Non-equilibrium doping of amorphous chalcogenides

The doping of amorphous chalcogenides to control their electronic properties, and specifically change the dominant charge carrier type from holes to electrons, has been a major research challenge for many years. Generally attempts to achieve this through doping during the glass forming phase have been frustrated by autocompensation effects mediated via charged defects resulting in Fermi level pinning. To date progress in this area has remained limited to Bi and Pb-doped GeX (X = S, Se, Te) systems where high doping levels (6-11%) have shown carrier type reversal (CTR) from p-type to n-type.We have readdressed the issue of CTR in amorphous chalcogenides by exploring the concept of non-equilibrium doping, corresponding to the introduction of dopants into the glass matrix following the formation of the network. To achieve this we utilize ion-implantation thereby enabling doping with a wide choice of ion species into any solid chalcogenide system. A number of amorphous chalcogenides systems have been studied including GeX, Ge2Sb2Te5 and GaLaS-based glasses deposited onto substrates via sputtering to form thin films. The structural, electrical and optical properties of the doped systems have been studied in detail revealing their significant modification and aiding understanding of the effect of non-equilibrium doping on these various systems. In particular a series of fabricated chalcogenide/n(p)-type Si devices enable detailed examination of the role of trap states in determining the electrical properties.We demonstrate the success of our non-equilibrium doping approach through achieving CTR in the GeX systems as lower doping concentrations than previously shown possible using conventional approaches. Furthermore, we demonstrate CTR for the first time in a non-GeX system, demonstrating the fabrication of pn-junction devices which are characterized for their electronic and optical behavior