra at about 313 eV, which we assign to a plasmon loss of δE = 17 eV in the Si overlayer, being excited by Pd Auger electrons of energy 330 eV.     

Cu/SiO2/Si(111)体系中Cu和Si的扩散及界面反应

室温下利用磁控溅射在p型Si(111)衬底上沉积了Cu薄膜. 利用X射线衍射和卢瑟福背散射分别对未退火以及在不同温度点退火后样品的结构进行了表征. 在此基础上，研究了Cu/SiO₂/Si(111)体系的扩散和界面反应. 实验结果表明：当退火温度高于450℃时出现明显的扩散现象，并且随着温度的升高，体系扩散现象会更加显著. 当退火温度低于450℃时没有铜硅化合物生成，当温度达到500℃时才有铜硅化合物生成. 关键词： 薄膜 扩散 界面反应 硅化物