BACK-SURFACE PASSIVATION OF POLYCRYSTALLINE CDSE THIN-FILM TRANSISTORS

The electrical characteristics of simple inverted-gate CdSe thin-film transistors (TFTs) were monitored after annealing and back-surface pas sivation treatments and the results were correlated with chemical anal ysis of the back interface. A dramatic increase in TFT current was obs erved after a short vacuum anneal at 320 degrees C. X-ray photoelectro n spectroscopy (XPS) analysis showed that the vacuum anneal removed pa rt of the oxide bonded to Se from the back (top) surface. This was con firmed by temperature-programed desorption data for oxidized CdSe, whi ch showed two distinct selenium oxide species desorbing above 300 degr ees C, XPS analysis showed that evaporated SiO, or SiO2 deposited by m icrowave plasma-enhanced chemical-vapor deposition, reacted strongly w ith an oxidized CdSe surface by removing the Se-bound oxygen present a fter an air anneal. Changes in the: width of the Cd 3d(5/2) peak sugge sted that the Cd bound oxygen was also transferred to the SiO during t he deposition of the oxide. A reoxidation of the CdSe/SiO interface oc curred after annealing in air at 350 degrees C. In contrast, silicon d ioxide deposited by e-beam evaporation did not react as strongly with the native oxide. The interfacial oxide was reduced by annealing in fa rming gas. A reduced CdSe surface with no Se bonded to oxygen and a na rrow Cd 3d(5/2) XPS peak was associated with a high density of donors at the interface. (C) 1998 American Vacuum Society. [s0734-2101(98)042 02-X].