A combination of spectroscopic and transport studies of the band tail states in bulk Ge₂Sb₂Te_(5-5x)Se_5x (GSST) phase change materials

The presence of defects in disordered materials can lead to the so-called band tails in the single-particle density of states (SDOS). These band tails are central to the understanding of the universal exponential behavior of optical absorption, known as Urbach tails, in disordered materials. Using temperature-dependent transport and angle resolved photoemission spectroscopy (ARPES) measurements, we have studied bulk Ge₂Sb₂Te_(5-5x)Se_5x(GSST), which are well-known phase change materials, both in the crystalline and amorphous states. In these samples, 5x represents the concentration of Se substituting Te. The SDOS derived from the ARPES data shows the presence of exponentially decaying density of sates at higher Se concentrations (x>0.5) even in the crystalline state. These band tails are expectedly temperature-dependent in the crystalline state. The band tails in the amorphous state are, however, essentially temperature-independent in the temperature range of our measurements. In contrast to the common perception that the materials with band tails display hopping transport, we observed power law behavior in the temperature dependence of the resistivity in the crystalline phase of the GSST at low temperatures. The charge transport, on the other hand, displays expected activated behavior in the amorphous phase.