Sergey Sadofev

Electronic coupling of optical excitations in organic/inorganic semiconductor hybrid structures

The epitaxial growth of small conjugated molecules on ZnO-based surfaces is studied. A weak substrate interaction allows for the preparation of organic layers with well-defined morphology and electronically intact interfaces without the need for extra passivation. Nonradiative energy transfer from inorganic quantum wells to various molecules is identified by optical spectroscopy. The strength of the dipole-dipole mediated coupling between Wannier and Frenkel excitons is as large as 2meV. In hybrid structures with type-II energy level alignment, charge separation occurs at the organic/inorganic interface as well. These findings render organic/ZnO hybrid structures interesting for light-emitting as well as photovoltaic applications benefiting from favorable properties of both material classes.

Molecular beam epitaxy of n-Zn(Mg)O as a low-damping plasmonic material at telecommunication wavelengths

We demonstrate that Zn(Mg)O:Ga layers can be grown by molecular beam epitaxy in a two-dimensional mode with high structural perfection up to Ga mole fractions of about 6.5%. The doping efficiency is practically 100% so that free-carrier concentrations of almost 1021 cm−3 can be realized providing a zero-crossover wavelength of the real part of the dielectric function as short as 1.36 μm, while the plasmonic damping does not exceed 50 meV. Structural, electrical, and optical data consistently demonstrate a profound change of the Ga incorporation mode beyond concentrations of 1021 cm−3 attended by deterioration of the plasmonic features.

Strong Coupling between Surface Plasmon Polaritons and Molecular Vibrations

The hybridization of different quasi-particles is an extensively studied subject; both from a practical as well as fundamental point of view. The resultant hybrid excitations exhibit new properties that are not available by the isolated constituents.

Ultrafast Nonlinear Response of Bulk Plasmons in Highly Doped ZnO Layers.

Longitudinal bulk plasmons in an n-doped ZnO layer system are studied by two-color femtosecond pump-probe spectroscopy in the midinfrared. The optical bulk plasmon resonance identified in linear reflectivity spectra undergoes a strong redshift and a limited broadening upon intraband excitation of electrons. The nonlinear changes of plasmon absorption decay on a time scale of 2 ps and originate from the intraband redistribution of electrons. Theoretical calculations explain the plasmon redshift by the transient increase of the ensemble-averaged electron mass and the concomitantly reduced plasma frequency in the hot electron plasma. The observed bulk plasmon nonlinearity holds strong potential for applications in plasmonics.

An Inorganic/Organic Semiconductor “Sandwich” Structure Grown by Molecular Beam Epitaxy

2009 WILEY-VCH Verlag Gmb Inorganic/organic semiconductor hybrid structures offer new functionalities that can not be achieved by the individual components alone. In order to benefit from the complementary properties of the two material systems, electronic coupling across the inorganic/organic interface is required. Nonradiative energy transfer as well as charge-carrier separation could be observed on properly designed specimens. These findings are not only interesting from a fundamental point of view, but are also of direct practical relevance. Combination of the high carrier mobility in inorganic semiconductors with the very strong and easily wavelength-tuneable absorption-emission features of organic molecules opens up new vistas for light-emitting or photovoltaic devices. Two approaches regarding the growth of semiconductor hybrid structures are pursued currently. One approach employs wet-chemically produced semiconductor nanocrystals, the other relies on epitaxial semiconductor quantum well (QW) structures. For the latter, ZnO/ZnMgO, InGaN/GaN, and GaAs/ AlGaAs have been employed successfully. High purity standards and excellent structural control are merits of epitaxial growth. However, the epitaxial hybrid structures fabricated so far have in common that a sole organic layer is merely deposited on top of the inorganic QW. Subsequent overgrowth by the inorganic material appeared to be impossible because typical temperatures applied in semiconductor epitaxy range between about 500 and 1000 8C and are thus not compatible with organic molecules. As found out recently, ZnO and some of its ternaries are remarkable exceptions as these compounds can be grown with high quality by molecular beam epitaxy (MBE) at temperatures as low as room temperature. Exploiting this unique potential, all-epitaxial periodic organic/inorganic hybrid structures come into reach. There are several crucial points, however, which have to be solved in order to prepare such hybrid structures. First, the molecules must survive the overgrowth with their electronic and optical properties remaining unchanged. ZnO is grown by radical-source MBE with the oxygen being provided by an rf-plasma source that generates a flux of highly reactive atomic species on the sample surface. It must be assured that the molecules survive such a harsh environment. Second, electronic coupling between the subcomponents must persist and, third, the inorganic overlayer should preferably grow in a coherent epitaxial mode. In this work, we will concentrate on the first two points. Moreover, we will demonstrate that ZnO/organic/ZnO sandwich structures form planar waveguides that support stimulated emission of the enclosed organic layer. The inorganic/organic sandwich (IOS) structures are grown under ultrahigh vacuum conditions in aMBE apparatus equipped with interconnected growth chambers for the two material systems. This ensures well-defined organic/inorganic interfaces free of extrinsic defects. For the inorganic part below the organic layer, the standard epitaxy regime is employed. At first, a 30-nm-thick nucleation layer, either ZnO or ZnMgO, is deposited at TS1⁄4 280 8C on an a-plane sapphire substrate and subsequently annealed at 560 8C. Then, a ZnO epilayer or ZnO/ZnMgO QW structure is grown at TS1⁄4 350 8C. Additional annealing steps (TS1⁄4 560 8C) at each side of the QW are introduced to assure atomically abrupt interfaces. The part atop the organic layer is a ZnO film with a typical thickness of 50–200 nm. For its growth, the substrate temperature is lowered to TS 100 8C. In the absence of the organic layer, a two-dimensional layer-by-layer mode is indeed established at such low temperatures. As depicted in Figure 1a, clear RHEED (Reflection high-energy electron diffraction) oscillations in the specular beam intensity are observed each of which corresponding to the deposition of one monolayer. Furthermore, the streaky RHEED diffraction pattern in the inset of Figure 1a is indicative of high crystalline perfection and an atomically flat surface. As organic molecule we chose 2,7-bis(biphenyl-4-yl)20,70-di-tert-butyl-9,90-spirobifluorene (SP6) with the chemical structure shown in Figure 1b. SP6 is amorphous in the condensed phase and considered as a promising candidate for organic solid-state lasers. The choice of SP6 is also triggered by the previous observation of strong excitonic coupling with ZnO/ ZnMgO QW structures. The molecules are deposited on the inorganic underlay at TS1⁄4 20 8C at a growth rate of 0.1 nmmin 1 resulting in closed and homogeneous films, both on ZnO and ZnMgO. The amorphous character of SP6 makes it unlikely that single-crystalline overgrowth of ZnO can be accomplished. Indeed, the RHEED pattern features Debye rings indicating that the ZnO atop of SP6 is polycrystalline. On the other hand, AFM images taken on completed ZnO/SP6/ZnO structures reveal that ZnO films with a very smooth surface morphology are formed. In the example depicted in Figure 1c, the root mean square roughness is only 0.7 nm. Therefore, though polycrystalline, the growth process is well-defined endorsing the choice of SP6 also from this point of view. The specific design used for assessing the viability of the ZnO/ SP6/ZnO IOS consists of a ZnO/ZnMgO QW structure followed by 5-nm-thick SP6 and 150-nm-thick ZnO (TS1⁄4 100 8C). The ZnMgO barrier below the QW is 600 nm thick, while the upper

Growth of wurtzite InN on bulk In2O3(111) wafers

A single phase InN epitaxial film is grown on a bulk In2O3(111) wafer by plasma-assisted molecular beam epitaxy. The InN/In2O3 orientation relationship is found to be (0001) ‖ (111) and [11¯00] ‖ [112¯]. High quality of the layer is confirmed by the small widths of the x-ray rocking curves, the sharp interfaces revealed by transmission electron microscopy, the narrow spectral width of the Raman E2h vibrational mode, and the position of the photoluminescence band close to the fundamental band gap of InN.

Demonstration of hyperbolic metamaterials at telecommunication wavelength using Ga-doped ZnO.

Hyperbolic metamaterials (HMMs) have attracted much attention because they allow for broadband enhancement of spontaneous emission and imaging below the diffraction limit. However, HMMs with traditional metals as metallic component are not suitable for applications in the infrared spectral range. Using Ga-doped ZnO, we demonstrate monolithic HMMs operating at infrared wavelengths. We identify the materials hyperbolic character by various optical measurements in combination with theoretical calculations. In particular, negative refraction of the extraordinary wave and propagation of light with wave vector values exceeding that of free-space are demonstrated in the entire telecommunication window. These findings reveal a considerable potential for creating novel functional elements at telecommunication wavelengths.

Cascade energy transfer versus charge separation in ladder-type oligo(p-phenylene)/ZnO hybrid structures for light-emitting applications

Usability of inorganic/organic semiconductor hybrid structures for light-emitting applications can be intrinsically limited by an unfavorable interfacial energy level alignment causing charge separation and nonradiative deactivation. Introducing cascaded energy transfer funneling away the excitation energy from the interface by transfer to a secondary acceptor molecule enables us to overcome this issue. We demonstrate a substantial recovery of the light output along with high inorganic-to-organic exciton conversion rates up to room temperature.

Uniform and Efficient UV-emitting ZnO/ZnMgO Multiple Quantum Wells Grown by Radical-Source Molecular Beam Epitaxy

Five-fold stacked ZnO/ZnMgO quantum wells are fabricated by radical-source molecular beam epitaxy on a-plane sapphire, employing low-temperature growth for the ternary component and appropriate annealing steps performed at each interface. Transmission electron microscopy images reveal that the ZnO/ZnMgO interfaces are abrupt and smooth on an atomic scale. Threading dislocations originating from the interfacial region between substrate and the ZnMgO nucleation layer are largely annihilated during growth of a subsequent 600 nm thick ZnMgO buffer. The residual dislocation density in the well region is sufficiently low to allow for efficient exciton emission up to room temperature.

Nanoscale transport of excitons at the interface between ZnO and a molecular monolayer

Time-resolved near-field optical microsopy maps exciton transport in a hybrid system. Within the 100 ps photoluminescence lifetime, an equilibrium distribution of surface and bound excitons displays lateral diffusion on a 50 nm length scale.