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

Optical in situ monitoring of hydrogen desorption from Ge(100) surfaces

Molecular hydrogen strongly interacts with vicinal Ge(100) surfaces during preparation in a metal organic vapor phase epitaxy reactor. According to X-ray photoemission spectroscopy and Fourier-transform infrared spectroscopy results, we identify two characteristic reflection anisotropy (RA) spectra for H-free and monohydride-terminated vicinal Ge(100) surfaces. RAS allows in situ monitoring of the surface termination and enables spectroscopic hydrogen kinetic desorption studies on the Ge(100) surface. Comparison of evaluated values for the activation energy and the pre-exponential factor of H desorption evaluated at different photon energies reflects that H unevenly affects the shape of the RA spectrum

MOVPE growth of GaP/GaPN core-shell nanowires: N incorporation, morphology and crystal structure

Dilute nitride III-V nanowires (NWs) possess great potential as building blocks in future optoelectronical and electrochemical devices. Here, we provide evidence for the growth of GaP/GaPN core-shell NWs via metalorganic vapor phase epitaxy, both on GaP(111)B and on GaP/Si (111) hetero-substrates. The NW morphology meets the common needs for use in applications, i.e. they are straight and vertically oriented to the substrate as well as homogeneous in length. Moreover, no parasitical island growth is observed. Nitrogen was found to be incorporated on group V sites as determined from transmission electron microscopy (TEM) and Raman spectroscopy. Together with the incorporation of N, the NWs exhibit strong photoluminescence in the visible range, which we attribute to radiative recombination at N-related deep states. Independently of the N incorporation, a peculiar facet formation was found, with {110} facets at the top and {112} at the bottom of the NWs. TEM reveals that this phenomenon is related to different stacking fault densities within the zinc blende structure, which lead to different effective surface energies for the bottom and the top of the NWs.This work was supported by the Deutsche Forschungsgemeinschaft (DFG, proj. no. HA 3096/4-2 & DA 396/6-2). We thank D Roßberg and D Flock for preparation of the TEM lamellae via FIB, as well as A Müller for technical support of the MOVPE system and W Dziony for AES measurements. We appreciate fruitful discussions with A Paszuk and A Nägelein

Impact of Rotational Twin Boundaries and Lattice Mismatch on III-V Nanowire Growth

Pseudomorphic planar III-V transition layers greatly facilitate the epitaxial integration of vapor liquid solid grown III-V nanowires (NW) on Si(111) substrates. Heteroepitaxial (111) layer growth, however, is commonly accompanied by the formation of rotational twins. We find that rotational twin boundaries (RTBs), which intersect the surface of GaP/Si(111) heterosubstrates, generally cause horizontal NW growth and may even suppress NW growth entirely. Away from RTBs, the NW growth direction switches from horizontal to vertical in the case of homoepitaxial GaP NWs, whereas heteroepitaxial GaAs NWs continue growing horizontally. To understand this rich phenomenology, we develop a model based on classical nucleation theory. Independent of the occurrence of RTBs and specific transition layers, our model can generally explain the prevalent observation of horizontal III V NW growth-in lattice mismatched systems and the high crystal quality of horizontal nanowires.This work was financially supported by the BMBF (Project No. 03SF0404A) and partly by the Spanish Ministry of Economy (Project TEC2014-54260-C3-2-P). C.K. and L.W. acknowledge the Thuringia Graduate School for Photovoltaics “Photograd” for financial support. The authors would like to thank A. Paszuk and A. Nagelein for valuable discussions as well as A. Muller and M. Biester for technical support, T. Nieszner for supporting the determination of the spatial direction of NWs, and D. Roßberg for preparing the TEM lamella

In situ control of Si(100) and Ge(100) surface preparation for the heteroepitaxy of III-V solar cell architectures

Si(100) and Ge(100) substrates essential for subsequent III-V integration were studied in the hydrogen ambient of a metalorganic vapor phase epitaxy reactor. Reflectance anisotropy spectroscopy (RAS) enabled us to distinguish characteristic configurations of vicinal Si(100) in situ: covered with oxide, cleaned by thermal removing in H2, and terminated with monohydrides when cooling in H2 ambient. RAS measurements during cooling in H2 ambient after the oxide removal process revealed a transition from the clean to the monohydride terminated Si(100) surface dependent on process temperature. For vicinal Ge(100) we observed a characteristic RA spectrum after annealing and cooling in H2 ambient. According to results from X-ray photo electron spectroscopy and Fourier-transform infrared spectroscopy the spectrum corresponds to the monohydride terminated Ge(100) surface

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

In situ control of As dimer orientation on Ge(100) surfaces

We investigated the preparation of single domain Ge(100):As surfaces in a metal-organic vapor phase epitaxy reactor. In situ reflection anisotropy spectra (RAS) of vicinal substrates change when arsenic is supplied either by tertiarybutylarsine or by background As4 during annealing. Low energy electron diffraction shows mutually perpendicular orientations of dimers, scanning tunneling microscopy reveals distinct differences in the step structure, and x-ray photoelectron spectroscopy confirms differences in the As coverage of the Ge(100): As samples. Their RAS signals consist of contributions related to As dimer orientation and to step structure, enabling precise in situ control over preparation of single domain Ge(100): As surfaces

In situ control of the GE(100)surface domain structure for III-V multijunction solar cells

Vicinal Ge(100) is the common substrate for state of the art multi-junction solar cells grown by metal-organic vapor phase epitaxy (MOVPE). While triple junction solar cells based on Ge(100) present efficiencies mayor que 40%, little is known about the microscopic III-V/Ge(100) nucleation and its interface formation. A suitable Ge(100) surface preparation prior to heteroepitaxy is crucial to achieve low defect densities in the III-V epilayers. Formation of single domain surfaces with double layer steps is required to avoid anti-phase domains in the III-V films. The step formation processes in MOVPE environment strongly depends on the major process parameters such as substrate temperature, H2 partial pressure, group V precursors [1], and reactor conditions. Detailed investigation of these processes on the Ge(100) surface by ultrahigh vacuum (UHV) based standard surface science tools are complicated due to the presence of H2 process gas. However, in situ surface characterization by reflection anisotropy spectroscopy (RAS) allowed us to study the MOVPE preparation of Ge(100) surfaces directly in dependence on the relevant process parameters [2, 3, 4]. A contamination free MOVPE to UHV transfer system [5] enabled correlation of the RA spectra to results from UHV-based surface science tools. In this paper, we established the characteristic RA spectra of vicinal Ge(100) surfaces terminated with monohydrides, arsenic and phosphorous. RAS enabled in situ control of oxide removal, H2 interaction and domain formation during MOVPE preparation

In situ controlled heteroepitaxy of single-domain GaP on As-modified Si(100)

Metalorganic vapor phase epitaxy of III-V compounds commonly involves arsenic. We study the formation of atomically well-ordered, As-modified Si(100) surfaces and subsequent growth of GaP/Si(100) quasisubstrates in situ with reflection anisotropy spectroscopy. Surface symmetry and chemical composition are measured by low energy electron diffraction and X-ray photoelectron spectroscopy, respectively. A twostep annealing procedure of initially monohydride-terminated, (1 × 2) reconstructed Si(100) in As leads to a predominantly (1 × 2) reconstructed surface. GaP nucleation succeeds analogously to As-free systems and epilayers free of antiphase disorder may be grown subsequently. The GaP sublattice orientation, however, is inverted with respect to GaP growth on monohydride-terminated Si(100)

The interface of GaP(100) and H_2O studied by photoemission and reflection anisotropy spectroscopy

We study the initial interaction of adsorbed H_2O 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