Optical properties of Ge–Sb–Te chalcogenides

We have used RF sputtering to grow amorphous films of GeTe, Sb2Te3, Ge2Sb2Te5, and Ge2Sb2Te7, and employed photothermal deflection spectroscopy (PDS) to characterize the optical absorption in these films. The PDS technique can measure the optical band gap, the exponential ‘band tails’, and the absorption due to deep defects in these materials. The gross features of the absorption of amorphous Ge2Sb2Te7 and Ge2Sb2Te5 are independent of the substrate temperature from about 290 K to 300 K. The optical band gaps (as measured at α = 104 cm−1) of amorphous Ge2Sb2Te5, and Ge2Sb2Te7 are all about 1.0 eV, although the absorption does depend slightly on the growth rate at room temperature. The band gaps of amorphous Sb2Te3 and GeTe are about 0.8 and 1.0 eV, respectively, consistent with previously published results.