Photoluminescence and photoluminescence excitation spectroscopy of multipleNd3+sites in Nd-implanted GaN

Site-selective photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies were carried out at 6 K on the ${\mathrm{Nd}}^{3+}$ emissions from the ${}^4{\overrightarrow{F_{3/2}}}^4{I}_{11/2}$ transition in Nd-implanted GaN grown by metal-organic chemical-vapor deposition. A detailed investigation of the ${\mathrm{Nd}}^{3+}$ PL spectra excited by selected wavelengths of above-gap and below-gap pump light has detected multiple, distinct ${}^4{\overrightarrow{F_{3/2}}}^4{I}_{11/2}$ ${\mathrm{Nd}}^{3+}$ emission spectra characteristic of five different ${\mathrm{Nd}}^{3+}$ sites in the Nd-implanted GaN. The site-selective PLE spectra detected at emission wavelengths characteristic of each of these distinct ${\mathrm{Nd}}^{3+}$ PL bands include spectral features representative of excitation by above-gap absorption, by direct sharp-line ${\mathrm{Nd}}^{3+}$ $4f$-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 ${\mathrm{Nd}}^{3+}$ sites. It is concluded that the ${\mathrm{Nd}}^{3+}$ site that is excited by direct sharp-line ${\mathrm{Nd}}^{3+}$ $4f$-shell absorption is the dominant or highest concentration ${\mathrm{Nd}}^{3+}$ center. The excitation mechanisms for the other four ${\mathrm{Nd}}^{3+}$ 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 ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Er}}^{3+}$ PL and PLE spectra in GaN, and the implications for the trap-mediated excitation mechanisms, are discussed.