Electrical and optical properties of amorphous vs crystalline GeTe films

Abstract

Ac and dc conductivity, tunneling, and optical transmission and reflectance studies have been made on amorphous and crystalline (rhombohedral and NaCl) GeTe films (80 Å to 10 μ thick) prepared by evaporation and by sputtering. The crystalline films behave like a p-type degenerate semiconductor with Fermi level ˜0.3-0.5 eV in the valence band, resistivity ˜10^-4 Ωcm, carrier concentration ˜10²⁰ to 10²¹/cm³, Hall mobility ˜25-120 cm²/V sec, and a small, positive (metallic) temperature coefficient of resistivity. Amorphous films show a high resistivity which increases exponentially from ˜10³ to >10¹⁰ Ω cm as the temperature decreases from 300-77 K, corresponding to an activation energy of 0.3-0.4 eV. The ac conductivity increases as ωⁿ(n<1) to approach a constant value above ω = 90 kc. Simultaneously, the activation energy for conduction decreases continuously to a nearly zero value. The conductivity also increases rapidly as a high power of the electric field at >10⁴ V/cm. The near infrared indices of amorphous and crystalline structures are 4.0 and 5.6, respectively. The optical absorption edge is nearly the same (˜0.8 eV) for the crystalline and amorphous films. A tentative explanation of the transport behavior of amorphous films is possible in terms of conduction by two parallel mechanisms: (a) in the conduction band by intrinsically excited carriers, and (b) by a hopping process in the trap-like localized levels above the Fermi level in the valence band.