Optimization of nucleation and buffer layer growth for improved GaN quality

By accurately optimizing the growth conditions for an oxygen doped AlN nucleation layer and for the subsequent epitaxial process the crystal quality of our GaN layers could be improved drastically. In X-ray diffraction analyses we observed FWHM values of 39 and 114 arcsec for the symmetric (0 0 4)- and asymmetric (1 1 4)-reflection, respectively. Consequently, the nominally undoped samples showed semi-insulating behavior in Hall measurements. By in situ deposition of a SiN interlayer, the dislocation density could be reduced by more than a factor of 2, reaching a value of 4×108cm-2 as confirmed by transmission electron microscopy and etch pit density counting. Samples with this low dislocation density showed an extremely narrow X-ray FWHM of 71 arcsec for the asymmetric (1 1 4)-reflection along with a narrow linewidth of 870μeV in photoluminescence (PL) for the donor bound exciton (D00X) at a temperature of 10 K. Atomic force microscopy yielded a very low rms roughness value of about 0.14 nm across a 4μm2 scan area. Finally the excellent crystal quality could be confirmed by growing AlGaN/AlN/GaN high electron mobility transistor structures with reverse breakdown voltages ⩾1000V and a very low sheet resistance of 330Ω/□.