High surface passivation quality and thermal stability of ALD Al2O3 on wet chemical grown ultra-thin SiO2 on silicon

In this work we present the surface passivation properties of pre-oxidized Si(100) surfaces compared to H-terminated Si(100) after atomic layer deposition (ALD) of Al2O3 thin films. Additionally the differences in surface passivation for n- and p-type silicon were investigated. The pre-oxidation was carried out in three different wet chemical solutions: (a) nitric acid (HNO3), (b) hydrochloric acid mixed with hydrogen peroxide (HCl/H2O2) and (c) a sulphuric acid with hydrogen peroxide (H2SO4/H2O2). The surface passivation quality was determined directly after deposition, after anneal at 400 °C in N2 atmosphere for 10 min and after direct firing at 850 °C in ambient for several seconds. Directly after deposition we find significantly higher passivation quality for the Al2O3 passivated samples with pre-oxidized surfaces than for H-terminated surfaces. After anneal and after directly firing the passivation quality for Al2O3 on H-terminated surfaces is slightly better, nevertheless the surface recombination velocity for Al2O3 on pre-oxidized silicon surfaces is still <15 cm/s for n-type Fz silicon and <70 cm/s for p-type Cz. A high quality surface passivation can therefore be achieved for silicon pre-oxidized with wet chemistries. The field-effect passivation for HF-last ALD Al2O3 exceeds the level of pre-oxidized samples after anneal as well as after direct firing.