Band alignment at GaxIn1-xP/AlyIn1-yP alloy interfaces from hybrid density functional theory calculations

The composition dependence of the natural band alignment at the GaxIn1-xP/AlyIn1-yP alloy interface is investigated via hybrid functional based density functional theory. The direct-indirect crossover for the GaxIn1-xP and AlyIn1-yP alloys is calculated to occur for x = 0.9 and y = 0.43. The calculated GaxIn1-xP/AlyIn1-yP interface band alignment shows a crossover from type-I to type-II with increasing Ga content x. The valence band offset is essentially positive irrespective of the alloy compositions, and amounts up to 0.56 eV. The conduction band offset varies between −0.85 and 1.16 eV

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

Photosynthesis is nature’s 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

Enhancement of the Zero Phonon Line emission from a Single NV-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 towards future diamond-based integrated quantum optical devices

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 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