Embedded Si nanoclusters in α-alumina synthesized by ion implantation: An investigation using depth dependent Doppler broadening spectroscopy

Embedded Si nanoclusters in optically transparent host materials have shown potential applications in opto-electronic devices. Ion implantation followed by annealing at higher temperatures is considered a promising way to produce embedded nanoclusters in variety of host materials. In the present work, α-alumina crystals were irradiated with Si ion beam (50 keV) with varying total dose (1 × 1016, 5 × 1016 and 1 × 1017 ions/cm2). The ion implantation and vacancy defect profiles in alumina have been calculated using computer code SRIM. The irradiated samples were annealed at 500 and 1000 °C in a reducing atmosphere for 30 min. Depth dependent Doppler broadening measurements using high purity Germanium (HPGe) detector coupled to a slow positron beam were carried out on as-irradiated as well as annealed samples. The line shape (S- and W-) parameters showing contribution from valence (low momentum) and core (high momentum) electrons were evaluated from the positron implantation energy dependent Doppler broadened spectra. The S-E profiles of as irradiated samples indicated the formation of open volume defects in the damaged region. On annealing, S-E profiles are modified in the damaged region that is attributed to the formation of vacancy clusters and Si nanoclusters as confirmed through S-W correlation plots. The S-E profiles have been fitted using variable energy positron fit (VEPFIT) to evaluate the characteristic S-parameter corresponding to the damaged region. It is observed that ∼90% of implanted positrons in the damaged region are confined to the Si nanoclusters due to their high affinity toward Si as compared to host material, α-alumina. The present study confirms that positrons are confined in embedded nanoclusters and acts as a self seeking probe for their characterization.