Effect of nanoporous silicon coating on silicon solar cell performance

The surface modification of silicon solar cells was used for improvement of photovoltaic characteristics of silicon solar cells. A screen-printed solar cell technology is used to fabricate n⁺-p silicon solar cell. Nanoporous silicon (PS) layer on n⁺-type Si wafers or on the frontal surface of (n⁺-p)Si solar cell was formed by electrochemical etching in HF-containing solution. The surface morphology, porosity, spectra of photoluminescence and reflectance of PS layers were analyzed. The photovoltaic characteristics of two silicon solar cell type with and without PS layer (PS/(n⁺-p)Si and (n⁺-p)Si cell) were measured and compared. The spectra of photosensitivity of cells were measured in the wavelength range of 300-1100 nm. An average reflection of the porous silicon layer, fabricated on a polished silicon surface, is decreased to 4%. A remarkable increment of the conversion efficiency by 20% have been achieved for PS/(n⁺-p)Si solar cell comparing to (n⁺-p)Si solar cell without PS layer. The results, related with improving of the performance of PS/(n⁺-p)Si solar cell, have been attributed to the effective antireflection and the wide-gap window role of nanoporous silicon on the silicon solar cell.     

Blue luminescence from porous layers produced by metal-assisted chemical etching on low-doped silicon

Photoluminescent porous layers were formed on highly resistive p-type silicon by a metal-assisted chemical etching method using K₂Cr₂O₇ as an oxidizing agent. A thin layer of Ag is deposited on the (1 0 0) Si surface prior to immersion in a solution of HF and K₂Cr₂O₇. The morphology of the porous silicon (PS) layer formed by this method as a function of etching time was investigated by scanning electron microscopy (SEM). It shows that the surface is formed by macropores filled with microporous silicon. The porous layers were characterized by backscattering spectrometry (BS) as a function of etching time in random and channelling mode. Channelling spectra show that the porous layer remains crystalline after etching. On the other hand, random and channelling spectra show that the deposited silver diffuses into the pore. Luminescence from metal-assisted chemically etched layers was measured. It was found that the PL intensity increases with increasing etching time. This behaviour is attributed to increase of the density of the silicon nanostructure. Finally, the PL spectra show two peaks of emission at 450 and 600 nm.     

Relaxing layers of silicon carbide grown on a silicon substrate by magnetron sputtering

Epitaxial layers of silicon carbide of the 3C-polytype are prepared by magnetron sputtering on Si(111) substrates of structural perfection with ω_θ = 1.4°. The crystalline structure and surface morphology of the 3C-SiC/Si(111) heterostructures depending on film thickness are studied by X-ray diffraction, Raman scattering, and atomic-force microscopy. It is found that the additional energy of ionized particles that is imparted to the Si(111) substrate during magnetron sputtering contributes the formation of strong C-C and β-SiC bonds, which hinders the crossing of the grain boundary by dislocations with increasing growth time.     

Studies on growth mechanism of HgCdTe epilayer on Si grown by HWE

In this paper, the mechanism of Hg_1−xCd_xTe/Si heterojunction grown by HWE (Hot Well Epitaxy) was studied. Opitical characterizations were shown with FTIR, the composition x = 0.39 was deduced by using MIR transmittance, the absorbing peak at 319.4 cm⁻¹ was measured by FIR transmittance, 319.4 cm⁻¹ confirmed the existence of Si–Te bond of at Si/HgCdTe interfacial layer. The I-V characteristics at both room temperature and 77 K of HgCdTe (n-type)/Si (p-type) heterojunction show that the good p-n heterojunction properties was obtained by using HWE. XRD study confirmed the formation of (111) oriented HgCdTe on (211) Si. Morphology of a cross section observed using EPMA indicates the columnar growth of HgCdTe. An analysis of interfcial layer by EPMA indicated presence of three layers composed of Si + Te, Si + Te + Hg and Si + Te + Cd + Hg. Among them, the most important one is the first layer. The problem of lattice mismatch and the difference of thermal expansion coefficient between Si and CdTe or HgTe may be improved by formation of Si–Te stable chemical bond through bybridization orbital bonding between Si and Te. The second and third layers are formed by evaporation-interdiffusion. Formation of the whole interfacial layer provides the appetency for the growth of (111) Hg_1−xCd_xTe epilayer on (211) Si substrate.     

Stress characterization of device layers and the underlying Si1−x Ge x virtual substrate with high-resolution micro-Raman spectroscopy

Silicon–germanium (Si–Ge) epitaxially grown mismatched heterostructures are becoming increasingly important for high-frequency microelectronics applications. One option under serious consideration is that of using Si–Ge virtual substrates, i.e., compositionally graded layers designed to accommodate the lattice mismatch between the underlying Si substrate and the overlying active epilayers(s). This assists in the prevention of misfit dislocations that can impact adversely on the active device regions. The stress in both device silicon cap layers and the underlying Si_1–x Ge _x virtual substrates is characterized with high-resolution micro-Raman spectroscopy (RS). The device layers of the samples studied composed of a 7-nm thick silicon channel, a 6-nm thick SiGe layer and were capped with a 7-nm thick silicon layer. The device layers are grown over a 1-m thick constant composition Si_0.70Ge_0.30 virtual substrate capping layer, and the Si-Ge virtual substrate is grown on a p⁺-type (0 0 1) silicon wafer with a thickness of about 500 m. RS measurement results with a 488-nm Ar⁺ visible laser source indicate that the Si_0.70Ge_0.30 capping layer at the virtual substrate is fully unstrained, while the top silicon cap layer is in extremely high tension. The use of a 325-nm HeCd UV laser for the RS measurements, which probes only a very small depth into the Si cap layer (approximately 9 nm) confirms this high tensile stress is in the top silicon cap layer. The tensile stress in the top silicon cap layer is estimated to be as large as 2.4 GPa by analyzing the shift of the Si Raman peak with respect to the standard strain-free silicon sample. The measured stress value is almost equal to the theoretically predicted tensile stress that should exist in the fully strained Si cap layer. This implies that the Si cap layer remains strained in samples with this structure.     

Effects of hydrogenation and aging on the optical properties in porous Si layers

The effects of hydrogenation and aging on the optical properties in porous Si (PS) layers were investigated by using photoluminescence (PL) measurements. When the hydrogenated PS layers were aged in air, the intensity of the PL spectrum increased. The emission peak for the hydrogenated PS layers shifted to higher energy with decreasing H₂/N₂ ratio. The relation of the dehydrogenized states in the as-formed PS surface to the quantum states of Si nanoparticles with relatively small sizes is discussed. These results indicate that the optical properties of PS layers are significantly affected by hydrogenation and aging.     

Formation of thin β-FeSi2 template layer for the epitaxial growth of thick film on Si (111) substrate

For the epitaxial growth of thick β-FeSi₂ films, we fabricated ultrathin β-FeSi₂ template layers (thinner than 20 nm) on Si (111) substrates with different methods. Surface morphology and crystallinity of the template layers were found to be dependent on the surface conditions of the substrate and the fabrication method. It was revealed that to form a smooth and continuous template, a hydrogen-terminated surface was better than that covered with a several-nanometer oxide layer. Using this surface, continuous (110)/(101)-oriented epitaxial template was obtained by depositing 6-nm iron at 400 °C and subsequent in situ annealing at 600 °C in MBE chamber, namely, a reaction deposition epitaxy (RDE) method. Co-deposition of iron and silicon with atomic ratio of Fe/Si=1/2 allowed the forming of template layers at further low temperature. Co-deposited template layers exhibited better crystallinity and morphology than those prepared by RDE. By using the optimized template layer, we succeeded in growing high-quality thick β-FeSi₂ films on Si (111) substrates with sharp β-FeSi₂/Si interface.     

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.     

Growth and characterisation of wide-bandgap,I-VII optoelectronic materials on silicon

γ-CuCl is a wide-bandgap (E_g = 3.395 eV), direct bandgap, semiconductor material with a cubic zincblende lattice structure. Its lattice constant, a_CuCl = 0.541 nm, means that the lattice mismatch to Si (a_Si = 0.543 nm) is < 0.5%. γ-CuCl on Si—the growth of a wide-bandgap, direct bandgap, optoelectronics material on silicon substrates is a novel material system, with compatibility to current Si based electronic/optoelectronics technologies.The authors report on early investigations consisting of the growth of polycrystalline, CuCl thin films with layer thicknesses of 100 nm to 1 μm on Si (100), Si (111), and quartz substrates by physical vapour deposition. X-ray diffraction (XRD) studies indicate that CuCl grows preferentially in the (111) direction but an epitaxial alignment with the substrate is also detected to a lesser extent in the case of Si (100). Photoluminescence (PL) and Cathodoluminescence (CL) reveal a strong room temperature Z₃ excitonic emission at ≈387 nm. X-ray microanalysis and XRD are used to investigate the effect of heat treatments on the CuCl thin films after deposition in the temperature range of 50 to 430∘C, (melting point of CuCl ≈ 430∘C). It is a found that a reaction occurs with Si on heating above 250∘C forming SiCl₄ and Cu.     

Electrochemical deposition of Ni and Cu onto monocrystalline n-Si(100) wafers and into nanopores in Si/SiO2 template

Nickel and copper were potentiostatically deposited onto monocrystalline n-Si (100) wafers and in nanoporous SiO₂/Si template from 0.5 M NiSO₄ + 0.5 M H₃BO₃ and 0.005 M CuSO₄ + 0.5 M H₃BO₃ solutions. Nanoporous SiO₂/Si template was formed by etching in dilute HF solution of ion tracks. The latter were produced by high-energy (380 MeV) Au⁺ ions bombardment of silicon oxide thermally grown on silicon (100) substrate. The deposition of metals was studied using cyclic voltammetry (CV), chronoamperometry; the structure and morphology of products were ex-situ investigated by SEM and XRD. The level of pores filling was controlled by deposition time. Electrodeposition occurred selectively into nanopores and the deposition on SiO₂ layer was excluded. It was found out that Ni and Cu electrodeposited into nanopores of SiO₂/Si system formed the same structures as at electrodeposition on the surface of monocrystalline n-Si—granules for Ni and scale-shaped particles for Cu deposits.     

Synthesis of vanadium dioxide films by a modified sol-gel process

Vanadium dioxide films have been grown on silicon substrates and on SiO₂ layers on silicon by a modified sol-gel process using methyl cellosolve as a solvent. We have failed to obtain vanadium dioxide layers on Pt/TiO _x /SiO₂/Si substrates. For all of the substrates studied, we have examined the effect of synthesis conditions (initial solution concentration, deposition procedure, and oxidation and reduction anneals) on the phase composition, thickness, and surface morphology of the films.     

Optical characterization of porous silicon and crystalline silicon by the Kramers-Kronig method

The electronic structure of porous silicon (PS) has been characterized by optical reflectance spectra analyses. Using a Cary-500 spectrometer, the reflectance spectra of PS are measured in the photon energy range of 0.4-6 eV. The spectral responses of optical constants are calculated for PS and Si by Kramers-Kronig analysis. The analysis clarified strong evidence for widening and direct bandgaps for PS samples. Also, the optical constants of PS layers as a function of porosity have been studied. Our results indicate that PS retains some of the characteristic optical features of crystalline Si. However, in the visible region, PS shows that the imaginary part of the complex refractive index increases, and the real part decreases as porosity increases. This feature could be related to the surface roughness of PS and its role in surface absorption and scattering.     

Interface structure of sputter deposited CNx film on silicon substrate

Amorphous carbon nitride (CN_x, x = 0.05) films were reactively sputtered on Si(100) substrate, and the interface structure was studied by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). In cross-sectional TEM a gray interlayer about 5 nm thick between the bulk CN_x film and silicon substrate is observed, and the interface is dense. A little Si impurity (< 1 at.%) is revealed in the films deposited for short time (7 s and 17 s) by XPS measurement. The in-depth XPS analyses indicate that there exists an interlayer with Si impurity above, and a sub-surface layer with C and N below the original surface of silicon substrate. The two layers have different chemical composition and bonding state.     

Design of porous silicon/PECVD SiOx antireflection coatings for silicon solar cells

The meso-porous silicon (PS) has become an interesting material owing to its potential applications in many fields, including optoelectronics and photovoltaics. PS layers were grown on the front surface of the n⁺ emitter of n⁺-p mono-crystalline Silicon junction. The thickness and the porosity of the PS layer were determined by an ellipsometer, as a function of time duration of anodization, and the variation law of the PS growth kinetics is established. Single layers PS antireflection coating (ARC) achieved around 9% of effective reflectivity in the wavelength range between 400 and 1000 nm on junction n⁺-p solar cells. To reduce the reflectivity and improve the stability and passivation properties of PS ARC, silicon oxide layers were deposited by PECVD on PS ARC. SiO_x layers of thickness of 105 nm combined with PS layer led to 3.8% effective reflectivity. V_oc measurements were carried out on all the samples by suns-V_oc method and showed an improvement of the quality of the passivation brought by the oxide layer. Using the experimental reflectivity results and taking into account the passivation quality of the samples, the PC1D simulations predict an enhancement of the photogenerated current exceeding 44%.     

Effect of carbon on lattice strain and hole mobility in Si1-x Ge x alloys

Pseudomorphic Si_1-x Ge _x and partially strain compensated layers with different Ge and C fractions have been grown at 500 °C by ultra high vacuum chemical vapor deposition on Si (100) substrates. The degree of strain compensation of the layers has been investigated by high resolution X-ray diffraction and simple application of the linear elasticity theory. The surface morphology of the layers has been characterized by atomic force microscopy. The dependence of Si–Si Raman mode vibrations on strain and composition of binary and ternary alloys have been explained with experimental and theoretically calculated results. The Hall hole mobility is found to increase with decreasing compressive strain or effective Ge content in the layer throughout the temperature range of 120–300 K.     

Influence of the surface morphology and microstructure on the biological properties of Ti-Si-C-N-O coatings

Detailed structural, microstructural, biofilm formation and cytotoxicity studies were performed on Ti-Si-C-ON hard coatings prepared by DC reactive magnetron sputtering, in order to evaluate the relation among these properties. Compositional analysis showed the existence of two distinct regimens; regime I: high C/Si atomic ratio (C/Si ≥ 1.42) and intermediate N/Ti atomic ratio; regime II: low C/Si atomic ratio (C/Si ≤ 0.49) and low N/Ti atomic ratio. The structural analysis revealed that, in regime I, films crystallized in a B1-NaCl crystal structure typical of TiC_0.2N_0.8. In regime II, the decrease of C/Si and increase in silicon concentration led to the formation of Ti-Si-C-ON along with a reduction of grain size in the films. Atomic force microscopy observations showed that the surface morphology of these Ti-Si-C-ON films became smoother when the silicon content increased and the nitrogen content decreased, which is consistent with the formation of nanosized clusters. Concerning biological properties, it was observed that cytotoxicity could be related with the titanium concentration while biofilm formation ability was found to be related with the surface morphology of the films.     

Influence of dielectric coverage on photovoltaic conversion of silicon solar cells obtained by epitaxial lateral overgrowth

This work presents an analysis of the influence of SiO₂ dielectric coverage of a Si substrate on the solar-cell efficiency of Si thin layers obtained by epitaxial lateral overgrowth (ELO). The layers were obtained by liquid phase epitaxy (LPE). All experiments were carried out under the following conditions: initial temperature of growth: 1193 K; temperature difference ??T = 60 K; ambient gas: Ar; metallic solvent: Sn+Al; cooling rates: 0.5 K/min and 1 K/min. To compare the influence of the interior reflectivity of photons, we used two types of dielectric masks in a shape of a grid etched in SiO₂ along the ??110?? and ??112?? directions on a p+ boron-doped (111) silicon substrate, where silicon dioxide covered 70 % and 80 % of the silicon surface, respectively. The results obtained in this work depict the correlation between the interior efficiency and percentage of SiO₂ coverage of the substrate of the ELO solar cells.     

Morphology and microstructure at different scales of porous silicon prepared by a nonconventional technique

In this work we present first results concerning the detailed structure of porous silicon (PS) layers prepared by a new method using a vapour-etching (VE)-based technique. Studies of the photoluminescence properties of VE-based PS show that the visible emission occurs at high energies as compared with PS prepared by conventional techniques. To understand the VE-based PS features, we need to point out the PS microstructure throughout its general morphology. For this purpose a microscopy multiscale study was done. Scanning, conventional transmission, and high-resolution transmission electron microscopes were employed. The investigations were made on PS films prepared from moderately and heavily doped n- and p-type silicon. SEM images show that VE-based PS layers are essentially formed of clusters like interconnected structures. TEM studies show that these clusters are composed of nanocrystallites with different shapes. The effect of the doping type of the starting Si substrate on the characteristics of the PS layers was examined (thickness, porosity, behavior). Pore propagation was found to depend on doping type. The crystallinity of the PS layers was also locally studied in depth.