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In the case of molecular beam epitaxy (MBE), the Mg acceptors are electrically active in the as‐grown material and a priori no additional annealing procedure is necessary. However, there are still some peculiarities in the electrical properties of ammonia‐process grown GaN:Mg and some annealing effect can be observed. Additionally, the character of weak temperature dependence in the vicinity of room temperature suggests that to describe the conduction process an additional conduction channel not related to the free carriers in the valence band must be taken into account. For these reasons, this article presents the results of low‐temperature resistivity and Hall Effect studies of Mg‐doped, ammonia‐process‐grown GaN. The studied samples are grown on low‐temperature buffers of GaN deposited on a sapphire substrate. High‐temperature annealing process (≈800 K) is carried out for all of them. The temperature dependences of the electrical transport properties before and after the annealing procedure are especially investigated at temperatures ranging from 10 up to 300 K. It is found that the low temperatures transport properties are sensitive to the annealing procedure and to describe the observed effects the hopping phenomena must be taken into account. [ABSTRACT FROM AUTHOR]

The polarized photoluminescence emitted on the edge of a series of aluminum‐rich (Al,Ga)N‐AlN quantum wells (QWs) grown by molecular beam epitaxy on AlN templates deposited by metal organic chemical vapor deposition on c‐plane sapphire is measured. The contrast and the principal axis of the emission diagrams for 2 nm‐thick (Al,Ga)N QWs grown for aluminum compositions between 40% and 90% are studied. The light is emitted on the edge of the QWs at wavelengths going from 280 nm down to 209 nm. The emission diagram, a change from oblate to prolate with respect to the in‐plane orientation, for an aluminum composition is found to occur around 72%, that is, at an emission wavelength of about 235 nm. The orientations and shapes of the edge‐emission diagrams indicate that the fluctuations of the composition of the (Al,Ga)N confining layer are deep enough for producing intravalence band mixings. This property, that acts in concert with the built‐in strain and quantum‐confined Stark effect, contributes to the anisotropy of the light emission when the aluminum composition reaches 60–70%, that is, for an emission wavelength of 260–235 nm. [ABSTRACT FROM AUTHOR]