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Communication Dans Un Congrès Année : 2007

Kelvin force microscopy on GaN wide gap materials

Résumé

Kelvin Force Microscopy (KFM) [1] enables to probe the contact potential difference between two materials, which is related to physical quantities such as work function differences and/or surface states. We address in this contribution the issue of quantitative measurements of such physical quantities using KFM in the case of GaN wide-gap materials. We will first focus on the z-spectroscopy of KFM measurements performed by nullifying the oscillation amplitude of the cantilever when the tip-surface capacitance is submitted to an electrostatic excitation at the cantilever resonance frequency. Even on metallic layers, an unexpected asymmetry of the V(z) curves (surface potential versus tip-substrate distance) can be observed experimentally, depending whether the electrostatic excitation is applied to the tip or to the substrate. This effect is demonstrated to originate in instrumental capacitive cross talks between the electrostatic excitation and the microscope photodiode signals. When suppressing these cross-talks, artefact free V(z) curves can be measured. Their monotonous variation at the scale of a few hundred of nanometers is interpreted in terms of contaminants forming a surface dipolar layer at the sample and tip.The scheme has been applied in a second step to n and p-type GaN layers grown by MOCVD on a (0001) sapphire substrate [2]. The n-type GaN (respectively p-type GaN) is doped by Si (Mg) with an effective doping level of 1.6e19 cm−3 (1e16 cm−3). To get a potential reference for KFM measurements, ohmic contacts on n-type GaN (resp. p-type GaN) were achieved using Ti/Al (resp. Ni/Au) metallization layers with thickness 20/200 nm (resp. 50/100 nm), followed by a Au thickening. Experiments show that the contact potential difference between n- and p-type GaN is ~0.8V. The lower value compared to GaN band-gap (3.4 eV) is attributed to surface-state induced band-bending at the oxidized GaN surface. This interpretation will be compared with transport experiments using conducting atomic force microscopy in the 150K-500K range.Finally, KFM experiments will be shown on AlGaN/GaN high-electron mobility transistor devices [4]. Surface potential maps here strongly depend on parasitic capacitive couplings between the cantilever and the device, which occur at the scale of a few tens of µm. In order to get quantitative measurements of surface potentials (and thus of charge distributions between source and drain contacts), the purpose is here to measure the transfer function of the tip-cantilever probe and use it as a deconvolution of experimental KFM images.[1] M. Nonnenmacher et al, Appl.Phy.Lett., 58,25,2921-2923 (1991).[2] M.A.di Forte-Poisson et al., Phys.Stat.Sol. (a), 203, 1, 185-193 (2006).[3] H.O.Jacobs et al., J. Appl.Phys., 84, 3, 1168-1173 (1998).[4] G.Koley et al., IEEE Transactions on Electron Devices, 50, 4, 886–893 (2003).
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Dates et versions

hal-00285289 , version 1 (05-06-2008)

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  • HAL Id : hal-00285289 , version 1

Citer

Sophie Barbet, Raphaël Aubry, D. Deresmes, Marie-Antoinette Di Forte-Poisson, Heinrich Diesinger, et al.. Kelvin force microscopy on GaN wide gap materials. Materials Research Society Fall Meeting, Symposium B : Nanoscale Phenomena in Functional Materials by Scanning Probe Microscopy, Nov 2007, Boston, MA, United States. ⟨hal-00285289⟩
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