Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2022 / Paper 1 / Section B / 9 / i

Use anisotropic alkaline wet etching of a suitably oriented crystalline silicon wafer. A KOH- or TMAH-based process etches crystallographic planes at different rates, leaving slow-etching $\{111\}$ facets as a pyramidal texture on a $\{100\}$ wafer. The facets redirect reflected light towards neighbouring surfaces, increasing the chance of absorption rather than specular escape.

Choose conditions that give a uniform texture without excessive removal or contamination, and clean the wafer before forming the junction. This mechanism depends on crystallinity: a multicrystalline or differently oriented wafer may need another texture process. The drawing alone does not establish the wafer orientation, so the standard pyramidal-texture method carries that explicit assumption.

Diffuse a donor such as phosphorus into the textured p-type surface, then control the thermal drive-in to form the shallow n-type emitter. A phosphorus-containing vapour source can supply the donor over the exposed area. Diffusion and activation create an n-on-p p-n junction; remove the phosphorus-rich surface glass and isolate the rear and wafer edges as required to avoid shunts.

An alternative is ion implantation followed by an activation and damage-repair anneal. The diffusion route is particularly convenient for large-area coverage of a textured wafer. The indicated roughly $1\mu \mathrm{m}$ layer is a depth target for process control, not an instruction to grow an undoped film and assume that it becomes n-type.

Deposit hydrogenated silicon nitride by plasma-enhanced chemical vapor deposition. A plasma activates a silicon-containing precursor and a nitrogen source, enabling a conformal thin film at a lower substrate temperature than a purely thermal process. Tune gas composition, power and time to control thickness, refractive index, stress and hydrogen content.

The film serves as an antireflection coating and can also provide chemical and field-effect surface passivation. As a simple optical check, a quarter-wave thickness is $d\simeq {\lambda }_{\mathrm{design}}/(4n)$; $d\simeq 75\mathrm{nm}$ and $n\simeq 2$ correspond to a design wavelength near $600\mathrm{nm}$. The textured surface and broadband illumination require a fuller optical optimization, so this check is an order-of-magnitude explanation rather than an exact reflectance calculation.