On the band diagram of Mg2Si/Si heterojunction as deduced from optical constants dispersions

The optical constant dispersions of ion-beam-synthesized Mg₂Si phase in Si matrix are obtained from the transmittance and reflectance spectra. Two types of samples are studied - one of them with Mg₂Si phase embedded in n-type (100)Si and the other with Mg₂Si phase embedded in p-type (100)Si. The formation of the phase is proved by Raman scattering and infrared transmittance measurements. From the interpretation of the optical constant dispersions, the energies of the transitions nearby the material band edge are determined. As a result the band diagram of the heterojunction Mg₂Si/Si is obtained. The results about the Mg₂Si band gap value are compared with the theoretically predicted and experimentally determined ones. The value of the conduction band offsets of Mg₂Si and Si is not reported by now.     

Study of surface morphology and optical properties of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> thin films with annealing

Nb₂O₅ films have been deposited on variety of substrates using the sol-gel dip coating technique. As-deposited films on all substrates are amorphous. Films were annealed under controlled ambience at 300, 400 and 600°C for 5 h. As-deposited films on glass substrate show uniform surface structure. The crystal structure and surface topography are found to depend strongly on the annealing temperature and nature of the substrates. The average grain size of 40 nm is observed in films annealed at 300°C. On annealing at 400°C increasing grain size and resulting fusing of them, enhanced surface roughness. Films deposited on NaCl substrates crystallized into a stable monoclinic phase and those deposited on single crystal Si substrates crystallized into hexagonal phase after annealing at 600°C. The as-deposited films show very high transmittance (>90%) in the visible region. The optical band gap is observed to increase from 4.35 eV when the films are in amorphous state to 4.87 eV on crystallization.     

Preparation of nanocomposite for optical application using ZnTe nanoparticles and a zero-birefringence polymer

Surface-modified ZnTe nanoparticles were mixed in a zero-birefringence polymer matrix. Transmission electron microscopy images revealed that aggregates of ZnTe nanoparticles with a diameter of ∼20 nm were uniformly dispersed in the polymer. The transmittance of ZnTe nanocomposites rapidly decreased at wavelengths shorter than the critical wavelength corresponding to the band gap of ZnTe nanoparticles, an effect which became significant as the volume fraction of particles increased. In this way, the optical characteristic of ZnTe nanoparticles was added to the polymer. The intrinsic zero-birefringence was confirmed in the heat-drawn ZnTe nanocomposites. As the ZnTe nanocomposites were left in air, a lowering of transmittance was observed. This was due to the oxidation of Zn and the resultant deposition of Te in the ZnTe nanocomposite, as the light absorption of Te is significant. The formation of oxygen non-permeable SiO₂ films onto the ZnTe nanocomposite by the sol-gel method was useful in preventing oxidation so that the decrement of transmittance decreased from 47.2% to 14.9% at 530 nm near the ZnTe band gap.     

Optical and structural properties of Cu-doped β-Ga2O3 films

The intrinsic and Cu-doped β-Ga₂O₃ films were grown on Si and quartz substrates by RF magnetron sputtering in an argon and oxygen mixture ambient. The effects of the Cu doping and the post thermal annealing on the optical and structural properties of the β-Ga₂O₃ films were studied. The surface morphology, microstructure, optical transmittance, optical absorption, optical energy gap and photoluminescence of the β-Ga₂O₃ films were significantly changed after Cu-doping. After post thermal annealing, Polycrystalline β-Ga₂O₃ films were obtained, the transmittance decreased. After Cu-doping, the grain size decreased, the crystal quality deteriorated and the optical band gap shrunk. The UV, blue and green emission bands were observed and discussed. The UV and blue emission were enhanced and a new blue emission peak centred at 475 nm appeared by Cu-doping.     

Synthesis and characterization of low cost willemite based glass–ceramic for opto-electronic applications

This paper presents a comprehensive study on thermal, structural and optical properties of novel willemite glass–ceramics. The precursor glass in the ZnO–SLS glass system was successfully prepared using conventional melt-quenching technique and willemite (Zn₂SiO₄) glass–ceramics were derived from this precursor glass by a control crystallization process. The effect of heat-treatment temperature on the phase transformation, morphology and size of Zn₂SiO₄ crystal phase was examined using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) techniques. Furthermore, fourier transform infrared reflection (FTIR) spectroscopy was used to evaluate the Zn₂SiO₄ crystal structural evolution. The average size of Zn₂SiO₄ crystallite obtained from calculation of XRD is found to be in the range 30–60 nm, whereas the grain size observed in FESEM is in range of 200–400 nm. The appearance of SiO₂, ZnO₄ and Zn–O–Si bands detected from FTIR indicate the formation of Zn₂SiO₄ crystal phase. Besides, the study of the optical band gap has found that optical band gap of the glass–ceramics decreased as the heat treatment temperature increased. The photoluminescence spectra of willemite glass–ceramics exhibit two different emissions around 525 nm (green) and 585 nm (yellow); exhibit a characteristic of broad absorption band around 260 nm. These two different spectra reveal that the luminescence performance of the willemite glass–ceramics was enhanced with the progression of heat treatment temperature due to different located energy levels of the β-Zn₂SiO₄ and α-Zn₂SiO₄ crystalline phase. Such luminescent glass–ceramics was expected to find potential applications in phosphors and opto-electronic devices.     

Narrow line-width filters based on rugate structure and antireflection coating

Filters that have high transmittance over the passband region of the reflectance spectrum are designed by combining rugate structures and antireflection (AR) coatings. It is found that, under certain conditions, the refractive index of the substrate could be “converted” to the air-side refractive index of the graded-refractive-index coating. A method for the fabrication of graded-index coatings by rapidly alternating deposition of low (SiO₂) and high (Nb₂O₅) refractive index materials is introduced, and this technology was used to fabricate a rugate structure. An AR coating with a refractive index of 1.23 was grown onto the rugate structure by glancing angle deposition technology. Optical measurements of the combined structure show excellent transmittances over the wavelength regions around the reflection band and high reflectances in the stopband.     

Optical properties of nano-silicon

We investigated the optical properties of silicon clusters and Si nanocrystallites using photolumine-scence (PL) and Raman scattering technique. Broad luminescence band in the red region was observed from Si-doped SiO₂ thin films deposited by co-sputtering of Si and SiO₂ onp-type Si (100) substrates, annealed in Ar and O₂ atmosphere. Nanocrystalline Si particles fabricated by pulsed plasma processing technique showed infrared luminescence from as grown film at room temperature. Raman spectra from these films consisted of broad band superimposed on a sharp line near 516 cm⁻¹ whose intensity, frequency, and width depend on the particle sizes arising from the phonon confinement in the nanocrystalline silicon. We also performed PL, Raman and resonantly excited PL measurements on porous silicon film to compare the optical properties of Si nanostructures grown by different techniques. An extensive computer simulation using empirical pseudo-potential method was carried out for 5–18 atoms Si clusters and the calculated gap energies were close to our PL data. Paper presented at the 5th IUMRS ICA98, October 1998, Bangalore.     

Characterization of mesoporous ZnO:SiO2 films obtained by the sol-gel method

ZnO:SiO₂ films are intensively investigated for optical and electronic applications. Additionally, porous ZnO:SiO₂ films are of great interest as catalyst and gas-sensing materials. The sol-gel method is an efficient and low-cost process for the deposition of meso- and microporous silica-based films. The present paper studies the effect of the withdrawal speed on the microstructure and optical properties of mesoporous ZnO:SiO₂ films obtained by the sol-gel method. The morphology of the films was investigated by atomic force microscopy and the overall structure was studied by X-ray diffraction. The structure and size of the zinc oxide nanoparticles embedded in the silica matrix were investigated in more detail by transmission electron microscopy. These techniques showed ZnO:SiO₂ films with crack-free mesoporous morphology and highly efficient embedding of ZnO nanoparticles with (100) preferred orientation. Furthermore, the optical transmittance (in the visible and near infrared regions) and the optical band gap value were observed to vary with withdrawal speed. It is shown that ZnO:SiO₂ nanocomposites films which possess ZnO particles exhibiting a (100) orientation, with possible special applications in non-linear optics, could be prepared by the low-temperature crystallization sol-gel method.     

Preparation of one-dimensional photonic crystals by sol-gel process for magneto-optical materials

One-dimensional photonic crystal (PC) of periodically alternating low (SiO₂) and high (TiO₂) refractive index materials was prepared by sol-gel dip coating method that controls the thickness of each layer with nanometer level. The photonic band gap of high reflectivity was verified at wavelengths between 590 nm and 820 nm, which became significant with increasing the number of bilayer. The UV-vis spectra, SEM image and glow discharge optical emission spectroscopy indicate the periodic structure of SiO₂/TiO₂ multilayer. The magnetic layer of CoFe₂O₄ was also prepared by a sol-gel dip coating method. After the annealing at 700 °C, the single phase of CoFe₂O₄ film with spinel structure, without any preferred crystalline orientation, was obtained. In addition, the one-dimensional magnetophotonic crystal (MPC), in which the magnetic defect layer of CoFe₂O₄ is introduced into the periodic structure, was prepared. The light was localized at the magnetic defect due to the interference of the multilayer film, and the localized transmittance was observed around 530 nm. The Faraday rotation of MPC shows a peak at ∼ 570 nm which is close to the localized peak of transmittance. This may correspond to the enhancement of Faraday rotation due to the localization of light.     

Growth and optical properties of Sn–Si nanocomposite thin films

The growth and optical properties of nanocomposite thin films comprising of nanocrystalline Sn and Si are reported. The nanocomposite films are produced by thermal annealing of bilayers of Sn and Si deposited on borosilicate glass substrates at various temperatures from 300 to 500 °C for 1 h in air. X-ray diffraction reveals that the as-deposited bilayers consist of nanocrystalline Sn films with a crystallite size of 30 nm, while the Si thin films are amorphous. There is onset of crystallinity in Si on annealing to 300 °C with the appearance of the (111) peak of the diamond cubic structure. The crystallite size of Si increases from 5 to 18 nm, whereas the Sn crystallite size decreases with increase in annealing temperature. Significantly, there is no evidence for any Sn–Si compound, and therefore it is concluded that the films are nanocomposites of Sn and Si. Measured spectral transmittance curves show that the films have high optical absorption in the as-deposited form which decreases on annealing to 300 °C. The films show almost 80 % transmission in the visible-near infrared region when the annealing temperature is increased to 500 °C. There is concomitant decrease in refractive index from 4.0, at 1750 nm, for the as-deposited film, to 1.88 for the film annealed at 500 °C. The optical band gap of the films increases on annealing (from 1.8 to ~2.9 eV at 500 °C). The Sn-Si nanocomposites have high refractive index, large band gap, and low optical absorption, and can therefore be used in many optical applications.     

Deposition and characteristics of CdO films with absolutely (200)-preferred orientation

CdO crystal thin films with (200)-preferred orientation have been prepared on Si and glass substrates by dc reactive magnetron sputtering method. At an optimum substrate temperature of 375 °C, the film has the best crystal quality. By the electrical and transmittance measurements the film shows large carrier concentration of 2.00×10²⁰/cm³, Hall mobility of 64 cm²/V s, resistivity of 4.87×10⁻⁴ Ω cm and a high average transmittance over 80% in the visible region together with a direct band gap of 2.43 eV. In view of the Burstein–Moss (BM) shift, theoretical calculations show that the film has a direct band gap of 2.17 eV, close to its intrinsic band gap of 2.2 eV. The photoluminescence (PL) measurement shows that the pure CdO film has no luminescence behavior, but it can alloy with ZnO to realize its applications in luminescent devices.     

Microstructure, optical and dielectric properties of compositional graded (Ba,Sr)TiO3 thin films derived by RF magnetron sputtering

Thin films of compositional graded Ba_1−x Sr _x TiO₃ (BST) (x decreasing from 0.3 to 0) were prepared on fused quartz and Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering. The microstructure of the graded BST thin films was characterized by X-ray diffraction (XRD). It indicates that the films were crystallized with peroveskite structure and (100) + (111) preferred orientation. The refractive index and the band gap were determined at room temperature in the wavelength 200–1100 nm from spectrophotometric measurements of the transmittance. The average value of the refractive index is found to be 2.17 for the graded BST films in the wavelength 400–1000 nm. The optical band gap of the graded BST film was 3.77 eV. The dielectric measurement showed that the dielectric constant and loss factor of the graded BST film was 318.04 and 0.028 at 100 KHz and room temperature.     

Sol–gel and rf sputtered AZO thin films: analysis of oxidation kinetics in harsh environment

Al-doped ZnO (AZO) thin film, which possess the advantages of low cost, low sheet resistance and high transmittance, are one of the most promising candidates to replace indium tin oxide films as the transparent electrode. However, oxidation causes a substantial increase in the sheet resistance of AZO film after exposing in ambient and especially, damp heat environment. In this work, we compare structural, optical, electrical properties and environmental stability between films prepared by two different methods: sol–gel and rf sputtering. Experimental results indicate that the properties of film can be affected by different deposition method. From the X-ray diffraction analysis, all films have hexagonal wurtzite crystal structure with different preferable orientation in two different methods. Optical transmittance spectra of the AZO films exhibited transparency higher than about 80 % within the visible wavelength region and the optical band gap (E_g) of these films was increased in sputtered film, probably due to the increase of carrier concentration. The better environmental stability was found in AZO film prepared by sputtering method. Improved surface morphology and enhancement of crystal orientation (110) was considered for this improvement.     

Piezoelectric properties of niobium-doped 18PMN-41PZ-41PT ceramics

The effects of various amounts of excess niobium ions added to the 18PMN-41PZ-41PT three-component ceramics system on the crystal structure, microstructure, polarization versus electric field, strain versus electric field and piezoelectric properties, have been studied. It was found that addition of niobium ions to 18PMN-41PZ-41PT induced a change in the crystal structure from rhombohedral to tetragonal phase. The excessive niobium ions also formed a micro-second phase which existed in the grain boundary, which could refine and uniform the grain. The refined and uniformed microstructure was found to increase the remanent polarization, saturation strain, planar coupling coefficient, K _p, and mechanical quality factor, the best value of K _p being 69%.     

升温对[100]cub弛豫铁电PMN-32PT单晶电畴组态演变的影响

采用热台偏光显微镜观察以ACRT+ Bridgman法获得、经过金相抛光处理的弛豫铁电PMN-32PT单晶[100]cub切型电畴组态,考察了未退火状态下[100]cub切型单晶的消光行为及连续升温对本征电畴与由抛光机械应力诱导非本征电畴组态的影响.结果表明,在升温过程中,PMN-32PT单晶非本征电畴在三方-四方铁电相变后消失;本征电畴组态的变化表现为两个阶段,第一阶段畴先细化后宽化,第二阶段进行铁电-顺电相变,相变过程缓慢,持续范围为168～177℃,且与三方-四方相变出现次序相反.     

Structural, morphological and optical properties of TiO2 films prepared using aqueous sol–gel methods

The influence of the preparation conditions on the structural, morphological and optical properties of TiO₂ thin films deposited on silicon substrate (Si), indium tin oxide coated glass (ITO) and alkali-free borosilicate glass (AFG), respectively is studied in this work. The X-ray diffraction analysis revealed that all TiO₂ samples had a polycrystalline structure. The TiO₂ films coated on Si showed a mixed phase of anatase and rutile while in the case of those on ITO and AFG only the pure anatase phase was observed. The crystallite size within the TiO₂ thin films varied with the calcinations temperature, solvent lateral chain and catalyst type. The optical transmittance, band gap, reflective index and porosity were strongly affected by the annealing temperature, substrate nature and solvent.     

Preparation and characterization of highly (1 1 1)-oriented (Pb,La)(Zr,Ti)O3 thin films by sol-gel processing

Lanthanum modified lead zirconate titanate (Pb_0.91La_0.09)(Zr_0.65Ti_0.35)O₃ (PLZT) ferroelectric thin films were grown on Pt/Ti/SiO₂/Si(1 0 0) and fused quartz substrates using a sol-gel method with rapid thermal annealing processing. The results showed that the highly (1 1 1)-oriented pervoskite PLZT thin film growth on Pt/Ti/SiO₂/Si(1 0 0) substrates. The electrical measurements were conducted on PLZT films in metal-ferroelectric-metal capacitor configuration. The PLZT thin films annealed at 600 °C showed well-saturated hysteresis loops with remanent polarization and coercive electric field values were 10.3 μC/cm² and 36 kV/cm, respectively, at an applied field of 300 kV/cm. At 100 kHz, the dielectric constant and dielectric loss of the film are 682 and 0.021, respectively. The PLZT thin film on fused quartz substrate, annealed at 600 °C, exhibited good optical transmittance, the band gap of optical direct transitions is 3.89 eV.     

Investigation of the substrate effect for Zr doped ZnO thin film deposition by thermionic vacuum arc technique

ZnO is a fundamental wide band gap semiconductor. Especially, doped elements change the optical properties of the ZnO thin film, drastically. Doped ZnO semiconductor is a promising materials for the transparent conductive oxide layer. Especially, Zr doped ZnO is a potential material for the high performance TCO. In this paper, ZnO semiconductors were doped with Zr element and microstructural, surface and optical properties of the Zr doped ZnO thin films were investigated. Zr doped ZnO thin films were deposited thermionic vacuum arc (TVA) technique. TVA is a rapid and high vacuum deposition method. A glass, polyethylene terephthalate and Si wafer (111) were used as a substrate material. Zr doped ZnO thin films deposited by TVA technique and their substrate effect investigated. As a results, deposited thin films has a high transparency. The crystal orientation of the films are in polycrystal formation. Especially, substrate crystal orientation strongly change the crystal formation of the films. Substrate crystal structure can change the optical band gap, microstructural properties and deposited layer formation. According to the atomic force microscopy and field emission scanning electron microscopy measurements, all deposited layer shows homogeneous, compact and low roughness. The band values of the deposited thin film were approximately found as to be 3.1–3.4 eV. According to the results, Zr elements created more optical defect and shifted to the band gap value towards to blue region.     

Investigations of gold surface state energy levels at the silicon-silicon dioxide interface by the MOS capacitance-voltage measurement technique

The influence of gold in the Si/SiO₂ system has been studied both theoretically and experimentally. The gold is found to introduce two regions of shallow acceptor surface state energy levels in the silicon band gap at the SiSiO₂ interface: one at 0.09 eV from the valence band edge and the other at 0.13 eV from the conduction band edge. It is tentatively postulated that both the surface state energy levels are due to gold atoms in substitutional sites.The two deep normal bulk gold energy levels in silicon do not seem to occur at the SiSiO₂ interface.Finally a model of gold diffusion is suggested, and gold is found to diffuse through the bulk silicon by a complex interstitial and substitutional mechanism.     

Influence of defects on the quality and optical studies of sodium p-nitrophenolate dihydrate, a nonlinear optical material

Nonlinear optical material of sodium p-nitrophenolate dihydrate was synthesized by employing the technique of controlled evaporation and the effect of temperature on growth morphology was investigated. Single crystal XRD analysis confirms that the crystals with different morphologies have the same lattice parameters. The high-resolution X-ray diffraction curves recorded by a multicrystal X-ray diffractometer revealed the presence of very low angle tilt boundaries. The green emission band at 524 nm is due to the existence of defects on the crystal which corroborated with HRXRD studies. The wide band gap of the SPND crystals confirms the large transmittance in the visible region.