Atomic layer deposited high-κ nanolaminates for silicon surface passivation

Abstract

Nanolaminates comprising of TiO2 or HfO2 sublayers within an Al2O3 matrix are grown with atomic layer deposition. These nanolaminates provide an improved silicon surface passivation compared to conventional Al2O3 films. The physical properties of the nanolaminates can be described with a dynamic growth model that considers initial and steady-state growth rates for the involved metal oxides. This model links the cycle ratios of the different atomic layer deposition precursors to the thickness and the material concentrations of the nanolaminate, which are determined by means of spectroscopic ellipsometry. Effective carrier lifetime measurements show that Al2O3-TiO2 nanolaminates achieve values of up to 6.0 ms at a TiO2 concentration of 0.2%. In Al2O3-HfO2 nanolaminates, a maximum effective carrier lifetime of 5.5 ms is reached at 7% HfO2. Electrical measurements show that the TiO2 incorporation causes strong hysteresis effects, which are linked to the trapping of negative charges and result in an enhanced field effect passivation. For the Al2O3-HfO2 nanolaminates, the capacitance data clearly show a very low density of interface traps (below 5·1010 eV−1·cm−2) and a reduction of the fixed charge density with increasing HfO2 concentration. Due to the low number of recombination centers near the surface, the reduced field effect passivation only had a minor impact on the effective carrier lifetime.

Symmetrical Al2O3-based passivation layers for p- and n-type silicon

“…On both p-and n-type substrates, similar Q fix values were obtained. HfO 2 is reported to have a very low density of positive fixed charges in a low range of 10 11 cm À 2 [14].…”

2D Mapping of Chemical and Field Effect Passivation of Al2O3 on Silicon Substrates

“…The different passivation stacks could hardly be detected in the lifetime map. Surface passivation with pure HfO 2 usually does not reach the performance of Al 2 O 3 [12,21]. However, an 1 nm thick HfO 2 interface layer underneath Al 2 O 3 is known to have little impact on the measured lifetime in high injection range [13].…”

“…After a megasonic clean, 20 nm thick symmetrical Al 2 O 3 passivation layers were deposited by thermal atomic layer deposition (ALD) at 150 °C. All samples were subsequently annealed in forming gas atmosphere (10 % H 2 , 90 % N 2 ) at 350 °C [12]. Aluminium contacts were deposited in a thermal evaporator system to prepare MIS structures.…”

Status of Al2O3/TiO2‐Based Antireflection and Surface Passivation for Silicon Solar Cells

“…The refractive index of laminated coatings consisting of alternating stacks of nanoscale Al 2 O 3 and TiO 2 sublayers grown by ALD can be controlled. [30,38,[50][51][52][53][54][55][56] If the refractive index of film materials is easy to adapt for optical coatings, it enables the flexible design of multilayer optical applications. The structure appears to be optically continuous when the layer thickness is much less than the wavelength of interest.…”

“…[23] Over the years, various material stacks with fixed charges were investigated for surface passivation of solar cells. [24][25][26][27][28][29][30][31][32] Passivation schemes using Al 2 O 3 /TiO 2 stacks with outstanding performance have been developed. This Review discusses the progress in the development and understanding of the properties of Al 2 O 3 /TiO 2 -based surface passivation schemes over the last few years.…”