Date of Award
Master of Materials Science and Engineering (MMatSE)
In the last decade, the phase change optical recording based upon chalcogenide glasses has advanced remarkably at full-scale commercial applications in the form of rewritable compact disks (CD-RWs). The principle of phase-change optical recording is based on a thermally induced reversible transformation between amorphous and crystalline phases. The search for recording materials possessing fast crystallization rate and long data retention has been pursued. Research results have revealed that Ge-Sb-Te compounds exhibit fast crystallization, which has been explained in terms of their single-phase structure.
In the present work, electrical properties of Ge-Sb-Te thin films using Impedance Spectroscopy, Current-Voltage, and Capacitance-Voltage techniques is presented. First, we studied the effect of the electrical contacts, deposited on top of Ge-Sb-Te films, on the measured electrical signal (Impedance Spectroscopy). To do this, three different configurations of the electrical contacts were prepared. The results show that the electrical contacts have no effect when the silver paste is placed on the gold electrodes away from the thin film under test. This guarantees no interaction between the silver paste and the film under test during the heating process. Also the results show that the order of depositing layers (material and the gold) is not important and what matters is the location of the silver paste on the gold electrodes.
Second, the Impedance Spectroscopy and Current-Voltage data shows similar resistance and relaxation frequency temperature dependence with activation energies 0.36 and 0.39 eV, respectively. Therefore the corresponding energy gap is estimated to be 0.72 eV and 0.78 eV. This result agrees with the optical gap (0.7 eV) reported in literature. In addition, the I-V curves show that the amorphous-crystalline transition temperature (Tc) is around 135 °C.
Third, the capacitance-Voltage measurements were performed on amorphous samples of In0.3Ge2-Sb2-Te5 and Ge2-Sb2-Te5 thin films in order to study the effect of doping on the electrical properties of Ge2-Sb2-Te5. The results show that inducing Indium to Ge2-Sb2-Te5 decrease the capacitance of the sample as the bias voltage increases for the whole range of temperatures. However, the same behavior is observed for undoped sample (Ge2-Sb2-Te5) only at temperatures close to the amorphous-crystalline transition temperature and at high applied bias voltages (≥ 15 volts). The behavior is attributed to the increase in electron concentration as the bias voltage and temperature increase. For In0.3Ge2-Sb2-Te5 sample, the indium can be considered as an additional source of free electrons. This would contribute to a further and significant decrease in the capacitance of In0.3Ge2-Sb2-Te5 sample, and eventually the capacitance becomes negative. The nonlinearity in the capacitance and conductivity could be related to the nucleation mechanism as the temperature becomes close to the amorphous-crystalline transition temperature. Finally in the conclusion I proposed some study to be conducted on Ge2Sb2Te5 films.
Humeid Al-Shamisi, Hessa Ali, "Preparation and Characterization of (GeSbTe)) Semiconductors for Phase-Change Optical Storage" (2008). Theses. 293.