Atomic level termination for passivation and functionalisation of silicon surfaces.

Nicholas E Grant,Marc Walker,Richard Jefferies,Daniel Hiller,Richard Beanland,Yisong Han,Alex I Pointon,John D Murphy

doi:10.1039/d0nr03860a

Abstract

Chemical treatments play an essential role in the formation of high quality interfaces between materials, including in semiconductor devices, and in the functionalisation of surfaces. We have investigated the effects of hydrogen and fluorine termination of (100)-orientation silicon surfaces over a range of length scales. At the centimetre scale, lifetime measurements show clean silicon surfaces can be temporarily passivated by a short treatment in both HF(2%) : HCl(2%) and HF(50%) solutions. The lifetime, and hence surface passivation, becomes better with immersion time in the former, and worse with immersion time in the latter. At the nanometre scale, X-ray photoelectron spectroscopy and atomic force microscopy show treatment with strong HF solutions results in a roughened fluorine-terminated surface. Subsequent superacid-derived surface passivation on different chemically treated surfaces shows considerably better passivation on surfaces treated with HF(2%) : HCl(2%) compared to HF. Lifetime data are modelled to understand the termination in terms of chemical and field effect passivation at the centimetre scale. Surfaces passivated with Al2O3 grown by atomic layer deposition behave similarly when either HF(2%) : HCl(2%) or HF(50%) are used as a pre-treatment, possibly because of the thin silicon dioxide interlayer which subsequently forms. Our study highlights that chemical pre-treatments can be extremely important in the creation of high quality functionalised surfaces.

Highlights

Silicon materials lie at the heart of many electronic devices, including the vast majority of photovoltaic solar cells
It is well established that a short treatment in a dilute hydrofluoric acid (HF) solution results in a metastable hydrogen-terminated surface,[7] Si–F bonds are considerably stronger than Si–H bonds,[7] and fluorine-terminated surfaces are known to result from treatments in concentrated HF.[8,9]
In the Electronic supplementary information (ESI) (Fig. S7†) we show that much worse surface passivation results when the samples are not treated with a HF-based solution after standard clean 1 (SC1) or Standard Clean 2 (SC2) cleaning

Summary

Introduction

Modification,[3] and for various optical devices, biomedical devices, and sensors.[4]. To the best of our knowledge, this can only currently be done by measuring the rate of surface recombination at the terminated silicon surface by a carrier lifetime measurement (such as by photoconductance decay[12] or by photoluminescence imaging13) that can perform measurements on the timescale of milliseconds or longer Providing these measurement techniques give a sufficient level of sensitivity, such techniques would be valuable in optimising the HF treatment, and maximising functionalisation of the silicon surface. We demonstrate that the effects of the chemical treatments are not always apparent, and are lost entirely when Al2O3 dielectric passivation is deposited on the treated surface by atomic layer deposition (ALD) To help understand this we use annular dark field scanning transmission electron microscopy (ADF-STEM) with energy dispersive X-ray (EDX) analysis to characterise the interfaces at the atomic scale

Results

Discussion

Findings

Conclusions