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The Nature of the III-V/Oxide Interface and its Impact on the ALD Growth of High-k Dielectrics for Advanced CMOS

III-V materials such as In0.53Ga0.47As are promising candidates for the scaling of n-channel MOS and MOSFET devices beyond the 22 nm node as a result of their high electron mobility. However, problems have been encountered when preparing gate oxide layers on this material resulting from the presence of native oxides which are significantly more complex than those found on silicon. Given the structural and chemical mismatch between the III-V and the gate oxide a completely defect-free interface would not be expected. However the problem is such that these oxides cause the formation of a relatively high density of interface states or defects which can act as charge traps thereby degrading device performance. With this in mind we are working towards the development of a pre-growth substrate treatment combined with an ALD growth process which minimizes the formation of such interface states or renders them electrically inactive. This paper will review what we know about In0.53Ga0.47As (100) surfaces and the nature of the native oxides and then describe some work in which the pre-growth chemical treatment of both n- and p-doped In0.53Ga0.47As surfaces using NH42S solutions of varying concentrations was performed in order to optimise the process in relation to subsequent MOS device performance [1]. Substrates were degreased and treated with NH42S solutions of concentration 22%, 10%, 5%, or 1% for 20 min at 295 K. Subsequently thin (nominally 8nm) Al2O3 layers were deposited by ALD and the MOS device then completed via the formation of Ni/Au gate contacts. Frequency dispersion studies of the C:V profiles suggests that the 10% solution gives the best structure in terms of interface state density (DIT = 2.5 x1012 cm−2 +/-1 x1012 cm−2) with the peak associated with the interface states being localised in the middle of the In0.53Ga0.47As energy gap. We have also studied the effect of delaying the ALD growth after the wet chemical treatment using XPS. These experiments suggest that if the sample is left for any appreciable time in the ambient, that a significant amount of oxide growth occurs. In addition, a feature associated with an elemental As species, such as As-dimer bonds, appears to form.

The paper will then briefly describe the use of ultra-thin (nominally 1 nm) Al2O3 interface control layers, designed to exploit the clean-up mechanism seen with the use of trimethyl aluminium in ALD, but also to create a new interface between the In0.53Ga0.47As channel and a selected high-k dielectric, HfO2 [2]. Finally, since plasma cleaning and plasma-assisted ALD provides a possibly advantageous approach, the paper will then describe some preliminary results which demonstrate the efficacy of using hydrogen plasma to prepare the In0.53Ga0.47As substrate prior to growth and also to initiate growth of a high-k dielectric directly on the plasma cleaned surface.


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Posted by: Martyn Pemble
October 21st, 2011 at 14:20

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