Effects of Ag/In additives and crystallization kinetics on the resistive characteristics of amorphous SbTe chalcogenide films

Chalcogenide films were able to be used as the recording layer of phase change recording media and in the application of phase change random access memory (PCRAM). The most attractive property of this material is its quick transformation between the amorphous and crystalline states, which phenomenon can accompany huge changes in the optical and electric properties. The reversible transformation between amorphous and crystalline phases was named as Ovonic Memory phenomenon and materials with such kind of properties were also named as the phase change materials. In practical applications, major efforts have been focused on increasing the crystallization speed and improvement on the optical or electrical contrast between amorphous and crystalline state. In the present study, there are two chalcogenide films being deposited on alkali-free glass with RF-sputtering method, pure SbTe films (ST) and Ag/In added SbTe films (AgInSbTe, AIST). In the first part, the microstructure and sheet resistivity of AIST films deposited with different parameters were analyzed. The results show that the as-deposited films possess amorphous structure no matter with what the sputtering parameter being adopted. The sheet resistivity measurement shows the amorphous films possess an extremely high resistivity and the temperature coefficient of resistivity (TCR) is negative. It is worth noting that the relationship of amorphous AIST films between the sheet resistivity and film thickness was found to against the classic size effect. In the second part, similar amorphous films were annealed isothermally at different temperatures to obtain their different crystallinity. The sheet resistance of the annealed specimens was measured at room temperature, where the sheet resistance of amorphous films can be 3 × 10⁴ higher than that of the crystalline films. As comparing X-ray diffraction patterns of AIST films to that of ST films, the sheet resistance change of the specimens can be correlated to the crystallization of amorphous phases, which transition temperature of the change in the sheet is at about 433 K for AIST films and 393 K for ST films. Through transmission electron microscopy (TEM) observations and Grazing-incidence X-ray diffraction (GI-XRD), the major phase in the crystalline ST films is the δ-Sb phase and the mixture of δ-Sb and AgSbTe₂ phases in the crystalline AIST films. Concerning of the thermal activation measurements, the activation energy and crystallization temperature were measured with the differential scanning calorimeter (DSC). The activation energy of AIST films is about 0.92 eV and that of ST films is about 0.82 eV, the crystallization temperature of AIST films is about 475 K and that of ST films is about 445 K. The result reveals Ag/In added SbTe films possesses high room temperature stability. Because the sheet resistance has been proven to change with the crystallinity, an apparatus was developed to estimate the activation energy and the Avrami exponent of crystallization through Johnson-Mehl-Avrami formulism. The activation energy is estimated to be about 0.815 eV and the Avrami exponent (n) is about 1.1 to 1.4. The exponent indicates that the crystal can grow freely and the sheet resistance will decrease dramatically after the impingement effects occurring. A model is proposed to explain why the sheet resistance decreases within a very short period and the homogeneous nucleation and free growth during isothermal annealing in this study.