Nonlinear optical response of nc-Si-SiO2 films studied with femtosecond four-wave mixing technique

Nonlinear optical properties of silicon nanocrystals (nc-Si) embedded in SiO2 films are investigated using time-resolved four-wave mixing technique with a femtosecond laser. the off-resonant third-order nonlinear susceptibility chi((3)) is observed to be 1.3 x 10(-10) esu at 800 nm. The relaxation time of the film is fast as short as 50 fs. The off-resonant nonlinearity is predominantly electronic in origin and enhanced due to quantum confinement.

Study on variable capacitance diode of (p)nc-Si : H/(n)c-Si heterojunction

Hydrogenated nanocrystalline silicon (nc-Si:H) layers of boron-doped increasing step by step was deposited on n-type crystalline silicon substrate using Plasma Enhanced Chemical Vapor Deposition (PECVD) system. After evaporating Ohm contact electrode on the side of substrate and on the side of nc-Si:H film, a structure of electrode/ (p)nc-Si:H/(n)c-Si/electrode was obtained. It is confirmed by electrical measurement such as I-V curve, C-V curve and DLTS that this is a variable capacitance diode. (C) 2003 Elsevier Science Ltd. All rights reserved.

Preparation and characterization of the stable nc-Si/a-Si : H films

High-quality nc-Si/a-Si:H diphasic films with improved stability were prepared by using the plasma-enhanced chemical vapor deposition technology. In comparison with typical amorphous silicon, the diphasic silicon films possess higher photoconductivity (two orders larger than that of the amorphous silicon film) and fairly good photosensitivity(the ratio of the photo-to dark-conductivity is about 10) and higher stability (the degradation of the photoconductivity is less than 10% after 24h long light soaking with 50 mW/cm(2) intensity at room temperature). In addition, the diphasic silicon film has a better light spectra response in the longer wavelength range. The improvement in photoelectronic properties may be attributed to: the existence of the disorder within the amorphous matrix, which breaks the momentum selection rule in the optical transition and, consequently, results in the large light absorption coefficient and high photosensitivity; the improved medium range order and low gap states density. Excess carriers generated in the amorphous matrix tend to recombine in the embedded crystallites, which suppresses nonradiative recombination within the amorphous matrix and reduces the subsequent defect creation.

Influence of coupling between Er3+, nc-Si and nonradiative centers on photoluminescence from Er3+-doped nc-Si/SiO2 films

Con-elation between nc-Si, Er3+ and nonradiative defects in Er-doped nc-Si/SiO2 films is studied. Upon the 514.5 run laser excitation, the samples exhibit a nanocrystal-related spectrum centered at around 750 nm and an Er3+ luminescence line at 1.54mum. With increasing Er3+ content in the films,the Er3+ emission becomes intense while the photoluminescence at 750 nm decreases. Hydrogen passivation of the samples is shown to result in increases of the two luminescence peaks. However, the effect of hydrogen treatment is different for the samples annealed at different temperatures. The experimental results show that the coupling between Er3+, nc-Si and noradiative centers has a great influence on photoluminescence from nc-Si/SiO2 < Er > films.

Numerical simulation of nc-Si : H/c-Si heterojunction solar cells

AMPS simulator, which was developed by Pennsylvania State University, has been used to simulate photovoltaic performances of nc-Si:H/c-Si solar cells. It is shown that interface states are essential factors prominently influencing open circuit voltages (V-OC) and fill factors (FF) of these structured solar cells. Short circuit current density (J(SC)) or spectral response seems more sensitive to the thickness of intrinsic a-Si:H buffer layers inserted into n(+)-nc-Si:H layer and p-c-Si substrates. Impacts of bandgap offset on solar cell performances have also been analyzed. As DeltaE(C) increases, degradation of VOC and FF owing to interface states are dramatically recovered. This implies that the interface state cannot merely be regarded as carrier recombination centres, and impacts of interfacial layer on devices need further investigation. Theoretical maximum efficiency of up to 31.17% (AM1.5,100mW/cm(2), 0.40-1.1mum) has been obtained with BSF structure, idealized light-trapping effect(R-F=0, R-B=1) and no interface states.

Study on optical band gap of boron-doped nc-Si : H film

The optical band gap (E-g) of the boron (B)-doped hydrogenated nano-crystalline silicon (nc-Si:H) films fabricated using plasma enhanced chemical vapor deposition (PECVD) was investigated in this work. The transmittance of the films were measured by spectrophotometric and the E-g was evaluated utilizing three different relations for comparison, namely: alphahnu=C(hnu-E-g)(3), alphahnu=B-0(hnu-E-g)(2), alphahnu=C-0(hnu-E-g)(2). Result showed that E-g decreases with the increasing of Boron doping ratio, hydrogen concentration, and substrate's temperature (T-s), respectively. E-g raises up with rf power density (P-d) from 0.45W.cm(-2) to 0.60w.cm(-2) and then drops to the end. These can be explained for E-g decreases with disorder in the films.

Impact of wide bandgap p-type nc-Si on the performance of a-Si solar cells

This paper reports the impact of a wide bandgap p-type hydrogenated nanocrystalline silicon (nc-Si:H) on the performances of hydrogenated amorphous silicon (a-Si:H) based solar cells. The player consists of nanometer-sized Si crystallites and has a wide effective bandgap determined mainly by the quantum size-confinement effect (QSE). By incorporation of this p-layer into the devices we have obtained high performances of a-Si:H top solar cells with V-infinity=1.045 V and FF=70.3 %, and much improved mid and bottom a-SiGe:H cells, deposited on stainless steel (SS) substrate. The effects of the band-edge mismatch at the p/i-interface on the I-V characteristics of the solar cells arc discussed on the bases of the density-functional approach and the AMPS model.

Carrier transport properties of the (n)nc-Si : H/(p)c-Si heterojunction

Phosphor-doped nano-crystalline silicon ((n))nc-Si:H) films are successfully grown on the p-type (100) oriented crystal silicon ((p) c-Si) substrate by conventional plasma-enhanced chemical vapor deposition method. The films are obtained using high H-2 diluted SiH4 as a reaction gas source and using PH3 as the doping gas source of phosphor atoms. Futhermore, the heterojunction diodes are also fabricated by using (n)nc-Si:H films and (p)c-Si substrate. I-V properties are investigated in the temperature range of 230-420K. The experimental results domenstrate that (n)nc-Si:H/(p) c-Si heterojunction is a typical abrupt heterojunction having good rectifing and temperature properties. Carrier transport mechanisms are tunneling - recombination model at forward bias voltages. In the range of low bias voltages ( V-F< 0.8 V), the current is determined by recombination at the (n)nc-Si:H side of the space charge region, while the current becomes tunneing at higher bias voltages( V-F>1.0 V). The present heterojunction has high reverse breakdown voltage ( > - 75 V) and low reverse current (approximate to nA).

Improved diphasic nc-si/a-si : H I-layer materials using PECVD

Two series of films has been prepared by using a new regime of plasma enhanced chemical vapor deposition (PECVD) in the region adjacent to the phase transition from amorphous to crystalline state. The photoelectronic properties of the films have been investigated as a function of crystalline fraction. In comparison with typical a-Si:H, these diphasic films with a crystalline fraction less than 0.3 show a similar optical absorption coefficient, higher mobility life-time product ( LT) and higher stability upon light soaking. By using the diphasic nc-Si/a-Si films as the intrinsic layer, a p-i-n junction solar cell has been prepared with an initial efficiency of 9. 10 % and a stabilized efficiency of 8.56 % (AM 1.5, 100 mW/cm(2)).

Z扫描法研究nc-Si/SiN_x多量子阱材料非线性光学特性

采用射频磁控反应溅射技术与热退火处理制备了nc-Si/SiN_x多量子阱材料.对样品进行了小角度XRD、Raman光谱、吸收光谱测试,研究了其结构和光学性质.采用皮秒脉冲激光单光束Z-扫描技术研究了样品在非共振吸收区的三阶非线性光学特性.实验结果表明,其非线性折射率为负值,非线性吸收属于双光子吸收.由实验数据计算得材料三阶非线性极化率为7.50×10~(-8)esu,该值比体硅材料的三阶非线性极化率大4个数量级.对材料光学非线性产生的机理进行了探讨,认为材料的非线性极化率的增加来源于材料量子限制效应增强.

nc-Si／SiN_x超晶格薄膜实现Nd

采用射频磁控反应溅射法制备a—Si／SiN。超晶格薄膜材料，热退火后形成纳米Si晶粒。把nc—Si／SiN，薄膜作为饱和吸收体插入Nd

Nonlinear Optical Properties of AI-Doped nc-Si-SiO_2 Composite Films

The nonlinear optical properties of Al-doped nc-Si-SiO_2 composite films have been investigated using the time-resolved four-wave mixing technique with a femtosecond laser. The off-resonant third-order nonlinear susceptibility is observed to be 1.0 × 10~(-10) esu at 800nm. The relaxation time of the optical nonlinearity in the films is as short as 60fs. The optical nonlinearity is enhanced due to the quantum confinement of electrons in Si nanocrystals embedded in the SiO_2 films. The enhanced optical nonlinearity does not originate from Al dopant because there are no Al clusters in the films.

nc-Ge/Si岛基光子晶体单点缺陷腔的数值模拟与分析

阐述了利用光子晶体单点缺陷微腔来提高Ge/Si纳米岛发光效率的机理.通过3D FDTD方法计算出在平板厚度为300nm时,谐振波长随a和r/a变化的规律,即当给定r/a,h时,波长随晶格常数成次线性增加;当给定a,h时,波长随r/a的增加而减小.并从理论上给予分析.

光子晶体对nc-Ge/Si岛发光增强的模拟

在Si基集成光电子学的发展中,高效的Si基光源是人们不懈追求的目标.但是Si材料的间接带隙特性导致其发光效率低,更谈不上受激发射.于是人们探索了多种Si基材料体系来提高Si材料的发光效率,并在不同程度上取得了重要的进展.在众多的Si基发光材料体系中,Ge/Si量子点材料,不仅生长工艺与标准的CMOS工艺有很好的兼容性,而且发光波长能够覆盖重要的光通信波段即1.3～1.55 μm,因此成为实现Si基发光器件的重要途径之一.但是目前这种材料的发光效率仍很低,所以提高其发光效率自然成为人们关注的焦点.如果将光子晶体引入到nc-Ge/Si材料中,它不仅可以改变材料本身的自发发射特性,而且可以改变发射的光子的提取效率,从而使材料的发光效率得到增强.提出了在Ge/Si量子点材料中引入光子晶体结构来提高其发光效率,包括光子晶体点缺陷腔结构和带边模式工作的完整光子晶体结构,并从理论上分析了发光效率提高的原理.针对发光波长在1.5 μm附近的材料结构,模拟出了相应的光子晶体的结构参数.从模拟结果可以看出,对于缺陷腔的光子晶体结构,采用单点缺陷微腔很好地实现了单模运作,但是微腔内有源材料的体积很小,因此得到的发光效率很低.而采用耦合缺陷腔的结构和H2腔都增加了腔内有源区的体积.但是耦合腔与H2腔相比,谐振腔模减少,主谐振模式的峰值强度增加,更容易实现单模发光.因而更适用于提高nc-Ge/Si的发光效率.而带边模式工作的光子晶体结构,尺寸较大,不需引入缺陷,工艺上更容易实现.

(p)nc-Si:H/(n)c-Si异质结变容二极管

采用等离子体增强化学气相沉积技术和电子束蒸发技术制备了一种新型的线性缓变异质结变容二极管--Au/Cr合金(电极)/multi-layer(p)nc-Si:H/(n)c-Si/(电极)Au/Ge合金结构.I-V,C-V,G-f以及DLTS的测试结果表明:其电容变化系数远大于单晶硅线性缓变异质结的电容变化系数,正向导电机制符合隧穿辅助辐射-复合模型,这是nc-Si:H层中nc-Si晶粒的量子效应所致;反向电流主要由异质结中空间电荷区的产生电流决定,且反向漏电流小,反向击穿电压高,表现出较好的整流特性.