Photoconductivity As a Probe of Defect Mediated Ferromagnetism In SnO2 Thin Films

Abstract

SnO2, a transparent oxide semiconductor, is of considerable interest for use in gas sensors and transparent displays. Also, it has been found that transition metal doped SnO2 is a room temperature ferromagnet, making it a candidate for spintronics devices. This study investigates the effects of changing growth conditions, namely substrate temperature, oxygen pressure, thickness, and growth rate on the structural, magnetic, photoconductive, and electrical properties of 5% Co doped and undoped SnO2 thin films grown by pulsed laser deposition, with the goal of relating the photoconductive and magnetic properties to defects. The magnitude and decay time of photoconductivity in SnO2 has been shown to depend on surface absorbed oxygen and on oxygen vacancies and defects at grain boundaries and within the material. Thus, we have studied photoconductivity to obtain information on the defects in doped SnO2 films. Additional information is obtained via electrical measurements to determine resistivity, carrier concentration, and mobility. These are correlated to structural and optical measurements of the films. For undoped SnO2, we found that changing growth temperature had large effects on the electrical and structural properties but that changing growth pressure had a smaller effect. The photoconductivity magnitude decreases, and the decay time increases with increasing growth temperature. The photoconductivity magnitude and decay time decrease with increasing growth pressure. These changes can be related to changes in crystallinity. For doped films, photoconductivity and magnetism are complex functions of growth conditions. Films grown at higher rates exhibit poorer crystallinity, small grain size, and lower magnetization. Films were ferromagnetic for growth temperatures >700C and growth pressures below 10-3 Torr, with the magnetic moment relatively thickness independent. The photoconductivity is largest for thinner films implying surface effects are important. The thickness independence of the magnetic moment thus suggests that the origin of ferromagnetism does not lie in the defects causing the photoconductivity. The carrier concentrations for doped and undoped films are similar; however, the photoconductivity was much larger, and the decay times much shorter for doped films. We conclude that the defects which cause the carriers differ from those contributing to the photoconduction.