New problems in nuclear microprobe analysis of materials

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Abstract

Advanced materials are being evaluated for use as novel radiation detectors and microelectronic devices, including, potentially, synthetic diamond radiation-hard detectors for high-energy physics experiments and tissue equivalent dosimeters. Use of a nuclear microprobe has allowed spatially resolved electrical properties of the detector material to be measured. However quantitative analysis requires good models for charge collection mechanisms by ion beam induced charge (IBIC). In fact, nuclear microprobe analysis is playing an increasingly prominent role in the analysis of detector materials and devices by IBIC, with secondary roles also being played by ionoluminescence (IL) and the traditional techniques of Rutherford backscattering and particle induced X-ray emission. In this paper, many recent applications are reviewed and some examples of applications of the nuclear microprobe to the study of new materials and devices are presented. Some of these applications involve wide band gap materials, such as GaN, as well as novel detectors for radiation dosimetry in cancer therapy, photovoltaic devices and other microelectronic devices.

Introduction

Characterisation of materials with a nuclear microprobe may be divided into two broad areas: characterisation of structural properties and characterisation of electrical properties. Many applications in these two areas have been comprehensively reviewed in recent papers that have covered structural and electrical characterisation [1], ion beam induced charge (IBIC) for electrical characterisation [2], applications involving diamond [3], charge transients from single ions [4] and a large number of earlier applications [5], [6]. This paper aims to review the applications of the nuclear microprobe to the analysis of synthetic materials that have occurred in the last 2–3 years. Comparisons are made between the well-established quantitative methods of ion beam analysis: Rutherford backscattering spectrometry (RBS), channeling contrast microscopy (CCM), particle induced X-ray emission (PIXE), etc., (to be known here as the classic methods) and the emerging methods of IBIC and ionoluminescence (IL) [7]. It is a significant feature of the work in the field for the period of this review that the traditional ability of the classic methods to provide quantitative results is now moving into the emerging methods. This is despite the considerably greater complexity of the required physical models.

It is a measure of the rise in importance of IBIC that although one of the earliest papers on IBIC was published in 1989 [8], with an early simple model for IBIC published in 1992 [9], in the past 3 years there have been 29 papers published on IBIC and its related technique of single event upsets (SEU), 30 papers on the classic methods, 2 on IL and 5 papers that combine IBIC with one of the classic methods. In fact, in the time interval since the last Conference on Nuclear Microprobe Technology and Applications, Santa Fe, 1996, only a few papers have appeared which utilise the classic methods alone.

A cursory comparison of the physical models employed for the analysis of RBS and PIXE spectra to those required for IBIC and IL reveals considerable differences and even some areas of missing theory. This brief review can only give a superficial overview of these topics. Furthermore, extensive past work has been done on single crystal silicon and related compounds and reviews of this work may be found in Refs. [2], [7], but there are now many new materials being considered for advanced devices. For example, in the case of diamond, several working devices have been reported including high voltage diodes [10], field effect transistors [11] and logic circuits [12]. Also, electrical measurements have been made on microscopic crystals [13] by employing ingenious techniques that would also be useful for IBIC. However, the greatest interest in diamond is in its potential use as a radiation hard detector material for high-energy physics experiments. The RD-42 consortium has been specifically established to achieve this aim and detailed studies of working detectors have been performed [14]. Most of this work has been done with non-spatially resolved techniques, typically involving irradiation with energetic electrons (0–2.283 MeV) from a 90Sr source [15]. However, the polycrystalline nature of synthetic diamond requires spatially resolved techniques for complete understanding of the rich phenomena associated with the electrical properties of this material. Some recent results are discussed below.

Section snippets

Classic methods

Wide band gap materials represent the materials science frontier. Ion beams are widely used to analyse these materials. GaN is a promising new material with many desirable applications such as blue lasers, high temperature microelectronic devices [16], [17] and possible heterostructures with other wide band-gap materials. Several issues in the growth of this material may be addressed with CCM and essential physical parameters analysed. As an example, the application of CCM to the study of

Conclusion

A great expansion in the number of applications for IBIC in the analysis of materials has occurred since the last nuclear microprobe conference in 1996. Many of these new applications have shown the great potential to obtain complimentary information about the specimen using different methods with the same instrument in order to obtain a full picture of both the structural and electrical properties of the material.

In the example on GaN, it would be of significant importance to measure the key

Acknowledgements

We wish to acknowledge all of our numerous collaborators for providing interesting specimens for the results from Melbourne described in this review. This work has been supported by grants from the Australian Research Council. Collaborative work with Chanyi Yang was supported by visiting fellowships from the University of Melbourne. Andrew Saint gratefully acknowledges the support of a JAERI visiting fellowship for collaborative experiments on heavy ion IBIC with the assistance of Dr. T. Hirao.

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