Light emission from lanthanide-doped AlN and AlxIn1-xN layers

Optical properties of lanthanide doped phosphors and semiconductors are of great interest especially in view of solid-state light emitting devices of the next generation. Radiative intra-4f electron transitions of trivalent lanthanide ions present sharp and well-de ned emission lines at the wavelengths from UV to IR part of the spectrum. However, their intensity is comparatively weak hitherto. In the scope of this dissertation, wide band gap III-nitride semiconductors, AlN and AlxIn1-xN, are chosen as host materials and doped with Pr, Sm, Tb and Tm ions respectively. According to our elaborate optoelectronic, structural and compositional characterisations, we attempt to nd an innovative guideline how to obtain the lanthanide luminescence and increase their intensity. In the light of crystal eld (CF) theory, electrostatic perturbation with a non-central symmetry on the lanthanide ions is a fundamental requirement for the selection of host material to achieve their intra-4f transitions. Splitting of low-temperature photoluminescence (PL) peaks from AlN:Sm and AlN:Tb layers reveals that most of the radiative lanthanide ions are substitutionally located in a C3v local symmetry. Physical and structural information of this architecture in AlN:Sm system (namely the e ective point charge of four surrounding N ions felt by the 4f -electrons of Sm ion and their spatial positions related to 4f -electron orbital of Sm ion) is determined through our preliminary e orts by using computer-assisted tting procedure. Once the host material is de ned, intensity of the lanthanide luminescence can be "extrinsically" enhanced by three ways. (1) Appropriate thermal treatment is a conventional technique for this purpose. The atomic rearrangements activated thereby can be considered as a joint reaction of rst order. By introducing a concept of "extended lanthanide luminescence centres" we are able to simplify the description of this intensity enhancement in AlN:Ln system with only two thermodynamic and kinetic parameters. From the PL spectra of AlN with and without lanthanide doping, we con rm that the luminescence generated by carrier recombination within O-associated defect states can be strengthened particularly after annealing at intermediate temperatures (300 600 °C). The peak of this defect luminescence covers the required energy for the excitation of lanthanide luminescence. Both of them exhibit therefore a correlated development. This result enables us to assist the excitation processes of lanthanide luminescence centre through (2) utilising available defect states within the band gap. Inspired of this, lanthanide luminescence can be further intensi ed by (3) establishing advii ditional excitation path via engineering the band structure of host material. This method is proven to be e ective in Al0.87In0.13N:Tm and Al0.84In0.16N:Pr systems. Due to decomposition at proper temperature, we observe an almost instantaneous formation of nano-sized In-rich AlxIn1-xN quantum dots (QDs) with subsequent comparatively slow coarsening. This coarsening permits us to modify the band gap energy of QDs by altering their size, which is a function of the annealing temperature and duration. If this band gap energy is in resonance with the 4f -levels to be excited in the lanthanide ions, luminescence intensity increases. An elaborate model, relating thermal formation of "extended lanthanide luminescence centres", time-dependent variation of band gap energy and resonant energy transfer, can su ciently describe the development of lanthanide luminescence intensity during the annealing.