We demonstrate a hard-x-ray microscope that does not use a lens and is not limited to a small field of view or an object of finite size. The method does not suffer any of the physical constraints, convergence problems, or defocus ambiguities that often arise in conventional phase-retrieval diffractive imaging techniques. Calculation times are about a thousand times shorter than in current iterative algorithms. We need no a priori knowledge about the object, which can be a transmission function with both modulus and phase components. The technique has revolutionary implications for x-ray imaging of all classes of specimen.

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 ...

We report first quantitative measurements by energy-selected imaging in a transmission electron microscope of In segregation within an uncapped islanded In0.25Ga0.75As layer grown epitaxially on GaAs. This layer has the lowest In concentration at which islanding occurs and, then, only after a flat approximately 3 nm alloy layer has been formed. In buildup by segregation at the surface of this initial flat layer is considered the driving force for islanding and, importantly, the segregation process introduces the characteristic delay seen before the Stranski-Krastanow transition. We observe strong inhomogeneous In enrichment within the islands (up to x(In) approximately 0.6 at the apex) and a simultaneous In depletion in the remaining flat layer.

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