Temperature effect on porous silicon luminescence

A theoretical surface-state model of porousilicon luminescence is proposed. The temperature effect on the PhotoLuminescence (PL) spectrum for pillar and spherical structures is considered, and it is found that the effect is dependent on the doping concentration, the excitation strength, and the shape and dimensions of the Si microstructure. The doping concentration has an effect on the PL intensity at high temperatures and the excitation strength has an effect on the PL intensity at low temperatures. The variations of the PL intensity with temperature are different for the pillar and spherical structures. At low temperatures the PL intensity increases in the pillar structure, while in the spherical structure the PL intensity decreases as the temperature increases, at high temperatures the PL intensities have a maximum for both models. The temperature, at which the PL intensity reaches its maximum, depends on the doping concentration. The PL spectrum has a broader peak structure in the spherical structure than in the pillar structure. The theoretical results are in agreement with experimental results.     

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.     

Morphological changes in porous silicon nanostructures: non-conventional photoluminescence shifts and correlation with optical absorption

In this paper, we show that the photoluminescence (PL) shifts of p-type porous silicon (PS) are mainly attributed to some morphological changes related to anodisation conditions. We discuss how differences in the stirring and nature of the electrolytic solution can lead to morphological changes of the PS layers. It has been found that when PS is formed in pure aqueous HF solution, it can exhibit a non-conventional and reproducible “porosity – PL peak relationship”. By correlating the PL spectral behaviour and PS morphology throughout a quantum-confinement model, we explain both conventional and non-conventional PL shifts. Correlation of PL and optical absorption (OA) shows that the PL peak energy and the optical absorption edge of PS exhibit the same trend with size effect. The spectral behaviour of OA with regard to that of PL is well analysed within the quantum-confinement model throughout the sizes and shapes of the nanocrystallites forming PS. The value of the effective band gap energy determined from the calculated lowest PL energy almost corresponds to that estimated from the optical absorption coefficient. These results suggest that the lowest radiative transition between the valence band and the conduction band corresponds to the largest luminescent wires, and that the radiative recombination process leading to the PL emission occurs in the c-Si crystallite core.     

蓝光发射多孔硅RTO过程中的尺寸分离效应

对用水热腐蚀技术制备的、具有蓝光发射的多孔硅样品在快速热氧化(RTO)处理前后其光致发光谱、硅纳米颗粒的大小及尺寸分布变化进行了研究.实验发现,新鲜多孔硅样品经过RTO处理后,其光致发光谱整体蓝移并由单发光峰分裂为两个发光峰;与此对应,样品中的硅纳米颗粒在整体减小的同时出现尺寸分离现象.这一结果表明,多孔硅中的短波长发射也具有强烈的尺寸相关性. 关键词：     

Excitation mechanisms and localization sites of erbium-doped porous silicon

Porous silicon (PS) is doped with erbium by electrochemical anodisation. The penetration of erbium into the PS layer is confirmed by Rutherford backscattering spectroscopy (RBS) and energy dispersive X-ray (EDX) measurements. Efficient green and infrared emissions were observed at room temperature. The investigations are focused on the evolutions versus temperature and pump intensity of the green photoluminescence (PL) corresponding to the ⁴S_3/2 → ⁴I_15/2 transition. It was found that an erbium related level defect can be involved on the excitation and emission processes of erbium. Pump intensity dependent PL studies revealed that for the electrochemical incorporation, most of the Er³⁺ ions are localized inside the Si nanocrystallites and not in stoichiometric SiO₂. The optical cross-section is close to that of erbium in Si nanocrystallites.     

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.     

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.     

Study of steady state and time resolved photoluminescence of thiol capped CdS nanocrystalline powders dispersed in N,N-dimethylformamide

The microwave (MW) assisted synthesis of thiol capped cadmium sulfide (CdS) nanocrystallites/quantum dots (QDs) was performed through the reaction of cadmium acetate with thiourea in N,N-dimethylformamide (DMF) by keeping the MW irradiation time fixed (40 s) in the presence of a thiol containing capping agent. Three capping agents, namely, benzyl mercaptan (BM), 1-butanethiol (BT) and 2-mercaptoethanol (ME) were used. The concentration of the precursors was varied to check the change in the average size of the thiol capped CdS nanocrystals formed. The nanocrystallites were characterized by usual procedure. The UV-vis absorption spectra and the photoluminescence (PL) spectra of the CdS nanocrystalline powders dispersed in DMF were studied. It was observed that with increase in concentration of the capping agent (BM), there is a shift in the nature of emission (PL) from trap associated PL to the band edge luminescence in the case of BM capped CdS nanocrystalline powders dispersed in DMF possibly due to better surface passivation. The relative PL quantum yield of the thiol capped CdS nanocrystalline powders dispersed in DMF was calculated under various experimental conditions. Time-correlated single-photon counting experiments were performed to study the time-resolved photoluminescence of the CdS nanocrystalline powders dispersed in DMF. The observed emission decay profiles have been simulated using the multiexponential model. The emission decay profiles for thiol capped CdS nanocrystalline powders dispersed in DMF depend on the nature of the capping agents (thiols) used to passivate the CdS nanocrystallites. The time resolved PL studies show that the average values of PL lifetime are related to the size and size distribution of the prepared thiol capped CdS nanocrystallites.     

Influence of mesoporous substrate morphology on the structural, optical and electrical properties of RF sputtered ZnO layer deposited over porous silicon nanostructure

Morphological, optical and transport characteristics of the RF sputtered zinc oxide (ZnO) thin films over the mesoporous silicon (PS) substrates have been studied. Effect of substrate porosity on the grain growth and transport properties of ZnO has been analyzed. Physical and optical properties of ZnO-PS structures were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy. Our experimental results indicate that on changing porosity of the PS substrates, regularity of the spatial distribution of the ZnO nanocrystallites can be controlled. While the morphology and grain size of ZnO depended strongly on the morphology and pore size of the PS substrates, the rectifying factors of the metal semiconductor junction were found to be different by a factor of 3. The deposition of semiconducting oxides on such mesoporous substrates/templates offers the possibility to control their properties and amplify their sensing response.     

Field-induced functions of porous Si as a confined system

Electroluminescent porous Si (PS) diodes exhibit various useful functions under a high-electric field. The experimental PS diodes are composed of thin semitransparent metal films, PS layers (about 500 nm thick in minimum), p- or n-type Si substrates and ohmic back contacts. Definite nonlinear electrical behavior (negative resistance and nonvolatile bistable memory effects) and cold electron emission phenomena appear in these PS diodes associated with the EL emission. Both the negative resistance and memory effects are related to the charging of Si nanocrystallites by field-induced carrier injection. The electron emission observed in the PS diodes formed on n+–Si substrates is caused by hot electrons tunnelling through the top contact. By an appropriate structural control of PS, the effective drift length under a high-field conduction is significantly increased, and then electrons are emitted ballistically. These functions reflect the activity of PS as a nanocrystalline confined system.     

Patterning and photoluminescence of CdS nanocrystallites on silk fibroin fiber

CdS nanocrystallites could be formed and assembled into nanoparticle strings and hexagons on natural silk fibroin fiber (SFF) through a room-temperature bio-inspired process. Herein, the biomaterial SFF served as reactive substrate, not only provides the in situ formation sites for CdS nanocrystallites, but also directs the arrangement of nanocrystalline CdS simultaneously. The photoluminescence (PL) of the resulting nanocomposites CdS/SFF is investigated extensively. The PL peaks observed from CdS nanoparticle strings are similar to those of separate CdS nanoparticles, corresponding to the band-edge emission of their individual building blocks (QD-CdS). Moreover, CdS nanoparticle hexagons perform a red-shifted and broadened emission peak.     

Photoluminescence activity of Yang and Secco etched multicrystalline silicon material

Ultraviolet and blue-green photoluminescence (PL) was investigated on multicrystalline silicon (mc-Si) samples chemically etched by Secco and Yang solutions. The samples were characterized by dislocation density (10⁵-10⁶ cm⁻²). The form of etched pits is triangular with Yang etch and like a honeycomb with Secco etch as observed with a scanning electron microscope (SEM). These textures of mc-Si wafers give a PL activity similar to that obtained with nanostructures of porous silicon (PS) as reported in the literature. The ultraviolet PL spectra observed with Yang etch shift to the blue-green spectrum range when applying Secco etch. In our experiments we have observed 3-5 μm diameter macro pores separated by a high density of nanowalls. These observations suggest that the origin of the PL activity are quantum dots resulting from the silicon nanocrystallites obtained after few minutes of chemical etching.     

多孔硅在30~180℃温区光致发光谱的研究

对长期存放在空气中的多孔硅样品和即时制备的多孔硅样品分别在室温~180℃下进行了光致发光（PL）温度效应的研究.两类样品的PL呈现不同的变化规律.前者的PL还表现为双峰,且长波PL峰随温度升高蓝移;后者的PL表现为单峰,且PL峰位随温度升高红移.基于量子限制效应对后者进行了解释;而前者难以用分立的量子限制和表面发光中心模型来解释,实验结果表明两种机制之间可能有较复杂的耦合效应发生.     

The mechanism of emission in the red photoluminescence band of porous silicon

Porous silicon (PS) exhibits several photoluminescence (PL) bands, whose spectral position and intensity depend strongly on the actual conditions of preparation of PS, its treatment, and subsequent use. The PS PL band peaking at about 1.8 eV and usually assigned to the intrinsic emission of silicon nanocrystals was studied. It was shown that the temperature-induced variation of the PL kinetics in the 80 to 300-K interval follows a complex pattern and depends noticeably on the actual point on the band profile. The temperature behavior of PL decay in the 1.8-eV band is determined by the electron-hole recombination rate within a nanocrystal and the cascade carrier transitions from small to large nanocrystals, with an attendant decrease in energy.     

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.     

High sensitivity of palladium on porous silicon MSM photodetector

In this work, the nanocrystalline porous silicon (PS) is prepared through the simple electrochemical etching of n-type Si (1 0 0) under the illumination of a 100 W incandescent white light. SEM, AFM, Raman and PL have been used to characterize the morphological and optical properties of the PS. SEM shows uniformed circular pores with estimated sizes, which range between 100 and 500 nm. AFM shows an increase in its surface roughness (about 6 times compared to c-Si). Raman spectra of the PS show a stronger peak with FWHM=4.3 cm⁻¹ and slight blueshift of 0.5 cm⁻¹ compared to Si. The room temperature photoluminescence (PL) peak corresponding to red emission is observed at 639.5 nm, which is due to the nano-scaled size of silicon through the quantum confinement effect. The size of the Si nanostructures is estimated to be around 7.8 nm from a quantized state effective mass theory. Thermally untreated palladium (Pd) finger contact was deposited on the PS to form MSM photodetector. Pd/PS MSM photodetector shows lower dark (two orders of magnitude) and higher photocurrent compared to a conventional Si device. Interestingly, Pd/PS MSM photodetector exhibits 158 times higher gain compared to the conventional Si device at 2.5 V.     

Effect of preliminary oxidation annealing on properties of porous silicon impregnated with a tungsten-tellurite glass activated by Er and Yb

The effect of preliminary oxidation annealing of porous silicon (PS) on photoluminescence (PL) under laser pumping at wavelengths of 532 and 980 nm, EPR, and transverse current transport in structures based on PS with a fused tungsten-tellurium glass (TTG) doped with Er and Yb has been studied. It has been shown that such annealing and the presence of silicon nanocrystals (nc-Si) in PS promote multiple PL enhancement for both Er ions in TTG and nc-Si in PS at wavelengths of 750 and 1540 nm, respectively. As TTG is fused into PS, P _b -centers of nonradiative recombination are suppressed, while retaining discrete electron tunneling through nc-Si grains in PS.     

Relaxing local modes and the theory of low-frequency Raman scattering in glasses

Raman scattering in glasses is investigated theoretically. The experimental Raman spectra of glasses exhibit a low-frequency peak (at ～10 cm^?1) that, as a rule, is attributed to vibrational modes of nanometer-sized structural units (nanocrystallites). It is established that the elastic moduli of nanocrystallites must necessarily be dependent on their sizes due to the Laplace pressure effect. A theory of the low-frequency peak is constructed using a realistic size distribution function of nanocrystallites with allowance made for the Laplace pressure effect and the dissipation of vibrational energy. Within this theory, the shape of the low-frequency peak and its evolution with temperature can be analyzed quantitatively. The proposed approach offers a physical interpretation of the experimental data and provides insight into the relation of the characteristic nanocrystallite sizes to the elastic moduli and surface tension coefficients of materials.     

Excitonic photoluminescence properties of nanocrystalline GaSb and Ga0.62In0.38Sb embedded in silica films

The GaSb and Ga_0.62In_0.38Sb nanocrystals were embedded in the SiO₂ films by radio-frequency magnetron co-sputtering and were grown on GaSb and Si substrates at different temperatures. We present results on the 10 K excitonic photoluminescence (PL) properties of nanocrystalline GaSb and Ga_0.62In_0.38Sb as a function of their size. The measurements show that the PL of the GaSb and Ga_0.62In_0.38Sb nanocrystallites follows the quantum confinement model very closely. By using deconvolution of PL spectra, origins of structures in PL were identified.     

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.