Heike Angermann

Wet-chemical passivation of Si(111)- and Si(100)-substrates

Abstract The influence of preparation-induced surface roughness, as well as the hydrogen and oxide coverage on electronic properties of Si(111) and Si(100) surfaces was investigated by combining various surface-sensitive techniques. Simultaneous surface photovoltage (SPV) and spectroscopic ellipsometry (SE) measurements, both in the ultraviolet/visible (UV–VIS) and the infrared (IR) spectroscopic region, yielded detailed information about intrinsic and extrinsic surface states on hydrogen (H)-terminated Si(111) and Si(100) surfaces, immediately after the wet-chemical preparation as well as during the initial oxidation. The energetic distributions of interface states D it ( E ) on Si(100) and Si(111) surfaces were correlated to the surface roughness 〈 d r 〉, the change of hydrogen coverage and the oxide growth on an atomic scale. As shown by these experiments, generally higher interface state densities D it, min were observed on Si(100) surfaces in comparison to Si(111). However, on Si(100) substrates a faster oxide growth and a significantly thicker final native oxide layer were found. The wet-chemical preparation methods of hydrogen or oxide passivated surfaces on Si(100) substrates were carefully optimized, resulting in smooth H-terminated surfaces (〈 d r 〉≈4 A and D it, min 10 cm −2 eV −1 ) and passivating oxide layers in the thickness range of 1–3 nm ( D it, min 11 eV −1 cm −2 ).

H-terminated silicon: spectroscopic ellipsometry measurements correlated to the surface electronic properties

Abstract A correlation was established between the morphological structure of the Si surface caused by chemical preparation and its electronic interface properties by combined monitoring of (i) the surface roughness and hydrogen and oxide coverage by ex situ (UV-VIS-IR) and in situ (UV-VIS) spectroscopic ellipsometry (SE), and (ii) the density D it,min and energetic distribution D it ( E ) of interface states by surface photovoltage (SPV) measurements. With these systematic investigations a wet-chemical H-termination procedure was optimized and very smooth Si(111) surfaces without any native oxide coverage were prepared, characterized by an intrinsic surface state distribution and a very low surface state density D it,min 10 cm −2 eV −1 . On these surfaces the resonant absorption due to the Si–H bonds was directly observed by infrared ellipsometry.

Wet-chemical treatment and electronic interface properties of silicon solar cell substrates

On textured n-type silicon substrates for solar cell manufacturing, the relation between light trapping behavior, structural imperfections, energetic distribution of interface state densities and interface recombination losses were investigated by applying surface sensitive techniques. The field-modulated surface photovoltage (SPV), in-situ photoluminescence (PL) measurements, total hemispherical UV-NIR-reflectance measurements and electron microscopy (SEM) were employed to yield detailed information on the influence of wet-chemical treatments on preparation induced micro-roughness and electronic properties of polished and textured silicon substrates. It was shown that isotropic as well as anisotropic etching of light trapping structures result in high surface micro-roughness and density of interface states. Removing damaged surface layers in the nm range by wet-chemical treatments, the density of these states and the related interface recombination loss can be reduced. In-situ PL measurements were applied to optimise HF-treatment times aimed at undamaged, oxide-free and hydrogen-terminated substrate surfaces as starting material for subsequent solar cell preparations.

Physical and Technological Aspects of a-Si:H/c-Si Hetero-Junction Solar Cells

We report on the basic properties of a-Si:H/c-Si hetero-junctions, their effects on the recombination of excess carriers and its influence on the a-Si:H/c-Si hetero-junction solar cells. For this purpose we measured the gap state density distribution in thin a-Si:H layers, determined its dependence on deposition temperature and doping by an improved version of near UV-photoelectron emission spectroscopy. Furthermore, the Fermi level position in the a-Si:H and the valence band offset were directly measured. In combination with interface specific methods such as surface photovoltage analysis and our numerical simulation program AFORS-HET, we are able to find out the optimum in wafer pretreatment, doping and deposition temperature for efficient a-Si:H/c-Si solar cells without an i-type a-Si:H buffer layer. By a deposition at 210degC with an emitter doping of 2000 ppm of B2 H6 on a well cleaned pyramidal structured c-Si(p) wafer we reached 19.8 % certified efficiency

Optimization of Interface Properties in a-Si:H/c-Si Heterojunction Solar Cells

We report on the optimization of hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells which were completely processed at temperatures below 230degC. Efficient solar cells based on both n-type and on p-type c-Si substrates were performed. In contrast to the approach from Sanyo no additional a-Si:H(i) buffer layer was used. Instead the conditions of the amorphous silicon preparation by conventional plasma enhanced chemical vapor deposition (PECVD) were optimized and several wet- or plasma chemical treatments were applied to improve the interface properties. The highest efficiencies so far are 17.4 % on p-type c-Si wafers and 19.8 % on n-type c-Si wafers

Interface State Densities and Surface Charge on Wet-Chemically Prepared Si(100) Surfaces

Introduction Due to the further miniaturization and thinning of films in device manufacturing, the wet-chemical conditioning of silicon wafer surfaces during each process must be specially optimized with regard to the prior and subsequent process steps. As recently reported, various chemical treatments using HF and NH4F solutions have been developed resulting in smooth, H-terminated Si(111) surfaces [1]. However, these methods are not so effective with the Si(100) surface typically used in device manufacturing, because of MOSFETs exhibit a higher reliability when fabricated on the Si(100) substrate. Therefore, the development of effective planarization and passivation methods for Si(100) substrate have received an increasing interest in recent years. The electronic interface properties of very thin oxide, epitaxial, and passivation layers are strongly influenced by the chemical integrity and morphological structure of the substrate surface prior to preparation. Therefore, controlling the flatness as well as the growth rate of native oxides on cleaned wafer surfaces is of great importance for the semiconductor process. Fast non-destructive methods are required for in situ characterization of wafers directly during the technological process. Up to now, however, a uniform, simple and comprehensive method of detecting the cleanness of a silicon surfaces has not been developed. By application of various spectroscopic and structural methods the morphology and the chemical structure of wet-chemically treated silicon substrates have intensively been investigated in the recent years. These measurements, however, are not sensitive enough to detect the small number (typically 10 to 10 eVcm) of silicon dangling bond defects which influence the electronic interface properties. In this paper a non destructive, very surface sensitive technique, the large-signal field-modulated surface photovoltage (SPV) was applied to yield detailed information on the influence of preparation induced surface morphology on electronic properties of H-terminated Si(100) surfaces, and their stability during storage in clean room air. Correlation between the density of interface states and the surface charge on initially H-terminated Si(100) surfaces and the native oxide and Si(−H)2 coverage during storage in clean room air were investigated and compared to recent findings on Si(111) surfaces.

Wet Chemical Oxidation of Silicon Surfaces Prior to the Deposition of All-PECVD AlOx/a-SiNx Passivation Stacks for Silicon Solar Cells

Aluminum oxide (AlOx) is currently under intensive investigation for use in surface passivation schemes in solar cells. AlOx films contain negative charges and therefore generate an accumulation layer on p-type silicon surfaces, which is very favorable for the rear side of p-type silicon solar cells as well as the p+-emitter at the front side of n-type silicon solar cells. However, it has been reported that quality of an interfacial silicon sub-oxide layer (SiOx), which is usually observed during deposition of AlOx on Silicon, strongly impacts the silicon/AlOx interface passivation properties [1]. The present work demonstrates that a convenient way to control the interface is to form thin wet chemical oxides of high quality prior to the deposition of AlOx/a-SiNx:H stacks by the plasma enhanced chemical vapor deposition (PECVD).

Wet-Chemical Conditioning of H-Terminated Silicon Solar Cell Substrates Investigated by Surface Photovoltage Measurements

For further enhancement of solar energy conversion efficiency the passivation of silicon (Si) substrate surfaces and interfaces of Si-based solar cell devices is a decisive precondition to reduce recombination losses of photogenerated charge carriers. These losses are mainly controlled by surface charges, the density and the character of rechargeable interface states (Dit) [], which are induced by defects localised in a small interlayer extending over only few Å. Therefore, the application of fast non-destructive methods for characterization of the electronic interface properties directly during the technological process has received an increasing interest in recent years.

Effect of wet-chemical substrate smoothing on passivation of ultrathin-SiO2/n-Si(111) interfaces prepared with atomic oxygen at thermal impact energies

Ultrathin SiO2 layers for potential applications in nano-scale electronic and photovoltaic devises were prepared by exposure to thermalized atomic oxygen under UHV conditions. Wet-chemical substrate pretreatment, layer deposition and annealing processes were applied to improve the electronic Si/SiO2 interface properties. This favourable effect of optimized wet-chemical pre-treatment can be preserved during the subsequent oxidation. The corresponding atomic-scale analysis of the electronic interface states after substrate pre-treatment and the subsequent silicon oxide layer formation is performed by field-modulated surface photovoltage (SPV), atomic force microscopy (AFM) and spectroscopic ellipsometry in the ultraviolet and visible region (UV-VIS-SE).

Surface Texturization and Interface Passivation of Mono-Crystalline Silicon Substrates by Wet Chemical Treatments

Continuously increasing prices for energy and general environmental concerns over the past few years have lead to an enormous growth of interest in alternative forms of energy and solar energy in specific. According to industry experts the manufacturing focus of the photovoltaic industry will be on silicon for at least another 5 to 10 years. The development of economically attractive silicon solar cells appropriates interface preparation and passivation methods to optimize the light trapping properties and to reduce recombination losses on structured interfaces [1]. The texturization of Si substrates, however, leads to a strong increase in crystallographic surface irregularities, resulting in a high density of rechargeable states and in high recombination losses on structured interfaces [2]. To reduce the density of these states, it is necessary to remove the damaged surface layer and to decrease the micro-roughness of structured surfaces on the nanometer scale.