Electron and hole traps in yttrium orthosilicate single crystals: The critical role of Si-unbound oxygen

Abstract

We studied the processes of hole and electron trapping in yttrium orthosilicate ${\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ single crystals using continuous wave and pulse electron spin resonance methods. We show that holes created by x-ray irradiation at low temperatures ($Tl80$ K) are preferably self-trapped at Si-unbound oxygen ions in the form of $\mathrm{O}{}^{\ensuremath{-}}$ centers. Under irradiation at higher temperatures (200--290 K), the holes are trapped at the Si-unbound oxygen ions in the vicinity of perturbing defects such as yttrium vacancies and impurity ions forming a variety of $\mathrm{O}{}^{\ensuremath{-}}$ centers with thermal stability up to room and higher temperatures. We have also found that under x-ray irradiation at $T$ 60 K, electrons are preferably trapped in the vicinity of Si-unbound oxygen ion vacancies and partly trapped also at Mo impurity ions in the form of $\mathrm{F}{}^{+}$-type and $\mathrm{Mo}{}^{5+}$ centers, respectively. The trapped electrons are thermally released from the $\mathrm{F}{}^{+}$ centers at 75--90 K, thus giving rise to a thermally stimulated luminescence peak at these temperatures. We assume that this process is realized without excitation of the electrons to the conduction band. The spectroscopic parameters ($g$ and hyperfine tensors) of all the investigated centers have been determined as well. Electron spin resonance measurements of electron and hole traps in the related compound lutetium orthosilicate $({\mathrm{Lu}}_{2}{\mathrm{SiO}}_{5})$ are discussed as well.