Influence of the SiO2 interlayer thickness on the density and polarity of charges in Si/SiO2/Al2O3 stacks as studied by optical second-harmonic generation

Abstract

By accurately tuning the SiO2 interlayer thickness the density and polarity of charges in Si/SiO2/Al2O3 stacks can be controlled. We report on the number density, polarity, and physical location of charges present in the stacks as studied by optical second-harmonic generation (SHG). Depending on the SiO2 interlayer thickness (1–150 nm) the effective charge density in the Si/SiO2/Al2O3 stacks ranges from 1013 to 1011 cm−2 for both n- and p-type silicon. The polarity of the charges switches from negative to positive around a SiO2 interlayer thickness of 5–10 nm at which point the effective charge density in the stacks is negligible. This switch in polarity is apparent from spectroscopic, time-dependent, and azimuthal SHG measurements. The observed trends in charge density and polarity can be explained by tunneling of electrons into defect states at the SiO2/Al2O3 interface as well as the presence of fixed and bulk charges at the Si/SiO2 interface and in the SiO2, respectively. This charge mechanism appears to hold generally for Si/SiO2/Al2O3 stacks as similar results were observed for SiO2 films prepared by various techniques.

“Zero-charge” SiO2/Al2O3 stacks for the simultaneous passivation of n+ and p+ doped silicon surfaces by atomic layer deposition

“…Next, we will investigate the possibility to optimize this passivation by precisely tuning the ALD SiO 2 interlayer thickness of the SiO 2 /Al 2 O 3 stack between 0 and 14.4 nm. It has been hypothesized, that an ALD SiO 2 interlayer of 2-4 nm thickness prevents the charge-injection from the Si into trap sites in the Al 2 O 3 layers [28]. Therefore, the negative Q f of SiO 2 /Al 2 O 3 stacks rapidly decreases towards virtually zero for these SiO 2 thicknesses.…”

“…It should be noted that also some positive charge formation in the SiO 2 film can occur (this is a "bulk" charge, instead of "interface charge" as is the case fore Al 2 O 3 films on Si). However, this charge density is relatively low ( $ 10 11 cm À 2 ) for ALD SiO 2 films and is therefore only partly contributing to the nullification of Q f the SiO 2 /Al 2 O 3 stacks [28]. Yet, due to these positive bulk charges in of the SiO 2 film, the overall charge polarity of the SiO 2 /Al 2 O 3 stacks becomes slightly positive for increasing ALD SiO 2 thicknesses (i.e., Q f changes from À 5 Á 10 12 cm À 2 for Al 2 O 3 only to Q f ¼(4 72) Â 10 11 cm À 2 for SiO 2 /Al 2 O 3 stacks with 12-16 nm thick SiO 2 interlayer [28]).…”

“…It has been reported by Dingemans et al [13] and Terlinden et al [28] that those stacks exhibit excellent chemical passivation levels (e.g., D it values o 10 11 eV À 1 cm À 2 at mid gap [32]), while their Q f can effectively be tuned from strongly negative up to zero or even positive values by carefully tuning the ALD SiO 2 thickness. It is hypothesized that for these stacks the SiO 2 interlayer acts as a (trap-assisted) electron tunnel barrier, preventing charge-injection from the c-Si base into the electron trap sites in the Al 2 O 3 [13,28]. The Al 2 O 3 plays a key role in the reduction of Si/SiO 2 interface defect states, which are effectively passivated by the diffusion of hydrogen from the Al 2 O 3 film to the Si/SiO 2 interface during annealing [33].…”

“…The Al 2 O 3 plays a key role in the reduction of Si/SiO 2 interface defect states, which are effectively passivated by the diffusion of hydrogen from the Al 2 O 3 film to the Si/SiO 2 interface during annealing [33]. ALD SiO 2 /Al 2 O 3 stacks have the advantage that they can be prepared at relatively low temperatures (e.g., 200°C), are conformal even over high-aspect ratio structures and can be prepared in a single deposition run [13,28,34]. The ALD stacks can potentially be used in high-volume manufacturing for the simultaneous passivation of both sides of IBC cells or bifacial cells.…”

“…In fact, various interlayers can be used to reduce the Q f of Al 2 O 3 layers to virtually zero. This includes SiO 2 layers prepared by ALD [13,28] and by plasmaenhanced chemical vapor deposition (PE-CVD) [20], thermallygrown SiO 2 [29], but also other materials such as HfO 2 [30]. Moreover, when SiO 2 is combined with other capping layers, such as in SiO 2 /SiN x or SiO 2 /Al 2 O 3 /SiN x stacks, very low charge densities can be obtained [7,29,31], although this strongly depends on the SiO 2 thickness and process conditions [7,29].…”