Low Effective Surface Recombination Velocities Research Articles

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.

Passivation of Al2O3 / TiO2 on monocrystalline Si with relatively low reflectance

Al2O3/TiO2 stack layers deposited by the plasma-enhanced atomic layer deposition enhance photoluminescence intensity by reducing effective surface recombination velocities on both n-type and p-type monocrystalline Si. The field effect of negative oxide charges in the dielectrics is responsible for the low effective surface recombination velocity. The dependence of the effective surface recombination velocity on the photoluminescence intensity is investigated by the 2D numerical simulation. The bilayer stacks without texture also reduce the AM1.5-weighted front side reflectance to 11.8%. The field-effect passivation of Al2O3/TiO2 films is further improved by a forming gas annealing due to the additional increase of the negative oxide charge density.

Atomic Layer Deposition (ALD) grown thin films for ultra-fine pitch pixel detectors

In this report we cover two special applications of Atomic Layer Deposition (ALD) thin films to solve these challenges of the very small size pixel detectors. First, we propose to passivate the p-type pixel detector with ALD grown Al2O3 field insulator with a negative oxide charge instead of using the commonly adopted p-stop or p-spray technologies with SiO2, and second, to use plasma-enhanced ALD grown titanium nitride (TiN) bias resistors instead of the punch through biasing structures. Surface passivation properties of Al2O3 field insulator was studied by Photoconductive Decay (PCD) method and our results indicate that after appropriate annealing Al2O3 provides equally low effective surface recombination velocity as thermally oxidized Si/SiO2 interface. Furthermore, with properly designed annealing steps, the TiN thin film resistors can be tuned to have up to several MΩ resistances with a few µm of physical size required in ultra-fine pitch pixel detectors.

Realization of improved efficiency on nanostructured multicrystalline silicon solar cells for mass production

We report the realization of both excellent optical and electrical properties of nanostructured multicrystalline silicon solar cells by a simple and industrially compatible technique of surface morphology modification. The nanostructures are prepared by Ag-catalyzed chemical etching and subsequent NaOH treatment with controllable geometrical parameters and surface area enhancement ratio. We have examined in detail the influence of different surface area enhancement ratios on reflectance, carrier recombination characteristics and cell performance. By conducting a quantitative analysis of these factors, we have successfully demonstrated a higher-than-traditional output performance of nanostructured multicrystalline silicon solar cells with a low average reflectance of 4.93%, a low effective surface recombination velocity of 6.59 m s−1, and a certified conversion efficiency of 17.75% on large size (156 × 156 mm2) silicon cells, which is ∼0.3% higher than the acid textured counterparts. The present work opens a potential prospect for the mass production of nanostructured solar cells with improved efficiencies.

Study of hydrogenated AlN as an anti‐reflective coating and for the effective surface passivation of silicon

AbstractStacks of aluminum oxide and silicon nitride are frequently used in silicon photovoltaics. In this Letter, we demonstrate that hydrogenated aluminum nitride can be an alternative to this dual‐layer stack. Deposited on 1 Ω cm p‐type FZ silicon, very low effective surface recombination velocities of 8 cm/s could be reached after firing at 820 °C. This excellent passivation is traced back to a high density of fixed charges at the interface of approximately –1 × 1012 cm–2 and a very low interface defect density below 5 × 1010 eV–1 cm–2. Furthermore, spectral ellipsometry measurements reveal that these aluminum nitride layers have ideal optical properties for use as anti‐reflective coatings. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-aligned local contacts through a-Si:H passivation layer

A novel method forms less than 100μm wide self-aligned local contacts through an amorphous silicon based passivation layer stack on crystalline silicon solar cells. Wire shading during the plasma deposition of the passivation layer enables local contact formation without the use of photolithography or laser processes. Subsequent full area aluminum evaporation and thermal annealing create good electrical contacts. The structure combines a low effective surface recombination velocity 36cm/s and high implied open circuit voltage 715mV with a good ohmic contact for the rear side of p-type crystalline silicon solar cells.

Comprehensive investigation of silicon surface passivation by a ‐Si:H and a ‐SiNx:H films

AbstractWe studied the passivation properties and microstructure of a ‐Si:H and a ‐SiNx:H films deposited by PECVD on p‐type and n‐type CZ silicon wafers for solar cells with planar and texturized surfaces. Focusing on the comparison between radio frequency (13.56 MHz) and microwave (2.45 GHz) excited plasma deposition, a systematic variation of the precursor gas flow ratio [NH3]/[SiH4] from the range suitable for the antireflection coating of solar cells (refractive index ∼ 2) up to a ‐Si:H, followed by a variation of the deposition as well as post‐deposition temperature between 150 and 400 °C was carried out. For all wafers and wafer surface structures, very low effective surface recombination velocities were achieved in the stoichiometry transition range between a ‐SiNx:H and a ‐Si:H. Additional improvement of the surface passivation by a ‐Si:H was achieved by post‐deposition annealing in N2 ambient. A correlation of this behavior to the microstructure of the passivation films is discussed for both plasma modes using measurements of the atomic bond densities and hydrogen content by means of the Fourier‐transform infrared spectroscopy (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dry fabrication process for heterojunction solar cells through in-situ plasma cleaning and passivation

We have studied low temperature plasma processes ( T sub≤200 °C) for the efficient cleaning and passivation of c-Si wafers, aiming for fully dry fabrication of heterojunction solar cells. We have experimented with H 2–SiF 4 plasmas in a standard RF PECVD reactor in order to etch the native oxide from the c-Si wafer and a thin a-Si:H layer was deposited from SiH 4 to passivate the c-Si surface. In-situ ellipsometry was used to optimize the process conditions for an efficient surface cleaning. Various plasma treatments were performed before a-Si:H deposition in order to reduce the surface recombination. Optimized process conditions resulted in high effective lifetime values ( τ eff≈1.55 ms), low effective surface recombination velocities ( S eff≤9 cm s −1) and high implicit open circuit voltages ( V oc≈0.713 V).

Near-Surface Channel Impedance Measurements, Open-Circuit Impedance Spectra, and Differential Capacitance vs Potential Measurements of the Fermi Level Position at Si/CH3CN Contacts

Near-surface channel impedance measurements, open-circuit impedance spectra, and differential capacitance vs potential measurements have been used to determine the barrier height of liquid contacts formed with n-type and p-type Si electrodes. Barrier heights were measured as the redox potential, E(A/A-), of a metallocene-based, one-electron, outer-sphere, acceptor/donor (A/A-) pair was varied in CH3CN solvent. The barrier heights of p-Si(111) electrodes in contact with CH3CN−Me10Fc+/0 (where Me10Fc is decamethylferrocene) or CH3CN-CoCp2+/0 (where CoCp2 is cobaltocene) were 0.69 ± 0.1 and 1.1 ± 0.1 V respectively. In contrast, barrier heights for n-Si(111)/CH3CN−Me10Fc+/0 and n-Si(111)/CH3CN-CoCp2+/0 contacts were 0.66 ± 0.1 and 0.09 ± 0.01 V, respectively. These measurements indicate that the barrier heights closely track changes in the electrochemical potential of the contact, instead of being relatively invariant to changes in the Fermi level of the contacting phase, as is observed for Si/metal Schottky barriers. These measurements also demonstrate that the low effective surface recombination velocity, S, for silicon in contact with CoCp2+/0 is primarily the result of an accumulation layer rather than solely being due to a low density of surface electrical defects.

Use of near-surface channel conductance and differential capacitance versus potential measurements to correlate inversion layer formation with low effective surface recombination velocities at n-Si/liquid contacts

Near-surface channel conductance measurements, differential capacitance versus potential measurements, and surface recombination velocity measurements have been performed on (111)- and (100)-oriented n-type Si samples in contact with nitrogen and/or liquid electrolyte solutions containing I2, I2/I−, ferrocene+/0, or decamethylferrocene+/0 in either methanol or tetrahydrofuran. Si/liquid contacts that displayed a low effective surface recombination velocity S corresponded to those that formed an inversion layer at the solid/liquid contact as indicated by channel conductance measurements or by differential capacitance versus potential measurements. Contacts that did not produce an inversion layer at the Si surface did not produce low effective S values. The observed behavior is consistent with the known energetics of Si/liquid contacts and provides an explanation for the low effective S values observed in these systems.

Effects of Interfacial Energetics on the Effective Surface Recombination Velocity of Si/Liquid Contacts

Photoconductivity decay data have been obtained for NH4F(aq)-etched Si(111) and for air-oxidized Si(111) surfaces in contact with solutions of methanol, tetrahydrofuran (THF), or acetonitrile containing either ferrocene+/0 (Fc+/0), [bis(pentamethylcyclopentadienyl)iron]+/0 (Me10Fc+/0), iodine (I2), or cobaltocene+/0 (CoCp2+/0). Carrier decay measurements were made under both low-level and high-level injection conditions using a contactless rf photoconductivity decay apparatus. When in contact with electrolyte solutions having either very positive (Fc+/0, I2/I-) or relatively negative (CoCp2+/0) Nernstian redox potentials with respect to the conduction-band edge of Si, Si surfaces exhibited low effective surface recombination velocities. In contrast, surfaces that were exposed only to N2(g) ambients or to electrolyte solutions that contained a mild oxidant (such as Me10Fc+/0) showed differing rf photoconductivity decay behavior depending on their different surface chemistry. Specifically, surfaces that pos...

Role of inversion layer formation in producing low effective surface recombination velocities at Si/liquid contacts

Photoconductivity decay lifetimes have been obtained for NH4F(aq)-etched Si(111) and for air-oxidized Si(111) surfaces in contact with solutions of CH3OH or tetrahydrofuran (THF) containing either ferrocene+/0 (Fc+/0), bis(pentamethylcyclopentadienyl) Fe+/0, or I2. Si surfaces in contact with electrolytes having Nernstian redox potentials >0 V versus the standard calomel electrode exhibited low effective surface recombination velocities regardless of the different surface chemistries, whereas those exposed only to N2(g) ambients or to electrolytes containing mild oxidants showed differing rf photoconductivity decay behavior depending on their different surface chemistry. The data reveal that formation of an inversion layer, and not a reduced density of electrical trap sites on the surface, is primarily responsible for the long charge-carrier lifetimes observed for Si surfaces in contact with CH3OH or THF electrolytes containing I2 or Fc+/0.

Optimization and characterization of remote plasma-enhanced chemical vapor deposition silicon nitride for the passivation of p-type crystalline silicon surfaces

In a recent letter [Lauinger et al., Appl. Phys. Lett. 68, 1232 (1996)] we have shown that record low effective surface recombination velocities Seff of 4 cm/s have been obtained at ISFH on low-resistivity (1 Ω cm) p-type crystalline silicon using microwave-excited remote plasma-enhanced chemical vapor deposition (RPECVD) of silicon nitride at low temperature (300–400 °C). As an important application, this technique allows a simple fabrication of rear-passivated high-efficiency silicon solar cells with monofacial or bifacial sensitivity. In this work, we present details of the required optimization of the PECVD parameters and a characterization of the resulting silicon nitride films. All deposition parameters are shown to strongly affect Seff as well as the stability of the films against the ultraviolet (UV) photons of terrestrial sunlight. A clear correlation between Seff and the film stoichiometry is observed, allowing a simple control and even a rough optimization of the surface passivation quality by measurements of the refractive index of the films. An optimum passivation and UV stability is obtained for silicon-rich silicon nitride films with a refractive index greater than 2.3.