Excitation and nonradiative deexcitation processes ofEr3+in crystalline Si

Abstract

A detailed investigation on the excitation and deexcitation processes of ${\mathrm{Er}}^{3+}$ in Si is reported. In particular, we explored Er pumping through electron-hole pair recombination and Er deexcitation through Auger processes transferring energy to either free or bound electrons and holes. Since Er donor behavior would result in a free-carrier concentration varying along its profile, experiments have been performed by embedding the whole Er profile within previously prepared $n$-doped or $p$-doped regions. Multiple P (B) implants were performed in $n$-type ($p$-type) Czochralski Si samples in order to realize uniform dopant concentrations from $4\ifmmode\times\else\texttimes\fi{}{10}^{16}$ to $1.2\ifmmode\times\else\texttimes\fi{}{10}^{18}/{\mathrm{cm}}^{3}$ at depths between 0.5 and 2.5 \ensuremath{\mu}m below the surface. These samples have been subsequently implanted with 4 MeV $3.3\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{E}\mathrm{r}/\mathrm{c}\mathrm{m}}^{2}$ and annealed at 900 \ifmmode^\circ\else\textdegree\fi{}C for 30 min. Free electrons or holes concentrations in the region where Er sits were measured by spreading resistance profiling. It has been found that the release of electrons or holes from shallow donors and acceptors, occurring at temperatures between 15 and 100 K, produces a strong reduction of both time decay and luminescence intensity at 1.54 \ensuremath{\mu}m. These phenomena are produced by Auger deexcitation of the ${\mathrm{Er}}^{3+}$ intra-$4f$ electrons with energy transfer to free carriers. The Auger coefficient of this process has been measured to be ${C}_{A}\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{\mathrm{cm}}^{3}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for both free electrons and free holes. Moreover, at 15 K (when the free carriers are frozen and the donor and acceptor levels occupied) the ${\mathrm{Er}}^{3+}$ time decay has been found to depend on the P (or B) concentrations. This is attributed to an impurity Auger deexcitation to electrons (or holes) bound to shallow donors (acceptors): the efficiency of this process has been determined to be two orders of magnitude smaller with respect to the Auger deexcitation with free carriers. Furthermore, at temperatures above 100 K a nonradiative back-transfer decay process, characterized by an activation energy of 0.15 eV, is seen to set in for both $p$-type and $n$-type samples. This suggests that the back-transfer process, which severely limits the high-temperature luminescence efficiency, is always completed by a thermalization of an electron trapped at an Er-related level to the conduction band. Finally, by analysis of the pump power dependence of time decay and luminescence yield at 15 K, we have found that excitation of Er through the recombination of an electron-hole pair is a very efficient process, characterized by an effective cross section of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}{\mathrm{cm}}^{2}$ and able to provide an internal quantum efficiency as high as 10% at low temperatures (15 K) and pump powers (below 1 mW). This efficiency is significantly reduced when, at higher temperatures and/or high pump powers, strong nonradiative decay processes set in. These phenomena are investigated in detail and their impact on device operation perspectives are analyzed and discussed.