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Recent progresses in epitaxial growth and fundamental studies on electrical and optical properties of high Al-content
AlGaN alloys with Si, Mg, and Zn doping are presented. For Si doped AlxGa1-xN, the Si activation energy was
determined for x = 0 up to 1, and the resistivity of n-AlxGa1-xN was found to increases by one order of magnitude when
Al content is increased by ~ 8%. From photoluminescence (PL) studies, three groups of deep impurity transitions were
observed, related with deep level acceptors involving cation vacancy and its complexes: (VIII)3-, (VIII-complex)2, and
(VIII-complex)1-, which are electron traps and compensating centers. By optimizing the growth processes to reduce the
densities of cation vacancy and its complexes, the n-type conductivity of AlxGa1-xN was significantly improved. A
record low room temperature n-type resistivity of 0.0075 Ω•cm has been obtained for Al0.7Ga0.3N, and n-type
conduction in pure AlN has also been achieved. We also review the electrical and optical measurement results of Mgdoped
AlGaN and AlN. It was found that the overall material quality and conductivity of Mg-doped AlN are strongly
correlated with the PL emission intensity of the nitrogen vacancy (VN
3+) related transition. Improved conductivity was
obtained by suppressing the VN3+ related emission line, which was attributed to the reduced hole compensation by VN3+.
With the identification of the emission peaks associated with VN3+ hole compensating centers, the p-type conductivity of high Al-content AlGaN alloys was improved by monitoring and suppressing the intensity of the VN3+ related emission
lines. P-type conduction in AlxGa1-xN (x > 0.7) was confirmed at elevated temperatures (> 700 K). The possibility of
using Zn as an alternative p-type dopant was also studied. It was found that contrary to the calculation, the energy level
of Zn acceptor in AlN was about 0.74 eV, which is 0.23 eV deeper than Mg level in AlN.
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