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

Abstract

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−xN 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 × 10¹⁷ to 3 × 10¹⁹ atoms/cm³ is obtained, with sharp dopant density transition profiles. In Si-doped Al_0.6Ga_0.4N epilayers, a room-temperature free electron concentration of ∼3 × 10¹⁹/cm³, an electron mobility of ∼27 cm²/V s, and an n-type resistivity of ∼7.5 m $\Omega \hspace{0.17em}\mathrm{cm}$ are obtained. The implications of these findings on electronic and photonic devices on single-crystal AlN substrates are discussed.