TECHNOLOGY SUBSTRATES


GaN layers are formed by selective area growth from the narrow sidewall of the patterned sapphire, so there is weak adhesion at the hetero-interface. Thus, mechanical failure can easily occur at the hetero-interface to yield free-standing GaN without additional processing.


Chemical mechanical polishing of our thick layer of GaN, which has been separated from sapphire, enables the formation of a 2-inch semi-polar substrate. According to measurements acquired with a scanning white light interferometer, the typical root mean square roughness of this free-standing GaN is just 0.2 nm, implying that the surface is atomically flat. X-ray diffraction indicates that the radius of lattice curvature of the substrate is approximately 4 m. The dislocation density depends on the orientation of GaN. It is low 108


cm-2 {1011}, and mid 106


for {2021}, low 107 cm-2


cm-2 for for {1122}. One of our goals is to reduce this density


by optimizing growth conditions and SiO2 mask geometry.


Our substrates have formed the foundation for the growth, by MOCVD, of an LED epiwafer structure featuring InGaN/GaN multiple quantum wells. X-ray diffraction measurements of this epistructure reveal satellite peaks, indicative of the abrupt heterointerface between the InGaN well and GaN barrier layers (see Figure 7). Meanwhile, transmission electron microscopy reveals the high quality of the active region.


We have also measured the current- voltage characteristics of these LED structures, with results showing that these devices can be fabricated on the {2021} GaN substrate. If the dimensions of this platform can improve, it could play a major role in increasing the performance of commercial InGaN LEDs.


£ The development of thick layers of semi-polar and non-polar GaN was carried out within the projects Regional Innovation Cluster Program (Global Type) and Low-Carbon Research Network in the MEXT Japan.


© 2014 Angel Business Communications. Permission required.


March 2014 www.compoundsemiconductor.net 51


Figure 7. X-ray diffraction and current-voltage curves of {2021}GaN LED. (a) Measured and simulated X-ray diffraction profiles of InGaN/GaN multiple quantum wells. The inset shows a transmission electron microscopy image of InGaN/GaN multiple quantum wells. (b) Typical current-voltage curves of {2021} GaN LED


Core researchers. From left to right: Keisuke Yamane, Yusuke Mitsui, Takashi Inagaki, Yusho Denpo, Yasuhiro Hashimoto and Narihito Okada


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