Study of photoconductivity and photoluminescence of organic/porous silicon complexes

In this paper, time-varying photoconductivity (PC) and the photoluminescence (PL) of different complexes were studied. Due to thick polymer layer hindering light penetrating into porous silicon (PS) layer, intrinsic PS luminescence in polymer/PS system disappeared. The physical origin of PL may be related to the recombination mechanisms involving surface defect states such as silicon oxide, siloxene. Due to carrier transfer controlled by different energy barrier, different devices prepared from different doped Si wafer showed opposite current-voltage characteristic.     

多孔硅的后处理及其发光特性

采用一种新颖而简便的方法,改善多孔硅的发光特性。该方法包括酸处理和阴极还原两步。实验证明通过对多孔硅进行酸处理,能有效提高多孔硅的发光强度;通过对多孔硅进行阴极还原处理,能明显改善多孔硅的发光稳定性,而且发光强度也得到了提高。综合酸处理和阴极还原两技术的特点,对所制备的多孔硅立即先进行酸处理,然后再对其进行阴极还原处理,结果表明该方法能较好地提高多孔硅的发光效率和发光稳定性。而且还对其发光机制进行了探讨。     

少子寿命和表面形貌与多孔硅发光性能的关系

研究了不同时间腐蚀的多孔硅的光致发光性能与多孔硅的表面形貌和少子寿命之间的关系。结果表明,多孔硅的发光来自与氧空位有关的缺陷,而多孔硅表面的氢原子能够钝化多孔硅表面的非辐射中心从而提高多孔硅的发光效率。多孔硅的空隙度随腐蚀时间的延长而增大,这也导致了多孔硅的少子寿命的降低,从而造成多孔硅的光致发光效率随多孔硅空隙度的增大以及少子寿命的降低而提高。另外,原子力显微照片表明长时间的腐蚀使多孔硅表面层被化学腐蚀,从而降低了多孔硅表面的粗糙度。     

Optical properties of neodymium incorporated in porous silicon

We report the optical properties of Nd-incorporated porous silicon. Photoluminescence and photoluminescence excitation measurements have been performed. Room temperature emission spectra from dried or annealed samples have been studied. While steady-state photoluminescence results indicate porous silicon light absorption by the Nd, the photoluminescence excitation shows a deficiency of energy transfer between porous silicon and Nd ions.     

多孔硅拉曼光谱的研究

采用电化学腐蚀的方法制备多孔硅。对不同实验条件下所得到的多孔硅的拉曼光谱进行了分析，确认多孔硅是具有纳米晶结构特征的材料，肯定了量子限制效应在多孔硅光致发光中的作用。     

Electrochemical deposition of cerium on porous silicon to improve photoluminescence properties

In this work, we present results for Cerium (Ce) doping effects on photoluminescence (PL) properties of porous silicon (PS). Cerium was deposited using electrochemical deposition on porous silicon prepared by electrochemical anodization of P-type (100) Si. From the photoluminescence spectroscopy, it was shown that porous silicon treated with cerium can lead to an increase of photoluminescence when they are irradiated by light compared to the porous silicon layer without cerium. In order to understand the contribution of cerium to the enhanced photoluminescence, energy dispersive X-ray (EDX) spectroscopy, Fourier transmission infrared spectroscopy (FTIR), X-ray diffraction (XRD) and atomic force microscopy (AFM) were performed, and it was shown that the improved photoluminescence may be attributed to the change of Si–H bonds into Si–O–Ce bonds and to a newly formed PS layer during electrochemical Ce coating.     

多孔硅微秒光致发光衰减

在微秒范围内测量了多孔硅的光致发光衰减,研究了衰减参数与发光波长、样品制备条件的关系,发现衰减参数与发射波长有关,但不依赖于样品制备条件。用发光三层模型解释了实验结果。     

An investigation of natural oxidation process on stain-etched nanoporous silicon by micro-Raman spectroscopy

Micro-Raman spectra of porous silicon (PS) samples as-formed, from stain etching process using heavily doped silicon wafers, and after 750 days storage in air were analyzed around Si peak (300-600 cm⁻¹) and at photoluminescence (PL) range (300-8000 cm⁻¹). The first-order Raman spectra in the vicinity of Si peak were fitted from phonon confinement model including a term taking into account the amorphous phase. This analysis allowed the determination of the correlation length, which corresponds to the crystallite size, also considering the PS natural oxidation process. The photoluminescence band, generated by Si crystallites located on the outermost part of the PS layer, was also fitted with a Gaussian distribution. In order to investigate the porous silicon nanostructure, the micro-Raman spectra were measured for different sets of porous silicon samples. These spectra showed good reproducibility and the effects of the natural oxidation at different periods. A slight decrease in the crystallite size was observed for all samples sets studied, while the spectral part related to the amorphous phase did not describe significant changes. The central position of PL band, analyzed after the oxidation process, exhibited consistently a shift to higher energies. In addition, top view high resolution scanning electron microscopy (HRSEM) images also confirmed a reasonable reproducibility and homogeneity. The results showed that after storing in air, natural oxidation can modify the Si crystallites size at the surface but not increase the amorphous phase.     

Comparative investigation of photoluminescence of silicon wire structures and silicon oxide films

Photoluminescence spectra and their dependence on temperature as well as Raman scattering spectra and Atomic Force Microscopy investigations have been used to study the peculiarities of the red photoluminescence band in low-dimensional Si structures, such as porous silicon and silicon oxide films. It has been shown that the red photoluminescence band of porous silicon is complex and can be decomposed into two elementary bands. It was discovered that elementary band intensities depend very much on surface morphology of porous silicon. The same positions of the photoluminescence bands are also observed in silicon oxide films for different oxide composition. Comparative investigation of the PL temperature dependences in porous silicon and silicon oxide films indicates that silicon-oxide defect related mechanisms of some elementary photoluminescence bands are involved.     

Effect of surface modification by thermally oxidization and HF etching on UV photoluminescence emission of porous silicon

Photoluminescence of porous silicon (PS) is instable due perhaps to the nanostructure modification in air. The controllable structure modification processes on the as-prepared PS were conducted by thermal oxidization and/or HF etching. The PL spectra taken from thermally oxidized PS showed a stable photoluminescence emission of 355 nm. The photoluminescence emission taken from both of PS and oxidized porous silicon (OPS) samples etched with HF were instable, which can be reversibly recovered by the HF etching procedure. The mechanism of UV photoluminescence is discussed and attributed to the transformation of luminescence centers from oxygen deficient defects to the oxygen excess defects in the thermal oxidized PS sample and surface absorbed silanol groups on PS samples during the chemical etched procedure.     

Photoluminescence quenching of porous silicon by noble metal adsorbates

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多孔硅的分形结构及其荧光特性

用扫描电子显微镜(SEM)对多孔硅的结构进行了分析.结果显示多孔硅具有分形特性,同计算机模拟结果一致;用荧光光谱仪,研究了多孔硅的荧光特性与激发波长的依赖关系.激发光谱测量结果发现,当激发波长从650 nm变到340 nm时,荧光谱峰位从红端780 nm连续蓝移到500 nm.综合分析说明：正是由于多孔硅的分形微结构以及量子限制效应,导致了多孔硅的荧光特性随激发波长改变的物理现象.     

激光辅助阳极化制备多孔硅的蓝光发射与红外研究

发现由n型单晶硅光照辅助阳极化制备的多孔硅在经过较长时间室温空气中放置氧化后，部分样品出现了蓝色光致荧光.荧光及红外透射、反射测量表明，在较强的氩离子488nm激光照射下制备的样品，经放置氧化后形成氧化层的组织较好，即氧化层中的应力较小、非晶程度相对较低、Si-O-Si的网络结构比较完整.而这样的氧化层是有利于多孔硅光致荧光中的蓝光发射的. 关键词：     

Peculiarities of photoluminescence of porous silicon with luminescent liquid crystal fillers

Photoluminescence spectra of porous silicon filled by luminescent liquid crystals 5CB and H109 were investigated. It was observed that there were photoluminescence bands corresponding to both porous silicon and liquid crystal in experimental spectra. In addition, the band corresponding to porous silicon increases in comparison with photoluminescence of porous silicon without the filler. Experimental results are explained by the radiating and nonradiating energy transfer from liquid crystal to porous silicon.     

The effect of Al and B on the luminescent property of porous silicon

In this paper, we studied the PL property and the surface morphology of porous silicon prepared from N-type single silicon wafers coated with and without Al film. By introducing the Al film on the surface of silicon wafer before etching, the morphology of porous silicon exhibits obvious discrimination compared with that of the conventional porous silicon, which can be explained by the formation mechanism of the samples, and the emission property of two-type porous silicon also showed the clear difference, which may be attributed to the discrepancy in the structural configuration of the samples. Furthermore, it was found that the blue emission decreased and the green emission was almost completely quenched after boron-particle deposition, which is attributed to the structural change or annihilation of the emission defects during annealing process.     

Laser-induced etching of silicon

Conventional fabrication method of porous silicon is anodisation of single crystal silicon in hydrofluoric acid. In this report, we show that it is possible to fabricate porous silicon by laser-induced etching. An earlier report by us has demonstrated the dependence of porous silicon photoluminescence characteristic on the etching laser wavelength [1]. Here we used 780 nm line from a diode laser as the etching source, and the optimum etching conditions were obtained. A simple model was proposed to explain the etching process. Scanning Electron Microscope (SEM) images of the samples support the proposed process.     

Thermal annealing dependence of some optical properties of plasma-modified porous silicon

The photoluminescence and reflectance of porous silicon (PS) with and without hydrocarbon (CH_x) deposition fabricated by plasma enhanced chemical vapour deposition (PECVD) technique have been investigated. The PS samples were then, annealed at temperatures between 200 and 800 °C. The influence of thermal annealing on optical properties of the hydrocarbon layer/porous silicon/silicon structure (CH_x/PS/Si) was studied by means of photoluminescence (PL) measurements, reflectivity and ellipsometry spectroscopy. The composition of the PS surface was monitored by transmission Fourier transform infrared (FTIR) spectroscopy. Photoluminescence and reflectance measurements were carried out before and after annealing on the carbonized samples for wavelengths between 250 and 1200 nm. A reduction of the reflectance in the ultraviolet region of the spectrum was observed for the hydrocarbon deposited polished silicon samples but an opposite behaviour was found in the case of the CH_x/PS ones. From the comparison of the photoluminescence and reflectance spectra, it was found that most of the contribution of the PL in the porous silicon came from its upper interface. The PL and reflectance spectra were found to be opposite to one another. Increasing the annealing temperature reduced the PL intensity and an increase in the ultraviolet reflectance was observed. These observations, consistent with a surface dominated emission process, suggest that the surface state of the PS is the principal determinant of the PL spectrum and the PL efficiency.     

High-resolution positron lifetime spectrometer with BaF2 scintillators

A transient negative photoconductivity phenomenon observed in silicon doped with gold atoms by an appropriate diffusion condition has been investigated. The transient negative photoconductivity could be observed with a diode doped with gold atoms under intrinsic light-pulse illumination. In the recovery process of the transient negative photoconductivity to a steady-state dark level, two kind of time constants were observed, which may be related with the emission of electrons from gold level. A dependence of the magnitude of the peak value on the ambient temperature was observed; a qualitative explanation is given.     

有机吸附物对多孔硅微腔发光的影响

理论上,采用Bruggeman有效介质近似,研究了有机吸附物对多孔硅微腔的折射率及其光致发光谱的影响.实验上,采用计算机控制的电化学腐蚀法制备了多孔硅微腔样品,并利用机械泵油的蒸气分子与该微腔样品进行相互作用.研究发现,多孔硅微腔发射的窄化光致发光谱对泵油蒸气分子的吸附与脱附很敏感,与之伴随的是该窄化光致发光谱发生明显的峰位移动(可达71nm)和强度变化.结合Bruggeman近似和表面态对多孔硅发光的影响,对实验结果进行了定性解释.实验结果与理论模拟结果符合较好. 关键词： Bruggeman近似 吸附物 多孔硅微腔 光致发光谱     

多孔硅后处理对其镶嵌染料光学特性的影响

对多孔硅进行真空退火处理和暴露大气快速退火处理,将有机染料香豆素102(C102)镶嵌其中,研究镶嵌复合膜发光特性的变化。通过比较多孔硅退火处理前、后傅立叶红外(FT-IR)吸收光谱的变化,从多孔硅与镶嵌染料分子间的能量传递方式角度出发,解释了PS表面态氧化能改善镶嵌复合膜发光特性的原因。实验通过改变多孔硅表面态,提高了复合膜的发光效率和多孔硅基体的透明程度,证明了多孔硅是一种良好的载体,在发展固体激光器方面有一定的应用,同时为实现硅基蓝绿发光开辟一条新的途径。