Scanning capacitance microscopy of GaN-based high electron mobility transistor structures: A practical guide

Chen Chen; Saptarsi Ghosh; Francesca Adams; Menno J Kappers; David J Wallis; Rachel A Oliver

doi:10.1016/j.ultramic.2023.113833

Scanning capacitance microscopy of GaN-based high electron mobility transistor structures: A practical guide

Ultramicroscopy. 2023 Dec:254:113833. doi: 10.1016/j.ultramic.2023.113833. Epub 2023 Aug 22.

Authors

Chen Chen¹, Saptarsi Ghosh², Francesca Adams², Menno J Kappers², David J Wallis³, Rachel A Oliver²

Affiliations

¹ Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom. Electronic address: cc2053@cam.ac.uk.
² Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom.
³ Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom; Centre for High Frequency Engineering, University of Cardiff, Cardiff CF24 3AA, United kingdom.

PMID: 37666104
DOI: 10.1016/j.ultramic.2023.113833

Abstract

The scanning capacitance microscope (SCM) is a powerful tool to characterise local electrical properties in GaN-based high electron mobility transistor (HEMT) structures with nanoscale resolution. We investigated the experimental setup and the imaging conditions to optimise the SCM contrast. As to the experimental setup, we show that the desired tip should be sharp (e.g., with the tip radius of ≤25nm) and its coating should be made of conductive doped diamond. Most importantly, its spring constant should be large to achieve stable tip-sample contact. The selected tip should be positioned close to both the edge and Ohmic contact of the sample. Regarding the imaging conditions, we also show that a dc bias should be applied in addition to an ac bias because the latter alone is not sufficient to deplete the two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure. The approximate range of the effective dc bias values was found by measuring the local dC/dV-V curves, yielding, after further optimisation, two optimised dc bias values which provide strong, but opposite, SCM contrast. In comparison, the selected ac bias value has no significant impact on the SCM contrast. The described methodology could potentially also be applied to other types of HEMT structures, and highly-doped samples.

Keywords: High electron mobility transistor structures; Plan-view characterisation; Scanning capacitance microscopy.