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New problems in nuclear microprobe analysis of materials
Nuclear Instruments and Methods in Physics Research B 158 (1999) 628±637
www.elsevier.nl/locate/nimb
New problems in nuclear microprobe analysis of materials David N. Jamieson a,*, Deborah R. Beckman a, Andrew A. Bettiol a, Jamie S. Laird a, Kin Kiong Lee a, Steven Prawer a, Andrew Saint a,1, Lachlan C.G. Witham a, Changyi Yang b a
School of Physics, Microanalytical Research Centre, University of Melbourne, Parkville 3052, Australia b Department of Physics, National University of Singapore, Singapore
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. Ó 1999 Elsevier Science B.V. All rights reserved.
1. 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
* Corresponding author. Tel.: +61-3-9344-5376; fax: +61-39347-4783; e-mail: [email protected] 1 Present address: GBC Scienti®c Equipment, 12 Monterey Road, Dandenong, Victoria, Australia
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
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methods of IBIC and ionoluminescence (IL) [7]. It is a signi®cant feature of the work in the ®eld 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 dierences and even some areas of missing theory. This brief review can only give a super®cial 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], ®eld eect 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 speci®cally 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 90 Sr source [15]. However, the polycrystalline nature of synthetic diamond requires spatially resolved
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techniques for complete understanding of the rich phenomena associated with the electrical properties of this material. Some recent results are discussed below. 2. Applications 2.1. 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 micro-structures in GaN, is shown in Fig. 1. Numerous hexagonal growth defects are visible, which probably arise from excessive nitridation of the substrate [18]. Improvements in the growth conditions lead to merging of these hexagonal defects into a smooth ®lm suitable for many applications. The CCM images of the hexagons reveal several interesting features, including excellent crystallinity, comparable to that of the surrounding smooth ®lm, with vmin 3%. The hexagons are about twice as thick as the surrounding ®lm, indicating faster growth. A surprising result is that the hexagons all reveal a larger surface peak compared to the surrounding ®lm. This may provide a clue to the mechanisms responsible for the hexagon growth. The CCM technique has been recently further developed with the development of the beam rocking technique in Oxford. Brie¯y, the technique involves the use of two sets of opposing scan coils on a nuclear microprobe system that allows the beam divergence to be scanned instead of the beam position. Provided the probe forming lens system aberrations are suciently small, the beam may be scanned over 1.5° while remaining within a region less then 300 lm in diameter on the specimen. This allows very accurate and convenient location of the orientation of planar and axial channeling
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Fig. 1. CCM analysis of hexagonal growth defects in epitaxial GaN ®lms on sapphire substrates. A 2.5 MeV He beam scanned over 200 ´ 200 lm2 , aligned with the á0 0 0 1ñ axis of the sapphire substrate was used for these measurements. The smooth curve is a simulation for 0.53 GaN on Al2 O3 .The optical image is for a dierent region compared to the CCM image.
minima. The theory and applications of this technique are reviewed in these proceedings [19]. 2.2. Ion beam induced charge and ionoluminescence The CCM analysis of GaN discussed above allowed useful structural properties of the material to be measured. However, the motivation for the synthesis of GaN is for eventual applications in microelectronic devices. As such, while the structural properties are of interest, it is the desirable electrical properties of the material that are of primary interest. Potentially, these properties may be conveniently measured by IBIC and IL, although most applications of these methods to date have involved non-quantitative imaging, or studies involving macroscopic models which do not fully utilise the underlying physics. This is perhaps because of the very wide diversity of physical phenomena involved in IBIC processes as can be seen from the list of recent applications in Table 1. It is very clear that one simple model for IBIC pro-
cesses is inadequate to account for all of the phenomena observed. A simpli®ed diagram of some of the many processes that must be taken into account for analysis of IBIC results from simple specimens is shown in Fig. 2. The ®gure also indicates the two possible analysis geometries: normal (or frontal) incidence where the beam is incident parallel to the electric ®eld in the depletion region (or parallel to an externally applied ®eld for specimens that lack a depletion region) and transverse (or lateral) incidence where the beam is incident at right angles to the electric ®eld. The surface topography, in particular the dead layer, will introduce signi®cant image contrast that does not derive from electrical characteristics as such. This is particularly true if the range of the ion beam is comparable with the layer thickness. Obviously, this eect can be reduced, allowing the interior of the specimen to be investigated, by choice of long range ions. In many cases, the one-dimensional model for IBIC put forward by Breese et al. [9] requires
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Table 1 Nuclear microprobe IBIC studies and analysis of electrical characteristics of materials Phenomenon
Beam
Analysis method or model
Reference
Charge trapping at Si/SiO2 interfaces owing to single ion irradiation SEU susceptibility in DRAMS
2 MeV He
[20]
2 MeV He
Radiation induced interface traps formed by the liberation of hydrogen ions Three-dimensional drift-diusion device simulator (MIDSIP: Mitsubishi Device Simulation Package) IBIC and EBIC images compared
2 MeV H
Hecht model for charge trapping
[23]
5 MeV H
Hecht model for charge trapping
[24]
5 MeV H
Qualitative evaluation
[25]
1.3 MeV H
Relative measurements from wells of dierent structures Computer model from ®rst principles
[26]
Charge collection eciency of grain boundaries in polycrystalline silicon solar cells Uniformity and beam induced degradation of semi-insulating GaAs radiation detectors Charge collection eciency of CdTe gamma ray detectors Micro-IBIC comparison of natural and arti®cial diamond Charge collection eciencies of dierent well structures in DRAMs Charge pulse height (charge collection eciency) of grain boundaries in polysilicon solar cells Localised ion eects in MOS devices Soft errors in scaled-down p±n junctions Charge collection in polysilicon solar cells
0.4±2.0 MeV H
2 MeV He 2 MeV He 3 MeV He 2 MeV He, H
Device damage due to oxide trapped holes Relative measurements of junctions of diering size and doping levels. Correlation with trace elements
[21] [22]
[27] [28] [29] [30]
Fig. 2. Schematic diagram of a selection of the phenomena required to be taken into consideration for accurate analysis of IBIC spectra.
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modi®cation for all but the simplest specimen. In the case of actual devices a full three-dimensional treatment is required owing to the three-dimensional structure of the device. Phenomena that must be treated include: charge drift and diusion in the three dimensions, charge trapping at surfaces and defects and funneling owing to the extension of the depletion region due to the plasma column created by ion irradiation. Funneling is most severe for heavy ions incident normal, however the phenomenon also occurs in the transverse mode. As an added complexity, many of the physical parameters typically employed in analytical models may vary signi®cantly across the specimen as a function of proximity to surfaces, interfaces or defects [31]. From the experimental perspective, several simple improvements have been made in Melbourne to improve the quality of IBIC measurements. One of the most signi®cant problems in IBIC experiments is the degradation of the signal by noise. One approach to noise reduction is to use of an in-vacuum preampli®er. A suitable preampli®er is the A250 manufactured by the Amptek
corporation, USA [32]. It is a hermetically sealed unit designed for use with an external FET, which itself may be cooled, that may be selected to match the characteristics of the specimen. The specimen is mounted in very close proximity to the preampli®er, thus reducing earthloops and noise pickup. A special 50 X feedthrough is used to bring the IBIC signals out of the vacuum chamber to the data acquisition system. It is also necessary to pay particular attention to the shaping time used for the ampli®er. This is because diusion of charge in the specimen may take place on time scales of greater than 1 ls and hence use of shorter shaping times may result in incomplete charge collection and misleading images. A recent example of the application of IBIC is shown in Fig. 3. In this case, an array of p±n diodes was evaluated for use as a microdosimeter for measurement of the distribution of radiation in medical therapy involving the precise targeting of radiation. The p±n diodes have a scale closely resembling that of typical body cells and knowledge of the conversion factor for the absorption of radiation by silicon compared to human tissue al-
Fig. 3. IBIC image of a p±n junction array. The pitch of the junctions is 30 lm. The highly complicated IBIC spectrum arises from the many regions of dierent charge collection eciency of the array. The grey scale aligned with the energy axis relates to the corresponding IBIC map. A 2 MeV He beam scanned over 100 ´ 100 lm2 was used for these measurements
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lows accurate dosimetry to be performed with such p±n diode arrays. Although many prior experiments had been done with unfocused beams, such as those from radioactive sources, the IBIC measurement shown in Fig. 3 allowed the full characterisation of the device to be performed [33]. Clearly, the highly non-uniform response of the device made the nuclear microprobe measurements essential. A further interesting application of IBIC has been to the imaging of charge transport properties of polysilicon solar cells where the properties of grain boundaries are of considerable interest and have been extensively investigated by electron and optical-based probes and secondary ion mass spectrometry (SIMS) [34]. IBIC work was ®rst done by Donolato et al. [22] who have also presented sophisticated analytical models for the IBIC signal [27]. Grain boundaries and other structures are clearly visible in a typical IBIC image, which are usually presented as median energy maps. In Fig. 4 a transverse image of a solar cell is shown. The image shows ecient charge collection from the surface depletion region (the collector) where charge drift is the collection mechanism. The image also shows very inecient charge collection from the substrate (the absorber) where charge diusion is the collection mechanism. Considerable non-uniformity is also noticeable in the structure of the depletion region. This is possibly a result of an uneven surface texture leading to the formation of an uneven depletion region during fabrication. This non-uniformity clearly signi®cantly degrades the cell performance.
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Building on the widespread use of unfocused radioactive sources by the RD-42 consortium, IBIC has had a signi®cant role in the study of diamond detector material by the group of Manfredotti et al. [35]. Important ®ndings include evidence for the fact that the pumping phenomenon, where the charge collection eciency increases as a function of radiation dose, is due to the ®lling of traps in the poor eciency grains of the polycrystalline material. However this process is accompanied by a decrease in eciency of the good grains that are fewer in number. The overall eciency becomes more uniform. A further application is discussed in Section 2.4 below. In some cases the surface region of a microelectronic device requires close scrutiny. Therefore IBIC with heavy ions can be used. In Fig. 5 an example of the use of a 15 MeV Si microprobe allows the surface structures of a SA3002 ROM to be imaged with an excellent signal to noise ratio compared to prior studies with light ions (see Ch. 7 of Ref. [7]). This is because the linear energy transfer (LET) of 15 MeV Si at the surface is 14 times larger than 2 MeV He previously used. However, because of the rapid build up of damage in the specimen, only a single pass of the beam across the specimen is possible. 2.3. Hydrogen analysis One of the most signi®cant elements in the control of electrical characterisitics of diamond and polysilicon is hydrogen. This is because in the successful synthesis of diamond by chemical vapour deposition (CVD), hydrogen is involved in
Fig. 4. IBIC image of the depletion region in a commercial polysilicon solar cell obtained in the transverse irradiation mode as shown by the schematic on the right. The surface absorber layer which is white (high eciency) sits on top of a thick p-type collector layer which is dark (low eciency) in the IBIC image. A 2 MeV He beam scanned over 150 ´ 50 lm2 was used for these measurements.
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Fig. 5. Heavy ion IBIC with 15 MeV Si microprobe on a SA3002 ROM. Only a single pass is possible owing to the rapid build-up of damage.
the suppression of the formation of undesirable sp2 bonds in favour of the diamond sp3 bonds. Also, dangling bonds in silicon, which contribute deep levels in the band gap, may be terminated by hydrogen to reduce the number of recombination sites. Several possibilities exist for the measurement of hydrogen with the nuclear microprobe. The ®rst is by nuclear reaction analysis (NRA), a classic method. An extensive review of NRA methods for detecting hydrogen with the nuclear microprobe has been given by Demortier [36]. Alternatively, elastic recoil detection analysis (ERDA) may be used where a heavy projectile knocks out the lighter H nucleus into a detector. The South African group has extensive experience with this method applied with a nuclear microprobe and has demonstrated a minimum detectable limit of about 50 ppm with a 20 MeV 28 Si beam focused to a 10 lm probe [37]. Impressive results were obtained suggesting enhanced H concentration at grain boundaries of CVD diamond where it has presumably been retained from the synthesis process. It would be of great interest to see if the charge trapping in the grain boundaries is reduced as a consequence of the retained hydrogen. A variant on the ERDA method is to use a forward-scattered incident H ion beam to knock out H from a thin specimen in symmetrical forward recoil. The forward-scattered H ion and the recoiling H from the specimen are usually detected in coincidence. Sj oland et al. describe a new development of this technique, speci®cally for nuclear microprobe systems, that has a sensitivity of
20 pg/cm2 [38]. Their system employs a novel segmented detector for enhanced eciency. While obviously restricted to thin specimens that allow transmission of the ion beam, the method clearly has potential for the analysis of hydrogen in thin, synthetic diamond ®lms that have many applications [39,40]. An example of the use of ERDA to map hydrogen in amorphous and polycrystalline silicon solar cells is shown in Fig. 6. In this case a glancing incidence 2.4 MeV He microprobe was employed to produce forward recoiling H from the specimen that was detected by an inexpensive silicon pin diode at a forward angle. The images display a range of interesting phenomena, prominent among which are the local ``hot spots'' of enhanced H concentration, some of which appear to be associated with co-localised deposits of metals as seen in RBS images obtained simultaneously. In these specimens, the observed H is localised in the surface layers of the cell, including the 140 nm surface passivation layer of TiO2 . Degradation of the cell from environmental attack by water can thus be studied with this method. Removal of the surface layers allows the H in the bulk to be studied allowing the localisation of H in grain boundaries to be observed [41]. 2.4. Combined methods As the ®nal part of this review, examples are given of applications of the nuclear microprobe to the study of materials that employ a combination
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Fig. 6. ERDA and RBS images of the hydrogen and metal distribution in amorphous and polycrystalline silicon solar cells. A 2.4 MeV He beam scanned over 900 ´ 900 lm2 was used for these measurements. In this context, ``metal'' means all elements heavier than silicon. The images were collected simultaneously with the beam incident 60° from the specimen surface normal, the RBS detector at a scattering angle of 150° and the ERDA detector at a scattering angle of 20°.
of the classic methods and IBIC or IL. Grain boundaries not only trap charge, but also provide particularly favourable paths for the diusion of impurities. Manfredotti et al. [42] sought correlations between the IBIC signals from a range of semiconductor materials and the corresponding elemental distributions obtained by PIXE. However, most impurities cannot be tolerated in microelectronic devices at concentrations as high as the minimum detectable limit for PIXE. Nevertheless, suggestive correlations were observed between regions of high eciency in polycrystalline diamond and regions also seen to be free from impurities seen by PIXE. As an extension of this work, Manfredotti et al. have also reported remarkable anti-correlations between IBIC maps and IL maps from polycrystalline diamond [43]. An important innovation in this work is the use of photon coincidence counting to reduce noise in the IL spectrum [44]. They observe regions of lower charge collection e-
ciency measured by IBIC which appear to be correlated with regions of stronger IL signals. This is attributed to the presence of traps that produce recombination through radiative transitions and it is dramatically concluded that radiative transitions are the main source of recombination in diamond. An application from the Melbourne group also sought correlations between trace elements from PIXE and IBIC signals in polycrystalline silicon solar cells [30]. While the observed correlations were suggestive, in some regions low IBIC eciency appeared to be correlated with enhanced concentrations of Fe, Cu and Ti, further work is required to determine the signi®cance of the correlations. The example of a combined study presented here, in Fig. 7, is of simultaneous imaging of the RBS, PIXE and IL signals from an IR LED. These images allow detailed evaluation of the variations in performance of the device as a function of position. For the future, the inclusion of IBIC in this list would provide a full structural, electrical and
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Fig. 7. Simultaneous RBS/PIXE and IL images of an IR LED. A 3 MeV H beam scanned over 400 ´ 400 lm2 was used for these measurements.
optical characterisation of this device. It also remains to perform ERDA measurements on this and the other devices discussed here to reveal the role of H. Few other analytical instruments can combine these tasks. 3. 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 dierent 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 signi®cant importance to measure the key electrical parameters cited above. There do not appear to be any reasons why this cannot eventually be done, provided suitable metal contact can be made to the GaN ®lms, and the quality of the material can be made high enough to allow for sucient charge to be collected from regions of interest. It is likely that IBIC characterisation of semiconductors will become as routine as is already the case with the classic methods. 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. The essential assistance of Roland Szymanski in maintaining the facilities in Melbourne is also gratefully acknowledged. References [1] D.N. Jamieson, Nucl. Instr. and Meth. B 136±138 (1998) 1. [2] M.B.H. Breese, Mater. Sci. and Eng. B 42 (1996) 67. [3] C. Manfredotti, F. Fizzotti, P. Polesello, E. Vittone, P. Rossi, G. Egeni, V. Rudello, I. Bogdanovic, M. Jaksic, V. Valkovic, Nucl. Instr. and Meth. B 130 (1997) 491. [4] F.W. Sexton, IEEE Trans. Nucl. Sci. 43 (1996) 687. [5] M. Takai, Nucl. Instr. and Meth. B 130 (1997) 466. [6] D.N. Jamieson, Nucl. Instr. and Meth. B 130 (1997) 706. [7] M.B.H. Breese, D.N. Jamieson, P.J.C. King, Materials Analysis with a Nuclear Microprobe, Wiley, New York, 1996. [8] D. Angell, B.B. Marsh, N. Cue, J.W. Miao, Nucl. Instr. and Meth. B 44 (1989) 172. [9] M.B.H. Breese, P.J.C. King, G.W. Grime, F. Watt, J. Appl. Phys. 72 (1992) 2097. [10] W. Ebert, A. Vescan, P. Gluche, T. Borst, E. Kohn, Diamond and Related Materials 6 (1997) 329. [11] L.Y.S. Pang, S.S.M. Chan, C. Johnston, P.R. Chalker, R.B. Jackman, Diamond and Related Materials 6 (1997) 333. [12] A. Hokazono, T. Ishikura, K. Nakamura, S. Yamashita, H. Kawarada, Diamond and Related Materials 6 (1997) 339.
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[32] http://www.amptek.com. [33] P.D. Bradley, A.B. Rosenfeld, K.K. Lee, D.N. Jamieson, G. Heiser S. Satoh, presented at IEEE Nuclear and Space Radiation Eects Conference (IEEE NSREC) Newport Bch, 20±24 July 1998, IEEE Trans. Nucl. Sci. 45 (1998) 2700±2710. [34] L.L. Kazmerski, Surf. Sci. Rep. 19 (1993) 169. [35] C. Manfredotti, F. Fizzotti, A. Lo Giudice, P. Polesello, E. Vittone, R. Lu, M. Jaksic, presented at the 6th International Conference on New Diamond Science and Technology, Pretoria, South Africa, 31 August±4 September 1998. [36] G. Demortier, Nucl. Instr. and Meth. B 104 (1995) 244. [37] R.D. Maclear, J.E. Butler, S.H. Connell, B.P. Doyle, I.Z. Machi, J.P.F. Sellschop, E. Sideras-Haddad, presented at the Diamond 98 conference, Pretoria, South Africa, September 1998. [38] A. Sj oland, P. Kristiansson, M. Elfman, K.G. Malmqvist, J. Pallon, R.J. Utui, C. Yang, Nucl. Instr. and Meth. B 124 (1997) 639. [39] R. Kalish, R. Richter, E. Cheifetz, A. Zalman, P. Yona, Appl. Phys. Let. 73 (1998) 46. [40] M. Cholewa, T. Kamiya, A. Saint, S. Prawer, G.J.F. Legge, J.E. Butler, D.J. Vestyck Jr., Diamond and Related Materials 7 (1998) 510. [41] C. Yang, E.J. Teo, T. Osipowicz, F. Watt, D. Jamieson, K.K. Lee, B. Tomcik, these proceedings (ICNMTA-6), Nucl. Instr. and Meth. B 158 (1999) 699. [42] C. Manfredotti, F. Fizzotti, E. Vittone, M. Boero, P. Polesello, S. Galassini, M. Jaksic, S. Fazinic, I. Bogdanovic, Nucl. Instr. and Meth. B 109/110 (1996) 555. [43] C. Manfredotti, G. Apostolo, G. Cinque, F. Fizzotti, A.L. Giudice, P. Polesello, M. Truccato, E. Vittone, G. Egeni, V. Rudello, P. Rossi, Diamond and Related Materials 7 (1998) 742. [44] C. Manfredotti, F. Fizzotti, A. Lo Giudice, P. Polesello, E. Vittone, M. Truccato, P. Rossi, presented at the 6th International Conference on New Diamond Science and Technology, Pretoria, South Africa, 31 August±4 September 1998.
www.elsevier.nl/locate/nimb
New problems in nuclear microprobe analysis of materials David N. Jamieson a,*, Deborah R. Beckman a, Andrew A. Bettiol a, Jamie S. Laird a, Kin Kiong Lee a, Steven Prawer a, Andrew Saint a,1, Lachlan C.G. Witham a, Changyi Yang b a
School of Physics, Microanalytical Research Centre, University of Melbourne, Parkville 3052, Australia b Department of Physics, National University of Singapore, Singapore
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. Ó 1999 Elsevier Science B.V. All rights reserved.
1. 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
* Corresponding author. Tel.: +61-3-9344-5376; fax: +61-39347-4783; e-mail: [email protected] 1 Present address: GBC Scienti®c Equipment, 12 Monterey Road, Dandenong, Victoria, Australia
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
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methods of IBIC and ionoluminescence (IL) [7]. It is a signi®cant feature of the work in the ®eld 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 dierences and even some areas of missing theory. This brief review can only give a super®cial 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], ®eld eect 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 speci®cally 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 90 Sr source [15]. However, the polycrystalline nature of synthetic diamond requires spatially resolved
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techniques for complete understanding of the rich phenomena associated with the electrical properties of this material. Some recent results are discussed below. 2. Applications 2.1. 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 micro-structures in GaN, is shown in Fig. 1. Numerous hexagonal growth defects are visible, which probably arise from excessive nitridation of the substrate [18]. Improvements in the growth conditions lead to merging of these hexagonal defects into a smooth ®lm suitable for many applications. The CCM images of the hexagons reveal several interesting features, including excellent crystallinity, comparable to that of the surrounding smooth ®lm, with vmin 3%. The hexagons are about twice as thick as the surrounding ®lm, indicating faster growth. A surprising result is that the hexagons all reveal a larger surface peak compared to the surrounding ®lm. This may provide a clue to the mechanisms responsible for the hexagon growth. The CCM technique has been recently further developed with the development of the beam rocking technique in Oxford. Brie¯y, the technique involves the use of two sets of opposing scan coils on a nuclear microprobe system that allows the beam divergence to be scanned instead of the beam position. Provided the probe forming lens system aberrations are suciently small, the beam may be scanned over 1.5° while remaining within a region less then 300 lm in diameter on the specimen. This allows very accurate and convenient location of the orientation of planar and axial channeling
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Fig. 1. CCM analysis of hexagonal growth defects in epitaxial GaN ®lms on sapphire substrates. A 2.5 MeV He beam scanned over 200 ´ 200 lm2 , aligned with the á0 0 0 1ñ axis of the sapphire substrate was used for these measurements. The smooth curve is a simulation for 0.53 GaN on Al2 O3 .The optical image is for a dierent region compared to the CCM image.
minima. The theory and applications of this technique are reviewed in these proceedings [19]. 2.2. Ion beam induced charge and ionoluminescence The CCM analysis of GaN discussed above allowed useful structural properties of the material to be measured. However, the motivation for the synthesis of GaN is for eventual applications in microelectronic devices. As such, while the structural properties are of interest, it is the desirable electrical properties of the material that are of primary interest. Potentially, these properties may be conveniently measured by IBIC and IL, although most applications of these methods to date have involved non-quantitative imaging, or studies involving macroscopic models which do not fully utilise the underlying physics. This is perhaps because of the very wide diversity of physical phenomena involved in IBIC processes as can be seen from the list of recent applications in Table 1. It is very clear that one simple model for IBIC pro-
cesses is inadequate to account for all of the phenomena observed. A simpli®ed diagram of some of the many processes that must be taken into account for analysis of IBIC results from simple specimens is shown in Fig. 2. The ®gure also indicates the two possible analysis geometries: normal (or frontal) incidence where the beam is incident parallel to the electric ®eld in the depletion region (or parallel to an externally applied ®eld for specimens that lack a depletion region) and transverse (or lateral) incidence where the beam is incident at right angles to the electric ®eld. The surface topography, in particular the dead layer, will introduce signi®cant image contrast that does not derive from electrical characteristics as such. This is particularly true if the range of the ion beam is comparable with the layer thickness. Obviously, this eect can be reduced, allowing the interior of the specimen to be investigated, by choice of long range ions. In many cases, the one-dimensional model for IBIC put forward by Breese et al. [9] requires
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Table 1 Nuclear microprobe IBIC studies and analysis of electrical characteristics of materials Phenomenon
Beam
Analysis method or model
Reference
Charge trapping at Si/SiO2 interfaces owing to single ion irradiation SEU susceptibility in DRAMS
2 MeV He
[20]
2 MeV He
Radiation induced interface traps formed by the liberation of hydrogen ions Three-dimensional drift-diusion device simulator (MIDSIP: Mitsubishi Device Simulation Package) IBIC and EBIC images compared
2 MeV H
Hecht model for charge trapping
[23]
5 MeV H
Hecht model for charge trapping
[24]
5 MeV H
Qualitative evaluation
[25]
1.3 MeV H
Relative measurements from wells of dierent structures Computer model from ®rst principles
[26]
Charge collection eciency of grain boundaries in polycrystalline silicon solar cells Uniformity and beam induced degradation of semi-insulating GaAs radiation detectors Charge collection eciency of CdTe gamma ray detectors Micro-IBIC comparison of natural and arti®cial diamond Charge collection eciencies of dierent well structures in DRAMs Charge pulse height (charge collection eciency) of grain boundaries in polysilicon solar cells Localised ion eects in MOS devices Soft errors in scaled-down p±n junctions Charge collection in polysilicon solar cells
0.4±2.0 MeV H
2 MeV He 2 MeV He 3 MeV He 2 MeV He, H
Device damage due to oxide trapped holes Relative measurements of junctions of diering size and doping levels. Correlation with trace elements
[21] [22]
[27] [28] [29] [30]
Fig. 2. Schematic diagram of a selection of the phenomena required to be taken into consideration for accurate analysis of IBIC spectra.
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modi®cation for all but the simplest specimen. In the case of actual devices a full three-dimensional treatment is required owing to the three-dimensional structure of the device. Phenomena that must be treated include: charge drift and diusion in the three dimensions, charge trapping at surfaces and defects and funneling owing to the extension of the depletion region due to the plasma column created by ion irradiation. Funneling is most severe for heavy ions incident normal, however the phenomenon also occurs in the transverse mode. As an added complexity, many of the physical parameters typically employed in analytical models may vary signi®cantly across the specimen as a function of proximity to surfaces, interfaces or defects [31]. From the experimental perspective, several simple improvements have been made in Melbourne to improve the quality of IBIC measurements. One of the most signi®cant problems in IBIC experiments is the degradation of the signal by noise. One approach to noise reduction is to use of an in-vacuum preampli®er. A suitable preampli®er is the A250 manufactured by the Amptek
corporation, USA [32]. It is a hermetically sealed unit designed for use with an external FET, which itself may be cooled, that may be selected to match the characteristics of the specimen. The specimen is mounted in very close proximity to the preampli®er, thus reducing earthloops and noise pickup. A special 50 X feedthrough is used to bring the IBIC signals out of the vacuum chamber to the data acquisition system. It is also necessary to pay particular attention to the shaping time used for the ampli®er. This is because diusion of charge in the specimen may take place on time scales of greater than 1 ls and hence use of shorter shaping times may result in incomplete charge collection and misleading images. A recent example of the application of IBIC is shown in Fig. 3. In this case, an array of p±n diodes was evaluated for use as a microdosimeter for measurement of the distribution of radiation in medical therapy involving the precise targeting of radiation. The p±n diodes have a scale closely resembling that of typical body cells and knowledge of the conversion factor for the absorption of radiation by silicon compared to human tissue al-
Fig. 3. IBIC image of a p±n junction array. The pitch of the junctions is 30 lm. The highly complicated IBIC spectrum arises from the many regions of dierent charge collection eciency of the array. The grey scale aligned with the energy axis relates to the corresponding IBIC map. A 2 MeV He beam scanned over 100 ´ 100 lm2 was used for these measurements
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lows accurate dosimetry to be performed with such p±n diode arrays. Although many prior experiments had been done with unfocused beams, such as those from radioactive sources, the IBIC measurement shown in Fig. 3 allowed the full characterisation of the device to be performed [33]. Clearly, the highly non-uniform response of the device made the nuclear microprobe measurements essential. A further interesting application of IBIC has been to the imaging of charge transport properties of polysilicon solar cells where the properties of grain boundaries are of considerable interest and have been extensively investigated by electron and optical-based probes and secondary ion mass spectrometry (SIMS) [34]. IBIC work was ®rst done by Donolato et al. [22] who have also presented sophisticated analytical models for the IBIC signal [27]. Grain boundaries and other structures are clearly visible in a typical IBIC image, which are usually presented as median energy maps. In Fig. 4 a transverse image of a solar cell is shown. The image shows ecient charge collection from the surface depletion region (the collector) where charge drift is the collection mechanism. The image also shows very inecient charge collection from the substrate (the absorber) where charge diusion is the collection mechanism. Considerable non-uniformity is also noticeable in the structure of the depletion region. This is possibly a result of an uneven surface texture leading to the formation of an uneven depletion region during fabrication. This non-uniformity clearly signi®cantly degrades the cell performance.
633
Building on the widespread use of unfocused radioactive sources by the RD-42 consortium, IBIC has had a signi®cant role in the study of diamond detector material by the group of Manfredotti et al. [35]. Important ®ndings include evidence for the fact that the pumping phenomenon, where the charge collection eciency increases as a function of radiation dose, is due to the ®lling of traps in the poor eciency grains of the polycrystalline material. However this process is accompanied by a decrease in eciency of the good grains that are fewer in number. The overall eciency becomes more uniform. A further application is discussed in Section 2.4 below. In some cases the surface region of a microelectronic device requires close scrutiny. Therefore IBIC with heavy ions can be used. In Fig. 5 an example of the use of a 15 MeV Si microprobe allows the surface structures of a SA3002 ROM to be imaged with an excellent signal to noise ratio compared to prior studies with light ions (see Ch. 7 of Ref. [7]). This is because the linear energy transfer (LET) of 15 MeV Si at the surface is 14 times larger than 2 MeV He previously used. However, because of the rapid build up of damage in the specimen, only a single pass of the beam across the specimen is possible. 2.3. Hydrogen analysis One of the most signi®cant elements in the control of electrical characterisitics of diamond and polysilicon is hydrogen. This is because in the successful synthesis of diamond by chemical vapour deposition (CVD), hydrogen is involved in
Fig. 4. IBIC image of the depletion region in a commercial polysilicon solar cell obtained in the transverse irradiation mode as shown by the schematic on the right. The surface absorber layer which is white (high eciency) sits on top of a thick p-type collector layer which is dark (low eciency) in the IBIC image. A 2 MeV He beam scanned over 150 ´ 50 lm2 was used for these measurements.
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Fig. 5. Heavy ion IBIC with 15 MeV Si microprobe on a SA3002 ROM. Only a single pass is possible owing to the rapid build-up of damage.
the suppression of the formation of undesirable sp2 bonds in favour of the diamond sp3 bonds. Also, dangling bonds in silicon, which contribute deep levels in the band gap, may be terminated by hydrogen to reduce the number of recombination sites. Several possibilities exist for the measurement of hydrogen with the nuclear microprobe. The ®rst is by nuclear reaction analysis (NRA), a classic method. An extensive review of NRA methods for detecting hydrogen with the nuclear microprobe has been given by Demortier [36]. Alternatively, elastic recoil detection analysis (ERDA) may be used where a heavy projectile knocks out the lighter H nucleus into a detector. The South African group has extensive experience with this method applied with a nuclear microprobe and has demonstrated a minimum detectable limit of about 50 ppm with a 20 MeV 28 Si beam focused to a 10 lm probe [37]. Impressive results were obtained suggesting enhanced H concentration at grain boundaries of CVD diamond where it has presumably been retained from the synthesis process. It would be of great interest to see if the charge trapping in the grain boundaries is reduced as a consequence of the retained hydrogen. A variant on the ERDA method is to use a forward-scattered incident H ion beam to knock out H from a thin specimen in symmetrical forward recoil. The forward-scattered H ion and the recoiling H from the specimen are usually detected in coincidence. Sj oland et al. describe a new development of this technique, speci®cally for nuclear microprobe systems, that has a sensitivity of
20 pg/cm2 [38]. Their system employs a novel segmented detector for enhanced eciency. While obviously restricted to thin specimens that allow transmission of the ion beam, the method clearly has potential for the analysis of hydrogen in thin, synthetic diamond ®lms that have many applications [39,40]. An example of the use of ERDA to map hydrogen in amorphous and polycrystalline silicon solar cells is shown in Fig. 6. In this case a glancing incidence 2.4 MeV He microprobe was employed to produce forward recoiling H from the specimen that was detected by an inexpensive silicon pin diode at a forward angle. The images display a range of interesting phenomena, prominent among which are the local ``hot spots'' of enhanced H concentration, some of which appear to be associated with co-localised deposits of metals as seen in RBS images obtained simultaneously. In these specimens, the observed H is localised in the surface layers of the cell, including the 140 nm surface passivation layer of TiO2 . Degradation of the cell from environmental attack by water can thus be studied with this method. Removal of the surface layers allows the H in the bulk to be studied allowing the localisation of H in grain boundaries to be observed [41]. 2.4. Combined methods As the ®nal part of this review, examples are given of applications of the nuclear microprobe to the study of materials that employ a combination
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Fig. 6. ERDA and RBS images of the hydrogen and metal distribution in amorphous and polycrystalline silicon solar cells. A 2.4 MeV He beam scanned over 900 ´ 900 lm2 was used for these measurements. In this context, ``metal'' means all elements heavier than silicon. The images were collected simultaneously with the beam incident 60° from the specimen surface normal, the RBS detector at a scattering angle of 150° and the ERDA detector at a scattering angle of 20°.
of the classic methods and IBIC or IL. Grain boundaries not only trap charge, but also provide particularly favourable paths for the diusion of impurities. Manfredotti et al. [42] sought correlations between the IBIC signals from a range of semiconductor materials and the corresponding elemental distributions obtained by PIXE. However, most impurities cannot be tolerated in microelectronic devices at concentrations as high as the minimum detectable limit for PIXE. Nevertheless, suggestive correlations were observed between regions of high eciency in polycrystalline diamond and regions also seen to be free from impurities seen by PIXE. As an extension of this work, Manfredotti et al. have also reported remarkable anti-correlations between IBIC maps and IL maps from polycrystalline diamond [43]. An important innovation in this work is the use of photon coincidence counting to reduce noise in the IL spectrum [44]. They observe regions of lower charge collection e-
ciency measured by IBIC which appear to be correlated with regions of stronger IL signals. This is attributed to the presence of traps that produce recombination through radiative transitions and it is dramatically concluded that radiative transitions are the main source of recombination in diamond. An application from the Melbourne group also sought correlations between trace elements from PIXE and IBIC signals in polycrystalline silicon solar cells [30]. While the observed correlations were suggestive, in some regions low IBIC eciency appeared to be correlated with enhanced concentrations of Fe, Cu and Ti, further work is required to determine the signi®cance of the correlations. The example of a combined study presented here, in Fig. 7, is of simultaneous imaging of the RBS, PIXE and IL signals from an IR LED. These images allow detailed evaluation of the variations in performance of the device as a function of position. For the future, the inclusion of IBIC in this list would provide a full structural, electrical and
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Fig. 7. Simultaneous RBS/PIXE and IL images of an IR LED. A 3 MeV H beam scanned over 400 ´ 400 lm2 was used for these measurements.
optical characterisation of this device. It also remains to perform ERDA measurements on this and the other devices discussed here to reveal the role of H. Few other analytical instruments can combine these tasks. 3. 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 dierent 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 signi®cant importance to measure the key electrical parameters cited above. There do not appear to be any reasons why this cannot eventually be done, provided suitable metal contact can be made to the GaN ®lms, and the quality of the material can be made high enough to allow for sucient charge to be collected from regions of interest. It is likely that IBIC characterisation of semiconductors will become as routine as is already the case with the classic methods. 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. The essential assistance of Roland Szymanski in maintaining the facilities in Melbourne is also gratefully acknowledged. References [1] D.N. Jamieson, Nucl. Instr. and Meth. B 136±138 (1998) 1. [2] M.B.H. Breese, Mater. Sci. and Eng. B 42 (1996) 67. [3] C. Manfredotti, F. Fizzotti, P. Polesello, E. Vittone, P. Rossi, G. Egeni, V. Rudello, I. Bogdanovic, M. Jaksic, V. Valkovic, Nucl. Instr. and Meth. B 130 (1997) 491. [4] F.W. Sexton, IEEE Trans. Nucl. Sci. 43 (1996) 687. [5] M. Takai, Nucl. Instr. and Meth. B 130 (1997) 466. [6] D.N. Jamieson, Nucl. Instr. and Meth. B 130 (1997) 706. [7] M.B.H. Breese, D.N. Jamieson, P.J.C. King, Materials Analysis with a Nuclear Microprobe, Wiley, New York, 1996. [8] D. Angell, B.B. Marsh, N. Cue, J.W. Miao, Nucl. Instr. and Meth. B 44 (1989) 172. [9] M.B.H. Breese, P.J.C. King, G.W. Grime, F. Watt, J. Appl. Phys. 72 (1992) 2097. [10] W. Ebert, A. Vescan, P. Gluche, T. Borst, E. Kohn, Diamond and Related Materials 6 (1997) 329. [11] L.Y.S. Pang, S.S.M. Chan, C. Johnston, P.R. Chalker, R.B. Jackman, Diamond and Related Materials 6 (1997) 333. [12] A. Hokazono, T. Ishikura, K. Nakamura, S. Yamashita, H. Kawarada, Diamond and Related Materials 6 (1997) 339.
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