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The gallium gradient in Cu(In,Ga)Se2 (CIGS) layers, which forms during the two industrially relevant deposition routes, the sequential and co-evaporation processes, plays a key role in the device performance of CIGS thin-film modules. In this contribution, we present a comprehensive study on the formation, nature, and consequences of gallium gradients in CIGS solar cells. The formation of gallium gradients is analyzed in real time during a rapid selenization process by in situ X-ray measurements. In addition, the gallium grading of a CIGS layer grown with an in-line co-evaporation process is analyzed by means of depth profiling with mass spectrometry. This gallium gradient of a real solar cell served as input data for device simulations. Depth-dependent occurrence of lateral inhomogeneities on the µm scale in CIGS deposited by the co-evaporation process was investigated by highly spatially resolved luminescence measurements on etched CIGS samples, which revealed a dependence of the optical bandgap, the quasi-Fermi level splitting, transition levels, and the vertical gallium gradient. Transmission electron microscopy analyses of CIGS cross-sections point to a difference in gallium content in the near surface region of neighboring grains. Migration barriers for a copper-vacancy-mediated indium and gallium diffusion in CuInSe2 and CuGaSe2 were calculated using density functional theory. The migration barrier for the InCu antisite in CuGaSe2 is significantly lower compared with the GaCu antisite in CuInSe2, which is in accordance with the experimentally observed Ga gradients in CIGS layers grown by co-evaporation and selenization processes. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The aim of this work is to provide an overview on the recent advances in the selective area growth (SAG) of () nanostructures by plasma assisted molecular beam epitaxy, focusing on their potential as building blocks for next generation LEDs. The first three sections deal with the basic growth mechanisms of SAG and the emission control in the entire ultraviolet to infrared range, including approaches for white light emission, using disks and thick segments on axial nanocolumns. SAG of axial nanostructures is developed on both /sapphire templates and -buffered (111). As an alternative to axial nanocolumns, section 4 reports on the growth and characterization of / core-shell structures on an ordered array of top-down patterned microrods. Finally, section 5 reports on the SAG of , with and without insertion, on semi-polar (11-22) and non-polar (11-20) templates. Upon SAG the high defect density present in the templates is strongly reduced as indicated by a dramatic improvement of the optical properties. In the case of SAG on non-polar (11-22) templates, the formation of nanostructures with a low aspect ratio took place allowing for the fabrication of high-quality, non-polar pseudo-templates by coalescence of these nanostructures. [ABSTRACT FROM AUTHOR]

Abstract: Polycrystalline Cu(In,Ga)Se2 (CIGS) thin films with thicknesses of 1.1μm, 2.4μm and 2.9μm were grown using an in-line co-evaporation process with a final Cu-poor composition. The different film thicknesses were achieved by a variation of the process speed under constant evaporation rates. We analyze CIGS thin films by means of highly spatially (<160nm) and spectrally resolved cathodoluminescence microscopy at low temperature (T=5K). The integral spectrum of the investigated samples is dominated by donor–acceptor-pair recombination around 1.15eV for the thinner samples and around 1.20eV for the thickest sample. The surface shows a Gaussian and therefore fully random distribution of the peak wavelength. An investigation of the cross-sections reveals a shift of the peak energies to lower energies towards the surface of the layers for all samples and thus visualizes the local vertical gradient of the Ga/(Ga+In) ratio of the samples via an optical method. The extent of this shift increases significantly from 13meV to 130meV with decreasing process speed. [Copyright &y& Elsevier]

Abstract: We have grown epitaxial CuGaSe2 layers by MOVPE on GaAs substrates. CuGaSe2 is the highest band gap member of the chalcopyrite Cu(In,Ga)Se2 family (CIGSe). CIGSe is used as an absorber in polycrystalline thin film solar cells. From growth experiments interrupted at different time steps we find that Ga diffusion from the substrate plays a major role in the early stages of growth. The epi-layers start to grow Ga-rich although the final film is dominated by Cu-excess and shows a homogeneous composition profile. After about 300nm film thickness Ga diffusion becomes less dominant and the Cu-excess leads to the formation of Cu selenides on the surface. The structure of the films and intentional conversion experiments show that those crystalline Cu selenides are transformed into epitaxial CuGaSe2 during growth. [Copyright &y& Elsevier]

• Epitaxial growth of AlN on Si (1 1 1) by pulsed reactive sputtering. • Change of N-precursor change growth mode from three-dimensionel to two-dimensional. • Change in growth mode approved by scanning transmission electron microscopy images. • Increase of ad-atom mobililty by use of NH 3. • Smooth surface morphology for growth with NH 3 with low roughness as 0.14 nm. We present a pulsed reactive magnetron sputter process for high quality AlN on Si (1 1 1) beneficially avoiding any high-temperature growth. Initially, metallic aluminium with a nominal thickness of about one monolayer is deposited at a substrate temperature around 850 °C in an Ar plasma followed by sputtering in an Ar/N plasma. For 250 nm thick AlN layers a surface roughness below 0.2 nm rms is obtained as determined by atomic force microscopy (AFM). Using an Al nucleation step prior to AlN growth substantially improves the crystalline properties of AlN. The FWHM values of the AlN (0 0 0 2) and the AlN (1 0 1 0) diffraction peaks with 0.45° and 0.86°, respectively, are comparable to state-of-the-art AlN on Si layers grown by metalorganic vapour phase epitaxy (MOVPE). Two different N-precursor gases, namely N 2 and NH 3 , lead to distinct layer qualities as revealed by atomic force microscopy and transmission electron microscopy. Only with NH 3 substantial lateral growth can be achieved at T = 850 °C which is mandatory to obtain smooth surface morphologies. In MOVPE such lateral AlN growth is typically only achieved at high growth temperatures (T > 1000 °C). [ABSTRACT FROM AUTHOR]

The structural and optical properties of InGaSb/GaP(001) type‐II quantum dots (QDs) grown by metalorganic vapor phase epitaxy (MOVPE) are studied. Growth strategies as growth interruption (GRI) after deposition of InGaSb and Sb‐flush prior to QD growth are used to tune the structural and optical properties of InGaSb QDs. The Sb‐flush affects the surface diffusion leading to more homogeneous QDs and to a reduction of defects. A ripening process during GRI occurs, where QD size is increased and QD‐luminescence remarkably improved. InGaSb QDs are embedded in GaP n + p‐diodes, employing an additional AlP barrier, and characterized electrically. A localization energy of 1.15 eV for holes in QDs is measured by using deep‐level transient spectroscopy (DLTS). The use of Sb in QD growth is found to decrease the associated QD capture cross‐section by one order of magnitude with respect to the one of In0.5Ga0.5As/GaP QDs. This leads to a hole storage time of almost 1 h at room temperature, which represents to date the record value for MOVPE‐grown QDs, making MOVPE of InGaSb/GaP related QDs a promising technology for QD‐based nano‐memories. InGaSb/GaP(001) quantum dots (QDs) grown by metalorganic vapor phase epitaxy (MOVPE) are investigated. The QD growth parameters are linked with their structural, optical, and electrical properties, leading to an increase of the hole storage time to almost 1 h by a factor of 14 compared to the previous record value. This value represents to date the record for MOVPE‐grown QDs. [ABSTRACT FROM AUTHOR]

Higher indium incorporation in self-organized triangular nanoprisms at the edges of InGaN/GaN core-shell nanorods is directly evidenced by spectral cathodoluminescence microscopy in a scanning transmission electron microscope. The nanoprisms are terminated by three 46 nm wide a-plane nanofacets with sharp interfaces forming a well-defined equilateral triangular base in the basal plane. Redshifted InGaN luminescence and brighter Z-contrast are resolved for these structures compared to the InGaN layers on the nanorod sidewalls, which is attributed to at least 4 % higher indium content. Detailed analysis of the inner optical and structural properties reveals luminescence contributions from 417 nm up to 500 nm peak wavelength proving the increasing indium concentration inside the nanoprism towards the nanorod surface. [ABSTRACT FROM AUTHOR]

Selective area epitaxial growth is an important technique, both for monolithic device integration as well as for defect reduction in heteroepitaxy of crystalline materials on foreign substrates. While surface engineering with masking materials or by surface structuring is an effective means for controlling the location of material growth, as well as for improving crystalline properties of epitaxial layers, the commonly involved integral substrate heating presents a limitation, e.g., due to constraints ofr the thermal budget applicable to existing device structures. As a solution, an epitaxial growth approach using a laser source only locally heating the selected growth area, in combination with metal-organic precursors to feed a pyrolithic chemical reaction (also known as metal-organic vapor phase epitaxy, MOVPE), is presented. Without masking or surface structuring, local epitaxial growth of III-V compound semiconductor layers on a 50-1500 µm length-scale, with high structural and optical quality, is demonstrated. We discuss general design rules for reactor chamber, laser heating, temperature measurement, sample manipulation, gas mixing, and distinguish laser-assisted local MOVPE from conventional planar growth for the important compound semiconductor GaAs. Surface de-oxidation prior to growth is mandatory to realize smooth island surfaces. Linear growth rates in the range 0.5-9 µm/h are demonstrated. With increasing island diameter, the probability for plastic deformation within the island increases, depending on reactor pressure. A step-flow mode on the island surface can be achieved by establishing a sufficiently small temperature gradient across the island.

We report on nanoscopic exploration of the luminescence from individual InP quantum dots (QDs) by means of highly spatially resolved cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM). A 7-fold layer stack with high-density InP quantum dots is embedded as an active medium membrane in an external-cavity surface-emitting laser. We characterize the vertical transfer of carriers within the periodic separate confinement heterostructure and determine the capture efficiency of carriers from the cladding layer into the quantum dot layers. Benefiting from the nanoscale resolution of our STEM-CL, we perform single-dot spectroscopy on single isolated QDs in the STEM lamella resolving the details of the excitonic structure of individual quantum dots. Executing highly spatially resolved spectrum line scans within the QD layers, we directly visualize the lateral transport, i . e ., the efficient lateral capture of carriers into an individual QD. We observe a characteristic change of the spectral fingerprint during this line scan, while the electron beam is approaching and subsequently receding from the quantum dot position. This directly correlates to the increase and decrease of the numbers of excess carriers reaching the dot, i.e. , altering the quantum dot population. The characteristic shift of emission energies visualize the renormalization of the ground-state energy of the single dot, and the intensity ratio of the excitonic recombinations verifies this change of the occupation and the state-filling.