Gallium nitride is a III-nitride semiconductor with direct, wide (3.4 eV) bandgap. The III-nitride semiconductor family has a wide range of applications in optoelectronics, high-power and high frequency devices. However, the material still faces challenges related to, e.g., light emission efficiency at high currents in the case of LEDs and extension of lifetime of laser diodes. The exact origins of these defects are still under debate. Positron annihilation spectroscopy is a powerful yet nondestructive tool in studying neutral and negative vacancy type defects in crystalline matter, such as, semiconductors . This is possible as positrons can get trapped at defects, and the annihilation radiation carries information of local atomic and electronic structure. In practice this, changes in positron lifetime and positron-electron momentum distribution are monitored to identify and quantify vacancy defects.
We present a combined experimental and computational study of vacancy-hydrogen complexes in GaN. We show that Ga vacancy-oxygen-multihydrogen complexes can be identified through their annihilation characteristics. These defects have been suggested to exist at abundant concentrations in MOCVD GaN thin films  and ammonothermal GaN bulk crystals . Importantly, Ga vacancies complexed with 1-2 hydrogen atoms are readily detected with positrons and introduce deep electronic states in the gap, while the VGa-3H are invisible to positrons and neutral. It seems that hydrogen can be stripped from the latter by a high enough current density , suggesting a possible cause for degradation of optical properties under lasing conditions.
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