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Fermi Level De-pinning In Metal-Semiconductor Contacts Via Nanometre-scale ALD Dielectric Films

Lookup NU author(s): Dr Peter King, Dr Erhan Arac, Srinivas Ganti, Sami Ramadan, Dr Kelvin Kwa, Dr Anders Barlow, Professor Peter Cumpson, Professor Anthony O'Neill


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The Schottky barrier height (SBH) at a metal/semiconductor interface is the energy barrier to current flow due to the difference in workfunction between the two materials, and is an encumbrance to device design. A major challenge for future electronic systems is to reduce the resulting electrical contact resistance at such interfaces. Even though the theory suggests that minimizing the SBH by selecting an appropriate metal with workfunction close to the electron affinity of the semiconductor should be possible, a phenomenon called ‘Fermi level pinning’ prevents it. One theory for the observed pinning of the Fermi level at metal/semiconductor junctions is that of ‘metal-induced-gap-states’ (MIGS): the tails of carrier wavefunctions decaying across the interface into the semiconductor. MIGS interaction with the semiconductor virtual states results in a pinning effect near the charge neutrality level of the semiconductor. A method to counteract MIGS is to insert an ultra-thin dielectric layer at the interface between metal and semiconductor. The optimum thickness of inserted film would suppress the MIGS at the interface, while being thin enough to allow a measureable tunneling current through. In this paper we show Fermi level de-pinning via dielectric films grown by thermal ALD. Ni/AlOx/Si back-to-back Schottky contacts were fabricated with varied thickness of AlOx film. Wide-angle XPS (WAXPS) was used to assess layer thicknesses and make complimentary band energy measurements. I-V measurements at different temperatures were used to determine the electrical properties, and to extract the SBH of the experimental specimens. The optimum thickness of AlOx film is found to be 20 ALD cycles, or ~0.9 nm for which Schottky barrier height is reduced by ~0.25 eV as shown in figure 1. We also compared the electrical properties of plasma and thermal ALD grown samples. The results indicated that high-growth-temperature thermal ALD samples displayed improved structural and electrical properties in comparison to low-growth-temperature and plasma-enhanced ALD modes.

Publication metadata

Author(s): King PJ, Arac E, Ganti S, Ramadan S, Kwa KSK, Barlow AJ, Cumpson PJ, Robertson J, O'Neill AG

Publication type: Conference Proceedings (inc. Abstract)

Publication status: Published

Conference Name: 12th International Baltic ALD 2014

Year of Conference: 2014