Magnesium (Mg) has been studied as a shallow acceptor in III-V semiconductors since Cho et al. first reported that it could be used as a p-type dopant for GaAs grown by molecular beam epitaxy (MBE) [1,2]. In 1982, Wood et al. studied that the dependence of Mg incorporation into GaAs as a function of the substrate temperature (Ts) and concluded that Mg incorporation coefficient (Ki) approached to unity below Ts of 500 ◦C, and got reduced at high Ts of 600 ◦C by three orders of magnitude to 0.3 × 10−3 [3]. From their results, the free hole concentration reduced from ∼10 to ∼10 cm−3 range. These substrate temperature dependent characteristics restrict the use of Mg. However, currently, GaAs molecular beam epitaxy (MBE) layers grown at low temperatures (LT) (200 ∼ 250 ◦C) are attracting much interest due to the minimizing of several common device problems such as backgating and sidegating to realize high-