A comparative study on electrical transport properties of thin films of Ge1Sb2Te4 and Ge2Sb2Te5 phase-change materials

We have investigated the electrical properties of Ge1Sb2Te4 and Ge2Sb2Te5 thin films in the temperature range of 27 to 200 °C. The optical bandgap values obtained from the measured absorption spectra for amorphous Ge1Sb2Te4 and Ge2Sb2Te5 are 0.70 and 0.90 eV, respectively. The results of the in situ temperature dependence of the sheet resistance demonstrate that both Ge1Sb2Te4 and Ge2Sb2Te5 thin films in the amorphous phase exhibit a temperature-activated electrical conductivity with activation energy values of 0.42 and 0.45 eV, respectively. These results show that our experimental results agree well with the predictions of Mott’s model. The temperature dependent carrier mobility was measured accurately using the surface acoustic wave method, which is an appropriate method for materials with low conductivities and low mobility. Typical values of drift mobility for amorphous Ge1Sb2Te4 and Ge2Sb2Te5 thin films at room temperature are round 10^-3 cm²/(Vs) and 10^-2 cm²/(Vs), respectively. The values of drift mobility remain small below 140 °C when the films are in an amorphous state. The drift mobility increases gradually as the temperature increases in the temperature range of 140–160 °C and shows dramatic changes above 160 °C. In general, the mobility of amorphous Ge2Sb2Te5 is 3 to 5 times higher than that of amorphous Ge1Sb2Te4. The smaller mobility for Ge1Sb2Te4 films might suggest that amorphous Ge1Sb2Te4 film has more vacancies, which results in more disordering- - as compared to that of Ge2Sb2Te5.