Surface Passivation Technique for Ge_1–<i>x</i>Sn_<i>x</i>-Based Diodes by GeO₂ Atomic Layer Deposition with Postoxidation Annealing

Ge1–xSnx/Ge1–x–ySixSny heterostructures enable monolithic integration of photonics and electronics on Si ultralarge-scale integration. The development of the Ge1–xSnx/Ge1–x–ySixSny resonant tunneling diode (RTD) composed of group-IV semiconductors is crucial for their use as oscillators in terahertz (THz) communication systems. Surface passivation techniques are crucial in enhancing the performance of these devices. This study investigates the atomic layer deposition (ALD) of GeO2 combined with postoxidation annealing (POA) for the surface passivation of Ge-based Group IV alloy semiconductor devices. Initially, we developed an ALD process for the deposition of GeO2 using tetraethoxygermanium and ozone as precursors. We investigated the GeO2 ALD condition, resulting in a growth rate of 0.35 Å/cycle at 200 °C. Subsequently, we demonstrated the effectiveness of the ALD-GeO2 passivation in conjunction with POA by fabricating mesa-structured Ge0.948Sn0.052/Ge p–n diodes. The incorporation of an ALD-GeO2 interlayer at the insulator/semiconductor interface and POA proved to be effective in suppressing the surface leakage current in Ge1–xSnx-based diodes. Furthermore, we fabricated Al/ALD-GeO2/p-Ge and Al/ALD-Al2O3/p-Ge metal-oxide-semiconductor capacitors (MOSCAPs) to clarify the mechanism of the reverse current reduction by introducing an ALD-GeO2 layer and POA. ALD-GeO2 MOSCAPs demonstrated a lower interface state density compared with ALD-Al2O3 MOSCAPs, along with a negative bias shift in the flat-band voltage by POA. The suppression of surface leakage current through the combined use of ALD-GeO2 and POA can be attributed to both chemical and field-effect passivation concepts. An ALD-GeO2 layer with POA is poised to become a key passivation technique for both Ge1–xSnx/Ge1–x–ySixSny p-RTD and versatile Ge-based Group IV alloy semiconductor devices.