Thomas Hannappel

Enhancement of the zero phonon line emission from a single nitrogen vacancy center in a nanodiamond via coupling to a photonic crystal cavity

Using a nanomanipulation technique a nanodiamond with a single nitrogen vacancy center is placed directly on the surface of a gallium phosphide photonic crystal cavity. A Purcell-enhancement of the fluorescence emission at the zero phonon line (ZPL) by a factor of 12.1 is observed. The ZPL coupling is a first crucial step toward future diamond-based integrated quantum optical devices.

Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure.

Photosynthesis is natures route to convert intermittent solar irradiation into storable energy, while its use for an industrial energy supply is impaired by low efficiency. Artificial photosynthesis provides a promising alternative for efficient robust carbon-neutral renewable energy generation. The approach of direct hydrogen generation by photoelectrochemical water splitting utilizes customized tandem absorber structures to mimic the Z-scheme of natural photosynthesis. Here a combined chemical surface transformation of a tandem structure and catalyst deposition at ambient temperature yields photocurrents approaching the theoretical limit of the absorber and results in a solar-to-hydrogen efficiency of 14%. The potentiostatically assisted photoelectrode efficiency is 17%. Present benchmarks for integrated systems are clearly exceeded. Details of the in situ interface transformation, the electronic improvement and chemical passivation are presented. The surface functionalization procedure is widely applicable and can be precisely controlled, allowing further developments of high-efficiency robust hydrogen generators.

Apparatus for investigating metalorganic chemical vapor deposition-grown semiconductors with ultrahigh-vacuum based techniques

An apparatus is described here in detail for the transfer of a sample from a metalorganic chemical vapor deposition (MOCVD) reactor to an ultrahigh-vacuum (UHV) chamber without introducing any contamination. The surface of the sample does not change during transfer as is borne out by the identical reflectance difference (RD) spectrum measured first in the MOCVD reactor, i.e., in situ, and afterwards again in the UHV chamber. Making use of the earlier apparatus a semiconductor can be grown in the MOCVD reactor and can afterwards be investigated with any desired tool of surface science, in particular also those that require UHV. All the data collected in UHV can be identified with the RD spectrum measured already in the MOCVD reactor. Several examples are presented here for data collection in UHV on III–V semiconductors grown in the MOCVD reactor. They illustrate the ease and reliability of the here described apparatus for contamination-free sample transfer. Signals are presented in particular for the genui...

In situ reflection anisotropy spectroscopy analysis of heteroepitaxial GaP films grown on Si(100)

In situ reflection anisotropy spectroscopy (RAS)/reflection difference spectroscopy was applied as a quantitative probe of antiphase domains in heteroepitaxial films deposited on Si(100). The in situ probe was deduced from the spectroscopic signature of the P-rich, homoepitaxial GaP(100) surface and its well-established atomic reconstruction via a comparative investigation using RAS (homoepitaxial versus heteroepitaxial). For that, we determined changes in temperature, surface reconstruction, atomic order, and excess phosphorus on the surface of the homoepitaxial GaP(100) samples to specifically change the RA spectra in terms of shape and intensity. According to the presence of antiphase disorder a linear reduction in the RAS signal occurred. In addition, RA spectra of the heteroepitaxially prepared GaP/Si(100) films contained characteristic deviations from RA spectra of homoepitaxial GaP(100). They originated from reflections at the additional GaP/Si(100) heterointerface. Simple interference affecting th...

Comparison of Direct Growth and Wafer Bonding for the Fabrication of GaInP/GaAs Dual-Junction Solar Cells on Silicon

Two different process technologies were investigated for the fabrication of high-efficiency GaInP/GaAs dual-junction solar cells on silicon: direct epitaxial growth and layer transfer combined with semiconductor wafer bonding. The intention of this research is to combine the advantages of high efficiencies in III-V tandem solar cells with the low cost of silicon. Direct epitaxial growth of a GaInP/GaAs dual-junction solar cell on a GaAsyP1-y buffer on silicon yielded a 1-sun efficiency of 16.4% (AM1.5g). Threading dislocations that result from the 4% lattice grading are still the main limitation to the device performance. In contrast, similar devices fabricated by semiconductor wafer bonding on n-type inactive Si reached efficiencies of 26.0% (AM1.5g) for a 4-cm2 solar cell device.

Photoelectrocatalysis: principles, nanoemitter applications and routes to bio-inspired systems

An overview on processes that are relevant in light-induced fuel generation, such as water photoelectrolysis or carbon dioxide reduction, is given. Considered processes encompass the photophysics of light absorption, excitation energy transfer to catalytically active sites and interfacial reactions at the catalyst/solution phase boundary. The two major routes envisaged for realization of photoelectrocatalytic systems, e.g. bio-inspired single photon catalysis and multiple photon inorganic or hybrid tandem cells, are outlined. For development of efficient tandem cell structures that are based on non-oxidic semiconductors, stabilization strategies are presented. Physical surface passivation is described using the recently introduced nanoemitter concept which is also applicable in photovoltaic (solid state or electrochemical) solar cells and first results with p-Si and p-InP thin films are presented. Solar-to-hydrogen efficiencies reach 12.1% for homoepitaxial InP thin films covered with Rh nanoislands. In the pursuit to develop biologically inspired systems, enzyme adsorption onto electrochemically nanostructured silicon surfaces is presented and tapping mode atomic force microscopy images of heterodimeric enzymes are shown. An outlook towards future envisaged systems is given.

In situ verification of single-domain III-V on Si(100) growth via metal-organic vapor phase epitaxy

Reflectance anisotropy spectroscopy (RAS) was used in situ for the quantification of antiphase domains on surfaces of thin GaP films deposited onto Si(100) by metal-organic vapor phase epitaxy (MOVPE). The preparation of a single-domain GaP∕Si(100) surface was determined via the analysis of RAS peak intensities in reference to the well-known P-rich surface reconstruction of homoepitaxially grown GaP(100). Both preprocessed Si(100) substrates and MOVPE as-grown GaP∕Si(100) films were also characterized ex situ by atomic force microscopy to identify the formation of mono- and diatomic surface steps and to analyze of the domain distribution, respectively.

RDS, LEED and STM of the P-rich and Ga-rich surfaces of GaP(100)

Abstract Reflectance difference spectroscopy was measured in the metal organic chemical vapor deposition reactor and also in UHV at 20 K. It revealed a characteristic negative peak at the low energy side that was indicative of the specific surface reconstruction. This peak disappeared completely if the sample was kept within a narrow intermediate temperature range. At 20 K the negative peak appeared at 2.4 eV for the Ga-terminated (2×4)-reconstructed surface and at 2.6 eV for the P-terminated (2×1)/(2×2)-reconstructed surface. RDS for the two different surface reconstructions displayed strong structures also in the range of the bulk transitions. A characteristic zig-zag pattern was observed in the STM image of the P-terminated surface.

Epitaxial III–V Films and Surfaces for Photoelectrocatalysis

Efficient photoelectrochemical devices for water splitting benefit from the highest material quality and dedicated surface preparation achieved by epitaxial growth. InP(100)-based half-cells show significant solar-to-hydrogen efficiencies, but require a bias due to insufficient voltage. Tandem absorber structures may provide both adequate potential and efficient utilization of the solar spectrum. We propose epitaxial dilute nitride GaPNAs photocathodes on Si(100) substrates to combine close-to-optimum limiting efficiency, lattice-matched growth, and established surface preparation. Prior to a discussion of the challenging III-V/Si(100) heterojunction, we describe the closely related epitaxial preparation of InP(100) surfaces and its beneficial impact on photoelectrochemical water-splitting performance. Analogies and specific differences to GaP(100) surfaces are discussed based on in situ reflectance anisotropy and on two-photon photoemission results. Preliminary experiments regarding GaP/Si(100) photoelectrochemistry and dilute nitride GaPN heteroepitaxy on Si(100) confirm the potential of the GaPNAs/Si tandem absorber structure for future water-splitting devices.

In situ investigation of hydrogen interacting with Si(100)

Silicon surfaces are subject to intense interaction with hydrogen ambient common in vapor phase epitaxy. We distinguish characteristic configurations of vicinal Si(100) by in situ reflectance anisotropy spectroscopy: covered by protective oxides, cleaned by thermal annealing, and the formation of monohydrides during cooling. Even above 1000 K, most dangling bonds of the (2×1)-reconstructed surface are saturated by hydrogen, while stability of Si–H bonds in the process gas ambient requires temperatures well below 750 K. Adjustment of hydrogen coverage employing alternative process gases provides experimental access to hydrogen adsorption and desorption characteristics valid for annealing in vapor phase epitaxy ambient.