The influence of light intensity on surface recombination in GaS single crystals

Gallium sulphide (GaS) is a layer structure semiconductor with relatively wide energy gap (E_g (295 K) = 2.5 eV and E_g (80 K) = 2.62 eV). It has potential applications in some areas of optoelectronics. This paper presents the investigations of the influence of light intensity on surface recombination velocity of charge carriers in GaS single crystals. To attain this purpose spectral dependences (between 420 and 550 nm) of absorption coefficients, reflectivity coefficients and photoconductivity were measured in vacuum. The investigations were performed for various light intensities in several temperatures from 80 to 333 K. The least square method was applied to fit the theoretical dependences of photoconductivity on wavelength and intensity of illumination at these temperatures. From the fittings the temperature and light intensity dependences of surface recombination velocity and bulk lifetime of charge carriers were obtained.     

Integrity of functional self-assembled monolayers on hydrogen-terminated silicon-on-insulator wafers

Silicon-on-insulator (SOI) wafers are commonly used to design microelectronics, energy conversion, and sensing devices. Thin solid films on the surfaces of SOI wafers have been a subject of numerous studies. However, SOI wafers modified by self-assembled monolayers (SAMs) that can also be used as functional device platforms have been investigated to a much lesser extent. In the present work, tert-butoxycarbonyl (t-boc, (CH₃)₃-C-O-C(O)-)-protected 1-amino-10-undecene monolayers were covalently attached to a H-terminated SOI (1 0 0) surface. The modified wafers were characterized by X-ray photoelectron spectroscopy to confirm the stability of the SAM/Si interface and the integrity of the secondary amine in the SAM. The transmission electron microscopy investigation suggested that this t-boc-protected 1-amino-10-undecene SAM produces atomically flat interface with the 2 μm single crystalline silicon of the SOI wafer, that the SiO_x and both available Si/SiO_x interfaces are preserved, and that the organic monolayers are stable, with apparent thickness of 1.7 nm, which is consistent with the result of the density functional theory modeling of the molecular features within a SAM.     

Chemical etching to dissolve dislocation cores in multicrystalline silicon

Multicrystalline silicon wafers are used for approximately half of all solar cells produced at present. These wafers typically have dislocation densities of up to ∼10⁶ cm⁻². Dislocations and associated impurities act as strong recombination centres for electron–hole pairs and are one of the major limiting factors in multicrystalline silicon substrate performance. In this work we have explored the possibility of using chemical methods to etch out the cores of dislocations from mc-Si wafers. We aim to maximise the aspect ratio of the depth of the etched structure to its diameter. We first investigate the Secco etch (1K₂Cr₂O₇ (0.15 M): 2HF (49%)) as a function of time and temperature. This etch removes material from dislocation cores much faster than grain boundaries or the bulk, and produces tubular holes at dislocations. Aspect ratios of up to ∼7:1 are achieved for ∼15 μm deep tubes. The aspect ratio decreases with tube depth and for ∼40 μm deep tubes is just ∼2:1, which is not suitable for use in bulk multicrystalline silicon photovoltaics. We have also investigated a range of etches based on weaker oxidising agents. An etch comprising 1I₂ (0.01 M): 2HF (49%) attacked dislocation cores, but its etching behaviour was extremely slow (<0.1 μm/h) and the pits produced had a low aspect ratio (<2:1).     

Triangular pattern formation on silicon through self-organization of GaN nanoparticles

Nanoparticles of gallium nitride, synthesized by a low-temperature reaction between triethyl gallium and ammonia, were introduced onto silicon wafers containing a thin layer of chemically prepared silicon dioxide. At room temperature, the nanoparticles form unstructured agglomerates on the surface. However, upon annealing the samples beyond the decomposition temperature of the silicon dioxide layer, the gallium nitride particles self-organize to form triangular structures. The pattern formation is attributed to the domain separation associated with the (1 × 1)-(7 × 7) surface phase transformation followed by selective incorporation of the nanoparticles.     

The study of the radiative lifetimes of 3pndF3 and 3pndD3 of Si I

The energy levels and lifetimes of 3pnd¹F₃ (n = 3-15) and 3pnd³D₃ (n = 3-18) of neutral silicon are calculated and predicted by means of the multichannel quantum defect theory (MQDT). In addition, this paper corrects the assignment mistakes of 3pnd¹F₃, 3pnd³F₃ for n > 8, and predicts many positions and lifetime values of the energy state which are not obtained in theory and experiment up to now, and analyzes the perturbation images between the channels.     

Solar cell fabrication using edge-defined film-fed growth (EFG) silicon wafers

The insufficient supply of polysilicon is limiting the growth of the segment of the photovoltaic industry using silicon materials. Because it is grown directly in the form of ribbon from a silicon melt, edge-defined film-fed growth (EFG) silicon ribbon is a promising alternative for cutting down wafer costs by reducing the polysilicon consumption and eliminating kerf loss. In this paper, we will discuss the various properties that can be achieved with for low cost and high-efficiency EFG silicon ribbon solar cell fabrication. Boron-doped p-type EFG ribbon silicon wafers with resistivities of 2-4 Ω cm and a size of 125 mm × 125 mm were used. The major fabrication steps we studied were mixed acid (HF, HNO₃, DI water) texturing, phosphorus diffusion with POCl₃, thermal oxide growth for surface passivation, laser process for edge isolation, and PECVD of SiN_x:H for surface passivation and antireflection coating. By optimizing the processing steps, we achieved a conversion efficiency, open circuit voltage, short circuit current, and fill factor as high as 14.5%, 584 mV, 32.1 mA/cm², and 77%, respectively.     

Application of the chemical vapor-etching in polycrystalline silicon solar cells

This paper reports a study of the application of chemical vapor-etching (CVE) for the rear surface and in the emitter of polycrystalline silicon (pc-Si) solar cells. The CVE technique consists of exposing pc-Si wafers to a mixture of HF/HNO₃. This technique is used to groove the rear surface of the pc-Si wafers for acid vapors rich in HNO₃ (HNO₃/HF > 1/4), in order to realize rear-buried metallic contacts (RBMC) and the formation of a porous silicon (PS) layer on the frontal surface of the cell for volume ratio of HNO₃/HF = 1/7. A significant increase of the spectral response in the long wavelength range was observed when a RBMC is formed. This increase was attributed to the reduction of the effective thickness of the base of the cells and grain boundary Al gettering. The achievement of a PS layer on the emitter of the pc-Si cells passivates the surface and reduces the reflectivity. The dark I-V characteristics of pc-Si cells with emitter-based PS show an important reduction of the reverse current together with an improvement of the rectifying behaviour. The I-V characteristic under AM1.5 illumination shows an enhancement of both short circuit current density and fill factor. The internal quantum efficiency is improved, particularly in the short wavelengths region.     

High-density-plasma (HDP)-CVD oxide to thermal oxide wafer bonding for strained silicon layer transfer applications

Direct wafer bonding between high-density-plasma chemical vapour deposited (HDP-CVD) oxide and thermal oxide (TO) has been investigated. HDP-CVD oxides, about 230 nm in thickness, were deposited on Si(0 0 1) control wafers and the wafers of interest that contain a thin strained silicon (sSi) layer on a so-called virtual substrate that is composed of relaxed SiGe (∼4 μm thick) on Si(0 0 1) wafers. The surfaces of the as-deposited HDP-CVD oxides on the Si control wafers were smooth with a root-mean-square (RMS) roughness of <1 nm, which is sufficiently smooth for direct wafer bonding. The surfaces of the sSi/SiGe/Si(0 0 1) substrates show an RMS roughness of >2 nm. After HDP-CVD oxide deposition on the sSi/SiGe/Si substrates, the RMS roughness of the oxide surfaces was also found to be the same, i.e., >2 nm. To use these wafers for direct bonding the RMS roughness had to be reduced below 1 nm, which was carried out using a chemo-mechanical polishing (CMP) step. After bonding the HDP-CVD oxides to thermally oxidized handle wafers, the bonded interfaces were mostly bubble- and void-free for the silicon control and the sSi/SiGe/Si(0 0 1) wafers. The bonded wafer pairs were then annealed at higher temperatures up to 800 °C and the bonded interfaces were still found to be almost bubble- and void-free. Thus, HDP-CVD oxide is quite suitable for direct wafer bonding and layer transfer of ultrathin sSi layers on oxidized Si wafers for the fabrication of novel sSOI substrates.     

Fabrication and characterization of polycrystalline silicon nanowires with silver-assistance by electroless deposition

Single crystal silicon wafers are widely used as the precursors to prepare silicon nanowires by employing a silver-assisted chemical etching process. In this work, we prepared polycrystalline silicon nanowire arrays by using solar-grade multicrystalline silicon wafers. The chemical composition and bonding on the surface of silicon nanowire arrays were characterized by Fourier Transform Infrared spectroscope, and X-ray photoelectron spectroscope. The photoluminescence spectra of silicon nanowires show red light emissions centered around 700 nm. Due to the passivation effect of Si dangling bonds by concentrated HNO₃ aqueous solution, the photoluminescence intensities are improved by 2 times. The influences of surface chemical states on the wettability of silicon nanowire arrays were also studied. We obtained a superhydrophobic surface on the as-etched silicon nanowire arrays without surface modification with any organic low-surface-energy materials, and realized the evolution from superhydrophobicity to superhydrophilicity via surface modifications with HNO₃ solutions.     

Time-domain modulated free-carrier absorption measurements of recombination process in silicon wafers

A time-domain modulated free-carrier absorption (MFCA) is developed both experimentally and theoretically to investigate the photo-carrier dynamics of silicon wafers illuminated by a square-wave-modulated super-band-gap laser beam. An explicit three-dimensional (3-D) theoretical expression for the temporal behavior of the MFCA signal is obtained by solving a 3-D carrier-diffusion equation The time-domain MFCA model is used to fit the experimental MFCA signals of p- and n-type Si wafers via a multi-parameter fitting procedure to determine simultaneously the electronic transport properties, that is, the bulk lifetime, the ambipolor diffusivity, and the front surface recombination velocity. The uncertainties of the fitted parameter values are estimated.     

The effect of etching time of porous silicon on solar cell performance

Porous silicon (PS) layers based on crystalline silicon (c-Si) n-type wafers with (1 0 0) orientation were prepared using electrochemical etching process at different etching times. The optimal etching time for fabricating the PS layers is 20 min. Nanopores were produced on the PS layer with an average diameter of 5.7 nm. These increased the porosity to 91%. The reduction in the average crystallite size was confirmed by an increase in the broadening of the FWHM as estimated from XRD measurements. The photoluminescence (PL) peaks intensities increased with increasing porosity and showed a greater blue shift in luminescence. Stronger Raman spectral intensity was observed, which shifted and broadened to a lower wave numbers of 514.5 cm⁻¹ as a function of etching time. The lowest effective reflectance of the PS layers was obtained at 20 min etching time. The PS exhibited excellent light-trapping at wavelengths ranging from 400 to 1000 nm. The fabrication of the solar cells based on the PS anti-reflection coating (ARC) layers achieved its highest efficiency at 15.50% at 20 min etching time. The I-V characteristics were studied under 100 mW/cm² illumination conditions.     

Luminescence of nanocrystalline ZnS:Cu

Temperature dependent luminescence and luminescence lifetime measurements are reported for nanocrystalline ZnS:Cu²⁺ particles. Based on the variation of the emission wavelength as a function of particle size (between 3.1 and 7.4 nm) and the low quenching temperature (T_q=135 K), the green emission band is assigned to recombination of an electron in a shallow trap and Cu²⁺. The reduction in lifetime of the green emission (from 20 μs at 4 K to 0.5 μs at 300 K) follows the temperature quenching of the emission. In addition to the green luminescence, a red emission band, previously only reported for bulk ZnS:Cu²⁺, is observed. The red emission is assigned to recombination of a deeply trapped electron and Cu²⁺. The lifetime of the red emission is longer (about 40 μs at 4 K) and the quenching temperature is higher.     

Construction of a novel multilayer system and its use for oriented immobilization of immunoglobulin G

In this contribution, we have developed a novel multilayer system composed of 3-aminopropyltrimethoxysilane (APTS), biotin, streptavidin and biotinylated protein A on the Si(1 0 0) surface to immobilize of immunoglobulin G (IgG) molecule. Existence of the first APTS layer covalently attached on the silicon surface was revealed by X-ray photoelectron spectroscopy (XPS). Average thickness of the APTS overlayer was estimated by spectroscopic ellipsometry analysis as 6.3 nm. The surface topography of the APTS overlayer observed by atomic force microscopy is found to be considerably different from the hydroxylated silicon surface and many additional islands of various heights are observed over the substrate. The water contact angle of the APTS overlayer was determined as 38°, whereas that of the hydroxylated silicon was obtained as 6°, indicating the difference in their surface hydrophobicility. Furthermore, multilayer studies on the APTS overlayer were carried out by using biotin, streptavidin and fluorescein-labeled biotinylated protein-A molecules. The fluorescence images obtained by fluorescence microscopy showed the formation of the multilayer on the Si(1 0 0) surface. The results indicated that the protein A-terminated surfaces can be used to immobilize IgG molecules in a highly oriented manner and maintain IgG molecular functional configuration on the multilayer system.     

调制激光致硅晶圆载流子辐射扫描成像试验研究

建立了调制激光诱发硅晶圆少数载流子密度波一维模型, 仿真分析了少数载流子输运参数对调制激光诱发载流子辐射信号频域响应的影响. 利用调制激光诱发载流子辐射扫描成像系统对含有表面划痕的硅晶圆进行了扫描成像试验研究. 通过少数载流子密度波模型与多参数拟合方法反求得到了扫描区域的输运参数二维分布图. 该方法得到的少数载流子寿命与利用传统光电导方法测量的少数载流子寿命结果相符; 分析了划痕对载流子输运参数造成的影响, 与光电导方法比较, 该方法可以测量不同位置的全部载流子输运参数且分辨率高.     

Reflectivity of porous-pyramids structured silicon surface

The antireflection of porous-pyramids structured silicon surface has been studied. The porous surface is formed by stain etching in HF/Fe(NO₃)₃ aqueous solution after textured in KOH/IPA solution. Reflectivity measurements show an overall reflectance of 4.2% for porous-pyramids textured silicon surface in the range from 400 to 900 nm. An optimal etching time of 30 min is obtained when both reflectivity and photo-generated carriers lifetime are considered. This technique may be probably used in the texturization process for high-efficiency silicon solar cells.     

Analysis of free carrier absorption measurement of electronic transport properties of silicon wafers

Sensitivity analysis of electronic transport property measurement of silicon wafers with modulated free carrier absorption (MFCA) technique and multi-parameter fitting procedure is performed. The sensitivity of the multi-parameter estimate employing the dependences of the MFCA amplitude and phase on the pump-probe-beam separation measured at several modulation frequencies covering an appropriate range is theoretically compared with that employing only the dependences of the MFCA amplitude and phase on the modulation frequency. Simulation results show that the dependences of the MFCA amplitude and phase on the pump-probe beam separation are more sensitive to the electronic transport properties of silicon wafers than the frequency dependences. The electronic transport properties of the silicon wafers determined with the two-beam separation dependence are therefore more accurate than that determined with the frequency dependence. Comparative experiments with a silicon wafer are performed and the carrier lifetime, the carrier diffusivity, and the front surface recombination velocity are determined simultaneously and unambiguously with both techniques.     

Dynamic photoluminescence lifetime imaging for the characterisation of silicon wafers

We present a fast and calibration‐free carrier lifetime imaging technique based on photoluminescence (PL) measurements using an InGaAs camera for the examination of crystalline silicon wafers. The carrier lifetime is determined from the time dependent luminescence emission after optical excitation. A ratio, including four PL images acquired at different times during the modulated excitation, is calculated and found to depend only on the camera integration time and the effective carrier lifetime. Therefore, the carrier lifetime is unambiguously determined by this ratio without knowing any additional wafer parameter. We demonstrate the applicability of the dynamic PL technique to multicrystalline silicon wafers. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Impact of nickel contamination on carrier recombination in n- and p-type crystalline silicon wafers

The effect of Ni surface contamination on carrier recombination after high temperature processing of crystalline silicon wafers has been studied for a range of n- and p-type resistivities. The results suggest that the presence of Ni precipitates at the wafer surfaces, formed during cooling, dominate the measured lifetimes. These precipitates exhibit a greater impact on the low-injection lifetime in p-type samples than in n-type. In addition, the injection-dependent lifetime curves for the n-type samples changed from increasing to decreasing with injection-level as the resistivity increased above approximately 10 Ω cm. In most cases, the surface recombination velocity attributable to the presence of these Ni precipitates at the oxidized surfaces increased linearly with the Ni dose. PACS 72.20.Jv; 73.20.Hb; 71.55.Cn     

Periodic patterning of silicon by direct nanosecond laser interference ablation

The production of periodic structures in silicon wafers by four-beam is presented. Because laser interference ablation is a single-step and cost-effective process, there is a great technological interest in the fabrication of these structures for their use as antireflection surfaces. Three different laser fluences are used to modify the silicon surface (0.8 J cm⁻², 1.3 J cm⁻², 2.0 J cm⁻²) creating bumps in the rim of the irradiated area. Laser induced periodic surface structures (LIPSS), in particular micro and nano-ripples, are also observed. Measurements of the reflectivity show a decrease in the reflectance for the samples processed with a laser fluence of 2.0 J cm⁻², probably caused by the appearance of the nano-ripples in the structured area, while bumps start to deteriorate.     

Nanocrystalline ZnO film deposited by ultrasonic spray on textured silicon substrate as an anti-reflection coating layer

A ZnO thin film was successfully synthesized on glass, flat surface and textured silicon substrates by chemical spray deposition. The textured silicon substrate was carried out using two solutions (NaOH/IPA and Na₂CO₃). Textured with Na₂CO₃ solution, the sample surface exhibits uniform pyramids with an average height of 5 μm. The properties and morphology of ZnO films were investigated. X-ray diffraction (XRD) spectra revealed a preferred orientation of the ZnO nanocrystalline film along the c-axis where the low value of the tensile strain 0.26% was obtained. SEM images show that all films display a granular, polycrystalline morphology. The morphology of the ZnO layers depends dramatically on the substrate used and follows the contours of the pyramids on the substrate surface. The average reflectance of the textured surface was found to be around 13% and it decreases dramatically to 2.57% after deposition of a ZnO antireflection coating. FT-IR peaks arising from the bonding between Zn–O are clearly represented using a silicon textured surface. A very intense photoluminescence (PL) emission peak is observed for ZnO/textured Si, revealing the good quality of the layer. The PL peak at 380.5 nm (UV emission) and the high-intensity PL peak at 427.5 nm are observed and a high luminescence occurs when using a textured Si substrate.