The origins and properties of intrinsic nonradiative recombination centers in wide bandgap GaN and AlGaN

The nonradiative lifetime (τ_NR) of the near-band-edge emission in various quality GaN samples is compared with the results of positron annihilation measurement, in order to identify the origin and to determine the capture-cross-section of the major intrinsic nonradiative recombination centers (NRCs). The room-temperature τ_NR of various n-type GaN samples increased with decreasing the concentration of divacancies composed of a Ga vacancy (V_Ga) and a N vacancy (V_N), namely, V_GaV_N. The τ_NR value also increased with increasing the diffusion length of positrons, which is almost proportional to the inverse third root of the gross concentration of all point defects. The results indicate that major intrinsic NRC in n-type GaN is V_GaV_N. From the relationship between its concentration and τ_NR, its hole capture-cross-section is estimated to be about 7 × 10⁻¹⁴ cm². Different from the case of 4H-SiC, the major NRCs in p-type and n-type GaN are different: the major NRCs in Mg-doped p-type GaN epilayers are assigned to multiple vacancies containing a V_Ga and two (or three) V_Ns, namely, V_Ga(V_N)_n (n = 2 or 3). The ion-implanted Mg-doped GaN films are found to contain larger size vacancy complexes such as (V_Ga)₃(V_N)₃. In analogy with GaN, major NRCs in Al_0.6Ga_0.4N alloys are assigned to vacancy complexes containing an Al vacancy or a V_Ga.