Stable field effect surface passivation of n-type Cz silicon

Surface recombination of carriers in solar cells can cause a significant reduction in their efficiency and is most commonly minimized by the deposition of surface dielectric layers which simultaneously perform two functions; efficient passivation of surface recombination and the provision of an effective anti-reflection layer. This can be difficult to achieve in practice since the conditions that produce an optimum anti-reflection coating are not necessarily the same as those required for effective passivation. In this work we describe the use of external electrical charging of dielectric layers which serves to improve their passivation properties without affecting their reflection properties. This provides a method by which, to some extent, the electrical and optical properties of the films can be decoupled so allowing better overall performance to be achieved. It is demonstrated that SiO2 and SiO 2/SiN stacks deposited on a silicon surface can provide a stable reduction of surface recombination when chemically treated, electrically charged using a corona discharge and then annealed at low temperature. Surface recombination velocity upper limits of 19 cm/s and 16 cm/s were inferred for single and double layers respectively on n-type, 5 Ωcm, Cz-Si. © 2013 The Authors

NEW RESULTS AND AN INTERPRETATION FOR SEM EBIC CONTRAST ARISING FROM INDIVIDUAL DISLOCATIONS IN SILICON.

The SEM EBIC contrast for individual screw and 60 degree dislocations formed in high-purity, n-type 10**1**5 cm** minus **3 silicon by deformation has been measured and found to vary with both specimen temperature and electron beam current. A new theory of recombination at dislocations has been developed and applied to the EBIC method. The new theory explains the experimental results and enables parameters associated with the recombination process at the individual dislocations, e. g. energy level, density of states, etc. , to be deduced

A technique for field effect surface passivation for silicon solar cells

The recombination of electric charge carriers at the surface of semiconductors is a major limiting factor in the efficiency of optoelectronic devices, in particular, solar cells. The reduction of such recombination, commonly referred to as surface passivation, is achieved by the combined effect of a reduction in the trap states present at the surface via a chemical component, and the reduction in the charge carriers available for a recombination process, via a field effect component. Here, we propose a technique to field effect passivate silicon surfaces using the electric field effect provided by alkali ions present in a capping oxide. This technique is shown to reduce surface recombination in a controlled manner, and to be highly stable. Surface recombination velocities in the range of 6-15 cm/s are demonstrated for 1 Ω cm n-type float zone silicon using this technique, and they are observed to be constant for over 300 days, without the use of any additional surface chemical treatment. A model of trapping-mediated ionic injection is used to describe the system, and activation energies of 1.8-2 eV are deduced for de-trapping of sodium and potassium alkali ionic species. © 2014 AIP Publishing LLC

Field emission from pyramidal cathodes covered in porous silicon

Square‐based pyramidal emitters formed by wet etching of p‐type silicon wafers have been anodized to give a thin surface layer of porous silicon. At the surface of such material are very small fibrils with widths ≤3 nm. Field emission measurements from pyramidal cathodes of plain and anodized silicon show a dramatic improvement, when the porous silicon is present. In this case, average peak emission currents of 25 μA have been obtained with the highest measured being 90 μA, improved uniformity between cathodes was produced and emission began at lower voltages. It was found that plain cathodes too blunt to emit did so when covered in porous silicon. The reasons why such silicon fibrils improve emission are discussed

DISLOCATION RECOMBINATION THEORY FOR SILICON AND INTERPRETATION OF EBIC CONTRAST IN TERMS OF FUNDAMENTAL DISLOCATION PARAMETERS.

A new theory is proposed for recombination at charged dislocations in semiconductors. This is applied to EBIC contrast from individual dislocations

Electron beam induced current investigations of transition metal impurities at extended defects in silicon

The electron beam induced current (EBIC) mode of a scanning electron microscopy is a useful technique for studying gettering of impurities to extended defects, its high sensitivity allowing very low impurity concentrations to be studied. Extended defects, when studied by EBIC, normally exhibit one of two different kinds of carrier recombination behavior. In the most common case this is accurately described by the Wilshaw model in which the recombination mechanism is charge controlled. Analyzed in terms of this physical model, quantitative EBIC experiments indicate that the small amount of recombination associated with deformation induced dislocations produced at 650°C or above and at stacking faults is due only to residual impurities, whereas a state intrinsic to the dislocation is produced by deformation at 420°C. There also exists a less common second type of recombination behavior, often associated with nickel contaminates, which can dominate at low temperatures. This type of recombination is less well understood and cannot be modeled by simple Shockley Read Hall recombination statistics

Field emission from pyramidal cathodes covered in porous silicon

Square‐based pyramidal emitters formed by wet etching of p‐type silicon wafers have been anodized to give a thin surface layer of porous silicon. At the surface of such material are very small fibrils with widths ≤3 nm. Field emission measurements from pyramidal cathodes of plain and anodized silicon show a dramatic improvement, when the porous silicon is present. In this case, average peak emission currents of 25 μA have been obtained with the highest measured being 90 μA, improved uniformity between cathodes was produced and emission began at lower voltages. It was found that plain cathodes too blunt to emit did so when covered in porous silicon. The reasons why such silicon fibrils improve emission are discussed

NEW RESULTS AND AN INTERPRETATION FOR SEM EBIC CONTRAST ARISING FROM INDIVIDUAL DISLOCATIONS IN SILICON.

The SEM EBIC contrast for individual screw and 60 degree dislocations formed in high-purity, n-type 10**1**5 cm** minus **3 silicon by deformation has been measured and found to vary with both specimen temperature and electron beam current. A new theory of recombination at dislocations has been developed and applied to the EBIC method. The new theory explains the experimental results and enables parameters associated with the recombination process at the individual dislocations, e. g. energy level, density of states, etc. , to be deduced

DISLOCATION RECOMBINATION THEORY FOR SILICON AND INTERPRETATION OF EBIC CONTRAST IN TERMS OF FUNDAMENTAL DISLOCATION PARAMETERS.

A new theory is proposed for recombination at charged dislocations in semiconductors. This is applied to EBIC contrast from individual dislocations

On the c-Si/SiO2 interface recombination parameters from photo-conductance decay measurements

The recombination of electric charge carriers at semiconductor surfaces continues to be a limiting factor in achieving high performance optoelectronic devices, including solar cells, laser diodes and photodetectors. The theoretical model and a solution algorithm for surface recombination have been previously reported. However, their successful application to experimental data for a wide range of both minority excess carrier concentrations and dielectric fixed charge densities has not previously been shown. Here, a parametrisation for the semiconductor-dielectric interface charge Qit is used in a Shockley-Read-Hall extended formalism to describe recombination at the c-Si/SiO2 interface, and estimate the physical parameters relating to the interface trap density Dit, and the electron and hole capture cross-sections σn and σp. This approach gives an excellent description of the experimental data without the need to invoke a surface damage region in the c-Si/SiO2 system. Band-gap tail states have been observed to limit strongly the effectiveness of field effect passivation. This approach provides a methodology to determine interface recombination parameters in any semiconductor-insulator system using macro scale measuring techniques