The structure and morphology of low growth temperature GaN nucleation layers have been studied using atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED), and transmission electron microscopy (TEM). The nucleation layers were grown at 600thinsp;deg;C by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) onchyphen;plane sapphire. The layers consist of predominantly cubic GaN (chyphen;GaN) with a high density of stacking faults and twins parallel to the film/substrate interface. The average grain size increases with increasing layer thickness and during the transition from low temperature (600thinsp;deg;C) to the high temperatures (1080thinsp;deg;C) necessary for the growth of device quality GaN. Upon heating to 1080thinsp;deg;C the nucleation layer partially converts to hexagonal GaN (hhyphen;GaN) while retaining a high stacking fault density. The mixed cubichyphen;hexagonal character of the nucleation layer region is sustained after subsequent highhyphen;temperature GaN growth. copy;1996 American Institute of Physics.
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