High-Quality Silicon Surface Passivation by Thermal-ALD Deposited Hafnium Oxide Films

Excellent silicon surface passivation is achieved by atomic layer deposition (ALD) grown hafnium oxide (HfO _x ) films on silicon surfaces (both n-type and p-type). It is inferred from the study that the silicon surface passivation by HfO _x thin films is a film-thickness-dependent quality and a minimum film thickness of ∼2 nm is essential to passivate the silicon surface. A good level of surface passivation (surface recombination velocity, SRV< 20 cm/s) can be achieved for film thickness, d >5 nm. However, the best results are obtained for hydrogen-annealed, ∼8.5-nm-thin HfO _x films. SRV as low as 3.5 cm/s (effective minority carrier lifetime, τ _eff ∼5 ms) and 4.4 cm/s (τ _eff ∼4 ms) are realized on p-type and n-type silicon surfaces, respectively. The injection level dependence of τ _eff reveals that HfO _x films provide better passivation for n-type silicon compared to p-type silicon at low injection levels. Hydrogen present in the annealing ambient first saturates the dangling bonds at the film/silicon interface and then affects the oxide charge density. Effective oxide charge density is positive in HfO _x /n-Si samples and negative in HfO _x /p-Si samples. This creates an accumulation condition near the silicon surface for both substrate-type situations and is responsible for effective passivation in both types of substrates. Thus, our study demonstrates that ALD-grown HfO _x films offer excellent passivation for silicon surfaces.