Metal-nanoparticle single-electron transistors fabricated using electromigration

We fabricate single-electron transistors from individual metal grains in a geometry that provides sufficient gate coupling to study more than 10 charge states of the nanoparticle. We achieve this by incorporating a nanoparticle into a gap created between two electrodes using electromigration, all on top of an oxidized aluminum gate. IV-curves of devices containing a gold particle 5-15 nm in diameter show single-electron-transistor behavior at 4.2 K and discrete level spectra at 0.1 K. We find that the levels are spin-degenerate and are filled as in a non-interacting picture.

People Involved

Kirill Bolotin, Ferdinand Kuemmeth, Abhay Pasupathy, Dan Ralph

Summary

Electron tunneling through single nanometer sized metal grains allows one to measure electron-in-a-box quantum states, and to study how these states are affected by a variety of different interactions inside metals [1]. In this project, we adapt a break junction technique [2] that was previously used to study transport properties of individual molecules [2,3] to contact a metal nanograin. This design enables better gate control of the grain’s charge, faster sample fabrication, and less parasitic capacitance in high-speed measurements than in previous experiments based on nanoholes in silicon nitride membranes [4,5].

We perform the fabrication by using electron-beam lithography (Leica VB6) on bilayer PMMA to define 100 nm-wide gold wires on top of an oxidized aluminum gate electrode. The wires are broken by electromigration to form a nm-sized gap. We then simply evaporate gold on top of the device so that nanoparticles can self-assemble under the influence of surface tension. After the evaporation some of the particles within the gap are close enough to both electrodes to yield a measurable tunnel current. Finally the device is protected by evaporation of aluminum oxide (30nm) without breaking vacuum.

I-V curves taken at 0.1 K show gate-dependent Coulomb blockade. When increasing the bias voltage beyond the Coulomb blockade threshold, the tunnel current increases in discrete steps: Each of these steps corresponds to the nanoparticle switching between a specific N- and N+1- electron quantum state, and shows up as a conductance line when plotting dI/dV as a function of bias and gate voltage. By tuning the gate voltage to different degeneracy points we can change the total number of electrons occupying the particle and measure the corresponding set of discrete conductance lines. This yields information on how the discrete level spectrum of the nanograin is affected by adding or removing electrons. For an Au particle 5-15 nm in size we find that the discrete levels are spin degenerate and are filled as in a non-interacting model [6]. This is consistent with weak exchange interaction in Au [7]. We can also observe how the energies of single-electron levels change when applying a magnetic field, and deduce individual g-factors ranging from 0.6 to 1.9. We have just started to introduce magnetic impurities into the nanograin and hope to be able to study how this interaction changes the electronic level spectra.

SEM of device

Figure 1:
(a) Top-view SEM image of the device geometry, with gold source and drain electrodes on top of an oxidized aluminum gate.
(b) Expanded view of the region outlined with a white rectangle in (a). A 10 nm gap made by electromigration is visible, along with deposited gold nanoparticles.
(c) Circuit schematic for the Au-nanoparticle SET.

Coulomb blockade diamonds

Figure 2: Differential conductance of a metal nanoparticle transistor vs. gate voltage and source drain bias, showing Coulomb-blockade “diamonds”.

discrete current steps

Figure 3: Tunnel current vs. bias voltage increases in discrete steps corresponding to single-electron states of the particle.

level filling

Figure 4: The level spectra of a gold particle at 3 adjacent charge states are very similar to each other due to weak exchange interaction.

References

  1. M. M. Deshmukh, Probing magnetism at the nanometer scale using tunneling spectroscopy, PhD. Thesis 2002 Cornell University and references therein.
  2. H. Park et al., Fabrication of metallic electrodes with nanometer separation by electromigration, Appl. Phys. Lett. 75, 301 (1999).
  3. J. Park, A. N. Pasupathy et al., Coulomb blockade and the Kondo effect in single-atom transistors, Nature 417, 722 (2002).
  4. D. C. Ralph, C. T. Black, and M. Tinkham, Spectroscopic Measurements of Discrete Electronic States in Single Metal Particles, Phys. Rev. Lett. 74, 3241-3244 (1995).
  5. M. M. Deshmukh, E. Bonet, A. N. Pasupathy, and D. C. Ralph, Equilibrium and nonequilibrium electron tunneling via discrete quantum states, Phys. Rev. B 65, 073301 (2002).
  6. K. I. Bolotin, F. Kuemmeth, A. N. Pasupathy and D. C. Ralph, Metal-nanoparticle single-electron transistors fabricated using electromigration, Appl. Phys. Lett. 84, 3154 (2004).
  7. D. A. Gorokhov and P. W. Brouwer, Fluctuations of g Factors in Metal Nanoparticles: Effects of Electron-Electron Interaction and Spin-Orbit Scattering, Phys. Rev. Lett. 91, 186602 (2003).

Last updated: 6/15/04

contact info