Photoluminescence and photoluminescence excitation spectroscopy of multiple sites in Nd-implanted GaN

Site-selective photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies were carried out at 6 K on the Nd³⁺ emissions from the ⁴F_3/2→⁴I_11/2 transition in Nd-implanted GaN grown by metal-organic chemical-vapor deposition. A detailed investigation of the Nd³⁺ PL spectra excited by selected wavelengths of above-gap and below-gap pump light has detected multiple, distinct ⁴F_3/2→⁴I_11/2 Nd³⁺ emission spectra characteristic of five different Nd³⁺ sites in the Nd-implanted GaN. The site-selective PLE spectra detected at emission wavelengths characteristic of each of these distinct Nd³⁺ PL bands include spectral features representative of excitation by above-gap absorption, by direct sharp-line Nd³⁺ 4f-shell absorption, and by broad, below-gap absorption bands attributable to defects and impurities and a possible isoelectronic trap associated with one of the five Nd³⁺ sites. It is concluded that the Nd³⁺ site that is excited by direct sharp-line Nd³⁺ 4f-shell absorption is the dominant or highest concentration Nd³⁺ center. The excitation mechanisms for the other four Nd³⁺ sites all involve the nonradiative transfer of energy from impurity- or defect-related traps to neighboring rare-earth atoms. These multiple excitation mechanisms closely parallel those observed previously in the PLE studies of GaN:Er. The similarities and relationships between the Nd³⁺ and Er³⁺ PL and PLE spectra in GaN, and the implications for the trap-mediated excitation mechanisms, are discussed.