The main activity of the HPRC Nitride Processing Lab is the fabrication of blue-laser diodes (LDs), and blue and UV light-emitting diodes (LEDs). Also, a fundamental research on the growth and physical properties of the nitrides is carried out.
In the present work, we investigated mechanical properties of bulk GaN substrate crystals and epitaxial structures grown on them. Growth methods were HVPE (Halide Vapor Phase Epitaxy) and MOVPE (Metalorganic Vapor Phase Epitaxy). Measuring techniques included Laser Profilometry for the curvature and X-Ray diffraction for the material parameters. Additionally, the EPD (Etch Pit Density) and the PL (Photoluminescence) were used. We developed a device that allows in-situ curvature measurements. It is based on the well-known MOS (Multi-beam Optical Sensor) technique. We also developed a simple, low cost laser profilometer for the ex-situ curvature measurements.
The aim of the present work was to establish relations between the material parameters such as the composition, lattice constant, layer thickness, dislocation density, and the bowing of structures. This is important, since large bowing and the stress can significantly affect the device parameters and is a big problem in device processing. Typically, epi-ready bulk GaN substrates have a radius of bowing of about 0.5 to 1.0 meters. This is due to the gradient of free-electrons concentration and the influence of polishing. Additional bowing is introduced by the growth of AlGaN and InGaN layers, due to the lattice mismatch. To give an example, 0.4μ m thick layer of Al_0.1Ga_0.9N on bulk GaN substrate causes bowing of about 0.25 m. This is the value typically seen in our LD structures.
To reduce the value of the bowing, various techniques can be used. Currently, we focus on mechanical properties of bulk GaN substrate crystals thickened by the HVPE method, as well as the AlGaN/InGaN layers and their influence on the bowing.