PO_x/Al₂O₃ Stacks for c-Si Surface Passivation: Material and Interface Properties

Abstract

Phosphorus oxide (PO x ) capped by aluminum oxide (Al2O3) has recently been discovered to provide excellent surface passivation of crystalline silicon (c-Si). In this work, insights into the passivation mechanism of PO x /Al2O3 stacks are gained through a systematic study of the influence of deposition temperature (T dep = 100–300 °C) and annealing temperature (T ann = 200–500 °C) on the material and interface properties. It is found that employing lower deposition temperatures enables an improved passivation quality after annealing. Bulk composition, density, and optical properties vary only slightly with deposition temperature, but bonding configurations are found to be sensitive to temperature and correlated with the interface defect density (D it), which is reduced at lower deposition temperature. The fixed charge density (Q f) is in the range of + (3–9) × 1012 cm–2 and is not significantly altered by annealing, which indicates that the positively charged entities are generated during deposition. In contrast, D it decreases by 3 orders of magnitude (∼1013 to ∼1010 eV–1 cm–2) upon annealing. This excellent chemical passivation is found to be related to surface passivation provided by hydrogen, and mixing of aluminum into the PO x layer, leading to the formation of AlPO4 upon annealing.

POx/Al2O3 stacks for surface passivation of Si and InP

“…The origin of the low D it enabled by PO x /Al 2 O 3 on silicon is found to be related to surface passivation provided by hydrogen and the formation of AlPO 4 upon annealing, leading to a decrease in D it by almost orders of magnitude from ~10 13 to ~10 10 eV − 1 cm − 2 [47]. On silicon a 100) wafers (data points with solid lines) and wurtzite InP nanowires (data points with dashed lines), as a function of annealing temperature (10 min in N 2 ), together with unpassivated semiconductor surfaces as reference.…”

“…Data on InP nanowires were adapted from Black et al [49] Note that the reported carrier lifetimes on Si were determined using photoconductance decay, while the carrier lifetimes on InP NWs were determined using time-resolved photoluminescence. high positive fixed charge of around 10 12 -10 13 cm − 2 is present, both after deposition and after annealing [47]. On InP NWs, PO x /Al 2 O 3 stacks induce an n-type field-effect, which indicates the presence of positive fixed charges [95].…”

“…The high positive fixed charge density is already present in the as-deposited state. Upon annealing, AlPO 4 is formed mainly in the PO x layer (see section 4.2), but the positive fixed charge density does not change [47]. This suggests that the mixing of the Al into the PO x layer does not significantly affect the fixed charge, possibly because the fixed charges are situated near the silicon interface.…”

“…Initial works employed plasma-enhanced atomic layer deposition (PE-ALD) to deposit the PO x /Al 2 O 3 stacks [45,46]. In later works, the PO x layer was deposited by a pulsed-flow plasma-enhanced chemical vapor deposition (PF-PECVD) process instead [47,48,62]. The details of these deposition processes will be discussed in section 3.1.…”

“…Recently, a novel stack of phosphorus oxide and aluminum oxide (PO x /Al 2 O 3 ) has been shown to provide excellent surface passivation of both Si [45][46][47][48] and InP [49] surfaces. On silicon, the PO x /Al 2 O 3 stacks have been shown to achieve a very low D it on the order of 10 10 eV − 1 cm − 2 and a high positive fixed charge density (Q f ) on the order of 10 12 -10 13 cm − 2 .…”