Oligonary Nitrides and Oxonitrides of Si,P, Al,and B in Combination with Rare Earth or Transition Metals as well as Molecular Precursor Compounds with Nitrido Bridges M‐N‐Si (M = Ti,Zr, Hf,W, Sn)

A novel synthetic approach is presented leading to hitherto unknown nitridosilicates, oxonitridosilicates, oxonitridoaluminosilicates, carbidonitridosilicates, as well as nitridoborates and oxonitridoborates of rare earth elements, alkali, and alkaline earth metals. Typically, the respective metals were reacted with silicon diimide, aluminum nitride, or poly(boron amide imide), respectively, under pure nitrogen atmosphere utilizing a radiofrequency furnace. Usually, the compounds are obtained within short reaction periods as coarsely crystalline products. Zink nitridophosphates of the sodalite structure type were obtained by the reaction of phosphorus nitride imide with zinc or zinc chalcogenides, respectively. Several molecular metal silylamides and imides containing nitridobridges between the metals and silicon were obtained by the reaction of differently chlorinated disilazanes with metal chlorides. During these investigations hitherto unknown bis(trimethylsilyl)ammonium salts have been discovered. Furthermore, we report about the synthesis of N‐silyl metal hydrazides.     

Solid-State Chemistry with Nonmetal Nitrides

Among the nonmetal nitrides, the polymeric binary compounds BN and Si₃N₄are of particular interest for the development of materials for high-performance applications. The outstanding features of both substances are their thermal, mechanical, and chemical stability, coupled with their low density. Because of their extremely low reactivity, boron and silicon nitride are hardly ever used as starting materials for the preparation of ternary nitrides, but are used primarily in the manufacture of crucibles or other vessels or as insulation materials. The chemistry of ternary and higher nonmetal nitrides that contain electropositive elements and are thus analogous with the oxo compounds such as borates, silicates, phosphates, or sulfates was neglected for many years. Starting from the recent successful preparation of pure P₃N₅, a further binary nonmetal nitride which shows similarities with Si₃N₄ with regard to both its structure and properties, this review deals systematically with the solid-state chemistry of ternary and higher phosphorus(V ) nitrides and the relationship between the various types of structure found in this class of substance and the resulting properties and possible applications. From the point of view of preparative solid-state chemistry the syntheses, structures, and properties of the binary nonmetal nitrides BN, Si₃N₄, and P₃N₅ will be compared and contrasted. The chemistry of the phosphorus(V ) nitrides leads us to expect that other nonmetals such as boron, silicon, sulfur, and carbon will also participate in a rich nitride chemistry, as initial reports indeed indicate.     

Advanced optical spectroscopy in nitrides and oxides: Some of the progress made during the past ten years

I review the recent progress made concerning the optical properties of (In,Al,Ga)N-based and (Zn,Mg)O-based epilayers and low dimensional systems like quantum wells, superlattices and microcavities.     

The structure of the surface compound CrN formed by cosegregation on a Fe–15%Cr–N(100) single crystal surface

Cosegregation is known to cause the formation of two-dimensional chemical compounds (surface compounds) which can be epitaxed to substrate surfaces of a suitable structure. In the present work the cosegregation-induced formation of the CrN surface compound on nitrided Fe–15%Cr–N(100) single crystal surfaces was studied by means of Auger electron spectroscopy and low-energy electron diffraction. Intensity versus energy spectra (I(E)) were measured and analysed fully dynamically to investigate the structural details of the CrN surface compound. It is found that nitrogen is segregated to the surface forming the sample's top layer and substantial amounts of chromium are cosegregated with nitrogen. Nitrogen atoms reside in four-fold symmetric hollow sites about 0.1 Å above the metallic substrate. There is a huge relative expansion of the distance between the first and second metal atom layers (Δd₁₂/d₀≈26%), while the distances between deeper layers are almost bulk-like. The small distance between the nitrogen and the top metal layer as well as the huge layer expansion Δd₁₂/d₀ are in agreement with results found for N/Cr(100).     

Facile One-step Synthesis of Zn1–xMnxSiN2 Nitride Semiconductor Solid Solutions via Solid-state Metathesis Reaction

We report on the synthesis of the II-IV-N₂ semiconductors ZnSiN₂, MnSiN₂, and the Zn_1–xMn_xSiN₂ solid solutions by a one-step solid-state metathesis reaction. The successful syntheses were carried out by reacting the corresponding metal halides with stoichiometric amounts of silicon nitride and lithium azide in sealed tantalum ampoules. After washing out the reaction byproduct LiCl, powder X-ray diffraction patterns were indexed with orthorhombic space group Pna2₁. Single phase products were obtained without applying external pressure and at a moderate reaction temperature of 700 °C. The resulting ZnSiN₂ was found to consist of nano-sized grains and needle-shaped nano-crystals. With increasing manganese content in the Zn_1–xMn_xSiN₂ solid solution, we found the reaction product to be increasingly crystalline. Both the cell parameters and the bandgap values across the different compositions of the solid solutions change linearly. The sample Zn_0.95Mn_0.05SiN₂ synthesized by means of solid-state metathesis reaction is an intense red emitter with a broad emission maximum at λ_max ≈ 619 nm when excited with ultraviolet light after annealing the sample at a pressure of 6 GPa and a temperature of 1200 °C.     

Untersuchungen im System Ta—Mo—N

An isothermal section at 1,100°C and at nitrogen pressures in the range of 1 to 300 bar of the system Ta-Mo-N has been examined by X-ray techniques. The composition range of the ternary Z-phase TaMoN has been investigated with respect to nitrogen content and metal composition. A new ternary phase was found at a composition of Ta₃MoN_3+x.Within the -Mo₂N-phase up to 25% of the Mo-atoms can be replaced by Ta-atoms at nitrogen pressures of 300 bar, while only a small amount of the Taatoms can be replaced by Mo-atoms within the Ta-nitrides up to nitrogen pressures of 300 bar. Up to about 50% of the Mo-atoms can be substituted by W-atoms within the Z-phase.

     

A comparison of commercial sources of epitaxial material for GaN HFETs fabrication

This paper describes a comparison of material and device results obtained from AlGaN/GaN epitaxial HFET wafers from three commercial sources. Although all three sources supplied material to the same nominal specification, X-ray diffraction, Hall effect and C–V profiling revealed significant differences between them. Wafers from two of the suppliers showed poor inter-device isolation characteristics, indicative of a conducting buffer layer. Wafers from the third supplier showed excellent inter-device isolation, but C–V measurements showed that the AlGaN was about twice as thick as specified, resulting in devices with high pinch-off voltages (−16 V). For the wafers with poor buffer isolation, RF measurements on 1.2 μm gate length devices gave values of f_T 5.0 GHz and values of f_max from 8.0 to 11.7 GHz (exact values depending on DC bias conditions), while for the wafer with over-thick AlGaN the corresponding values were 8.0 and 20.0 GHz.     

Influence of Be on N composition in Be-doped InGaAsN grown by RF plasma-assisted molecular beam epitaxy

Undoped and Be-doped InGaAsN layers were grown on GaAs substrates under the same growth conditions by radio frequency plasma-assisted molecular beam epitaxy. Increased tensile strain (Δa/a=3×10⁻³) was observed for Be-doped InGaAsN layers, compared to undoped InGaAsN layers. The strain is shown to originate from the increase in N composition related to Be incorporation, rather than solely from Be atoms substituting Ga atom sites (Be_Ga). A possible reason is the high Be–N bond strength, which inhibits the loss of N from the growth surface during epitaxial growth, thereby increasing the N composition in the Be-doped InGaAsN layer.     

Profiling band structure in GaN devices by electron holography

Electron holography in a field emission gun transmission electron microscope has been used to profile the inner potential V₀ across GaN/x nm In_0.1Ga_0.9N/GaN/(0 0 0 1) sapphire samples (x=10, 40 nm) grown by molecular beam epitaxy and viewed in cross-section. Results are presented which suggest a decrease in V₀ of 3–4 V across the InGaN layer in the [0 0 0 1] direction. It is proposed that the results can be explained by charge accumulation across the InGaN layer and that the opposing contributions due to piezoelectric and polarisation fields are effectively masked by Fermi level pinning.