Self-Trapped-Exciton Radiative Recombination in β–Ga₂O₃: Impact of Two Concurrent Nonradiative Auger Processes

The peculiarities of radiative and nonradiative processes associated with self-trapped intrinsic eXcitons in the excited β-Ga₂O₃ crystals are studied via time-resolved techniques of induced absorption, transient grating, and photoluminescence (PL) at room temperature. The excitation above the bandgap is produced by laser pulses with linear light polarization parallel and orthogonal in the (–201) and (001) planes. We elucidate that the nonradiative recombination rate occurring in the eXciton prevails over its radiative emission rate in a wide range of free carrier concentration composed of excited and equilibrium electrons. Hence, the nonradiative recombination has no effect on the strong anisotropy and the shape of the eXciton emission band. However, we find out that the conventional ABC model of electron effective lifetime is insufficient for explanation of the excitation dependences. Inclusion of two nonradiative Auger mechanisms in a modified ABC formula provides excellent agreement of these dependences. We conclude that the trap-assisted Auger process is in proportion to the free electron density with coefficient B = 1.1 × 10^–11 cm³/s and appears at low/intermediate excitation, while the triple-particle Auger process is in proportion to Δn² with coefficient C = 8 × 10^–30 cm⁶/s and appears at high excitation conditions. The transition between two Auger mechanisms is accompanied by a rise of the eXciton diffusivity in preferred crystallographic directions where the radiative PL intensity is maximal. The diffusion length LD in these directions can reach values ~300 nm, but, at high excitations, LD becomes limited by Auger lifetimes. These findings pave the way for the implementation of self-trapped eXcitons into specific optoelectronic devices.. Authors: Grivickas, Vytautas [Vilnius University (Lithuania)] (ORCID:000000018234121X); Ščajev, Patrik [Vilnius University (Lithuania)] (ORCID:0000000195880570); Miasojedovas, Saulius [Vilnius University (Lithuania)]; Voss, Lars [Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)] (ORCID:0000000263498219); Grivickas, Paulius [Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)]. DOE Contract: AC52-07NA27344. Subjects: 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Auger recombination mechanism; anisotropic radiative emission; electrical conductivity; excitons; light; nonradiative lifetime; polarization; recombination; self-trapped polaron; ultrawide bandgap electronic materials