Surface Doping Concentration Research Articles

Very low surface recombination velocity of boron doped emitter passivated with plasma-enhanced chemical-vapor-deposited AlOx layers

In this work, we present a systematic study of the surface recombination velocity of boron-diffused Si wafer passivated with plasma-enhanced chemical-vapor-deposited (PECVD) AlOx layers. Saturation current densities in the range of 5.2–38fAcm−2 (at 300K) were achieved on planar surfaces. In particular, we present an industrially relevant boron emitter, allowing for about 700mV open circuit voltage on textured surfaces, after a firing process. This high passivation quality could be achieved using AlOx passivation layers deposited by PECVD. The passivation quality is found to be equivalent to AlOx layers deposited by plasma-assisted atomic-layer-deposition. A wide range of surface doping concentration (2.5–70×1018cm−3) was investigated. The emitters used here allow a high resolution in the determination of the surface recombination velocity upper limit. Our simulations, based on Fermi-Dirac statistics, indicate that only very shallow emitters can be used to further increase the resolution on the determination of the surface recombination velocity.

First-principles theoretical analysis of transition-metal doping of ZnSe quantum dots

We present a systematic analysis of the underlying mechanism of transition-metal doping in ZnSe nanocrystals, using first-principles density functional theory calculations. Our analysis focuses on the adsorption and surface segregation of Mn dopants on ZnSe nanocrystal surface facets. We find that the chemical potentials of the growth precursor species determine the surface structure and morphology of the nanocrystals. We report binding energies for Mn adsorption onto ZnSe surfaces and find that all the anion-rich surfaces contribute toward dopant adsorption onto ZnSe nanocrystal surface facets. Beyond a critical value of dopant surface coverage, these adsorbed dopants may induce structural transitions in low-Miller-index surface facets, resulting in morphological transitions of the ZnSe nanocrystals. In addition, the dopant binding-energy dependence on the dopant surface concentration explains the doping difficulties during nanocrystal growth. Finally, we report surface segregation energy profiles for Mn dopant segregation on low-Miller-index ZnSe nanocrystal surface facets. We find that, under conditions that render ZnSe(001)-(2 × 1) as the dominant dopable surface of ZnSe nanocrystals, Mn dopants do not have a tendency to segregate on this surface; this guarantees that the dopants remain incorporated into the core regions of the nanocrystal instead of escaping to the surface.

Dopant depletion in the near surface region of thermally prepared silicon (100) in UHV

Degenerately doped (arsenic) n-type hydrogen terminated silicon (100) samples were prepared using various heat treatments for ultrahigh vacuum scanning tunneling microscopy (STM) and spectroscopy (STS) analysis. Samples heat treated to 1050 °C were found to have a consistent level of doping throughout the bulk and near surface regions. STS revealed tunneling through dopant states consistent with degenerately doped samples. SIMS profiling and HREELS measurements confirmed dopant and carrier concentrations, respectively. Samples heated to 1250 °C were found to have a reduced concentration of dopants in the near surface region. STS measurements showed shifted I/V curves and the loss of tunneling through dopant states in the band gap, indicating reduced dopant concentrations. Observations were confirmed by SIMS and HREELS where depleted dopants and reduced carrier concentrations were measured. The effect of the varying surface dopant concentrations on the STM imaging characteristics of dangling bonds on hydrogen terminated surfaces was also investigated. Understanding the effect of thermal processing on near surface dopant atom concentrations will permit better control over equilibrium charge occupation and charging characteristics of dangling bond midgap states on H:silicon.

A novel method for crystalline silicon solar cells with low contact resistance and antireflection coating by an oxidized Mg layer

One of the key issues in the solar industry is lowering dopant concentration of emitter for high-efficiency crystalline solar cells. However, it is well known that a low surface concentration of dopants results in poor contact formation between the front Ag electrode and the n-layer of Si. In this paper, an evaporated Mg layer is used to reduce series resistance of c-Si solar cells. A layer of Mg metal is deposited on a lightly doped n-type Si emitter by evaporation. Ag electrode is screen printed to collect the generated electrons. Small work function difference between Mg and n-type silicon reduces the contact resistance. During a co-firing process, Mg is oxidized, and the oxidized layer serves as an antireflection layer. The measurement of an Ag/Mg/n-Si solar cell shows that Voc, Jsc, FF, and efficiency are 602 mV, 36.9 mA/cm2, 80.1%, and 17.75%, respectively. It can be applied to the manufacturing of low-cost, simple, and high-efficiency solar cells.

Printed contact on emitter with low dopant surface concentration

Pastes and inks to contact silicon solar cell emitters have been further developed since decades and considerable improvements have been achieved. According to different R&D roadmaps for silicon solar cells emitter sheet resistances will further increase and the surface dopant concentration will decrease as narrower contact lines support such a development. This leads to new requirements of contact pastes. In this paper an improved contact ink based on silver will be presented which is able to contact phosphorous emitters with low surface dopant concentrations of ND = 8á1018 cm-3.

Theoretical Energy Distributions of Electrons from a Large Exponential-Doping GaAs Photocathode

Theoretical calculation indicates that the large exponential-doping GaAs photocathodes have a much narrower electron energy distribution than traditional GaAs NEA cathodes, and the excellent performance attributes to the special structure characters of the band-bending region and lower negative electron affinity of the new-type GaAs photocathodes. The effects of surface doping concentration and work function on the energy distribution are discussed in details, and the FWHM of the energy distribution is less than 100meV. The simulation results indicate that the large exponential-doping mode further improves the features of the electron energy spreads for GaAs photocathodes, which may meet the further demand of next generation of electron guns.

Formation of lateral thin-film 700-V insulated-gate bipolar transistors by using retrograde p-well double implantation scheme

Formation of lateral thin-film 700-V insulated-gate bipolar transistors (IGBTs) on Si-semiconductor-on-insulator (Si-SOI) substrates by using retrograde p-well double implantation scheme has been proposed. With a low thermal budget for p-well formation, retrograde high-energy boron implantation is employed to cause a high dopant concentration in the bulk region, and additional low-energy boron implantation is carried out for alleviating the surface punch-through of MOSFET (metal-oxide-semiconductor-field-effect-transistor) structure. As compared to the conventional p-well formation process, the retrograde double implantation scheme can lead to better latchup immunity, due to a considerably higher dopant concentration and thus lower well resistance in the p-well bulk region. In addition, owing to a lower surface dopant concentration, the retrograde double implantation scheme can lead to better on-state characteristics than the conventional process, while achieving comparable blocking voltage.

Mg and Si dopant incorporation and segregation in GaN

AbstractThe surface segregation of Mg and Si dopant species and their atomic incorporation sites in GaN films grown by metal‐organic vapour phase epitaxy (MOVPE) on sapphire (0001) substrates have been analysed by X‐ray photoemission spectro‐microscopy and X‐ray standing waves (XSW). As revealed by spectro‐microscopy, both Mg and Si tend to segregate to the surface. In case of Mg, an enhanced surface dopant concentration is found even after sputter‐removal of several tens of nanometres, which confirms a segregation mechanism proposed earlier [S. Figge et al., Appl. Phys. Lett. 81, 4748 (2002)]. Si doping has been found to result in the formation of facetted grooves in the GaN films. The surface silicon concentration at the facets is determined to be about 2.5 times higher as compared to the planar (0001) surface by micro‐spectroscopy. XSW results show that with increasing Mg dopant concentration, non‐substitutional lattice sites are progressively occupied. The results can quantitatively be explained by Mg atoms incorporated in an anti‐bixbyite‐like structure and is related to inversion domain boundaries. For Si doping, no evidence is found for the occupation of non‐substitutional sites. However, a decrease in crystal quality is observed with increasing Si concentration.magnified image X‐ray photoemission micrograph of a Si doped GaN film showing a facetted groove at the lower left corner of the image. The local spectra (see inset) reveal Si enrichment at the facets.

Photoemission of graded-doping GaN photocathode**Project supported by the National Natural Science Foundation of China (Grant No. 60871012) and the Research Fund of Nanjing University of Science and Technology (Grant No. 2010ZYTS032).

We study the photoemission process of graded-doping GaN photocathode and find that the built-in electric fields can increase the escape probability and the effective diffusion length of photo-generated electrons, which results in the enhancement of quantum efficiency. The intervalley scattering mechanism and the lattice scattering mechanism in high electric fields are also investigated. To prevent negative differential mobility from appearing, the surface doping concentration needs to be optimized, and it is calculated to be 3.19×1017 cm−3. The graded-doping GaN photocathode with higher performance can be realized by further optimizing the doping profile.

Optoelectronic characteristics of Cu(In,Ga)(S,Se) 2 thin film solar cells obtained from varied chalcogenization processes

We investigated industrially produced chalcopyrite solar cells based on the absorber modifications Cu(In,Ga)Se 2 and Cu(In,Ga)(S,Se) 2 in order to study the nature of the experimentally verified efficiency improvement, mainly caused by an increased open circuit voltage. We show that the introduction of sulfur during the absorber formation via rapid thermal processing leads to a substantial lowering of the surface doping concentration and widening of the space charge region (SCR). Temperature dependent diode analysis revealed a reduction of the SCR recombination in (Se,S) devices which would lead to a larger splitting of quasi-Fermi levels and hence to an increased open circuit voltage as compared to neat Cu(In,Ga)Se 2 devices.

Modelling carrier recombination in highly phosphorus-doped industrial emitters

Abstract One important parameter for modelling emitter recombination is the surface recombination velocity (SRV), which strongly depends on the surface doping concentration and the applied surface passivation. However, for highly phosphorus-doped surfaces with concentrations in excess of 2×1020 cm-3, where not all of the dopant is electrically activated, data is hardly available in the literature. Moreover, the bulk carrier lifetime in such supersaturated near surface regions is unknown. We use an analytical model to describe silicon-nitride-passivated phosphorus-diffused emitters. The model shows excellent agreement with a recently presented numerical solver, deviating less than 1%. In both cases we apply a Fermi-Dirac statistics correction and account for band gap narrowing to calculate the intrinsic carrier density. Our results from measurements of the emitter dark saturation current density indicate that either the SRV or the local carrier lifetime in the supersaturated region are strongly affected by the doping concentration, even if it exceeds the dopant activation limit by far. Assuming only Auger recombination in the supersaturated region, we derive an upper limit for the SRV that depends on the chemical phosphorus surface concentration.

High efficient n‐type back‐junction back‐contact silicon solar cells with screen‐printed Al‐alloyed emitter and effective emitter passivation study

AbstractN‐type back‐junction back‐contact (BJ–BC) silicon solar cells have been successfully introduced in industry by Sunpower with efficiencies up to 23.4% and are being investigated by several research groups. For such n‐type bulk Si solar cells, the formation of the p+ emitter is still an issue. A traditional method is boron diffusion, which needs high temperature processes to form the emitter and to remove the silicate glass boron skin. In recent years, people have shown excellent results on n‐type front contact rear junction cells with a screen‐printed Al‐alloyed emitter. In this work, we demonstrate the use of such emitters on n‐type BJ–BC silicon solar cells. Different pitch sizes and emitter fractions have been studied. Clear trends of the short‐circuit current densities have been observed. Light beam induced current technique is used to investigate this phenomenon. Different front surface field (FSF) profiles were applied to our cells. The shallower FSF with a lower surface doping concentration results in lower front surface recombination, which results in the best cell performance. Efficiencies of 19.1% under 1 Sun on BJ–BC solar cells with an aperture area of 2 cm × 2 cm have been achieved on n‐type FZ and CZ silicon wafers. Good short‐circuit current densities and fill factors have been obtained. Compared to the boron‐diffused emitter BJ–BC solar cells, relatively low open‐circuit voltages were obtained. Better quality emitter with more effective cleaning process steps and passivation layers have been developed, to result in a further increase in cell efficiency in the future work. Copyright © 2010 John Wiley & Sons, Ltd.

Silicon PV devices based on a single step for doping, anti-reflection and surface passivation

A simplified process for photovoltaic device fabrication is explored. The simplified process incorporates a single step wherein a thin film of phosphorus doped spin on dopant (SOD) serves as dopant source, anti-reflection coating (ARC) and a surface passivation layer. Detailed fabrication and photovoltaic device characterization results are presented. Device results for solar cells fabricated on low minority carrier lifetime Cz-substrates and high minority carrier lifetime Fz (polished and chemically textured substrates) are presented. Using this simplified process, photovoltaic devices with conversion efficiency of over 15% on c-Si wafers have been demonstrated. Further improvements in efficiency are possible by reduction in series resistance, control of surface dopant concentration and a rear passivated electrode configuration.

Diameter-dependent dopant location in silicon and germanium nanowires

We report studies defining the diameter-dependent location of electrically active dopants in silicon (Si) and germanium (Ge) nanowires (NWs) prepared by nanocluster catalyzed vapor-liquid-solid (VLS) growth without measurable competing homogeneous decomposition and surface overcoating. The location of active dopants was assessed from electrical transport measurements before and after removal of controlled thicknesses of material from NW surfaces by low-temperature chemical oxidation and etching. These measurements show a well-defined transition from bulk-like to surface doping as the diameter is decreased <22-25 nm for n- and p-type Si NWs, although the surface dopant concentration is also enriched in the larger diameter Si NWs. Similar diameter-dependent results were also observed for n-type Ge NWs, suggesting that surface dopant segregation may be general for small diameter NWs synthesized by the VLS approach. Natural surface doping of small diameter semiconductor NWs is distinct from many top-down fabricated NWs, explains enhanced transport properties of these NWs and could yield robust properties in ultrasmall devices often dominated by random dopant fluctuations.

Formation of n-channel polycrystalline-Si thin-film transistors by using retrograde channel scheme with double implantation

Excellent n-channel poly-Si thin-film transistors (poly-Si TFTs) have been formed by using retrograde channel scheme with channel doping implantation and extra counter-doping implantation. As compared to the conventional sample with undoped channel layer, a much smaller leakage current can be achieved by boron-doping the poly-Si channel layer, due to a significantly reduced depletion region. However, the on-state characteristics are degraded. A retrograde channel scheme, implemented by further phosphorus counter-doping the surface of the boron-doped channel layer, is proposed for lowering the channel surface doping concentration without changing the bulk channel doping concentration. By using the retrograde channel scheme, an off-state leakage current as low as that for the normal channel-doping scheme may be achieved, while yielding excellent on-state I– V transfer characteristics.

Phosphorus emitter and metal - grid optimization for homogeneous (n+p) and double-diffused (n++n+p) emitter silicon solar cells

This work focuses on studying two types of structure: homogeneous and double-diffused emitter silicon solar cells. The emitter collection efficiencies and the recombination current densities were studied for a wide range of surface dopant concentrations and thicknesses. The frontal metal-grid was optimized for each emitter, considering the dependence on the metal-semiconductor contact resistivity and on the emitter sheet resistance. The best efficiency for n+p structures, η≈ 25.5%, is found for emitters with thicknesses between (0.5-3) µm and surface doping concentrations in the range 2 x 10(19) cm-3- 4 x 10(18) cm-3; while the n++n+p structure a maximum efficiency of η≈ 26.0% was identified for an even wider range of emitter profiles.

Comprehensive analysis of advanced solar cell contacts consisting of printed fine‐line seed layers thickened by silver plating

AbstractThis work presents a detailed analysis of a new two‐layer process to contact industrial solar cells. However, most of the results seem to be transferable to standard screen print paste contacts. The seed layer was created by a pad or screen printer and thickened by light‐induced plating (LIP) of silver. These contact structures were investigated microscopically to gain a better understanding of the observed electrical parameters. A review of the present microscopic contact formation model for flat surfaces is presented. This model was extended and applied to surfaces textured with random pyramids. This analysis has revealed two new types of silver crystallites which can be described by a crystallographic model. The dependence of the silver crystallite density on the surface doping concentration was investigated. Next, the dependence of the contact resistance on the width of the seed layer was measured showing that the contact resistivity increases with a reduction of the seed layer width. These results have been further approved by an analysis of SEM images of wet‐chemically etched contacts examining the density of crystallites and the fraction of removed SiNx layer. Contact resistance RC measurements before and after LIP of silver showed surprisingly a positive influence of the plating process on RC. A detailed microscopical analysis revealed four new possible current flow paths due to the LIP of a conventional contact or a seed layer. The results led to an extension of the existing model for a screen‐printed contact. Copyright © 2008 John Wiley &amp; Sons, Ltd.

First-principles theoretical analysis of dopant adsorption and diffusion on surfaces of ZnSe nanocrystals

We report results from a detailed analysis of dopant adsorption and diffusion on surfaces of ZnSe nanocrystals and discuss their implications for dopant incorporation into the growing nanocrystals. Using first-principles density functional theory calculations, we find that the binding energy for Mn dopant adsorption onto various surface sites of the dominant dopable surface, ZnSe(0 0 1)-(2 × 1), increases with increasing dopant surface concentration. Due to low activation barriers, dopant atoms can migrate fast along the Se dimer rows without substantial surface relaxation, while their diffusion across the dimer rows is governed by a high-barrier pathway.

Study on the Effect of RTA Ambient to Shallow N+/P Junction Formation using PH3 Plasma Doping

ABSTRACTPlasma doping (PLAD) process utilizing PH3 plasma to fabricate n-type junction with supplied bias of −1 kV and doping time of 60 sec under the room temperature is presented. The RTA process is performed at 900 °C for 10 sec. A defect-free surface is corroborated by TEM and DXRD analyses, and examined SIMS profiles reveal that shallow n+ junctions are formed with surface doping concentration of 1021atoms/cm3. The junction depth increases in proportion to the O2 gas flow when the N2 flow is fixed during the RTA process, resulting in a decreased sheet resistance. Measured doping profiles and the sheet resistance confirm that the n+ junction depth less than 52 nm and minimum sheet resistance of 313 Ω/□ are feasible.

Phosphorus Double-Diffused Emitter Silicon Solar Cells: Emitter and Metal-Grid Theoretical Optimizations on the Contour Plot View

Despite many theoretical optimizations have been presented, a reoptimization aiming to completely understand Gaussian profile and Phosphorus emitters was performed, considering the currently adopted parameters and light trapping effect. This work is focused on studying doublediffused (n++n+) emitter solar cells (DD), keeping constant a low sheet resistance under the metal-contacted region, 13W/square. The metal-grid optimization was done taking into account the dependence on the metalsemiconductor contact resistance and on each studied emitter sheet resistance. The optimum emitters presented high quality: high collection efficiency and low total emitter recombination (<12 fA/cm2). Complete structures (n++n+p) were also studied. The best efficiencies, h=25.7 - 26.0% were found for emitter surface doping concentration and thickness ranges, Ns=1x1018cm-3 - 1x1019cm-3 and 0.5mm<We<10mm, respectively.