A hydrogen pathway for electronic processes in amorphous silicon

The characteristic difference in the diamagnetic fractions of hydrogen in doped and undoped amorphous hydrogenated silicon (a-Si:H), as obtained through MuSR modelling, is investigated by first reviewing recent calculations of the electronic energies of interstitial hydrogen. This results firstly in the identification of a charge transfer process between dopant atoms and hydrogen interstitials that provides a new mechanism for dopant passivation. It is concluded, however, that the increase in the diamagnetic fraction with temperature in doped a-Si:H can only be explained by the diffusion of hydrogen interstitials to charged dangling bond sites. These two interactions, of interstitial hydrogen and dopants on the one hand, and hydrogen and dangling bond defects on the other, are then used to propose a hydrogen pathway between dopants and dangling bond defects that explains why the process of doping amorphous silicon is always accompanied by the formation of charged dangling bonds.     

Characterization of SiNx/a-Si:H crystalline silicon surface passivation under UV light exposure

One of the most promising solution for crystalline silicon surface passivation in solar cell fabrication consists in a low temperature (< 400 °C) Plasma Enhanced Chemical Vapor Deposition of a double layer composed by intrinsic hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (SiN_x). Due to the high amount of hydrogen in the gas mixture during the double layer deposition, the passivation process results particularly useful in case of multi-crystalline silicon substrates in which hydrogenation of grain boundaries is very needed. In turn the presence of hydrogen inside both amorphous layers can induce metastability effects. To evaluate these effects we have investigated the stability of the silicon surface passivation obtained by the double layer under ultraviolet light exposure. In particular we have verified that this double layer is effective to passivate both p- and n-type crystalline silicon surface by measuring minority carrier high lifetime, via photoconductance-decay. To get better inside the passivation mechanisms, strongly connected to the charge laying inside the SiN_x layer, we have collected the Infrared spectra of the SiN_x/a-Si:H/c-Si structures and we have monitored the capacitance-voltage profiles of Al/SiN_x/a-Si:H/c-Si Metal Insulator Semiconductor structures at different stages of UltraViolet (UV) light exposure. Finally we have verified the stability of the double passivation layer applied to the front side of solar cell devices by measuring their photovoltaic parameters during the UV light exposure.     

Amorphous silicon carbide heterojunction solar cells on p-type substrates

The performance of silicon heterojunction (SHJ) solar cells is discussed in this paper in regard to their dependence on the applied amorphous silicon layers, their thicknesses and surface morphology. The emitter system investigated in this work consists of an n-doped, hydrogenized, amorphous silicon carbide a-SiC:H(n) layer with or without a pure, hydrogenized, intrinsic, amorphous silicon a-Si:H(i) intermediate layer. All solar cells were fabricated on p-type FZ-silicon and feature a high-efficiency backside consisting of a SiO₂ passivation layer and a diffused local boron back surface field, allowing us to focus only on the effects of the front side emitter system. The highest solar cell efficiency achieved within this work is 18.5%, which is one of the highest values for SHJ-solar cells using p-type substrates. A dependence of the passivation quality on the surface morphology was only observed for solar cells including an a-Si:H(i) layer. It could be shown that the fill factor suffers from a reduction due to a reduced pseudo fill factor for emitter thicknesses below 11 nm due to a lower passivation quality and/or a higher potential for shunting thorough the a-Si emitter to the crystalline wafer with the conductive indium tin oxide layer. Furthermore, the influence of a variation of the doping gas flow (PH₃) during the plasma enhanced chemical vapor deposition of the doped amorphous silicon carbide a-SiC:H(n) on the solar cell current-voltage characteristic-parameter has been investigated. We could demonstrate that a-SiC:H(n) shows in principle the same dependence on PH₃-flow as pure a-Si:H(n).     

SiNx/a-SiCx:H passivation layers for p- and n-type crystalline silicon wafers

In this work we present a detailed investigation of Si surface passivation obtained by a PECVD double dielectric layer, composed of intrinsic hydrogenated amorphous silicon-carbon (a-SiC_x:H), followed by a silicon nitride (SiN_x). The double layers have been deposited on p- and n-type of mono- and multi-crystalline silicon wafers. IR spectra have been carried out to evaluate the structure of a-SiC_x:H layers on monocrystalline wafers. The passivation effects have been studied performing the following measurements: the photoconductance decay, to measure contactlessly the effective lifetime of passived mono and multi Si wafers; the capacitance voltage profile of Al/SiN_x/Si, Al/a-SiC_x:H/Si and Al/SiN_x/a-SiC_x:H/Si MIS structures, to estimate the field effect at the dielectric/silicon interface and individuate the passivation mechanism on silicon surfaces. It has been found that the mechanism of the surface passivation depends on the doping type of the silicon wafer. Indeed from C-V measurements it has been realized that the great amount of positive charge within the SiN_x is able to promote an inversion layer if it is deposited on a-SiC_x:H/Si p-type and an accumulation if it is grown on a-SiC_x:H/Si n-type.     

Diffusion of Phosphorus in silicon dioxide from a spin-on source

The diffusion of phosphorus in silicon dioxide was investigated. A phosphosilicate glass which was deposited from a solution containing a silicic ester was used as the source. Using radioactive ³²P it was possible to determine the dopant profile in silicon dioxide by measuring the β activity. The profiles obtained cannot be described by the usual solution of the diffusion equation. The phosphorus mobility in silicon dioxide was found to be markedly dependent on the oxygen content of the ambient gas during diffusion. Pre-annealing in nitrogen of the SiO₂ layer into which the diffusion took place similarly influences the diffusion profile. It is presumed that a free acid of phosphorus from the doping solution participates in the diffusion process.     

Electrical properties/Doping efficiency of doped microcrystalline silicon layers prepared by hot-wire chemical vapor deposition

Microcrystalline silicon (μc-Si) doped films were prepared by hot-wire chemical vapor deposition (HWCVD) to investigate the doping efficiency. The incorporation probability of different dopant atoms into the solid-phase is always increasing with the doping gas concentrations, but very different for the doping gases used: trimethylboron (TMB), boron trifluoride (BF₃) and phosphine (PH₃). At the same doping gas concentration in the process gas the incorporation of phosphorus atoms into the solid μc-Si phase is much larger than that of boron atoms with respect to the dissociation probability of the doping gases. The electron and hole concentrations, estimated from Hall measurements, are directly related to the solid phase concentration of the doping atoms and independent of the type of dopant and the doping gas used. This results in an equal doping efficiency of about 20 % for the incorporated B and P atoms in doped HWCVD μc-Si films. For the dopant atom concentration regime investigated the doping efficiency of B atoms is in good agreement with corresponding PECVD doping efficiencies however, the doping efficiency of P atoms is considerably lower for our n-doped films.     

High-Quality YBa2Cu3O7−x Films with CeO2/YSZ Buffer Layers on 2-Inch Si Wafers Deposited by Pulsed Laser

The pulsed laser deposition (PLD) process is shown for in situ reproducibly fabricating YBa₂Cu₃O_7?x (YBCO) superconducting films with yttrium-stabilized zirconia (YSZ) and CeO₂ buffer layers, nonsuperconducting crystalline YBa₂Cu₃O_7?x (YBCO*) passivation layer, and silver contact film on 2-inch silicon wafers. Variations of less than ±7% in film thickness have been obtained for this multilayer growth over the whole wafer. The YBCO films on 2-inch silicon wafers have homogeneous superconducting properties with zero resistance temperature T _c0 from 88.4 K to 88.9 K. and critical current density J _c at 77 K and zero field from 2.5 × 10⁶ to 7× 10⁶ A/cm². The YSZ, CeO₂ and YBCO layers grow epitaxially on silicon wafers. Full widths at half maximum (FWHMs) of (113) reflections of 40 nm thick YBCO layer from φ-scan patterns are only 1.71° and 1.85° corresponding to the center and edge of the wafer, respectively. These results are very promising for developing high-quality high-T _c superconducting devices on large-area silicon wafers.     

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%.     

Thick-layer growth of n+ silicon on n− silicon substrate by liquid-phase epitaxy

A solution technique is proposed for the growth of thick epitaxial layers with high doping of arsenic on undoped silicon substrates. The interface between the epitaxial layer grown and the substrate was investigated by spreading resistance measurements and a quadruple crystal X-ray diffractometer. The results showed that (a) a fairly abrupt change of the n-type dopant arsenic concentration was obtained at the interface between the epitaxial layer and the silicon substrate, and (b) no lattice mismatch between the epitaxial layer and the substrate was found from the results of the rocking curves by the quadruple crystal X-ray diffractometer, even if the arsenic-doping level in the epitaxial layer was high, 2×10¹⁹ atoms cm⁻³. This revised version was published online in July 2006 with corrections to the Cover Date.     

Transfer of physically-based models from process to device simulations: Application to advanced SOI MOSFETs

Dopant implantation, followed by spike annealing is one of the main focus areas in the simulation of silicon processing due to its ability to form highly-activated ultra-shallow junctions. Coupled with the growing interest in the use of silicon-on-insulator (SOI) wafers, modelling and simulation of the influence of SOI structure on damage evolution and ultra-shallow junction formation on one hand, and on electrical MOSFET device characteristics on the other hand, are required.In this work, physically-based models of dopant implantation and diffusion, including amorphization, defect interactions and evolution, as well as dopant-defect interactions in both bulk silicon and SOI are integrated within a unique simulation tool to model the different physical mechanisms involved in the process of ultra-shallow junction formation.The application to 65 nm SOI MOSFET devices demonstrated the strong impact of the process simulation models on the simulated electrical device characteristics, in particular for both defect evolution and defect dopant interaction with the additional silicon/buried oxide (Si/BOX) interface. Simulation results of the threshold voltage (V_th) and the variation of the on- and off-state currents of the explored structures are in good agreement with experimental data and can provide important insight for optimizing the process in both bulk silicon and SOI technologies.     

Interfacial Passivation of the p‐Doped Hole‐Transporting Layer Using General Insulating Polymers for High‐Performance Inverted Perovskite Solar Cells

Organic–inorganic lead halide perovskite solar cells (PVSCs), as a competing technology with traditional inorganic solar cells, have now realized a high power conversion efficiency (PCE) of 22.1%. In PVSCs, interfacial carrier recombination is one of the dominant energy‐loss mechanisms, which also results in the simultaneous loss of potential efficiency. In this work, for planar inverted PVSCs, the carrier recombination is dominated by the dopant concentration in the p‐doped hole transport layers (HTLs), since the F4‐TCNQ dopant induces more charge traps and electronic transmission channels, thus leading to a decrease in open‐circuit voltages (V_OC). This issue is efficiently overcome by inserting a thin insulating polymer layer (poly(methyl methacrylate) or polystyrene) as a passivation layer with an appropriate thickness, which allows for increases in the V_OC without significantly sacrificing the fill factor. It is believed that the passivation layer attributes to the passivation of interfacial recombination and the suppression of current leakage at the perovskite/HTL interface. By manipulating this interfacial passivation technique, a high PCE of 20.3% is achieved without hysteresis. Consequently, this versatile interfacial passivation methodology is highly useful for further improving the performance of planar inverted PVSCs.     

Surface morphology and structural analysis of fluorocarbon nano-rings formation through EBL and SiO2 plasma etching

This paper reports the formation of nano-scale ring-shaped fluorocarbon macromolecules during silicon dioxide SiO₂ reactive ion etching (RIE). This nanostructure was created on a SiO₂ substrate with poly methyl methacrylate (PMMA) mask during the RIE process, using trifluoromethane (CHF₃) and oxygen etchants. Variation in etching time results in the creation of square, double concentric, and flower-shaped nano-rings around SiO₂ micro-pits. In addition, increasing the etching times leads to an increase in ring width. The formation of these nano-rings is shown by a deposition of passivation layer, consisting of silicon oxide, Si_xO_y and fluorocarbon, C_xF_y, on sidewalls during SiO₂ etching in fluorocarbon plasma. Field Emission Scanning Electron Microscopy (FESEM) and Energy-dispersive X-ray (EDX) were utilized to investigate the morphology and the structure of the nano-rings. Results show that the flower-shaped nano-rings were created on the surface of silicon for 8 min of etching time. These fluorocarbon nano-rings could be used as nano-scale templates.     

Sintering behavior and dielectric properties of Ce doped strontium barium niobate ceramics with silica sintering additive

Sr_0.5Ba_0.5Nb₂O₆ ceramics with and without CeO₂ dopant were prepared by a partial co-precipitation method and a liquid phase sintering process. The cooperative effects of Ce doping and silica sintering additive on the sintering behaviors and the dielectric properties of Sr_0.5Ba_0.5Nb₂O₆ ceramics was investigated. It was observed that the lattice parameters of a-axis and c-axis of Sr_0.5Ba_0.5Nb₂O₆ ceramics decrease with the increase of Ce dopant, namely contraction of crystal cell volume occurs. Amorphous silica used for sintering additive can effectively restrain abnormal grain growth and prevent the rise of sintering activation energies caused by Ce doping, but Ce doping has more effect on the average size of the grains and the dielectric properties than the silica sintering additive when the Ce dopant and the silica sintering additive are introduced. Both the Curie temperatures and the maximum of dielectric constant at Tc decrease as the Ce³⁺ concentration increases.     

Boron doping of silicon rich carbides: Electrical properties

Boron doped multilayers based on silicon carbide/silicon rich carbide, aimed at the formation of silicon nanodots for photovoltaic applications, are studied. X-ray diffraction confirms the formation of crystallized Si and 3C-SiC nanodomains. Fourier Transform Infrared spectroscopy indicates the occurrence of remarkable interdiffusion between adjacent layers. However, the investigated material retains memory of the initial dopant distribution. Electrical measurements suggest the presence of an unintentional dopant impurity in the intrinsic SiC matrix. The overall volume concentration of nanodots is determined by optical simulation and is shown not to contribute to lateral conduction. Remarkable higher room temperature dark conductivity is obtained in the multilayer that includes a boron doped well, rather than boron doped barrier, indicating efficient doping in the former case. Room temperature lateral dark conductivity up to 10^?3 S/cm is measured on the multilayer with boron doped barrier and well. The result compares favorably with silicon dioxide and makes SiC encouraging for application in photovoltaic devices.     

Investigation of charges carrier density in phosphorus and boron doped SiNx:H layers for crystalline silicon solar cells

Dielectric layers are of major importance in crystalline silicon solar cells processing, especially as anti-reflection coatings and for surface passivation purposes. In this paper we investigate the fixed charge densities (Q_fix) and the effective lifetimes (τ_eff) of phosphorus (P) and boron (B) doped silicon nitride layers deposited by plasma-enhanced chemical vapour deposition. P-doped layers exhibit a higher τ_eff than standard undoped layers. In contrast, B-doped layers exhibit lower τ_eff. A strong Q_fix decrease is to be seen when increasing the P content within the film. Based on numerical simulations we also demonstrate that the passivation obtained with P- and B-doped layers are limited by the interface states rather than by the fixed charges.     

Silicon nanocrystals produced by solid phase crystallisation of superlattices for photovoltaic applications

Silicon nanocrystals in a dielectric matrix can form a material with higher band gap than that of bulk crystalline silicon and can therefore be applied as stable top solar cells for an all-silicon based tandem solar cell. In this review article we focus on one proven method to fabricate such structures, the superlattice approach, as cost-efficiency seems to be possible which is essential for photovoltaic applications. We comprehensively discuss the different challenges for competing material systems such as SiO₂, Si₃N₄ and SiC and give an overview on what is known so far in terms of electro-optical performance of the materials. So far, devices using silicon nanocrystals have been realised either on silicon wafers, or using in-situ doping in the superlattice deposition which may hinder the nanocrystal formation. Nevertheless, V_oc of up to 518 mV has been shown on such devices. In this paper we also present a membrane structure which allows the investigation of the electrical and photovoltaic properties of nanocrystal quantum dot layers independently from the substrate and unaffected by dopant diffusion. The device structure provides full flexibility in the material choice of both, i.e. electron and hole, contacts.     

Silicon heterojunction solar cells: Optimization of emitter and contact properties from analytical calculation and numerical simulation

The key constituent of silicon heterojunction solar cells, the amorphous silicon/crystalline silicon heterojunction (a-Si:H/c-Si), offers a high open-circuit voltage (V_oc) potential providing that both the interface defect passivation and the band bending in the c-Si absorber are sufficient. We detail here analytical calculations of the equilibrium band bending in c-Si (ψ_c-Si) in Transparent Conductive Oxide (TCO)/a-Si:H emitter/c-Si absorber structures. We studied the variation of some electronic parameters (density of states, work function) according to relevant experimental values. This study introduces a discussion on the optimization of the doped emitter layer in relation with the work function of the TCO. In particular, we argue on the advantage of having a highly defective (p)a-Si:H emitter layer that maximizes ψ_c-Si and reduces the influence of the TCO on V_oc.     

Dual‐Additive Assisted Chemical Vapor Deposition for the Growth of Mn‐Doped 2D MoS2 with Tunable Electronic Properties

Doping of bulk silicon and III–V materials has paved the foundation of the current semiconductor industry. Controlled doping of 2D semiconductors, which can also be used to tune their bandgap and type of carrier thus changing their electronic, optical, and catalytic properties, remains challenging. Here the substitutional doping of nonlike element dopant (Mn) at the Mo sites of 2D MoS₂ is reported to tune its electronic and catalytic properties. The key for the successful incorporation of Mn into the MoS₂ lattice stems from the development of a new growth technology called dual‐additive chemical vapor deposition. First, the addition of a MnO₂ additive to the MoS₂ growth process reshapes the morphology and increases lateral size of Mn‐doped MoS₂. Second, a NaCl additive helps in promoting the substitutional doping and increases the concentration of Mn dopant to 1.7 at%. Because Mn has more valance electrons than Mo, its doping into MoS₂ shifts the Fermi level toward the conduction band, resulting in improved electrical contact in field effect transistors. Mn doping also increases the hydrogen evolution activity of MoS₂ electrocatalysts. This work provides a growth method for doping nonlike elements into 2D MoS₂ and potentially many other 2D materials to modify their properties.     

Formation and properties of porous silicon/titania nanostructures

Porous silicon/titania structures have been prepared for the first time by a sol-gel process in which a porous silicon layer was produced on single-crystal p-type silicon wafers and the titania was obtained from Ti-containing sol. The formation of TiO₂, predominantly in the form of anatase, on the porous silicon surface was demonstrated by X-ray diffraction and energy dispersive X-ray analysis. The porous layers were found to contain carbon in addition to the host elements (Si, Ti, and O). Increasing the pore volume through the thermal oxidation of the porous silicon and dissolution of the oxide layer had little effect on the final Ti content, whereas the average pore diameter increased twofold, and the photoluminescence intensity in the porous silicon increased by 20 times.     

Structural properties of porous silicon/SnO2:F heterostructures

In this work we present structural studies made on SnO₂ deposited on macroporous silicon structures. The porous silicon substrates were prepared by anodization of p-type silicon wafers. The SnO₂ doped layers were synthesized by the sol-gel method from SnCl₄·5H₂O-ethanolic precursor, where the effect of fluorine doping level on structural properties was investigated. The fundamental structural parameters of tin oxide such as the lattice parameter and the crystallite size were studied in correlation with the dopant concentration. In addition, the effect of fluorine incorporation into the structure of tin oxide was analyzed on the basis of theoretical calculations that take into account the structural factor. The results obtained indicate that incorporation of fluorine occurs only at substitutional sites for SnO₂ deposited on porous silicon.