Natural white light

White light-emitting diodes (LEDs) have tremendous potential for energy-efficient illumination. However, achieving white light emission with a 'colour temperature' that is not too blue is a serious challenge.

Credit: © 2010 AIP

In_xGa_1−xN compound semiconductor alloys are currently considered to be one of the most promising material systems for white LEDs, as they offer a broad direct bandgap that varies from the near-infrared (0.6 eV, InN) to the near-ultraviolet (3.4 eV, GaN). In principle this allows high-quality white LEDs to be realized by mixing In_xGa_1−xN devices emitting at blue, green and red wavelengths. The problem is the dramatic drop in emission efficiency at longer (green and red) wavelengths. Because of the large lattice mismatch between InN and GaN (∼11%) and the polar nature of their crystal structures, long-wavelength devices based on high-In-content In_xGa_1−xN/GaN quantum wells have an unavoidably high density of defects and huge internal piezoelectric fields (>1 MV cm⁻¹). Reducing the strain and the internal electric fields of such devices is therefore a significant goal in the realization of In_xGa_1−xN-based white LEDs.

Hon-Way Lin and co-workers at the National Tsing-Hua University in Taiwan have now demonstrated a monolithic In_xGa_1−xN-based white LED using an approach that overcomes these problems (Appl. Phys. Lett. 97, 073101; 2010). Their success relies on the use of a self-assembled nanorod geometry to eliminate the material strain. The team fabricated an array of In_xGa_1−xN/GaN nanorod heterostructures by plasma-assisted molecular beam epitaxy. Emission of natural white light was realized by designing each GaN nanorod p–n junction to contain a stack of several nanodisk InGaN emitters, each emitting a different colour due to their varying thickness and composition.

At small drive currents, the observed electroluminescence was 'spotty' because only a small number of nanorods were electrically injected by the p-type contact pad (see image). However, the integrated electroluminescence intensity was found to increase linearly as a function of injection current, eventually becoming a uniform natural white (a colour temperature of 6,000 K) at drive currents of 20 mA and above.