Raman scattering and photoluminescence study of porous silicon formed on n-type silicon

We report Raman scattering and photoluminescence studies on porous silicon film formed on n-type silicon. The Raman spectra over the sample surface exhibit considerable variation whereas the photoluminescence spectra are practically identical. Our results indicate that, well inside the film surface, it consists of spherical nanocrystals of typical diameter ≈ 100Å, while on the edge these nanocrystals are ? 300Å. We further observe that there is no correlation between the photoluminescence peak position and the nanocrystal diameter. This suggests that the origin of the photoluminescence is due to radiative recombination between defect states in the bulk as well as on the surface of the nanocrystal.     

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.     

Mn离子注入Mg掺杂GaN的微结构和光学特性研究

通过Mn离子注入Mg掺杂GaN外延层制备了铁磁性GaN∶Mn薄膜,利用拉曼散射和光致发光谱研究了退火温度对薄膜微结构和光学特性的影响。拉曼谱测试显示由离子注入相关缺陷引起了新的声子模,分析认为Mn离子相关的局域振动(LVM)紧邻Ehigh2峰。光致发光谱观察到位于1.69,2.54和2.96eV的3个新的发光峰,分析认为2.96eV的发光峰来自MgGa-VN复合体深施主能级和Mg的浅受主能级之间的辐射复合跃迁,2.54eV的发光峰来自浅施主能级和深受主能级之间的辐射复合跃迁,对于位于1.69eV的新发光峰不排除来自MgGa-VN复合体深施主能级和Mn相关深受主能级之间辐射复合跃迁的贡献。     

Optical properties of the low-energy Ge-implanted and annealed SiO2 films

Ultraviolet–visible light emissions from nanocrystalline (nc) Ge-embedded SiO₂ films fabricated by ion implantation and rapid annealing techniques are studied as a function of different fabricating conditions (implanting dose and annealing temperature). The samples exhibit seven photoluminescence (PL) peaks peaked at 1.68, 1.75, 1.84, 1.93, 2.00, 2.70, and 3.10 eV. There are also two excitation bands in the PL excitation (PLE) spectra peaked at 4.90 and 5.19 eV. Raman spectra are employed to observe and understand structural variations in SiO₂ matrix during the formation of nc-Ge and defects. Within the frame of the quantum confinement (QC) theory and Oswald ripening growth model, the origin and evolution of the seven PL peaks and two PLE bands are identified clearly demonstrated by the proposed energy-level and generated process diagrams. Our results indicate that both the implanting dose and annealing temperature play a dominant role in modulating the optical properties of the nc-Ge-embedded SiO₂ films.     

Correlation of structural and optoelectronic properties of thin film silicon prepared at the transition from microcrystalline to amorphous growth

Photovoltaic properties of 4 µm thick microcrystalline silicon p–i–n solar cells have been studied, over a range of crystallinity determined using Raman spectroscopy. Low-crystallinity material (below 10%) appears to absorb disproportionately strongly in the infrared, possibly due to increased light scattering or to relaxation of the crystal momentum selection rule. A minimum in solar cell efficiency is observed under AM1.5 illumination when V_OC ≈ 580 mV, with blue response most strongly affected. This is consistent with a reduction in electron mobility to a value below that of amorphous silicon for low-crystallinity material, in agreement with time-of-flight measurements.     

Enhanced photo luminescence from porous silicon on texturized surface

Porous silicon (PS) was formed on both polished and texturized single crystal silicon (100) by anodic etching. Photoluminescences (PL) from both of these silicon surfaces were measured and compared. A two-fold enhancement of PL from textured silicon surface was obtained. This enhancement could be ascribed to the geometry of the textured surface.     

Superhydrophilicity and photocatalytic property of nanocrystalline titania sol-gel films

Nanocrystalline titania thin films were prepared by deposition and annealing of titania sol-gel precursor materials, both non-hydrated solutions and hydrosols. The sol-gels were immobilized on glass substrates applying doctor blade and dip-coating deposition techniques. The nanoparticle size, determined by the full-width at half-maximum of the X-ray diffraction peaks as well as by shifting and broadening of the Raman modes, strongly depends on the precursor solution. Significant differences were also confirmed on the surface characteristics, roughness and complexity of the films by atomic force microscopy. The films present high activity towards photodecomposition of methyl orange azo-dye together with enhanced photo-induced superhydrophilicity under illumination with ultraviolet light. Judicious choice of the precursor material and efficient control of the deposition parameters may lead to the optimization of their photocatalytic and/or superhydrophilic properties.     

纳晶硅氧化层中的陷阱态(英文)

由Si-H键钝化的多孔硅的光致荧光(PL)发光频移遵循量子受限效应,随着纳米结构尺寸的变小PL发光频率从红外蓝移到紫外。多孔硅被氧化后,PL发光带的中心波长被钉扎在700nm~750nm范围,且强度明显增加。计算表明,氧化后的Si=O键或Si—O—Si键能在展宽的导带下方形成电子陷阱态。由此提出量子受限与氧化陷阱态模型可以很好地解释PL发光的钉扎和增强效应。该模型中的电子陷阱态扮演了重要的角色。     

光电化学刻蚀n型多孔硅过程中的介质增强荧光效应

光电化学阳极刻蚀n型单晶硅过程中,在电解液中加入适量钛溶胶,制得多孔硅（PS）,经荧光光谱（PL）检测,发光强度明显增强;傅里叶红外光谱（FT-IR）检测发现,在多孔硅结构中出现了亚甲基的伸缩振动吸收;用扫描电镜（SEM）和X射线光电子能谱（XPS）表征手段对其进行表征,发现表面的裂缝区域明显加大。在Fe（CN）63＋/Fe（CN）64-溶液中的测量了复合电极的光电流-电压关系,结果显示,用添加钛凝胶制备的多孔硅电极具有更好的光电流特性。     

多孔硅的高效可见光发射来自其表面

用扫描电镜继续对发光多孔硅的阴极射线发光进行了系统的研究,得到了其阴极荧光发射部位、强度分布显微照片,发现在样品的表面层脱落处（暴露着多孔层大量微孔）无阴极射线发光产生,只有表面层未脱落处才有阴极射线发光;对样品的截面实验研究同样清楚地表明多孔硅样品的阴极射线发光只来源于其表面层,多孔层、硅单晶衬底区域不发生阴极射线发光．实验还提供了阴极射线发光强度在截面上随深度变化情况显微照片．阴极射线发光光谱表明其光谱峰值位置在680nm处,相似于多孔硅的光致发光．实验结果再次表明多孔硅的可见光来源于其表面层中的荧光物质．     

Optical properties of plasma-assisted molecular beam epitaxy grown InN/sapphire

The optical properties of as-grown InN/sapphire films prepared by plasma assisted molecular beam epitaxy (PA-MBE) are characterized by photoluminescence (PL), Raman scattering (RS) and infrared (IR) reflectance techniques. The PL measurements have consistently exhibited lower values of InN band gaps providing clear indications of electron concentration dependent peak energy shifts and widths. The phonon modes identified by RS are found to be in good agreement with the grazing inelastic X-ray scattering measurements and ab initio lattice dynamical calculations. An effective medium theory used to analyze IR reflectance spectra of InN/sapphire films has provided reasonable estimates of free charge carrier concentrations.     

Variation of microstructure and transport properties with filament temperature of HWCVD prepared silicon thin films

Thin films of hydrogenated silicon are prepared by varying the filament temperature (T_F) (1600-1900 °C) at a deposition rate of 8-12 Å/s without using any hydrogen dilution. While the films deposited at low T_F are amorphous in nature, those deposited at higher T_F (≥ 1800 °C) contain nanocrystallites embedded in the amorphous network. The optical band gap (E₀₄) of the films (~ 1.89-1.99 eV) is slightly higher compared to the regular films, which is attributed to the improved short and medium range order as well as the presence of low density amorphous tissues in the grain boundary regions. The films show improved stability under long term light exposure due to more ordered structure and presence of hydrogen mostly as strong Si-H bonds.     

Thin film deposition on plastic substrates using silicon nanoparticles and laser nanoforming

Reduced melting temperature of nanoparticles is utilized to deposit thin polycrystalline silicon (c-Si) films on plastic substrates by using a laser beam without damaging the substrate. An aqueous dispersion of 5 nm silicon nanoparticles was used as precursor. A Nd:YAG (1064 nm wavelength) laser operating in continuous wave (CW) mode was used for thin film formation. Polycrystalline Si films were deposited on flexible as well as rigid plastic substrates in both air and argon ambients. The films were analyzed by optical microscopy for film formation, scanning electron microscopy (SEM) for microstructural features, energy dispersive spectroscopy (EDS) for impurities, X-ray photoelectron spectroscopy (XPS) for composition and bond information of the recrystallized film and Raman spectroscopy for estimating shift from amorphous to more crystalline phase. Raman spectroscopy showed a shift from amorphous to more crystalline phases with increasing both the laser power and irradiation time during laser recrystallization step.     

Origin of visible photoluminescence from porous silicon as studied by Raman spectroscopy

In this paper we discuss the different models proposed to explain the visible luminescence in porous silicon (PS). We review our recent photoluminescence and Raman studies on PS as a function of different preparation conditions and isochronal thermal annealing. Our results can be explained by a hybrid model which incorporates both nanostructures for quantum confinement and silicon complexes (such as SiH _x and siloxene) and defects at Si/SiO₂ interfaces as luminescent centres.     

DC conductivity of amorphous-nanocrystalline silicon thin films

The direct current (DC) conductivity of amorphous-nanocrystalline Si films deposited by the plasma enhanced chemical vapour deposition method was studied as a function of the structural properties obtained by Raman spectroscopy and grazing incidence small angle X-ray scattering (GISAXS).The crystalline fraction estimated from the Raman spectra altered between 0 and 60% while the average size of the crystals varied from 2 to 7 nm, however, the size distribution was wide i.e. smaller and larger crystals were also present.GISAXS showed a signal that corresponds to “particles” with values for the gyration radius close to the average crystal sizes, between 2 and 6 nm. Samples with higher crystalline fraction had elongated “particles” that are larger when situated closer to the sample surface, which indicates a columnar structure.The DC conductivity had a nearly constant, low value up to some 30% of crystal fraction. A further increase of the crystal fraction resulted in an abrupt increase of the conductivity in a narrow interval of crystal fraction. Above this interval, conductivity was much higher and remained constant in that range. This result is in perfect agreement with the percolation threshold obtained by model calculation for a six-fold coordinated cubic lattice that appears at 32% of crystal fraction. A certain scattering of the experimental data around the predicted values was discussed as a possible consequence of the variation of the individual crystal size and shape by change of the crystal fraction and/or non-uniformity of depth distribution.     

Optical properties of single-walled carbon nanotubes functionalized with copolymer poly(3,4-ethylenedioxythiophene-co-pyrene)

Optical properties are reported for composites based on single-walled carbon nanotubes (SWNTs) and copolymer poly(3,4-ethylenedioxythiophene-co-pyrene) (PEDOT-Py) prepared by chemical polymerization of two monomers in the presence of carbon nanotubes. A charge transfer between SWNTs and the PEDOT-Py copolymer was demonstrated by Raman scattering. The increase in the relative intensity of the Raman lines peaked at 440–577 cm⁻¹, which were assigned to the ethylenedioxy ring vibrational modes, indicated a significant hindrance steric in the case of the composites based on the PEDOT-Py copolymer and metallic SWNTs. The increase in the absorbance of IR band peaked at 984 cm⁻¹ occurred simultaneously with the disappearance of the IR band at 1639 cm⁻¹. This finding was a consequence of the formation of new covalent bonds between SWNTs and the thiophene and benzene rings of the repeating units of the PEDOT-Py copolymer. The photoluminescence (PL) quenching process of the PEDOT-Py copolymer was induced by semiconducting SWNTs. The PL quenching of PEDOT-Py copolymer in the presence of SWNTs was demonstrated based on the energy level diagrams of the two constituents of the PEDOT-Py/SWNTs composite material.     

Mn掺杂ZnO纳米线的拉曼散射和光致发光特性

研究了不同Mn掺杂含量的ZnO纳米线在室温条件下的拉曼散射和光致发光性能,发现Mn掺杂入ZnO后引入了部分应力,在其拉曼光谱中表现出拉曼峰的位置发生偏移,Mn的掺杂含量越高,峰偏移得越明显.Mn的掺杂对ZnO纳米线的发光性能也有影响,尽管掺杂后仍保持有较为明显的紫外发光峰,但是,随着Mn含量的增加,紫外发光峰的强度降低,并且半峰宽逐渐增大.此外,Mn的掺杂明显地改变了ZnO紫外发光峰的位置.     

Comparative study of electro-physical properties of heterostructures containing PECVD nanocrystalline and anodic porous silicon layers

We performed a comparative study of the electro-physical properties of heterostructures containing PECVD nanocrystalline silicon (nc-Si) and electro-chemically etched porous silicon (PS) layers over a wide range of thicknesses, in terms of their energy parameters. Based on the proposed analytical expressions and the experimental current-voltage and capacitance-voltage characteristics, we studied the characteristics of the surface states at the nc-Si(or PS) interfaces in Pd-nc-Si(or PS)-p-Si heterostructures. The results revealed that the surface states play an essential role in the carrier transport in both types of heterostructures that were investigated.     

制备方法对锌掺杂多孔硅蓝光发射强度及稳定性的影响

分别采用浸渍法和电镀法对多孔硅薄膜进行了锌掺杂.用扫描探针显微镜研究了多孔硅掺杂前后的表面形貌,用荧光分光光度计分析了样品的光致发光特性,发现锌掺杂增强了多孔硅的蓝光发射,且在420nm附近出现了一个小峰,样品放置一个月后,发光强度和峰位变化很小.红外吸收谱表明锌掺杂后,Si-O-Si键、Si2O-SiH键、H2Si-O2键的振动增强,且引入了Zn-O键.锌掺杂多孔硅发射蓝光是由于掺杂后多孔硅无定形程度增大,应力增大,表面进一步被氧化,使纳米硅粒中激发的电子-空穴对在SiOx层中或纳米硅粒与SiOx层界面的发光中心复合发光造成的,420nm处的发光峰是由锌填隙引起浅施主能级上的电子到价带跃迁造成的,同时分析了电镀法掺杂锌的优越性.     

Au/Si-NPA复合纳米体系的SERS增强能力研究

利用具有准周期结构的硅纳米孔柱阵列(silicon nanoprous pillar array,Si-NPA)为衬底使用浸渍法制备优化Au/Si-NPA活性基底。并利用最优化制备的Au/Si-NPA活性基底对罗丹明6G(Rhoda-mine 6G,R6G)进行探测,研究其表面增强拉曼散射(surface enhanced Raman scattering,SERS)光谱并对其增强原理进行解释。