New information on the electron-hole wave functions in InAs-GaAs self-assembled quantum dots is deduced from Stark effect spectroscopy. Most unexpectedly it is shown that the hole is localized towards the top of the dot, above the electron, an alignment that is inverted relative to the predictions of all recent calculations. We are able to obtain new information on the structure and composition of buried quantum dots from modeling of the data. We also demonstrate that the excited state transitions arise from lateral quantization and that tuning through the inhomogeneous distribution of dot energies can be achieved by variation of electric field. 68.90. + g, 73.50.Pz, Self-assembled InAs-GaAs quantum dots (QDs) provide nearly ideal examples of zero-dimensional semiconductor systems [1] and are hence of considerable contemporary interest for the study of new physics and potential device applications. However, very little is known experimentally about the nature of the QD carrier wave functions and their response to applied fields. Numerous calculations of the electronic structure of QDs have been performed [2][3][4][5], but in the absence of definitive structural information they assume idealized QD shapes, usually pyramidal [6]. However there is evidence that in many cases the dots more closely approximate to lens shaped [7], and may also contain significant concentrations of Ga [8], rather than being pure InAs. In view of the uncertainties in shape and composition, the applicability of the calculated electronic structure to real dots must at best be regarded as approximate at the present time.Consequently, experimental information on the nature of the wave functions is urgently required, to provide a reliable guide to theory, and a firm basis for the interpretation of experiments. In this paper we demonstrate that photocurrent spectroscopy under electric field F provides important, new information on the carrier wave functions, and by comparison with theory, on the composition, shape and effective height of the dots. We show that the QDs possess a permanent dipole moment, implying a finite spatial separation of the electron and hole for F 0. Contrary to the predictions of all recent calculations, we demonstrate that the holes are localized above the electrons in the QDs. This "inverted" alignment can only be explained by assuming nonuniform Ga incorporation in the nominally InAs QDs. As a result of our work the extensive previous theoretical modeling of the electronic structure of InAs QDs will need to be reexamined.Two types of dots were studied, both grown by molecular-beam epitaxy on ͑001͒ GaAs substrates at 500 ± C. The first type (samples A C) was deposited at 0.01 monolayers per second (ML͞s) to give a density ഠ1.5 3 10 9 cm 22 , base size 18 nm, and height 8.5 nm [ Fig. 1(a)], as determined from transmission electron microscopy (TEM). The second type (sample D) had a higher deposition rate of 0.4 ML͞s, resulting in a density ഠ5 3 10 10 cm 22 and size 15 3 3.5 nm. The asymmetric shaped QDs, sitting on an ...

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We present analytical calculations of the potential in a two-dimensional electron gas (2DEG) generated by patterned polygon gates on the surface of a heterostructure. They give the bare and screened potentials and reveal the effect of different boundary conditions on the surface. The formulas for the bare electrostatic potential from patterned gates are simple enough to be plotted in spreadsheets; they give threshold voltages, estimates of the region occupied by the 2DEG, and the energies of some collective infra-red excitations. We also consider the screened potentials in linear response, where no part of the 2DEG is fully depleted, which can again be found within an electrostatic approximation. The behavior of the exposed surface between the gates affects the potential strongly. Surface states provide perfect pinning of the Fermi energy in the "equipotential" model, the usual assumption, but this requires charge to move to the surface from the 2DEG. The charge on the surface is held fixed in response to a gate voltage in the "frozen" model, which typically gives a lower cutoff voltage but stronger confinement of electrons in a split-gate wire. The difference between the two models is large, emphasizing that accurate modeling needs a thorough understanding of the surface states.

We have calculated the piezoelectric coupling between a two-dimensional electron gas and the stress field due to a lateral surface superlattice, a periodic striped gate. The stress is assumed to arise from differential contraction between the metal gate and semiconductor. The piezoelectric potential is several times larger than the deformation potential and generally gives the dominant coupling. It depends on the orientation of the device and vanishes on a (100) surface if current flows parallel to a crystallographic axis. Most devices, however, are fabricated parallel to {011} cleavage planes and in this case the piezoelectric potential is at a maximum. We also discuss different elastic models for the gate and sources of screening, which include the partlyoccupied donors in a typical GaAs-AlGaAs heterostructure.

We have quantitatively established a fundamental limitation on the ultimate spatial resolution of the perfect lens (thin metal slab) in the near field. This limitation stems from the spatial dispersion of the dielectric response of the Fermi liquid of electrons with Coulomb interaction in the metal. We discuss possible applications in nanoimaging, nanophotolithography, and nanospectroscopy.