# Link between graphene features and the resulting functionality of quasi-van der Waals Zn3P2

## TL;DR

This paper explores how graphene substrates can improve the growth of Zn3P2 thin films for solar cells by reducing defects and enhancing crystal quality.

## Contribution

The study identifies optimal graphene substrates and growth conditions for high-quality Zn3P2 thin films using quasi-van der Waals epitaxy.

## Key findings

- Triangular grain nucleation occurs on atomic steps of graphene substrates with minimal point defects.
- H-CVD graphene on the Si-face of 6H-SiC produces the highest-quality Zn3P2 thin films with large grain sizes.
- Higher growth temperatures improve Zn3P2 crystal quality, as shown by enhanced photoluminescence.

## Abstract

Zn3P2, made from earth-abundant elements, is a promising candidate for thin-film solar cells but faces limitations due to difficulties in achieving n-type doping and its large lattice mismatch with commercial substrates and a high thermal expansion coefficient, causing defects and cracks. Graphene substrates can address these challenges thanks to its weak van der Waals interactions with Zn3P2 allowing for mechanical transfer of the thin film and strain-free growth. This study compares five graphene substrates for quasi-van der Waals epitaxial (q-vdWe) growth of polycrystalline Zn3P2 thin films using molecular beam epitaxy. Surface features like steps and wrinkles on graphene were identified as main nucleation sites for Zn3P2, provided the graphene has minimal point defects. The highest-quality thin films, with the largest grain sizes, were grown on H-CVD graphene on the Si-face of 6H-SiC, featuring solely terraces of atomic height. All substrates showed comparable growth windows for crystalline Zn3P2, with higher growth temperatures improving crystal quality, as indicated by enhanced photoluminescence. Cryo-cathodoluminescence measurements revealed spatially localized sub-bandgap emissions, potentially linked to localized strain fields at grain boundaries of up to ±3% as identified by cross-sectional transmission electron microscopy. This work provides insights into advantages and drawbacks of utilising q-vdWe to produce Zn3P2 thin films for solar cell applications and highlights the effects of graphene substrate choice and growth parameters on Zn3P2 film quality.

Zn3P2 growth on graphene: triangular grain nucleation on atomic steps of graphene substrates and its growth window.