Modelling of Kelvin probe surface voltage and photovoltage in dielectric-semiconductor interfaces

The characterisation of dielectric-semiconductor interfaces via Kelvin probe surface voltage and photovoltage has become a widespread method of extracting the electrical properties influencing optoelectronic devices. Kelvin probe offers a versatile, contactless and vacuum-less technique able to provide useful insights into the electronic structure of semiconductor surfaces. Semiconductor theory has long been used to explain the observations from surface voltage measurements, often by making large assumptions about the characteristics of the system. In this work I report an updated theoretical treatment to model the results of Kelvin probe surface voltage and photovoltage measurements including four critical mechanisms: the concentration of charge stored in interface surface states, the charge stored in different locations of a surface dielectric thin film, the changes to effective lifetime and excess carrier density as a result of charge redistribution, and the non-uniformity of charge observed on most large scale thin film coatings used for passivation and optical improvement in optoelectronic devices. A full model is drawn and solved analytically to exemplify the role that these mechanisms have in surface voltage characterisation. The treatment in this work provides crucial understanding of the mechanisms that give rise to surface potential in semiconductors. As such this work will help the design and development of better optoelectronic devices

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

Potassium ions in SiO2: electrets for silicon surface passivation

This manuscript reports an experimental and theoretical study of the transport of potassium ions in thin silicon dioxide films. While alkali contamination was largely researched in the context of MOSFET instability, recent reports indicate that potassium ions can be embedded into oxide films to produce dielectric materials with permanent electric charge, also known as electrets. These electrets are integral to a number of applications, including the passivation of silicon surfaces for optoelectronic devices. In this work, electric field assisted migration of ions is used to rapidly drive K+ into SiO2 and produce effective passivation of silicon surfaces. Charge concentrations of up to ~5 × 1012 e cm−2 have been achieved. This charge was seen to be stable for over 1500 d, with decay time constants as high as 17 000 d, producing an effectively passivated oxide–silicon interface with SRV &lt; 7 cm s−1, in 1 Ω cm n-type material. This level of charge stability and passivation effectiveness has not been previously reported. Overall, this is a new and promising methodology to enhance surface passivation for the industrial manufacture of silicon optoelectronic devices.</p

Potassium ions in SiO2: electrets for silicon surface passivation

This manuscript reports an experimental and theoretical study of the transport of potassium ions in thin silicon dioxide films. While alkali contamination was largely researched in the context of MOSFET instability, recent reports indicate that potassium ions can be embedded into oxide films to produce dielectric materials with permanent electric charge, also known as electrets. These electrets are integral to a number of applications, including the passivation of silicon surfaces for optoelectronic devices. In this work, electric field assisted migration of ions is used to rapidly drive K+ into SiO2 and produce effective passivation of silicon surfaces. Charge concentrations of up to ~5 × 1012 e cm−2 have been achieved. This charge was seen to be stable for over 1500 d, with decay time constants as high as 17 000 d, producing an effectively passivated oxide–silicon interface with SRV −1, in 1 andOmega; cm n-type material. This level of charge stability and passivation effectiveness has not been previously reported. Overall, this is a new and promising methodology to enhance surface passivation for the industrial manufacture of silicon optoelectronic devices.</p

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

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

Very low surface recombination velocity in n-type c-Si using extrinsic field effect passivation

In this article, field-effect surface passivation is characterised as either intrinsic or extrinsic, depending on the origin of the charges present in passivation dielectric layers. The surface recombination velocity of float zone, 1 Ω cm, n-type silicon was reduced to 0.15cm/s, the lowest ever observed for a passivating double layer consisting of thermally grown silicon dioxide and plasma enhanced chemical vapour deposited silicon nitride. This result was obtained by enhancing the intrinsic chemical and field-effect passivation of the dielectric layers with uniform, extrinsic field-effect passivation induced by corona discharge. The position and stability of charges, both intrinsic and extrinsic, were characterised and their passivation effect was seen stable for two months with surface recombination velocity &lt;2cm/s. Finally, the intrinsic and extrinsic components of passivation were analysed independently. Hydrogenation occurring during nitride deposition was seen to reduce the density of interfacial defect states from ∼5×10 10cm-2eV-1 to ∼5×109 cm-2eV-1, providing a decrease in surface recombination velocity by a factor of 2.5. The intrinsic charge in the dielectric double layer provided a decrease by a factor of 4, while the corona discharge extrinsic field-effect passivation provided a further decrease by a factor of 3. © 2014 AIP Publishing LLC

Lowest surface recombination in n-type oxidised crystalline silicon by means of extrinsic field effect passivation

Surface recombination remains a major factor limiting the efficiency of silicon solar cells. The post-processing of dielectric films used as surface coatings has been previously demonstrated an effective technique to improve their passivation quality. In this paper extrinsic methods are demonstrated to produce the lowest reported surface recombination velocity in solar relevant n-type silicon. Recombination velocities below 2.8 cm/s at an injection of 1015 cm-3, are achieved using extrinsic field-effect passivation, or &lt; 0.6 cm/s when using combined extrinsic chemical and extrinsic field effect passivation. These are equivalent to emitter saturation current densities J0e&lt;1.4 fA/cm2 and 0.6 fA/cm2

Lowest surface recombination in n-type oxidised crystalline silicon by means of extrinsic field effect passivation

Surface recombination remains a major factor limiting the efficiency of silicon solar cells. The post-processing of dielectric films used as surface coatings has been previously demonstrated an effective technique to improve their passivation quality. In this paper extrinsic methods are demonstrated to produce the lowest reported surface recombination velocity in solar relevant n-type silicon. Recombination velocities below 2.8 cm/s at an injection of 1015 cm-3, are achieved using extrinsic field-effect passivation, or < 0.6 cm/s when using combined extrinsic chemical and extrinsic field effect passivation. These are equivalent to emitter saturation current densities J0e<1.4 fA/cm2 and 0.6 fA/cm2

Surface passivation provided by an alneal through sio2/tio2 bilayer

The process of annealing a SiO2 dielectric layer coated in aluminium, termed the alneal, is known to produce some of the most effective surface passivation for silicon. However, it is traditionally performed on a SiO2 dielectric coating which has sub-optimal anti-reflection properties. In this work it is shown that it is possible to achieve alneal passivation through a double layer SiO2/TiO2 stack. TiO2 was investigated as it is one of the most effective antireflection coatings available and its deposition technology is advanced and cost effective. Here, the alneal was carried out on n-type ~40 Ωcm Cz-Si coated with a SiO2/TiO2 dielectric stack. In the best case, the alneal produced a lifetime increase from ~15 μs to 3084 μs, which equates to a SRV ≤ 10 cm/s or a J0e of 32 fA/cm2. This increase in lifetime was comparable to that achieved by a conventional alneal on a single layer SiO2 specimen. It was also found that the thicker the TiO2, the less effective the alneal was at passivating. Lastly, the passivation achieved after the alneal on the SiO2/TiO2 bilayer was found to be more stable than that achieved on the conventional single SiO2 layer