Some silicon-based heterostructures for optical applications

In this paper, we will present our recent research on the growth and characterization of some Si-based heterostructures for optical and photonic devices. The heterostructures to be discussed are ZnO nanorods on Si, SiO₂, and other substrates such as SiN and sapphire. We will also consider strained Si_1−xGe_x/Si heterostructures for Si optoelectronics. The performance and functionality extension of Si technology for photonic applications due to the development of such heterostructures will be presented. We will focus on the results of structural and optical characterization in relation to device properties. The structural characterization includes x-ray diffraction for assessment of the crystallinity and stress in the films and secondary ion mass spectrometry for chemical analysis. The optical properties and electronic structure were investigated by using photoluminescence. The device application of these thin film structures includes detectors, lasers, and light emitting devices. Some of the Si-based heterostructures to be presented include devices emitting and detecting up to the blue-green and violet wave lengths.     

The effect of thermal annealing on the structure of nanocrystalline zinc sulfide films

The effect of thermal annealing on the structure of nanocrystalline ZnS films obtained at negative (centigrade) condensation temperatures on NaCl, Si, and SiO₂ substrates has been studied. It is shown that the structure of zinc sulfide films on Si and SiO₂ substrates differs from that of films on NaCl substrates. The hexagonal phase appears in the films on NaCl and disappears upon annealing. The appearance and disappearance of this phase is due to the effect of stresses in the film-substrate system.     

Density Functional Theory Modeling of (001)Si–Oxynitride Interfaces

Four interface models for crystalline oxynitride on (001)Si substrates are proposed and investigated. All four models are proposed to model thin oxynitride films on Si substrates, according to experimental findings and theoretical studies on amorphous oxynitride films and nitrogenated SiO₂. State-free insulating interfaces were obtained by expanding the bulk oxynitride cell by approximately 12% and 1% along the [100] and [010] axes, respectively, and interfacing it with (001)Si. Results demonstrate state-free insulating interfaces for all models with, however, valence-band offsets slightly above or below 1 eV. The significant decrease in the valence-band offsets is mainly attributed to the significant expansion of the oxynitride’s lattice constant to lattice-matched (001)Si, as well as to the high concentration of nitrogen atoms in the oxynitride bulk.     

Properties of Sb2S3 and Sb2Se3 thin films obtained by pulsed laser ablation

The properties of Sb₂S₃ and Sb₂Se₃ thin films of variable thickness deposited onto Al₂O₃, Si, and KCl substrates are investigated by the method of pulsed laser ablation. The samples are obtained at a substrate temperature of 180°C in a vacuum chamber with a residual pressure of 10^?5 Torr. The thickness of the films amounted to 40–1500 nm. The structure of the bulk material of the targets and films is investigated by the methods of X-ray diffraction and transmission high-energy electron diffraction, respectively. The electrical properties of the films are investigated in the temperature range of 253–310 K. It is shown that the films have semiconductor properties. The structural features of the films determine their optical parameters.     

Phase Selection in Mn–Si Alloys by Fast Solid-State Reaction with Enhanced Skyrmion Stability

B20-type transition-metal silicides or germanides are noncentrosymmetric materials hosting magnetic skyrmions, which are promising information carriers in spintronic devices. The prerequisite is to prepare thin films on technology-relevant substrates with magnetic skyrmions stabilized at a broad temperature and magnetic-field working window. A canonical example is the B20-MnSi film grown on Si substrates. However, the as-yet unavoidable contamination with MnSi_1.7 occurs due to the lower nucleation temperature of this phase. In this work, a simple and efficient method to overcome this problem and prepare single-phase MnSi films on Si substrates is reported. It is based on the millisecond reaction between metallic Mn and Si using flash-lamp annealing (FLA). By controlling the FLA energy density, single-phase MnSi or MnSi_1.7 or their mixture can be grown at will. Compared with bulk MnSi, the prepared MnSi films show an increased Curie temperature of up to 41 K. In particular, the magnetic skyrmions are stable over a much wider temperature and magnetic-field range than reported previously. The results constitute a novel phase selection approach for alloys and can help to enhance specific functional properties, such as the stability of magnetic skyrmions.     

Plasma-assisted chemical vapor deposited silicon oxynitride as an alternative material for gate dielectric in MOS devices

Silicon oxynitride films have been deposited on Si substrates at 200 °C by a remote-plasma-assisted process in a RF-plasma CVD reactor using Si(OC₂H₅)₄ (TEOS) as a precursor and nitrogen as gas ambient. During deposition the Si substrates were biased with negative voltages of −120 and −600 V or were under no DC bias and the influence of this voltage on the film properties has been considered. Film parameters, such as density, chemical bonds, refractive index, composition, oxide and interface charge densities of the deposited dielectric films have been estimated by analysis of the results from the infrared (IR) spectroscopy, spectral ellipsometry (SE) and capacitance-voltage (C-V) measurements. The IR and SE results have proven the films are oxynitrides of silicon with predominantly oxide network. The analysis of the capacitance-voltage characteristics has shown that the dielectric charge densities increase with increasing DC bias but they remain considerably low in comparison to that for a standard SiO₂/Si structure before any annealing steps.     

SiGe MOSFET structures on silicon-on-sapphire substrates grown by ultra-high vacuum chemical vapor deposition

SiGe heterostructures on silicon-on-sapphire (SOS) substrates were investigated to determine the advantages of combining these two technologies. Devicequality epitaxial layer structures were grown by ultra-high vacuum chemical vapor deposition (UHV/CVD) on silicon-on-sapphire substrates having a very low density of microtwin defects. Enhancements in device performance comparable to similar SiGe devices on bulk Si substrates were achieved, even though significant interdiffusion of Si and Ge had occurred during device fabrication processes at T>850°C. These results emphasize the need for low temperature fabrication processes to fully exploit SiGe heterostructures for device applications.     

Characterization of sputtered nano-crystalline zirconium carbide as a diffusion barrier for Cu metallization

Zirconium carbide (ZrC) films were grown on Si (100) substrates using magnetron sputtering where the growth temperature (T_s) was varied from 25°C to 290°C. The microstructure and resistivity of the as-deposited ZrC films were examined. The results reveal that nano-crystalline ZrC films with grain size less than 5 nm were fabricated only at 29°C, which can be explained by a repeated nucleation mechanism. For thermal stability characterization, the stacked structure of Cu/ZrC/Si was subsequently subject to thermal treatments at temperatures from 300°C to 900°C for 30 min in a vacuum tube. The stacked samples were shown to be thermally stable up to about 800°C from Auger electron spectroscopy (AES) and x-ray diffraction (XRD). The diffusion coefficient and activation energy of Cu and Si in the ZrC barrier were also derived. It indicated that Si has a lower activation energy than Cu resulting in faster diffusion. The device completely fails at 900°C, and the mechanism is discussed in this paper.     

Electrical characteristics of palladium silicide

Resistivity and Hall mobility measurements were performed on Pd₂Si films grown on <100〉 and <111〉 oriented silicon substrates as a function of temperature and thickness of the films. The results show that Pd₂Si has metallic character. The Debye temperature of Pd₂Si was found to be 120±20°K and the concentration of the charge carriers, which are electrons, is 4 × 10²¹ cm^?3. The bulk value of the resistivity at room temperature is 25–30 ωΩcm and the Hall mobility is 50–60 cm² V^?1 sec^?1, both depending on the structure of Pd₂Si which is known to be epitaxial on <111〉 substrate but not on <100〉 substrate. This structural difference is clearly reflected by the electrical characteristics.     

Plasmon–phonon coupling in the infrared reflectance spectra of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> films

The results of studies of optical reflection in the far- and mid-infrared spectral regions are reported. The reflectance of five Bi₂Se₃ topological insulator films grown by molecular-beam epitaxy on Si(111) substrates is measured. The characteristic parameters of phonons and plasmons are determined by means of dispersion analysis for multilayer structures. It is found that the plasma frequency in a layer close to the Si–film interface is noticeably higher than that in the film bulk. Calculations of the loss function show that plasmon–phonon coupling plays an important role in Bi₂Se₃ films. The attenuated total internal reflection method is used to determine the frequency of the surface plasmon–phonon mode.     

Growth and characterization of superconducting V3Si on Si and Al2O3 by molecular beam epitaxial techniques

A study of the growth parameters governing the nucleation of metastable superconducting A15 V₃Si on Si and A1₂O₃ is presented. Nominally, 500Å films of V_1-xSi_x were produced through codeposition of V and Si onto heated (111) Si and (1102) A1₂O₃ substrates. Samples were prepared in a custom-built ultrahigh vacuum (UHV) chamber containing dual e-beam evaporation sources and a high temperature substrate heater. V and Si fluxes were adjusted to result in the desired average film composition. V_0.75Si_0.25 films prepared at temperatures in excess of 550° C on Si show significant reaction with the substrate and are nonsuperconducting while similar films grown on A1₂O₃ exhibit superconducting transition temperatures(@#@ T_c @#@) approaching bulk values for V₃Si (16.6-17.1 K). Codeposition at temperatures between 350 and 550° C results in superconducting films on Si substrates while growth at lower temperatures results in nonsuperconducting films. Lowering the growth temperature to 400° C has been shown throughex situ transmission electron microscopy (TEM) and Auger compositional profiling to minimize the reaction with the Si substrate while still activating the surface migration processes needed to nucleate A15 V₃Si. Variation of film composition aboutx = 0.25 is shown to result in nonsuperconducting films for highx and superconducting films with T_c approaching the bulk V value (5.4 K) for lowx. Finally, lowering the V_0.75Si_0.25 deposition rate is shown to raise T_c.     

In situ studies of epitaxial silicon growth by gas source molecular beam epitaxy

The value of in situ monitoring to study growth dynamics and surface reaction kinetics in a gas source molecular beam epitaxy process is illustrated with reference to the growth of Si films on Si(001) substrates using a beam of disilane (Si₂H₆). By using a combination of reflection high-energy electron diffraction (RHEED) and reflection anisotropy spectroscopy (RAS), we show first how morphological (long-range order) and local electronic structure effects can be separated in the evaluation of growth dynamics. This involves the measurement of step density changes by RHEED concomitantly with the variation in domain coverage on the Si(001) (2×1)+(1×2) reconstructed surface by RAS. This approach is then extended to investigate the kinetics of hydrogen desorption, which is the rate-limiting step in Si growth from Si₂H₆. It is shown that over a significant temperature range, zeroth-order kinetics are obeyed and this is explained on the basis of a step-mediated desorption process. Finally we show how this influences the growth rate on substrates of differing degrees of vicinality. © 1997 John Wiley & Sons, Ltd.     

Selective CdTe Nanoheteroepitaxial Growth on Si(100) Substrates Using the Close-Spaced Sublimation Technique Without the Use of a Mask

The development of HgCdTe detectors requires high sensitivity, small pixel size, low defect density, long-term thermal-cycling reliability, and large-area substrates. CdTe bulk substrates were initially used for epitaxial growth of HgCdTe films. However, CdTe has a lattice mismatch with long-wavelength infrared (LWIR) and middle-wavelength infrared (MWIR) HgCdTe that results in detrimental dislocation densities above mid-10⁶ cm^?2. This work explores the use of CdTe/Si as a possible substrate for HgCdTe detectors. Although there is a 19% lattice mismatch between CdTe and Si, the nanoheteroepitaxy (NHE) technique makes it possible to grow CdTe on Si substrates with fewer defects at the CdTe/Si interface. In this work, Si(100) was patterned using photolithography and dry etching to create 500-nm to 1-μm pillars. CdTe was selectively deposited on the pillar surfaces using the close-spaced sublimation (CSS) technique. Scanning electron microscopy (SEM) was used to characterize the CdTe selective growth and grain morphology, and transmission electron microscopy (TEM) was used to analyze the structure and quality of the grains. CdTe selectivity was achieved for most of the substrate and source temperatures used in this study. The ability to selectively deposit CdTe on patterned Si(100) substrates without the use of a mask or seed layer has not been observed before using the CSS technique. The results from this study confirm that CSS has the potential to be an effective and low-cost technique for selective nanoheteroepitaxial growth of CdTe films on Si(100) substrates for infrared detector applications.     

Cyclic Annealing During Metalorganic Vapor-Phase Epitaxial Growth of (211)B CdTe on (211) Si Substrates

High-quality (211)B CdTe buffer layers on Si substrates are required to enable Hg_1–x Cd _x Te growth and device fabrication on lattice-mismatched Si substrates. Metalorganic vapor-phase epitaxy (MOVPE) of (211)B CdTe on Si substrates using Ge and ZnTe interlayers has been achieved. Cyclic annealing has been used during growth of thick CdTe layers in order to improve crystal quality. The best (211)B CdTe/Si films grown in this study display a low x-ray diffraction (XRD) rocking-curve full-width at half-maximum (FWHM) of 85 arcsec and etch pit density (EPD) of 2 × 10⁶ cm⁻². These values are the best reported for MOVPE-grown (211) CdTe/Si and are comparable to those for state-of-the-art molecular beam epitaxy (MBE)-grown CdTe/Si.     

Artificial GeSi substrates for heteroepitaxy: Achievements and problems

It is desirable to have a set of substrates which are based on Si and ensure growth of heterostructures with various lattice parameters in order to develop electronic devices composed of semiconductor materials whose epitaxial growth is reasonably well developed. Such substrates are typically referred to as artificial. In this paper, a comparative analysis of various methods for the fabrication of artificial substrates (heterostructures), in which the relaxation of stresses is based on the introduction of misfit dislocations, is performed. Based on published and new experimental data, the mechanisms for attaining a low density of threading dislocations in plastically relaxed films represented by heterostructures composed of GeSi and an Si buffer layer grown at low temperatures are analyzed. The problems and results of another group of methods for obtaining artificial substrates which gained favor recently and become known as “compliant” or “soft” substrates are discussed. The most important electrical parameters of Si and GeSi films grown on artificial substrates are considered.     

Infrared Optical Characterization of PLT Thin Films for Applications in Uncooled Infrared Detectors

Lanthanum-modified lead titanate (PLT) thin films have been grown directly on Pt/Ti/SiO₂/Si (100) and LaNiO₃/Si (100) substrates by a modified sol-gel method. X-ray diffraction analysis shows that the PLT thin films are polycrystalline. The infrared optical properties of the thin films were investigated using infrared spectroscopic ellipsometry (IRSE) in the spectral range of 2.5–12.5 μm. By fitting the measured ellipsometric parameter (tan Ψ and cosΔ) data with a three phase model (air/PLT/Pt) for the PLT thin films on Pt/Ti/SiO₂/Si (100) and a four phase model (air/PLT/LNO/Si) for the PLT thin films on LaNiO₃/Si (100) substrates, and a derived classical dispersion relation for the thin films, the optical constants and thicknesses of the thin films have been simultaneously obtained. The refractive index and extinction coefficient of the PLT thin films on Pt/Ti/SiO₂/Si (100) substrates are slightly larger than those on LaNiO₃/Si (100) substrates. Given the infrared semitransparent metal of Nickel currently used, the absorption of the Ni/PLT/Pt and Ni/PLT/LNO/Si multilayer thin films in this study is very large around 3.0 μm and 5.7 μm wavelength range and decrease to 15% or 20% in the 8–12.5 μm wavelength region.     

8% Efficient thin‐film polycrystalline‐silicon solar cells based on aluminum‐ induced crystallization and thermal CVD

A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from polycrystalline‐silicon (pc‐Si) thin films on inexpensive substrates. We recently showed promising solar cell results using pc‐Si layers obtained by aluminum‐induced crystallization (AIC) of amorphous silicon in combination with thermal chemical vapor deposition (CVD). To obtain highly efficient pc‐Si solar cells, however, the material quality has to be optimized and cell processes different from those applied for standard bulk‐Si solar cells have to be developed. In this work, we present the different process steps that we recently developed to enhance the efficiency of pc‐Si solar cells on alumina substrates made by AIC in combination with thermal CVD. Our present pc‐Si solar cell process yields cells in substrate configuration with efficiencies so far of up to 8·0%. Spin‐on oxides are used to smoothen the alumina substrate surface to enhance the electronic quality of the absorber layers. The cells have heterojunction emitters consisting of thin a‐Si layers that yield much higher V_oc values than classical diffused emitters. Base and emitter contacts are on top of the cell in interdigitated finger patterns, leading to fill factors above 70%. The front surface of the cells is plasma textured to increase the current density. Our present pc‐Si solar cell efficiency of 8% together with the fast progression that we have made over the last few years indicate the large potential of pc‐Si solar cells based on the AIC seed layer approach. Copyright © 2007 John Wiley & Sons, Ltd.     

Surface Defect Suppression for High Color Purity Light-Emitting Diode of Free-Standing Single-Crystal Perovskite Film

High color purity is one of the important features for single-crystal metal halide perovskite light-emitting diode (LED). Despite single-crystal perovskite showing low bulk defect concentration, single-crystal perovskite LEDs do not exhibit high color purity advantage due to the absence of effective surface defect passivation. Herein, one fully wrapped structure is proposed to passivate the surface of the free-standing CsPbBr₃ single-crystal films. The surface of CsPbBr₃ single-crystal films is wrapped by ultra-thin polymethyl methacrylate, precisely controlling the thickness. The single-crystal perovskite film device can achieve high color purity with a full width at half maximum (FWHM) of 15.8 nm) and a large luminescent area of 2.25 mm². It is observed that surface passivation is due to interaction of CO bond in polymer chains with the Lewis acid PbBr₂. The passivated perovskite single-crystal films significantly improve carrier lifetime and suppress surface defects. It is noteworthy that the passivated free-standing single-crystal perovskite films are feasible to build up a vertical LED device structure, avoiding the edge glowing and short-circuiting of the LED device. This study demonstrates the large luminescent area of the high-quality millimetre-scale free-standing single-crystal films for wide color gamut display and vertical optoelectronic devices.     

Mg2Si buffer layers on Si(100) prepared by a simple evaporation method

The formation of Mg₂Si(100), a_o= 6.39Å, on Si(100) substrates has been investigated. Mg was first evaporated onto Si(100) surfaces and Mg₂Si (100) films were formed in a subsequent annealing process. The Mg₂Si layers were characterized by x-ray diffraction and transmission electron microscopy analysis. Optical and scanning electron microscopy analysis show the surface morphology to be smooth. The films are stable under thermal cycling and exhibit low resistivity. Epitaxial films of Mg₂Si on Si(100) could be an ideal substrate for mercury cadmium telluride and antimonide based III-V semiconductor for mid-infrared devices because of its close lattice matching (the lattice misfit factor is less than 1.5%).     

无掩模硅图形衬底上3C-SiC的异质外延生长

采用低压化学气相沉积方法在无掩模的硅图形衬底上异质外延生长3C-SiC.硅图形衬底采用光刻和ICP刻蚀得到.图形由平行长条状沟槽和台面组成,其中沟槽宽度为1～10μm不同间隔,沟槽之间距离为1～10μm不同间隔.对于在不同的沟槽和台面尺寸区域3C-SiC的生长进行了详细研究.采用扫描电镜分别观察了不同区域的生长形貌,分析了图形衬底结构上SiC的生长行为.其中合并生长形成的空气隙结构可以释放由Si和SiC晶格失配引起的应力,从而可以用来解决SiC生长中的晶片翘曲问题,进行厚膜生长.XRD结果表明此无掩模硅图形衬底上得到3C-SiC(111)取向生长.