Design of near-unity transmittance dielectric/Ag/ITO electrodes for GaN-based light-emitting diodes

We designed a near-unity transmittance dielectric/Ag/ITO electrode for high-efficiency GaN-based light-emitting diodes by using the scattering matrix method. The transmittance of an ultrathin metal layer, sandwiched between a dielectric layer and an ITO layer, was investigated as a function of the thickness and the optical constant of each constituent layer. Three different metals (Ag, Au, and Al) were examined as the metal layer. The analytical simulation indicated that the transmittance of a dielectric/metal/ITO multilayer film is maximized with an approximately 10-nm-thick Ag layer. Additionally, the transmittance also tends to increase as the refractive index of the upper dielectric layer increases. By tailoring the thickness of the dielectric layer and the ITO layer, the dielectric/Ag/ITO structure yielded a transmittance of 0.97, which surpasses the maximum transmittance (0.91) of a single ITO film. Furthermore, this extraordinary transmittance was present for other visible wavelengths of light, including violet and green colors. A complex phasor diagram model confirmed that the transmittance of the dielectric/metal/ITO multilayer film is influenced by the interference of reflected partial waves. These numerical findings underpin a rational design principle for metal-based multilayer films that are utilized as transparent electrodes for the development of efficient light-emitting diodes and solar cell devices.     

高次谐波体声波谐振器的有效机电耦合系数

高次谐波体声波谐振器HBAR (High-overtone Bulk Acoustic Resonator)由基底、压电薄膜和上下电极组成,系统地研究了它们的结构参数(厚度)和性能参数(特性阻抗)对HBAR的重要性能参数有效机电耦合系数K_eff2的影响。在谐振频率附近,通过将HBAR的分布参数等效电路简化为集总参数等效电路得到了它们之间的关系表达式,分析了K_eff2在所关心频率最近谐振点的变化情况。结果表明,保持压电薄膜厚度不变,连续增加基底厚度,K_eff2呈振荡(非单调)下降,当基底厚度达到一定值时K_eff2与厚度成反比下降;保持基底厚度不变,连续增加压电薄膜厚度,K_eff2的峰值随基底和压电层的特性阻抗之比增加快速下降,到达极小值后缓慢增加;选择低阻抗的熔融石英作为基底可以获得较大的K_eff2;与Al电极相比,Au电极选择适当厚度可以获得较高的K_eff2。上述揭示的一些规律为HBAR的优化设计提供了理论依据。      

一维光子晶体中亚波长缺陷膜对Goos-Hnchen位移的调制特性

      利用传输矩阵法分析一维光子晶体中亚波长缺陷膜对缺陷模频率处Goos-Hnchen位移的调制特性,讨论了亚波长缺陷膜的厚度、磁导率及介电常量对一维光子晶体缺陷模频率处的Goos-Hnchen位移的影响.研究发现：一层几何厚度极小的亚波长薄膜即可非常灵敏地调制一维光子晶体缺陷模频率处Goos-Hnchen位移的位置及其大小;并且当亚波长缺陷膜为左手材料时,Goos-Hnchen位移随亚波长缺陷膜物理参量的变化趋势与普通右手材料时的情形完全相反.     

Transparent conductive ITO/Ag/ITO multilayer electrodes deposited by sputtering at room temperature

ITO/Ag/ITO multilayers have been prepared onto conventional soda lime glass substrates by sputtering at room temperature. The optical and electrical characteristics of single layer and multilayer structures have been investigated as a function of the Ag and ITO film thicknesses. Transmittance and sheet resistance values are found mainly dependent on the Ag film thickness; whereas the wavelength range at which the maximum transmittance is achieved can be changed by adjusting the ITO films thickness. ITO/Ag/ITO electrodes with sheet resistance below 6 Ω/sq have been obtained for Ag film thickness above 10 nm and ITO layers thickness in the 30-50 nm range. These multilayers also show high transmittance in the visible spectral region, above 90% by discounting the glass substrate, with a maximum that is located at higher wavelengths for thicker ITO.     

Effect of GZO thickness and annealing temperature on the structural,electrical and optical properties of GZO/Ag/GZO sandwich films

The GZO/Ag/GZO sandwich films were deposited on glass substrates by RF magnetron sputtering of Ga-doped ZnO (GZO) and ion-beam sputtering of Ag at room temperature. The effect of GZO thickness and annealing temperature on the structural, electrical and optical properties of these sandwich films was investigated. The microstructures of the films were studied by X-ray diffraction (XRD). X-ray diffraction measurements indicate that the GZO layers in the sandwich films are polycrystalline with the ZnO hexagonal structure and have a preferred orientation with the c-axis perpendicular to the substrates. For the sandwich film with upper and under GZO thickness of 40 and 30 nm, respectively, it owns the maximum figure of merit of 5.3 × 10⁻² Ω⁻¹ with a resistivity of 5.6 × 10⁻⁵ Ω cm and an average transmittance of 90.7%. The electrical property of the sandwich films is improved by post annealing in vacuum. Comparing with the as-deposited sandwich film, the film annealed in vacuum has a remarkable 42.8% decrease in resistivity. The sandwich film annealed at the temperature of 350 °C in vacuum shows a sheet resistance of 5 Ω/sq and a transmittance of 92.7%, and the figure of merit achieved is 9.3 × 10⁻² Ω⁻¹.     

Effective electromechanical coupling coefficient of high-overtone bulk acoustic resonator

A high-overtone bulk acoustic resonator(HEAR) is composed of a substrate,a piezoelectric film and upper and lower electrodes,the influences of their structure parameter(thickness) and performance parameter(characteristic impedance) on effective electromechanical coupling coefficient K_(eff)~2 are investigated systematically.The relationship between K_(eff)~2 and these parameters is obtained by a lumped parameter equivalent circuit instead of distributed parameter equivalent circuit near the resonant frequency,and K_(eff)~2 at the resonance frequency closest to the given frequency is analyzed.The results show that K_(eff)~2 declines rapidly and oscillatorily with the continuous increase of the substrate thickness when the piezoelectric film thickness is fixed,and decreases inversely proportion to the thickness when the substrate thickness is greater than a certain value.With the ratio of the characteristic impedance of the substrate to the piezoelectric layer increasing,the maximum of K_(eff)~2 obtained from the variation curve of K_(eff)~2 with the continuous increase of the piezoelectric film thickness decreases rapidly before reaching the minimum value,and later increases slowly.Fused silica with low impedance is appropriate as the substrate of HBAR to get a larger K_(eff)~2.Compared with Al electrode,Au electrode can obtain larger K_(eff)~2 when the appropriate electrode thickness is selected.The revealed laws above mentioned provide the theoretical basis for optimizing parameters of HBAR.     

Fabrication and characterization of Ag intermediate transparent and conducting TiON/Ag/TiON multilayer films

Silver (Ag) intermediate transparent and conducting TiON/Ag/TiON (TAgT) films were deposited by RF and DC magnetron sputtering on glass substrates. Changes in the optoelectrical properties of the films were investigated as a function of Ag thickness. The thickness of the Ag film varied from 5 to 20 nm.In XRD patterns, the TAgT films showed characteristic diffraction peaks for Ag (111), (200), (220), and (311) planes, while peaks for TiON were not observed. Thus, it was concluded that the Ag interlayer did not affect the crystalline structures of the upper TiON films. However, electrical resistivity was dependent on the thickness of the Ag interlayer. For TiON 50/Ag 20/TiON 30 nm films, electrical resistivity decreased to as low as 3.3 × 10^− 4 Ωcm. The optical transmittance was also influenced by the Ag interlayer. As the Ag thickness increased, the optical transmittance decreased to as low as 45% for TiON 50/Ag 20/TiON 30 nm films. From observed figure of merit and work function, it is concluded that a TAgT film with a 5 nm-thick Ag interlayer is a good candidate for use as a transparent electrode in OLEDs and flat panel displays.     

Transmission properties of frequency selective structures with air gaps

The transmission properties of compound frequency selective structures with dielectric slab and air gaps were investigated by computation and experimentation.Mechanism analyses were also carried out.Results show that the air gaps have a distinct influence on the transmission properties.Resonant frequency of the structure would increase rapidly when the air gap appears.After the gap gets larger to a specific value,generally 1/5 wavelength corresponding to the resonant frequency,the transmission properties would change periodically with the gap thickness.The change of transmission properties in one period has a close relationship with the dielectric thickness.These results provide a new method for designing a bandpass radome of large incidence angle and low loss with the concept of stealth shield radome.     

Effect of dielectric layer on ferroelectric responses of P(VDF-TrFE) thin films

The dielectric layer in the sandwich structural device plays a very important role in determining the electrical properties of the ferroelectric film. In this paper, we investigate the effect of the dielectric layers with different thicknesses on switching performance of ferroelectric P(VDF-TrFE) thin films. The hysteresis loops become slanting with increasing thickness of the dielectric layer. A negative slope of the ‘real’ hysteresis loop is apparently observed which demonstrates negative capacitance effect caused by the dielectric layer. This behavior is simulated qualitatively by the Weiss mean field model considering an interfacial dielectric layer in series with a ferroelectric layer. The agreement between experiments and simulations supports that negative capacitance results from the positive feedback among electric dipoles. Furthermore, the switching time of the ferroelectric film increases with the increase of dielectric layer thickness. This study shows that the ferroelectric sandwich structure provides great potential towards low power negative capacitance devices.     

基于银/二氧化钛复合膜的表面等离子共振气体传感器

表面等离子体共振是一种免标记的传感技术,当介质周围的介电常数发生改变时,则SPR谐振光谱特性也会随之改变。因此表面等离子体共振传感技术已广泛应用于生物化学和环境监测等领域。由于二氧化钛（TiO₂）覆盖层不仅可以保护金属层,还能调谐SPR谐振的光谱强度和谐振波长于近红外波段,应用于1550 nm的光纤传感,其氧化还原反应还能使其用于检测气体。由于氢气易燃易爆性,随着氢能源的广泛应用,因此对低浓度氢气检测技术研究具有特殊的意义。提出一种可更换银/二氧化钛复合膜的表面等离子共振的气体传感器,研究了SPR传感器在1 550 nm近红外波段对气体的敏感特性。研制了可更换银/二氧化钛复合膜的表面等离子共振（SPR）气体传感器。研究了在近红外波段对气体的表面等离子体共振光谱特性。仿真计算Kretschmann棱镜耦合的四层结构模型的共振光谱强度与银膜厚度,二氧化钛厚度和棱镜材料的关系,优化了Ag和TiO₂层的厚度以获得最大灵敏度,得到的最佳膜厚是45 nm Ag和110 nm TiO₂。Ag/TiO₂薄膜设计为可更换的一次性气敏膜,采用蒸镀和溅射方法镀膜,制备成本文所使用的 SPR传感器。利用Ag/TiO₂薄膜在复合界面产生SP共振光谱的移动,对气体进行测试。采用Kretschmann棱镜耦合结构的光谱波长检测实验系统。固定光源和入射角,测量波长的偏移量。宽光源（波长范围：1 462～1 662 nm）通过环形器、准直器,照射到棱镜和可更换的Ag/TiO₂敏感膜,经全反射(TIR)后,再由高反射镜反射回传感膜,并以相同的TIR角和光路再次反射回到准直器,从而被光谱仪检测。实验结果表明,Ag/TiO₂复合膜可以调谐共振波长到1 550 nm近红外波段,增强该传感器的光谱灵敏度,低浓度（14.7%～25%）氢气下的灵敏度可达-8.305 nm·%-1。并且可通过更换气敏膜检测不同的气体,增加生物相容性和气体传感能力。     

粉末靶制备的AZO/Ag/AZO透明导电薄膜的光电性能

室温下,采用射频磁控溅射AZO粉末靶和Ag靶在玻璃基底上制备Ag层厚度分别为12 nm和15 nm两组对称结构掺铝氧化锌/银/掺铝氧化锌(AZO/Ag/AZO)透明导电薄膜,研究了Ag层和AZO层厚度对薄膜光电性能的影响。结果表明:3层薄膜的可见光区平均透光率达到了80%,550 nm处的最高透过率达到了88%,方块电阻小于5 Ω/□。Ag层厚度是影响AZO/Ag/AZO薄膜光电性能的主要因素,AZO层的厚度对薄膜光学性能影响较大。     

金属/电介质颗粒复合介质的非线性交流响应

推广T矩阵法来研究二组分颗粒复合介质的非线性交流响应，解析推导出基频、三次及五次谐波频率下的有效线性介电常数及三阶和五阶非线性极化率的解析表达式.当组分颗粒的介电常数为实数时，表达式同以前微扰理论完全一致.进一步数值计算了金属-电介质复合介质的有效非线性交流响应，数值结果表明，体系的三阶、五阶非线性极化率在表面等离子共振频率附近有明显增强，而且，随着体积分数提高，共振峰增强且伴随着共振频率红移现象.还进一步讨论了维度效应对体系非线性交流响应的影响. 关键词： T矩阵方法 复合介质 非线性交流响应     

Smith-Purcell radiation from a charge running near the surface of a photonic crystal

Smith-Purcell radiation from a charge running near a photonic crystal is calculated for a slab system of a periodic array of dielectric spheres, with the photonic band effect taken into account exactly. The radiation spectrum has a series of resonantly enhanced structures, which are shown to arise accompanying the excitation of the photonic bands. It is also shown that the overall intensity of the emission band does not depend very strongly on the slab thickness but the height of the resonant peaks increases progressively with thickness due to the enhanced Q value of excited photonic bands.     

A transparent electromagnetic-shielding film based on one-dimensional metal–dielectric periodic structures

In this study, we designed and fabricated optical materials consisting of alternating ITO and Ag layers. This approach is considered to be a promising way to obtain a light-weight, ultrathin and transparent shielding medium, which not only transmits visible light but also inhibits the transmission of microwaves, despite the fact that the total thickness of the Ag film is much larger than the skin depth in the visible range and less than that in the microwave region. Theoretical results suggest that a high dielectric/metal thickness ratio can enhance the broadband and improve the transmittance in the optical range. Accordingly, the central wavelength was found to be red-shifted with increasing dielectric/metal thickness ratio. A physical mechanism behind the controlling transmission of visible light is also proposed. Meanwhile, the electromagnetic shielding effectiveness of the prepared structures was found to exceed 40 dB in the range from 0.1 GHz to 18 GHz, even reaching up to 70 dB at 0.1 GHz, which is far higher than that of a single ITO film of the same thickness.     

异质界面对Ca(Mg1/3Nb2/3)O3/CaTiO3叠层薄膜结构和介电性能的影响

采用异质叠层方式制备出一定厚度的Ca(Mg_1/3Nb_2/3)O₃/CaTiO₃(CMN/CT)叠层薄膜,研究了异质界面对薄膜结构、微观形貌及介电性能的影响及其规律.根据实验测试结果,提出CMN/CT叠层薄膜的模拟等效电路,建立介电常数和介电损耗的理论计算公式.结果表明:CMN/CT异质叠层薄膜具有完全正交钙钛矿结构,结构致密,厚度均匀,薄膜中存在独立的CMN和CT相.异质界面处存在过渡层,随着薄膜中异质界面个数增加,介电常数增大,介电损耗减小.减小界面过渡层的厚度,有利于提高CMN/CT叠层薄膜的介电性能.     

Fabrication and analysis of ZnO thin film bulk acoustic resonators

This study employs RF magnetron sputter technique to deposit high C-axis preferred orientation ZnO thin film on silicon substrate, which is then used as the piezoelectric thin film for a thin film bulk acoustic resonator (FBAR). Electrical properties of the FBAR component were investigated by sputtering a ZnO thin film on various bottom electrode materials, as well as varying sputter power, sputter pressure, substrate temperature, argon and oxygen flow rate ratio, so that structural parameters of each layer were changed. The experimental results show that when sputter power is 200 W, sputter pressure is 10 mTorr, substrate temperature is 300 °C, and argon to oxygen ratio is 4:6, the ZnO thin film has high C-axis preferred orientation. The FBAR component made in this experiment show that different bottom electrode materials have great impact on components. In the experiment, the Pt bottom electrode resonant frequency was clearly lower than the Mo bottom electrode resonant frequency, because Pt has higher mass density and lower acoustic wave rate. The component resonant frequency will decrease as ZnO thin film thickness increases; when top electrode thickness is higher, its resonant frequency also drops, due to top electrode mass loading effect and increased acoustic wave path. Therefore, ZnO thin film and top/bottom electrode thickness can be fine-tuned according to the required resonant frequency.     

Numerical simulation of optical bi-stability in antiferromagnetic sandwich structure

The magnetically optical bi-stability, a third-order nonlinear response, is investigated on an antiferromagnetic (AF) sandwich structure, where an AF film is sandwiched between two dielectric films. The configuration with the AF anisotropy axis and external static magnetic field both in the interfaces and normal to the incident plane is used. The incident wave is taken as a TE wave with its electric component transverse to the incident plane. We find that bistable switches can appear only in a finite frequency range and an incident angle range for a given regime of incident power, which means that there are the critical incident angle and frequency. The power threshold value for the bi-stability increases with the incident angle. In addition, the bi-stability also easily is modulated by the external magnetic field.     

Water vapour sorption by mim systems

In studying the properties of the sandwich system Al-Al₂O₃-Au(Al) the capacitance of the system was found to depend on the humidity of the environment. The humidity induced capacitance change is reversible and can be explained assuming a process of physical sorption of water vapours by Al₂O₃ dielectric film. The sorption rate is independent of the oxide film thickness, which leads to a conclusion that mainly the sorption in the surface layer of the oxide is involved in the process. The sorption rate is, however, decreasing with increasing thickness of the upper evaporated electrode, which can be taken as an evidence of the fact that the water molecules penetrate to the oxide films through the upper metal layer whose coverage is not complete due to the porosity of the oxide film underneath.On the basis of our experimental results we come to a conclusion that even when the Al₂O₃ films are prepared by oxidation in a 3% solution of tartaric acid, their structure is porous. Proceeding from a simplified picture of the dielectric structure we propose an equivalent electrical scheme of the Al-Al₂O₃-Au Al system. The frequency characteristics measured at different environment humidities correspond to the dependences calculated for the proposed equivalent circuit.     

电极材料及偏压极性对氧化物介质击穿行为的影响及机制

忆阻器和能量存储电容器具有相同的三明治结构,然而两个器件需要的操作电压有明显差异,因此在同一个器件中,研究操作电压的影响因素并对操作电压进行调控,实现器件在不同领域的应用是十分必要的一个工作.本文利用反应磁控溅射技术在ITO导电玻璃、Pt/Si基底上生长了多晶ZrO_2和非晶TaO_x薄膜,选用不同金属材料Au, Ag和Al用作上电极构建了多种金属/氧化物介质/金属三明治结构的电容器,研究了器件在不同偏压极性下的击穿强度.结果发现:底电极是ITO的ZrO_2基电容器在负偏压下的击穿电场比Pt电极器件稍大.不管底电极是ITO还是Pt, Ag作为上电极时器件的击穿强度均存在明显的偏压极性依赖性,正偏压下的击穿电场减小了一个数量级;相反,在Al作为上电极的Al/TaO_x/Pt器件中,正向偏压比负向偏压下的击穿电场增加了近2倍.上述器件的不同击穿行为分别可以由氧化物电极和介质界面层间氧的迁移和重排、电化学活性金属电极的溶解迁移和还原以及化学活性金属电极与氧化物界面的氧化还原反应来解释.该实验结果对有不同操作电压要求的器件,如忆阻器和介质储能电容器等在器件设计和操作方面具有指导意义.     

POROUS AMORPHOUS FLUOROPOLYMER FILMS WITH ULTRALOW DIELECTRIC CONSTANT

With the development of ultralarge scale integrated circuit, new interlayer dielectrics with low dielectric constant for multilevel interconnections are required, instead of conventional SiO₂ films. For the sake of seeking perfect dielectrics, amorphous fluoropolymer (AF) thin film with a thickness of about 0.9μm has been prepared by spin-coating method, following the principle of phase separation. By capacitance-voltage (C-V) measurements the dielectric constant of the thin film is equal to 1.57 at 1 MHz, which is attributed to numerous pores contained in the film matrix. X-ray photoelectron spectroscopy (XPS) spectra show that after annealing, about 71% CF₃ groups in the AF film have decomposed into CF₂, CF, etc. This leads to the increase of CF₂ groups by three times and CF groups by 8% in the AF film. In a word, compared with the film without being annealed, about 25% carbon, 7% fluorine and 12% oxygen atoms will be lost after annealing at 400℃ for 30min.