Mbe Growth And Donor Doping Of Coherent Ultrawide Bandgap Algan Alloy Layers On Single-Crystal Aln Substrates

Single-crystal Aluminum Nitride (AlN) crystals enable the epitaxial growth of ultrawide bandgap Al(Ga)N alloys with drastically lower extended defect densities. Here, we report the plasma-MBE growth conditions for high Al-composition AlGaN alloys on single-crystal AlN substrates. An AlGaN growth guideline map is developed, leading to pseudomorphic AlxGa1-xN epitaxial layers with x similar to 0.6-1.0 Al contents at a growth rate of similar to 0.3 mu m/h. These epitaxial layers exhibit atomic steps, indicating step flow epitaxial growth, and room-temperature band edge emission from similar to 4.5 to 5.9eV. Growth conditions are identified in which the background impurity concentrations of O, C, Si, and H in the MBE layers are found to be very near or below detection limits. An interesting Si segregation and gettering behavior is observed at the epitaxial AlGaN/AlN heterojunction with significant implications for the formation and transport of 2D electron or hole gases. Well-controlled intentional Si doping ranging from similar to 2 x 10(17) to 3 x 10(19) atoms/cm(3) is obtained, with sharp dopant density transition profiles. In Si-doped Al0.6Ga0.4N epilayers, a room-temperature free electron concentration of similar to 3 x 10(19)/cm(3), an electron mobility of similar to 27cm(2)/Vs, and an n-type resistivity of similar to 7.5 mOmegacm are obtained. The implications of these findings on electronic and photonic devices on single-crystal AlN substrates are discussed.