PHYSICAL REVIE%' 8 VOLUME 34, NUMBER 3 1 AUGUST 1986 Variable-energy positron-beam studies of Ni implanted with He K. G. Lynn, D. M. Chen, Bent Nielsen, and R. Pareja Brookhauen Xationa/ Laboratory, Upton, Rem Fork, 11973 S. Mycrs Sandia Xationa/ Laboratories, A/buquerque, ¹m Mexico 87185 (Received 23 January 1986) Variable-energy positron-beam studies have been made on well-annealed polycrystalline Ni sam- ples implanted with 30-, 90-, and 180-keV He ions. The positron-annihilation characteristics were measured with a solid-state Ge detector at a number of different incident-positron energies and after isochronal annealing at various temperatures. The Doppler broadening of the annihilation photons was found to be strongly influenced by the ~He implantations. The data indicate that trapping of the positrons occurred predominantly at small He bubbles. The variation of the broadening with incident-positron energy was sensitive to the depth distribution of the traps. A diffusion model as- suming a square concentration-defect profile was developed and analytically fitted to the parametrized momentum data. These fitted results were compared to Monte Carlo range calcula- tions for He in Ni, and fairly good agreement was found. This investigation demonstrates the capa- bilities of positron annihilation for nondestructive depth profiling in ion-implanted systems. In ad- dition, it establishes paraBels between the trapping behavior of positrons and that reported elsewhere for hydrogen, thereby augmenting the present level of understanding of the technologically impor- tant trapping of hydrogen by the bubbles. I. INTRODUCTION With the development of controllable energy positron beams it is now possible to profile defects, overlayers, and interfaces in the near-surface region (g 1 pm) of materi- als. A few initial studies of this type have been performed by various researchers. ' We have attempted to semi- quantitatively study the effect of helium implantation on the positron-annihilation characteristics in well-charac- terized polycrystalhne Ni samples. In the present investigation measurements were made on the annihilation characteristics of low-energy positron beams in Ni following ion implantation of ~He. This study was motivated by two considerations. First, ion- implanted He in Ni is known to agglomerate into very stable nanometer-sized bubbles, and the depth distribution of these entities can be varied in a known fashion by changing the implantation energy; hence, if such bubbles trap the positrons with an associated observable modifica- tion of the annihilation behavior, then specimens of this kind can be used to test the profiling capabilities of the positron analysis. The second objective was to examine in detail the trapping behavior of positrons at the He bub- bles, within which the atomic density of the He is believed to be of order 10 (Ref. 3). Such information is expected to enhance the current understanding of the strong hydro- gen trapping which often occurs at such bubbles, an ef- fect with strong relevance to plasma-wall interactions in fusion energy. To accomplish these goals we implanted monoenergetic positrons into the Ni samples at various energies. After the thermalization (or energy loss) processes ( (10 psec) the implanted positron eventually annihilates with an elec- tron, producing predominantly two annihilation y rays. Measurements of the positron's lifetime, the angular dis- tribution of the annihilation photons, or the Doppler broadening of the y-ray spectrum provide useful informa- tion on the environment of the positron at the time of an- nihilation. In this experiment the last quantity was stud- ied. To actually profile the implanted region one mea- sures the Doppler spectrum at various incident positron energies. The change from unim planted Ni is then greatest when the mean range of the positron profile over- laps the He-implanted region. The Dopper broadening of the y-ray spectrum is deter- mined by the momentum distribution of the e+-e sys- tem prior to annihilation. For a perfect crystal the spec- trum results from positrons diffusing within the lattice and subsequently annihilating with both conduction and core electrons. In simple metals the Doppler-broadened energy spectrum can be approximated with a central para- bolic region representing the conduction-electron contri- bution superimposed with a broad but smaller core- electron part. If a sample contains open-volume defects, there is a finite probability for positrons to be trapped in these regions. The total electron density as well as the core fraction are reduced at the site of the trapped posi- tron. As a result there is a narrowing of the y spectrum. The peak height and shape of the resulting Doppler- broadened spectra are different depending on the type of defect where the positron is localized when annihilation occurs. Such a spectrum can be characterized by a line- shape parameter S which simply divides the total spec- trum into the central region. This generally provides one with the ability to determine the fraction of positrons which annihilate with these defects. One can also esti- 34