Positronium formation via excitonlike states on Si and Ge surfaces

Abstract

Recent experiments combining lifetime and laser spectroscopy of positronium (Ps) show that these atoms are emitted from p-Si(100) at a rate that depends on the sample temperature, suggesting a thermal activation process, but with an energy that does not, precluding direct thermal activation as the emission mechanism. Moreover, the amount of Ps emitted is substantially increased if the target is irradiated with 532 nm laser light just prior to the implantation of the positrons. Our interpretation of these data was that the Ps was emitted via an exciton-like positron-electron surface state, not dissimilar to an electronic surface exciton observed on Si in two-photon photoemission measurements. The hypothesis that this state may be populated by electrons from one of the occupied electronic surface states, either thermally or by laser excitation, is consistent with our observations and suggests that one should expect a high Ps yield at room temperature from an n-doped Si(100) sample, since in this case the same surface states will already be occupied. Here we present data obtained with an n-Si(100) target that supports our model, and also reveals the unexpected result that the kinetic energy of Ps emitted from this material actually decreases when it is heated, an effect we attribute to shifts in the surface energy levels due to the presence of a high density of thermally generated electrons. We show data obtained with a Ge(100) sample that corroborate the idea that the effects we observe are related to surface electron states, and hence should occur for any indirect band gap semiconductor with dangling bond states. Our model is further confirmed by the observation that the Ps yield from p-Si depends linearly on the surface density of the implanted positrons due to the effects of electron-hole pairs created as the positrons slow down.

Experimental progress in positronium laser physics

“…Since Ps is not expected to exist in the bulk of any such materials, it was assumed that Ps formation was occurring via the same thermal desorption process in all cases. However, recent experiments have shown that this is not the case: simultaneous measurements of the Ps emission yield and energy from Si and Ge surfaces has shown that while the yield does exhibit the same Arrhenius type of thermal dependence, the Ps energy is not thermal in nature [113,114]. The exact mechanism underlying this process is not yet known but the formation of an exciton-like Ps state (PsX) on the surface has been hypothesized [111], analogous to electronic surface exciton (X) formation [276].…”

“…Both thermal and optical excitation result in the emission of Ps with a nearly constant energy. Indeed, in some cases the Ps energy has been observed to decrease at higher temperatures [113,114], completely ruling out a thermal desorption model. This may be due to a modification of the electronic surface energy levels when there are many electrons present.…”

“…This does mean, however, that it is relatively easy to measure Ps energies via the Dopplerbroadened width of 1 3 S 1 → 2 3 P J transitions. This can be done in different geometries to obtain the Ps energies perpendicular or parallel to the surface of Ps converters [109,113]. Such measurements can be used, with appropriate additional parameters, such as laser delays, to obtain Ps thermalization and emission rates [110].…”

“…This parameter is proportional to the amount of long-lived Ps present, but should not be mistaken for the actual Ps fraction [113]. The integration regions defined by A, B and C are selected according to the type of detector used and the processes being studied.…”

Caracterización de nanoestructuras sub-superficiales utilizando espectroscopía de aniquilación de positrones

“…La espectroscopía de aniquilación de positrones (PAS) es una poderosa y versátil herramienta para el estudio de distintos tipos de materiales nivel nanoestructural: metales, semiconductores, polímeros ya que posee características únicas debido a su alta sensibilidad a la presencia de defectos de tamaño atómico y nanométrico tales como vacancias, aglomerados de vacancias o nanohuecos (voids) [1,3]. Asimismo, PAS posibilita la identificación y caracterización de cada uno de estos defectos siendo posible obtener, además, información experimental detallada de la estructura electrónica y atómica de la región estudiada por los positrones.…”