Nuclear reaction Analysis (NRA) (Continued)
| In this lecture I am going to discuss the resonance method of NRA. As mentioned in the previous lecture, a resonance peak occurs in many nuclear reactions, which is a function of incident energy. Depth profile is done by measuring the reaction yield as a function of probing ion beam. In practice, the resonances are observed by varying the incident ion energy in small steps and the yield is measured with fluence. Let us first see the following ideal case of resonance as depicted schematically in Fig. m5.10. | |||||||||||||||||||||||||||||||||||
/5.16.png) | |||||||||||||||||||||||||||||||||||
FIGURE m5.10 Concentration profile using resonance reaction of NRA | |||||||||||||||||||||||||||||||||||
| In this case (as seen in lower plot), incident ions of energy E (larger than ER, the resonance energy for reaction with any impurity at trace level) are slowed down until ER is reached at a depth x. The nuclear reaction occurs at this depth at a rate proportional to the impurity concentration. If the angle between the incident ion and the surface normal of any sample is α, the depth x and the energy of the incident ion can be correlated with the following equation:5 | |||||||||||||||||||||||||||||||||||
/equ5_18.png){kind=small} | |||||||||||||||||||||||||||||||||||
| The yield curve as shown in as shown in above figure can be converted into the desired concentration profile by simply changing scales of yield and energy to corresponding scales of concentration and depth, respectively. | |||||||||||||||||||||||||||||||||||
| Let us look at the following example: | |||||||||||||||||||||||||||||||||||
| One classic example to determine depth profile of H implanted in Al2O3 using 19F as a probe beam.7 it is known that a strong resonance occurs at ~16.4 MeV when a reaction between H and F takes place [shown in Fig. m5.11]. | |||||||||||||||||||||||||||||||||||
/m5_17.png) | |||||||||||||||||||||||||||||||||||
FIGURE m5.11 H(19F, α γ) reaction indicating resonance at 16.4 MeV in H implanted Al2O3. (shown in the inset) | |||||||||||||||||||||||||||||||||||
| The depth profile extracted using above equation is depicted in figure m5.12 below considering the reaction yield within the depth d1 to d2. | |||||||||||||||||||||||||||||||||||
/m5_18.png) | |||||||||||||||||||||||||||||||||||
FIGURE m5.12 Schematic depth profile of the above system. | |||||||||||||||||||||||||||||||||||
| Some of the important reactions, which are being routinely used in NRA are listed below: | |||||||||||||||||||||||||||||||||||
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| NRA is used for measuring absolute concentration (atoms/cm2) of light impurities and also provides absolute calibration for other surface sensitive techniques like Secondary ion mass spectrometry, Auger electron spectroscopy etc. | |||||||||||||||||||||||||||||||||||