S. J. Rhee

Trap-mediated excitation of Er3+ photoluminescence in Er-implanted GaN

Site-selective photoluminescence (PL) spectra obtained at 6 K from the 1540 nm 4I13/2→4I15/2 emissions characteristic of four distinct Er3+ centers in Er-implanted films of GaN are compared with the Er3+ PL excited by 325 nm above-gap pump light. Two of the site-selective 1540 nm Er3+ PL spectra pumped by below-gap, trap-mediated excitation bands dominate the Er3+ PL spectrum excited by above-gap light. A third broad band-excited spectrum and a fourth spectrum pumped by direct Er3+ 4f-band absorption are apparently not strongly excited by above-gap light. These results indicate that trap-mediated excitation dominates above-gap pumping of Er3+ emission in GaN:Er, and suggest an explanation for the reduced thermal quenching of Er3+ emission in GaN.

Observation of multiple Er3+ sites in Er-implanted GaN by site-selective photoluminescence excitation spectroscopy

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the 1540 nm 4I13/2→4I15/2 emission of Er3+ in Er-implanted films of GaN grown by metalorganic chemical vapor deposition. The PLE spectra exhibit several broad below-gap absorption bands, which excite three distinct site-selective Er3+ PL spectra. The excitation of two of the site-selective Er PL bands involves optical absorption by defects or background impurities, rather than direct intra-f shell absorption, with subsequent nonradiative transfer of the energy to nearby Er3+ luminescence centers. The characteristics of the PLE spectrum of the third site-selective PL band suggest that an exciton bound at an Er-related trap is involved in the excitation mechanism.

Selective enhancement of 1540 nm Er3+ emission centers in Er-implanted GaN by Mg codoping

The ∼1540 nm 4I13/2 to 4I15/2 Er3+ photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Er-implanted Mg-doped GaN reveal a selective enhancement of one of the nine different Er3+ centers observed previously in PL and PLE studies of Er-implanted undoped GaN. These Er3+ PL spectra are excited selectively by pump wavelengths that correspond to broadband, below-gap absorption bands associated with different Er3+ centers. In the Er-implanted, Mg-doped GaN, both the 1540 nm PL spectrum characteristic of the so-called violet-pumped Er3+ center and the ∼2.8–3.4 eV (violet) PLE band that enables its selective excitation are significantly enhanced by Mg doping. In addition, the violet-pumped PL center dominates the above-gap-excited Er3+ PL spectrum of Er-implanted Mg-doped GaN, whereas it was nearly unobserveable under above-gap excitation in Er-implanted undoped GaN. These results confirm our hypothesis that appropriate codopants can increase the efficiency of trap-mediated above-gap excitatio...

PHOTOLUMINESCENCE EXCITATION SPECTROSCOPY OF FREE-TO-BOUND TRANSITIONS IN UNDOPED GAN GROWN BY HYDRIDE VAPOR PHASE EPITAXY

Temperature-dependent photoluminescence excitation (PLE) spectroscopy has been carried out on the yellow band (YB) in GaN. The 5 K PLE spectra demonstrate that the exciting light must have photon energy large enough to generate free carriers or carriers localized on shallow impurities in order to excite the YB effectively. With increasing temperatures, progressively deeper energy levels can be thermally ionized, enabling extrinsic absorption by these deeper levels to generate the free holes required to excite the YB emission. The broad below-band gap PLE response then exhibits thermally activated onsets attributed to these free-to-bound transitions. One such onset corresponds to the well-known 205 meV acceptor, and a second onset provides conclusive evidence for the existence of a previously unconfirmed ∼120 meV impurity or defect level in GaN.

Effects of material growth technique and Mg doping on Er3+ photoluminescence in Er-implanted GaN

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the ∼1540 nm 4I13/2–4I15/2 emissions of Er3+ in Er-implanted and annealed GaN. These studies revealed the existence of multiple Er3+ centers and associated PL spectra in Er-implanted GaN films grown by metalorganic chemical vapor deposition, hydride vapor phase epitaxy, and molecular beam epitaxy. The results demonstrate that the multiple Er3+PL centers and below-gap defect-related absorption bands by which they are selectively excited are universal features of Er-implanted GaN grown by different techniques. It is suggested that implantation-induced defects common to all the GaN samples are responsible for the Er site distortions that give rise to the distinctive, selectively excited Er3+PL spectra. The investigations of selectively excited Er3+PL and PLE spectra have also been extended to Er-implanted samples of Mg-doped GaN grown by various techniques. In each of these samples, the so-called viol...

Site-selective photoluminescence excitation and photoluminescence spectroscopy of Er-implanted wurtzite GaN

Site-selective photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy have been carried out at 6K on the ∼1540 nm 4 I 15/2 → 4 I 15/2 emissions of Er 3+ in Er-implanted GaN. The PLE spectra exhibit several broad, below-gap, defect- or impurity-related absorption bands which excite three distinct site-selective Er + PL spectra. The near-band edge spectral position and lineshape of the PLE spectrum of one of the site-selective PL bands suggest that this Er site forms a trap level within the band gap and an exciton bound at this trap is involved in the excitation mechanism. In addition, high resolution PLE spectra obtained with a tunable laser in the 810 nm spectral range reveal a set of sharp PLE peaks due to the 4 I 15/2 → 4 I 9/2 internal Er + ƒ-band absorption superimposed on the broad defect PLE band. The site-selective PL spectrum excited by the sharp line ∼810 nm Er + intra-ƒ shell PLE bands is characteristic of a fourth distinct Er + site. The simple structure of the site-selective PL and PLE spectra associated with direct intra-ƒ shell absorption suggests that the optically active Er site responsible for these spectra is of high symmetry in wurtzite GaN and that it could be attributed to a single Er atom on a Ga site.

Growth and characterization of Y2O3:Eu on Si and yttria-stabilized zirconia

Y2O3 films doped with Eu are grown by metalorganic chemical vapor deposition on Si(100) and yttria-stabilized zirconia (YSZ) (100). The mismatch is only 2.6% between the lattice constant of YSZ and half the lattice constant of Y2O3. The samples are characterized by x-ray diffraction, scanning electron microscopy, Rutherford backscattering, and photoluminescence. The films deposited on Si are polycrystalline. When films are deposited on YSZ, the Y2O3(100) direction is aligned with YSZ(100). The luminescence and the narrow x-ray diffraction lines indicate that a Y2O3:Eu film with high crystallinity is obtained without annealing.

Site-Selective Photoluminescence Spectroscopy of Er-Implanted Wurtzite Gan Under Various Annealing Conditions

The ~1540 nm 2/ longrightarro 2/ emissions of E in Er-implanted GaN annealed at temperatures in the 400 to 100 range were investigated to gain a better understanding of the formation and dissociation processes of the various E sites and the recovery of damage caused by the implantation with increasing annealing temperature ( ).The monotonic increase in the intensity of the broad defect photoluminescence(PL) bands with incresing proves that these are stable radiative recombination centers introduced by the implantation and annealing process. Theser centers cannot be attributed to implantation-induced damage that is removed by post-implantation annealing. Selective wavelength pumpling of PL spectra at 6K reveals the existence of at least nine different E sites in this Er-implanted semiconductor. Most pf these E PL centers are attributed to complexed of Er atoms with defects and impurities which are thermally activated at different . Only one of the nine observed E PL centers can be pumped by direct 4f absorption and this indicates that it is highest concentration E center and it represents most of the optically active E in the implanted sample. The fact that this E center cannot be strongly pumped by above-gap light or broad band below-gap absorption indicates that it is an isolated center, i.e not complexed with defects or impurities, The 4f-pumped P: spectrum appears at annealing temperatures as low as 40, and although its intensity increase monotonically with increasing the wavelengths and linewidths of its characteristic peaks asre unaltered. The observation of this high quality E PL spectrum at low annealing temperatures illustrates that the crystalline structure of GaN is not rendered amorphous by the ion implantation. The increase of the PL intensities of the various E sites with increasing is due to the removal of competing nonradiative channels with annealing. with annealing.annealing.

Characterization of as-grown and ion-implanted GaN by photoluminescence and photoluminescence excitation spectroscopy

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy have been used to characterize as-grown and ion-implanted epitaxial films of GaN grown on sapphire and SiC substrates by MOCVD, and on sapphire substrates by HVPE. Our studies have included a variety of near-band edge PL bands and deeper PL bands such as the ubiquitous yellow band which is present at varying strength in most as-grown films of GaN, and several types of deep emission bands introduced by the implantation of Er, Cr, and isovalent impurities such as As and P. PLE spectroscopy of the deep PL bands such as the yellow band, the broad implantation-induced defect bands, and the intra-f-shell emissions of Er/sup 3+/ been used to detect extrinsic (below gap) absorption bands due to defects and impurities over an extremely broad spectral range. Comparison of the PLE spectra of these deep PL bands and the temperature dependences of the PL and PLE spectra provide new insights concerning the competing excitation and recombination mechanisms for the deep centers in as-grown and ion-implanted GaN.