Correlation of electrical and optical properties in dually Cd⁺ and N⁺ ion-implanted GaAs

Cd in GaAs is an acceptor atom and has the largest atomic diameter among the four commonly-used group-II shallow acceptor impurities (Be, Mg, Zn and Cd). The activation energy of Cd (34.7 meV) is also the largest one in the above four impurities. When Cd is doped by ion implantation, the effects of lattice distortion are expected to be apparently different from those samples ion-implanted by acceptor impurities with smaller atomic diameter. In order to compensate the lattice expansion and simultaneously to adjust the crystal stoichiometry, dual incorporation of Cd and nitrogen (N) was carried out into GaAs. Ion implantation of Cd was made at room temperature, using three energies (400 keV, 210 keV, 110 keV) to establish a flat distribution. The spatial profile of N atoms was adjusted so as to match that of Cd ones. The concentration of Cd and N atoms, [Cd] and [N] varied between 1 × 10¹⁶ cm^-3 and 1 × 10²⁰ cm^-3. Two type of samples, i.e., solely Cd⁺ ion-implanted and dually (Cd⁺ + N⁺) ion-implanted with [Cd] = [N] were prepared. For characterization, Hall effects and photoluminescence (PL) measurements were performed at room temperature and 2 K, respectively. Hall effects measurements revealed that for dually ion-implanted samples, the highest activation efficiency was ∼ 40% for [Cd] (= [N]) = 1 × 10¹⁸ cm^-3. PL measurements indicated that [g-g] and [g-g]_i (i = 2, 3, α, β, ...), the emissions due to the multiple energy levels of acceptor-acceptor pairs are significantly suppressed by the incorporation of N atoms. For [Cd] = [N] ≥ 1 × 10¹⁹ cm^-3, a moderately deep emission denoted by (Cd, N) is formed at around 1.45-1.41 eV. PL measurements using a Ge detector indicated that (Cd, N) is increasingly red-shifted in energy and its intensity is enhanced with increasing [Cd] = [N]. (Cd, N) becomes a dominant emission for [Cd] = [N] = 1 × 10²⁰ cm^-3. The steep reduction of net hole carrier concentration observed for [Cd]/[N] ≤ 1 was ascribed to the formation of (Cd, N) which is presumed to be a novel radiative complex center between acceptor and isoelectronic atoms in GaAs.