Unpassivated Surface Research Articles

Epitaxial Wafer Equivalent Solar Cells with Overgrown SiO2 Reflector

Crystalline silicon thin film solar cells, based on the epitaxial wafer equivalent, require a reflecting interlayer between substrate and active layers to increase the generated current and reach similar efficiencies as wafer based solar cells. With the epitaxial lateral overgrowth technique, a reflecting dielectric layer can be implemented. In this paper results of solar cells with overgrown, patterned SiO2 films are shown. A beneficial optical effect due to the interlayer and also a reduced effective diffusion length within the epitaxially grown silicon layer are observed. Cells with reflecting interlayer and non-optimized absorber layer thicknesses therefore exhibit lower efficiencies than cells without SiO2. Slightly higher currents are observed with textured front sides. Significantly increased effective diffusion lengths and cell performances can be reached with non-merged active silicon layers. Reasons might be the avoidance of defects generated at merging points and of unpassivated surface areas in voids remaining where two growth fronts merge.

INFLUENCE OF SURFACE RECOMBINATION ON PHOTOVOLTAIC CONVERSION PROCESSES IN SILICON PHOTOSENSITIVE STRUCTURES

The influence of the front near-surface isotypic n+-n-junction on the recombination characteristics of silicon photosensitive structures was experimentally investigated. It was shown that the presence of near-surface isotypic junction in such structures provides low velocity of effective surface recombination even in the case unpassivated surface. It was found that after complete etching of heavily doped n+-layer of near-surface isotypic n+-n-junction the effective surface recombination velocity increases due to the fact that the dominant recombination mechanisms in this case are recombination via surface recombination centres and recombination in the depletion layer of spacecharge region.

Nanopendeo coalescence overgrowth of GaN on etched nanorod array

AbstractThis paper reports the characteristics of gallium nitride layers that have been grown on top of an etched array of nanorods. Nanoimprint lithography has been used to create nanorods on a wafer‐scale that are subsequently passivated to allow selective regrowth from their tips by MOVPE. The first epitaxial growth step favoured lateral growth and the second resulted in a planar coalesced layer of gallium nitride. It was found that smooth planarised overgrowth required careful timing of the growth transition between the two steps and that the crystallographic quality of the overgrown layer was very sensitive to the nanorod template properties, in particular the amount of unpassivated nanorod surface. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Large Surface Dipole Moments in ZnO Nanorods

A self-consistent linear scaling three-dimensional fragment (LS3DF) method is used to study the dipole moments and internal electric fields of large ZnO nanorods. Our ab initio calculations reveal that the ZnO nanorod with unpassivated (1010) side surface has a side surface contribution per Zn-O dimer 10 times larger than the bulk contribution per Zn-O bond. Detailed analysis is used to decompose the total dipole moment into different contributions. It is found that the total dipole moment of a nanorod can be calculated from the different parts with their dielectric screening described by a continuous model. We also show the effect of the dipole moment on the interior electronic structure of the nanorod.

Carrier mobilities in multicrystalline silicon wafers made from UMG‐Si

AbstractWe investigate majority‐ and minority‐carrier mobilities in multicrystalline silicon (mc‐Si) made from upgraded metallurgical‐grade (UMG) feedstock. Since UMG‐Si contains high amounts of both boron and phosphorus, a decrease of the carrier mobility due to increased scattering at ionized impurities is expected. Minority‐carrier mobilities are determined by measuring effective carrier lifetimes τeff on as‐cut wafers, where τeff is limited by carrier diffusion to the unpassivated surfaces. By examining a wafer cut vertically from the mc‐Si ingot, we indeed find a reduction in minority‐carrier mobility μmin with increasing dopant density. In addition, we find a further strong reduction of μmin in the transition region from p ‐ to n ‐type silicon. Similar results are obtained regarding the majority‐carrier mobility μmaj, which is investigated by combining measurements of the resistivity ρ and the equilibrium hole concentration p0 (equilibrium electron concentration n0 in n ‐type material) obtained from electrochemical capacitance‐voltage measurements. Apart from an overall reduction in μmaj compared to values measured in non‐compensated p ‐type mc‐Si, an additional pronounced decrease of the mobility with increasing compensation level is observed. This additional reduction can be explained by reduced screening of the ionized scattering centers. We propose a parameterization of the experimental data based on the Brooks‐Herring equation and find excellent agreement between the two (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mobility in ultrathin SOI MOSFET and pseudo-MOSFET: Impact of the potential at both interfaces

The mobility-thickness dependence in SOI films is clarified. Measurements in fully depleted SOI MOSFETs show that the low-field mobility at the front channel decreases by thinning the Si film or by sweeping the back gate from depletion into accumulation. We demonstrate that this mobility degradation is only apparent, being related to the potential value at the surface facing the channel. This opposite-surface potential induces an intrinsic vertical field which adds to the usual gate-related field. The mobility drop simply indicates a deviation from the low-field condition which cannot be achieved. We propose an updated model for proper extraction and interpretation of the low-field mobility. Pseudo-MOSFET results reveal the existence of a similar additional vertical field in bare SOI wafers, induced by charges present on the unpassivated surface. This intrinsic field increases in thinner films and affects pseudo-MOSFET conduction. The mobility decrease measured in SOI wafers with thinner films reflects the increasing impact of the intrinsic field and does not imply any degradation in quality of film-BOX interface.

Effects of Short-Term DC-Bias-Induced Stress on n-GaN/AlGaN/GaN MOSHEMTs With Liquid-Phase-Deposited $\hbox{Al}_{2}\hbox{O}_{3}$ as a Gate Dielectric

This paper presents a comparative study of the degradation of dc characteristics and drain current collapse under dc-bias stress in passivated metal-oxide-semiconductor high-electron mobility transistor (MOSHEMT), unpassivated HEMT, and passivated HEMT devices. The Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> oxide thin film that is used as a gate dielectric and a passivation layer in MOSHEMTs is prepared by a simple, low-cost, and low-temperature liquid-phase deposition (LPD) technique. All devices are subjected to short-term dc-bias stress to investigate the reliability of the oxide and its passivation effect. In the case of MOSHEMTs and passivated HEMTs, the gradual reduction in drain current is found within 20-h drain-bias stress, which is apparently caused by the hot-electron injection and trapping in the buffer, and a barrier layer that is operated at a high drain voltage. However, faster degradation is found in unpassivated HEMTs, and some devices are permanently damaged due to the degradation of unpassivated surface states. Nonetheless, the current is partially recovered for all devices after gate stress, and no damage to the MOSHEMTs is observed. Therefore, it is believed that the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin film that is prepared through the LPD technique is effective as a gate dielectric and as a surface passivation layer in reducing device degradation during dc-bias stress and in diminishing the current collapse effect in MOSHEMTs.

Formation of Surface Traps on Quantum Dots by Bidentate Chelation and Their Application in Low‐Potential Electrochemiluminescent Biosensing

Bidentate chelation, meso-2,3-dimercaptosuccinic acid (DMSA), was used as a stabilizer for the synthesis of CdTe quantum dots (QDs). The bidentate chelate QDs, characterized with FT-IR, PL, and UV/Vis spectroscopy; element analysis; and high-resolution transmission electron microscope, exhibited surface traps due to the large surface/volume ratio of QD particle and the steric hindrance of the DMSA molecule. The unpassivated surface of the QDs produced a narrower band gap than the core and electrochemiluminescent (ECL) emission at relatively low cathodic potential. In air-saturated pH 7.0 buffer, the QDs immobilized on electrode surface showed an intense ECL emission peak at -0.85 V (vs. Ag/AgCl). H(2)O(2) produced from electrochemical reduction of dissolved oxygen was demonstrated to be the co-reactant, which avoided the need of strong oxidant as the co-reactant and produced a sensitive analytical method for peroxidase-related analytes. Using hydroquinone/horseradish peroxidase/H(2)O(2) as a model system, a new, reagentless, phenolic, ECL biosensor for hydroquinone was constructed, based on the quenching effect of ECL emission of QDs by consumption of co-reactant H(2)O(2). The biosensor showed a linear range of 0.2-10 μM with acceptable stability and reproducibility. This work opens new avenues in the search for new ECL emitters with excellent analytical performance and makes QDs a more attractive alternative in biosensing.

Shape-Enhanced Photocatalytic Activity of Single-Crystalline Anatase TiO2 (101) Nanobelts

Particle size is generally considered to be the primary factor in the design of nanocrystal photocatalysts, because the reduction of particle size increases the number of active sites. However, the benefit from the size reduction can be canceled by a higher electron-hole recombination rate due to the confined space in sphere-shaped nanoparticles. Here we report a mechanistic study on a novel nanobelt structure that overcomes the drawback of sphere-shaped nanoparticles. Single-crystalline anatase TiO(2) nanobelts with two dominant surfaces of (101) facet exhibit enhanced photocatalytic activity over the nanosphere counterparts with an identical crystal phase and similar specific surface area. The ab initio density functional theory (DFT) calculations show that the exposed (101) facet of the nanobelts yields an enhanced reactivity with molecular O(2), facilitating the generation of superoxide radical. Moreover, the nanobelts exhibit a lower electron-hole recombination rate than the nanospheres due to the following three reasons: (i) greater charge mobility in the nanobelts, which is enabled along the longitudinal dimension of the crystals; (ii) fewer localized states near the band edges and in the bandgap due to fewer unpassivated surface states in the nanobelts; and (iii) enhanced charge separation due to trapping of photogenerated electrons by chemisorbed molecular O(2) on the (101) facet. Our results suggest that the photocatalysis efficiency of nanocrystals can be significantly improved by tailoring the shape and the surface structure of nanocrystals, which provides a new concept for rational design and development of high-performance photocatalysts.

Analysis of a Comb-Drive Actuator Taking the Depletion Layer into Consideration

We have analyzed the effect of penetration of the electric field into the semiconductor on the electrical and mechanical characteristics of comb-drive actuators. In this study we found two specific effects due to the depletion layer: (1) the electro-mechanical conversion factor is a function of the depletion layer and becomes smaller than that for the “ideal conductor” assumption, and (2) an additional stiffness appears in the mechanical system. Following the change in the electro-mechanical conversion factor, the resonance peak becomes lower than that of an ideal conductor. These effects are relatively small when the gap between the comb-fingers and the substrate is of the order of a micrometer. However if the gap is 0.1μm, the electro-mechanical conversion factor decreases by about 2.8%. In this analysis, we have taken only the depletion effect into account; however, for a more complete analysis we should also consider the surface traps induced by unpassivated surface bonds.

Self-selective recovery of photoluminescence in amphiphilic polymer encapsulated PbS quantum dots

Self-selected recovery of the photoluminescence (PL) of amphiphilic polymer encapsulated PbS quantum dots (QDs) was observed in water for the first time and possible mechanisms were proposed based on investigations by means of transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction and fluorescence spectroscopy. Water-soluble PbS QDs were synthesized by transferring monodispersed QDs capped with hydrophobic ligands of oleylamine from an organic solvent into water via amphiphilic polymers poly(maleic anhydride-alt-1-octadecene-co-poly(ethylene glycol)). The water transfer process leads to a double size distribution (5.6 ± 0.9 nm and 2.7 ± 0.4 nm), attributed to ligand etching together with Ostwald ripening, as well as the fast decay of PL. The automatic recovery of the PL in PbS QDs stored in water in the dark for 3 months was only observed for the subset of smaller QDs and is largely due to the removal of surface defects with aging, as evidenced by the decreased percentage of unpassivated surface atoms from XPS studies. In contrast, the PL of the subset of larger QDs in the same sample does not self-recover in water and can only be slightly recovered by transferring them into environments with less external quenches. The results strongly suggest that it is the surface defect in the larger QDs themselves, introduced during Ostwald ripening, that is primarily responsible for their non-emitting status or rather low PL intensity under different conditions. The increase of unpassivated Pb atoms in larger PbS QDs after the 3 month aging has been confirmed by XPS, which explains their non-recovery behavior in water. The PL-recovered QD sample in water is very stable and shows comparable photostability to the initial QDs dispersed in an organic phase.

Modeling of negatively charged states at the Ge surface and interfaces

Modeling based on surface charge neutrality predicts that the Ge surface tends to be p-type, irrespective of the bulk conductivity. This is a consequence of the fact that the Ge band gap is small and the charge neutrality level lies low in the gap very close to the valence band, probably determined by low-lying unpassivated surface dangling bond acceptors or other defects. According to the model, the acceptor defects build negative charge, inverting the surface of n-type Ge at no gate bias for low doping concentration (&amp;lt;1016 cm−3) and moderate or high interface state densities (&amp;gt;5×1011 eV−1 cm−2). This is predicted to cause undesired positive threshold voltage shift in the range of 0.2–0.4 V in Ge p-channel field effect transistors. The model also predicts that inversion in n-channel field effect transistors is inhibited, which could be related to the observed poor performance of these devices.

Competition between zero-phonon and phonon-assisted luminescence in colloidal CdSe quantum dots

We determine the low-temperature optical properties of dark-exciton states in CdSe/CdS nanocrystal quantum dots (NQDs). By using resonant laser excitation we distinguish zero-phonon from phonon-assisted photoluminescence. The NQDs show a decreasing zero-phonon intensity with decreasing temperature, resulting in a redshift of the nonresonant photoluminescence. This redshift is undone by application of a magnetic field. Our results show that dark-exciton luminescence originates from the intricate competition of phonon-assisted and zero-phonon transitions, the latter of which are enhanced by dark--bright-exciton mixing due to unpassivated surface states or a magnetic field.

Electrical transport in a two-dimensional electron and hole gas on aSi(001)−(2×1)surface

$\text{Si}(001)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ surface is one of the many two-dimensional systems of scientific and applied interest. Due to its asymmetric dimer reconstruction, transport through this surface can be considered in two distinct directions, i.e., along and perpendicular to the paired dimer rows. We calculate the zero-bias conductance of these surface states under flatband condition and find that conduction along the dimer row direction is significant due to strong orbital hybridization. Additionally, we find that the surface conductance is orders of magnitude higher than the bulk conductance close to the band edges for the unpassivated surface at room temperature. Thus, we propose that the transport through these surface states may be the dominant conduction mechanisms in the recently reported scanning tunneling microscopy of silicon nanomembranes. The zero-bias conductance is also calculated for the weakly interacting dangling-bond wires along and perpendicular to the dimer row direction and similar trends are obtained. The extended H\uckel theory is used for the electronic structure calculations.

Molybdenum–phosphorus compounds based passivator to control corrosion of hot dip galvanized coated rebars exposed in simulated concrete pore solution

Hot dip galvanized (HDG) coating passivated with molybdenum–phosphorus compounds based passivator exhibits high degree of corrosion protection exposed in chloride contaminated simulated concrete pore solution (SPS). The coating develops very stable corrosion products and its corrosion potential gets greatly ennobled due to the formation of a complex passive layer. This composite layer has been identified as hydrozincite (HZ) embedded with several other compounds. The studies reveal that the presence of passive films on HDG makes the HZ layer more impervious and less soluble in the test electrolyte. The layer also helps in faster development of hydrozincite film on HDG surface. A faster healing of defects in hydrozincite layer takes place for passivated than the unpassivated surface. Electrochemical impedance spectroscopy (EIS), Raman Spectroscopy, Scanning Electron Microscopy (SEM), energy dispersive X-ray analysis (EDXA), and X-ray diffraction (XRD) studies etc. have been performed to study the kinetics and understand the mechanism of formation of hydrozincite at the corroding interface. These studies are expected to provide a new area in development of lesser polluting protective passive layer on galvanized coatings.

Magnetic coupling properties of Mn-doped ZnO nanowires: First-principles calculations

Based on the density functional theory, we study the magnetic coupling properties of Mn-doped ZnO nanowires. For the nanowires with passivated surfaces, the antiferromagnetic state is found and the Mn atoms have a clustering tendency. When the distance between two Mn atoms is large, the system energetically favors the paramagnetic or spin-glass state. For the nanowires with unpassivated surfaces, the ferromagnetic (FM) coupling states appear between the two nearest Mn atoms, and the zinc vacancies can further stabilize the FM states between them. The electrons with enough concentration possibly mediate the FM coupling due to the negative exchange splitting of conduction band minimum induced by the s-d coupling, which could be useful in nanomaterial design for spintronics.

From Cd-Rich to Se-Rich − the Manipulation of CdSe Nanocrystal Surface Stoichiometry

We report a protocol for manipulating the surface composition of CdSe nanocrystals. By combining the successive ion layer adhesion and reaction (SILAR) method developed by Li et al. J. Am. Chem. Soc. 2003, 125, 12567 with a phosphine-free selenium precursor, the surface stoichiometry of CdSe can be tunably altered from Cd- to Se-rich. By changing the overall surface stoichiometry, we demonstrate ligand binding to specific surface sites. Tertiary phosphines produce a dramatic enhancement in photoluminescence quantum yield of CdSe particles with Se-rich surfaces but have little effect on Cd-rich surfaces. Unpassivated selenium surface sites are also shown to be a cause of the photobrightening behavior of CdSe nanocrystals.

Soft nitridation of GaAs(100) by hydrazine sulfide solutions: Effect on surface recombination and surface barrier

The effect of nitridation of GaAs(100) by hydrazine sulfide solutions on the surface recombination velocity and surface barrier has been studied using photoluminescence and photoreflectance spectroscopies. Nitridation produces a decrease of surface recombination velocity by a factor of 26. After three years of air exposure, the recombination velocity is still smaller than for the naturally oxidized surface by a factor of 11. The observed effect is caused by a continuous nitride monolayer bonded with the GaAs substrate. The surface Fermi level is still pinned near midgap, which is attributed to residual unpassivated surface defects.

Surface morphological evolution on single crystal films by strong anisotropic drift diffusion under capillary and electromigration forces

The morphological evolution of voids at unpassivated surfaces and the sidewalls of single crystal metallic films is investigated via computer simulations by using a mathematical model based on fundamental postulates of irreversible thermodynamics. The effect of drift-diffusion anisotropy on the development of surface morphological scenarios is explored under the action of electromigration (EM) and capillary forces, utilizing numerous combinations of the surface texture and the direction of the applied electric field. Analytical expressions for the interconnect catastrophic failure time due to the EM-induced transgranular wedge-shaped voids, the propagation velocity of surface solitary waves, and the incubation time of the regenerative oscillatory surface waves are deduced under the severe instability regimes, by inverse normalization procedures applied to the outputs of the extensive computer simulation experiments.

Passivation of the surface of rear contact solar cells by porous silicon

In this paper we analyse the passivation of the front surface of p-Si interdigitated rear contacts solar cell (IBC) by a thin porous silicon (PS) layer. Effectively, an efficiency improvement of 87% in relative was observed after porous silicon layer formation on the front surface of the IBC cell. The origin of surface passivation by the PS layer was studied by Laser Beam Induced Current (LBIC) method. The front surface of rear contacts cell with thin porous silicon layer was scanned by a modulated red laser beam in presence of a permanent light with different wavelengths and intensities. It was shown that without permanent illumination, the photocurrent of the cell with PS layer is very low, even lower than for a cell with unpassivated surface. However with short permanent wavelength illumination a strong increase of photocurrent was observed (8–10 times!). The light-dependent porous silicon passivation phenomenon is explained by a significant negative charge accumulation at the PS/p-Si interface traps under illumination. This leads to the formation of a hi-low (p +/p) junction at the front surface of the cell and to the reduction of the front surface recombination rate, like in Front Surface Field Solar Cell.