Mechanical stress studies of amorphous GexSb40−xS60 film

Mechanical stress in the GexSb40−xS60 film/Si substrate system has been examined before and after film treatment (illumination and/or annealing). The intrinsic stress in the film was tensile. Stress dependence on the atomic percentage Ge content x shows two extremes. The minimum, at x≈15, has been associated with the so-called ‘stress-free’ composition. We suggest that the ‘stress-free’ phase in these films has the same origin as Boolchand’s intermediate phase, which is characteristic of chalcogenide glasses. Such a state could be realised with the presence of an extra structural unit, the quasi-tetrahedral S = Sb (S1/2)3, seen in these films in addition to the main pyramidal SbS3 and tetrahedral GeS4 units. The maximum at x≈27 is attributed to the well known threshold behaviour of the physical properties of such ternary systems. Illumination of the films results in a decrease of the stress for x>15 and we propose that this is mainly related to structural reorganization of the edge-shared GeS4 tetrahedra. The stress was increased by the process of annealing. In the Ge-poor region, we suggest that this increase is related to both the SbS3-chain rearrangements and the increased chain number and length due to a transformation of the quasi-tetrahedra into pyramids. The sharp increase in stress at x≈27 is attributed to a rearrangement of the GeS4 edge- and corner-sharing tetrahedra, that initiates a process of nanoscale phase separation upon thermal annealing. The effect of the double treatment, i.e. both annealing and illumination of the layers, is dependent on the order in which they are carried out.