We examine unintentional incorporation of Al during the growth of molecular beam epitaxy (MBE) related to N gas introduction. In spite of the closed shutter of Al cell, we observe Al incorporation in the epitaxial layer with a concentration up to 1 x 10(18) cm(-3). The concentration depends on the N-2 gas flow rate and Al source temperature. The concentration is suppressed by the reduction of the Al cell temperature. Shutter control of the N cell, As beam equivalent pressure and the operation of the RF plasma Power have no impact on that. The introduction of the large amount of N can modify the beam dispersion of Al in the MBE chamber, which will cause the extrinsic Al incorporation. The unintentional impurity incorporation can induce material deteriorations. We thus suggest that the growth should be carried out with the decreased Al cell temperature and the N gas flow rate as low as possible. (C) 2008 Elsevier B.V. All rights reserved.