In-depth RBS study of optical layers based on nanostructured silicon

The particular structure of porous silicon (PS), which can be described as an amorphous matrix in which silicon nanocrystals are embedded, makes this material very suitable for its use in many different fields, including optoelectronics and biological applications. In the present work, Rutherford backscattering spectroscopy (RBS) measurements and optical characterization are carried out on PS layers in order to determine their in-depth compositional profile, homogeneity, porosity, oxidation degree and overall optical behavior. The experimental results show quite good in-depth homogeneity of the PS layers since only slight porosity and oxidation degree gradients have been observed, further supported by optical measurements. Additionally, RBS measurements were used to confirm the existence of a layer of low porosity at the PS/silicon interface independently of the formation current density.     

Hydrothermal synthesis of hollow twinning ZnO microstructures

Two kinds of hollow twinning ZnO microstructures were synthesized through a simple hydrothermal method without additional templates or any surfactants. Dumbbell‐like and shuttle‐like ZnO microstructures with hollows were obtained by changing the materials source. The products were characterized by X‐ray power diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high‐resolution transmission electron microscopy (HRTEM). It was found that different precursors may be responsible for the formation of two different morphologies. Based on the time‐dependent experiments, we investigated the growth process of these hollow twinning structures and found the “Ostwald‐ripening process” played an important role. The interesting point of this growth process was that the interface of the two twinning structure performed as the activate center where the Ostwald‐ripening process carried out. We also investigated the luminescent properties of the as‐obtained products by photoluminescence (PL) spectroscopy. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of PbS nano‐microcrystals with different morphologies and their optical properties

PbS nano‐microcrystals were prepared from Pb(OAc)₂·3H₂O and sulfur in a solution without any surfactant using the solvothermal process. Different morphologies, mainly including polyhedron microcrystals and sphere‐like assemblies, were characterized using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). PbS nano‐microcrystals with cubic crystal structure were detected using X‐ray diffraction (XRD), electron diffraction (ED) and high resolution transmission electron micrograph (HRTEM) techniques. The optical properties were investigated by ultraviolet‐visible (UV‐vis) spectroscopy, and photoluminescence spectroscopy (PL). The UV‐vis absorption peaks of PbS exhibited a large blue‐shift and the PL spectra had a strong and broad emission bands centered at 408 nm. The crystal growth mechanism of PbS was also discussed.     

Influence of initial growth parameters on the structural and optical properties of GaAs on (001) Si

The structural and optical properties of GaAs on (001) Si substrates were investigated by transmission electron microscopy (TEM) and low-temperature photoluminescence (PL). It was found that the success of the two-step growth technique is controlled by the quality (morphology and defect density) of the low-temperature grown AlGaAs nucleation layer. GaAs epilayers grown on low V/III ratio AlGaAs nucleation layers exhibit improved surface morphologies and structural properties. These results were confirmed by optical measurements where it was shown that the best PL response was obtained from GaAs epilayers in which the initial AlGaAs nucleation layers were deposited at a low V/III ratio.     

Photoluminescence of Si or Ge nanocrystallites embedded in silicon oxide

This paper presents the results of photoluminescence, its temperature dependence and Raman scattering investigations on magnetron co-sputtered silicon oxide films with (or without) embedded Si (or Ge) nanocrystallites. It is shown the oxide related defect origin of the visible PL centers peaked at 1.7, 2.06 and 2.30 eV. The infrared PL band centered at 1.44–1.58 eV in Si–SiO_x, system has been analyzed within a quantum confinement PL model. Comparative PL investigation of Ge–SiO_x system has confirmed that high energy visible PL bands (1.60–1.70 and 2.30 eV) are connected with oxide related defects in SiO_x. The PL band in the spectral range of 0.75–0.85 eV in Ge–SiO_x system is attributed to exciton recombination inside of Ge NCs.     

MgZnO/ZnO异质结构的发光性质研究

本文利用等离子体辅助分子束外延(P-MBE)技术在蓝宝石 (Al2O3) 衬底上生长了Mg0.12Zn0.88O(100nm)/ZnO (20nm) /Mg0.12Zn0.88O (40nm) 异质结构,测得样品的X射线衍射谱表明,在34.56°的位置出现很强的(002)方向衍射峰,其半高宽度为0.20°,比Mg0.12Zn0.88O合金薄膜的半高宽度0.15°明显展宽.通过光致发光谱研究了MgZnO/ZnO/MgZnO异质结构的光学性质,室温下测得在370nm(3.35eV)位置有很强的紫外发光,而在348nm (3.56eV)的位置处有一个较弱的发光,这两个峰分别被归结于来自ZnO层和MgZnO盖层的发光.室温下的吸收光谱中,在上述两个峰的位置附近分别存在很明显的吸收,指示了带边吸收来自于MgZnO和ZnO两种材料.通过变温发光谱研究了异质结构中载流子弛豫、复合的规律.随着温度增加,来自于ZnO层和MgZnO层的发光强度比增加,这归结为MgZnO/ZnO异质结构存在界面势垒所致.     

Synthesis of radial‐like ZnO structure by hydrothermal method with ZnSO4·7H2O and Zn(CH3COO)2·2H2O as zinc sources

Radial‐like ZnO structures were prepared using zinc sulfate (ZnSO₄·7H₂O) and zinc acetate [Zn(CH₃COO)₂·2H₂O] as zinc sources by a facile template‐free hydrothermal method in this paper. Structural and optical properties of radial‐like ZnO structures are characterized by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV‐vis spectrophotometer and photoluminescence measurement (PL). It has been found that the distinct surface morphologies of radial‐like ZnO structures grown by different zinc sources. Slim radial‐like ZnO with a hexagonal wurtzite structure is grown by using ZnSO₄·7H₂O as zinc sources, whereas coarse radial‐like ZnO with zincite structure is achieved by zinc acetate. The UV‐vis absorption spectra of them both display an obvious and significant absorption in the ultraviolet region. The room temperature PL spectra of ZnO structures grown by two different zinc sources possess a common feature that consists of a strong ultraviolet (UV) peak and visible emission band.     

Effect of strain in the barrier layer on structural and optical properties of highly strained In0.77Ga0.23As/InGaAs multiple quantum wells

We have investigated the effect of the barrier strain in +1.65%-strained In_0.77Ga_0.23As/InGaAs multiple quantum wells (MQWs) on the structural and optical properties by means of double-crystal X-ray diffraction, transmission electron microscopy (TEM), and room-temperature photoluminescence (PL). The optimum condition of the barrier layer deduced from the X-ray and the PL measurements was nearly lattice-matching, i.e., strain from −0.40 to +0.20% is required for the sharp X-ray diffraction satellite peaks and from −0.17 to +0.14% for large PL intensity. Under compressive strain in the barrier layer, misfit dislocations are introduced into the MQW structures. In the case of tensile strain, however, threading dislocations originating from the thickness undulations in the wells and the barriers are observed. The TEM studies reveal that the thickness undulations are induced by the compositional modulation. The undulation and modulation are enhanced by increasing the tensile strain in the barrier layers. These results indicate that the strain-compensation does not work well on the MQW containing such highly strained InGaAs wells.     

Photoluminescence of Si nanocrystallites in different types of matrices

This paper presents the comparative investigation of photoluminescence (PL) and its temperature dependence for rf-magnetron co-sputtered Si-enriched SiO_x systems and amorphous Si films prepared by hot-wire CVD method with Si nanocrystallites of different sizes. It is shown that PL spectra of Si–SiO_x films consist of the five PL bands peaked at 1.30, 1.50, 1.76, 2.05 and 2.32 eV. Amorphous Si films with Si nanocrystallites are characterized by three PL bands only peaked at 1.35, 1.50 and 1.76 eV. The peak position of the 1.50 eV PL band shifts with the change of Si quantum dot sizes and it is attributed to exciton recombination inside of Si quantum dots. The nature of four other PL bands is discussed as well.     

One‐step green synthesis by organic molten salt method and optical properties of CdS nanosheets

Cadmium sulfide (CdS) nanosheets were synthesized by an environment friendly, “green” organic molten salt (OMS) method at 220 °C. The as‐synthesized products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS), respectively. The XRD results reveal that the as‐synthesized CdS nanosheets are of the hexagonal wurtzite structure and the CdS nanosheets grow along the c‐axis. The SEM results indicate that the diameters and thickness of the CdS nanosheets are about 20–40 nm and 5–10 nm, respectively. The optical properties of the CdS nanosheets were investigated by ultraviolet–visible (UV‐Vis) spectroscopy and photoluminescence (PL) spectroscopy. The ultraviolet–visible spectrum exhibits two excitonic peaks with a step‐like absorption and the photoluminescence spectrum shows a green emission peak centered at around 524 nm. A possible growth mechanism of CdS nanosheets was discussed.     

Controlled synthesis of various morphologies of nanostructured zinc oxide: flower,nanoplate, and urchin

Nanoplates, flower‐like nanostructure of ZnO were successfully synthesized by employing ZnSO₄·7H₂O, NaOH as the starting materials at 120°C under hydrothermal condition. Keeping the same parameters, ZnO urchin shape was obtained by addition of vitamin C at 190°C. Characterizations were carried out by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) at room temperature. Selected area electron diffraction (SAED) pattern confirms that the product is single crystalline nature. The possible formation mechanisms for synthesized ZnO nanosturcture with various morphologies have also been proposed. PL spectrum from the ZnO flower‐like structures reveals weak UV emission and strong green emission. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Improvement of amorphous silicon n-i-p solar cells by incorporating double-layer hydrogenated nanocrystalline silicon structure

We develop a double-layer p-type hydrogenated nanocrystalline silicon (p-nc-Si:H) structure consisting of a low hydrogen diluted i/p buffer layer and a high hydrogen diluted p-layer to improve the hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells. The electrical, optical and structural properties of p-nc-Si:H films with different hydrogen dilution ratio (R_H) are investigated. High conductivity, low activation energy and wide band gap are achieved for the thin films. Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the thin films contain nanocrystallites with grain size around 3-5 nm embedded in the amorphous silicon matrix. By inserting a p-nc-Si:H buffer layer at the i/p interface, the overall performance of the solar cell is improved significantly compared to the bufferless cell. The improvement is correlated with the reduction of the density of defect states at the i/p interface.     

Structural,Raman, and photoluminescence properties of double‐shelled coaxial nanocables of In2O3 core with ZnO and AZO shells

We synthesized In₂O₃/ZnO/Al‐doped ZnO (AZO) core‐double shell nanowires, in which the inner shell (ZnO) and the outer shell (AZO) have been subsequently deposited on the core In₂O₃ nanowires. With their one‐dimensional morphology being preserved, the X‐ray diffraction (XRD), lattice‐resolved transmission electron microscopy (TEM) image, selected area electron diffraction, and Raman spectrum coincidentally revealed that the shell was comprised of hexagonal ZnO phase. In addition, TEM‐EDX investigation revealed the presence of Al elements in the shell region. The thermal annealing at 700 °C did not significantly change the nanowire morphology, however, the XRD spectrum indicated that the ZnO phase was crystallized by the annealing. PL spectrum of the 700 °C‐annealed In₂O₃/ZnO/AZO core‐double shell nanowires was comprised of three Gaussian bands at approximately 2.1 eV, 2.4 eV, and 3.0 eV, respectively. The integrated intensities of 2.1 eV‐, 2.4 eV‐, and 3.0 eV‐bands were decreased by the thermal annealing. This study will pave the road to the preparation and applicaition of double‐shelled nanowires. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effects of ZnO coating on the photoluminescence properties of porous silicon for the advanced optoelectronic devices

In the present work we investigate, the role of zinc oxide (ZnO) thin films passivating layer deposited by rf magnetron sputtering at room temperature on low (18%) and high (80%) porosity porous silicon (PS). The micro-Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) analysis have been carried out to understand the effect of ZnO films coating on PS. A systematic investigation from Raman spectroscopy suggests the formation of a good quality ZnO wurtzite structure on PS. The photoluminescence (PL) measurements on PS and ZnO coated PS shows a red, blue and UV emission bands at around ～1.8, ～2.78 and ～3.2 eV. An enhancement of all PL emission bands have been achieved after ZnO films deposition on high porosity PS.     

Structure of SiC‐Quantum Wells Studied by TEM and CBED

Multi‐quantum well structures of 3C/4H‐SiC polytypes grown either on stepped or on on‐axis hexagonal SiC by molecular beam epitaxy have been investigated by conventional and high resolution TEM. The 3C‐SiC layers were nearly free of defects and the interface between different polytypes was abrupt. For the 3C‐SiC layers the strain state and the lattice parameters have been investigated to a high accuracy by convergent beam electron diffraction (CBED).     

Optical properties of flower‐like CdS prepared by a hydrothermal method with SDBS as surfactant

In this article, flower‐like CdS structures have been prepared by a hydrothermal method with SDBS as surfactant. The influences of different experimental conditions on the morphologies, UV‐Vis and fluorescence properties of CdS have been investigated. The performances of CdS have been analyzed by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), ultraviolet‐visible (UV–Vis) and room‐temperature photoluminescence (PL). The XRD result indicates that the flower‐like CdS structures are of hexagonal phase. The FESEM results indicate that the main role of SDBS is to make the CdS crystals assemble together to form the flower‐like structures. The UV–Vis results show CdS has a strong absorption in the ultraviolet region and visible‐light region. The PL results show CdS has two emission peaks, respectively at 461 nm and 553 nm. The growth mechanism for the formation of flower‐like CdS structures is also described.     

前驱体法制备氮氧化硅纳米线及其光学性能研究

在Ni催化剂的存在下,通过SiCl4的水解氨解反应并在1300℃氨气气氛中进行热氮化处理制得了无定形氮氧化硅纳米线.产物经X射线衍射(XRD)、热重-差示扫描量热(TG-DSC)、扫描电镜(SEM)、透射电镜(TEM)、能量色散谱(EDS)和选区电子衍射(SAED)等表征手段进行分析,结果表明纳米线为无定形结构,直径为100～150nm.在波长为220nm的光激发下,产物的光致发光光谱(PL)在563nm和289nm处分别出现了一个强的绿光发光峰和一个弱的紫光发光峰.对纳米线的生长机理进行分析,表明纳米线的生长遵循气-液-固(VLS)机制控制模式.     

RF plasma treatment of porous silicon

The effect of oxygen and hydrogen rf plasmas on the photoluminescence (PL) spectra of porous silicon (PS) was studied. PS samples were prepared from p-type, (100) oriented silicon wafers with a resistivity of 0.03 Ω cm by electrochemical anodization in an ethanol-containing solution. The plasma treatments were carried out in a planar reactor for 150 min, whereas the samples were placed on the ground electrode and heated to 250°C. The PL was excited by the 548 nm line of Hg lamp and measured using a monochromator and lock-in amplifier. Both hydrogen and oxygen plasma treatments led to a decrease in PL intensity. The decrease was with a factor of 5 after hydrogen and 2.5 after oxygen plasma treatment. It was established by means of Auger electron spectroscopy (AES) that both plasma treatments led to an increase in the oxygen content and to a decrease in the carbon content on the PS surface. However, no direct correlation between the AES data and the PL intensity could be established. We suggest that the appearance of plasma-induced electronic defects, rather than the change in the surface chemistry, accounts for the PL quenching.     

Modelling of point defects in monoclinic zirconia

We present the results of plane wave density functional theory calculations of oxygen vacancies and interstitial oxygen atoms in monoclinic zirconia. After calculating the incorporation energies and structures of interstitial oxygen atoms and formation energies of neutral oxygen vacancies, we consider the electron affinities and ionisation potentials of these defects. These properties are especially important at the silicon/oxide interface in MOSFET devices, where silicon may serve as an electron and hole source. The results demonstrate that interstitial oxygen atoms and positively charged oxygen vacancies can trap electrons if the electron source (band offset) is higher than ∼2 eV above the top of the zirconia valence band.     

Catalyst‐free thermally‐evaporated growth and optical properties of ZnO nanowires on Si,GaN and sapphire substrates

Vertically well‐aligned zinc oxide nanowires (NWs) with high density were successfully synthesized on Si, sapphire and GaN/sapphire substrates by thermal evaporation of zinc powders without catalysts or additives. The growth behavior of ZnO NWs was strongly dependent on the substrate materials. The effects of the substrate position on the structures and properties of ZnO NWs were primarily discussed. The morphology and crystallinity of the resultant NWs were studied by scanning electron microscope, transmission electronic microscope and X‐ray diffraction. The photoluminescence (PL) characteristics of the ZnO NWs on the different substrates were studied. The results showed that the as‐grown ZnO NWs exhibit a sharp and strong ultraviolet emission at 3.27 eV and a very weak green emission at around 2.48 eV, indicating that the a‐synthesized NWs have excellent PL properties with good crystalline quality and can be an ideal candidate for making luminescent devices. By comparison of PL spectra, we revealed that the green‐to‐UV emission intensity ratios were considerably dependent on the substrate materials, which was explained by the difference in the structural morphology of the produced nanowires.