MBE growth and donor doping of coherent ultrawide bandgap AlGaN alloy layers on single-crystal AlN substrates

摘要

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 Al_xGa_(1-x)N epitaxial layers with x ～0.6-1.0 Al contents at a growth rate of ～0.3 μm/h. These epitaxial layers exhibit atomic steps, indicating step flow epitaxial growth, and room-temperature band edge emission from ～4.5 to 5.9 eV. 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 ～2 × 10~(17) to 3 × 10~(19) atoms/cm~3 is obtained, with sharp dopant density transition profiles. In Si-doped Al_(0.6)Ga_(0.4)N epilayers, a room-temperature free electron concentration of ～3 × 10~(19)/cm~3, an electron mobility of ～27 cm~2/Vs, and an n-type resistivity of ～7.5 m Ω cm are obtained. The implications of these findings on electronic and photonic devices on single-crystal AlN substrates are discussed.