S.S. Klein

A model for ion-irradiation induced hydrogen loss from organic materials

In the study of interfacial diffusion processes in polymer light-emitting diodes, the use of high-energy ion-scattering techniques can be of great value due to the possibility of quantitative elemental depth profiling. However, ion irradiation of polymers is known to cause various degradation effects, including the loss of hydrogen. Since the hydrogen loss determines the accuracy of depth profiling, it is an interesting subject for study in order to define experimental conditions in which the degradation is suppressed. The loss of hydrogen from porphyrins (organic solar cells) has been measured by means of elastic recoil detection analysis with 2, 4, and 7.6 MeV He+ beams. A theoretical model is proposed in which the hydrogen loss is described through the formation and recombination of free hydrogen radicals. A distinct difference is introduced between direct recombination processes and the diffusion of radicals out of the ion track.

Separate determination of concentration profiles for atoms with different masses by simultaneous measurement of scattered projectile and recoil energies

Abstract When the energies of a scattered projectile and the recoiling target nucleus are measured both at the same time, the energy of the lightest particle can be used to select the mass while the energy of the heaviest particle is most sensitive to the depth from which both particles emerge. This is shown for α particles of 30 MeV scattered at 82° on 12C, 13C, 16O and 27Al and the corresponding recoils. The depth resolution of about 50 nm is comparable to that obtained in the measurement of recoil energies from the elastic scattering of heavy ions. The selectivity is almost perfect even between the carbon isotopes. Given suitable improvements, the sensitivity may reach the ppm region.

Improved depth resolution in CERDA by recoil time of flight measurement

Detector limitations to the energy resolution can be avoided by measuring the time of flight over a sufficient distance. For 6 MeV 12C ions the energy resolution has been improved from 70 to 20 keV on a 3 m flight path. The depth resolution obtained in CERDA (coincident elastic recoil detection analysis) with a 12.6 MeV beam from the Eindhoven AVF cyclotron improves by a smaller factor (from 50 to 30 nm) because the depth resolution is now dominated by kinematic effects of beam emittance and detection solid angle. Without sensitivity loss the depth resolution may be further improved to 15 nm selecting the beam in a more effective way.

Recoil selection by pulse shape discrimination in elastic recoil detection analysis with α-particles (α-ERDA)

Abstract In elastic recoil detection analysis (ERDA) with α-particles, recoils can be separated from scattered projectiles by using detectors with a thin depletion layer and a pulse shape discrimination (PSD) circuit [S.S. Klein and H.A. Rijken, Nucl. Instr. and Meth. B 66 (1992) 393]. Improved performance is obtained with a multi-parameter data acquisition system; recoil events are more easily and accurately selected and sensitivity is improved by a factor of 5 to 1 × 10 14 at/cm 2 for oxygen on amorphous silicon. The typical surface depth resolution is 10 nm. Two alternative PSD circuits are proposed neither of them using fast electronics. Examples are given of profiling carbon and oxygen in thin films of Si, C and SiO x N y .

Elastic recoil detection analysis with high energy alpha beams

Abstract Detection of low Z recoils (C, N, O) from scattering of high energy alpha particles (10–15 MeV) is compared with conventional ERDA with heavy ion projectiles. Recoils and scattered a particles are separated using pulse shape discrimination or coincidence methods. Submonolayer sensitivity and depth resolution in the nm range is obtained. Depth/mass ambiguity may be avoided.

Improvements of the eindhoven proton microbeam

The EUT microbeam is produced from a cyclotron beam by a 1×1 mm object diaphragm and a quadruplet lens system of high demagnification. To limit the effect of chromatic aberrations the beam energy spread can be reduced by tuning the beam transport system to a dispersive mode and the aperture of the lens system can be reduced. With the transport system in the normal achromatic mode, using 3.5 MeV protons a spot of 60×40 μm with a beam current of 150 nA is obtained. In the dispersive mode a spot of 10×10 μm with a beam current of 2 nA can be achieved. The microprobe is used for scanning PIXE analysis and three dimensional NRA and RBS.

Subnanosecond timing with ion-implanted detectors

The energy resolution of ion-implanted charged particle detectors may be improved by decreasing the thickness of the implanted detector window to minimize energy straggling. Because of the resistance of this layer, however, the timing depends on the position of entry. Two solutions to this conflict between energy resolution and time resolution are studied: evaporating a very thin aluminum layer on the detector window and fabricating a rectangular detector. Both solutions are shown to be successful with a total time resolution in the low subnanosecond region (< 200 ps).

Ion microscopy using magnifying ion TRANSPORT systems and position sensitive detectors

Abstract Microanalysis by detection of charged particles may be speeded up by several orders of magnitude when position sensitive detectors are used at the focus of a magnifying lens system. Using beam optics calculations a resolution on the sample of 40 × 7 μm 2 appears possible with available quadrupoles with an acceptance of 0.375 msr selecting particle energy within a 1.2% range. Even submicron resolution may be possible when energy is selected within a 0.3% range. The projection system has an appreciable field of view: sample regions of the order of 1 mm 2 may be studied simultaneously when appropriate detectors are used.

Elastic recoil detection of light elements (C, N, O) with high energy (10–15 MeV) He beams

Abstract Elastic recoil detection (ERD) of carbon, nitrogen and oxygen from thin films on thick substrates was demonstrated using 10–15 MeV He2+ ions and thin films detectors. Non-Rutherford scattering was used to alter the relative sensitivities for carbon, nitrogen and oxygen by selecting specific recoil angles and incident energies. A depth resolution of 15 nm near the surface can be easily achieved as well as probing depths up to approximately 500 nm.

Elastic recoil selection by pulse shape analysis

Abstract Recoils ejected by α-particles are detected by low-resistivity detectors at low bias. Pulses from scattered α-particles that cross the depletion layer are rejected by pulse shape analysis. Using 500 Ω cm detectors the maximum pulse height recorded for α-particles corresponds to an energy of 4 MeV. Further suppression of pulses caused by α-particles awaits the construction of detectors optimized for operation at low bias as well as the development of faster electronics. Using a 13.4 MeV α-particle beam oxygen has been detected from layers as deep as 250 nm. Sensitivity was improved to 5 × 1014 atoms cm 2 and a surface depth resolution of 12 nm was obtained for N in Si2N0.86O3. The depth range and sensitivity may be further improved using higher bombarding energies.