Glassy transformation and structural change in oxygen doped GeSb ₂Te₄ films

The aim of this work is to study the amorphous to crystalline transition in pure and oxygen doped (in the range of 0-15 at%) GeSb₂Te₄ films in the process of isothermal heating. The transition was followed by x-ray diffraction and impedance measurements. The results show that in the material without oxygen, the initial nuclei have the GeSb₄Te ₇ composition, which could appear due to local fluctuations in the film composition. With increasing annealing time these nuclei transform into the crystalline GeSb₂Te₄ phase. In films with an oxygen concentration in the range of 2-10%, the oxygen is located on tetrahedral interstitial sites and acts as the nucleation centre, producing nuclei with a single GeSb₂Te₄ composition, which grow until crystallisation of the films is completed. In films with oxygen concentration above 10 at%, germanium is partially oxidised. Upon isothermal annealing, the amorphous films first crystallises into the fcc GeSb₂Te₄ phase. Once this phase is completed, a new Sb₂Te₃ crystalline phase starts to form until crystallisation is completed. These phenomena explain anomalous behaviours of the Avrami plot in films with high oxygen content.