Showing papers by "Songyou Wang published in 2021"

Optical Properties of Solar Absorber Materials and Structures

Abstract: As the key approach to enhance the efficient application of solar energy, solar selective absorbers have been extensively investigated in the past years. With great efforts contributed by scientists and engineers all around the world, new materials and excellent structures were achieved in solid progress to stimulate applications in broad fields. In this book, we will present an overview of both theory and experimental methods to fulfill the high-efficiency solar absorber devices. It begins with a historical description of the study and development for the spectrally selective solar absorber materials and structures based on the optical principles and methods in past decades. The optical properties of metals and dielectric materials are addressed to provide the background on how to realize high performance of the solar absorber devices applied in the solar energy field. In the following sections, different types of materials and structures, including the experimental methods, are discussed for practical construction and fabrication of the solar absorber devices, aiming at maximally harvest the solar energy, at the same time to suppress the heat-emission loss effectively. The optical principles and methods used to evaluate the performance of solar absorber devices with broad applications in different physical conditions are presented. The book will be suitable for graduate students in applied physics fields with valuables reference also for the researchers working actively in the solar energy fields.

Effects of interlayer coupling on the excitons and electronic structures of WS2/hBN/MoS2 van der Waals heterostructures

Abstract: Inserting hexagonal boron nitride (hBN) as barrier layers into bilayer transition metal dichalcogenides heterointerface has been proved an efficient method to improve two dimensional tunneling optoelectronic device performance. Nevertheless, the physical picture of interlayer coupling effect during incorporation of monolayer (1L-) hBN is not explicit yet. In this article, spectroscopic ellipsometry was used to experimentally obtain the broadband excitonic and critical point properties of WS2/MoS2 and WS2/hBN/MoS2 van der Waals heterostructures. We find that 1L-hBN can only slightly block the interlayer electron transfer from WS2 layer to MoS2 layer. Moreover, insertion of 1L-hBN weakens the interlayer coupling effect by releasing quantum confinement and reducing efficient dielectric screening. Consequently, the exciton binding energies in WS2/hBN/MoS2 heterostructures blueshift comparing to those in WS2/MoS2 heterostructures. In this exciton binding energies tuning process, the reducing dielectric screening effect plays a leading role. In the meantime, the quasi-particle (QP) bandgap remains unchanged before and after 1L-hBN insertion, which is attributed to released quantum confinement and decreased dielectric screening effects canceling each other. Unchanged QP bandgap as along with blueshift exciton binding energies lead to the redshift exciton transition energies in WS2/hBN/MoS2 heterostructures.

Structural features of chalcogenide glass SiTe: An ovonic threshold switching material

Abstract: The state-of-the-art phase-change memory is usually composed of ovonic threshold switching (OTS) material and ovonic memory switching (OMS) material for selective and data storage, respectively. OMS materials have been intensely studied, while the knowledge of the OTS mechanism is still inadequate. In this article, we have explored the local structure and electronic property of a simple OTS material, the amorphous (a-) SiTe, by first-principles calculations. The results reveal that most of the atoms in a-SiTe obey the “8-N” rule in contrast to a-GeTe, a well-studied OMS material. 76.5% of Si-centered configurations are in the form of randomly distributed tetrahedral clusters, while Te-centered configurations are relatively disordered without notable conformation. Furthermore, a large number of fivefold rings are found in a-SiTe. All of these structural characteristics lead to the high stability of a-SiTe, prohibiting its crystallization. In addition, the p state of Te also contributes much to the mid-gap states, which may be relevant for OTS behavior. Our findings provide an in-depth understanding of the structural signature and electronic properties of a-SiTe, having important implications for the design and applications of OTS materials.

Unraveling the structural and bonding nature of antimony sesquichalcogenide glass for electronic and photonic applications

Abstract: Sb-Based phase-change materials have exhibited tremendous advantages in both data storage and reconfigurable photonic devices. Despite the intensive studies on their structures and properties in the crystalline state, the widely used amorphous phase remains elusive. Here, we investigate amorphous Sb2Te3, Sb2Se3, and Sb2S3 through ab initio calculations to link their unique properties to the local structure and bonding nature. We discover that Sb forms shorter and stronger bonds with Se and S than Te, and the average bonding angles of Se (92.0°) and S (94.1°) show larger distortion than that of Te (91.5°). This leads to larger Peierls-like distortion in Sb2Se3 and Sb2S3. On the other hand, more charge transfer and void fraction are presented, opening band gaps and leading to different electronic and optical properties. In contrast, Sb2Te3, due to its semiconducting behavior and low thermal stability, enables its application in phase-change memory. Our results reveal the physics of vastly different electronic and optical properties induced by S, Se, and Te alloying, providing an effective strategy for materials design.

Manipulation of electronic property of epitaxial graphene on SiC substrate by Pb intercalation

Abstract: Manipulating the electronic properties of graphene has been a subject of great interest since it can aid material design to extend the applications of graphene to many different areas In this paper, we systematically investigate the effect of lead (Pb) intercalation on the structural and electronic properties of epitaxial graphene on the SiC(0001) substrate We show that the band structure of Pb-intercalated few-layer graphene can be effectively tuned through changing intercalation conditions, such as coverage, location of Pb, and the initial number of graphene layers Lead intercalation at the interface between the buffer layer (BL) and the SiC substrate decouples the BL from the substrate and transforms the BL into a $p$-doped graphene layer We also show that Pb atoms tend to donate electrons to neighboring layers, leading to an $n$-doping graphene layer and a small gap in the Dirac cone under a sufficiently high Pb coverage This paper provides useful guidance for manipulating the electronic properties of graphene layers on the SiC substrate

High brightness silicon nanocrystal white light-emitting diode with luminance of 2060 cd/m2.

Abstract: High brightness Si nanocrystal white light-emitting diodes (WLED) based on differentially passivated silicon nanocrystals (SiNCs) are reported. The active layer was made by mixing freestanding SiNCs with hydrogen silsesquioxane, followed by annealing at moderately high temperatures, which finally led to a continuous spectral light emission covering red, green and blue regimes. The photoluminescence quantum yield (PLQY) of the active layer was 11.4%. The SiNC WLED was composed of a front electrode, electron transfer layer, front charge confinement layer, highly luminescent active layer, rear charge confinement layer, hole transfer layer, textured p-type Si substrate and aluminum rear electrode from top to bottom. The peak luminance of the SiNC WLED achieved was 2060 cd/m2. The turn-on voltage was 3.7 V. The chromaticity of the SiNC WLED indicated white light emission that could be adjusted by changing the annealing temperature of the active layer with color temperatures ranging from 3686 to 5291 K.

Characterizations of electronic and optical properties of Sb-based phase-change material stabilized by alloying Cr

Abstract: The application of monatomic Sb material in the phase-change memory has been greatly compromised due to easy crystallization at room temperature. In this work, we alloy 10 at. % Cr into Sb, so that the crystallization temperature of the amorphous Cr10Sb90 thin film has been raised to above 130 °C and the crystallinity can be controlled by different annealing temperatures. We find that Cr10Sb90 thin films possess relatively large electrical and optical contrasts between the amorphous (a-) and crystalline (c-) states, e.g., the resistance of the a-film decreases by three orders of magnitude after crystallization and the real part of the dielectric function of glass is much larger than that of the crystal in the wavelength range of 300 to 1650 nm. The first-principles simulations reveal that Cr doping leads to a more disordered a-state and the Cr–Sb bonds appear to be stronger than Sb–Sb bonds, which explains the enhanced stability of a-Cr10Sb90. Our findings demonstrate that alloying with Cr is an effective way to improve the stability of phase-change materials in the memory applications without damaging the desired properties of these materials.

A coma-free super-high resolution optical spectrometer using 44 high dispersion sub-gratings.

Abstract: Unlike the single grating Czerny–Turner configuration spectrometers, a super-high spectral resolution optical spectrometer with zero coma aberration is first experimentally demonstrated by using a compound integrated diffraction grating module consisting of 44 high dispersion sub-gratings and a two-dimensional backside-illuminated charge-coupled device array photodetector. The demonstrated super-high resolution spectrometer gives 0.005 nm (5 pm) spectral resolution in ultra-violet range and 0.01 nm spectral resolution in the visible range, as well as a uniform efficiency of diffraction in a broad 200 nm to 1000 nm wavelength region. Our new zero-off-axis spectrometer configuration has the unique merit that enables it to be used for a wide range of spectral sensing and measurement applications.

Luminescence mechanism in hydrogenated silicon quantum dots with a single oxygen ligand

Abstract: Though photoluminescence (PL) of Si quantum dots (QDs) has been known for decades and both theoretical and experimental studies have been extensive, their luminescence mechanism has not been elaborated. Several models have been proposed to explain the mechanism. A deep insight into the origin of light emissions in Si QDs is necessary. This work calculated the ground- and excited state properties of hydrogenated Si QDs with various diameters, including full hydrogen passivation, single SiO ligands, single epoxide and coexisting SiO and epoxide structures in order to investigate the dominant contribution states for luminescence. The results revealed that even a single oxygen atom in hydrogenated Si QDs can dramatically change their electronic and optical properties. Intriguingly, we found that a size-independent emission, the strongest among all possible emissions, was induced by the single SiO passivated Si-QDs. In non-oxidized Si-QDs exhibiting a core-related size-tunable emission, the luminescence properties can be modulated by the ligands of Si QDs, and a very small number of oxygen ligands can strongly influence the luminescence of nanocrystalline silicon. Our findings deepen the understanding of the PL mechanism of Si QDs and can further promote the development of silicon-based optoelectronic devices.

High-accuracy and rapid azimuth calibration for polarizing elements in ellipsometry by differential spectral analysis on the ellipse azimuth

Abstract: We propose an accurate and rapid azimuth calibration method for polarizing elements in ellipsometry. Over 200 calibrations were achieved simultaneously at different wavelength points in a spectral range of 550-650 nm without any calibrated element. The azimuth of the polarizer was determined from the differential spectral analysis on the ellipse azimuth of reflected light. The information of the ellipse azimuth is experimentally acquired in the spectral range by a rotating polarizing element and a spectrometer. The presented method was performed and verified with Si and Au bulk, respectively, showing reliability and feasibility for efficient and reliable calibration in ellipsometry.

Method for Analyzing the Measurement Error with Respect to Azimuth and Incident Angle for the Rotating Polarizer Analyzer Ellipsometer

Abstract: We proposed a method to study the effects of azimuth and the incident angle on the accuracy and stability of rotating polarizer analyzer ellipsometer (RPAE) with bulk Au The dielectric function was obtained at various incident angles in a range of 55°–80° and analyzed with the spectrum of the principal angle The initial orientations of rotating polarizing elements were deviated by a series of angles to act as the azimuthal errors in various modes The spectroscopic measurements were performed in a wavelength range of 300–800 nm with an interval of 10 nm The repeatedly-measured ellipsometric parameters and determined dielectric constants were recorded monochromatically at wavelengths of 350, 550, and 750 nm The mean absolute relative error was employed to evaluate quantitatively the performance of instrument Apart from the RPAE, the experimental error analysis implemented in this work is also applicable to other rotating element ellipsometers

Quality Improvement of Laser-Induced Periodic Ripple Structures on Silicon Using a Bismuth-Indium Alloy Film

Abstract: In this work, a new buffer layer material, a bismuth-indium (Bi-In) alloy, was utilized to improve the quality of large-area, laser-induced periodic ripple structures on silicon. Better-defined ripple structures and larger modification areas were obtained at different scanning speeds by pre-depositing a Bi-In film. The single-spot investigations indicated that ripple structures were much easier to form on silicon coated with the Bi-In film under laser fluences of 2.04 and 2.55 J/cm2 at a fixed pulse number of 200 in comparison with on bare silicon. A physical model in terms of the excellent thermal conductivity contributed by the free electrons in the Bi-In film homogenizing the thermal distribution caused by the laser irradiation in the early stage of the formation of laser-induced periodic surface structures was proposed to explain the above phenomena. The results show that the Bi-In film enabled a wider range of laser fluences to generate periodic structures and helped to form regular ripple structures on the silicon. In addition, the modulation effects of the laser fluence and pulse number on surface structures were studied experimentally and are discussed in detail.

High-Performance Ellipsometry With 2-D Expanded Channels for Spectroscopy and Polarization Analysis

Abstract: We constructed and investigated a high-speed broadband spectroscopic ellipsometric system with 12 polarization channels and a 2-D charge-coupled device. The system without any mechanically moving component completes the measurements of more than 10 000 polarization signals at 889 data points in a wavelength range of 400–800 nm within 150 ms, with a spectral resolution better than 1 nm. An integrated analyzer consisting of 12 subanalyzers was employed to obtain the light of different polarization states simultaneously. The spectral distribution of different polarization channels was acquired by the spectral data acquisition system in parallel mode. Two kinds of data processing methods were applied to analyze the light intensities of different channels to obtain the ellipsometric information and other physical parameters of the material over a broad spectral range. According to the analysis of measurement results of gold and silicon bulk, and tantalum pentoxide film, the reliability of the proposed instrument was verified, showing application prospects in the field where in situ spectral monitoring is required.