Study of free-standing GaN substrates prepared by hydride vapor phase epitaxy technology

Free-standing GaN substrates has been prepared by hydride vapor phase epitaxy (HVPE) technology. Round substrates with diameters up to 1.5 inch and thickness up to 350 micron can be achieved. Various measurement tools, including X-ray diffraction, atomic force microscopy (AFM), and etch pit density (EPD) measurement, have been applied to characterize the optical and the structural properties of these substrates. The typical full width at half maximum of the PL spectra and the [002] X-ray diffraction peak was 10 nm and 120 arcsec respectively, compared to the typical values of 10 nm and 200 arcsec obtained on epitaxial GaN layers grown by metalorganic chemical vapor deposition (MOCVD) technique on Al₂O₃ substrates. Yellow bands usually detected in PL spectra can be eliminated, indicating reduction of related point defects. AFM measurements revealed a surface roughness of 5 nm or larger after growth and separation. With proper polish, the surface roughness can be reduced to less than 1 nm. EPDs on these substrates can be as low as in the 10⁵ cm^-2 range, compared to the typical values of 10⁸-10¹⁰ cm^-2observed in MOCVD GaN layers grown on Al₂O₃ substrates. Without doping, the free-standing GaN substrates showed electron concentrations less than 10¹⁶ cm^-3. But with proper doping, electron concentration as high as 10¹⁹ cm^-3, and hence resistivity lower than 0.1 Ω-cm, can be achieved. Maximum electron mobilities higher than 800 cm²/V-sec and 400 cm²/V-sec were detected in lightly doped and heavily doped substrates respectively. In this paper, the procedure of producing these free-standing GaN substrates will be briefly described, and the above measurement results, along with their indications, will be discussed.