In contrast to organic reactions, which can almost always be described in terms of a single multiplicity, in organometallic systems, quite often more than one state may be involved. The phenomenon of two states of different multiplicities that determine the minimum-energy pathway of a reaction is classified as two-state reactivity (TSR). As an example, the ion/molecule reactions of 'bare' transition-metal-monoxide cations with dihydrogen and hydrocarbons have been analyzed in terms of the corresponding potential-energy hypersurfaces. It turns out that, besides classical factors, such as the barrier heights, the spin-orbit coupling factor is essential, since curve crossing between the high-and low-spin states constitutes a distinct mechanistic step along the reaction coordinates. Thus, TSR may evolve as a new paradigm for describing the chemistry of coordinatively unsaturated transition-metal complexes. This concept may contribute to the understanding of organometallic chemistry in general and for the development of oxidation catalysts in particular.Introduction. -The role of concepts which can provide new insight in chemical problems can hardly be overemphasized, and the present article describes the two-statereactivity (TSR) paradigm and its manifestation in H-H and C-H bond activation by 'bare' 0x0-cations MO+ of the late, first-row transition metals Mn-Cu [l] [2].With a few notable exceptions, for example 0, and CH,, organic species generally possess low-spin ground states, and their reactions proceed on a single potential energy surface; this will be referred to as single-state reactivity (SSR). To conceptualize the key features of SSR, it is in principle sufficient to understand the transition structures (TSs), their entropic requirements, and the associated barrier heights. Indeed, the arsenal of reactivity paradigms that has evolved in organic chemistry, such as linear free energy relationships [3], frontier molecular orbital theory [4], the Woodward-Hoffmann rules [5], and the valence bond (VB) crossing diagrams [6], all are related to SSR. Thus, our intuition derives from the experience with surfaces of a single multiplicity. In fact, even photochemical reactivity in the sense of cleavage and formation of chemical bonds is basically treated on a single (though excited) potential-energy hypersurface. Nevertheless, it is clear that SSR is only one aspect in chemical reactivity.Coordinatively unsaturated transition-metal compounds often possess high-spin ground states and nearby low-spin excited states [7-91. As a result of the adjacency of these spin states, the reactivity of these compounds may generally involve (at least) two states in which the ground state must not necessarily be the most reactive one. TSR is characterized by a crossing of two potential-energy hypersurfaces of different multiplic-

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A Sn-doped (100) β-Ga2O3 epitaxial layer was grown via metal-organic vapor phase epitaxy (MOVPE) onto a singlecrystal, Mg-doped semi-insulating (100) β-Ga2O3 substrate. Ga2O3-based Metal-Oxide-Semiconductor Field effect Transistors (MOSFETs) with a 2 µm gate length (LG), 3.4 µm source-drain spacing (LSD) and 0.6 µm gate-drain spacing (LGD) were fabricated and characterized. Devices were observed to hold a gate-to-drain voltage of 230 V in the off-state. The gate-to-drain electric field corresponds to 3.8 MV/cm, which is the highest reported for any transistor and surpassing bulk GaN and SiC theoretical limits. Further performance projections are made based on layout, process, and material optimizations to be considered in future iterations.

The potential energy surface (PES) corresponding to the reaction of the iron cation with ethane, which represents a prototype of the activation of C−C and C−H bonds in alkanes by transition metal cations, has been investigated employing the recently suggested hybrid density functional theory/Hartree−Fock method (B3LYP) combined with reasonably large one-particle basis sets. The performance of this computational approach has been calibrated against experimentally known Fe+−R binding energies of fragments R relevant to the [Fe,C2,H6]+ PES and against the relative energies of the possible exit channels. Both the C−C and C−H bond activation branches of the PES are characterized by a low barrier for the first step, the insertion of the iron cation into a C−C and C−H bond, respectively. Rate determining are the second steps which in the C−C bond activation branch corresponds to an [1,3]-H shift leading to a complex between FeCH2 + and methane. Along the C−H activation reaction coordinate, no transition state corresponding to a β-hydrogen shift resulting in a dihydrido species could be located, even though such a step has been often postulated. The decisive step is rather a concerted saddle point connecting the C−H inserted species directly with a complex of Fe+ with molecular hydrogen and ethylene. The mechanistic scenario provided by our calculations is in concert with all experimental information and allows for the first time a detailed and consistent view on the mechanistic details of this import reaction sequence. It further demonstrates the usefulness of the B3LYP approach for describing even complex electronic situations such as present in open-shell transition metal compounds.

We studied the growth of Si-and Sn-doped homoepitaxial β-Ga 2 O 3 layers on (010)-oriented substrates by metal organic vapor phase epitaxy (MOVPE). At optimal growth conditions (850 • C, 5 mbar) the layers were smooth with RMS roughness values of ∼600 pm. A microstructural study by transmission electron microscopy (TEM) revealed a very high crystalline perfection of the layers. No dislocations or planar defects were observed within the field of view of TEM. Using Si as dopant, the free electron concentration could be varied in a range between 1 × 10 17 and 8 × 10 19 cm −3 , while with Sn the doping range was restricted to 4 × 10 17 −1 × 10 19 cm −3 . This was explained by a pronounced Sn memory effect in the MOVPE reactor that hampers achieving low carrier densities and by incorporation issues that limit the doping efficiency at high Sn doping levels. The electron mobility for a given doping density increased from ∼50 cm 2 /Vs for n = 8 × 10 19 cm −3 to ∼130 cm 2 /Vs for n = 1 × 10 17 cm −3 independently of the dopant. These values match the best literature data relative to β- Transparent Oxide Semiconductors (TSOs) are an emerging class of materials, which combine high electrical conductivity with transparency down to the deep UV region.1,2 Among TSOs, monoclinic β-Ga 2 O 3 is one of the most interesting compounds thanks to a wide bandgap (∼4.8 eV) that leads to a calculated electric breakdown field strength of 8 MV/cm. 3,4 The most promising application of β-Ga 2 O 3 is in the field of high power electronics, where it is predicted to outperform the leading technology based on SiC and GaN. A key advantage of β-Ga 2 O 3 is that native substrates can be fabricated from bulk single crystals grown from the melt by Floating Zone (FZ), 5 Edge-Defined Film Fed Growth (EFG) 6 and Czochralski (CZ) 7,8 methods. The growth of homoepitaxial β-Ga 2 O 3 has been mainly investigated by molecular beam epitaxy (MBE), 9-11 and only rarely by metal organic vapor phase epitaxy (MOVPE).12-14 For both epitaxial techniques Schottky diodes and field-effect transistors have been realized. 15,16 The structural quality of (100) β-Ga 2 O 3 layers grown by MOVPE has been relatively poor so far, with maximum electron mobilities of ∼40 cm 2 /Vs, 13 contrary to values achieved for layers grown by MBE on (010)-oriented substrates that are comparable to those of bulk material (>100 cm 2 /Vs). 10 A further improvement of the growth of β-Ga 2 O 3 by MOVPE is desirable, since MOVPE is more suitable for large-scale production.Here we report on the MOVPE-growth of Si-and Sn-doped epitaxial β-Ga 2 O 3 layers on (010) β-Ga 2 O 3 substrates. We selected this substrate orientation to investigate whether it promotes the growth of layers with higher crystalline perfection. Moreover, thermal conductivity in β-Ga 2 O 3 is anisotropic with the highest value along the [010] direction (27.0 W/mK at RT) and the lowest one along the [100] direction (10.9 W/mK at RT). 17 Heat dissipation in devices fabricated on (010)-oriented substrates is then predicted to be a...