Henning Döscher

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.

Sunlight absorption in water – efficiency and design implications for photoelectrochemical devices

Sunlight absorption in water has a critical impact on solar fuel generation by direct photoelectrolysis because devices are commonly illuminated through the aqueous electrolyte. We show the relevant reference spectra, calculate fundamental solar-to-hydrogen efficiency prospects, and discuss the design implications for unassisted solar water-splitting devices.

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...

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.

Solar-to-hydrogen efficiency: shining light on photoelectrochemical device performance

Illumination characteristics from artificial sources strongly influence the experimental performance of solar water-splitting devices, with the highest impact on tandem structures designed for optimum conversion efficiency. We highlight quantitative and qualitative flaws of common characterization techniques, discuss their impact on research results and strategy, and demonstrate approaches toward advanced measurement accuracy.

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.

Domain-sensitive in situ observation of layer-by-layer removal at Si(100) in H2 ambient

Double-layer step formation on Si(100) substrates is a crucial prerequisite for antiphase-domain free III?V compound semiconductor heteroepitaxy. Due to its unequaled relevance in microelectronics, the (100) oriented surface of silicon is by far the most studied semiconductor surface. However, Si(100) preparation in hydrogen process gas ambient, which is commonly employed for Si and III?V device preparation, is completely different from preparation in ultra-high vacuum due to strong interaction between H2 and the Si surface, leading to a kinetically driven different step formation. Here, we observe chemical layer-by-layer removal of surface atoms from the terraces at the Si(100) surface during annealing in hydrogen ambient. Mutually perpendicularly oriented dimers on subsequently removed monolayers induce oscillations in the in situ reflection anisotropy spectroscopy (RAS) signal. Scanning tunneling microscopy measurements support a model, where surface atom removal proceeds by formation and anisotropic expansion of vacancy islands on the terraces. We determined an activation energy Ed of 2.75???0.20?eV for Si etching in H2 ambient by transient in situ RAS measurements. In situ control of the highly reactive Si(100) surface preparation is essential for subsequent defect-free III?V heteroepitaxy.

Si(100) surfaces in a hydrogen-based process ambient

We studied the atomic surface properties of Si(100) during preparation in a (metal-organic) vapor phase epitaxy (VPE) reactor and the impact of the hydrogen ambient. Absorption lines in Fourier-transform infrared spectra were identified as stretch modes of coupled Si–H monohydrides, in agreement with Si-dimers observed by scanning tunneling microscopy. The polarization dependence of the antisymmetric stretch mode distinguished different dimer orientations and verified a clear preference for one of the (2×1)/(1×2) surface reconstruction domains. Tip-induced H-desorption proved the complete saturation of dangling bonds after VPE-preparation. In situ reflectance anisotropy spectroscopy showed the absence of Si–H bonds at elevated annealing temperature.

Quantitative investigation of hydrogen bonds on Si(100) surfaces prepared by vapor phase epitaxy

The authors investigated Si(100) surfaces prepared by vapor phase epitaxy (VPE) using Fourier transform infrared spectroscopy (FTIR) in an attenuated total reflection configuration and low energy electron diffraction (LEED). They detected the symmetric and antisymmetric stretch modes of the H–Si–Si–H monohydrides using FTIR in agreement with the associated (2×1)/(1×2) LEED patterns. Polarized FTIR measurements verified the surface character of the observed hydrogen bonds. Exchanging the process gas in our VPE reactor to argon at an intermediate temperature of around 700 °C showed the impact of the hydrogen ambient during the cooling phase at the end of the process. The authors were able to obtain a strong preference of one of the two possible surface domains by variation of the cooling procedure and quantified the domain ratio by comparison of the absorption due to the antisymmetric modes in polarized spectra parallel and perpendicular to the plane of incidence.