n-Type doping of SnO2 thin films by Sb ion implantation

Abstract

The effects of Sb implantation into textured thin films of SnO₂ prepared by RF sputtering have been characterised by a range of techniques including secondary ion mass spectrometry (SIMS), X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS), infrared reflectance, X-ray diffraction and conductivity measurements. The depth distribution of implanted ¹²¹Sb ions at low dose is characterised by a truncated bell-shaped distribution, with mean implantation ranges of 250 Å for 90 keV implantation and 370 Å for 150 keV implantation. However, owing to surface sputtering, the maximum in the implantation profile shifts toward the surface with increasing ion dose and for 90 keV ions at doses above 3 × 10¹⁶ cm^–2, the implantation profile is sputter limited with maximum implant concentration at the surface. Electrical activation of the implanted ions is achieved at temperatures lower than those required to cause complete sintering of the small grains of the as-deposited films. The surface concentration of Sb seen in XPS increases during post-annealing treatments for Sb doses up to 3 × 10^l5cm^–2, but decreases at the higher dose of 3 × 10¹⁶ cm ^–2. These observations are discussed in terms of Sb segregation and Sb₄O₆ evaporation during the post-annealing treatment. The effects of Sb in introducing carriers into the surface region are investigated both by conductivity measurements and by UPS. The latter technique reveals directly occupation of Sn 5s conduction band states. Sb-implanted SnO₂ thin films can be used as sensors for detection of methane at much lower temperatures than is possible with undoped films.