It is difficult to obtain p-type conductivity in beryllium-doped gallium nitride. Even when the material exhibits p-type conductivity, it tends to be highly compensated. Beryllium-doped gallium nitride samples grown by molecular beam epitaxy were investigated using deep level optical spectroscopy (DLOS), photoluminescence (PL), and positron annihilation spectroscopy (PAS) in connection with an annealing study in an attempt to correlate compensation and PL features with microscopic defects.
Interestingly, both DAP PL and a DLOS indicate an energy level that if interpreted as an acceptor would yield an optical activation energy of beryllium in gallium nitride of about 100meV. These signatures are missing in all as-grown gallium-polar gallium nitride doped with beryllium at levels below 2×1014 cm-3. Upon annealing in pure nitrogen or forming gas, the samples clearly exhibit the DAP at 3.38 eV associated with a shallow Be acceptor, but the samples remain semi-insulating. Interestingly, all nitrogen-polar as-grown samples exhibit the DAP emission at 3.38eV. We will discuss more about the effect of annealing on the apparent optical activation of beryllium and the shift of the photoluminescence peak.
DLOS and PAS studies suggest that gallium vacancies and/or gallium-related vacancies are related to compensation in beryllium doped gallium nitride samples. For heavy beryllium doped gallium nitride, there is a correlation between PL at 2.3-2.4eV and a beryllium-related deep acceptor complex. This is supported by PAS studies and DLOS studies. Additionally, there is a correlation between donor-acceptor pair (DAP) at 3.38eV, beryllium concentration, and yellow-red photoluminescence at 2.0 or 2.2eV.