Determination of Stress in Laterally Overgrown GaN and Underlying Diamond Stripes of Different Dimensions by Visible Raman Spectroscopy
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Wide bandgap semiconductors forming High Electron Mobility Transistors (HEMT) have shown promise in RF technology, space science, and many other sectors which require high power. The local heat produced in the HEMT structure during its operation lowers its efficiency. However, this efficiency may be increased if a highly thermally conductive thin film such as diamond grown via Chemical Vapor Deposition (CVD) can be combined with the HEMT. Epitaxial Lateral Overgrowth (ELO) of GaN on selectively deposited diamond stripes offers the additional benefit that it does not include a barrier layer between the GaN and the Diamond interface. High thermal conductivity across the GaN-Diamond interface is expected because there is no barrier layer in between them. High thermal conductivity between GaN and Diamond will greatly improve thermal management within RF and power electronic HEMTs.
The way ELO GaN grows over the diamond stripes and the high lattice and thermal mismatches between GaN and diamond have led to the present investigation about the nature of the stress distribution on the GaN and diamond stripes. The primary objective is to ascertain the stress distribution at the ELO GaN and window GaN, and on the diamond stripes near the GaNDiamond interface. The stress on the diamond stripes without ELO GaN has been previously determined, but this stress distribution should change after the ELO GaN layer is deposited. Furthermore, different dimensions of the stripes are expected to influence the stress distribution at the surfaces in question. Since stress significantly impacts device performance and dislocation density within the layers, results from this research will be very useful in estimating the overall efficiency of HEMT devices with and without diamond stripes.
The present studies determined the ELO GaN stress distribution by Raman spectroscopy over the diamond stripes and validated the results with Transmission Electron Microscopy and COMSOL modelling. The lower tensile stress above the diamond stripes is attributed to uncoalesced GaN wings and various defects present near the coalescence region. The gap located in between overgrown GaN and diamond is also contributing to lower ELO GaN stress. In addition to GaN stress, the underlying diamond and Si stress behavior has been mapped. The diamond stress was found compressive with maximum value at the center of the stripe while the Si substrate was found to be relaxed due to the intermediate nitride layers in between diamond and Si substrate. Furthermore, ELO and coherently grown GaN quality are compared by presenting defect distribution and symmetry breaking Raman modes. Finally, the stress distribution along the GaN growth direction is discussed which will enable the determination of the ELO GaN conditions needed to optimally grow high thermal transport III-Nitride devices.