Christian Höhn

The interface of GaP(100) and H2O studied by photoemission and reflection anisotropy spectroscopy

We study the initial interaction of adsorbed H2O with P-rich and Ga-rich GaP(100) surfaces. Atomically well defined surfaces are prepared by metal-organic vapour phase epitaxy and transferred contamination-free to ultra-high vacuum, where water is adsorbed at room temperature. Finally, the surfaces are annealed in vapour phase ambient. During all steps, the impact on the surface properties is monitored with in situ reflection anisotropy spectroscopy (RAS). Photoelectron spectroscopy and low-energy electron diffraction are applied for further in system studies. After exposure up to saturation of the RA spectra, the Ga-rich (2 × 4) surface reconstruction exhibits a sub-monolayer coverage in form of a mixture of molecularly and dissociatively adsorbed water. For the p(2 × 2)/c(4 × 2) P-rich surface reconstruction, a new c(2 × 2) superstructure forms upon adsorption and the uptake of adsorbate is significantly reduced when compared to the Ga-rich surface. Our findings show that microscopic surface reconstructions of GaP(100) greatly impact the mechanism of initial interface formation with water, which could benefit the design of e.g. photoelectrochemical water splitting devices.

Formation of GaP/Si(100) Heterointerfaces in the Presence of Inherent Reactor Residuals.

Adequate silicon preparation is a prerequisite for defect-free III-V growth on Si. We transfer the silicon processing from clean to GaP containing metalorganic vapor phase epitaxy reactors, where we monitor the entire process in situ with reflection anisotropy spectroscopy and analyze the chemical composition of the surface with X-ray photoelectron spectroscopy. Beyond a certain submonolayer threshold value of (Ga,P) residuals found on the Si(100) surface, GaP grows with an inverted majority sublattice. Analogously to III-V growth on two-domain substrates, the coexistence of Si-Ga and Si-P interfacial bonds at terraces of the same type causes antiphase disorder in GaP epilayers.

Materials for light-induced water splitting: In situ controlled surface preparation of GaPN epilayers grown lattice-matched on Si(100)

Energy storage is a key challenge in solar-driven renewable energy conversion. We promote a photochemical diode based on dilute nitride GaPN grown lattice-matched on Si(100), which could reach both high photovoltaic efficiencies and evolve hydrogen directly without external bias. Homoepitaxial GaP(100) surface preparation was shown to have a significant impact on the semiconductor-water interface formation. Here, we grow a thin, pseudomorphic GaP nucleation buffer on almost single-domain Si(100) prior to GaPN growth and compare the GaP_(0.98)N_(0.02)/Si(100) surface preparation to established P- and Ga-rich surfaces of GaP/Si(100). We apply reflection anisotropy spectroscopy to study the surface preparation of GaP_(0.98)N_(0.02) in situ in vapor phase epitaxy ambient and benchmark the signals to low energy electron diffraction, photoelectron spectroscopy, and x-ray diffraction. While the preparation of the Ga-rich surface is hardly influenced by the presence of the nitrogen precursor 1,1-dimethylhydrazine (UDMH), we find that stabilization with UDMH after growth hinders well-defined formation of the V-rich GaP_(0.98)N_(0.02)/Si(100) surface. Additional features in the reflection anisotropy spectra are suggested to be related to nitrogen incorporation in the GaP bulk.

Lifetime and performance of InGaAsP and InGaAs absorbers for low bandgap tandem solar cells

Time resolved photoluminescence (TRPL) measurements were used to evaluate the lifetimes of the low bandgap absorber materials InGaAsP (1.03 eV) and InGaAs (0.73 eV) embedded between InP barriers. A low bandgap tandem solar cell based on these absorber materials has been developed. The cell is designed to work below an InGaP / GaAs high bandgap tandem solar cell. Tandem solar cells grown with these absorber materials reached efficiencies above 10% (in-house) below a 4-¿m-thick GaAs filter under 35 suns concentration.

Water-induced modifications of GaP(100) and InP(100) surfaces studied by photoelectron spectroscopy and reflection anisotropy spectroscopy

In this work, we investigate the initial interaction of water and oxygen with different surface reconstructions of GaP(100) applying photoelectron spectroscopy, low-energy electron diffraction, and reflection anisotropy spectroscopy. Surfaces were prepared by metal-organic vapour phase epitaxy, transferred to ultra-high vacuum, and exposed to oxygen or water vapour at room temperature. The (2 4) reconstructed, Ga-rich surface is more sensitive and reactive to adsorption, bearing a less ordered surface reconstruction upon exposure and indicating a mixture of dissociative and molecular water adsorption. The p(2 2)=c(4 2) P-rich surface, on the other hand, is less reactive, but shows a new surface symmetry after water adsorption. Correlating findings of photoelectron spectroscopy with reflection anisotropy spectroscopy could pave the way towards optical in-situ monitoring of electrochemical surface modifications with reflection anisotropy spectroscopy.

An experimental-theoretical atomic-scale study - in situ analysis of III–V on Si(100) growth for hybrid solar cells

We consider GaP/Si(100) as quasi-substrate for III-V-on-silicon growth targeting solar energy exploration in dual junction devices for both photovoltaics as well as photoelectrochemical tandem diodes with optimum bandgaps. We prepare Si(100) surfaces with majority domains of either type, grow thin GaP layers free of anti-phase disorder, find that abrupt Si-P interfaces are favored over abrupt Si-Ga interfaces and, finally, observe an RAS signal attributed to N incorporation in GaPN/Si(100). Combining in situ reflection anisotropy spectroscopy during metalorganic vapor phase epitaxy with UHV-based surface techniques and ab initio DFT calculations, we aim to understand the interface formation at the atomic scale.