Luminescent and magnetic properties of rare-earth (RE) elements doped in insulators and metals have been well investigated and they have been successfully applied to practically-used fluorescent substances and magnets.  In these applications, however, either luminescent or magnetic property has been independently used.  Furthermore, research on the RE-doped materials has been based on experienced trial-and-error, not on material design by precise control of RE doping and understanding of energy-transfer mechanism.

RE-doped semiconductors are a new class of materials with various promising potentials.  RE ions doped in semiconductors exhibit a characteristic emission due to the intra-4f shell transitions of RE ions.  The intra-4f shell transitions give rise to sharp emission lines whose wavelengths are largely independent of both the host material and temperature.  This stability occurs because the filled outer 5s and 5p electron shells screen transitions within the inner 4f electron shell from the interaction with the host.  We have intensively investigated RE-doped III-V semiconductors grown by atomically-controlled organometallic vapor phase epitaxy (OMVPE) and fabricated new types of light-emitting diodes (LEDs) with the materials.

In Er,O-codoped GaAs (GaAs:Er,O), one kind of Er center is formed selectively as an Er atom located at the Ga sublattice with two adjacent O atoms (hereafter referred as Er-2O) together with two As atoms, resulting in drastic enhancement in intensity of Er-related luminescence due to the intra-4f shell transitions of Er³⁺ ions [1].  GaAs:Er,O homostructure and double-heterostructure (DH) LEDs were fabricated by OMVPE, which exhibited successfully 1.5 mm electroluminescence (EL) due to the Er-2O center under forward bias at room temperature [2,3].  The dependence of the EL intensity on the injection current density indicated extremely large excitation cross section of Er ions by current injection (1~2 x 10^-15 cm²) in the LED, which is larger by five orders in magnitude than optical excitation cross section in conventional Er-doped fiber amplifiers (10^-20~10^-21 cm²). 

RE-doped GaN has been identified as a promising candidate for the realization of white LEDs, displays, and lasers.  Eu³⁺ ions have been widely used as red-emitting phosphors for cathode ray-tube and plasma display panels.  In these applications, the ions are doped into an insulator and red emission is obtained mainly by optical excitation.  GaN is an attractive host material for Eu doping because its wide bandgap allows visible wavelength emission from Eu ions and reduces the thermal quenching effect for the emission intensity.  We have grown Eu-doped GaN (GaN:Eu) by OMVPE and observed successfully bright red emission due to the intra-4fshelltransitions of Eu³⁺ ions from a LED with the GaN:Eu as an active layer[4].  The main emission line with a half-width of less than 1 nm was observed at 621 nm, which can be assigned to the ⁵D₀–⁷F₂ transition of Eu³⁺ ions.  Notably, no band-edge and defect luminescence was observed under bias conditions, indicating that the Eu luminescence is caused by the ultrafast energy transfer from the GaN host to the Eu³⁺ ions.  These results suggest a novel method of realizing GaN-based red LEDs, which are an alternative to conventional toxic As-containing AlGaInP/GaAs red LEDs, and a monolithic device composed of red, green andblue GaN-based LEDs for full-color display or lighting technology.

Significant improvement in device performance has been achieved by optimizing growth parameters and device structures.  The atmospheric-pressure growth of GaN:Eu drastically increased the light output power of the Eu emission, which is due to the increased number of optically active Eu centers and efficient energy transfer by the reduced non-radiative processes in the GaN host [5].  Furthermore, Eu,Mg-codoped GaN (GaN:Eu,Mg) showed an approximately five-fold improved Eu intensity at room temperature [6,7].  An additional emission center created by Mg codoping and its unique behavior under thermal annealing were also observed.  With increasing active layer thickness in the LED, on the other hand, the output power increased monotonically.  In the presentation, current status of the RE-doped semiconductors and their LEDs is reviewed.

References: [1] K. Takahei and A. Taguchi, J. Appl. Phys. 74, 1979 (1993).  [2] A. Koizumi, Y. Fujiwara, A. Urakami, K. Inoue, T. Yoshikane and Y. Takeda, Appl. Phys. Lett. 83, 4521 (2003).  [3] A. Koizumi, K. Inoue, Y. Fujiwara, T. Yoshikane, A. Urakami and Y. Takeda, Jpn. J. Appl. Phys. 42, 2223 (2003).  [4] A. Nishikawa, T. Kawasaki, N. Furukawa, Y. Terai, and Y. Fujiwara, Appl. Phys. Express 2, 071004 (2009).  [5] A. Nishikawa, N. Furukawa, T. Kawasaki, Y. Terai, and Y. Fujiwara, Appl. Phys. Lett. 97, 051113 (2010).  [6] D. Lee, A. Nishikawa, Y. Terai, and Y. Fujiwara, Appl. Phys. Lett. 100, 171904(2012).  [7] D. Lee, R. Wakamatsu, Y. Terai, and Y. Fujiwara, Appl. Phys. Lett. 102, 141904(2013).

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