High mobility yttrium doped cadmium oxide thin films

Donor doped CdO thin films on c-plane sapphire are prepared by reactive co-sputtering from Cd-metal and Y-metal targets which are driven using pulsed-dc and RF power respectively. Intrinsic CdO exhibits a carrier density of 1.8 × 10¹⁹ cm^-3 and a mobility of 330 cm² V^-1 s^-1. By increasing the Y-flux, carrier density values can be increased smoothly and reproducibly to a maximum value of 3.3 × 10²⁰ cm^-3. Mobility increases with Y flux, and exhibits a broad plateau between approximately 5 × 10¹⁹ cm^-3 and 2 × 10²⁰ cm^-3. Higher carrier concentrations produce a sharp drop in mobility. The increase in mobility is attributed to a reduction of intrinsic donors (i.e., oxygen vacancies) with increasing carrier density while the ultimate decrease in mobility results from a combination of factors including cadmium vacancies, reduced crystal quality, and smaller crystallite sizes, all of which accompany carrier density values greater than the mid 10²⁰ cm^-3 range. This work demonstrates that CdO thin films can be prepared by magnetron sputtering with transport properties and crystal quality that are comparable to those grown using molecular beam epitaxy.