Ag-BaO薄膜在电场作用下的可见——近红外波段光学吸收特性

对超额Ba激活的Ag-BaO复合薄膜在外加垂直表面电场作用下的光学吸收特性进行了测量.结果显示,薄膜在可见—近红外光波段存在两个吸收峰,其中近红外光区的吸收峰强度随垂直表面电场的作用而降低.理论分析表明,可见光区的主吸收峰源于埋藏在BaO半导体中的Ag超微粒子的表面等离激元共振吸收;近红外光区的次吸收峰则由BaO半导体基质中杂质能级的光吸收引起,杂质能级的产生与超额Ba在BaO晶体中造成的氧缺位有关.在外加垂直表面电场作用下,BaO基质中的杂质发生电离,并导致杂质能级上束缚电子浓度减小,表现为薄膜在与杂质 关键词： 光吸收 金属超微粒子半导体复合薄膜 表面等离激元 杂质能级     

Ag-BaO薄膜在垂直表面电场作用下的光吸收增强

Ag-BaO薄膜是金属纳米微粒埋藏于半导体介质中的功能复合薄膜,它具有超快的光电时间响应,可以检测超短激光脉冲,在Ag-BaO薄膜表面加入垂直电场,可以提高薄膜的光电发射效率,在垂直表面电场作用下近紫外波段光吸收有较明显增强现象,在波长λ=303nm处15V电压作用下光吸收增强6.5%,30V电压作用下增强18%。这种光吸收增强是由于在电场作用下,薄膜的能带结构发生倾斜,以及在强电场下能级分裂。光吸收涉及被激发电子在倾斜能带间隧穿几率的增加,和被激发电子在这些分裂能级间的跃迁。     

纳米表面二维周期半圆凹槽增强硅薄膜太阳能电池光吸收

利用时域有限差分方法, 研究硅薄膜上下表面周期半圆凹槽结构对于太阳光吸收的增强效应. 研究发现这种结构可以实现太阳光宽波段的光吸收增强, 通过调节SiO₂表面减反层厚度和凹槽半径长度来实现薄膜太阳能电池最大的光吸收, 并实现了波长在300-1000 nm范围的太阳光吸收总能量比没有这种 结构下硅薄膜光吸收提高了约117%. 关键词： 硅薄膜 半圆凹槽 吸收增强     

新型二维拉胀材料SiGeS的理论预测及其光电性质

二维材料由于其在力学、电学以及光学等领域的潜在应用而受到广泛关注.基于第一性原理计算,通过有序地排列SiH3SGeH3的Si-S-Ge骨架,设计了一种全新的二维材料SiGeS.单层SiGeS具有良好的能量、动力学以及热力学稳定性. SiGeS具有非常罕见的负泊松比.此外,单层SiGeS是间接带隙半导体,其带隙值为1.95 eV.在应变的作用下, SiGeS可转变为带隙范围为1.32—1.58 eV的直接带隙半导体,可被应用在光学或半导体领域.同时,本征SiGeS拥有优异光吸收能力,其最高光吸收系数可达约10~5 cm^–1,吸收范围主要在可见光到紫外波段.在应变下,光吸收范围可覆盖到整个红外波段.这些有趣的性质使得SiGeS成为一种多功能材料,有望被用于纳米电子、纳米力学以及纳米光学等领域.     

非晶态As_2S_8半导体薄膜的光激励现象的研究及应用

实验研究了非晶态As2S8半导体薄膜在紫外汞灯和He_Cd激光照射下的光诱导现象,证实了光照射后的As2S8薄膜的折射率增大,体积缩小,可见光吸收谱的吸收带蓝移。在此基础上,应用光折变效应试制了As2S8条波导,观测到As2S8条波导的光阻断效应,实现了光_光效应的开关功能。     

镶嵌在氢化氮化硅中纳米非晶硅粒子光吸收的模拟

采用量子限制效应模型对镶嵌有纳米非晶硅粒子的氢化氮化硅薄膜的光吸收进行了理论模拟,探讨了由吸收谱分析给出该结构薄膜光学参数的方法,并通过对不同氮含量样品的讨论给出了量子限制效应和纳米硅粒子表面的结构无序对薄膜光吸收特性的影响规律。分析结果表明,随氮含量的增加,薄膜有效光学带隙增大,该结果与薄膜中纳米硅粒子平均尺寸的减小引起的量子限制效应的增强相关,而小粒度纳米硅粒子比例增加所引入的较高微观结构无序度和较多缺陷将会导致薄膜低能吸收区吸收系数增加。     

降低表面等离子激元寄生吸收损失的途径研究

从纳米Ag颗粒表面等离子激元光学及表面高能电场特性两方面入手,较为系统地研究了周围介质的导电特性对表面等离子激元的影响.通过对复合薄膜紫外-可见-近红外光谱及表面增强拉曼散射光谱的分析,指出绝缘性的Al₂O₃介质薄膜能够起到良好的表面电场定域效果,且不会引入附加的光吸收损失;而导电性的ITO薄膜则会引入表面价电子的溢出损失,加速了表面电场的衰逝,同时引起长波方向上显著的光吸收损失.研究还表明致密的Al₂O₃介质薄膜能够起到良好的屏蔽作用,且纳米Ag颗粒表面等离子激元特性仅受最近邻材料特性的影响.研究结果为在硅基薄膜太阳电池中实现对纳米Ag颗粒的阻挡、寄生光吸收损失的降低以及表面高能电场的利用,提供了一条有效的解决途径.     

激光等离子体效应对薄膜损伤特性的影响

高能激光的发展对光学元件的抗损伤能力要求越来越高,其中光学薄膜是最薄弱的环节之一。实验研究了激光的聚焦位置对石英基片上HfO2/SiO2减反射薄膜损伤形貌的影响,研究发现:激光等离子体的高压冲击波对薄膜产生强烈的冲击剥离效应,其压强随膨胀半径的增加而迅速减小。激光等离子体光谱的辐射波长小于入射激光波长,这会增强薄膜对辐射光能量的吸收;位于深紫外波段、能量大于HfO2薄膜带隙的光子能量,将被薄膜直接吸收,从而加剧薄膜的电离破坏。激光等离子体的辐射效应和冲击波效应的共同作用决定了薄膜的损伤形貌。当激光聚焦到薄膜表面时,冲击波压强极大会使薄膜发生大面积的电离去除,同时基底发生击穿;当两者距离大到一定距离时,冲击波只会使得中心处小面积薄膜发生剥离,基底未出现断裂。     

非晶硅纳米圆柱团超表面的双峰近完美吸收效应

利用非晶硅纳米圆柱团超表面,模拟实现了可见光波段的双峰近完美吸收效应.采用离散偶极子近似法,研究了电偶极、磁偶极和电四极对纳米圆柱团超表面散射截面的贡献.模拟分析了非晶硅纳米圆柱团超表面的透射、反射和吸收光谱和电场分布,调整电偶极、磁偶极和电四极米氏共振光谱使其重合并产生相干耦合作用,进而产生电场局域增强效应,实现可见光波段双峰近完美吸收效应,且其对入射角不敏感.非晶硅纳米圆柱团超表面双峰近完美吸收效应具有低损耗、角度不敏感等特性,有望被应用于光学隔离、能量采集等纳米光子学领域.     

第三讲 半导体的激子效应及其在光电子器件中的应用

人们对半导体中的电子空穴对在库仑互作用下形成的激子态及其有关的物理性质进行了深入研究．激子效应对半导体中的光吸收、发光、激射和光学非线性作用等物理过程具有重要影响，并在半导体光电子器件的研究和开发中得到了重要的应用．与半导体体材料相比，在量子化的低维电子结构中，激子的束缚能要大得多，激子效应增强，而且在较高温度或在电场作用下更稳定．这对制作利用激子效应的光电子器件非常有利．近年来量子阱、量子点等低维结构研究获得飞速的进展，已大大促进了激子效应在新型半导体光源和半导体非线性光电子器件领域的应用．     

电子回旋共振等离子体增强沉积氟化非晶碳薄膜的光学性质

用紫外可见光透射光谱(UV-VIS)并结合键结构的X射线光电子能谱(XPS)和红外谱(FTIR)分析,研究了电子回旋共振等离子体增强化学气相沉积法制备的氟化非晶碳薄膜的光吸收和光学带隙性质.在微波功率为140—700W、源气体CHF₃∶C₆H₆比例为1∶1—10∶1条件下沉积的薄膜,光学带隙在1.76—2.85eV之间.薄膜中氟的引入对吸收边和光学带隙产生较大的影响,吸收边随氟含量的提高而增大,光学带隙则主要取决于CF键的含量,是由于强电负 关键词： 氟化非晶碳薄膜 光吸收与光学带隙 电子回旋共振等离子体     

Optical properties of hexagonal boron nitride thin films deposited by radio frequency bias magnetron sputtering

The optical properties of hexagonal boron nitride (h-BN) thin films were studied in this paper. The films were characterized by Fourier transform infrared spectroscopy, UV--visible transmittance and reflection spectra. h-BN thin films with a wide optical band gap E_g (5.86 eV for the as-deposited film and 5.97 eV for the annealed film) approaching h-BN single crystal were successfully prepared by radio frequency (RF) bias magnetron sputtering and post-deposition annealing at 970~K. The optical absorption behaviour of h-BN films accords with the typical optical absorption characteristics of amorphous materials when fitting is made by the Urbach tail model. The annealed film shows satisfactory structure stability. However, high temperature still has a significant effect on the optical absorption properties, refractive index n, and optical conductivity σ of h-BN thin films. The blue-shift of the optical absorption edge and the increase of E_g probably result from stress relaxation in the film under high temperatures. In addition, it is found that the refractive index clearly exhibits different trends in the visible and ultraviolet regions. Previous calculational results of optical conductivity of h-BN films are confirmed in our experimental results.     

Structural and optoelectronic properties of amorphous GaN thin films

Amorphous gallium nitride (a-GaN) thin films were deposited on glass substrate by electron beam evaporation technique at room temperature and high vacuum using N ₂ as carrier gas. The structural properties of the films was studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). It was clear from XRD spectra and SEM study that the GaN thin films were amorphous. The absorbance, transmittance and reflectance spectra of these films were measured in the wavelength range of 300–2200 nm. The absorption coefficient spectral analysis in the sharp absorption region revealed a direct band gap of E _g = 3:1 eV. The data analysis allowed the determination of the dispersive optical parameters by calculating the refractive index. The oscillator energy E ₀ and the dispersion energy E _d, which is a measure of the average strength of inter-band optical transition or the oscillator strength, were determined. Electrical conductivity of a-GaN was measured in a different range of temperatures. Then, activation energy of a-GaN thin films was calculated which equalled E _a = 0:434 eV.     

Optical characterisation of CuInSe2 thin films prepared by two-stage process

Optical absorption spectra of CuInSe₂ chalcopyrite semiconductor films prepared using a two-stage technique were investigated. In addition to absorption measurements, energy-dispersive analysis of X-rays (EDAX) and X-ray diffraction measurements (XRD) were also performed. Direct bandgap energy values for the CuInSe₂ films were derived from the variation of (αhν)² with energy. All the measurements were performed on samples with various Cu/In ratios. It was determined from the absorption measurements that the materials have strong absorption at the fundamental band edge. The E_g values showed an increasing trend with decreasing Cu/In ratios. Received: 26 May 2000 / Accepted: 31 October 2000 / Published online: 10 January 2001     

Thermally and optically induced effects on sub-band gap absorption in nanocrystalline CdSe (nc-CdSe) thin films

Nanocrystalline cadmium selenide (nc-CdSe) thin films have been prepared by thermal evaporation using the inert gas condensation (IGC) technique. Transmission electron microscopy (TEM) studies show that the CdSe nanocrystals (NCs) are spherical in shape. Constant photocurrent method (CPM) is used to determine the value of absorption coefficient in low absorption region of as-deposited, annealed and light soaked nc-CdSe thin films. Values of optical band gap (E_g) have been determined by using Tauc's relation for as-deposited, annealed and light soaked nc-CdSe thin films from the α values evaluated from reflection and transmission measurements. We have used a derivative procedure to sub-gap absorption spectra in order to get information on the energetic distribution of the occupied density of states below Fermi level. Constants such as optical gap E_g, Urbach edge E_u have been obtained from CPM spectra in as-deposited, annealed and light soaked nc-CdSe thin films. Concentration of defect states has also been calculated in as-deposited, annealed and light soaked nc-CdSe thin films.     

Studies on the electrical conductivity, optical absorption and x-ray diffraction in bismuth thin film

Spectroscopically pure bismuth is evaporated onto glass substrates at different substrate temperature using a Hind Hivac coating plant. The electrical conductivity of bismuth thin films, prepared at different substrate temperatures is measured and thermal activation energy is evaluated. From the recorded optical absorption spectrum in the ultraviolet and visible regions optical band gapE _g is determined. X-ray diffractograms are recorded and lattice parameters are determined.     

Effect of balanced and unbalanced magnetron sputtering processes on the properties of SnO2 thin films

A comparative study has been carried on the role of balanced magnetron (BM) and unbalanced magnetron (UBM) sputtering processes on the properties of SnO₂ thin films. The oxygen partial pressure, substrate temperature and deposition pressure were kept 20%, 700 °C and 30 mTorr, respectively and the applied RF power varied in the range of 150–250 W. It is observed that the UBM deposition causes significant effect on the structural, electrical and optical properties of SnO₂ thin films than BM as evidenced by X-ray diffraction, C-V, Spectroscopic Ellipsometer and Photoluminescence measurements. The value of band gap (E_g) of the films deposited at 150 W in UBM is found as E_g = 3.83 eV which is much higher than the value of E_g = 3.69 eV as observed in BM sputtering indicating that UBM sputtering results in good crystalline quality. Further, the C-V measurements of SnO₂ thin films deposited using UBM at high power 250 W show hysteresis with large flat band shift indicating that these thin films can be used for the fabrication of memory device. The observed results have been attributed to different mechanisms which exist simultaneously under unbalanced magnetron sputtering due to ion bombardment of growing SnO₂ thin film by energetic Ar+ ions.     

Laser synthesis of nanostructures based on transition metal oxides

Nanostructures based on iron oxides in the form of thin films were synthesized while laser chemical vapor deposition (LCVD) of elements from iron carbonyl vapors (Fe(CO)₅) under the action of Ar⁺ laser radiation (λ_L = 488 nm) on the Si substrate surface with power density about 10² W/cm² and vapor pressure 666 Pa. Analysis of surface morphology and relief of the deposited films was carried out with scanning electron microscopy (SEM) and atomic force microscopy (AFM). This analysis demonstrated their cluster structure with average size no more than 100 nm. It was found out that the thicker the deposited film, the larger sizes of clusters with more oxides of higher oxidized phases were formed. The film thickness (d) was 10 and 28 nm. The deposited films exhibited semiconductor properties in the range 170-340 K which were stipulated by oxide content with different oxidized phases. The width of the band gap E_g depends on oxide content in the deposited film and was varied in the range 0.30-0.64 eV at an electrical field of 1.6 × 10³ V/m. The band gap E_g was varied in the range 0.46-0.58 eV at an electrical field of 45 V/m. The band gap which is stipulated by impurities in iron oxides E_i was varied in the range 0.009-0.026 eV at an electrical field of 1.6 × 10³ V/m and was varied in the range 0-0.16 eV at an electrical field 45 V/m. These narrow band gap semiconductor thin films displayed of the quantum dimensional effect.