We report on spectral and time-resolved photoluminescence ͑PL͒ studies performed on Eu-doped GaN prepared by solid-source molecular-beam epitaxy. Using above-gap excitation, the integrated PL intensity of the main Eu 3ϩ line at 622.3 nm ( 5 D 0 → 7 F 2 transition͒ decreased by nearly 90% between 14 K and room temperature. Using below-gap excitation, the integrated intensity of this line decreased by only ϳ50% for the same temperature range. In addition, the Eu 3ϩ PL spectrum and decay dynamics changed significantly compared to above-gap excitation. These results suggest the existence of different Eu 3ϩ centers with distinct optical properties. Photoluminescence excitation measurements revealed resonant intra-4 f absorption lines of Eu 3ϩ ions, as well as a broad excitation band centered at ϳ400 nm. This broad excitation band overlaps higher lying intra-4 f Eu 3ϩ energy levels, providing an efficient pathway for carrier-mediated excitation of Eu 3ϩ ions in The visible and infrared light emissions from rare-earthdoped GaN ͑GaN:RE͒ are of significant current interest for applications in thin-film electroluminescence ͑EL͒ devices. [1][2][3][4] For achieving red light emission, the 5 D 0 → 7 F 2 intra-4 f transition of trivalent Eu 3ϩ ions seems most promising. Intense red photoluminescence ͑PL͒ around 622 nm from GaN:Eu ͑as-grown and ion-implanted͒ has been reported from several research groups. [1][2][3][4][5][6][7][8][9] In addition, several EL device structures based on GaN:Eu have been demonstrated. [1][2][3][4][5] The optimization of present EL devices, however, requires a more detailed understanding of the incorporation, excitation, and emission properties of Eu 3ϩ ions in the GaN host matrix.Several studies have recently appeared focusing on the preparation and optical properties of GaN:Eu. 4 -11 Based on the comparison to RE ions in other III-V semiconductors ͑e.g., InP:Yb, 12 GaAs:Er 13 ͒, the most probable lattice location for Eu 3ϩ ions in GaN are ͑substitutional͒ Ga sites, which have C 3V symmetry. However, significant differences in the Eu 3ϩ PL properties have been observed depending on the material preparation. Monteiro et al. 7 studied Euimplanted GaN and Eu in situ doped GaN grown by metalorganic chemical vapor deposition. They observed significant differences in the Eu 3ϩ PL properties, including the number of emission lines associated with the 5 D 0 → 7 F 2 transition. Based on optical spectroscopy and Rutherford backscattering studies, the authors concluded that the local symmetry of the Eu 3ϩ ions has to be lower than C 3V symmetry. 7 Bang et al. 9 studied Eu-doped GaN prepared by gas-source molecularbeam epitaxy ͑MBE͒ and concluded, based on extended x-ray absorption fine-structure data, that Eu 3ϩ occupies Ga sites with C 3V symmetry. It was also suggested that more than one local environment of Eu 3ϩ ions may exist in the investigated GaN samples.In this letter, we present PL results on GaN:Eu prepared by solid-source MBE, which provide spectroscopic evidence for the existence of different Eu...

The temperature dependent behavior of continuous-wave and time-resolved photoluminescence of Eu-doped GaN in the visible region is measured for both the 5 D 0 → 7 F 2 and 5 D 0 → 7 F 3 transitions. The radiative decay of these transitions, following pulsed laser excitation of the GaN host, is monitored by a grating spectrometer and photomultiplier tube detector system. In addition to these two radiative energy transfer pathways within Eu 3ϩ , the data reveal two nonradiative energy transfer paths between Eu 3ϩ and the host GaN. Decay constants for the relaxation processes are extracted from the data using a numerically solved rate equation model. Although the dominant radiative relaxation processes decayed with a temperature insensitive decay constant of 166 s, a prominent role for nonradiative transfer between Eu 3ϩ and impurities within the GaN host was deduced above 180 K.

Photoluminescence ͑PL͒ excitation spectroscopy mapped the photoexcitation wavelength dependence of the red luminescence ͑ 5 D 0 → 7 F 2 ͒ from GaN:Eu. Time-resolved PL measurements revealed that for excitation at the GaN bound exciton energy, the decay transients are almost temperature insensitive between 86 K and 300 K, indicating an efficient energy transfer process. However, for excitation energies above or below the GaN bound exciton energy, the decaying luminescence indicates excitation wavelength-and temperature-dependent energy transfer influenced by intrinsic and Eu 3+ -related defects. Native and rare earth-induced defects participate in the energy transfer processes that lead to red light emission in GaN:Eu. 6 An impurity band spreading 370 meV below the conduction band of GaN:Eu has been observed to arise from such defects. 7 Previous measurements of the extended x-ray absorption fine structure show that Er 3+ and Eu 3+ dopants assume a substitutional Ga site with C 3 symmetry. 8,9 Sharp, otherwise forbidden 4f emission lines from Eu 3+ are allowed by symmetry breaking in the GaN host. Nyein et al. recently performed time-resolved photoluminescence ͑PL͒ studies of these emission lines by using both above-and below-band gap excitation. 10 Photoluminescence excitation ͑PLE͒ spectroscopy indicated the impurity band is involved in the energy transfer between the GaN host and the Eu 3+ dopants. In this paper, visible and UV wavelength PLE measurements of GaN:Eu evaluate the excitation wavelength-dependent energy transfer between the GaN host or defects and the Eu 3+ dopants, while temperature-dependent, time-resolved PL ͑TRPL͒ measurements investigate energy transfer and carrier relaxation dynamics.A Eu-doped GaN film was deposited on a p-Si ͑111͒ substrate by solid-source MBE. A thin GaN buffer layer was first deposited at a substrate temperature of 600°C before the main growth took place at 800°C for about 2 h. Details of the growth conditions can be found elsewhere. 4 The Eu cell temperature was 400°C, resulting in a Eu concentration of 8.8ϫ 10 20 cm -3 ͑1 at. % ͒ estimated by secondary ion mass spectrometry. The thickness of the GaN:Eu layer is approximately 2.4 µm. PL spectra were excited by a He-Cd laser ͑E p = 3.815 eV͒, and the tunable light source for PLE spectroscopy was a xenon arc lamp dispersed through an Acton 150 mm monochromator. The luminescence was analyzed by a 0.75 m focal length SPEX single grating monochromator and detected by a thermoelectrically cooled photomultiplier tube ͑Hamamatsu R928͒. Standard lock-in techniques were used for collecting both PL and PLE signals. The pulsed laser source for TRPL measurements was an optical parametric amplifier ͑OPA͒, pumped by a 1 kHz regenerative amplifier seeded by an 80 MHz Ti:sapphire oscillator operating at 800 nm. In this experiment the OPA was tuned between E p = 3.02-4.14 eV while maintaining a pulse intensity of 600 J/cm 2 and pulse width less than 200 fs. The luminescence from the GaN:Eu sample was collected by two UV lenses, spectrall...

Selective enhancement of electroluminescent emission from high-energy transitions in Tm-doped GaN has been observed to be a strong function of GaN growth temperature. GaN:Tm thin films have been grown by molecular beam epitaxy at temperatures from 100 to 700 °C. At low growth temperatures (100–200 °C) the low energy (infrared-801 nm) transition dominates, while at higher growth temperatures (400–700 °C) the high energy (blue-477 nm) transition dominates. For films grown at low temperatures the main emission excitation mechanism is impact excitation, while for films grown at higher temperatures (⩾600 °C) the main excitation mechanism appears to be lattice impact ionization followed by energy transfer to Tm ions.

The magnetic properties of Tm-doped AlxGa1−xN (0⩽x⩽1) alloys grown by solid-source molecular beam epitaxy were studied by hysteresis measurements and shown to exhibit ferromagnetic behavior at room temperature. The measured magnetization was strongly dependent on the Al content and reached a maximum for x=0.62. Previously reported photoluminescence measurements on these films yielded a blue emission at 465nm with peak intensity at the same Al content. Both magnetic and optical properties are directly correlated with the alloy compositional fluctuation found in undoped AlxGa1−xN alloys.