458 O. Maksimov et al. / Materials Chemistry and Physics 100 (2006) 457–459Fig. 1. RHEED patterns for: (A) GaAs substrate after oxide desorption at 500 ◦C, (B) GaAs substrate after 5 min of nitridation at 400 ◦C, (C) GaN buffer layerdeposited at 600 ◦C, (D) GaN ﬁlm grown at 750 ◦C.an arc pattern after approximately 5 min indicating develop-ment of a preferred out-of-plane orientation in a disorderedlayer, Fig. 1B. Since it does not change when the wafer isrotated around the surface normal, layer is not oriented in-plane. Spot-like features with hexagonal symmetry developafter, approximately, 10 min. This reconstruction does not sig-niﬁcantly change when the wafer is exposed to nitrogen plasmafor a longer period of time. Therefore, we limit nitridationto 15 min.In the next step we close nitrogen plasma source shutterand increase wafer temperature to 600 ◦C. Annealing, ∼30 min,sharpens diffraction spots demonstrating recrystallization of ␣-GaN phase, Fig. 1C. The diffraction spots are broad signifyingthat very defective GaN layer forms at the beginning. How-ever, they become signiﬁcantly sharper and elongated duringthe growth of a relatively thin (50-nm) buffer layer, indicatingthat GaN buffer has a better crystalline quality and a smoothersurface.Finally, wafer temperature is raised to 750 ◦C for GaNgrowth. A slightly diffused (1 × 1) reconstruction is observedduring the ﬁlm growth, Fig. 1D.Crystalline quality of the GaN ﬁlms is studied by X-raydiffraction (XRD). All the ﬁlms are deposited in one growthrun under identical conditions and differ only in the nitridationtemperature (A 400 ◦C, B 500 ◦C, C 550 ◦C, D 600 ◦C). A XRDθ–2θ scan demonstrates that low temperature nitridation pro-motes growth of c-oriented ␣-GaN, Fig. 2A. Mis-oriented grains( 1 0 1 1 , 1 0 1 2 , 1 1 2 0 , 1 0 1 3 ) and cubic ␤-GaN inclu-sions ( 0 0 2 ) develop when nitridation is performed at 500 ◦C,Fig. 2B. The intensity of 0 0 0 2 diffraction decreases whileother peaks become more pronounced with the further increaseof nitridation temperature indicating degradation of crystallinequality of the ﬁlm, Fig. 2C and D. This trend is, most prob-ably, due to the surface etching that is activated by substratetemperature during nitridation . It results in a rough defec-tive epilayer/substrate interface and can promote polycrystallinegrowth.In conclusion, we demonstrate that crystalline quality of GaNﬁlms grown on [0 0 1] GaAs substrates is extremely sensitive tonitridation conditions. Nitridation has to be performed at low-temperature (400 ◦C) to achieve c-oriented ␣-GaN. Higher sub-strate temperature promotes formation of mis-oriented domainsand ␤-GaN inclusions.Fig. 2. XRD θ–2θ scans of ∼2 m thick GaN ﬁlms grown on a GaAs substrate.Substrate nitridation is performed at (A) 400 ◦C, (B) 500 ◦C, (C) 550 ◦C, (D)600 ◦C.
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