Temperature Dependent Thermal Properties in LEDs for Solid State Lighting Available to Purchase

Over 20% of electricity in US is used by lighting. Solid state lighting (SSL) efficiency can surpass that of incandescent and fluorescent lighting techniques. Nonetheless SSL efficiency is greatly reduced at high temperatures that result from inadequate heat dissipation. SSL requires blue and green light emitting diodes (LEDs) made from Gallium Nitride (GaN) and Indium Gallium Nitride (InGaN) to eventually generate white light. Conduction within the LED is a major thermal resistance for heat dissipation, and motivates study of thermal properties of LED materials, including GaN and InGaN. Bulk thermal properties are poor estimates of thin film properties due to increased boundary and defect scattering of phonons in the films. By examining real nitride based LED architectures with the 3-omega technique, thin film thermal conductivities of nucleation, buffer, contact, and active regions were measured from 100–400K. We find that the AlN nucleation layer is a bottleneck to heat transfer, having a thermal conductivity (κ) two orders of magnitude less than bulk crystalline AlN. Further, the temperature dependent behavior is characteristic of an amorphous solid. TEM images of the AlN layer show a very high dislocation density (4×10¹⁰ cm⁻²). We hypothesize that scattering from these dislocations as well as the film boundaries, causes the observed behavior.