Graphic method for numerical analysis of a periodically stratified thin-film omnidirectional reflector

We introduce a graphic method for analyzing a periodically stratified multilayer omnidirectional reflector. When the dispersive refractive indices of the materials are known, the contours of constant omnibandwidth and constant center wavelength with respect to layer thickness can be numerically and graphically presented for analysis. Examples of this procedure for TiO2/SiO2 and Si/SiO2 periodically layered systems for the visible and near-infrared regions are given. A comparison of omnidirectional reflectors of quarter-wave and non-quarter-wave layer thicknesses is made by the graphic method.     

Design of omnidirectional high reflectors with quarter-wave dielectric stacks for optical telecommunication bands

A design for omnidirectional high reflectors with quarter-wave dielectric stacks in the optical telecommunication band that uses conventional optical thin-film design theory is described. The omnidirectional bandwidth is derived in units of wavelength and investigated as a function of its high- and low-refractive-index values in the near infrared. The results show that the high refractive index should be larger than 2.26 for an omnidirectional high reflector and that the low refractive index for maximum onmidirectional bandwidth should be approximately 1.5. It is shown that one can design broad-bandwidth omnidirectional high reflectors for S, C, and L bands for optical telecommunication simply by connecting the band edges of omnidirectional high reflectors.     

Design of omnidirectional reflectors based on a cascaded one-dimensional photonic crystal structure

The omnidirectional transmission properties of photonic heterostructures composed of cascaded one-dimensional photonic crystals (1DPCs) with the same materials and different thickness ratios of the alternate high- and low-refractive index layers were studied theoretically. A criterion for designing omnidirectional reflectors with a maximum bandgap width is presented for heterostructures with an arbitrary number of cascaded 1DPCs. Omnidirectional reflectors based on two and three cascaded 1DPCs are designed according to the criterion.     

Short reflectors operating at the fundamental and second harmonics on 128 degree LiNbO3

In this work, we study numerically the operation of surface acoustic wave (SAW) reflectors comprising a small number of electrodes on the 128 degree YX-cut lithium niobate (LiNbO3) substrate. The electrodes have a finite thickness, and they are either open circuited or grounded. The center-to-center distance between adjacent electrodes d corresponds roughly either to half of the characteristic wavelength d proportional to lambda0/2 or to d proportional to lambda0, for the reflectors operating at the fundamental and second harmonic modes, respectively. We use software based on the finite-element and boundary-element methods (FEM/BEM) for numerical experiments with a tailored test structure having 3 interdigital transducers (IDTs), simulating experimental conditions with an incident wave and reflected and transmitted SAWs. Using the fast Fourier transform (FFT) and time-gating techniques, calculation of the Y-parameters in a wide frequency range with rather a small step allows us to determine the reflection coefficients, and to estimate the energy loss due to bulk-wave scattering. The detailed dependences of the attenuation and reflectivity on the metallization ratio and the electrode thickness are given for the classic 128 degree-cut of LiNbO3.     

Stabilization and operation of porous silicon photonic structures from near-ultraviolet to near-infrared using high-pressure water vapor annealing

The effects of high-pressure water vapor annealing (HWA), electrochemical oxidation, and substrate resistivity on the properties of porous silicon Bragg mirrors and photoluminescent cavities have been investigated. The photonic structures treated by HWA show very good stability upon ageing in air whereas as-formed structures exhibit significant drifts in their optical properties. Using HWA with lightly doped porous silicon, the structure transparency is enhanced sufficiently to enable the possible photonic operation in the near-ultraviolet. However, the index contrast achievable with these structures is very low in the visible and near-infrared. Useful index contrasts in this range can be achieved with either lightly doped porous silicon treated by electrochemical oxidation and HWA or heavily doped porous silicon treated by HWA.     

Narrow-band interference filters with unconventional spacer layers

Classical narrow-band interference dielectric filters with all-dielectric reflectors have quarter-wave stacks separated by a half-wave thickness (or a multiple-) spacer layer. These filters are essentially Fabry-Perot filters; hence the theory developed for those filters applies in full. The theory of narrow-band interference dielectric filters with unconventional spacer layers is presented. This spacer layer consists of two different materials. The optical features of these filters are compared with the features of Fabry-Perot filters. The influence of the errors of the layers on spectral characteristics is analyzed. The theory presented can be applied to any spectral range as well as to any thin-film material, including absorbing and nonlinear materials.     

Multidirectional lateral gradient films with position-dependent photonic signatures made from porous silicon

We describe here the fabrication of laterally graded porous silicon films which display gradients of photonic reflectance peaks spanning the optical spectrum. We demonstrate that up to three of these gradients can be overlayed to produce multidirectional photonic gradients with position-dependent spectral bar-codes. Each gradient is generated by asymmetric anodisation of silicon using temporal variations (sinusoidal or square-wave) in current density affording rugate and Bragg reflectors, respectively. The fabricated optical structures and the quality of the photonic resonances are characterised by optical reflectivity measurements and scanning electron microscopy. We finally remove the pSi gradient layers from the silicon substrate by applying an electropolishing current and embed the free-standing pSi membranes in polydimethylsiloxane to form flexible and foldable photonic films.     

Mode selection for a piezoelectric layer in a bulk-acoustic-wave composite acoustic resonator

An analysis is made of the mode selection for a thin piezoelectric layer in a bulk-acoustic-wave composite layered acoustic resonator by transformation of the impedance using a set of quarter-wave layers. Depending on the number of layers, the impedance of the substrate on which the piezoelectric layer is deposited may be abruptly increased or sharply reduced. This serves to simulate a “fixed” or “free” surface. As a result, a mechanically strong resonator structure is produced, operating at the natural frequencies of a thin piezoelectric layer. A numerical simulation is made of multilayer structures formed by alternating quarter-wave layers of LiNbO₃ and LiTaO₃. It is shown that if these layers are of micron thickness and there is a sufficiently large number of them, the acoustic properties of the substrate do not influence the frequency characteristics and Q factor of the thin piezoelectric vibrating layer in the microwave range. Pis’ma Zh. Tekh. Fiz. 23, 35–41 (October 12, 1997)     

Quantum efficiency of silicon photodiodes in the near-infrared spectral range

The quantum efficiency of silicon photodiodes and factors that might be responsible for the drop in quantum efficiency in the near-infrared spectral range were analyzed. It was shown that poor reflectivity from the rear surface of the die could account for a decrease in Si photodiode quantum efficiency in near-infrared spectral range by more than 20%. The photodiode quantum efficiency was modeled with an appropriate representation for the carrier-collection efficiency dependence on the die penetration depth. A corrected analytical expression for calculating the photodiode quantum efficiency is given. Some methods to improve the quantum efficiency of silicon photodiodes in near-infrared spectral range are discussed.     

Analysis and fabrication of hybrid metal-dielectric omnidirectional Bragg reflectors

We describe chalcogenide glass and polymer based Bragg reflectors with a metallic underlayer and use a transfer matrix model to analyze their performance. The angle-averaged reflectance of a hybrid mirror approaches unity for only a few periods and is much higher than that for a nonmetallized Bragg reflector or for the metallic layer alone. For an angle-averaged reflectance greater than 0.99, the addition of a metallic underlayer enables nearly a tripling of the omnidirectional bandwidth (from approximately 110 to approximately 305 nm) concurrent with a significant reduction in the number of required periods (from 10.5 to 4.5). Hybrid mirrors of 4.5 periods, with a 50 nm Au underlayer and overall thickness of approximately 2 microm, were fabricated atop silicon substrates and characterized. They exhibit an omnidirectional stop band in the 1450-1750 nm wavelength range, in good agreement with theoretical predictions.     

热氧化多孔硅制备及其干涉特性研究

采用电化学阳极氧化法制备彩色薄层多孔硅,经高温热氧化处理后形成稳定的热氧化多孔硅.研究电化学制备条件对热氧化多孔硅的干涉效应和光学厚度的影响,分析热氧化处理前后多孔硅的稳定性.结果表明,在可见光波长范围内,所制备的热氧化多孔硅反射光谱出现一定规律性的干涉条纹,表现出明显的反射干涉现象;随阳极氧化时间、电流密度和HF浓度增大,热氧化多孔硅光学厚度呈增大趋势,当阳极氧化时间为30s、电流密度为520mA/cm2、v(HF):v(C2H5OH)为2:1～5:2时,制备的热氧化多孔硅干涉条纹均匀且光学厚度较大;热氧化处理后,多孔硅结构中的Si-Hx键被Si-O键所取代,其反射干涉特性非常稳定.     

基于光谱干涉椭偏法的薄膜厚度测量

薄膜厚度的测量在芯片制造和集成电路等领域中发挥着重要作用。椭偏法具备高测量精度的优点,利用宽谱测量方式可得到全光谱的椭偏参数,实现纳米级薄膜的厚度测量。为解决半导体领域常见的透明硅基底上薄膜厚度测量的问题并消除硅层的叠加信号,本文通过偏振分离式光谱干涉椭偏系统,搭建马赫曾德实验光路,实现了近红外波段硅基底上膜厚的测量,以100 nm厚度的二氧化硅薄膜为样品,实现了纳米级的测量精度。本文所提出的测量方法适用于透明或非透明基底的薄膜厚度测量,避免了检测过程的矫正步骤或光源更换,可应用于化学气相沉积、分子束外延等薄膜制备工艺和技术的成品的高精度检测。     

GZO厚度对非晶硅电池中复合背电极的影响

在非晶硅太阳能电池中加入复合背电极是提高非晶硅太阳能电池光电转换效率和稳定性的有效手段.本文利用磁控溅射技术在非晶硅薄膜太阳能电池上制备了ZnO :Ga(GZO)/Al复合背电极,研究了GZO厚度对GZO薄膜光电性质及非晶硅电池中GZO/Al复合背电极性能的影响.研究表明：随着GZO层厚度的增加,GZO薄膜的光电性质均表现出较高水平,适合制备GZO/Al复合背电极;相较于单层Al背电极的非晶硅太阳能电池,具有GZO/Al复合背电极的太阳能电池性能大幅提高.当GZO层厚度为100 nm时,太阳能电池的短路电流(I_SC)、开路电压(V_OC)和填充因子(FF)分别达到8.66 mA,1.62 V和54.7%.     

Morphology and microstructure at different scales of porous silicon prepared by a nonconventional technique

In this work we present first results concerning the detailed structure of porous silicon (PS) layers prepared by a new method using a vapour-etching (VE)-based technique. Studies of the photoluminescence properties of VE-based PS show that the visible emission occurs at high energies as compared with PS prepared by conventional techniques. To understand the VE-based PS features, we need to point out the PS microstructure throughout its general morphology. For this purpose a microscopy multiscale study was done. Scanning, conventional transmission, and high-resolution transmission electron microscopes were employed. The investigations were made on PS films prepared from moderately and heavily doped n- and p-type silicon. SEM images show that VE-based PS layers are essentially formed of clusters like interconnected structures. TEM studies show that these clusters are composed of nanocrystallites with different shapes. The effect of the doping type of the starting Si substrate on the characteristics of the PS layers was examined (thickness, porosity, behavior). Pore propagation was found to depend on doping type. The crystallinity of the PS layers was also locally studied in depth.     

Growth, structure and epitaxy of ultrathin NiO films on Ag(001)

NiO ultrathin films have been grown on Ag(001) by Ni deposition in an O₂ atmosphere. The thickness range 5–50 ML has been investigated. X-ray photoelectron spectroscopy has been used to study the stoichiometric composition and chemical purity of the oxide films. We found completely oxidized stoichiometric NiO films. Their contamination has been found to be limited to the topmost layers. Photoelectron diffraction has given information concerning the local crystal structure of the films. The film atomic geometry has been found to be the same independent of thickness in the 0–50 ML range. The films have the expected (001) rock-salt structure with the same in plane orientation as the Ag(001) substrate. Specular X-ray reflectivity has allowed a very accurate thickness evaluation and has given information on the width of the density gradients at the film–substrate and vacuum–film interfaces, found to be of the order of a few atomic layers.     

Platinum/carbon multilayer reflectors for soft-x-ray optics

We have fabricated platinum/carbon (Pt/C) multilayer reflectors with 2d spacaings between 50 and 200 ?, using an electron-beam evaporator. We investigated the effects of 2d values, the number of layer pairs, substrate temperature, coatings, and the long-term stability on the reflectivity performance by using characteristic x rays and monochromatized synchrotron radiation in the 0.8-8-keV region. In this study we show that Pt/C multilayers with 10-20 layer pairs exhibit high and stable soft-x-ray reflectivity. The interfacial roughness was measured in the range of 5 ? and becomes lower for structures deposited at liquid-nitrogen temperatures. Coating these reflectors with a 100-?-thick platinum layer increased the grazing angle reflectivity without significantly lowering the Bragg peak reflectivity.     

Structure and surface morphology of Si1 — x Ge x layers grown on Si/sapphire by molecular beam epitaxy using a sublimation silicon source and gaseous germanium source

Si_{1 ? x} Ge _x epilayers have been grown on silicon-on-sapphire structures by molecular beam epitaxy using a sublimation silicon source and gaseous germanium source. Si_{1 ? x} Ge _x layers grown directly on sapphire substrates had poorer structural perfection, so Si buffer layers were used in subsequent growth runs. Using X-ray photoelectron spectroscopy, we determined the concentration-depth profiles of Si, Ge, and background impurities across the layers. Varying the Si buffer thickness in the range 50–300 nm was shown to have no effect on the structure of the SiGe layers. Single-crystal SiGe layers were obtained at substrate temperatures in the range 360–410°C. Varying the germanium concentration in the range 5–25% had no effect on the structure of the layers but slightly increased their roughness.     

Fast Melting of Amorphous Silicon Carbide Induced by Nanosecond Laser Pulse

We report the first experimental detailed study of laser induced surface melting on the nanoscale time scale of amorphous silicon carbide layers produced by ion implantation. Time-resolved reflectivity has been used to observe the fast liquid–solid–liquid transition features, and transmission electron microscopy (TEM) was used in order to study the structure resulting after the fast solidification following the laser induced melting. By means of the evaluation of the laser fluences required to induce melting of amorphous layers of different thickness on top of a crystalline substrate, we evaluated the thermal diffusion coefficient and the melting point of the amorphous material which occurred much lower than for crystalline material. Moreover, we give evidence of amorphous-to-crystal transitions occurring in the solid phase on the nanosecond time scale, for laser irradiation at fluences below the melting threshold. A quite different crystalline structure is observed for crystallization from the liquid phase than from the solid phase.     

Orientation study of iron phthalocyanine (FePc) thin films deposited on silicon substrate investigated by atomic force microscopy and micro-Raman spectroscopy

In this article, we present temperature and orientation study of iron phthalocyanine (FePc) thin films with different thickness deposited on silicon substrate. The organic thin films were obtained by the quasi-molecular beam evaporation. The micro-Raman scattering spectra of FePc thin layers were investigated in the spectral range 550–1,800 cm⁻¹ using 488-nm excitation wavelength. The Raman scattering and atomic force microscopy studies were performed at room temperature before and after annealing process. Annealing process of thin layers was carried out at 453 K for 6 h. From polarized Raman spectra using surface Raman mapping procedure the information on distribution of polymorphic phases of FePc layers has been carried out. Moreover, the obtained results showed the influence of the annealing process on the ordering of the molecular structure of thin films deposited on silicon substrate. For the very thin layers we did not observe the change of the polymorphic phase but only reordering of the thin layers and change of molecular structure to intermediate phase. Using atomic force microscopy method, we observed arrangement of the thin layers structure connected with the change of roughness of the thin layers after annealing process. The obtained results indicate that the structure of thin layer deposited on silicon substrate is strongly affected by the annealing process.     

Polarization-dependent angular-optical reflectance in solar-selective SnOx:F/Al2O3/Al reflector surfaces

Polarization-dependent angular-optical properties of spectrally selective reflector surfaces of fluorine-doped tin oxide (SnOx:F) deposited pyrolytically on anodized aluminum are reported. The angular-reflectance measurements, for which both s- and p-polarized light are used in the solar wavelength range 0.3-2.5 microm, reveal strong spectral selectivity, and the angular behavior is highly dependent on the polarizing component of the incident beam, the total film thickness, and the individual thickness of the Al2O3 and the SnO2:F layers. The anodic A12O3 layers were produced electrochemically and varied between 100 and 205 nm in thickness. The SnOx:F films were grown pyrolytically at a temperature of 400 degrees C with film thicknesses varying in the range 180-320 nm. The reflectors were aimed at silicon solar cells, and good spectrally selective reflector characteristics were achieved with these thinly preanodized, SnOx:F/Al samples; that is, high cell reflectance was obtained for wavelengths below 1.1 microm and low thermal reflectance for wavelengths above 1.1 microm, with the best samples having values of 0.80 and 0.42, respectively, at near-normal angles of incidence. This corresponds to an anodic layer thickness of 155 nm. Both the angular calculations and the experimental measurements show that the cell reflectance is relatively insensitive to the incidence angle, and a low thermal reflectance is maintained up to an angle of approximately 60 degrees.