Improving ALD-Al₂O₃ Surface Passivation of Si Utilizing Pre-Existing SiO_x

Abstract

Al 2 O 3 has rapidly become the surface passivation material of choice for p+ layers of solar cells because of its high negative fixed charge, good long-term and thermal stability, and no parasitic absorption. In this article, the surface saturation current density, fixed charge, and interface state density are compared for Al 2 O 3 deposited on Si substrates where the pre-existing out-of-thebox SiO x layer was not removed, with substrates where the SiO x was removed by hydrofluoric acid. The depositions are performed by atomic layer deposition at temperatures in the 150-300 °C range, using trimethylaluminium, H 2 O, and O 3 as precursors. The samples where the native oxide was not removed achieve a higher level of surface passivation for every tested deposition temperature, with the sample deposited at 200 °C exhibiting a surface saturation current density of only 0.9 fA/cm 2 after annealing, a fixed charge of −4.2 × 10 12 cm −2 , and a density of interface states of 9.8 × 10 9 cm −2 eV −1 . Capacitance and conductance voltage characteristics reveal a strong correlation between the surface saturation current density and the density of interface states and fixed charges. It is also determined that the long-term stability of the surface passivation depends on the deposition temperature, with higher deposition temperatures resulting in improved long-term stability. The results indicate that H-terminated Si prior to Al 2 O 3 deposition may have a detrimental effect on the surface passivation.

A Low‐Temperature Batch Process for the Deposition of High‐Quality Conformal Alumina Thin Films for Electronic Applications

“…This indicates the presence of fixed positive charges (ions) in the oxide, which appear to decrease with increasing deposition temperature (increase in ). For the 125 °C deposition, the flat band voltage extracted from this half cycle is −0.96 V, approaching the calculated value of −0.44 V. With the 150 °C deposition, increases above the calculated value to −0.02 V. Typically, ALD‐alumina layers deposited at >150 °C are associated with the presence of fixed negative charges, [ 29,30 ] as observed for the 150 °C deposition with . The positive‐to‐negative sweep results in which is unstable under repeated voltage cycling (shaded area of Figure 6a).…”

“…The degree to which these traps are reduced under negative bias, and consequently the stability of the dielectric under initial bias stress, also appears to be dependent on deposition temperature, with little change seen between sweeps 1 and 3 for the 150 °C deposition (1.49 × 10 12 to 1.41 × 10 12 cm −2 ). In general, the density of trapped electrons is typical of previous studies of the Si–SiO x –alumina interface, [ 30,33,34 ] despite the lower temperature for the MVD process compared with conventional ALD.…”

“…With the 150 °C deposition, V fb increases above the calculated value to À0.02 V. Typically, ALD-alumina layers deposited at >150 °C are associated with the presence of fixed negative charges, [29,30] as observed for the 150 °C deposition with qV fb > Φ MS . The positive-to-negative sweep results in V fb > 0V, which is unstable under repeated voltage cycling (shaded area of Figure 6a).…”

“…This could be explained by the thin native SiO x layer governing the near‐interface trap states. [ 30,34 ] The incomplete adsorption of TMA molecules during the initial growth cycles has been used to explain high negative charge density near the interface. [ 33 ] For lower deposition temperatures, where the incomplete reaction of TMA molecules may be expected in the bulk of the films, a higher density of positive fixed charge is observed (negative shift in ), but also correlated with an increase in electron traps (increased hysteresis).…”

“…However, this difference in temperature dependence will be present in any system utilizing a carrier gas to introduce precursor to the chamber, indicating that a flow‐type system capable of producing high‐quality films in a higher temperature regime may have very different reaction kinetics when the temperature is reduced. [ 30,31,35,36 ] In summary, this simple model shows that pump‐type reactor designs have inherent advantages in terms of reaction kinetics at low deposition temperatures, which in turn influence deposition uniformity and conformality.…”