Study of laser crystallization and recording properties of oxygen doped Ge:Sb:Te films

The aim of this work is to study the mechanism of the amorphous-to- crystalline phase transformation in Ge ₁ Sb ₂ Te ₄ films doped with oxygen using nanosecond laser pulses under isothermal annealing in the time scale of minutes. Experimental results show that the nucleation time, t _nucl (minimum laser pulse duration for starting the laser-induced crystallization) depends on the oxygen concentration in the films. For those films with compositions in the range of 2-8 at.%, t _nucl is shorter than that observed in films free of oxygen. In contrast, in films with oxygen in the range of 10-28 at.%, t _nucl is longer than in the reference sample. Reflection and X-ray measurements on minute time scale have shown that in the films without oxygen annealed under isothermal conditions, the nucleation of the Ge ₁ Sb ₄ Te ₇ metastable phase is first observed, which is subsequently transformed into the Ge ₁ Sb ₂ Te ₄ crystalline phase. This effect increases the nucleation time in laser-crystallized materials. For films with 2-8 at.% of oxygen the first nucleation phase to be observed is the crystalline Ge ₁ Sb ₂ Te ₄ , where it is assumed that the oxygen acts as the center of nucleation, therefore decreasing the nucleation time. In the films with oxygen concentration above 10 at.%, the thermal treatments leads to the formation of stable amorphous GeO ₂ . This decreases the amount of available Ge and leads to the formation of crystalline Sb ₂ Te ₃ . The phase segregation in films with more than 10 at.% of oxygen, results in an increase in the nucleation and crystallization times in laser-induced crystallization. This crystallization behavior allows the possibility of having multilevel laser recording.