Hydrogen passivation of silicon/silicon oxide interface by atomic layer deposited hafnium oxide and impact of silicon oxide underlayer

HfO₂ synthesized by atomic layer deposition (ALD) can be used as a passivation material for photodetectors. This paper shows a significant reduction of density of interface traps at the Si/SiO₂ interface using ALD HfO₂. This is explained by a chemical passivation effect due to presence of hydrogen from water used in the ALD process. Furthermore, ALD HfO₂ layers appear negatively charged which generate an additional field effect passivation. The impact of the SiO₂ underlayer is also discussed by comparing a chemical silicon oxide to a standard thermal silicon oxide. It is shown that chemical silicon oxide can act as a reservoir of hydrogen atoms which helps to reduce the density of defects close to the Si/SiO₂ interface. This result demonstrates the importance of the surface preparation before the ALD of HfO₂ in the passivation scheme. Finally, this work shows the correlation between negatively charged defects and Si–O–Hf bonds at the SiO₂/HfO₂ interface. A passivation stack composed of chemical oxide permits to reach both a low density of interface traps (∼1.0 × 10¹¹ cm⁻² eV⁻¹) and a negative charge density (∼−1.0 × 10¹¹ cm⁻²). This stack provides both chemical and field effect passivation of the silicon surface.