Effect of indium doping on Ge2Sb2Te5 thin films for phase-change optical storage

The influence of In doping on the crystallization kinetics of Ge₂Sb₂Te₅ has been investigated using four-point-probe electrical resistance measurements, grazing incidence X-ray diffraction (XRD), X-ray reflectometry (XRR), variable incident angle spectroscopic ellipsometry, a static tester, and atomic force microscopy. For a stoichiometric Ge₂Sb₂Te₅ alloy doped with 3% In, the amorphous-to-crystalline transition is observed at 150 °C in the sheet resistance measurements. XRD reveals the formation of a predominant NaCl-type Ge₂Sb₂Te₅ phase during the amorphous-to-crystalline transition together with small amounts of crystalline In₂Te₃. Density values of 5.88±0.05 g cm^-3 and 6.22±0.05 g cm^-3 are measured by XRR for the film in the amorphous and crystalline states, respectively. Perfect erasure can be achieved by laser pulses longer than 165 ns. The retarded crystallization, as compared with the undoped Ge₂Sb₂Te₅ alloy, is attributed to the observed phase segregation. Sufficient optical contrast is exhibited and can be correlated with the large density change upon crystallization. PACS 68.55.-a; 78.20.-e; 78.66.Jg