Alloying, co-doping, and annealing effects on the magnetic and optical properties of MOCVD-grown Ga1-xMnxN

Recent theoretical work for Ga1-xMnxN predicts ferromagnetism in this materials system with Curie temperatures above room temperature. Ferromagnetic behavior observed in Ga1-xMnxN is still controversial, as there are conflicting experimental reports owing to the disparity in crystalline quality and phase purity of Ga1-xMnxN produced by different methods. In this work, metal-organic chemical vapor deposition (MOCVD) has been used to grow high-quality epitaxial films of Ga1-xMnxN of varying thickness and manganese doping levels using Cp-2 Mn as the Mn source. Crystalline quality and phase purity were determined by high-resolution X-ray diffraction, indicating that no macroscopic second phases are formed. Atomic force microscopy revealed MOCVD-like step flow growth patterns and a mean surface roughness of 0.378 nm in optimally grown films, which is close to that from the as-grown template layer of 0.330 nm. No change in the growth mechanism and morphology with Mn incorporation is observed. A uniform Mn concentration in the epitaxial layers is confirmed by secondary ion mass spectroscopy. SQUID measurements showed an apparent room temperature ferromagnetic hysteresis with saturation magnetizations of over 2 mu B/Mn at x = 0.008, which decreases with increasing Mn incorporation. Upon high-temperature annealing, numerous changes are observed in these properties, including an increase in surface roughness due to surface decomposition and a large decrease in the magnetic signature. A similar decrease in the magnetic signature is observed upon co-doping with the shallow donor silicon during the growth process. These results demonstrate the critical importance of controlling the Fermi level relative to the Mn2+/3+ acceptor level in Ga1-xMnxN in order to achieve strong ferromagnetism. (c) 2005 Elsevier B.V. All rights reserved.