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We describe experiments on Si doping in Al x Ga 1−x N grown by gas source molecular beam epitaxy with ammonia and silane. Growth conditions that minimize self-compensation were used to assure Si incorporation at a level of 2 ϫ 10 20 cm −3 for the entire range of compositions investigated, from x = 0.56 to 1.0. These conditions resulted in donor concentrations of ϳ1 ϫ 10 19 cm −3 up to x = 0.85. Layers of Al x Ga 1−x N up to x = 0.85 show good mobility and low resistivity. In these layers, the activation energy, E a , of Si stays below ϳ25 meV and Si can be considered a shallow donor. For AlN content above x = 0.85 the donor activation energy increases to E a ϳ 250 meV in AlN. The change in donor activation energy correlates with increased incorporation of oxygen and carbon.

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