(poster) Electric Field Distribution in Black Silicon Needles Under Reverse Bias: A TCAD Simulation Study

Black silicon (b-Si) – a nanotextured silicon surface in the dimensions below the wavelength
of incident light – is an attractive absorption-enhancing surface treatment for high-field
silicon photodetectors such as avalanche photodiodes (APDs). However, the behavior of its
high-aspect-ratio nanoscale needles under high reverse bias remains poorly understood.
While earlier b-Si simulations have primarily focused on equilibrium field-effect passivation
[1], [2] – how fixed charges in Al2O3 repel carriers from needle surfaces – the redistribution
of electric field under applied reverse bias and its implications for leakage, trap-assisted
generation, and breakdown have not been systematically studied. We present a simulation
study examining how b-Si needle geometry governs electric field enhancement and
localization under high reverse bias.
TCAD simulations with b-Si geometries (width, height, pitch) extracted from SEM images
(Fig. 1a) are used to study how reverse bias redistributes the field in and around the b-Si layer
under realistic substrate doping and Al2O3 passivation with fixed interface charge. Initial
results (Fig. 1b) reveal two distinct regimes: at zero bias, field localization is dominated by
the fixed Al2O3 charge, creating hotspots at needle tips and inter-needle valleys, but
surprisingly under high reverse bias, the sub-array bulk field increases by several orders of
magnitude while the field inside the needle bulk remains comparatively low. Notably, the
peak field is spatially confined to discrete regions just below the inter-needle valleys rather
than distributed uniformly, identifying these regions as the likely sites for microplasma
formation and trap-assisted leakage current.
We will present a systematic study of varying needle geometry – width, height, and pitch –
to map how the transition from a planar to a nanostructured surface drives field localization
under bias. These findings will be discussed in the context of experimentally fabricated b-Si
APDs, where the identified field localization mechanisms directly impact avalanche
uniformity, breakdown onset, and dark current.