Multiple color capability from rare earth-doped gallium nitride

Rare earth (RE) doping of GaN has led to a new full color thin film electroluminescent (TFEL) phosphor system. GaN films doped with Eu, Er, and Tm dopants emit pure red, green, and blue emission colors, respectively. As a host for RE luminescent centers, GaN possesses many properties which are ideal for bright multiple color TFEL. Specifically, GaN has excellent high field transport characteristics, is chemically and thermally rugged, and incorporates well the RE dopants. X-ray absorption measurements have shown that even at RE dopant levels exceeding 0.1 at.% the majority of RE dopants occupy a strongly bonded substitutional site on the Ga sublattice. According to RE crystal field theory this tetrahedrally bonded site allows optical activation and emission from RE 4f–4f inner-shell electronic transitions. Monte Carlo calculations of GaN carrier transport have shown that at ∼2 MV cm−1 applied field the average electron possesses 2.6 eV energy which is adequate for exciting blue emission. GaN:Er TFEL devices have exhibited a brightness of 500–1000 cd/m2 at ∼540 nm. In addition to pure colors, mixed colors can be achieved by doping with a combination of REs. For example, co-doping with Er and Tm results in an emission spectrum which is perceived by the human eye as a blue–green (turquoise) hue. Multiple color capability in a single device has also been demonstrated by adjusting the bias voltage (in a co-doped GaN:Er,Eu layer) or by switching the bias polarity (in a stacked two layer GaN:Er/GaN:Eu structure). The combination of pure or mixed color emission, the availability of bias controlled color, and the potential for white light emission indicate that GaN:RE TFEL devices have enormous potential for display applications.