As a contribution to (transparent) bipolar oxide electronics, vertical pn heterojunction diodes were prepared by plasma-assisted molecular beam epitaxy of unintentionally doped p-type SnO layers with hole concentrations ranging from p=1018 to 1019 cm−3 on unintentionally doped n-type β-Ga2O3(−201) substrates with an electron concentration of n=2.0×1017 cm−3. The SnO layers consist of (001)-oriented grains without in-plane epitaxial relation to the substrate. After subsequent contact processing and mesa-etching (which drastically reduced the reverse current spreading in the SnO layer and associated high leakage), electrical characterization by current–voltage and capacitance–voltage measurement was performed. The results reveal a type-I band alignment and junction transport by thermionic emission in forward bias. A rectification of 2×108 at ±1 V, an ideality factor of 1.16, a differential specific on-resistance of 3.9 m Ω cm2, and a built-in voltage of 0.96 V were determined. The pn-junction isolation prevented parallel conduction in the highly conductive Ga2O3 substrate during van-der-Pauw Hall measurements of the SnO layer on top, highlighting the potential for decoupling the p-type functionality in lateral transport devices from that of the underlying n-type substrate. The measured maximum reverse breakdown voltage of the diodes of 66 V corresponds to a peak breakdown field of 2.2 MV/cm in the Ga2O3-depletion region and suggests the low bandgap of the SnO (≈0.7 eV) not to be the limiting factor for breakdown. Higher breakdown voltages that are required in high-voltage devices could be achieved by reducing the donor concentration in the β-Ga2O3 toward the interface to increase the depletion width, as well as improving the contact geometry to reduce field crowding.
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