Optical and microstructural investigations of porous silicon

Raman scattering and photoluminescence (PL) measurements on (100) oriented n-type crystalline silicon (c-Si) and porous silicon (PS) samples were carried out. PS samples were prepared by anodic etching of c-Si under the illumination of light for different etching times of 30, 60 and 90 min. Raman scattering from the optical phonon in PS showed the redshift of the phonon frequency, broadening and increased asymmetry of the Raman mode on increasing the etching time. Using the phonon confinement model, the average diameter of Si nanocrystallites has been estimated as 2.9, 2.6 and 2.3 nm for 30, 60 and 90 min samples, respectively. Similar size of Si crystallites has been confirmed from the high resolution transmission electron microscopy (HRTEM). Using 2TO phonon mode intensity, we conjectured that the disordered Si region around the pores present in 30 min PS dissolved on etching for 90 min. The photoluminescence (PL) from PS increased in intensity and blue shifted with etching time from 2.1–2.3 eV. Blue shifting of PL is consistent with quantum confinement of electron in Si nanocrystallites and their sizes are estimated as 2.4, 2.3 and 2.1 nm for 30, 60 and 90 min PS, respectively which are smaller than the Raman estimated sizes due to temperature effect. Unambiguous dominance of quantum confinement effect is reported in these PS samples.     

永冲电化学腐蚀制备n型多孔硅及其发光性能

采用脉冲电化学腐蚀法,以n型单晶硅为衬底制备多孔硅（n—PS）,通过扫描电镜（SEM）、室温500—700nm范围内荧光光谱,系统研究腐蚀时间、占空比和脉冲频率对n-PS的结构形貌和可见光区室温光致发光特性（PL）的影响,结果表明,相比直流电化学腐蚀方法,脉冲腐蚀能获得孔径分布均匀且发光强度更高的多孔硅;随腐蚀时间、占空比和脉冲频率等腐蚀条件的变化,其发光峰位及发光强度均有明显改变;当等效腐蚀时间为30min、占空比为0．5、脉冲频率为10Hz时,制备的n—PS的PL强度较高,发光性能较好。     

多孔硅与聚甲基丙烯酸甲酯复合光致发光特性研究

多孔硅与有机材料复合可以改善多孔硅的光致发光特性。用化学腐蚀的方法制备了多孔硅，通过不同方法实现了多孔硅与聚甲基丙烯酸甲酯(PMMA)的复合。实验结果表明，用旋涂法实现的PMMA固化后再与多孔硅复合而制得的样品的结果最好，它与原始的多孔硅样品相比，发光峰发生了蓝移而且发光强度下降很小。PMMA层有限的厚度和PMMA对多孔硅表面的保护使复合后发光强度下降很小。制备的多孔／PMMA复合体系的发光强度几乎不随时间而下降，这可能是由于PMMA有效地隔绝多孔硅与空气的接触，保护了多孔硅的表面，不会产生更多的悬挂键。     

Porous silicon nanowires fabricated by electrochemical and laser-induced etching

Electrochemical and laser-induced etching processes were simultaneously used to synthesize the nanowires structure of porous silicon (PS). Surface morphology and structural properties of nanostructured silicon were characterized by using scanning electron microscopy (SEM) and atomic forces microscopy (AFM) images. Nanowires with dimensions of few nanometers were formed on the whole etched surface. The optical properties of silicon nanostructures were studied. Raman spectra were shifted and broadened relatively to 519.9 cm⁻¹ of PS prepared by electrochemical etching, and shifted to 517.2 cm⁻¹ for laser-induced etching process and to 508.9 cm⁻¹ for electrochemical and laser etching simultaneously. Blue shift luminescence was observed at 649.6 nm for PS produced by electrochemical etching, and at 629.5 nm for laser-induced etching. PS produced a blue shift at 626.5 nm using both etching procedures simultaneously. X-Ray diffraction (XRD) was used to investigate the crystallites size of the PS as well as to provide an estimate of the degree of crystallinty of the etched sample.     

Optical and structural analysis of porous silicon coated with GZO films using rf magnetron sputtering

In the production of porous silicon (PS) to optoelectronic application one of the most significant constrains is the surface defects passivation. In the present work we investigate, gallium-doped zinc oxide (GZO) thin films deposited by rf magnetron sputtering at room temperature on PS obtained with different etching times. The X-ray diffraction (XRD), Fourier transform infrared (FTIR) and atomic force microscopy (AFM) analysis have been carried out to understand the effect of GZO films coating on PS. Further, the XRD analysis suggests the formation of a good crystalline quality of the GZO films on PS. From AFM investigation we observe that the surface roughness increases after GZO film coating. The photoluminescence (PL) measurements on PS and GZO films deposited PS shows three emission peaks at around 1.9 eV (red-band), 2.78 eV (blue-band) and 3.2 eV (UV-band). PL enhancement in the blue and ultraviolet (UV) region has been achieved after GZO films deposition, which might be originated from a contribution of the near-band-edge recombination from GZO.     

Physical and electronic properties of ZnO:Al/porous silicon

UV, violet and blue-green photoluminescence has been achieved at room temperature (RT) from ZnO:Al (AZO) films deposited by radio frequency (rf) co-sputtering. As the ZnO target power increases from 100 W, the violet luminescence vanishes and the blue and green-blue luminescences appear. The most intense UV and blue-green luminescence is obtained for the films deposited at higher sputtering powers depending upon the stoichiometry of the films as well as the crystalline quality. The as-prepared porous silicon (PS) emission band lies in the blue-green spectral region and is blue shifted due to the AZO deposition. The current–voltage characteristics of AZO/PS heterostructures have been studied. The ideality factor is found to be 19 and the series resistance as determined from the forward characteristics is 36 MΩ.     

Enhanced photoluminescence from porous silicon passivated with an ultrathin aluminum film

The photoluminescence (PL) of the porous silicon (PS) can be enhanced by coating it with an ultrathin aluminum (Al) film. The PL intensity of PS was found to increase up to ~ 67% by radio frequency (RF) sputter deposition of 5.2 nm Al film on PS. Fourier transform infrared (FTIR) spectroscopy analysis results suggest that the PL enhancement is related to change of Si–H and Si–O–Si bonds into Si–Al bonds as well as the increase in the carrier concentration participating in the radiative recombination under photoexcitation. On the other hand, the PL of the Al-passivated PS was found to be significantly deteriorated by postannealing owing to the thermal oxidation of the Al layer during annealing.     

Formation of nanosize poly(p-phenylene vinylene) in porous silicon substrate

We report the results of optical investigations in porous silicon (PS)/poly(p-phenylene vinylene) (PPV) systems obtained by filling the pores of silicon wafers with polymer.By scanning electron microscopy (SEM), IR, and Raman spectroscopy, we observed that the porous silicon layer was thoroughly filled by the polymer with no significant change in the structure of the materials. This suggests that there is no interaction between the components. On the other hand, the photoluminescence (PL) spectra of the devices investigated at different temperatures (from 11 to 290 K) showed that both materials are active at low temperatures. Porous silicon has a band located at 398 nm while PPV has two bands at 528 and 570 nm. As the temperature increases, the PL intensity of porous silicon decreases and that PPV is blue shifted. A new band emerging at 473 nm may indicate an energy transfer from the porous silicon to PPV, involving short segments of the polymer. The band of PPV located at 515 nm becomes more dominant and indicates that the nanosize polymer films are formed in the pores of the silicon layer, in agreement with the results obtained by SEM, IR, and Raman analyses.     

Blue luminescence from porous layers produced by metal-assisted chemical etching on low-doped silicon

Photoluminescent porous layers were formed on highly resistive p-type silicon by a metal-assisted chemical etching method using K₂Cr₂O₇ as an oxidizing agent. A thin layer of Ag is deposited on the (1 0 0) Si surface prior to immersion in a solution of HF and K₂Cr₂O₇. The morphology of the porous silicon (PS) layer formed by this method as a function of etching time was investigated by scanning electron microscopy (SEM). It shows that the surface is formed by macropores filled with microporous silicon. The porous layers were characterized by backscattering spectrometry (BS) as a function of etching time in random and channelling mode. Channelling spectra show that the porous layer remains crystalline after etching. On the other hand, random and channelling spectra show that the deposited silver diffuses into the pore. Luminescence from metal-assisted chemically etched layers was measured. It was found that the PL intensity increases with increasing etching time. This behaviour is attributed to increase of the density of the silicon nanostructure. Finally, the PL spectra show two peaks of emission at 450 and 600 nm.     

Visible luminescence in photo-electrochemically etched p-type porous silicon: Effect of illumination wavelength

The effect of low power density of ~ 5 μW/cm² monochromatic light of different wavelengths on the visible photoluminescence (PL) properties of photo-electrochemically formed p-type porous silicon (PS) has been investigated. Two-peak PL “red” and “green” is resolved in PS samples etched under blue-green wavelength illumination; 480, 533 and 580 nm. It is found that the weight of “green” PL has maxima for the sample illuminated with 533 nm wavelength. Whereas, PL spectra of PS prepared under the influence of red illumination or in dark does not exhibit “green” PL band, but shows considerable enhancement in the “red” PL peak intensity. Fourier transform infrared (FTIR) spectroscopic analysis reveals the relationship between the structures of chemical bonding in PS and the observed PL behavior. In particular, the PL efficiency is highly affected by the alteration of the relative content of hydride, oxide and hydroxyl species. Moreover, relative content of hydroxyl group with respect to oxide bonding is seen to have strong relationship to the blue PL. Although, the estimated energy gap value of PS samples shows a considerable enlargement with respect to that of bulk c-Si, the FTIR, low temperature PL and Raman measurements and analysis have inconsistency with quantum confinement of PS.     

The optical properties of porous silicon produced by metal-assisted anodic etching

Porous silicon (PS) was obtained from n-type (100) mono-crystalline silicon wafers with different metal using two different illumination conditions. The visible photoluminescence (PL) may come from defect-related radiative centers on PS surface and adsorbed hydrogen atoms may be associated to the elimination of irradiative centers on PS surface, which can be proved by the infrared absorption spectra. The metal can be used as catalytic role to increase the etching rate under back illumination, but under front illumination, the metal can cancel light-generated carrier leading to the decrease of etching rate during anodic etching. Furthermore, the change of minority carrier lifetime is opposite to the change of PL efficiency of PS, which can be Confirmed by the results of μ-PCD measurements.     

Effect of etch-treatment upon the intensity and peak position of photoluminescence spectra for anodic alumina films with ordered nanopore array

Porous anodic alumina membranes (AAMs) were prepared in oxalic acid and then carried on an etch-treatment in phosphoric acid. Using the etch-treatment the photoluminescence (PL) intensity of AAMs increases by a factor of 1/3. The effect of etch-treatment upon the intensity and peak position of photoluminescence (PL) spectra was investigated. It was found that the intensity of the photoluminescence (PL) spectra increased with the etching time increasing. A PL spectrum can be divided into two subbands with the peak at 434 and 460 nm, respectively. As the etching time prolongs, the intensity of the peak of 434 nm subband increases and that of the 460 nm subband rises firstly and then decreases. It can be explained by that two luminescence centers (F and F⁺ centers) coexist in AAMs. F centers are concentrated in the surface layer and F⁺ centers are enriched in the depth of pore wall. The increment of the PL intensity comes from the contribution of F⁺ photoluminescence centers concentrated in the depth of pore wall in AAMs. This work will be beneficial to improving the photoluminescence intensity and understanding the light-emitting mechanisms for related materials.     

Al-assisted Anodic Etched Porous Silicon

The photoluminescence (PL) properties and the surface morphologies of the porous silicon (PS) prepared from the wafers whose front sides were coated with or without Al film were studied. Furthermore, the Fourier Transforms Infrared (FTIR) and the Raman spectra were carried out. By introducing the Al film onto the front side of the wafer before the anodic etching, the surface morphology of the PS was quite different from that of conventional PS, which can be explained by the formation mechanism of the PS. The different PL properties of the PS may be attributed to the discrepancy in the structural configuration of the samples.     

Influence of SnO2 coating and annealing on the luminescence properties of porous silicon nanowires

Porous silicon (PS)-core/SnO₂-shell nanowires (NWs) were synthesized by a two step process: electrochemical anodization of silicon followed by atomic layer deposition of SnO₂. The photoluminescence spectrum of the PS nanowires showed a broad blue green emission band centered at approximately 510 nm. PL measurement also showed that the blue green emission was enhanced by SnO₂ coating and enhanced further by thermal annealing. It appeared that annealing in a reducing atmosphere was more efficient in increasing the blue green emission intensity than annealing in an oxidizing atmosphere. Energy-dispersive X-ray spectroscopy revealed that the enhancement in the blue green emission by annealing in a reducing atmosphere was attributed to the formation of Sn interstitials in the PS cores due to the dissociation of the SnO₂ shells followed by the diffusion of the Sn atoms, generated as a result of the dissociation of SnO₂, into the PS cores during the annealing process.     

Influence of Acetone Vapor on the Photoluminescence Behavior of Porous Silicon

The adsorption of acetone molecules at the surface of porous silicon is found to result in a blueshift of the photoluminescence (PL) maximum by 50–90 nm, depending on the time of exposure to acetone vapor (1 to 15.5 h). Under UV laser excitation, the PL intensity in the samples exposed to acetone vapor initially decreases and then rises, reaching a level which is substantially higher than that before the exposure to acetone vapor. The PL spectra and the time variations of the integrated PL intensity and intensity at a fixed wavelength are thoroughly studied for the samples exposed to acetone vapor for various lengths of time. Under UV irradiation, the original PL intensity is partially restored, and the PL maximum gradually returns to its original position. A model describing the spectral characteristics and transient behavior of PL in porous silicon is proposed.     

The synthesis and photoluminescence properties of selenium-treated porous silicon nanowire arrays

Here we prepared vertical and single crystalline porous silicon nanowire (SiNW) arrays using the silver-assisted electroless etching method. The selenization was carried out by annealing the samples in vacuum with selenium atmosphere. The selenization treatment at 700?°C is useful for investigating the photoluminescence (PL) properties of porous SiNWs, with an enhancement of 30 times observed. The observed PL peaks blue-shift to 650 nm and the decomposition of the spectrum reveals that three PL bands with different origins are obtained. It is proved that selenization treatment could remove the Si-H bonds on the surface and form Si-Se bonds, which could increase the absorbance of the SiNWs and also enhance the stability of the PL intensity. These Se-treated porous SiNWs may be useful as nanoscale optoelectronic devices.     

Luminescent Properties of Porous Si Passivated by Diamond Film and DLC Film

Surface passivation methods for porous Si (PS) surfaces, i.e., depositing diamond film or diamond-like carbon (DLC) film on PS surfaces, were attempted. Two emission bands, weak blue band and strong red band existed in the PL spectrum of diamond film coated on PS, were discovered by the photoluminescence measurements. The luminescent mechanism and stability were discussed. The results indicated that diamond film may stabilize the PL wavelength and intensity of PS, and therefore could become a promising passivation film of porous Si. The PL properties of PS coated by DLC films, including hydrogenated diamond like carbon (DLC:H) film and nitrogen doped DLC film (DLC:N) were also studied in this paper. The DLC films may stabilize the PL of PS, but the photoluminescent intensity was obviously weaker than that of diamond film coated PS.     

Low mechanical pressure during electrochemical etching: induced modification in optical and structural properties of n-type porous silicon

In this study, n-type porous silicon (PS) layers are formed in the dark with the assistance of a low mechanical pressure during electrochemical etching process. Pressure-induced stress/strain modifies the resistivity of the silicon substrate to enhance the etching process. Under the same equivalent etching condition, pressure-assisted etching can yield PS layer with stronger room temperature photoluminescence intensity than the layer formed by ordinary electrochemical etching. The porosity of pressure-assisted etched PS layers is found to be much higher than that of ordinary etched layer. Fourier transformation infrared absorption spectroscopy and grazing incidence X-ray diffraction measurements and analysis show that application of the pressure during electrochemical etching promotes the degree of oxidation and reduces the crystallites size of the PS layer. The effect of the pressure during etching process on the surface topography of PS is revealed by scanning electron microscopy imaging.     

Estimation of oxide related electron trap energy of porous silicon nanostructures

Estimation of electron trap energy (E_t), with respect to bulk Si valence band, of oxidized porous silicon (PS) nanostructures is reported. Photoluminescence (PL) spectra of oxidized PS prepared with different formation parameters have been investigated and the room temperature PL characteristics have been successfully explained on the basis of oxide related trap assisted transitions. PL peak energy for the oxidized samples with low porosity exhibited a blue shift with increasing formation current density (J). For the high porosity samples double peaks appeared in the PL spectra. One of these peaks remained constant at ∼730 nm while the other was blue shifted with increase in J. Evolution of PS nanostructure was correlated to the formation parameters using a simple growth mechanism. PS nanostructure was modelled as an array of regular hexagonal pores and the average value of E_t was estimated to be 1.67 eV.     

Photoluminescence properties of porous silicon nanocomposites

Different porous silicon (PS) layers were impregnated with rhodamine 6G (Rh) solution in order to form Rh/PS nanocomposites. The effect of the porous matrix (fresh, oxidised, type p⁺) on the propriety of photoluminescence (PL) has been investigated. It was found that the luminescence of this nanocomposite is provided by an energy transfer from PS nanocristallites to rhodamine and from interaction of dye molecules with the chemical species on the Si surface. An antistokes PL has been observed using a He–Ne laser excitation. The dependence of its intensity via power excitation suggests a process of two-step two-photon absorption as origin of this PL. Nanocomposite formed by PS and ZnO has been also investigated. We show that this transparent oxide does not degrade the skeleton of PS and does not inhibit PL. The PL band shifts to high energy and the intensity becomes stable providing the passivation of the dangling bonds on the Si-surface by ZnO.