Tomohiro Inaba

Substantial enhancement of red emission intensity by embedding Eu-doped GaN into a microcavity

We investigate resonantly excited photoluminescence from a Eu,O-codoped GaN layer embedded into a microcavity, consisting of an AlGaN/GaN distributed Bragg reflector and a Ag reflecting mirror. The microcavity is responsible for a 18.6-fold increase of the Eu emission intensity at ∼10K, and a 21-fold increase at room temperature. We systematically investigate the origin of this enhancement, and we conclude that it is due to the combination of several effects including, the lifetime shortening of the Eu emission, the strain-induced piezoelectric effect, and the increased extraction and excitation field efficiencies. This study paves the way for an alternative method to enhance the photoluminescence intensity in rare-earth doped semiconductor structures.

Emission enhancement and its mechanism of Eu-doped GaN by strain engineering

Eu-doped GaN (GaN:Eu) is an attractive material for red emission from GaN based light-emitting diodes (LEDs). In GaN:Eu, the Eu ions form multiple luminescent centers that differ in fractional percentage and affinity for capturing energy from the GaN host. We investigated the effects of compressive strain on the PL intensity from GaN:Eu using superlattices. It was found that the PL intensity from a particular Eu center, which is brightest under current injection, could be controlled by varying the magnitude of the in-plane compressive strain. This resulted in a 1.9 times enhancement of the PL intensity. Moreover, it was revealed that this enhancement was mainly due to an increase in the fractional percentage of this Eu luminescent center. This observation suggests that the fractional percentage of the Eu luminescent center could be manipulated by the in-plane compressive strain for the development of brighter GaN:Eu based red LEDs.

Quantitative study of energy-transfer mechanism in Eu,O-codoped GaN by time-resolved photoluminescence spectroscopy

In order to investigate the excitation processes in Eu,O-codoped GaN (GaN:Eu,O), the time-resolved photoluminescence signal including the rising part is analyzed. A rate equation is developed based upon a model for the excitation processes in GaN:Eu to fit the experimental data. The non-radiative recombination rate of the trap state in the GaN host, the energy transfer rate between the Eu3+ ions and the GaN host, the radiative transition probability of Eu3+ ion, as well as the ratio of the number of luminescent sites (OMVPE 4α and OMVPE 4β), are simultaneously determined. It is revealed and quantified that radiative transition probability of the Eu ion is the bottleneck for the enhancement of light output from GaN:Eu. We also evaluate the effect of the growth conditions on the luminescent efficiency of GaN:Eu quantitatively, and find the correlation between emission intensity of GaN:Eu and the fitting parameters introduced in our model.

Drastic enhancement of Eu emission from red light-emitting Eu-doped GaN in a microcavity

Eu-doped GaN (GaN:Eu) has been identified as a promising red emitter. A GaN:Eu layer was confined in a microcavity consisting of a Ag mirror and an AlGaN/GaN distributed Bragg reflector (DBR), resulting in drastic enhancement of Eu emission intensity.