APCVD titanium-dioxide and its affect on the passivation of the silicon – silicon-dioxide interface.

Presented by: Andrew ThomsonSupervisor: Dr Keith Mcintosh

Introduction

No subtitles sorry.About me: • Completed my undergrad at UC, Christchurch in 2005.• BE (hons), in Electrical and Computer Engineering.• Honours project, created the worlds second quantised

conductance atomic switch.Currently in my 15th month of my Ph.d here at CSES working with the TiO2 passivation group.

Presentation Outline

TiO2 overview – then why, what and how.How to understand my results.Characterisation of depositions on thick oxides.Thin oxides and my discovery!Summary and conclusion.

Project outline

Understand how the deposition of APCVD TiO2 affect the Si–SiO2 interface. Understand the underlying mechanisms.Seek to optimise the passivation of the Si–SiO2–TiO2stack. Ultimate goal – provide a cheaper alternative to the industry standard PECVD silicon-nitride.

What is an APCVD

What is passivation?

Stops the recombination process of free carries .Particularly important at the surface.Number of mechanisms.Important to this is recombination at defect sites.These defects can be passivated by hydrogen.

TiO2 and silicon solar cells

TiO2 has a refractive index well suited for use as an AR coating. Used widely in the PV industry, 1970’s till the mid 1990’s.Applied to low-cost screen-printed cells with heavy front diffusion. Since been superseded by PECVD silicon-nitride (Si3N4).Hydrogen and charge content in the film, provides improved passivation.

Why TiO2 – Project motivation

APCVD TiO2 is 4-10 times cheaper than SiN.It is an in-line belt process, Si3N4 is a batch.It’s affect on the passivation of the Si–SiO2 interface are not understood.We managed to procure an APCVD with industry support.

Potential for passivation improvement

Adding charge to the TiO2 films – doping of the filmsAdding hydrogen to the films – in-situ FGADeposition optimisation:• Temperature of deposition.• Flow ratios.• Total gas flows.• Belt speed.

Temperature affect on optical constants

Lifetime measurements – Background

Process of calculating effective lifetime.High effective lifetime = good, high J0e = bad.Effective lifetime is a measure of total recombination.J0e measure of recombination at surface and emitter.

J0eV σ Δn τeff

Affect of TiO2 deposition

Sample used:• FZ, 1000 Ω-cm, n-type, planar and textured, 250um.• HF-Nitric to remove damage.• Light phosphorous diffusions 104-130 Ω/.• 1100 degree oxidation and anneal (tox ~ 100nm).

Brain-storming Mechanisms

Stress generated at the interface.• Thermal and intrinsic stress is generated at all film interfaces.

Contamination.• The pizza oven has lots of contaminants, samples on a steel belt.

Humidity degradation.• H2O is reactant at temperatures.

Hydrogen loss.• Temperatures we use are close to that of a FGA.

Stress: different thickness and temperature depositions.Contamination: process samples without gases.Hydrogen: perform post-depositions FGA.Humidity: processing with H2O gases only. Test were performed on planar and textured samples.

Degradation Testing Methods

Results summary

FGA reversible degradation was observedFor thick oxides:• No contamination was observed.• Humidity degradation was observed – insignificant when

processing.• No permanent degradation for planar samples.• Permanent degradation for textured samples.

Possible that the degradation for textured samples is stress related.

Sanity Check

Move to thinner oxides

Repeat results for thinner oxides.Was not able to obtain reliable results.No repeatable results over multiple batches.Processing Conditions: • FZ, 3-5 Ω-cm, n-type, planar and textured, 650um.• HF-Nitric to remove damage.• Light phosphorous diffusions 130-150 Ω/.• 950 degree oxidation and 1050 degree anneal (tox = 15-

25nm).

Lifetime stability observation

Sample were degrading with time.Made experiments uncontrolled.Numerous experiments were performed to determine cause.Finally a break-through!!!

TIDLS Curve

Temperature controlled lifetime measurements.Precise measurements of changing lifetime taken.Experiment repeated for multiple temperatures.Initial curve.

Chi^2/DoF R^2----------------------------0.00015 0.99859----------------------------

Arrhenius Plot

Reaction rate equivalent to 1 / decay constant.Plot log(rate) vs. 1 / Temperature (K).Slope corresponds to an activation energy.Use Arrhenius equation. Example plot.

Possible mechanisms

Calculated activation energy ~ 0.1eV.Decay not observed in non-diffused samples. Probably phosphorus related.Not passivation of shallow phosphorus donors as the activation energy is 1 – 1.45eV.Any suggestions???

Repeated for well passivated samples

Previous samples had poor J0e fits.Repeated for samples new samples.Processing difference:• Higher temperature oxidation 1100 degrees. • Oxide thickness still 15-25nm

Effect on J0e can be observed.

Summary and conclusions

Planar samples only suffer reversible degradation.Textured show permanent degradation.Phosphorous diffused sample degrade post FGA.This is a surface related effect.

Questions??? – And Thanks!!!

Please come and question me rigorously at the afternoon tea break. Any feedback would be greatly appreciated. Thanks also to all the wonderful technicians and lab workers in the department, especially Nina, Chris, and Neil. Also thanks to my supervisor Keith McIntosh and the rest of the passivation group for the helpful input and advice.If you want references come see me – this is just a DERF.