Atomic layer deposition of HfO2 as a charge-lean capping layer material for SiO2-modulation acceptor doping of silicon

Modulation doping of SiO2 by Al-induced acceptor states is a promising alternative to conventional impurity doping for silicon nanostructures, enabling the introduction of free holes in Si without direct impurity incorporation into the lattice. SiO2 modulation doping of Si is achieved by a short high-temperature anneal of a tunnel-SiO2 layer coated with an AlOx monolayer by atomic layer deposition (ALD). However, this ultra-thin modulation doping (MD) stack is highly susceptible to degradation when exposed to ambient air. In this work, we investigated ALD hafnium oxide (HfO2) as a reliable, charge-lean capping layer to protect the MD stack and preserve its doping properties. We optimized the ALD-HfO2 deposition process using tetrakis(ethylmethylamino)hafnium (TEMAHf) and various oxygen co-reactants (H2O, O3, O2-plasma) as well as different deposition temperatures and studied the effects of post-deposition RTA. Thermal ALD with H2O as the oxygen reactant at 200 °C yields HfO2 films with superior electrical properties, including low positive fixed charge densities (<1 × 1012 cm−2), minimal hysteresis, and high permittivity (ɛ = 17). The optimized HfO2 film was successfully integrated into modulation-doped metal–oxide–semiconductor capacitors (MD MOS-caps) to evaluate their effectiveness as capping layers. Notably, in situ capping, where the HfO2 layer is deposited immediately after the AlOX without breaking vacuum, results in higher modulation doping efficiency and demonstrated higher negative fixed charge densities compared to ex situ capping. These findings demonstrate that optimized ALD-HfO2 can serve as a charge-lean capping material, enhancing the stability and performance of modulation-doped Si nanostructures by effectively protecting the ultra-thin MD stack from ambient degradation.