Band gap engineering of atomically thin two-dimensional semiconductors

Atomically thin two-dimensional （2D） layered materials have potential applications in nanoelectronics, nanophoton- ics, and integrated optoelectronics. Band gap engineering of these 2D semiconductors is critical for their broad applications in high-performance integrated devices, such as broad-band photodetectors, multi-color light emitting diodes （LEDs）, and high-efficiency photovoltaic devices. In this review, we will summarize the recent progress on the controlled growth of composition modulated atomically thin 2D semiconductor alloys with band gaps tuned in a wide range, as well as their induced applications in broadly tunable optoelectronic components. The band gap engineered 2D semiconductors could open up an exciting opportunity for probing their fundamental physical properties in 2D systems and may find diverse applications in functional electronic/optoelectronic devices.

An Effective Thin Reservoir Identification Method for Fine Oilfield Development

Thin reservoirs prediction method such as spectral inversion has drawn considerable attention in recent years. In order to avoid extracting wavelets within the whole field area purposeless and to make the filtered data has preferable fidelity as well as signal-to-noise ratio, an effective structural constrained thin reservoir description method which combines spectral inversion and wide-band Ricker wavelet filtering technology has been proposed in this paper. The method given here is more credible and is suitable for the prediction of middle-deep thin reservoirs. We take LD-A structure within Bohai Bay Basin as an example to show the implement of our method. Several sets of thin sand layers which are hardly to recognize originally have been finally identified. Also, with the application of this method, a high-production thin reservoir of LD-B structure has been identified accurately, which provides credible information for subsequent fine oil exploration and development.

THEORY AND EXPERIMENTAL INVESTIGAION OF FLEXURAL WAVE PROPAGATION IN THIN RECTANGULAR PLATE WITH PERIODIC STRUCTURE

With the idea of the phononic crystals, a thin rectangular plate with two-dimensional periodic structure is designed. Flexural wave band structures of such a plate with infinite structure are calculated with the plane-wave expansion （PWE） method, and directional band gaps are found in the ΓX direction. The acceleration frequency response in the ΓX direction of such a plate with finite structure is simulated with the finite element method and verified with a vibration experiment. The frequency ranges of sharp drops in the calculated and measured acceleration frequency response curves are in basic agreement with those in the band structures. Thin plate is a widely used component in the engineering structures. The existence of band gaps in such periodic structures gives a new idea in vibration control of thin plates.

Fabrication and Characterization of PZN-4.5PT Inorganic Perovskites Nanoparticles Thin Films Deposited on P-Type Silicon Substrate

This work involves an investigation of nanostructures, microelectronic properties and domain engineering of nanoparticles thin layers of Pb(Zn1/ 3Nb2/3)O3-PbTiO3 (PZN-PT) ferroelectric single crystals deposited on nanostructured silicon substrate. In this study, devices made from PZN-4.5PT nanoparticles thin films successfully deposited on silicon substrate have been studied and discussed. SEM images show the formation of local black circles and hexagonal shapes probably due to the nucleation of a new Si-gel component or phase induced by annealing. Micro Xray Fluorescence mapping shows that the high values of Si and B atoms (≅7 and 4 normalized unit respectively) can be explained by the fact that the substrate is p-type silicon. The most interesting result of optical measurements is the very good absorption for all the thin films in UV, Visible and NIR regions with values from 70% to 90% in UV, from 75% to 93% in Visible and NIR. Tauc plots present particularities (rarely encountered behavior) with different segments or absorption changes showing the presence of multiple band gaps coming from the heterogeneity of the thin films (nanowires, gel and nanoparticles). Their values are 1.9 and 2.8 eV for DKRN-Gel, 2.1 and 3.1 eV for DKRN-UD and 2.1 and 3.2 eV for DKRN-D) corresponding respectively to the band gap of nanowires and that of the gel while the last ones correspond to the undoped and doped nanoparticles (3.1 and 3.2 eV respectively).

Effect of Substrate on CuS/PVA Nanocomposite Thin Films Deposited on Glass and Silicon Substrate

Transition metal chalcogenide nanocomposite thin films deposited by chemical routes are currently attracting wide attention being inexpensive, simple and have utility for large area applications. The role of substrate becomes very important in film deposition as well as in controlling their properties due to strain induced properties modification and lattice mismatch. CuS/PVA nanocomposite thin films were successfully deposited on glass and silicon substrates using sol-gel technique. Thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), UV-visible (UV-VIS) and Raman spectroscopy. Structural data confirm the amorphous nature of as grown films which transform into crystalline films after annealing at 200°C. The degree of crystallinity seems to be better in film deposited on silicon substrate in comparison to those grown over glass substrate with average crystallite sizes ≅?4.00 nm and 7.00 nm for films deposited on glass and silicon substrate respectively. Atomic force microscopy (AFM) images in dynamic as well as contact modes display nanoparticles embedded in polymer network. The films surface roughness parameters quantitatively estimated from AFM micrographs are compared. Raman spectra show a sharp peak at ≅474 cm¯1 assigned to S-S stretching mode of S2 ions in films grown on both substrates and associated as due to presence of hexagonal (covellite) crystal structure. Optical band gaps of thin film on glass and silicon substrate are 2.10 eV and 2.02 eV respectively. The effect of substrate on the measured properties is discussed.

Optoelectronic Characterization of Chemical Bath Deposited Cd_xCo_1-xS Thin Film

Cadmium Cobalt Sulphide (CdxCo1-xS) thin film was deposited on microscopic glass substrate using chemical bath deposition technique at room temperature from aqueous solutions of Cadmium Chloride, Cobalt Chloride and Thiourea in which ammonium solution was used as complexing agents. The optical properties were characterized using the absorbance and transmission measurement from Unico UV-2102 PC spectrophotometer, at normal incidence of light in the wavelength range of 200 - 1000 nm. We report the deposition and optimization of the growth parameter with respect to time which showed that the band gap energy and the composition verified from the extended Vegard’s law are highly dependent on deposition time. The average transmittance of the film in VIS-NIR region ranges between 30% and 78% with absorbance range of 0.15 - 0.47 within the same wavelength range. The film was also observed to exhibit poor reflectance (11 x = 0.75;0.83 and 0.94), respectively. Other optical and dielectric properties of the films were also characterized. Based on the exhibited properties of the film, it can be concluded that it is a promising material for selective coatings for solar cells;effective coatings for poultry houses;use as antireflective coating materials, and for fabrication of optoelectronic devices.

Modeling of polycrystalline ZnO thin-film transistors with a consideration of the deep and tail states

We report a model of the carrier transport and the subgap density of states in a polycrystalline ZnO film for simulating a polycrystalline ZnO thin film transistor. This simple model considering the deep and the band tail states reproduces well the characteristics of polycrystalline ZnO thin film transistors. Furthermore, using the developed model, we study the effects of defect parameters on the electrical performances of the polycrystalline ZnO thin film transistors.

Role of Substrate Temperatures on Structural, Optical, Wetting and Electrical Transport Properties of CdS Thin Films

Cadmium sulphide thin films were deposited on the glass substrate via simple spray pyrolysis technique. The substrate temperatures (Ts) have been varied from 250℃ to 350℃ and concentration of precursor’s solution of cadmium chloride and thiourea was optimized. The X-ray patterns and morphological studies of CdS thin films indicated that films are crystalline in nature with hexagonal crystal structure. The grain size calculated and found to be 250.12 to 349.61 nm. The optical spectra exhibited high transmittance and band gap varied from 2.41 eV to 2.39 eV. The angle of contact measured and found to be hydrophilicity behaviour. The electrical conductivity and thermoelectric power have been measured with two probe method. It was found that CdS thin films were semiconducting in nature with n-type.

Electronic Modeling and Optical Properties of CuIn_0.5Ga_0.5Se₂Thin Film Solar Cell

In this work, the band structure and optical-related properties of CuIn0.5Ga0.5Se2 thin film are presented. The calculation is performed by the full-potential linearized augmented plane wave (FPLAPW) method. The spin-orbit coupling is considered. The result for the dielectric function is in good agreement with earlier experimental measurements and simulations. Based on the complex dielectric function, the dielectric constant, the absorption coefficient, the complex refractive index and the reflectivity at normal incidence are explored. We found that they are comparable with the earlier results.

High Sensitivity of Porous Cu-Doped SnO2 Thin Films to Methanol

Porous Cu-doped SnO2 thin films were synthesized by the sol-gel dip-coating method for enhancing methanol sensing performance. The effect of Cu doping concentration on the SnO2 sensibility was investigated. XRD data confirm that the fabricated SnO2 films are polycrystalline with tetragonal rutile crystal structure. AFM and SEM micrographs confirmed the roughness and the porosity of SnO2 surface, respectively. UV-Vis spectrum shows that SnO2 thin films exhibit high transmittance in the visible region ~95%. The band gap (3.80 - 3.92 eV) and the optical thickness (893 - 131 nm) of prepared films were calculated from transmittance data. The sensing results demonstrate that SnO2 films have a high sensitivity and a fast response to methanol. In particular, 3% Cu-SnO2 films have a higher sensitivity (98%), faster response (10-2 s) and shorter recovery time (18 s) than other films.

Preparation and Characterization of TiO₂and SiO₂Thin Films

Although scaling will continue for couple of decades but device geometries reaches to atomic size and limitation of quantum mechanical physical boundaries. To address these problems there is need of innovation in material science & engineering, device structure, and new nano devices based on different principle of physics. So TiO2 thin films have been grown on well clean N-type silicon substrates via a sol–gel spin coating method. MOS capacitor were fabricated and characterized with SiO2 and TiO2 as dielectric material on N-type silicon wafer. The thickness was measured by stylus profiler and found to be 510 ? and 528 ? for SiO2 and TiO2 respectively. Some of the material parameters were found from the measured Capacitance -Voltage (C-V) curve obtained by SUPREM-III (Stanford University Process Engineering Model Version 0-83) for SiO2 and C-V Keithly 590 analyzer for TiO2 thin films. The result shows that obtained TiO2 film present a dielectric constant of approximately 80. The refractive index was found to be 2.4 and optical constant was 5.43 obtained from Ellipsometry. Band gap 3.6 eV of TiO2 was calculated by spectrophotometer and Surface morphology was obtained using Scanning Electron Microscope (SEM-JEOL) micrograph. The aluminum (Al) metal was deposited by the thermal evaporation system on the back side of the sample for the ohmic contact. Analysis shows that TiO2 may be acceptable as a viable substitute for high k dielectric in order to prevent the tunneling current problems.

Optical, Structural and Morphological Properties of Photocatalytic ZnO Thin Films Deposited by Pray Pyrolysis Technique

Photocatalytic ZnO thin films have been deposited onto glass substrate by spray pyrolysis technique. The sprayed solution consists of 0.1 M of zinc acetate dihydrate dissolved in double distilled water and sprays onto ultrasonically cleaned glass substrates maintained at 350°C, through an air-atomizing nozzle. The X-ray diffraction (XRD), scanning electron microscopy (SEM), EDX and UV-VIS spectrophotometer were applied to describe the structural, morphological, compositional and optical properties of ZnO catalyst. XRD analysis confirms that the films were found to be single phase hexagonal wurtzite structure. The SEM micrograph of the films is shown highly uniform, crack free and found to be fiber like structures. The optical transmittance spectra of the ZnO thin films were found to be transparent to visible light and the average optical transmittance was greater than 85%. The direct optical band gap energy values of the films shift towards the lower energy as a consequence of the thermal annealing. The Urbach energy of the films was found to increase with annealing temperature. The refractive index of the films was calculated and the refractive index dispersion curve of the films obeys the single oscillator model. The values of oscillatory energy Eo, dispersion energy Ed, and static dielectric constant εs for the ZnO thin films were determined. The films were evaluated for their ability to degrade methylene blue. The Langmuir-Hinshelwood kinetic model was used to interpret quantitatively the observed kinetic experimental result. The photocatalytic activity of ZnO thin films was enhanced by annealing temperature.

Angle-Resolved and Resonant Photoemission Study of the Valence Bands of α-La(0001) on W(110)

We report a photoelectron spectroscopic study of the valence bands of double hexagonal-close-packed (dhcp) α-La(0001) films epitaxially grown on W(110) at room temperature. The La 5d photoemission cross section in the photon energy region from 20 eV to 130 eV was obtained and the valence-band structure of α-La was determined. Except for 4f-related structures, the valence-band structures of dhcp α-La and dhcp β-Ce were found to resemble each other. From the band structure, the crystal structure of the La film was confirmed. No evidence for the existence of a 5d-like surface state near the Fermi energy at the point of the surface Brillouin zone was obtained and a 6s band bottom was identified.

Ultraviolet and Deep-Ultraviolet Emissions from c-MgxZn1-xO/MgO Ultrathin Multilayer Heterostructures

Cubic phase MgxZn1-x O/MgO multilayer heterostructures （c-Mgx Zn1-xO/MgO MHs） are grown on Si（100） and quartz substrates by reactive electron beam evaporation at low temperature （250℃）. Cross-sectional morphology observations by field-emission scanning electron microscopy show the legible interfaces of c-MgxZn1-x O/MgO MHs. X-ray diffraction demonstrates that c-MgxZn1-xO/MgO MHs are of highly （100）-oriented. Optical trans- mission investigations of c-Mgx Zn1-x O/MgO MHs on quartz substrates reveal the coexistence of the two phases, c-MgxZn1-xO and MgO. Photoluminescence examination indicates the emergence of deep-ultraviolet emission centred at about 290nm along with the blue shift of the ultraviolet emission from 405nm to 39Gnm when the nominal thickness of c-MgxZn1-xO well layers of MHs is diminished to 3nm, which is probably originated from quantum confinement effect.

用于红外焦平面探测器的2.95~5.05μm带通滤光膜

中波红外焦平面探测器因在红外制导导弹、红外夜视仪等军事领域的重要性而被广泛关注,其中带通滤光膜具有滤除杂散光和保护探测器的作用。本研究的目的是针对中波红外焦平面探测器滤光膜的关键技术问题,通过分析、选材、优化等方法,制备出性能良好的中波红外带通滤光膜。以Ge为高折射率材料和基底,以SiO为低折射率材料,设计了带通膜系结构。不仅实现了在0°入射角下对2.95~5.05μm波段大于92%的高透过率、通带宽度优于3.7~4.8μm的常规应用,而且对2.95~5.05μm之外的其它波段也能达到良好的截止效果。经测试,其光洁度、牢固性等各方面性能良好,可以很好地应用于中波红外焦平面探测器。

矩形波宽带通滤光膜研究

对矩形波宽带通滤光片进行了深入研究,提出了一种设计、制备矩形波宽带通滤光片的方法。使用该方法设计并制备了400 nm~1 100 nm波段,中心波长λ_(0)=515 nm,透射带λ=λ_(0)±25 nm,透射带平均透射率τ≥92%,截止带λ=400 nm~475 nm、λ=555 nm~1 100 nm,截止带透射率小于0.1%的矩形波宽带通OD3-A滤光片。对样片光谱进行了测试,结果满足需求。该方法设计、制备矩形波宽带通滤光片克服了F-P型窄带滤光膜监控精度要求高、通带宽带窄、成本高以及传统长、短波截止膜组合方式膜层总厚度过大、通带透过率低、波形矩形度差的缺点。

退火温度对蓝宝石衬底上Mg_(2)Si薄膜质量和光学性质的影响

采用磁控溅射法在蓝宝石衬底上制备了结晶良好的Mg2Si多晶薄膜,研究了退火温度(375~475℃)对薄膜晶体结构、表面形貌、拉曼光谱和光学性质的影响。X射线衍射(XRD)结果表明,当退火温度为400 ℃时Mg_(2)Si(220)衍射峰强度最强,样品结晶质量最好,未见明显可观测的MgO相。扫描电镜(SEM)结果表明,所有样品表面均呈现清晰可见的规则六边形,且退火温度对形貌影响较小。拉曼光谱结果显示所有样品均呈现出Mg_(2)Si薄膜的特征峰(256 cm^(-1)附近的F_(2g)振动模),同时出现345 cm^(-1)附近的F_(1u)(LO)声子模,表明生成样品均为结晶良好的Mg_(2)Si薄膜。对薄膜光学性质的研究结果表明,随着退火温度升高,样品光学带隙先增大后减小。

硫化锑太阳电池的数值模拟分析

利用Afors-het一维器件模拟仿真软件,研究传统CdS/Sb2S3异质结太阳电池器件中Sb2S3吸收层和CdS缓冲层厚度、带隙、吸收层受主浓度以及缺陷对电池性能的影响。结果表明,一定厚度的吸收层可以提高器件的短路电流密度,但过厚的吸收层会减小填充因子。研究CdS薄膜发现,过厚的CdS对电池的开路电压,短路电流密度以及填充因子损害较大。Sb2S3最优的带隙宽度在1.5~1.6 eV之间。提高Sb2S3受主浓度可以有效改善开路电压,但施主缺陷态密度与缺陷态在能级中能量的增加将会使电池效率降低。同时模拟结果表明,当吸收层中载流子寿命达到10-7s时,电池的短路电流密度可以得到明显改善。

幂指数棱台声子晶体对薄板振动弯曲波的调控特性研究

针对薄板结构的振动控制,提出了一种幂指数棱台声子晶体构型,并对其带隙的产生机理和影响因素进行了分析,结果表明提出的幂指数棱台声子晶体具有三个弯曲波完全带隙,其中第二带隙宽度可达850 Hz。联合数值仿真和试验方法对声子晶体的弯曲波带隙进行了验证。随着棱台结构高度的增加,三个带隙的带宽扩大。棱台的幂函数幂次升高会使带隙的起始频率与终止频率降低,而边缘厚度的增高会弱化能量聚焦效应使带隙的宽度逐渐变窄。具有线缺陷的声子晶体板可使带隙频段内的弯曲波沿着设计路径传播。