Deposition of p-type metal oxide semiconductors for large-area electronic display and solar cell applications

As we move into the era of the ‘Internet of Things’, there is a continuous drive to develop electronic materials and devices with better performance and a greater focus on improved flexibility and form factor. Current large-area electronics is still dominated by silicon based technologies, which, with their relatively low performance, limits the advance and development of both the display and solar cell industries. This has led to the development of alternative metal oxide technologies. However, whilst very good n-type materials have now been developed, there is still an absence of good p-type materials, which are essential to allow development of the inorganic, non-silicon based p-n junctions and CMOS technology, required by the solar cell and display industries respectively. The inherent material properties of metal oxides traditionally make this difficult, with most exhibiting poor p-type behaviour, due to low mobilities and high carrier localisation. However, Cu₂O and SnO have emerged as promising, naturally p-type metal oxides, with lower hole localisation due to hybridisation of the valence band maximum. Therefore, work within this project has been focussed on the deposition of these materials using industry compatible, chemical deposition processes: atomic layer deposition (ALD) for SnO, due to its very narrow process window and the superior stoichiometric control associated with this deposition technique; and plasma-enhanced chemical vapour deposition (PECVD) for Cu₂O, which as a technique used in current silicon based production lines, is therefore, highly compatible with the large-area electronics industry. Firstly, using a novel Sn precursor, the successful fabrication of p-type SnO thin film transistors (TFTs) with an on-off ratio of 4×10⁴, field effect mobility of 0.6 cm²(Vs)⁻¹ and SS of 0.6 V/dec has been demonstrated. In order to achieve this performance, the ALD process required multiple Sn pulses per cycle, which increases the precursor dosage and leads to improved film crystallinity. The performance was further improved by post deposition vacuum annealing and alumina passivation. Secondly, work attempting to deposit Cu₂O using Cu(acac)₂, encountered significant difficulties linked to the need to deposit the precursor as a liquid for this particular PECVD system, resulting in high carbon contamination and inhibited growth. Therefore, focus turned to an alternative precursor (Cu(hfac)(tmvs)) and the successful deposition of p-type Cu₂O using PECVD has been demonstrated, with films exhibiting a Hall mobility of 1.48 cm²(Vs)⁻¹, resistivity of 1.92×10⁵ Ωcm and carrier concentration of 3.09×10¹³ cm⁻³. The significance of the substrate pretreatment has been identified by the existence of two distinct growth regimes, with further optimisation of the substrate state required to allow successful fabrication of p-type devices. However, with thicknesses of ∼40 nm achieved from a single 1.75 ms pulse of the Cu(hfac)(tmvs) precursor, the work shown here represents an exciting opportunity for rapid growth of p-type Cu₂O films using a deposition technique highly suitable for industry.

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2021

Author dc:creator

• Gomersall, Daisy

Advisor dc:contributor.advisor

• Flewitt, Andrew

• PECVD
• metal oxides

Licence

• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

Author Identifier

0000-0003-4275-732X

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/339101