Low-Cost Substrates for High Performance Nanorod Array LEDs

A radical departure from the conventional approaches to dislocation reduction in substrates for GaN-based LEDs is needed to meet the 2025 DOE/SSL goal of a cost-competitive, long-lifetime white light emitter with better than 50% product system efficiency and a spectral dependence that reproduces that of sunlight.  Whereas the primary emphasis in efforts aimed at improving the internal quantum efficiency (IQE) of GaN-based LEDs has been on reducing the threading dislocation density, these approaches including epitaxial lateral overgrowth, bulk GaN substrates and bulk SiC substrates — inevitably increase the materials or manufacturing costs markedly.  This project is designed to exploit the relief of lattice mismatch strain and the expulsion of dislocations that are characteristic of nanoheteroepitaxy in the growth of heteroepitaxial device structures on nanoscale substrates to expand the spectral range of efficient GaN-based LEDs to include the entire visible spectrum, thereby eliminating the efficiency losses associated with phosphor down-conversion.  The investigators have demonstrated the fabrication of uniform arrays of GaN nanorods using a low-cost process that does not involve foreign catalysts or direct-write nanolithography.  This achievement is the foundation of the first phase of the present project, in which dislocation-free compositionally graded nanorod substrates will be fabricated on sapphire substrates. In the second phase, the graded nanorod process will be transferred to low-cost silicon substrates metallized with refractory metallic nitrides (TM-N) that are compatible with the semiconducting nitrides.  These TM-N/silicon substrates combine high electrical and thermal conductivity with an integral reflective backside ohmic contact to the n-GaN underlayer. Metallic TM-N phases such as TiN, ZrN and VN are robust materials on which epitaxial GaN may be grown at conventional growth temperatures.  Although the crystalline texture (mosaicity) of GaN films grown on these metallic substrates is not ideal, each active nanorod device structure (30-100 nm in diameter) is seeded by only one grain of the GaN/TM-N underlayer, and is therefore expected to be free of extended defects.  Furthermore, the natural growth morphology consisting of prismatic side facets and pyramidal cap facets induces the strain-controlled formation of a 1-D array of (In,Ga)N quantum dots along the central axis of the nanorod, spatially removed from the surface.  Finally, the nanorod geometry is expected to allow for the formation of dislocation-free, compositionally-graded (In,Ga)N and (Al,Ga)N nanorod array substrates that will substantially broaden the span of nitride compositions and their combinations that can be utilized in coherent heterostructures designed for high IQE visible and uv LEDs.

Content dated 2/08