Investigation of electrical and optical measurements of silicon nanocrystals embedded in SiO2 matrix

Samples containing silicon nanocrystals (Si-nc) with radius of approximately 3 nm embedded in SiO₂ exhibit a strong PL emission around 750 nm because of the quantum confinement of the exciton inside the Si-nc. To characterize the contribution of absorption by Si-nc to the photo-electric efficiency of silicon, metal–oxide–semiconductor structures were fabricated, without and with Si-nc embedded in silicon oxide and electrically characterized in the dark and under illumination. Samples containing Si-nc show a relatively large photocurrent compared to samples without Si-nc. Electron trapping and de-trapping in Si-nc was also observed by capacitance–frequency measurements showing a decrease of photo-generated electrons under illumination at high frequency. The efficiency of the photo-generated electrons from Si-nc was evaluated and shows a high EQE below 650 nm suggesting that Si-nc could be an interesting candidate for an the third generation photovoltaic solar cells.     

Near-infrared photoluminescence in germanium oxide enclosed germanium nano- and micro-crystals

We have studied the near-infrared photoluminescence properties of free-standing germanium nano-crystals (20?nm on average) and micro-crystals (60?μm on average) at 80-300?K. Two peaks were observed at ～1.0 and ～1.4?eV from both the nano-?and micro-crystals. The integrated PL (I(PL)) intensity of the nano-crystals is about an order of magnitude stronger than that of the micro-crystals and the I(PL) is also enhanced by ageing in air for both crystals. The ～1.0?eV peak position does not change with either the crystal size or temperature. We suggest that the deep traps located at the interfacial region between the surface GeO(2) layer and the bulk crystal Ge is responsible for the near-infrared PL.     

Sub-50 nm positioning of organic compounds onto silicon oxide patterns fabricated by local oxidation nanolithography

We present a process to fabricate molecule-based nanostructures by merging a bottom-up interaction and a top-down nanolithography. Direct nanoscale positioning arises from the attractive electrostatic interactions between the molecules and silicon dioxide nanopatterns. Local oxidation nanolithography is used to fabricate silicon oxide domains with variable gap separations ranging from 40?nm to several microns in length. We demonstrate that an ionic tetrathiafulvalene (TTF) semiconductor can be directed from a macroscopic liquid solution (1?μM) and selectively deposited onto predefined nanoscale regions of a 1?cm(2) silicon chip with?an accuracy of 40?nm.     

Visible electroluminescence from silicon nanoclusters embedded in chlorinated silicon nitride thin films

Visible electroluminescence (EL) has been obtained from devices with active layers of silicon nanocrystals embedded in chlorinated silicon nitride (Si-nc/SiN_x:Cl) thin films, deposited by remote plasma enhanced chemical vapour deposition, using SiCl₄/NH₃/H₂/Ar. The active nc-Si/SiN_x:Cl film was sandwiched between Al contacts and a transparent conductive contact of ZnO_x:Al deposited by the pyrosol process. White EL centred at around 600 nm was observed, with a turn-on voltage of 5 V, and the intensity increasing as a function of voltage. Recombination between electron-hole pairs generated in the Si-nc by electron impact ionization is proposed as the EL mechanism.     

Large-area 2D periodic crystalline silicon nanodome arrays on nanoimprinted glass exhibiting photonic band structure effects

Two-dimensional silicon nanodome arrays are prepared on large areas up to 50 cm2 exhibiting photonic band structure effects in the near-infrared and visible wavelength region by downscaling a recently developed fabrication method based on nanoimprint-patterned glass, high-rate electron-beam evaporation of silicon, self-organized solid phase crystallization and wet-chemical etching. The silicon nanodomes, arranged in square lattice geometry with 300 nm lattice constant, are optically characterized by angular resolved reflection measurements, allowing the partial determination of the photonic band structure. This experimentally determined band structure agrees well with the outcome of three-dimensional optical finite-element simulations. A 16% photonic bandgap is predicted for an optimized geometry of the silicon nanodome arrays. By variation of the duration of the selective etching step, the geometry as well as the optical properties of the periodic silicon nanodome arrays can be controlled systematically.     

Polarization strategies to improve the emission of Si-based light sources emitting at 1.55 μm

We present a electroluminescence (EL) study of the Si-rich silicon oxide (SRSO) LEDs with and without Er³⁺ ions under different polarization schemes: direct current (DC) and pulsed voltage (PV). The power efficiency of the devices and their main optical limitations are presented. We show that under PV polarization scheme, the devices achieve one order of magnitude superior performance in comparison with DC. Time-resolved measurements have shown that this enhancement is met only for active layers in which annealing temperature is high enough (>1000 °C) for silicon nanocrystal (Si-nc) formation. Modeling of the system with rate equations has been done and excitation cross-sections for both Si-nc and Er³⁺ ions have been extracted.     

Optical studies of electrodeposited ZnCuTe ternary nanowire arrays

The optical properties of electrodeposited zinc copper telluride (ZnCuTe) ternary nanowires on ITO substrate using polycarbonate membrane (Whatman) of diameter 200,100 and 50?nm have been studied and reported in this paper. Scanning electron microscopy confirmed the formation of the standing nanowires having uniform diameter equal to the diameter of the template used. UV–vis absorption and photoluminescence (PL) spectroscopy were used for optical studies. The optical band gaps of 200, 100 and 50?nm have been calculated as 3.19, 3.39 and 3.57?eV, respectively using UV–vis spectroscopy. The UV–visible absorption spectrometry reveals the absorption spectra of 200, 100 and 50?nm shows a blue shift. UV–visible absorption depicts that the band gap increases with decrease in the diameter size of the nanowires. Several broad emission lines have been observed over a wide wavelength range (390–690?nm) of visible light spectrum in the PL spectra of ZnCuTe nanowires of diameter 200, 100 and 50?nm. A good emission peak at around 615?nm has been observed in all nanowires.     

Evidence of localized amorphous silicon clustering from Raman depth-probing of silicon nanocrystals in fused silica

Silicon nanocrystals (Si-nc) and amorphous silicon (α-Si) produced by silicon implantation in fused silica have been studied by micro-Raman spectroscopy. Information regarding the Raman signature of the α-Si phonon excitation was extracted from Raman depth-probing measurements using the phenomenological phonon confinement model. The spectral deconvolution of the Raman measurements recorded at different laser focusing depths takes into account both the Si-nc size variation and the Si-nc spatial distribution within the sample. The phonon peak associated with α-Si around 470?cm(-1) is greatest for in-sample laser focusing, indicating that the formation of amorphous silicon is more important in the region containing a high concentration of silicon excess, where large Si-nc are located. As also observed for Si-nc systems prepared by SiO(x) layer deposition, this result demonstrates the presence of α-Si in high excess Si implanted Si-nc systems.     

锗纳米镶嵌薄膜的电致发光及其机制

采和射频磁控溅射技术,在Ge纳米镍嵌薄膜的基础上制备出电致发光器件。器件的结构为半透明Au膜／Ge纳米镍嵌薄膜／p-Si基片。当正向邻居坟大于6V时,用肉眼可以观察到可见的电致发光,但在反向偏压下探测不到光发射。所测电致发光谱中只有一个发光峰,峰位在510nm（2．4eV,绿光）,并且随着正向偏坟的升高,峰位不发生移动;对于不同温度退火的样品,峰位也保持不变。根据分析结果讨论了可能的电致发光机制。     

CdSe胶质量子点的电致发光特性研究

采用胶体化学法合成硒化镉(CdSe)胶质量子点, 在此基础上制成了以CdSe胶质量子点为有源层, 结构为ITO/ZnS/CdSe/ZnS/Al的电致发光(EL)器件. 透射电镜测量表明量子点的尺寸为4.3 nm, 扫描电子显微镜测量ZnS薄膜和Al薄膜结果显示表面均较为平整, 由器件结构的X射线衍射分析观察到了CdSe(111)、ZnS(111)等晶面的衍射, 表明器件中包含了CdSe量子点和ZnS绝缘层材料. 光致发光谱表征胶质量子点的室温发光峰位于614 nm, 电致发光测量得到器件在室温下的发光波长位于450 ~ 850 nm, 峰值在800 nm附近. 本文对电致发光机制及其与光致发光谱的区别进行了讨论.     

Characterization of plasmonic hole arrays as transparent electrical contacts for organic photovoltaics using high-brightness Fourier transform methods

We present a methodology for probing light-matter interactions in prototype photovoltaic devices consisting of an organic semiconductor active layer with a semitransparent metal electrical contact exhibiting surface plasmon-based enhanced optical transmission. We achieve high-spectral irradiance in a spot size of less than 100?μm using a high-brightness laser-driven light source and appropriate coupling optics. Spatially resolved Fourier transform photocurrent spectroscopy in the visible and near-infrared spectral regions allows us to measure external quantum efficiency with high sensitivity in small-area devices (<1?mm²). This allows for rapid fabrication of variable-pitch sub-wavelength hole arrays in metal films for use as transparent electrical contacts, and evaluation of the evanescent and propagating mode coupling to resonances in the active layer.     

Visible photocatalytic properties of vanadium doped zinc oxide aerogel nanopowder

Vanadium-doped zinc oxide nanoparticles have been synthesized by sol-gel method. In our approach the water for hydrolysis used in the synthesis of nanopowder was slowly released followed by a thermal drying in ethyl alcohol at 250 °C. The obtained nanopowder was characterized by various techniques such as particle size analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and photoluminescence (PL). In the as-prepared state, the powder with an average particle size of 25 nm presents a strong luminescence band in the visible range. From photoluminescence excitation (PLE) the energy position of the obtained PL band depends on the excitation wavelength and this PL band can be also observed under visible excitations. This result is very promising for visible photo catalysis applications, which was confirmed by methylene blue photo-degradation using visible lamp as a light source.     

Infrared light emission from porous silicon

Very intense broad sub-bandgap infrared (IR) light emission around 1,550 nm was observed on porous silicon by photoluminescence (PL) measurements. The integrated intensity of the IR signal is two orders of magnitude higher than that of the band–band emission in Cz silicon. PL measurements with the sample immersed in different media, e.g., in HF and H₂O₂, confirmed that the broad IR band originates from the Si/SiO _x interface. Electroluminescence spectroscopy was carried out on a porous silicon p–n junction sample contacted with indium-tin oxide. The IR band was detected at room temperature at both forward and reverse bias. The results indicate that radiative recombination through interface states is very efficient at room temperature.     

太阳电池缺陷EL检测及电性能分析

基于电致发光（EL）的理论,利用红外检测的方法,通过CCD近红外相机实验检测出晶体硅太阳电池中存在的隐性缺陷,如隐裂、断栅、电阻不均匀、花片等,并将EL图像与可见光下电池图像进行了对比。对存在缺陷的太阳电池进行了伏安特性测试,得出隐裂缺陷对太阳电池伏安特性、填充因子、效率等性能的影响,也证明电致发光技术检测太阳电池缺陷的准确性。     

Structural,Transport and Optical Properties of Boron-doped Zinc Oxide Nanocrystalline

The paper has reported the structural,transport and optical properties of boron doped zinc oxide(ZnO:B) thin films grown on glass substrate by sol-gel spin coating process.It is observed from the analysis of the X-ray diffraction(XRD) results that the crystalline quality of the films is improved with increasing B concentration.A crystallite size of ～17 nm is obtained for B doped films.A minimum resistivity of 7.9×10-4 Ω.cm is obtained at 0.6 at.% of B concentration in the ZnO:B films.Ionized and intragrain cluster scattering are found to dominate the scattering mechanism in ZnO:B films.Optical interference pattern in transmittance spectra shows good homogeneity with a transparency of ～88% in the visible region.The band gap of the films is increased from 3.24 to 3.35 eV with increasing B concentration.Band gap widening is analyzed in terms of Burstein-Moss shift.The origin of the broad band photoluminescence(PL) spectra is explained in terms of the intragrain cluster scattering.     

胶体量子点多色LED的设计、制备和表征

量子点LED采用胶体量子点为LED发光层,通过调节量子点的尺寸可以制作出覆盖可见以及近红外光谱的量子点LED(QD-LED),而且量子点LED器件发出的光谱范围很窄(光谱半高宽可达30 nm).为了研究不同发光颜色的QD-LED器件特性,本文采用具有523 nm和608 nm发光波长的CdSe/ZnS核壳型量子点为发光层、poly-TPD为空穴传输层、ZnO为电子传输层,制备了量子点红光和绿光LED并讨论了器件的相关特性.这些结果对量子点LED在飞机驾驶舱以及医疗器械照明方面的应用提供了参考,但要满足商业化的需求其寿命、亮度以及效率还需要进一步的提高.     

Photoanode Activity of ZnO Nanotube Based Dye-Sensitized Solar Cells

Vertical ZnO nanotube (ZNT) arrays were synthesized onto an indium doped tin oxide (ITO) glass substrate by a simple electrochemical deposition technique followed by a selective etching process. Scanning electron microscopy (SEM) showed formation of well-faceted hexagonal ZNT arrays spreading uniformly over a largearea. X-ray diffraction (XRD) of ZNT layer showed substantially higher intensity for the (0002) diffractionpeak, indicating that the ZnO crystallites were well aligned with their c-axis. Profilometer measurements ofthe ZNT layer showed an average thickness of ～7 μm. Diameter size distribution (DSD) analysis showedthat ZNTs exhibited a narrow diameter size distribution in the range of 65–120 nm and centered at ～75nm. The photoluminescence (PL) spectrum measurement showed violet and blue luminescence peaks thatwere centered at 410 and 480 nm, respectively, indicating the presence of internal defects. Ultra-violet (UV)spectroscopy showed major absorbance peak at ～348 nm, exhibiting an increase in energy gap value of 3.4 eV.By employing the formed ZNTs as the photo-anode for a dye-sensitized solar cell (DSSC), a full-sun conversion efficiency of 1.01% was achieved with a fill factor of 54%. Quantum efficiency studies showed the maximumof incident photon-to-electron conversion efficiency in a visible region located at 520–550 nm range.     

Multilayer Graphene–GeSn Quantum Well Heterostructure SWIR Light Source

The problem of light source always prevents silicon‐based photonics from achieving a final integration. Although some optical pump lasers have been reported in recent years, an electrical pumping laser is considered as the ultimate solution. To fabricate a Si‐based laser, there are some crucial obstacles that need to be solved such as difficulties in material epitaxy, light absorption by metal electrodes, and compatibility with the existing complementary metal–oxide–semiconductor transistor process. Here, a multilayer graphene and GeSn/Ge quantum well (QW) heterostructure is designed and fabricated as a Si‐based light source. Specially designed Ge_0.9Sn_0.1/Ge QWs are used as active layer, which achieves a photoluminescence (PL) peak at 2050 nm. Graphene, which has a high transmittance for all bands of light, lessens the burden of growing thick cladding layer and perfectly breaks the deadlock of light disappearance in metal contacts. The electroluminescence (EL) spectrum of the device is achieved at a peak of 2100 nm under an injection current density of 100 A cm^?2. Both the PL and EL measurements show the heterostructure has good performance as a short‐wave infrared (SWIR) light source. Therefore, the results provides a good alternative for the light source in silicon‐based photonics.     

Functional properties of silicon nanocrystals in oxygen-rich amorphous matrices formed by laser irradiation of substoichiometric silicon oxides

Silicon-based nanostructured thin films have been obtained through laser irradiation of amorphous silicon oxides. The optoelectronic properties of nanostructured films largely differ from their amorphous counterpart, exhibiting optical gap narrowing, wavelength-dependent spectral modification of the photoluminescence (PL) and conduction mechanism variations. In particular, following the hydrogen effusion and related defect density increase, a spectral red shift and PL intensity quenching is detected for λ_exc=514.5 nm, whereas PL enhancement and spectral blue shift is observed at λ_exc=632.8 nm. Different thermal activation regimes of conductivity are also detected by conductivity measurements.Such results are discussed in terms of microstructural changes from an hydrogenated amorphous network to a nanostructured two-phase material where wavelength-selective excitation of radiative recombination channels and temperature-dependent conductivity paths occurs.     

The effect of annealing on the electroluminescence of SiO<Subscript>2</Subscript> layers with excess silicon

We have studied the effect of annealing on the electroluminescence (EL) spectrum of Si-SiO₂ structures containing excess ion-implanted silicon in the oxide layer. The implantation of 150-keV silicon ions to doses in the range from 5×10¹⁶ to 3×10¹⁷ cm⁻² leads to the appearance of an intense emission band at 2.7 eV in the EL spectrum. The postimplantation annealing leads to a decrease in the intensity of this band and to the appearance of a new EL band at 1.6 eV assigned to radiative transitions in defect centers formed at the boundaries between silicon nanoclusters and silicon dioxide.