Growth of conductive Si-doped α-Ga₂O₃ by suboxide molecular-beam epitaxy

We report a two-step film-growth process using suboxide molecular-beam epitaxy (S-MBE) that produces Si-doped α-Ga₂O₃ with record transport properties. The method involves growing a relaxed α-(Al_xGa_1−x)₂O₃ buffer layer on m-plane sapphire at a relatively high substrate temperature (T_sub), ∼750 °C, followed by an Si-doped α-Ga₂O₃ overlayer grown at lower T_sub, ∼500 °C. The high T_sub allows the ∼3.6% lattice-mismatched α-(Al_xGa_1−x)₂O₃ buffer with x = 0.08 ± 0.02 to remain epitaxial and phase pure during relaxation to form a pseudosubstrate for the overgrowth of α-Ga₂O₃. The optimal conditions for the subsequent growth of Si-doped α-Ga₂O₃ by S-MBE are 425 °C ≤ T_sub ≤ 500 °C and P_{80% O3} = 5 × 10⁻⁶ Torr. Si-doped α-Ga₂O₃ films grown with this method at T_sub > 550 °C are always insulating. Secondary-ion mass spectrometry confirms that both the insulating and conductive films have uniform silicon incorporation. In conductive films with 10¹⁹ ≤ N_Si ≤ 10²⁰ cm⁻³, the incorporated silicon is ∼100% electrically active. At N_Si ≤ 10¹⁹ cm⁻³, the carrier concentration (n) plummets. A maximum Hall mobility (μ) = 90 cm 2 V·s at room-temperature is measured in a film with n = 2.9 × 10¹⁹ cm⁻³ and a maximum conductivity (σ) = 650 S/cm at room-temperature in a film with n = 4.8 × 10¹⁹ cm⁻³. A threading dislocation density of (5.6 ± 0.6) × 10¹⁰ cm⁻² is revealed by scanning transmission electron microscopy, showing that there is still enormous room to improve the electrical properties of doped α-Ga₂O₃ thin films.