Effective Surface Recombination Velocity Seff Research Articles

Comparison of three titanium-precursors for atomic-layer-deposited TiO2 for passivating contacts on silicon

Atomic layer-deposited (ALD) TiO2 thin films on silicon were deposited using titanium tetrachloride (TiCl4), titanium tetraisopropoxide (TTIP), and tetrakis(dimethylamino)titanium (TDMAT) together with water vapor as the oxidant at temperatures ranging between 75 and 250 °C. The Si surface passivation quality of as-deposited and isothermally annealed samples was compared using photoconductance lifetime measurements in order to calculate their effective surface recombination velocities Seff. A low Seff of 3.9 cm/s (J0s=24fA/cm2) is achieved for as-deposited TiCl4-TiO2 at 75 °C when a chemically grown (i.e., from RCA cleaning) SiOx interface layer is present. Depositing TTIP-TiO2 at 200 °C on a chemically grown SiOx interface layer yields equivalent Seff values; however, in this case, TTIP-TiO2 requires a 5–15 min postdeposition forming gas anneal at 250 °C. In contrast, TDMAT-TiO2 was not found to provide a similar level of passivation with/without a chemically grown SiOx interface layer and postdeposition anneal. Modeling of the effective lifetime curves was used to determine the magnitude of the effective charge densities Qf in the TiO2 films. In all cases, Qf was found to be of the order of ∼1011 q cm−2, meaning field-effect passivation arising from ALD TiO2 is relatively weak. By comparing the material properties of the various TiO2 films using ellipsometry, photothermal deflection spectroscopy, Raman spectroscopy, elastic recoil detection analysis, x-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, we find experimental support for the role of Cl (in conjunction with hydrogen) playing a beneficial role in passivating dangling bond defects at the Si surface. It is concluded that low deposition temperature TiCl4 processes are advantageous, by providing the lowest Seff without any postanneal and a comparatively high growth per cycle (GPC).

Demonstration of Excellent Crystalline Silicon Surface Passivation (S < 1.27 cm s−1) by High‐Rate DC‐Sputtered Hydrogenated Amorphous Silicon

The surface passivation quality of intrinsic hydrogenated amorphous silicon (i‐a‐Si:H) layers deposited by constant DC power‐facing target sputtering (FTS) technology is investigated. An n‐type crystalline silicon wafer passivated with a 42 nm‐thick i‐a‐Si:H shows an effective carrier lifetime of 11 ms, which corresponds to a low upper limit effective surface recombination velocity Seff of 1.27 cm s−1. An ultrathin 5 nm‐thick i‐a‐Si:H film achieves an implied open‐circuit voltage (iVoc) of 730 mV at a high deposition rate of 31 nm min−1. It is found that constant DC‐FTS does not cause sputtering damage under high deposition rate conditions. This work demonstrates that i‐a‐Si:H deposited by constant DC‐FTS can achieve the same passivation quality as i‐a‐Si:H deposited by plasma‐enhanced chemical vapor deposition or catalytic chemical vapor deposition, paving the way for the fabrication of silicon heterojunction solar cells without the use of explosive and toxic gases.

Hole‐Selective Ultrathin Al‐Doped SiOx Passivation Layer Formed by Immersing in Aluminum Nitrate Aqueous Solution

Herein, ultrathin Al‐doped SiOx layer formed by immersing in Al(NO3)3 aqueous solution is introduced. This layer enables high‐level surface passivation with a maximum effective surface recombination velocity (Seff,max) < 16 cm s−1 at an excess carrier density (Δn) of 1 × 1015 cm−3 for 2.5 Ω cm n‐type c‐Si without any other processes such as film deposition in a vacuum chamber, high‐temperature annealing, and hydrogenation. This passivation effect presumably comes from negative fixed charges localized in Al‐doped SiOx layers, forming inversion layers due to upward Si band bending at the SiOx/Si interface. In addition, the Al‐doped SiOx layers facilitate accumulated holes at the interface to tunnel through the layers even though the applied bias is extremely low. The high tunnel current density of holes accumulated in the inversion layers implies that these ultrathin Al‐doped SiOx layers are expected to be favorably implemented for high‐efficiency passivating‐contact c‐Si solar cells.

Passivation and Electrical Properties of Alumina Layers Deposited by Atomic‐Layer Deposition with Different Precursors

The electrical and passivation properties of alumina layers deposited by the atomic layer deposition (ALD) technique with different precursors at different temperatures are analyzed. Herein, a significantly larger density of the negative fixed charge in alumina layers deposited with ozone in comparison to those prepared with water is observed. A part of this negative charge can be successfully removed by the annealing of the samples already at around 250 °C and this negative charge is tentatively ascribed to oxygen interstitials. The effective surface recombination velocity (s eff) is below 2 cm s−1 in all as-deposited alumina layers independent of the deposition conditions and it is significantly smaller in layers prepared with water. The higher s eff in films prepared with ozone can be explained by the higher density of the interface states in these samples as obtained from impedance measurements performed at different ac frequencies and at different biases. The conduction mechanisms in alumina films are also investigated and their difference is discussed.

Effect of the thickness of amorphous silicon carbide interlayer on the passivation of c-Ge surface by aluminium oxide films

Surface passivation is of paramount importance in photovoltaic devices based on high-quality crystalline absorbers such as crystalline germanium (c-Ge). In this work, we report on the surface passivation of p-type c-Ge by an amorphous silicon carbide/aluminium oxide film stack (a-SiCx/Al2O3). In particular, we focus on the impact of the thin (0, 1, 2, and 4 nm) a-SiCx interlayer which is used to provide chemical passivation while the Al2O3 is responsible for the field-effect passivation due to its high negative fixed charge density. The measured effective surface recombination velocity (Seff) values show that the introduction of the a-SiCx interlayer improves surface passivation, but the thinner the a-SiCx film the better. High frequency capacitance-voltage characterization leads to the determination of the fixed charge density (Qf) which indicates that the a-SiCx is shielding the negative charge located at the first nanometers of the Al2O3 film, as suggested by chemical characterization of the interface. As a consequence, the best result is Seff = 18 cm/s for the case of 1 nm of a-SiCx.

Revised parametrization of the recombination velocity at SiO2/SiNx-passivated phosphorus-diffused surfaces

We investigate the effective surface recombination velocity Seff of alkaline textured, Phosphorus-diffused and thermal SiO2/SiNx passivated surfaces with an emphasis on the impact of the thermal oxidation temperature. The application of a recent calibration procedure for the carrier lifetime measurements enables a precise determination of the dark saturation current density. The experimental results include 25 diffusion/oxidation process combinations that cover a wide range of final surface concentration levels Ns between 3⋅1020 cm−3 and 1⋅1019 cm−3, using oxidation temperatures Tox from 650 °C to 900 °C. This yields a data set that enables a revision of the commonly applied parameterization of the effective surface recombination velocity of this passivation scheme using numerical simulation. In addition, the impact of fixed surface charges is modeled for separating field effect and chemical passivation properties. Also, the role of the oxidation temperature on the passivation quality is investigated.

Effective suppression of nano-textured black silicon surface recombination channels through sputtered aluminum oxide: A comparison study with ALD grown films.

This accepted manuscript was withdrawn on 2 November 2020 on the author's request.In this work, the effective surface passivation of nano-textured silicon (NT-Si) using sputtered aluminum oxide (AlOx) films has been demonstrated and established a comparison with atomic layer deposition (ALD) grown AlOx films. Silver assisted wet chemical etching (SAWCE) is used to prepare optically black NT-Si. Sub-wavelength size features of NT-Si surface optimized to control the surface defect states which act as trap centres for charge carriers and proportional to the surface area. Sputtered and ALD grown AlOx films used to passivate the NT-Si surfaces are analyzed primarily in context of available trap centres. The AlOx/NT-Si interface is quantified by the density of interface trap states (Dit) and the total fixed charge (Qf) that correlated with effective surface recombination velocity (Seff) of carriers. The estimated Dit and Qf of sputtered AlOx films are ~4 x 1011 eV-1cm-2 and ~7 x 1012 cm-2, respectively which results in the Seff of ~37 cm/s due to the field-effect passivation. While the Dit and Qf of ALD grown AlOx films are 1.1 x 1011 eV-1cm-2 and ~7.8 x 1012 cm-2 results in the Seff of ~22 cm/s owing to both the chemical and field-effect passivation.

Influence of layer thickness on passivation properties in SiOx/Al2O3 stacks

SiOx/Al2O3 passivation stacks have shown the potential to effectively passivate both n- and p-type surfaces as the charge density may be controlled via the SiOx layer thickness. Here, the passivation quality of the SiOx/Al2O3 stacks is investigated as the function of SiOx layer thickness and Al2O3 layer thickness. The SiOx/Al2O3 passivation stacks were deposited on n- and p-type Si using plasma-enhanced atomic layer deposition and the effective surface recombination velocity (Seff), interface defect density (Dit), and effective fixed charge density (Qeff) were measured. The level of passivation was highest when the surface was passivated with only Al2O3 (1.1 cm/s and 4.8 cm/s for n-type and p-type, respectively). Seff increased with increasing SiOx thickness, reached a maximum value, and then decreased for thicker SiOx layers. The low Seff values correlate with accumulation or inversion while maximum Seff appears at the depletion/edge of inversion. Increasing the Al2O3 thickness also lowered Seff, both when used on its own and in SiOx/Al2O3 stacks but did not affect where the maximum Seff was located. The defect density was low (Dit<1011eV−1cm−2) irrespective of SiOx thickness and the n-type wafers showed an overall lower defect density than p-type wafers. The SiOx layer lowered the defect density for n-type wafers and Dit<1010eV−1cm−2 could be observed. The SiOx layer thickness greatly affected the effective fixed charge density (Qeff) which became more positive as the thickness increased. The doping type had an influence on the resulting Qeff with the n-type Si becoming more negatively charged than p-type.

Enhanced field effect passivation of c-Si surface via introduction of trap centers: Case of hafnium and aluminium oxide bilayer films deposited by thermal ALD

Field effect passivation of crystalline silicon surface is controlled by tailoring the built-in charge in a bilayer dielectric system consisting of hafnium oxide (HfO2) and aluminium oxide (Al2O3). The effective surface recombination velocity (Seff, max) ~ 10 cm/s is achieved at intermediate bulk injection levels with thermal ALD deposited HfO2 (top)/Al2O3 (bottom) bilayer system on n-type silicon with individual layer thickness between 3 and 7 nm. The best realized Seff,max value is lower by a factor of ~ 2.5 with respect to the single Al2O3 layer of similar thickness deposited under the same experimental conditions. The improved field effect passivation is quantified by enhanced effective charge density for the bilayer system in comparison with the corresponding value for single Al2O3 layer. The introduction of extra trap centers at the interface of the bilayer system is primarily responsible for the enriched field effect passivation, however, the sequence of the dielectric layers is important.

Effect of the Ti/Si ratio of spin coating solutions on surface passivation of crystalline silicon by TiOx–SiOx composite films

Passivation films or antireflection coatings are generally prepared using costly vacuum or high-temperature processes. Thus, we report the preparation of TiOx–SiOx composite films by novel spin coatable solutions for the synthesis of low-cost passivation coating materials. The desired films were formed by varying the mixing ratios of TiOx and SiOx, and the resulting films exhibited excellent surface passivation properties. For the p-type wafer, an optimal effective surface recombination velocity (Seff) of 93 cm/s was achieved at , while a surface recombination current density (J0s) of 195 fA/cm2 was obtained. In contrast, for the n-type wafer, an Seff of 27 cm/s and a J0s of 38 fA/cm2 were achieved at . This excellent surface passivation effect could be attributed to the low interface state density and high positive fixed charge density. Furthermore, the thickness of the interfacial SiOx layer was determined to be important for obtaining the desired surface passivation effect.

Surface passivation of silicon nanowires based metal nano-particle assisted chemical etching for photovoltaic applications

Metal Nano-particle Assisted Chemical Etching (MNpACE) is an extraordinary developed wet etching method for producing uniform semiconductor nanostructure (silicon nanowires) from patterned metallic film on crystalline silicon surface. The metal films facilitate the etching in HF and H2O2 solution and produce silicon nanowires (SiNWs).The creation of different SiNWs morphologies by changing the etching time and its effects on optical and optoelectronic properties was investigated. The combination effect of formed SiNWs and stain etching treatment in acid (HF/HNO3/H2O) solution on the surface morphology of Si wafers as well as on the optical and optoelectronic properties especially a PL response at 640nm are presented. As a results, the effective lifetime (τeff) and surface recombination velocity (Seff) evolution of SiNWs after stain etching treatment showed significant improvements and less than 1% reflectance was achieved over the wavelength range of 400–800nm and more than 36% reduction was observed compared to untreated surface. It has, thus, been demonstrated that all these factors may lead to improved energy efficiency from 8% to nearly 14.2% for a cell with SiNWs treated in acid (HF/HNO3/H2O) solution.

Extremely low surface recombination in 1 Ω cm n‐type monocrystalline silicon

A key requirement in the recent development of highly efficient silicon solar cells is the outstanding passivation of their surfaces. In this work, plasma enhanced chemical vapour deposition of a triple layer dielectric consisting of amorphous silicon, silicon oxide and silicon nitride, charged extrinsically using corona, has been used to demonstrate extremely low surface recombination. Assuming Richter's parametrisation for bulk lifetime, an effective surface recombination velocity Seff = 0.1 cm/s at Δn = 1015 cm–3 has been obtained for planar, float zone, n ‐type, 1 Ω cm silicon. This equates to a saturation current density J0s = 0.3 fA/cm2, and a 1‐sun implied open‐circuit voltage of 738 mV. These surface recombination parameters are among the lowest reported for 1 Ω cm c‐Si. A combination of impedance spectroscopy and corona‐lifetime measurements shows that the outstanding chemical passivation is due to the small hole capture cross section for states at the interface between the Si and a‐Si layer which are hydrogenated during nitride deposition. (© 2016 The Authors. Phys. Status Solidi RRL published by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of PECVD SiNx/SiOyNx–Si interface property on surface passivation of silicon wafer**Project supported by the National High Technology Research and Development Program of China (Grant No. 2015AA050302) and the National Natural Science Foundation of China (Grant No. 61306076).

It is studied in this paper that the electrical characteristics of the interface between SiOyNx/SiNx stack and silicon wafer affect silicon surface passivation. The effects of precursor flow ratio and deposition temperature of the SiOyNx layer on interface parameters, such as interface state density Dit and fixed charge Qf, and the surface passivation quality of silicon are observed. Capacitance–voltage measurements reveal that inserting a thin SiOyNx layer between the SiNx and the silicon wafer can suppress Qf in the film and Dit at the interface. The positive Qf and Dit and a high surface recombination velocity in stacks are observed to increase with the introduced oxygen and minimal hydrogen in the SiOyNx film increasing. Prepared by deposition at a low temperature and a low ratio of N2O/SiH4 flow rate, the SiOyNx/SiNx stacks result in a low effective surface recombination velocity (Seff) of 6 cm/s on a p-type 1 Ω·cm–5 Ω·cm FZ silicon wafer. The positive relationship between Seff and Dit suggests that the saturation of the interface defect is the main passivation mechanism although the field-effect passivation provided by the fixed charges also make a contribution to it.

Long-term Stability of Al2O3 Passivated Black Silicon

In this work we report on the long-term stability of black silicon surfaces passivated with atomic layer deposited (ALD) 20nm thick Al2O3 films on p- and n-type FZ c-Si substrates. The results are directly compared with random pyramid textured counterparts. The effective surface recombination velocity Seff has been measured within a time frame of one year after activation of surface passivation. The results demonstrate that after an initial slight degradation during the first month Seff values stabilize around 45 and 25cm/s on p- and n-type black silicon samples, respectively. These values are enough to guarantee stable high efficiency in interdigitated back-contacted (IBC) c-Si(n) solar cells (> 24.5%) using black silicon nanostructures on the front side. Similar, although weaker, losses are also observed in surface passivation on textured samples covered by Al2O3 with equal thickness, indicating that the origin of the instability might be independent of surface morphology.

SiOyNx/SiNx stack: a promising surface passivation layer for high‐efficiency and potential‐induced degradation resistant mc‐silicon solar cells

AbstractA thin SiOyNx film was inserted below a conventional SiNx antireflection coating used in c‐Si solar cells in order to improve the surface passivation and the solar cell's resistance to potential‐induced degradation (PID). The effect of varying the flow ratio of the N2O and SiH4 precursors and the deposition temperature for the SiOyNx thin film upon material properties were systematically investigated. An excellent surface passivation was obtained on FZ p‐type polished silicon wafers, with the best results obtained with a SiOyNx film deposited at a very low temperature of 130 °C and with an optical refractive index of 1.8. In the SiOyNx/SiNx stack structure, a SiOyNx film with ~6 nm thickness is sufficient to provide excellent surface passivation with an effective surface recombination velocity Seff < 2 cm/s. Furthermore, we applied the optimized SiOyNx/SiNx stack on multicrystalline Si solar cells as a surface passivation and antireflection coating, resulting in a 0.5% absolute average conversion efficiency gain compared with that of reference cells with conventional SiNx coating. Moreover, the cells with the SiOyNx/SiNx stack layers show a significant increase in their resistance to PID. Nearly zero degradation in shunt resistance was obtained after 24 h in a PID test, while a single SiNx‐coated silicon solar cell showed almost 50% degradation after 24 h. Copyright © 2016 John Wiley & Sons, Ltd.

Influence of annealing in H atmosphere on the electrical properties of Al2O3 layers grown on p-type Si by the atomic layer deposition technique

In the present study the electrical properties of 100nm and 400nm alumina films grown by the atomic layer deposition technique on p-type Si before and after a post-deposition annealing at 440°C and after a dc H plasma treatment at different temperatures are investigated. We show that the density of interface states is below 2×1010cm−2 in these samples and this value is significantly lower compared to that reported previously in thinner alumina layers (below 50nm). The effective minority carrier lifetime τg,eff and the effective surface recombination velocity seff in untreated p-type Si samples with 100nm and 400nm aluminum oxide is comparable with those obtained after thermal oxidation of 90nm SiO2. Both, a post-deposition annealing in forming gas (nitrogen/hydrogen) at elevated temperatures and a dc H-plasma treatment at temperatures close to room temperature lead to the introduction of negatively charged defects in alumina films. The results obtained in samples annealed in different atmospheres at different temperatures or subjected to a dc H plasma treatment allow us to correlate these centers with H-related defects. By comparing with theory we tentatively assign them to negatively charged interstitial H atoms.

A method for determining average damage depth of sawn crystalline silicon wafers.

The depth of surface damage (or simply, damage) in crystalline silicon wafers, caused by wire sawing of ingots, is determined by performing a series of minority carrier lifetime (MCLT) measurements. Samples are sequentially etched to remove thin layers from each surface and MCLT is measured after each etch step. The thickness-removed (δt) at which the lifetime reaches a peak value corresponds to the damage depth. This technique also allows the damage to be quantified in terms of effective surface recombination velocity (Seff). To accomplish this, the MCLT data are converted into an Seff vs δt plot, which represents a quantitative distribution of the degree of damage within the surface layer. We describe a wafer preparation procedure to attain reproducible etching and MCLT measurement results. We also describe important characteristics of an etchant used for controllably removing thin layers from the wafer surfaces. Some typical results showing changes in the MCLT vs δt plots for different cutting parameters are given.

Quantifying carrier recombination at grain boundaries in multicrystalline silicon wafers through photoluminescence imaging

We present a method based on steady state photoluminescence (PL) imaging and modelling of the PL intensity profile across a grain boundary (GB) using 2D finite element analysis, to quantify the recombination strength of a GB in terms of the effective surface recombination velocity (Seff). This quantity is a more meaningful and absolute measure of the recombination activity of a GB compared to the commonly used signal contrast, which can strongly depend on other sample parameters, such as the intra-grain bulk lifetime. The method also allows the injection dependence of the Seff of a given GB to be explicitly determined. The method is particularly useful for studying the responses of GBs to different cell processing steps, such as phosphorus gettering and hydrogenation. The method is demonstrated on double-side passivated multicrystalline wafers, both before and after gettering, and single-side passivated wafers with a strongly non-uniform carrier density profile depth-wise. Good agreement is found between the measured PL profile and the simulated PL profile for both cases. We demonstrate that single-side passivated wafers allow more recombination active grain boundaries to be analysed with less unwanted influence from nearby features. The sensitivity limits and other practical constraints of the method are also discussed.

Silicon Surface Passivation by Sputtered Aluminium Oxide: Influence of Annealing Temperature and Ambient Gas

Aluminium oxide (AlOx) films have achieved excellent passivation on crystalline-silicon surfaces. Irrespective of the deposition method, an anneal is required to activate the passivation. In this work, the reaction kinetics of the annealing process for radiofrequency reactively-sputtered AlOx is investigated. The effectiveness of the activation anneal critically depend on the ambient gas. A gas mixture of N2 and H2 is found to be the best annealing ambient for sputtered Al2O3 films, leading to an effective surface recombination velocity Seff of ∼5 cm/s. The experiments indicate that the surface passivation anneal follows an activation energy EA close to 1.1 eV.

On effective surface recombination parameters

This paper examines two effective surface recombination parameters: the effective surface recombination velocity Seff and the surface saturation current density J0s. The dependence of Seff and J0s on surface charge Q, surface dopant concentration Ns, and interface parameters is derived. It is shown that for crystalline silicon at 300 K in low-injection, Seff is independent of Ns only when Q2/Ns < 1900 cm in accumulation and Q2/Ns < 1600 cm in depletion; otherwise Seff increases with Ns. These conditions are rarely satisfied in undiffused wafers but sometimes satisfied in heavily diffused wafers when coated with lowly charged films. Under the same conditions, J0s is independent of Ns when Q2/Ns > 1.5 × 107 cm for accumulation and Q1.85/Ns > 1.5 × 106 cm for inversion. These conditions are commonly satisfied in undiffused wafers but rarely in diffused wafers. We conclude that for undiffused silicon, J0s is superior to the conventional Seff as a metric for quantifying the surface passivation, whereas for diffused silicon, the merit in using J0s or Seff (or neither) depends on the sample. Experimental examples are given that illustrate the merits and flaws of J0s and Seff.