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Improved single-electron pumping with surface acoustic waves in ZnO/GaAs systems

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For a piezoelectric substrate, a surface acoustic wave (SAW) is a combined mechanical and potential wave propagating along the surface. In a GaAs/AlGaAs heterostructure, SAWs are able to pump single electron from a high-mobility two-dimensional electron gas (2DEG) [1]. The GaAs substrate, as a weak piezoelectric material (piezoelectric coupling coefficine K2≃0.06%), requires a high power to of an RF signal generate a strong SAW . However, a high power cause a electromagnetic wave crosstalk disturbing the result. We use a thin layer ZnO to improve the SAW amplitude and suppress the crosstalk,

We have developed a room-temperature technique for sputtering ZnO on to the GaAs surface [2] . However, we found that the sputtering process strongly depletes high mobility 2D electron or hole gases in a GaAs/AlGaAs heterostructure at liquid helium temperatures, presumably through implantation of ions. We find that depositing a 25 nm-thick aluminium oxide layer by ALD before sputtering the ZnO is sufficient to protect the 2DEG. With the help of the good piezoelectric coupling coefficient of ZnO (K2≃1%), the SAW resonance on the ZnO/GaAs substrate (scattering parameter ∆S11 =10 dB) is much stronger than the pure GaAs substrate (∆S11=2 dB). In the ZnO-coated devices, The SAW -delayed pinch-off voltage from 100mV in GaAs increase to 350mV in ZnO/GaAs system. The minimum RF power required for SAW pumping was reduced from 7 dBm to ˗4 dBm and the minimum RF power to observe SAW single-electron pumping decreased from 10 dBm to 3 dBm. These results show the great enhancement of the SAW amplitude. Furthermore, the ZnO enhancement of the amplitude is so great that SAWs can propagate even under the surface of liquid helium, where mass-loading normally damps it out very strongly. When the device is dipped into liquid helium, the SAW pumping current oscillates with time, which we explain by the heat dissipation from the SAW that generates a thin helium gas buffer layer on the interface between the ZnO and liquid helium. In future, we will use the ZnO sputtering technique to further investigate SAW pumping spectroscopically. A comprehensive explanation of the SAW single-electron pumping process, and improvement of SAW strength, will benefit the development of SAW quantum technology, such as the manipulation of SAW flying qubits [3] in quantum computation [4].

[1] V. I. Talyanskii, J. M. Shilton, M. Pepper, C. G. Smith, C. J. B. Ford, E. H. Linfield, D. A. Ritchie and G. A. C. Jones, Phys. Rev. B 56 , 15180 (1997). [2] J. Pedros, L. Garcia-Gancedo, C. J. B. Ford, C. H. W. Barnes, J. P. Griffiths, G. A. C. Jones and A. Flewitt, J. Appl. Phys. 110, 103501 (2011). [3] R. P. G. McNeil, M. Kataoka, C. J. B. Ford, C. H. W. Barnes, D. Anderson, G. A. C. Jones, I. Farrer and D. A. Ritchie, Nature 477, 439 (2011); Hermelin, S. et al. ibid, p.435. [4] C. H. W. Barnes, J. M. Shilton, and A. M. Robinson, Phys. Rev. B 62 , 8410 (2000).

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