The spin of a single electron subject to a static magnetic field provides a natural two-level system that is suitable for use as a quantum bit, the fundamental logical unit in a quantum computer. Semiconductor quantum dots fabricated by strain driven self-assembly are particularly attractive for the realization of spin quantum bits, as they can be controllably positioned, electronically coupled and embedded into active devices. It has been predicted that the atomic-like electronic structure of such quantum dots suppresses coupling of the spin to the solid-state quantum dot environment, thus protecting the 'spin' quantum information against decoherence. Here we demonstrate a single electron spin memory device in which the electron spin can be programmed by frequency selective optical excitation. We use the device to prepare single electron spins in semiconductor quantum dots with a well defined orientation, and directly measure the intrinsic spin flip time and its dependence on magnetic field. A very long spin lifetime is obtained, with a lower limit of about 20 milliseconds at a magnetic field of 4 tesla and at 1 kelvin.

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 ...

Silicon crystallized in the usual cubic (diamond) lattice structure has dominated the electronics industry for more than half a century. However, cubic silicon (Si), germanium (Ge) and SiGe-alloys are all indirect bandgap semiconductors that cannot emit light efficiently. Accordingly, achieving efficient light emission from group-IV materials has been a holy grail 1 in silicon technology for decades and, despite tremendous efforts 2-5 , it has remained elusive 6 . Here, we demonstrate efficient light emission from direct bandgap hexagonal Ge and SiGe alloys. We measure a sub nanosecond, temperature insensitive radiative recombination lifetime and observe a similar emission yield to direct bandgap III-V semiconductors. Moreover, we demonstrate how by controlling the composition of the hexagonal SiGe alloy, the emission wavelength can be continuously tuned in a broad range, while preserving a direct bandgap. Our experimental findings are shown to be in excellent quantitative agreement with the ab initio theory. Hexagonal SiGe embodies an ideal material system to fully unite electronic and optoelectronic functionalities on a single chip, opening the way towards novel device concepts and information processing technologies.Silicon has been the workhorse of the semiconductor industry since it has many highly advantageous physical, electronic and technological properties. However, due to its indirect bandgap, silicon cannot emit light efficientlya property that has seriously constrained potential for applications to electronics and passive optical circuitry 7-9 . Silicon technology can only reach its full application potential when heterogeneously supplemented 10 with an efficient, direct bandgap light emitter.The band structure of cubic Si, presented in Fig. 1a is very well known, having the lowest conduction band (CB) minimum close to the X-point and a second lowest * These authors contributed equally to this work. † Correspondence to E.P.A.M.(e.p.a.m.bakkers@tue.nl).minimum at the L-point.As such, it is the archetypal example of an indirect bandgap semiconductor, that, notwithstanding many great efforts 3-6 , cannot be used for efficient light emission.By modifying the crystal structure from cubic to hexagonal, the symmetry along the 111 crystal direction changes fundamentally, with the consequence that the L-point bands are folded back onto the Γ-point. As shown in Fig. 1b, for hexagonal Si (Hex-Si) this results in a local CB minimum at the Γ-point, with an energy close to 1.7 eV 11-13 . Clearly, Hex-Si remains indirect since the lowest energy CB minimum is at the M-point, close to 1.1 eV. Cubic Ge also has an indirect bandgap but, unlike Si, the lowest CB minimum is situated at the L-point, as shown in Fig. 1c. As a consequence, for Hex-Ge the band folding effect results in a direct bandgap at the Γ-point with a magnitude close to 0.3 eV, as shown in the calculated band structure in Fig. 1d 14 .To investigate how the direct bandgap energy can be tuned by alloying Ge with Si, we calculated the band structures of He...