Invited talk abstract

Local vibrational mode (LVM) spectroscopy is a direct method for the identification of important impurities as well as their complexes in semiconductor matrices. In this work, we concentrate on studies of Mg and Be acceptors in GaN semiconductor and their interactions with oxygen and hydrogen. Bulk, high-pressure grown GaN crystals doped intentionally with Mg or/and Be are investigated by Fourier transform infrared spectroscopy and electrical measurements. Mg doped samples exhibit either slightly p-type or semi-insulating behaviour. Be-doped and Be+Mg co-doped samples are always semi-insulating. Bulk GaN crystals grown by means of high-pressure synthesis contain oxygen donors in relatively large concentrations (~5x10^19 cm-3). We believe that these donors are responsible for the high compensation rate of the investigated materials. In beryllium doped samples, a local vibrational mode at 789 cm-1 is observed. It is attributed to "isolated" electrically active beryllium. This mode is the analogue of the 656 cm-1 mode observed by Raman spectroscopy in magnesium doped GaN and assigned to "isolated" Mg [1, 2]. In magnesium doped GaN, several correlated modes are attributed to magnesium-oxygen complexes. In beryllium doped samples, evidences of beryllium-oxygen complexes are also observed. In Mg+Be co-doped samples both of these features together with the 789 cm-1 mode can be seen. The complexing of magnesium with hydrogen in GaN is well established in MOVPE as grown materials; it gives rise to a local vibrational mode at 3125 cm-1 [3]. This mode is rarely observed in GaN:Mg as grown crystals elaborated by high-pressure synthesis whereas it is clearly observed in samples annealed in hydrogen ambient. The analogue mode in beryllium doped samples has not been detected even in samples annealed under hydrogen atmosphere.

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