Micro‐photoluminescence spectroscopy on heavily‐doped layers of silicon solar cells

We report and explain the photoluminescence spectra emitted from silicon solar cells with heavily‐doped layers at the surface. A micro‐photoluminescence spectroscopy system is employed to investigate the total spectrum emitted from both the heavily‐doped layer and the silicon substrate with micron‐scale spatial resolution. The two regions of the device give rise to separate photoluminescence peaks, due to band‐gap narrowing effects in the highly‐doped layer. Two key parameters, the absorption depth of the excitation wavelength, and the sample temperature, are shown to be critical to reveal the separate signatures from the two regions. Finally, this technique is applied to locally diffused and laser‐doped regions on silicon solar cell pre‐cursors, demonstrating the potential value of this micron‐scale technique in studying and optimizing locally doped regions. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Fano resonance in heavily doped porous silicon

Unexpectedly, the Fano resonance caused by the interference of continuum electron excitations with the longitudinal optical (LO) phonons was observed in random porous Si by Raman scattering. The analysis of the experimental data shows that the electron states trapped at the Si SiO₂ interface dominate in the observed Raman scattering. The gap energy associated with the interface states was determined. Copyright © 2011 John Wiley & Sons, Ltd.     

A 3‐state defect model for light‐induced degradation in boron‐doped float‐zone silicon

We report on a light‐induced bulk defect activation and subsequent deactivation in boron doped float‐zone silicon that can be described by a 3‐state model. During treatment at elevated temperature and illumination, a sample first converts from an initial high lifetime state into a degraded low lifetime state and then shows a recovery reaction leading to a third high lifetime state that is then stable under degradation conditions. Furthermore, it is shown that reverse reactions into the initial state appear to be possible both from the degraded as well as the regenerated state. An injection dependent analysis of lifetime data yields a defect capture cross section ratio of ～20 suggesting a positively charged defect. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Modeling the formation of local highly aluminum‐doped silicon regions by rapid thermal annealing of screen‐printed aluminum

The formation of local highly aluminum‐doped (Al‐p⁺) regions by rapid thermal annealing (firing) of screen‐printed aluminum strongly depends on the temperature profile and the contact geometry. We measure the local Al‐p⁺ layer thickness W_Al‐p+ as a function of the point and line contact size. Using quantitative yet simple analytical modeling, the time‐dependent silicon concentration in the Al melt is described by elementary differential equations. From this we calculate W_Al‐p+ and find agreement with the measurements. In contrast to the formation of full area Al‐p⁺ layers we find a smaller silicon concentration at the end of the firing process compared to the equilibrium concentration. This is a result of the process dynamics such as the dissolution rate of solid silicon and the transport of silicon in the Al melt. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Aligned silicon nanowires with fine‐tunable tilting angles by metal‐assisted chemical etching on off‐cut wafers

The morphology control of aligned silicon nanowires (SiNWs) is highly desirable as SiNWs demonstrated high prospect in a variety of applications. Besides the control over length, shape and distribution of aligned SiNWs, the fine‐tuning of tilting angles thereof also attracted intense interest. Up to now, only several discrete tilting angles have been reported. In this Letter, the ability to fine‐tune the tilting angle of SiNWs is demonstrated and the range that can be achieved is identified. Our technique employs the anisotropic characteristic of the etching process using custom‐produced off‐cut Si wafers of various orientations as substrates. With this technique, a uniquely favoured etching direction can result and the tilting angle can be precisely controlled. Tilted SiNWs with tilting angles from 0° to 50° relative to the wafer normal were obtained. The mechanism of the tilting angle manipulation is also discussed. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Light‐induced degradation in indium‐doped silicon

Light‐induced degradation of charge carrier lifetime was observed in indium‐doped silicon. After defect formation, an annealing step at 200 °C for 10 min deactivates the defect and the initial charge carrier lifetime is fully recovered. The observed time range of the defect kinetics is similar to the well known defect kinetics of the light‐induced degradation in boron‐doped samples. Differences between defect formation in boron‐ and indium‐doped silicon are detected and discussed. A new model based on an acceptor self‐interstitial A_Si–Si_i defect is proposed and established with experimental findings and existing ab‐initio simulations.

     

Light‐induced degradation in copper‐contaminated gallium‐doped silicon

To date, gallium‐doped Czochralski (Cz) silicon has constituted a solar cell bulk material free of light‐induced degradation. However, we measure light‐induced degradation in gallium‐doped Cz silicon in the presence of copper impurities. The measured degradation depends on the copper concentration and the material resistivity. Gallium‐doped Cz silicon is found to be less sensitive to copper impurities than boron‐doped Cz silicon, emphasizing the role of boron in the formation of copper‐related light‐induced degradation. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

掺铜及掺铝多孔硅的光致发光及其发光机理

分别将特定杂质铜和铝引入多孔硅后，观察到了杂质铜和铝所引起的附加发光带：对于没有掺铜的多孔硅，其光致发光谱只有一个发光带；而掺过铜的多孔硅，其光致发光谱出现两个发光带，其中能量较低的发光带随主发光带而变化。在掺铝多孔硅的光致发光谱中，则出现4个与铝杂质能级有关发光带。我们认为上述与杂质有关的发光带是由截流子在杂质深能级上复合所致。     

A comparative study of surface‐enhanced Raman scattering from silver‐coated anodic aluminum oxide and porous silicon

Three types of Ag‐coated arrays from porous anodic aluminum oxide (AAO) were prepared and studied as substrates for surface‐enhanced Raman scattering (SERS). They were compared with Ag‐coated porous silicon (PSi) samples. AAO‐based substrates were prepared by the vapor deposition of silver directly onto the surface of porous AAO with different morphologies of the pores, whereas SERS‐active island films on the PSi were prepared by immersion plating. The resulting metallic nanostructures were characterized by UV‐vis absorption spectroscopy and scanning electron microscopy (SEM). Thermal evaporation leads to the formation of granular arrays of Ag nanoparticles on the surface of AAO. SERS activity of the substrates was tested using water‐soluble cationic Zn(II)‐tetrakis (4‐N‐methylpyridyl) porphyrin (ZnTMPyP4) as a probe molecule. The results indicate that all AAO‐based substrates studied here exhibit some degree of SERS activity. Noteworthy, for excitation at 532 nm, signals from AAO‐based substrates were comparable with those from the PSi‐based ones, whereas for 441.6 nm excitation they were about twice higher. The strongest SERS‐enhancement at 441.6 nm excitationwas provided by the AAO substrates with silver deposited on the monolith (originally nonporous) side of AAO. Preferential SERS‐enhancement of the bands ascribed to the vibrations of the N‐methylpyridinium group of ZnTMPyP4 when going to blue excitation was found. Copyright © 2010 John Wiley & Sons, Ltd.     

3d过渡金属Co掺杂核壳结构硅纳米线的第一性原理研究

基于密度泛函理论的第一性原理计算,研究了横截面为五边形和六边形的核壳结构硅纳米线的过渡金属Co原子替代掺杂.通过比较形成能发现,核心位置掺杂、壳层单链掺杂以及外壳层全替代掺杂的硅纳米线都具有稳定性,其中核心位置掺杂结构的稳定性最高.掺杂体系均呈现金属性,随着掺杂浓度的增加,电导通道数增加.Co原子掺杂的硅纳米线呈现铁磁性,具有磁矩.Bader电荷分析表明,电荷从Si原子转移至过渡金属Co原子.与自由态时过渡金属Co原子的磁矩相比,体系中Co原子的磁矩有所降低,这主要是由Co原子4s轨道向3d/4p轨道的电荷转移以及4s,3d,4p的上自旋电子转移至下自旋导致的.     

Regularities of the photoluminescence of porous silicon after chemical etching in HF

The effect of chemical treatment of porous silicon samples by HF on its photoluminescence and its evolution with time in sample aging in air is investigated. It is shown that the effect of HF on the luminescence parameters depends on the duration of the treatment and the initial photoluminescence intensity of the sample. It is found that chemical etching in HF accelerates the growth of the total luminescence intensity in aging of the sample in air. The evolution of the photoluminescence spectrum in aging of the sample in air after chemical etching can be explained within the framework of the quantum-size model of the luminescence of porous silicon. Presented at the Fall Meeting of the Material Research Society, December 1–5, 1997, Boston, USA Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 423–427, May–June, 1999.     

Atom probe tomography of size‐controlled phosphorus doped silicon nanocrystals

Doping of silicon nanocrystals is essential to control their electronic and optical properties. The incorporation of an impurity into a silicon nanovolume is a nontrivial task due to the self‐purification effect. Here, a systematic atom probe tomography study of the phosphorus distribution and incorporation in size‐controlled silicon nanocrystals embedded in silicon dioxide is presented. Qualitatively, it turns out that the phosphorus distribution in the system follows a universal, nanocrystal‐size independent trend: phosphorus‐enrichment at the interface with a substantial phosphorus‐incorporation in the silicon nanocrystal as small as 2 nm in diameter. This clearly contradicts strict self‐purification. These observations are explained by the bulk‐solubility and ‐segregation behaviour, kinetic effects related to the diffusion lengths, and nanoscale interface strain. The quantitative determination of the amount of phosphorus atoms per quantum dot enables a systematic understanding of phosphorus‐induced effects on optical and electronic properties of silicon nanovolumes.     

Surface characterization of porous silicon after pore opening processes inducing chemical modifications

In this paper, we present a study on the porous silicon surface with the aim of filling porous silicon layers with organics. We discuss on two processes used to remove the outer parasitic layer created during the porous silicon formation. We demonstrate that these etching processes influences the surface properties, in particular wetting ability. By XPS and infrared absorption spectroscopy studies, we show that a SF₆ plasma treatment does not modify irreversibly the chemistry of porous silicon surface, nor the surface morphology. We also point out that NaOH etching does bring significant morphological modifications and influences the hydrophilicity of the porous silicon surface. This last treatment increases the polar groups (SiO) concentration on the pore surface and therefore allows a better filling of a porous silicon layer with organics, like dibromo-EDOT which can be thermally converted into PEDOT.     

Luminescence from porous silicon doped with erbium–ytterbium complexes

The study of photoluminescence (PL) from porous silicon (PS) containing complexes of gadolinium oxychloride with Er³⁺- and Er³⁺–Yb³⁺ is reported. The concentration dependencies of PL intensity of PS with Er³⁺ containing complex have been studied. The dependencies have retained the main features that are characteristic of the pure complex for both IR and visible regions of the PL spectra. This allows interpretation of PL processes in complex-containing PS lthrough the concept of multiplication of low-energy electron excitations and cross-relaxation degradation of higher excited states. It has been shown that introducing Yb³⁺ ions into the complex significantly increases the PL intensity. Mechanisms associated with defect formation, the intrinsic conversion of excitation energy within Yb³⁺, and the conversion within Er³⁺ ions followed by transferring of excitation energy to the Yb³⁺ ions has been considered. The PL polarization with excitation in the visible is reported as well.     

Raman study of Fano interference in p‐type doped silicon

As the silicon industry continues to push the limits of device dimensions, tools such as Raman spectroscopy are ideal to analyze and characterize the doped silicon channels. The effect of inter‐valence band transitions on the zone center optical phonon in heavily p‐type doped silicon is studied by Raman spectroscopy for a wide range of excitation wavelengths extending from the red (632.8 nm) into the ultra‐violet (325 nm). The asymmetry in the one‐phonon Raman lineshape is attributed to a Fano interference involving the overlap of a continuum of electronic excitations with a discrete phonon state. We identify a transition above and below the one‐dimensional critical point (E = 3.4 eV) in the electronic excitation spectrum of silicon. The relationship between the anisotropic silicon band structure and the penetration depth is discussed in the context of possible device applications. Copyright © 2010 John Wiley & Sons, Ltd.     

Multi‐crystalline silicon solar cells with metal‐assisted nano‐texturing using HNO3 as hole injection agent

In this study, metal‐assisted etching (MAE) with nitric acid (HNO₃) as a hole injecting agent has been employed to texture multi‐crystalline silicon wafers. It was previously proven that addition of HNO₃ enabled control of surface texturing so as to form nano‐cone shaped structures rather than nanowires. The process parameters optimized for optically efficient texturing have been applied to multi‐crystalline wafers. Fabrication of p‐type Al:BSF cells have been carried out on textured samples with thermal SiO₂/PECVD‐SiN_x stack passivation and screen printed metallization. Firing process has been optimized in order to obtain the best contact formation. Finally, j_sc enhancement of 0.9 mA/cm² and 0.6% absolute increase in the efficiency have been achieved. This proves that the optimized MAE texture process can be successfully used in multi‐crystalline wafer texturing with standard passivation methods.

J –V curves and SEM images of the nano and iso‐textured samples. j_sc enhancement of 0.9 mA/cm² together with 0.6% absolute efficiency gain was observed on nano‐textured samples.     

Stiffness properties of porous silicon nanowires fabricated by electrochemical and laser-induced etching

Nanowires with dimensions of few nanometers were formed on the whole etched surface. The optical analysis of silicon nanostructures was studied. Blue shift luminescence was observed at 660 nm for PS produced by electrochemical etching, and at 629 nm for laser-induced etching. PS produced a blue shift at 622 nm using both etching procedures simultaneously. X-ray diffraction (XRD) was used to investigate the crystallites size of PS as well as to provide an estimate of the degree of crystallinty of the etched sample. Refractive index, optical dielectric constant, bulk modulus and elasticity are calculated to investigate the optical and stiffness properties of PS nanowires, respectively. The elastic constants and the short-range force constants of PS are investigated.     

Excellent passivation of thin silicon wafers by HF‐free hydrogen plasma etching using an industrial ICPECVD tool

In this work, hydrogen plasma etching of surface oxides was successfully accomplished on thin (～100 µm) planar n‐type Czochralski silicon wafers prior to intrinsic hydrogenated amorphous silicon [a‐Si:H(i)] deposition for heterojunction solar cells, using an industrial inductively coupled plasma‐enhanced chemical vapour deposition (ICPECVD) platform. The plasma etching process is intended as a dry alternative to the conventional wet‐chemical hydrofluoric acid (HF) dip for solar cell processing. After symmetrical deposition of an a‐Si:H(i) passivation layer, high effective carrier lifetimes of up to 3.7 ms are obtained, which are equivalent to effective surface recombination velocities of 1.3 cm s^–1 and an implied open‐circuit voltage (V_oc) of 741 mV. The passivation quality is excellent and comparable to other high quality a‐Si:H(i) passivation. High‐resolution transmission electron microscopy shows evidence of plasma‐silicon interactions and a sub‐nanometre interfacial layer. Using electron energy‐loss spectroscopy, this layer is further investigated and confirmed to be hydrogenated suboxide layers. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Fabrication of CoFe_2O_4 ferrite nanowire arrays in porous silicon template and their local magnetic properties

CoFe_2O_4 ferrite nanowire arrays are fabricated in porous silicon templates. The porous silicon templates are prepared via metal-assisted chemical etching with gold(Au) nanoparticles as the catalyst. Subsequently, CoFe_2O_4 ferrite nanowires are successfully synthesized into porous silicon templates by the sol–gel method. The magnetic hysteresis loop of nanowire array shows an isotropic feature of magnetic properties. The coercivity and squareness ratio(M_r/M_s) of ensemble nanowires are found to be 630 Oe(1 Oe = 79.5775 A·m~(-1) and 0.4 respectively. However, the first-order reversal curve(FORC) is adopted to reveal the probability density function of local magnetostatic properties(i.e., interwire interaction field and coercivity). The FORC diagram shows an obvious distribution feature for interaction field and coercivity. The local coercivity with a value of about 1000 Oe is found to have the highest probability.     

稀土掺杂多孔硅的蓝光发射

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