Abstract
Physical properties of the chalcogenides based on GaSbTe system were investigated and their feasibility as phase-change recording media was also evaluated. Thin films of compositions between the Sb7Te3-GaSb and Sb2Te3-GaSb pseudo-binary tielines were prepared by an RF co-sputtering method. The laser-crystallized structures of the GaSbTe films were identified by X-ray diffraction. All the crystallized films show rhombohedral structures of space group R-3m with continuous variation in lattice constants. The model of six-layer hexagonal lattice was proposed. The crystallization kinetics of amorphous GaSbTe powders and films were studied by non-isothermal differential scanning calorimetry (DSC) and isothermal time-resolved transmission, respectively. In DSC studies, the crystallization temperature and activation energy increase with increasing GaSb content. The kinetic exponents confirm the crystallization mechanism to be diffusion-controlled growth with decreased nucleation and show higher values at the SbTe-rich compositions. In transmission studies, crystallization of GaSbTe films reveals the two-step feature: initial surface nucleation and coarsening followed by one-dimensional grain growth. The kinetic parameters of the first stage show much correspondence with those of non-isothermal cases. Optical properties of amorphous and crystalline GaSbTe films were analyzed by a spectrometer and ellipsometry. The optical spectra of binary SbTe films show higher reflectance and lower absorptance at the crystalline state, while that of GaSb film shows the reverse case. Compositional deviation from binary SbTe to GaSb reduces the reflectivity ratio between crystalline and amorphous states but increases the absorption ratio instead. The changes in optical spectra can be interpreted in terms of the optical constants, and more explicitly, the optical bandgaps. The addition of GaSb into binary SbTe tends to diminish the changes in peak amplitudes and peak locations of both n and k values during the phase transformation, leading to lower optical contrast. The absorption coefficients of all studied films reveal allowed indirect optical transitions at either amorphous or crystalline state. After crystallization, the optical bandgaps of amorphous films are greatly reduced. Phase-change recording disks based on the GaSbTe recording media were prepared in the DVD+RW format and evaluated by a dynamic tester. Enhanced recrystallization has been demonstrated in the disks with Sb7Te3-rich recording layer, and the approximated composition Ga2Sb5Te3 exhibits a nearly complete erasure (-40 dB). Relationship between dynamic characteristics and lattice constants of the GaSbTe recording media has been proposed.