Strain-free GaAs/AlGaAs semiconductor quantum dots (QDs) grown by droplet etching and nanohole infilling (DENI) are highly promising candidates for the on-demand generation of indistinguishable and entangled photon sources. The spectroscopic fingerprint and quantum optical properties of QDs are significantly influenced by their morphology. The effects of nanohole geometry and infilled material on the exciton binding energies and fine structure splitting are well-understood. However, a comprehensive understanding of GaAs/AlGaAs QD morphology remains elusive. To address this, we employ high-resolution scanning transmission electron microscopy (STEM) and reverse engineering through selective chemical etching and atomic force microscopy (AFM). Cross-sectional STEM of uncapped QDs reveals an inverted conical nanohole with Al-rich sidewalls and defect-free interfaces. Subsequent selective chemical etching and AFM measurements further reveal asymmetries in element distribution. This study enhances the understanding of DENI QD morphology and provides a fundamental three-dimensional structural model for simulating and optimizing their optoelectronic properties.

The complex behavior of magnetoconductance of Bi films grown epitaxially on Si(111) with a thickness of 20-100 bilayers (BL) was measured at T= 9 K in magnetic fields up to B = 4 T, oriented in-plane parallel and perpendicular to the electric dc current I. Contributions to magnetoconductance (MC) by diffuse scattering, by weak localization (WL) as well as by weak antilocalization (WAL) were identified. All these components to MC turned out to be isotropic in two dimensions, i.e., no dependence on angle between B and I within the surface plane was found. Only for B perpendicular to I an increase of MC was detected that is, to first approximation, proportional to B-2. It is ascribed to ballistic scattering between the Rashba-split interfaces that allow Umklapp scattering without spin flip. While MC within the surface states, dominant at small thicknesses, d, shows negligible diffuse scattering under the chosen geometry, their quantum corrections are characterized by WAL with alpha = -0.3 and a coupling strength that decays alpha 1/d with layer thickness. The admixing of quantized bulk states, which dominates MC above 50 BL, not only increases diffuse scattering, it introduces WL in combination with WAL. Presumably due to hybridization with the surface states, it also modifies strongly the WAL component for d > 60 BL. Thus our findings suggest an intriguing interplay in magnetotransport between 2D and quantized 3D states at the Fermi surface of ultrathin bismuth quantum films and provide further deep insight into the electronic transport in quantized and partly spin split bands.

The magnetoconductance of Bi films grown epitaxially on Si(111) for a film thickness between 10 and 100 bilayers (BL) was investigated at a temperature of T = 9 K in magnetic fields up to 4 T oriented perpendicular to the surface plane. The thickness dependence of magnetoconductance (MC) and Hall resistivity was investigated in order to derive thickness dependent charge carrier concentrations as well as their mobilities and to identify corrections by weak antilocalization (WAL) to magnetoconductance. While the electronic transport in ultrathin films up to 30 bilayers (BL) turned out to take place mainly within the surface states, contributions of (bulk derived) quantum well states mix in at larger thicknesses and dominate incoherent transport above 70 BL. On the contrary, for the WAL contribution at magnetic fields normal to the surface, scattering within the surface states dominates at all thicknesses, as evident from the gradual change from values of alpha = -0.35 to alpha = -1 as a function of thickness. This finding reflects the decrease of coupling between the two interfaces going from a single combined conduction channel to two independent channels at the highest film thickness. Quick changes of both parts of magnetoconductance as a function of film thickness at the thinnest films seem to be strain induced by the Bi/Si interface. These results will advance the understanding of the transport properties of Bi thin films and reveal exotic quantum phenomena.

Epitaxial Bi(111) films were subject to many and partly even controversial studies on the semimetal-semiconductor transition triggered by a robust quantum confinement. The residual conductance was ascribed to conducting surface channels. We investigated ultrathin crystalline Bi films on Si(111) as a function of film thickness d between 20 and 100 bilayers by means of electric transport measurements. Varying temperature and magnetic field, we disentangled two transport channels. One remains indeed metallic at all thicknesses investigated and exhibits a slightly increasing conductance as a function of d, whereas the second is activated with a d(-1) dependence of the activation energy, indicating a quasiharmonic confining potential. Both channels reflect the electronic properties of the entire film and do not allow us to strictly separate surface and bulk states. While there is clearly no bulk conductivity, the activated channel is consistently described as electronic excitation into the partly occupied quantum well states, which are also responsible for the metallic conductance and preferentially located close to both interfaces of the film.

In this paper, we use the gold and platinum marker method, to confirm our preliminary finding that aluminum indiffusion into Si crystal during so-called aluminum gettering (AlG) can result in the injection of vacancies into Si crystal. Depth profiles of Au in p-type Si have been studied by the DLTS after diffusion at 850 degrees C for 2h prior to and subsequent to AlG. The donor concentration attributed to Aus is about two orders of magnitude higher compared with that measured in samples without AlG. Similar results were also observed for Pt. The increase in Pts or Aus concentration in the silicon bulk provides the most direct evidence so far that AlG with 400nm Al layer, for 50min at 1100 degrees C injects vacancies with non-equilibrium concentrations of about 1015cm3. The possible mechanism of vacancies generation is discussed.

This contribution summarizes recent efforts to apply transmission electron microscopy (TEM) techniques to recombination-active extended defects present in a low density. In order to locate individual defects, electron beam induced current (EBIC) is applied in situ in a focused ion beam (FIB) machine combined with a scanning electron microscope. Using this approach defect densities down to about 10cm-2 are accessible while a target accuracy of better than 50nm is achieved. First applications described here include metal impurity related defects in multicrystalline silicon, recombination and charge collection at NiSi2 platelets, internal gettering of copper by NiSi2 precipitates and site-determination of copper atoms in NiSi2.

Deep level transient spectroscopy is used to study light-induced reactions of residual iron impurities after aluminum gettering (AlG) in crystalline silicon. White-light illumination at room temperature leads to the formation of a defect which is associated with a donor level at 0.33 eV above the valence band. This defect is stable up to about 175 °C where it dissociates reversibly in case of small iron concentrations and irreversibly for high iron concentrations. Since marker experiments using gold and platinum diffusion show a high vacancy concentration after AlG a tentative identification of the new defect as the metastable iron-vacancy pair is proposed.

Multicrystalline silicon materials for photovoltaic applications inherently contain extended defects like grain boundaries, dislocations, microdefects and in some cases also second phase precipitates due to high concentrations of light elements (carbon, nitrogen or oxygen) and transition metal impurities. The latter are known to reduce the minority carrier lifetime and hence should be removed by gettering during solar cell processing. This paper discusses the influence of extended defects on the spatial distribution of copper- and nickel-related silicide precipitates for a model system containing a small angle grain boundary and in one part silicon oxide pecipitates partly associated with punched-out dislocations. Phosphorus-diffusion gettering under conditions of mostly precipitated metal impurities is discussed in terms of quantitative simulations. It is shown that two regimes can be distinguished where gettering kinetics are either limited by precipitate dissolution or phosphorus in-diffusion. Finally, binding of metal impurities to dislocations is considered and its effect on gettering kinetics is illustrated in terms of gettering simulations.

The efficiency of solar cells produced from crystalline silicon materials is considerably affected by the presence of metal impurities. In order to reduce the concentration of metal impurities, gettering processes as phosphorus diffusion gettering (PDG) and aluminum gettering (AlG) are routinely included in solar cell processing. Further development and optimization of gettering schemes has to ground on physics-based simulations of gettering processes. In this contribution we use quantitative simulations to compare the efficiency and kinetics of PDG and AlG in the presence of precipitates for interstitially dissolved metals, like iron, at different gettering conditions. Recently measured segregation coefficients of iron in liquid AlSi with respect to crystalline silicon are used in order to compare with PDG under typical conditions. It is shown that kinetics of both, PDG and AlG, can be separated into two regimes: (i) at low temperatures kinetics are limited by precipitate dissolution, and (ii) at high temperatures kinetics of AlG is mainly limited by metal impurity diffusion while phosphorus in-diffusion is the limiting factor of PDG.

The electronic properties of present-day multicrystalline silicon (mc-Si) materials for photovoltaic applications are strongly influenced by point defects, their mutual interaction and their interaction with dislocations and grain boundaries. This paper presents results from fundamental investigations of metal impurity interaction with extended defects, namely a small-angle grain boundary and bulk microdefects. It is shown that the distribution of copper suicide precipitates closely follows the density of bulk microdefects indicating the underlying physics of 'good' and 'bad' grains frequently observed in mc-Si. Co-precipitation of copper and nickel in the same samples leads to virtually the same distribution of multi-metal silicide precipitates which according to light-beam induced current measurements show the same recombination activity as single-metal suicide particles. Transmission electron microscopy is used to show that for copper-rich and nickel-rich conditions two types of silicides co-exist, i.e. Cu3Si precipitates containing a small amount of nickel and NiSi2 precipitates containing some copper. Finally, phosphorus-diffusion gettering (PDG) is discussed as the main gettering process used in present-day silicon photovoltaics. Special emphasis is put on the effect of extended defects and their interaction with metal impurities on PDG kinetics. It is shown that different limiting processes will be simultaneously operative in mc-Si as a result of inhomogeneous bulk defect distributions. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Deep level transient spectroscopy is used to study the segregation of Fe from crystalline Si to an Al:Si liquid at its surface, which is the basic mechanism of aluminum gettering used in silicon photovoltaics. The measured segregation coefficient is smaller than estimates from the binary Fe:Si and Al:Fe phase diagrams. This apparent discrepancy originates from the ternary character of the system where the solubility of Fe in Si in equilibrium with the Al-doped α-FeSi2 has to be taken as a reference. Our data suggest that this solubility exceeds that in the binary Fe:Si system by two orders of magnitude.