Electron traps in rutile

Abstract

Undoped samples of single crystal rutile and rutile powder have been studied by the methods of photoluminescence, thermoluminescence and thermally stimulated conductivity. Crystal samples doped with chromium, iron, manganese, cobalt and nickel and powder samples doped with chromium and iron were also studied. Additionally capacitance-voltage, current-voltage and deep level transient spectroscopy measurements were made on Schottky diodes fabricated on rutile made semiconducting by reduction and niobium doping. Undoped, chromium and nickel doped samples all showed one form of PL temperature dependence in which the luminescence was quenched above 190 K. Iron, cobalt and manganese doped samples showed a different dependence in which the intensity decreased from 80 K. The chromium doped powders showed an increase in photoluminescence intensity with chromium doping which supports the view of several workers that chromium provides the luminescence centre in rutile. Evidence was found in thermoluminescence spectra for nine different trapping levels. Thermoluminescence measurements on powders indicated that the spectra were composed of the same peaks found in the crystal samples, although they were not distinct. DLTS spectra from a niobium doped rutile crystal showed one dominant maximum. The variation of this maximum with the DLTS rate window gave an activation energy and a capture cross-section in excellent agreement with the values for the dominant TSL peak and suggests that the simple insulator model used for TSL analysis applies well to rutile. The DLTS result allows the results for trap depths derived from TSL measurements to be viewed with some confidence.