Structural and electrical properties of undoped and doped α‐Ga2O3 thin films grown by pulsed laser deposition on m‐plane sapphire in a two‐step process are presented. A buffer layer of undoped α‐Ga2O3 is introduced below the electrically active thin film to improve the crystal quality and enable the stabilization of the α‐phase at lower substrate temperatures for sufficient dopant incorporation. Donor doping of the active layers with tin, germanium, and silicon, respectively, is realized below a critical substrate temperature of 600 °C. Depth‐resolved X‐ray photoelectron spectroscopy measurements on tin‐doped samples reveal a lower amount of tin in the bulk thin film compared to the surface and a lower tin incorporation for higher substrate temperatures, indicating desorption or float‐up processes that determine the dopant incorporation. Electron mobilities as high as 17 cm2 V−1 s−1 (at 1.4 × 10 19 cm − 3 ) and 37 cm2 V−1 s−1 (at 3.7 × 10 18 cm − 3 ) are achieved for tin‐ and germanium doping, respectively. Further, a narrow window of suitable annealing temperature from 680 to 700 K for obtaining ohmic Ti/Al/Au layer stacks is identified. For higher annealing temperatures, a deterioration of the electrical properties of the thin films is observed suggesting the need for developing low temperature contacting procedures for α‐Ga2O3‐based devices.
Phase-pure α-Ga2O3 thin films with high surface quality and crystallinity have been grown on m-plane sapphire using pulsed laser deposition (PLD). Therefore, the influence of growth temperature, oxygen background pressure, and film thickness on the structural properties is investigated to determine the growth window for phase-pure corundum structured α-Ga2O3. Samples were analyzed using x-ray diffraction (XRD), atomic force microscopy (AFM), and spectroscopic ellipsometry measurements. A distinct growth window in favor of phase-pure (10.0)-oriented α-Ga2O3 for growth temperatures above 480 °C and low oxygen partial pressures p(O2) of 3 × 10−4 mbar is identified. The growth rate increases significantly with increasing oxygen pressure. Furthermore, it shows an Arrhenius-like decrease for lower temperatures, caused by the increasing desorption of volatile Ga2O suboxides. It was found that for thicker layers, the growth of monoclinic β-Ga2O3 is promoted likely facilitated by the c-facets of the α-Ga2O3 grains. This leads to a (010)-oriented island growth corroborated by stripe-like features in AFM scans and a corresponding in-plane orientation confirmed by XRD ϕ-scans. For oxygen partial pressures above 3 × 10−4 mbar and Tg < 540 °C, the formation of mixed (10.0) α-Ga2O3 and spinel-defective (110)-γ-Ga2O3 manifesting as defective inclusions was observed independent of the layer thickness d. A corresponding p(O2) − d − T phase diagram for the growth of Ga2O3 on m-plane sapphire by PLD is provided.
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