Arresting photodegradation of porous silicon by a polymer coating

Light soaking in air rapidly decreases photoluminescence (PL) of porous silicon (PS) and increases electron spin resonance (ESR) signal. In vacuum, a short light exposure (<2700 s) increases PL and decreases ESR, but longer exposures again degrade the PL. We could arrest the light-induced degradation over long periods by applying a thin polymer coating, which resulted in constant PL and ESR intensities. The PL intensity of coated PS is comparable to the PL intensity of a fresh PS sample in air. FTIR spectrum suggests new bond formations at the PS/polymer interface that may be responsible for PL stability.     

Controlling surface states and photoluminescence of porous silicon by low-energy-ion irradiation

Porous silicon (PS) was irradiated by three kinds of low-energy ions with different chemical activity, namely argon ions, nitrogen ions and oxygen ions. The chemical activity of ions has significant effect on the surface states and photoluminescence (PL) properties of PS, The photoluminescence quenching after argon ions and nitrogen ions irradiation is ascribed to the broken Si-Si bonds, while the PL recovery is attributed to the oxidation of Si-H back bonds. Oxygen ions irradiation leads to the formation of a SiO_x layer with oxygen defects and PS shows different PL evolution than PS irradiated by argon ions and nitrogen ions.     

The Influence of Immersion of Porous Silicon in Aqueous Solutions of Fe(NO<Subscript>3</Subscript>)<Subscript>3</Subscript> on Photoluminescence during Long Storage

The interaction of porous silicon (PS) with aqueous solutions of Fe(NO₃)₃ with different molar (M) concentrations causes introduction of iron ions into silicon pores (PS–Fe), formation of adsorbed iron coatings with different thicknesses, and an increase in the stability of PS layers, which is important for development of device structures based of PS. To treat PS layers with solutions by the immersion method, it is necessary to determine how this affects the spectral composition and intensity of photoluminescence (PL), as well as the kinetics of PL changes during long storage under atmospheric conditions. Upon treatment of freshly prepared PS by immersion into in a Fe(NO₃)₃ aqueous solution, it was found that, after short-term storage (up to 5 days) of the PS samples, the PL intensity increases by 7.5 and 3–3.6 times at low (0.2 M) and high (0.7–0.8 M) concentrations of Fe(NO₃)₃, respectively, compared to the PL intensity of an untreated PS layer. After long-term storage (4 months), the PL intensity of PS–Fe samples with concentrations of 0.1–0.2 and 0.7–0.8 M was observed to considerably increase (by 8–18 times) with unchanged position of the PL peak with respect to untreated PS. However, at the Fe(NO₃)₃ concentration of 0.3 М, the PL intensity decreases and the PL peak shifts to the blue, which is explained by incomplete coverage of the PS surface by an adsorbed iron layer. The kinetics of PL spectra during long-term storage is analyzed, and a model is proposed to explain the PL intensity and spectral composition.     

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.     

Photoluminescence studies on porous silicon/polymer heterostructure

Hybrid devices formed by filling porous silicon with MEH-PPV or poly [2-methoxy-5(2-ethylhexyloxy-p-phenylenevinylene)] have been investigated in this work. Analyses of the structures by scanning electron microscopy (SEM) demonstrated that the porous silicon layer was filled by the polymer with no significant change of the structures except that the polymer was infiltrated in the pores. The photoluminescence (PL) of the structures at 300 K showed that the emission intensity was very high as compared with that of the MEH-PPV films on different substrates such as crystalline silicon (c-Si) and indium tin oxide (ITO). The PL peak in the MEH-PPV/porous silicon composite structure is found to be shifted towards higher energy in comparison with porous silicon PL. A number of possibilities are discussed to explain the observations.     

Depth dependence of photoluminescence and chemical bonding in porous silicon

Porous silicon (PS) is studied by stepwise peeling of the surface layer to clarify the non-uniformity in the photoluminescence (PL) and correlate it with the in-depth chemical bonding and structure of the 30 μm thick layer. The PL intensity grows by an order of magnitude after the peeling off of the first 10 μm and decreases five times in the next 5 μm while the peak maximum position shifts from 730 to 800 nm. X-ray photoelectron spectroscopy (XPS) measurements show that Si–Si and Si–O bonds are present both on the surface and below, and the preferential oxidation state of silicon changes from 3+ and 4+ on the surface to 1+ and 2+ below 10 μm. Using Raman spectroscopy silicon nanocrystals are shown to exist. Their mean size can be estimated at about 3 nm. These results show that the strongest PL comes from a region in the PS layer where silicon nanocrystallites are surrounded by oxides with a low level of oxidation and not from the strongly oxidized surface layer.     

Photoluminescence properties of Tb in porous silicon

Terbium (Tb³⁺)/porous silicon (PS) nanocomposites have been formed by impregnation of PS layer in chloride solution of terbium. Complete and uniform penetration of Tb³⁺ into PS layer is confirmed by Rutherford backscattering spectrometry (RBS) study. Photoluminescence (PL) spectrum shows that Tb³⁺ ions emit highly in the green region, while the PL band of PS is quenched. The emission of Tb³⁺ ions depends strongly on the excitation energy and shows a high efficiency at 488 nm corresponding to the maximum absorption band in terbium. A systematic study of the PL versus annealing temperature was performed. It shows an important improvement of the PL intensity for 700°C temperature annealing.     

Characterization of nanostructured CuO–porous silicon matrix formed on copper-coated silicon substrate via electrochemical etching

A pulsed anodic etching method has been utilized for nanostructuring of a copper-coated p-type (100) silicon substrate, using HF-based solution as electrolyte. Scanning electron microscopy reveals the formation of a nanostructured matrix that consists of island-like textures with nanosize grains grown onto fiber-like columnar structures separated with etch pits of grooved porous structures. Spatial micro-Raman scattering analysis indicates that the island-like texture is composed of single-phase cupric oxide (CuO) nanocrystals, while the grooved porous structure is barely related to formation of porous silicon (PS). X-ray diffraction shows that both the grown CuO nanostructures and the etched silicon layer have the same preferred (220) orientation. Chemical composition obtained by means of X-ray photoelectron spectroscopic (XPS) analysis confirms the presence of the single-phase CuO on the surface of the patterned CuO–PS matrix. As compared to PS formed on the bare silicon substrate, the room-temperature photoluminescence (PL) from the CuO–PS matrix exhibits an additional weak ‘blue’ PL band as well as a blue shift in the PL band of PS (S-band). This has been revealed from XPS analysis to be associated with the enhancement in the SiO₂ content as well as formation of the carbonyl group on the surface in the case of the CuO–PS matrix.     

Effect of contact with air on the photoluminescence spectrum of porous silicon

A study has been made of the transformation of photoluminescence (PL) spectra of porous silicon (PS) induced by its ageing, including the early stages of contact with air. The sample was prepared under conditions that minimized this contact, and spectral measurements were carried out in a high vacuum or in liquid nitrogen. The PS PL spectra obtained under continuous measurement in high vacuum are always dominated by one emission band of PS nanoelements, which shifts toward shorter wavelengths with ageing by 150 nm. At 80 K, the band intensity is considerably higher than at 300 K, and this difference grows with ageing. Exposure of a sample to air for a few tens of seconds is long enough to strongly transform its time-resolved PL spectra, which is evidence of a change in the sample surface. The effect of immersion of PS samples in liquid nitrogen on PL spectra is associated not only with their cooling, but also with the field of adsorbed nitrogen molecules, whose influence becomes weaker with increasing thickness of the oxidized near-surface layer. The variation of the spectral properties and kinetics of the long-wavelength PS PL band with temperature, medium (liquid nitrogen or vacuum), and exposure time suggest that these factors affect carrier migration between silicon nanoelements.     

Improvement of photoluminescence and electrical properties of porous silicon layer treated with lanthanum

The influence of surface treatment of porous silicon (PS) in lanthanum (La) containing solution during different times on its photoluminescence and electrical properties has been investigated. For this purpose, chemical composition, structural, vibrational, photoluminescence and electrical characteristics of the porous silicon layer with and without lanthanum were examined using X-ray diffractometry (XRD), energy dispersive X-ray (EDX) spectroscopy, Fourier transmission infrared (FTIR) spectroscopy, photoluminescence (PL) spectroscopy and current–voltage (I–V) measurements. The results indicate that porous silicon layers treated with lanthanum exhibit an enhancement of photoluminescence intensity and show an improvement current intensity compared to untreated porous silicon layer.     

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。     

多孔硅蓝光发射与发光机制

在制备出光致发光能量为２．７ｅＶ的发射蓝光多孔硅的基础上对它进行了较系统的研究：测量了它的光致发光时间分辨光谱，用傅里叶交换红外光谱分析了其表面吸附原子的局域振动模，研究了γ射线辐照对其发光的影响，并与发红、黄光的多孔硅作了对比，通过空气中长期存放、激光和紫外线照射的方法，研究了光致发光峰能量为２．７ｅＶ的多孔硅发光稳定性．我们及其它文献中报道的多孔硅蓝光发射的实验结果与量子限制模型矛盾，但能用量子限制／发光中心模型解释．我们认为多孔硅的２．７ｅＶ发光是多孔硅中包裹纳米硅的ＳｉＯ_x层中某种特征发光中心引起的． 关键词：     

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.     

Emission mechanisms in stabilized iron-passivated porous silicon: Temperature and laser power dependences

Photoluminescence (PL) measurements of porous silicon (PS) and iron-porous silicon nanocomposites (PS/Fe) with stable optical properties versus temperature and laser power density have been investigated. The presence of iron in PS matrix is confirmed by Raman spectroscopy. The PL intensity of PS and PS/Fe increases at low temperature, the evolution of integrated PL intensity follows the modified Arrhenius model. The incorporation of iron in PS matrix reduces the activation energy traducing the existence of shallow levels related to iron atoms. Also, the temperature dependence of the porous silicon PL peak position follows a linear evolution at high temperature and a quadratic one at low temperature. Such evolution is due to the thermal carriers' redistribution and an energy transfer. Similarly, we have compared the laser power dependence of the PL in PS and PS/Fe layers. The results prove that the recombination process in PS is realised through the lower energy traps localised in the electronic gap. However, the observed emission in PS/Fe is essentially due to direct transitions. So, we can conclude that the presence of iron in PS matrix induces a strong modification of the PL mechanisms.     

氧化处理多孔硅的光致发光和发光衰减

用沸硫酸对多孔硅进行了处理,测量了它们的红外吸收光谱、光致发光光谱和发光衰减.对氢、氧的作用进行了讨论,发现氧的加入可使发光强度增加两个数量级,发光衰减变快.用三层发光模型对结果进行了讨论.     

Luminescence properties of Er in SiN/PS : Er

The decrease in luminescence from host porous silicon (PS) by thermal annealing prevents the optical activation of Er ions. We prepared a SiN layer on erbium-doped porous silicon (PS : Er) as the capping layer by photo-chemical vapor deposition (photo-CVD). After deposition of SiN, the sample was annealed in pure Ar atmosphere for optical activation. We observed an Er-related emission at 1532 nm with a full-width at half-maximum (FWHM) of 10 nm at 18 K from the sample with the SiN layer. In contrast, no emission was observed from the sample without the SiN layer. At 300 K, the peak intensity of Er³⁺-related photoluminescence (PL) for the sample annealed at 1100°C decreased to 40.0% of that observed at 18 K. From these results, it was found that the SiN layer on PS:Er is useful for both host PS and Er-related 1.5 μm luminescences.     

Optoelectronic characterization of ZnS/PS systems

ZnS thin films are deposited on porous silicon （PS） substrates with different porosities by pulsed laser deposition （PLD）. The photoluminescence （PL） spectra of the samples are measured at room temperature. The results show that the PL intensity of PS after deposition of ZnS increases and is associated with a blue shift. With the increase of PS porosity, a green emission at about 550 nm is observed in the PL spectra of ZnS/PS systems, which may be ascribed to the defect-center luminescence of ZnS films. Junction current- voltage （I-V） characteristics were studied. The rectifying behavior of I-V characteristics indicates the formation of ZnS/PS heterojunctions, and the forward current is seen to increase when the PS porosity is increased.     

Photoluminescence enhancement and stabilisation of porous silicon passivated by iron

Iron is incorporated in porous silicon (PS) by impregnation method using Fe(NO₃)₃ aqueous solution. The presence of iron in PS matrix is shown from energy-dispersive X-ray (EDX) analysis and Fourier transform infrared (FTIR) measurements. The optical properties of PS and PS-doped iron are studied by photoluminescence (PL). The iron deposited in PS quenched the silicon dangling bonds then increased the PL intensity. The PL peak intensity of impregnated PS is seven times stronger than that in normal PS. Upon exposing iron-PS sample to ambient air, there is no significant change in peak position but the PL intensity increases during the first 3 weeks and then stabilises. The stability is attributed to passivation of the Si nanocrystallites by iron.     

多孔硅的表面碳膜钝化

报道对多孔硅（ＰＳ）进行碳膜（ＣＦ）钝化处理的结果。红外吸收光谱表明,经钝化处理样品的表面由Ｓｉ－Ｃ、Ｓｉ－Ｎ和Ｓｉ－Ｏ键所覆盖;荧光我谱表明,经钝化处理的样品较未处理的样品发光强度提高４￣４．５倍,且发光峰位明显蓝移;存放实验显示,经钝化处理的样品发光强度稳定、发光峰位不变。这些结果表明正丁胺可以在多孔硅表面形成优良的钝化碳膜,是一种十分有效的多孔硅后处理途径。     

多孔硅的微结构与发光特性研究

利用原子力显微镜（AFM）和光致荧光（PL）光谱对一系列直流腐蚀和脉冲腐蚀的多孔硅的微结构及发光特性进行了对比研究．表面和侧面的AFM结果表明，多孔硅表面呈“小山”状，有许多小的、突出的硅颗粒．在相同的腐蚀条件（等效）下，脉冲腐蚀的样品表面Si颗粒更加尖锐、突出，侧面的线状结构更明显，多孔硅层更厚．对应的PL谱，脉冲腐蚀的样品发光更强．量子限制效应的理论可以比较成功地解释这个结果 关键词：