Textured Si Research Articles

Study of Optical Properties of Nanocrystalline Zinc Phosphide Thin films

Zinc phosphide (Zn3P2), a II–V group semiconductor compound has long been considered as a very interesting and suitable candidate for several applications especially in photovoltaics. Nanocrystalline Zn3P2 thin films were successfully synthesized on glass and different silicon-based substrates by a combined physico-chemical method. The properties and morphology of Zn3P2 films were investigated. SEM images showed that all films displayed a nanoscale granular, polycrystalline morphology. It depends dramatically on the surface state of the substrate and follows the substrate its morphology. X-ray diffraction (XRD) spectra revealed a preferred orientation of the Zn3P2 nanocrystalline film along the (102) and (405) direction. The transmittance of the Zn3P2 films was found to be high of the order of 87%. The optical gap of the film was determined using the optical transmission spectra and the obtained optical band gap value is 1.96 eV. A decrease of the average reflectance from 15% to 4.25%, after deposition of the nanocrystalline Zn3P2 on a textured Si substrate, was observed. This result was compared to the one obtained with SiNx layer deposited on the same Si substrate. A difference of 1% was noticed. We assume that the combination of nanocrystalline Zn3P2 thin film and a textured Silicon surface could increase the efficiency of photovoltaic silicon solar cell by using Zn3P2 as an antireflection layer.

UV–visible light detection with TiO2 thin film deposited on chemically textured p-Si substrate

This paper reports a study on the photodetection properties of TiO2 thin film deposited on chemically textured p-Si substrate and is compared to those deposited on pristine p-Si substrate. The structural properties of both the heterostructures were investigated using XRD analysis. FESEM images confirmed the deposition of TiO2 thin film on chemically pyramidal textured Si substrate. The average total reflectance of the textured Si substrate was reached to ~ 7.6% in the wavelength range of 300–900 nm. It was further decreased to ~ 6.5% after the deposition of 55 nm thick TiO2 on the top of the textured Si substrate. A systematic study was carried out to correlate the structural, optical and electrical properties of both the heterostructures. The electrical parameters of the heterojunction diodes were measured and compared under dark and illuminated conditions using UV and solar simulated light. UV as well as visible light detection property of the heterostructure of TiO2 thin film deposited on pyramidal textured Si substrate was improved compared to the one deposited on pristine Si by the factor 2.0 and 1.86, respectively, under the bias of − 2 V.

Colossal Broad Band Antireflection from Conformally Grown Cu2O on Pyramidally Textured Si and its Photovoltaic Applications

In this paper, we show the efficacy of a conformally grown, self‐organized Cu2O overlayer on chemically synthesized, pyramidally textured silicon substrate for a colossal suppression of optical reflection loss. In particular, we demonstrate that p‐Cu2O film deposited on a chemically textured n‐Si substrate shows an ultra‐low average surface reflectance (≈0.25%) over the entire spectral range (300–2400 nm), whereas the same turns out to be ≈25% for the film deposited on an as‐obtained n‐type Si substrate. It is observed that in the former case, Cu2O grows in the form of leaf‐like nanostructures, which is attributed to be responsible for such a low reflectance. In addition, under both configurations, the p‐Cu2O/n‐Si heterostructure shows a non‐linear current–voltage characteristics, clearly suggesting the formation of barrier junctions. Further, it is found that a solar cell fabricated with p‐Cu2O overlayer grown on a pyramidally textured n‐Si substrate shows an eight times higher efficiency compared to the one grown on an as‐obtained n‐Si substrate. This study will pave the way to design cost‐effective and efficient copper oxide‐based solar cells.

Fabrication of low reflective nanopore-type black Si layer using one-step Ni-assisted chemical etching for Si solar cell application

Nanotextured Si fabricated through metal-assisted chemical etching (MACE) technique exhibits a promising potential for producing antireflective layer for photovoltaic (PV) application. In this study, a novel single-step nickel (Ni) assisted etching technique was applied to produce an antireflective, nonporous Si (black Si) in an aqueous solution containing hydrofluoric acid (HF), hydrogen peroxide (H2O2) and NiSO4 at 40 °C. Field emission scanning electron microscope was used to characterize different morphologies of the textured Si. Optical reflection measurements of samples were carried out to compare the reflectivity of different morphologies. Results indicated that vertical as well as horizontal pores with nanosized diameters were bored in the Si wafer after 1 h treatment in the etching solution containing different molar ratios of H2O2 to HF. Increasing H2O2 concentration in electrochemical etching solution had a considerable influence on the morphology due to higher injection of positive charges from Ni atoms onto the Si surface. Optimized concentration of H2O2 led to formation of an antireflective layer with 2.1% reflectance of incident light.

Heterojunction Hybrid Solar Cells by Formation of Conformal Contacts between PEDOT:PSS and Periodic Silicon Nanopyramid Arrays.

Surface nanotexturing with excellent light-trapping property is expected to significantly increase the conversion efficiency of solar cells. However, limited by the serious surface recombination arising from the greatly enlarged surface area, the silicon (Si) nanotexturing-based solar cells cannot yet achieve satisfactory high efficiency, which is more prominent in organic/Si hybrid solar cells (HSCs) where a uniform polymer layer can rarely be conformably coated on nanotextured substrate. Here, the HSCs featuring advanced surface texture of periodic upright nanopyramid (UNP) arrays and hole-conductive conjugated polymers, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), are investigated. The tetramethylammonium hydroxide etching is used to smooth the surface morphologies of the Si-UNPs, leading to reduced surface defect states. The uniform Si-UNPs together with silane chemical-incorporated PEDOT:PSS solution enable the simultaneous realization of excellent broadband light absorption as well as enhanced electrical contact between the textured Si and the conducting polymer. The resulting PEDOT:PSS/Si HSCs textured with UNP arrays show a promising power conversion efficiency of 13.8%, significantly higher than 12.1% of the cells based on the-state-of-the-art surface texture with random pyramids. These results provide a viable route toward shape-controlled nanotexturing-based high-performance organic/Si HSCs.

Surface Plasmon Excited on Imprintable Thin-Film Metallic Glasses for Surface-Enhanced Raman Scattering Applications

Metallic glasses (MGs) are a class of amorphous alloys in contrast with crystalline metals and provide a challenge of engineering applications for unique structure and properties. However, plasmonic applications remain a virgin area for MGs. In this work, we discovered that certain compositions of gold-based MGs possessed negative dielectric constants and could be used as plasmonic materials. Furthermore, with a low glass-transition temperature of gold-based thin-film MGs (TFMGs), we were able to fabricate large dimensions of nanostructures using an inexpensive thermal imprint method in air instead of other costly lithography methods. We performed both measurements and simulations to demonstrate that our designed nanostructures were suitable for surface-enhanced Raman scattering (SERS) applications. In addition, in the absence of grain boundaries in amorphous TFMGs, damping due to increased scattering at grain boundaries does not occur, and SERS could be improved. Also, compared to regular SERS substrates of textured Si with deposited Au films, imprinted Au-based TFMGs provided complete coverage of Si underneath, and the vibrational signal of Si lattice would not show in Raman spectra to possibly overlap signals of analyte and decrease the accuracy of sensing. Our results suggested new avenues for applying a low-cost and high-throughput method on TFMGs to fabricate large dimensions of substrates for plasmonic applications.

Evaluation of the charge carriers’ transportation through the grown MgFe2O4 on the nanotextured silicon surface in the presence of hydrogen gases

Here, we report, for the first time, the solution growth of n-type MgFe2O4 spinel ferrite nanoparticles on nano-textured p-type silicon (100) substrate. Water soluble MgFe2O4 spinel ferrite nanoparticles are prepared by polyol technique. The MgFe2O4 spinel ferrite nanoparticles were characterized by transmission electron microscopy both at low and high resolution, selected area electron diffraction, energy dispersive spectroscopy, and Raman spectroscopy. The texturing of the surface of the Si(100) substrate to create well-organized nanopillar-like shapes was carried out by a nanosecond laser lithography technique. The water soluble MgFe2O4 spinel ferrite nanoparticles were deposited on the textured Si(100) substrate. The sample was characterized by scanning electron microscopy and X-ray diffraction. The charge carriers transport mechanism was discussed in terms of the variation of the barrier height under the exposure to various gases. A comparison between the MgFe2O4 spinel ferrite nanoparticles grown on planar and textured p-type Si(100) surfaces showed that the latter demonstrated an improvement of gas sensitivity and accelerated response time. These two characteristics, combined with high performance stability, make these sensors promising for practical applications.

The optoelectronic properties of developed p–n junction on the textured silicon surface for improving the solar cell performance

Here, we report the development of the solar cell with a hybrid p-n junction based on poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) deposited on the textured Si surface. Texturing was accomplished by a laser processing technique, allowing the formation of nanopyramids on the Si surface, leading to ultralow reflectance. The PEDOT:PSS solution was deposited on the textured Si-wafer via a spin-coating technique. The evaluation of the I–V characteristics under illumination demonstrated an improvement of the short-circuit current density of the PEDOT:PSS/textured nanopyramid Si-substrate by 61% compared to that of the device based on the planar Si-substrate. The effect of the nanopyramid aspect ratio (height/diameter) of the textured Si-substrate on the optical absorption and reflection was studied to determine the optimum aspect ratio to achieve the maximum rate of the light entrapment. It was demonstrated that the increase of the aspect ratio leads to an enhancement of the short-circuit current and the filling factor. The hybrid device developed shows a promise for future applications in photovoltaic solar cell devices.

Optical Modeling of Ultrathin Silicon Solar Cells with PC1D

We show that the optical diffusion model embedded in the numerical simulation program PC1D systematically underestimates light‐generated currents in thin textured Si solar cells. The demonstration exploits the exact analytical solution to minority‐carrier transport in uniformly doped regions and the fact that, at the first pass of internal light, the average optical propagation angle, θ, with respect to the device normal is determined by surface texture. We provide a simple correction procedure to remove the aforementioned systematical error. One can so reliably model with PC1D the optical performance of textured thin devices at standard Lambertian regimen (θ = 60° at all wavelengths, λ) that starts at either the first or the second pass of trapped light. We exploit such a possibility to scrutinize with PC1D a reported non‐standard Lambertian optical model, where θ varies with λ.

Growth and characterization of textured well-faceted ZnO on planar Si(100), planar Si(111), and textured Si(100) substrates for solar cell applications.

In this work, textured, well-faceted ZnO materials grown on planar Si(100), planar Si(111), and textured Si(100) substrates by low-pressure chemical vapor deposition (LPCVD) were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and cathode luminescence (CL) measurements. The results show that ZnO grown on planar Si(100), planar Si(111), and textured Si(100) substrates favor the growth of ZnO(110) ridge-like, ZnO(002) pyramid-like, and ZnO(101) pyramidal-tip structures, respectively. This could be attributed to the constraints of the lattice mismatch between the ZnO and Si unit cells. The average grain size of ZnO on the planar Si(100) substrate is slightly larger than that on the planar Si(111) substrate, while both of them are much larger than that on the textured Si(100) substrate. The average grain sizes (about 10–50 nm) of the ZnO grown on the different silicon substrates decreases with the increase of their strains. These results are shown to strongly correlate with the results from the SEM, AFM, and CL as well. The reflectance spectra of these three samples show that the antireflection function provided by theses samples mostly results from the nanometer-scaled texture of the ZnO films, while the micrometer-scaled texture of the Si substrate has a limited contribution. The results of this work provide important information for optimized growth of textured and well-faceted ZnO grown on wafer-based silicon solar cells and can be utilized for efficiency enhancement and optimization of device materials and structures, such as heterojunction with intrinsic thin layer (HIT) solar cells.

Passivation of textured crystalline silicon surfaces by catalytic CVD silicon nitride films and catalytic phosphorus doping

Silicon nitride (SiNx) films formed by catalytic chemical vapor deposition (Cat-CVD) and phosphorus (P)-doped layers formed by catalytic impurity doping (Cat-doping) are applied for the passivation of pyramid-shaped textured crystalline Si (c-Si) surfaces formed by anisotropic etching in alkaline solution. Lower surface recombination velocities (SRVs) tend to be obtained when smaller pyramids are formed on c-Si surfaces. P Cat-doping is effective for reducing the SRV of textured c-Si surfaces as in the case of flat c-Si surfaces. We realize SRVs of textured c-Si surfaces of ∼8.0 and ∼6.7 cm/s for only SiNx passivation and for the combination of SiNx and P Cat-doping, respectively. These structures also have high optical transparency and low Auger recombination loss, and are of great worth in application for the surface passivation of interdigitated back-contact c-Si solar cells.

Minority Carrier Lifetime Estimation by Photoconductance Decay for Silicon Wafers Immersed in HF

The measurement of minority carrier bulk lifetimes of Si wafers immersed in HF is known to have an immersion time dependence, which is specially strong for high lifetime wafers and can vary significantly for different surface treatments performed previously. These complications can make the lifetime measurement become difficult to interpret and inaccurate. Therefore, a method for extracting the bulk lifetime from the measured curves of lifetime versus immersion time is suggested. A model of the reaction between Si and HF is considered, consisting in a first-order reaction between Si and HF for polished wafers and two first-order reactions for textured wafers. Experiments on polished and textured p-type Si wafers were conducted with the photoconductance decay technique to test the model proposed and to show that it is more adequate to estimate the bulk lifetime by using the reactions models than to report it as the lifetime measured at a particular immersion time, which is the usual procedure.

Boron-doped p-BaSi2/n-Si solar cells formed on textured n-Si(0 0 1) with a pyramid structure consisting of {1 1 1} facets

BaSi2 films were fabricated on textured Si(001) substrates that consisted of {111} facets using molecular beam epitaxy. The light-trapping effect of these films and their performance when incorporated into solar cells were measured. X-ray diffraction and reflectivity measurements showed that the BaSi2 films were grown epitaxially on the textured Si(001) substrate and confirmed the light-trapping effect. The critical thickness over which BaSi2 relaxes increased from approximately 50 to 100nm when comparing the BaSi2 films on a flat Si(111) substrate and the textured substrate, respectively. p-BaSi2/n-Si solar cells were fabricated with varying BaSi2 layer thickness and with hole concentrations in the range between 2.0×1018 and 4.6×1018cm−3. These cells exhibited a maximum energy conversion efficiency of 4.62% with an open-circuit voltage of 0.30V and a short-circuit current density of 27.6mA/cm2 when the p-BaSi2 layer was 75nm-thick. These results indicated that the use of BaSi2 films on textured Si(001) substrates in solar cells shows great promise.

Laser Processing of Low Optical Reflection Micro/Nano-patterned Si Substrates for SERS

ABSTRACTLight collection efficiency and specific molecular detection are crucial factors for the performance of bio-chemical molecule sensors. In this paper, the low optical-reflection silicon (Si) substrates which combine reduced optical reflectivity by light trapping effect and high Raman enhancement ability of gold nanoparticles (Au-NPs) coated textured Si substrates are investigated for surface-enhanced Raman scattering (SERS). A fast, single-step and highly controllable nanosecond (ns) laser processing technique is employed to fabricate textured Si substrates under ambient conditions. Parallel arrays of micro-pyramids are fabricated on Si surface by direct laser writing two-dimensional structures. SEM micrographs clearly show well-ordered surface features in the form of micro-pyramid shape with well-defined sharp tips on the laser processed Si substrates. The aggregation of Si micro/nanoparticles on Si surface forms nanocavities and nanogaps and further enhances the surface roughness in order to minimize the optical reflection. The low optical reflection Si substrates exhibit optical reflection below 15% over a broad wavelength range from 300 nm to 1200 nm. The textured Si substrates with high signal reproducibility are successfully applied as SERS substrates to detect a very small concentration of Rhodamine B molecules with an average enhancement factor of the order of ∼107. The low optical reflection and SERS signal amplification are also altered by the variation of laser pulse energy resulting into low optical reflection and high SERS signal intensity over the entire laser-patterned area. The effect of surface roughness on water contact angle was studied after the modification with Polydimethylsiloxane (PDMS), the surfaces show perfect superhydrophobicity with almost no water adhesion. This approach provides a novel high-speed and cost-effective method for fabricating SERS substrate with micro/nano-scale surfaces roughness and low optical reflection for high Raman signal enhancement.

Gold-Hyperdoped Black Silicon With High IR Absorption by Femtosecond Laser Irradiation

Gold (Au)-doped-textured silicon (Si) material with a thermostable absorption below bandgap (>50%) is obtained by femtosecond laser irradiation. Although the concentration of Au impurity (1019 cm−3 ) in textured Si is at least four orders of magnitude greater than the solid solubility of Au in crystalline Si, the sheet carrier density (approximately 1010 cm−2) in Au-doped Si is very low due to a self-compensation effect of Au impurity in Si material. The infrared absorption of Au-doped Si is related to laser-induced-structural defects and sub-band absorption of deep energy levels of Au in Si, which is determined by temperature-dependent Hall Effect measurement. Besides supersaturated doping of Au, a gold silicide phase is formed at textured Si surface.

Efficiency enhancement of silicon solar cells with vertically aligned ZnO nanorod arrays as an antireflective layer

Vertically aligned ZnO nanorods grown by the hydrothermal method have been explored as an antireflection layer on polished, textured, and antireflection coating (ARC) coated textured silicon (Si) wafers. Average reflectance (from 380 to 1100 nm) of polished and textured Si wafers reduced from 32 to 9% and 14 to 2%, respectively. With nanorods, multiple light interactions and good optical impedance matching with graded refractive index (effective medium) from air to Si favored for light confinement in Si. Optimized nanorods on ARC coated textured Si cell led to an enhancement of photocurrent from 34.30 to 36.38 mA/cm2 and efficiency from 15.11 to 16.43%.

Back- and Front-side Texturing for Light-management in Perovskite / Silicon-heterojunction Tandem Solar Cells

The perovskite / silicon-heterojunction (SHJ) tandem solar cell can theoretically overcome the efficiencies of the single junction solar cells with experimental results not far behind. Here, we use optical simulations to analyze the potential of such currently feasible monolithic perovskite / SHJ tandem devices. We consider three different device designs in the optical simulations: planar device, device built on back-side textured Si wafer, and device with textured UV Nanoimprint Lithography light management foil. For each of these three designs, the current matching point is simulated to evaluate device efficiencies. While the device with back-side textured Si wafer causes light trapping and therefore enhanced photocurrent generation for longer wavelengths, the anti-reflection foils prove to be overall more efficient as the foil significantly reduces the reflection in almost full wavelength range.

Characterization of Nanostructured n-ZnO/p-Si Heterojunction Prepared by a Simple Sol–Gel Method

The nanostructured ZnO film was prepared on a texturized Si wafer by a simple sol–gel method to fabricate n-ZnO/p-Si heterojunction photoelectric device. The novel sol–gel method is cheap and convenient. The structural, optical and electrical properties of the nanostructured ZnO film were studied by XRD, SEM, XPS, PL, UV–Vis spectrophotometer and Hall effect measurement. The current–voltage (I–V) curve of nanostructured ZnO/p-Si heterojunction device shows good rectifying behavior. Good photoelectric behavior is obtained.

X-ray photoelectron spectroscopic studies of CeO2 thin films deposited on Ni-W (100), c-Al2O3 (0001) and Si (100) substrates

The oxidation states of CeO2 films, deposited on biaxially textured Ni-W(100), c-Al2O3(0001) and Si(100) substrates, were investigated by using X-ray photoelectron spectroscopy (XPS). The CeO2 films were deposited by RF magnetron sputtering under same deposition conditions. X-ray diffraction (XRD) studies of the films revealed that preferred (200) orientation of CeO2 was observed in case of Ni-W substrate while (111) orientation on Al2O3 and Si substrates. The O1s and Ce3d spectra reveal that the films mostly consist of Ce4+ oxidation states with ∼72.6%, 74.7% and 74.9% on Ni-W, Al2O3 and Si substrates, respectively. The less Ce4+ content in case of Ni-W substrate has been attributed to the migration of oxygen atoms from film to the metallic substrate. The decrease of O2p states in the valence band spectra also in support with the presence of more oxygen vacancies in CeO2/Ni-W compared to that in other substrates. Further, the position of valance band maxima is almost identical (∼2.6 eV) for all the deposited films indicating that all films are n-type in nature.

Characterization of Nickel Silicide Formation by a Selective Electroless Plating Process for Crystalline Silicon Solar Cells

The nickel (Ni) electroless plating process is an encouraging method for selectively depositing Ni onto silicon (Si) wafers to form front electrodes of Si solar cells. Ni has the ability to block diffusion in copper (Cu), which is used for further thickening of the Ni contacts. The contact between Ni and Si can be optimized by an additional sintering step to form nickel-silicide (NiSix). This paper presents the characterization of NiSix according to bath conditions, deposition time, and sintering process. The selective electroless deposition of Ni was achieved by plating from an alkaline nickel chloride (NiCl2) bath with ammonia as the pH control and sodium hypophosphite as the reducing agent. Thin clusters of Ni were deposited on top of Si, which ensured the formation of uniform Ni seed layers with better surface coverage on both textured and non-textured Si surfaces. The dominant phase of NiSi that offered the lowest series resistance was confirmed by X-ray diffraction (XRD) analysis at sintering temperatures of 350 oC and 400 oC. Selective deposition of Cu over the Ni seed layer was achieved by light induced plating arrangements, and thin finger lines with widths up to 28.4 μm were realized. (Received July 1, 2015)