Elsevier

Vacuum

Volume 50, Issues 3–4, 1 July 1998, Pages 451-462
Vacuum

Pore structure investigations in porous silicon by ion beam analytical methods

https://doi.org/10.1016/S0042-207X(98)00080-3Get rights and content

Abstract

Ion beam analytical methods (e.g. Rutherford Backscattering, RBS, Elastic Recoil Detection, ERD, or Nuclear Reaction Analysis, NRA) are widely used for quantitative determination of the depth distribution of elements. In porous samples however, where the diameter of the pores is a few tens of nm (i.e. unresolvable even by the most sophisticated microbeam), the measured depth distribution differs from the real one and this difference strongly depends on the pore structure.

Introduction

Ion beam analytical methods, IBA, based on the detection of various emitted particles produced by incident ions of a few MeV energy, e.g. (Rutherford) Backscattering Spectrometry, (R)BS, and Nuclear Reaction Analysis, NRA, are widely used for quantitative determination of the depth distribution of elements.[1]Their powerful ability is based on the following facts : (i) The yield, Y, of scattered ions in BS or nuclear reaction products in NRA is proportional to the concentration of the investigated element ; (ii) Ions lose energy along their path and this loss is proportional to the depth in the sample. The depth distribution can be evaluated from the energy spectrum of emitted ions (yield vs emitted ion energy, Y(Ed)). An example is given in Fig. 1. Excitation curves corresponding to a sharp resonance in the cross section of the nuclear reaction or elastic scattering (yield vs incident ion energy, Y(E0)) also represent the depth profiles (see, e.g. Fig. 2).Porous silicon (PS) layers are produced on the surface of crystalline Si wafers by anodization in a HF based solution. Because of its visible luminescence PS nowadays is in the focus of extensive research activity. Depending on the parameters of their preparation (composition of electrolyte, current density during anodization, doping type and orientation of the Si wafer, etc.), three main types of PS layers can be obtained : (i) Spongy : the pores run in random directions and their diameter, d, is small (1–10 nm) ; (ii) Columnar : the pores of larger diameter (5–20 nm) run parallel with each other and perpendicular to the sample surface (in <100> oriented Si wafers) ; (iii) Mixed : the pores are more or less ordered into columns, but their directions are not strictly parallel and they are surrounded by numerous side branches. The volume fraction of pores ( porosity), P, can be in the range of 30–90% and the internal surface of the pore walls is huge, ∼600 m2\cm3 for spongy and ∼200 m2\cm3 for columnar PS. The average inter-pore distance, D, is determined by P and d and can be approximated as :D≈d1−P For porous Si samples, where the diameter of the pores is a few tens of nm (i.e. unresolvable even by the most sophisticated microbeam), one might anticipate that IBA cannot give any information on the pore structure, since the ions do not interact with the holes. Both the reaction or scattering probability of ions, κ, and the energy loss along unit pathlength, S, will be reduced in the same extent given by the porosity :κporous=(1−P)κcompactSporous=(1−P)ScompactThis means, that in first order approximation a porous layer will appear in these measurements to be identical with a compact one, just the energy loss should be transformed into depth scale with a reduced density :ρporous=(1−P)ρcompact According to homogeneous approximation, in IBA measurements the PS layer results in a depth distribution for silicon that is almost identical with that of the compact substrate, hence it might be difficult to distinguish them in the obtained depth profiles. It might be useful to decorate the wall of the pores before the measurements by an impurity which depth profile can be determined more precisely. On the other hand, when PS is aged in atmosphere the huge internal surface becomes quickly oxidized or it is contaminated by a hydrocarbon layer under real vacuum conditions. These undesirable processes frequently help to carry out the measurements without any additional pre-treatments.The ‘‘classical’’ energy spread contributions in IBA methods (like energy straggling, multiple scattering, geometrical spread, detector energy resolution, etc.) for homogeneous materials are well known. The cumulated effect of these contributions can be calculated with ∼10% precision using the DEPTH code.[2]In the next sections it will be shown, that the porous structure itself causes new ‘‘structure induced’’ energy spread contributions. The yield is also influenced by the porous structure.The aim of the present work is to give a short review on the most significant effects observed so far and point out their origin. It will be shown, that the structure induced energy spread is significant and easily separable from the classical ones through their different angular dependencies. Comparing experimental and calculated energy spread vs tilt angle curves we can get valuable information on the porous structure itself.

Section snippets

Basic processes leading to structure induced effects

Deviations from the homogeneous approximation, the ‘‘structure induced effects’’, are based on the following processes.

Experimentally observed structure induced effects

So far the following effects of the porous structure were demonstrated experimentally in IBA measurements.

Summary

In the special case of porous silicon it was demonstrated by several examples that IBA methods are sensitive to the porous structure of the samples. To perform the measurements, the sample frequently have to contain an interface between the porous layer and the substrate or had to be decorated by different contaminants.All the presented structure induced effects strongly depend on the pore structure, therefore provide information on the pore structure itself. To get detailed information from

Acknowledgements

The work was supported by Hungarian OTKA Grants No. T019147 and F019165. The authors are indebted to Dr Zs Horváth and Dr É Vázsonyi for their helpful discussions and to the technological laboratory of KFKI-Research Institute for Materials Science for providing the samples used in their original measurements.3, 4, 6, 9

References (9)

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