Surface and bulk properties of magnetically doped GaN and their dependence on the growth conditions (original) (raw)

2005

Abstract

MnGaN and CrGaN thin films were grown by molecular beam epitaxy on MOCVD GaN(0001)/sapphire(0001) and sapphire(0001) substrates respectively. Dependence of structural and magnetic properties of MnGaN and CrGaN samples on Ga/N flux ratios were investigated. Scanning tunneling microscopy studies of c-GaN(001) were performed to understand the surface reconstructions of this material, as Mn doped c-GaN has been predicted to have a Curie temperature about 6% higher compared to Mn-doped wurtzite GaN.[1] MnGaN samples were grown on MOCVD GaN(0001)/sapphire(0001) at substrate temperature of 550°C under different Ga/N flux ratios leading to 4 different growth regimes: N-rich, slight metal-rich, metal-rich, and Ga-rich. Mn incorporation and hence magnetic properties clearly depend on the growth conditions. The N-rich grown sample exhibits much larger magnetization compared to the other samples. Ga-rich magnetization is attributed to accumulates, but N-rich magnetization is attributed to carrier-mediated ferromagnetism and/or ferromagnetism due to clusters. Influence of the growth conditions on structural and magnetic properties of CrGaN on sapphire(0001) has been investigated. CrGaN samples are grown under different Ga/N flux ratio of 65% to 100% at substrate temperatures of 650 and 700°C by rf N-plasma molecular beam epitaxy. The Cr/Ga flux ratio is set to either 3% or 5%. These growth parameters allow to vary over a range of growth conditions from N-rich to Ga-rich. Surface conditions during growth influence the surface morphology and magnetic properties of CrGaN films. In particular, we show that N-rich and metal-rich growth conditions result in room temperature ferromagnetism. Scanning tunneling microscopy studies have been performed on c-GaN(001) grown by rf N-plasma molecular beam epitaxy. Scanning tunneling microscopy studies of c-GaN (001) reveal several surface reconstructions including 4x11, c(4x20), 4x9, c(4x16), 4x7, and c(4x12). These reconstructions depend on the surface Ga coverage with 4x11 having the highest Ga coverage and c(4x12) having the lowest Ga coverage per unit area. The reconstruction which is most commonly found on the surface is c(4x16). Scanning tunneling spectroscopy results suggest that c(4x16) reconstruction is in metallic state.

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