High energy heavy ions in materials characterization at NSC Pelletron

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ELSEVIER

Nuclear Instruments and Methods in Physics Research B 136138 (1998) 729-735

High energy heavy ions in materials characterization Pelletron D.K. Avasthi

at NSC

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Abstract High energy heavy ions available at 1.5MV Pelletron at Nuclear Science Centre (NSC), Delhi, are being extensively used for research in materials science. The energetic heavy ions play an important role in materials characterization and in materials modification. High energy ion backscattering and elastic recoil detection (ERD) provide identification as well as depth profiling of high Z and low Z elements, respectively. Elastic recoils produced by the impingement of energetic ions have vital role in characterization of materials by the technique of ERD. A variety of high energy ions have been used at the NSC Pelletron for the analysis of different types of materials. H depth profiling in diamond-like carbon (DLC) and diamond films provide valuable information on the growth of these films and their dependence on deposition conditions. The developments for ion beam analysis at NSC are briefly outlined. Ion beam techniques provide an excellent opportunity for in situ monitoring of the modification of materials in some specific cases. Elastic recoils pro0 1998 Elsevier Science B.V. duced by heavy ions have also been used for stopping power measurements.

1. Introduction Energetic (MeV) heavy ion irradiation of materials causes modification to their properties, which is distinctly different from that produced by low energy ions in the keV energy region. Therefore, ion beam modification of materials produced by swift heavy ions has attracted a large number of researchers. It is desirable to keep in view the type

of possible interactions of the ions with materials. Ions interact with material either by the process of elastic collision referred to as nuclear stopping or by inelastic collisions referred to as electronic stop-

’ Fax: +0091 11 6893666; tel.: +0091 11 6893955; e-mail: [email protected]. 0168-583X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PIISO168-583X(97)00815-X

ping. The ions undergo elastic collisions with the atoms of the material in the process of nuclear stopping, as a result of which the atoms are displaced from their sites, creating vacancies and displaced atoms in interstitial sites. This process dominates at low energy. The other process, i.e. the electronic stopping of ions in materials, dominates when the ion energy is such that its velocity is close to or greater than the velocity of the valence electrons. In the electronic stopping, the incident ions make inelastic collisions with the atoms of the material and the atoms either get excited or get ionized. The process of electronic excitation of the atoms causes different effects in different materials. It creates columnar defects in high T, materials [l], cylindrical tracks in amorphization in some materials,

polymers [2], phase transi-

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tions [3], structural changes [4], etc. Therefore, the energies of tens of MeV to 200 MeV of different ions available with 15 UD Pelletron [5,6] at Nuclear Science Centre (NSC) provides a unique opportunity to materials scientists to modify the materials preferentially via electronic excitation process. The use of ion beam techniques for materials analysis is another aspect of ions in materials. The non-destructive nature of ion beam analysis is a key positive feature as compared to other techniques like SIMS. There is a possibility of radiation damage or materials modification in the sample by irradiation during ion beam analysis. This however can be looked as blessing in disguise as it allows us to monitor the modification of materials by the ion beam. It has been observed that the contents of H, N, 0 do change [779] under ion irradiation. Ion beam techniques offer the possibility of monitoring such effects. Among various ion beam techniques, Rutherford backscattering spectrometry (RBS) [lo] and elastic recoil detection analysis (ERDA) [l I] attract special attention due to their capability of depth profiling high Z and low Z elements with a reasonable depth resolution (lo-20 nm) and sensitivity (0.1 at.%). The developments, especially in ERDA in the last decade, made it possible to expand the span of mass (of elements), which can be analyzed by this method. Other techniques such as nuclear reaction analysis [12,13] are applicable to one element at a time. In this talk, the developments and activities in ion beam techniques, and online monitoring of ion induced changes, carried out at NSC, are discussed.

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The lighter elements in a sample however can be detected by the ERDA technique. In this technique, when ions are incident on a tilted sample, it imparts energy to the atoms (in the sample) in the collision, as a result of which the atoms recoil in forward direction. These recoils are detected in a detector located at forward angle beyond the tilt angle of the sample. The detector has a stopper foil of an appropriate thickness to stop the unwanted scattered particle and heavier recoils so that only light mass recoils can be detected. The use of stopper foil can be avoided in some specific cases when the incident ion is heavier than the atoms of the sample and the detector is kept beyond the maximum scattering angle as discussed in the following section. 2.1. ERDA

iu CItrunmission

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The analysis of thin, self-supporting films in ERDA is quite simple. One need not keep the sample tilted, instead it is kept perpendicular with respect to the incident ion beam. One such example [14] is the analysis of a thin self-supporting carbon film by ERDA with 100 MeV I ions. The detection angle was more than the maximum scattering angle so that elastically scattered ions do not reach the detector. It indicated the capability of analyz-

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1200 C

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2. Elastic recoil detection analysis

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RBS technique has the limitation of poor sensitivity for light elements (say for atomic numbers < 14 or so). The use of heavy ions can improve the sensitivity due to a larger cross section but this also meets a limitation, which is that high mass ions cannot scatter back from a lighter mass and hence do not provide any signal. For example, Si ions cannot scatter back from light ions such as C, N, 0, Mg, Al, etc.

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i

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075

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Fig. I. ERDA spectrum [16] of a thin self-supporting C foil with thin deposition of Ca. The experiment was performed in a transmission geometry using 100 MeV Ag ions. Identified elements are marked on the peaks.

D. K. Avasthi I Nucl. Instr. and Meth. in Phys. Res. B 136-138

ing the light elements C, N, 0, etc. of adjacent masses in a thin self-supporting film. A similar approach has been adopted by Assmann and Maier [IS] and Kabiraj et al. 1161.A typical recoil spectrum in the case of a thin, self-supporting film of Ca deposited on a thin C backing is shown in Fig. 1. 2.2. ERDA in a reflection geometry 2.2.1. Simultaneous detection of H, N and Si The detection of neighboring mass elements by conventional ERD is possible only in the case of a thin, self-supporting film. If the sample is thick or it is on a substrate, the recoil energies of different elements overlap. It has been shown by Avasthi et al. [17] that a few masses of elements can be distinguished in conventional ERD, if the masses are well separated as in the case of a-SiN,:H film. It is achieved by the appropriate choice of thickness of stopper foil and proper choice of the substrate. 90 MeV Ni ions were used for the characterization. In the above example, the film was on a Fe substrate and the stopper foil thickness was chosen to stop the Fe recoils and the elastically scattered Ni ions.

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The sample was kept in a reflection geometry. The recoil spectrum from the sample is shown in Fig. 2. 2.2.2. Simultaneous detection of H, C, N and 0 in a DLC_film We noticed in the example in Section 2.1 that the simultaneous detection of neighboring mass light elements is possible by conventional ERDA in case of a thin, self-supporting foil. The situation, however, does not remain the same in the case of thick films or thin films on a thick substrate. In case of a thick film, the recoil energies of the neighboring elements overlap which results in loss of mass resolution. In the case of a thin film on a substrate, one needs to have a stopper foil to stop unwanted recoils and scattered ions. The use of a stopper foil causes energy straggling due to which recoil energies of the neighboring elements overlap, resulting in poor mass resolution. There are various ways to overcome this problem. Particle identification techniques such as (1) simultaneous measurements of time and energy by time of flight (TOF) [l&19], telescope detectors [20,21], Bragg curve spectrometer [22] and magnetic spectrometer [23] are techniques to discriminate neighboring mass elements. Detectors based on the gaseous ionization chamber were developed at NSC for materials characterization. The detector

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Fig. 2. ERDA spectrum of a-SiN,:H film on a thick stainless steel substrate. The broad bump corresponds to N and Si recoils and the sharp peak corresponds to H recoils. N recoils are distinguished as those above the dotted curve.

Fig. 3. Two dimensional film of DLC.

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plot of the recoils from a thin

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telescope consists of two detectors, AE and E. The details of these are available elsewhere [24] and references therein. One example of simultaneous detection of H. C, N and 0 in a diamond-like carbon (DLC) film with a AE gaseous - E semiconductor detector telescope is reported by Avasthi et al. [24]. Fig. 3 shows a typical bi-dimensional plot which separates out H, C, N and 0. A Bragg curve spectrometer consists of a gaseous ionization chamber with an entrance polymer window and anode to collect the signal. It has a uniform transverse electric field produced by field shaping electrodes. The ionization signal produced by the ions is processed in two channels of electronics; one with large time constant giving the total energy of the ion and the other with a short time constant giving a method of Z discrimination. One such detector has been developed [25] at NSC and is in use for materials characterization. 2.2.3. Large area position sensitive detector The requirements of sensitivity and depth resolution are contradictory to each other in the sense that if one wants to improve one, the other deteriorates. In an ERD experiment, one may like to have high sensitivity to achieve the analysis of the sample at smaller dose, so that radiation damage effects are minimized on the sample. This can be achieved by the use of a large area position sensitive detector. Large area provides high sensitivity and at the same time the depth resolution is not effected because the kinematic broadening is corrected by the software with the help of the position sensitive feature of the detector. Its use was first demonstrated by Assmann et al. [26]. We at NSC, Delhi, plan to fabricate a large area position sensitive detector and implement the necessary software for kinematic correction.

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noticed that evolution of H, N, 0, etc. takes place in the process of irradiation [7-91. The changes in H concentration were observed in diamond, DLC, porous Si, a-Si films, etc. Therefore, ion irradiation offers a possibility to alter the hydrogen concentration to the desired level in hydrogenous systems. This process, however, will always be combined with structural changes and, therefore, requires detailed investigation. Fig. 4 shows a typical example [27] of the loss of H during ion irradiation in the case of a diamond film. It indicates that the film prepared at pressure below 30 Torr are more radiation resistant in terms of the H content. It clearly shows that transport of H atoms due to ion irradiation can be monitored in situ. It has been noticed [28] in an experiment at large electronic excitation that a thin film of Fe absorbs 0 at the surface and it further diffuses in the film. This study was possible with the use of a large area position sensitive detector [26] with kinematic correction. Large electronic excitation was generated by 243 MeV Au ions impinging on the Fe film. 3.2. On-line monitoring

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Since ion beam techniques can measure the concentration of elements in a thin film, it is possible to monitor the changes (decrease) in the concentration of the elements due to sputtering. In

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3. Dynamic monitoring during ion irradiation 3.1. On-line nlonitorblg tion

of’ desorption

and absorp-

The ion beam techniques described in Section 2 can be exploited to monitor changes in concentration of elements during ion irradiation. It has been

Fig. 4. Plot for H concentration observed as a function of ion dose in case of DLC film bombarded by 85 MeV Ni ions.

the case of insulators [29], the expected sputtering rate can be as high as 1000 atoms per incident ion. The sensitivity limit of the technique allows us to measure sputtering of a few hundreds of sputtered atoms per incident ion. 3.3. On-line interficce

monitoring

of ion induced

Qects

at

There is a possibility of monitoring the ion beam mixing effects at the interface with the use of on-line ERDA. by choosing the experimental parameters carefully.

4. Summary of some important ion beam studies in materials at NSC Some important results of ion beam utilization in materials research carried out at NSC are outlined. 4.1. Porous silicon An energetic heavy ion beam provided useful information on the elemental constituents of a porous silicon (PS) layer [30]. It showed unusually large amount of carbon. which was consistent with a previous report [31]. The uniqueness of the measurement was that it required only one experiment using a detector telescope with ERDA, as compared to other measurements [32], where two or more than two experiments were performed for the same objective. The high energy heavy ion (85 MeV Ni ions) irradiation at a dose of 2 x lOI3 ions/cm’ caused significant reduction [33] in the photo luminescence of the PS layer. It was accompanied by the disappearance of H-SOH and Si-O-Si bonds and a reduction in the intensity of Si-H bonds. The latter was associated with H loss under irradiation which was monitored on-line by ERDA. In other work [34], irradiation with lower energy lighter ions ( 10 MeV Si) produced a desirable change by increasing the PL intensity significantly. It was noticed that the PL from the PS layer was highly stable with time. The changes were associated with an increase in !&OH bonds.

4.2. Valuable injkmation and diamond films

on growth

of DLC,films

Ion beam analysis was carried out on a large number of DLC and diamond films prepared under different conditions. These studies gave valuable information [35] on the growth of these films related to the H concentration. The H content was determined [27] in the films deposited under various deposition conditions such as gas flow, chamber pressure. substrate temperature, etc. The H concentration, in general, was found to be higher in the DLC films and in the diamond films having non-diamond carbon impurities. The H concentration in diamond films was found to be
Normally, it is believed that the implanted species in a high Z matrix can be depth profiled by conventional heavy ion ERD. It has been shown by us [39] that in specific cases where the implanted species is near C, this may give misleading results. It is due to the fact that C is often incorporated during the process of ion implantation. It was shown [39] that such cases can be examined unambiguously by the use of a telescope detector with ERD. 4.4. Dependence phase

of implanted element profile on the

The depth profiling of B in B-implanted pure fee phase stainless steel and mixed phase (fee and

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bee) stainless steel was carried out to investigate different tribological properties of the two specimens. It was found [40] that the mixed phase stainless steel sample has a narrower B profile and a higher hardness. 4.5. StudI> sumpIes

qf’ H in hydrogenated Pdlsemiconductor

Hydrogen depth profiling [41] of hydrogenated and as-deposited Pd films on semiconductor (Si and GaAs) substrates was carried out to understand the changes in the properties of the hydrogenated device. The study revealed that the hydrogen is present in the Pd and the semiconductor substrate even in the as-deposited samples. The H content goes up after hydrogenation. The study also indicated that p-type Si absorbs more H than n-type Si.

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The energy loss dE in a foil of known thickness (placed in front of one of the detectors) is determined by the energy shift in recoils A and B. The technique was further modified by Bhagwat and Avasthi [44]. in which a twin detector mount was designed and fabricated in such a way that the recoils subtend an equal angle at both the detectors. The evolved method was utilized in a series of experiments [4548] on the stopping power measurements.

6. Conclusion

An alloy of Zr, V and Fe which is a non-evaporable getter (NEG) material, is used in vacuum applications for pumping [42]. A saturated NEG strip and a virgin NEG strip were taken for the intake study of H and other gases during vacuum pumping. It clearly indicated that H was absorbed by the strip in appreciable amount. Such analytical studies using ion beams reveal information on the process of adsorption and diffusion.

High energy heavy ions at the NSC Pelletron are utilized for materials modification as well as for materials analysis. The ion beam techniques utilized at the NSC Pelletron are outlined along with some suitable examples of characterization of PS, DLC, diamond films, NEG strip and implanted samples, carried out at NSC. High energy heavy ions make it possible to analyze and depth profile neighboring-mass light elements such as B, C, N, 0, etc. which, in general. is difficult by other techniques. Monitoring of the changes in concentration of light gaseous species like H, N, 0, etc. is possible during ion irradiation. This is likely to provide better understanding of materials modification caused by ion beams. A scenario of wide variety of research work with energetic ion beams is presented.

5. Elastic recoils for dE/ds measurements

Acknowledgements

Stopping power (dE/ds) measurements are of wide interest due to their vast applications. The knowledge of dElds is needed in materials analysis by RBS and ERDA, lifetime measurement of nuclear states by the Doppler shift attenuation method, understanding of ion-atom collisions. etc. A novel approach was suggested and demonstrated by Nath et al. [43] to measure the stopping power of foils for different ions. The elastic recoils produced by the main beam were used for this study, which had definite advantages in terms of saving experimental time and in the study of the dE/ds dependence on the atomic number of the ion.

The help by my colleagues Mr. D. Kabiraj and Mr. E.T. Subramaniyam in implementation of ERDA and development of necessary software is gratefully acknowledged. The financial support from Department of Science and Technology for development of facilities for research in materials science with ion beam is gratefully acknowledged.

4.4. H depth pmjiling in u NEG strip

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