ALD-deposited SiO2 layers on p-type germanium substrates: material characterization and surface passivation properties

Dielectric surface passivation of crystalline germanium (c-Ge) substrates is of paramount importance for the fabrication of high quality devices. Aluminum oxide (Al2O3) deposited by atomic layer deposition (ALD) has demonstrated excellent passivating properties. As an alternative, silicon oxide (SiO2) layers deposited by the same technique have been investigated for the passivation of low doped n-type c-Ge. In this work, we extend the application of ALD-deposited SiO2 layers for the passivation of 1.2 Ω·cm (NA= 2·1015 cm-3) p-type c-Ge substrates. Firstly, we characterize the SiO2 layers leading to stoichiometric layers with less defects when they are deposited at 200 ºC. Next, we determine effective surface recombination velocity (Seff) on c-Ge of 40 nm SiO2 layers which decreases with post-deposition annealings (PDA) up to 51 cm/s, similar to the passivation quality obtained with Al2O3 layers. Interestingly, the best result is obtained after a 600 ºC annealing demonstrating that SiO2 has a better thermal stability than its Al2O3 counterparts which degrades beyond 450 ºC. Interface state density (Dit) and fixed charge (Qf) measured by capacitance-voltage characterization suggest that the improvement with PDA is due to a decrease in Dit while Qf is maintained constant and positive. Additionally, XPS measurements at the interface indicates the creation of GeOx compound at the SiO2/c-Ge which could be the reason behind the better thermal stability.