     Device operation characteristics of light-emitting devices and transistors deteriorate owing to heat generation and resultant increase in temperature of the devices. For example, the population distribution in excitonic states varies with an increase in phonon occupation or phonon temperature, and the change in the distribution reduces the rate of radiation and efficiency. The effects of crystal defects on thermal energy transport have been extensively discussed in many articles. However, microscopic local imaging of heat transport at a specific defect is still a challenge. In Raman measurements, mode-separated information of phonons can be obtained, which is an advantage compared with the pump-probe method of photoreflectivity. It is known that in Raman scattering measurements, two spectra obtained with two laser beams of different wavelengths provide the depth profile of the strain field of a two-layer structure, while analyses of heat transport at a heterointerface or crystal defects require the additional element of simultaneous incidence of two laser beams, the pump (heating) and probe system, and the control of irradiation positions in order to distinguish the heating and probing positions.

     GaInN and AlInN alloys are key materials for optical devices in green gap and high power transistors. Characteristic properties of local strain, fluctuation of energy gap, carrier dynamics, and so forth due to spatial nonuniformity of alloy composition are now under investigation, while nonuniform thermal conduction remains unclear. In this report we show nonuniform heat or phonon conductance in the nonuniform field of GaInN alloy.

A Ga~0.84~In~0.16~N (110nm) film grown on a GaN layer grown on a sapphire substrate was characterized by micro Raman spectroscopy at room temperature with controlling spot positions of two color lasers: 325 nm for electronic excitation and phonon generation and 532 nm for Raman probe. High sensitivity, high spectral resolution, and high stability of the measurement system were achieved by controlling the fluctuation of circumstance temperature less than 1 degree. The irradiation spot of the probing laser was controlled to be less than 0.65 micrometer. Using this system, we have observed anisotropic heat transport in the Ga~0.84~In~0.16~N/GaN-heterostructure by analyzing images of Raman signals for E~2~(high) and A~1~(LO) modes in 8 – 15 micrometer square regions. The 325-nm laser is absorbed in the GaInN layer. The energy relaxation of generated carriers to the band edges yielded a local heating in the GaInN layer. When the probing of Raman signal was performed by the same laser beam, the energy shifts of Raman peaks provided the increase in the temperature in the GaInN layer. When the Raman signal was probed by the 532-nm laser, which was penetrated to the GaN underlayer, the increase in the temperature of the GaN layer was probed. We have observed triangle structures in microscopic mapping images of photoluminescence intensity and Raman shift for the E~2~(high) mode from the GaInN layer, which reveals the locations of misfit dislocations (MDs) in the vicinity of the GaInN/GaN interface. It was found that spatially uniform increase in temperature was observed for the GaInN layer, whereas in the temperature increase in the GaN layer, clear triangular structures corresponding to the PL mapping image of the GaInN layer were observed. The temperature increase beneath the MDs is lower than that of the outside areas, which reveals the blocking of energy transport from the GaInN layer to GaN layer in the vicinity of misfit dislocations on the heterointerface. This presentation provides a methodology of heat transport analysis using Raman scattering, which can be applied to various targets of superlattices, threading dislocations, and other various defects and structures down to submicrometer scale.


Local Heat Energy Transport Analyses in Gainn/gan Heterostructure by Microscopic Raman Imaging Employing Simultaneous Irradiation of Two Laser Beams

Division / Sponsors: 
Electronic & Photonic Packaging Division (EPPD)

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2020/volume/84041

InterPACK2020 - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.