Reactive Epitaxial Formation of a Mg–P–Zn Ternary Semiconductor in Mg/Zn₃P₂ Solar Cells

Abstract

Zinc phosphide (Zn3P2) has attracted considerable attention as an environmentally benign and earth-abundant photoabsorber for thin-film photovoltaics. It is known that interdiffusion occurs at the Mg/Zn3P2 interface, which is a component of the record device, but the micro- and nanoscopic structures of the interface after interdiffusion have been controversial for over three decades. Here, we report on the formation of a Mg–P–Zn ternary semiconductor, Mg­(Mg x Zn1–x )2P2, at the Mg/Zn3P2 interface. Interestingly, Mg­(Mg x Zn1–x )2P2 is epitaxially grown on Zn3P2 with the orientation relationship of [21̅1̅0](0001)Mg(Mg x Zn1–x )||[100]­(011)Zn3P2 due to interdiffusion. The lattice mismatch of the Mg­(Mg x Zn1–x )2P2 layer on the Zn3P2 substrate is less than 0.5%, and this is favorable for carrier transport across the interface. Mg­(Mg x Zn1–x )2P2 is the material suggested as “n-type Mg-doped Zn3P2” or “a Mg–P–Zn alloy” in the previous studies. Thus, only the optimization of Mg treatment as conducted in the previous studies is insufficient for the improvement of the cell performance. This work clarified that a suitable microstructure and band structure around Mg­(Mg x Zn1–x )2P2/Zn3P2 heterointerface should be established.

Rotated domains in selective area epitaxy grown Zn3P2: formation mechanism and functionality

“…1,3,6,16,17 MBE growth of Zn 3 P 2 utilises relatively low growth temperatures as compared to other commonly used techniques. 21 The lower growth temperatures help mitigate the potential post-growth strain build up due to mismatching CTE, which could otherwise result in defect formation of the grown material. Nanostructures, on the other hand, have been shown to allow for additional elastic strain relaxation mechanisms due to their reduced dimensions, as well as supressing misfit dislocation formation due to their limited interface area (efficiency being a function of lattice mismatch and interface area).…”

Multiple morphologies and functionality of nanowires made from earth-abundant zinc phosphide

“…Extrinsic doping, particularly n-type, is not yet fully unravelled. [13][14][15] To overcome these challenges, the nanowire morphology shows promising perspectives. First, their small diameter allows for high-quality epitaxial growth on lattice mismatched substrates through radial stress relaxation.…”

“…30 In principle, the precise flux control in MBE should also enable the controlled incorporation of extrinsic dopants such as Ag, Mg and In. 13,14,[33][34][35] Previously, in addition to thin films and bulk crystals, Zn 3 P 2 has been obtained in the form of randomly oriented nanowires, nanoribbons, and nanotrumpets. [36][37][38][39][40][41][42][43] In most of these studies, nanowires were produced through chemical vapour deposition (CVD) or by a thermochemical method relying on a quartz capsule containing the precursors being heated in a furnace.…”

Towards defect-free thin films of the earth-abundant absorber zinc phosphide by nanopatterning

“…MBE also provides a path for precise incorporation of extrinsic dopants, such as magnesium or silver. [19][20][21][22] Several substrates have been employed for the epitaxial growth of zinc phosphide thin lms. 12,14,17,23 For example, we previously showed that single-crystal zinc phosphide akes can nucleate and grow defect-free on graphene as the interactions are restricted to van der Waals.…”