The basic binary compound semiconductors of the nitride material system, also referred to as nitrides, are: galliumnitride (GaN), aluminum nitride (AlN), indium nitride (InN) and boron nitride (BN), while the latter one is still immature as a semiconductor material . The nitride material system covers a broad range of band gap energies, corresponding to an optical spectrum reaching from infrared (0.8 eV for InN) to ultra violet (6.2 eV for AlN). The benefits for optoelectronic applications is obvious because the nitrides cover the whole optical spectrum and they are direct band gap materials in their basic configurations . The blue Light Emitting Diode (LED), arguably the greatest optoelectronic advance of the past 25 years , nowadays entirely relies on the nitride material system in the commercial sector, what explains the dominance of the nitrides on today’s optoelectronic market. State-of-the-art white LEDs, which base on blue LEDs plus phosphor, achieve an efficiency of around 250 lm/W and hence they outperform the classical light bulb (≈ 16 lm/W) and fluorescent tubes (≈ 100 lm/W) by far, additionally benefiting from an increased longevity in comparison to compact fluorescent lights [2, 3].
6.2. Selective Etching of p-GaN
Completely removing the p-GaN without attacking the underlying AlGaN barrier re- quires highly selective etch process, i.e. a large etch rate for p-GaN is desired, whereas the etch rate of AlGaN should ideally be zero. Achieving different etch rates for mate- rials is primarily based on chemical reactions in contrast to physical etching by sputter- ing. Therefore, a dry-etch process in an inductively coupled plasma reactive ion etching (ICP-RIE) tool is preferred, in which the plasma density can be independently con- trolled through the ICP power. The dry-etching agent of choice for galliumnitride is chlorine, which shows a high etch rate for GaN and, slightly weaker, for AlGaN . Oxygen acts as a retarding agent in the dry-etching process with higher effectiveness for AlGaN compared to GaN  and can be used to adjust the selectivity. Another possi- bility is the use of a fluorine chemistry as retarding agent . For both, a competing process between the passivation of the surface by an oxide/fluoride due to chemical reactions, and etching by sputtering this passivation through ion bombardment and at- tacking the underlying material is established. These two processes need to be balanced to achieve high selectivity. This type of process was first introduced by Lee et al. in 2000  with a Cl 2 /O 2 /Ar chemistry leading to a selectivity of 24 and improved by Han et al.  with a Cl 2 /O 2 /N 2 chemistry and a selectivity of 60. Particularly the process from Han et al. has seen growing interest recently and shows very good re- sults [117, 125–127].
The present thesis addresses the fabrication and characterisation of nanowires con- sisting of the indium galliumnitride (InGaN) material system. The group III-nitrides have become established in application for optoelectronic devices in the last years and are of great interest for research and industry. Particularly the use of galliumnitride (GaN) - InGaN heterostructure layers as the basis for light emitting diodes (LEDs) has to be pointed out. They are not only utilized as energy-efficient light sources but further as blue lasers in Blu-ray players and as background lighting in displays. Cre- ating nanostructures, especially nanowires, out of this material system is promising to boost device efficiencies. Applied in the field of photovoltaics, nanowires can help to diminish reflection losses and increase the fraction of absorbed light due to their unique morphology and high surface-to-volume ratio. Furthermore, nanowires, unlike planar layers, enable to reduce the integration of stacking faults during heteroepitaxy on substrates like silicon or sapphire.
Chapter 2 Introduction Group III-nitrides have been considered as a promising material base for semiconductor device applications since 1970. The group III-nitrides, with the binary compounds aluminum nitride (AlN), galliumnitride (GaN), and indium nitride (InN) are excellent candidates for optoelectronic applications such as blue-, UV- and IR-light emitting diodes, because they form a continuous alloy system (InGaN, InAlN, and AlGaN) whose direct optical bandgap for the hexagonal wurtzite phase ranges from 0.7 eV for InN over 3.4 eV for GaN to 6.2 eV for AlN [4, 5] as depicted in Figure 2.1. The wide band gap with large breakdown electric fields makes the material suitable for high power applications as well. Heterostructures with a discontinuity in total polarization are used to build high electron mobility transistors (HEMT) based on a 2-dimensional electron gas [4, 6].
Galliumnitride GaN turned out to be one of the most important semiconductors in modern technology.  Most available GaN based semiconductor devices use GaN deposited by heteroepitaxy. These thin nitride films contain large defect concentrations depending on substrate material.  For growth of homoepitaxial layers, an almost defect free GaN single crystal would be the ideal substrate. [3-5] Standard crystal growth methods (i.e. Czochralski-process) are inapplicable since nitrides would decompose. To avoid this decomposition, synthesis under elevated pressure is favorable. Reactions of liquid Ga with gaseous nitrogen at higher pressure would require extremely high pressures (up to 15000 bar) at temperatures up to 1500 °C. Using Na-flux as solvent, temperature can be lowered to 750 °C.  Changing reaction gas to ammonia, lower pressures and even lower temperatures are sufficient for synthesis of crystalline nitride materials.  The solvent ammonia is less polar and less protic in comparison to water but nevertheless, many inorganic compounds are soluble in (liquid) NH 3 . Similar to hydrothermal recrystallization of oxides, single crystals of nitrides are accessible in supercritical ammonia.  Furthermore, ammonothermal synthesis is a quite controllable and reproducible process. The resulting reaction pressure is dependent on the size of reaction vessel, filling degree and temperature. Ammonia itself can react as three-basic acid to form amides, imides or nitrides, whereas the reaction rates are higher and crystallinity of the products is improved when supercritical ammonia is employed.  Early investigations of the solubility of metals in ammonia showed that alkali and alkaline-earth metals as well as lanthanides dissolve well in liquid ammonia (ref. eq. 1). [10-16]
Semiconducting oxides and nitrides are, due to their large band gap energies, the two most interesting groups to employ in optoelectronic devices operating in the visible and UV spectral range. There are to mention Zinc Oxide (ZnO) and Gallium Oxide (Ga2O3) on the oxide side and GalliumNitride (GaN) together with Indium Nitride (InN) and Aluminum Nitride (AlN) for the nitrides. These semiconducting materials combine unique properties on their crystallography and growth mechanisms, as well as on their optical, electrical and magnetic properties. Hence it is not surprising that with these materials it was possible to build novel displays, light emitters, data storages, bio- and environmental-sensors and energy generating- or saving-devices. For any device application one has to solve problems related to the growth mechanisms of the materials. Defect characterization of the materials is a necessity, since relevant physical properties are affected by intrinsic and extrinsic defects. There are various characterization tools ranging from the electrical- or optical- and magnetic methods to microscopy’s such as electron- or atomic force microscopy which give information on the structural- or surface-properties. The choice which one suits best to achieve the given purpose depends on the specific information one needs.
All these methods are typically parameterized for bulk structures and are applied to het- erostructures with finite size (e.g. micro- and nanostructures). Ideally, the parameters are de- termined entirely from consistent experimental input. For the group-III-nitrides, however, many of the key band parameters have not been conclusively determined until now despite extensive research effort in this field. 85 ; 86 In a comprehensive review Vurgaftman and Meyer summarized the field of III-V semiconductors in 2001 and recommended up-to-date band parameters for all common compounds and their alloys including the nitrides. 87 But only two years later they re- alized that most nitride parameters had already been superseded. Therefore, they published a revised and updated description of the band parameters for nitride-containing semiconductors in 2003. 61 While this update includes evidence supporting a revision of the band gap of InN from its former value of 1.9 eV to a significantly lower value around 0.7 eV, 88 – 92 they had to concede that in many cases experimental information on certain parameters was simply not available. 61 This was mostly due to growth-related difficulties in producing high-quality samples for unam- biguous characterization. In the meantime the quality of, e.g., wurtzite InN samples has greatly improved 85 and even the growth of the zinc-blende phase has advanced. 93 Nevertheless, many of the basic material properties of the group-III nitrides are still undetermined or, at least, con- troversial. On the theoretical side, certain limitations of density-functional theory (DFT) in the local-density approximation (LDA) or generalized gradient approximation—currently the most
On the other hand, for films thicknesses of several microns the growth of the c-BN phase is found to be superimposed by an increasing amount of the hexagonal phase. The reason for this phenomenon could be traced back by atomic force microscopy measurements to an increasing roughness during film growth which, due to our experimental geometry, leads to shaded areas where the low-energy ion bombardment necessary to grow c-BN is strongly suppressed. Furthermore, according to High Resolution Transmission Electronmicroscopy (HRTEM) measurements, c-BN films are also found to be nanocrystalline (typical crystal size 5nm) with a high density of defects and grain boundaries making electronic applications with their need for a high degree of crystal perfection practically impossible. It has been demonstrated that a promising alternative way to enhance the crystalline quality as well as surface quality of c-BN films is significant increase of the deposition temperature up to the order of 1000°C. To obtain the cubic phase at temperatures > 800°C on top of Si substrates, it is necessary to apply a two-stage preparation process with a 50 nm c-BN seed layer grown first at a medium substrate temperature 420°C followed by the deposition at elevated temperatures. It is a relevant contribution of the present work to show that 1000°C- type temperatures does not cause Si diffusion from the substrate into the BN film. Thus, a low level of impurities (Ar ~ l at.%; Si < 0.1 at.%; Fe, W < 0.01 at.%) indicates the good quality of boron nitride films deposited by the dual beam technique even at high temperatures.
Monomerie diiodides of 2,4,6-tri(rm -butyl)phenyl = (s-M es), 2,4,6-triisopropylphenyl = (Trip)- and 2,4-di-terf-butyl-6-methylphenyl = (D B M P)-gallium and -indium were prepared by the reaction o f the lithiated aryls with MI3 (M = Ga, In) in hexane/Et20 . s-M esInI2 (1), s-M es2InI (2), s-M esG aI2 (3), s-M es2GaI (4), Trip2InI (5) and (D B M P )2InI (6) were obtained and characterized by their mass and NM R spectra ( !H, 13C), and/or by elem ental analyses. A n X-ray crystal structure analysis was carried out for 1.
Neutrale Dialkyl-Indiumhydrazide mit heterozyklischer Struktur waren in der Literatur bis heute unbekannt. Mit dem in dieser Arbeit erstmals syn- thetisierten Dimethylindiumphenylhydrazid (12) l¨aßt sich zeigen, daß diese Verbindung die von Aluminium und Gallium am h¨aufigsten angenomme- nen Strukturen mit viergliedigen Heterozyklen und zwei exozyklischen N– N-Bindungen bevorzugt. F¨ur Indiumhydrazide tritt allerdings, ¨ahnlich wie bei den Aluminiumhydraziden und im Unterschied zu den Galliumhydra- ziden, die zuvor beschriebene Aufspaltung in zwei Isomere nicht auf. Ein zweites Indiumhydrazid wurde mit Dimethylindium-tert-buylhydrazid (11) dargestellt. Die erhaltenen Einkristalle waren jedoch in allen F¨allen unl¨osbar verzwillingt, so daß die Struktur nicht vollst¨andig zu vern¨unftigen G¨utefak- toren zu verfeinern war. Die vorl¨aufigen Ergebnisse zeigen aber, daß auch hier eine Verbindung mit viergliedrigem Heterozyklus vorliegt. Die 1 H-NMR-
niumkomplex. Die Abweichungen betragen für M - O im M ittel 0,07 Ä, für M - N aber nur 0,035 Ä. U m diese Abstände einhalten zu können, ist die Spannweite des Liganden („bite“) N ---0 von 2,64 (Ga) auf 2,59 Ä (Al) verengt. Die Winkel an den M etallatom en weichen durchwegs in kaum verschiedener Weise von der 90°- bzw. 180°-Norm ab. Insgesam t ist in diesem Zusam menhang er staunlich, daß die sechs Sauerstoffatome in den von Orvig et al. synthetisierten Aluminium- und Gallium hydroxypyridinon-Chelatkom plexen sich facial um das Zentralmetall ordnen [39-42].
In summary, a series of low condensed carbon nitride materials based on heptazine units were synthesized showing photocatalytic activity for the hydrogen evolution reaction. Though we were not able to unambiguously elucidate the composition and structure of the melamine samples calcined for an increasing period of time, we have accumulated evidence that the calcined samples are mixtures of melem and melem oligomers. TEM analysis of the samples calcined for a short period of time as well as for intermediate times invariably shows the presence of melem in the samples. However, another crystalline phase was observed which likely is another modification of melem, possibly a planar structure. The melem oligomers presumably resemble melon on a local level as well as with respect to their planar layered structure, but seem to be essentially amorphous. Nevertheless, the solubility of the calcined samples is quite low. Three kinds of solvents (non-polar, polar aprotic and polar protic solvents) were tested, but only DMSO slightly dissolves parts of the samples which contain melem. The photocatalytic activities of these calcined melamine samples in the hydrogen evolution reaction are smaller than that of melon.
Over the last few decades, main-group nitrides have significantly gained importance in the field of high-performance functional materials due to their exceptional chemical stability and properties. [1–3] With the spotlight focused on light element-based nitrides qualified for a multitude of technological applications by their structural variety and strong covalence, carbon nitrides have boomed owing to their specific chemical properties. Since the “harder than diamond” fever has been evoked by the work of Liu and Cohen, much effort has been made to synthesize dense 3D phases of binary carbon nitride, C 3 N 4 , which was predicted to show very low compressibility and superhardness. [4–8] Carbon nitride imide (C 2 N 2 (NH)), presented in 2007, was the first described crystalline 3D carbon nitride network and showed a defect wurtzite-type structure.  Recently, low-density 2D carbon nitrides have also been attracting interest owing to their manifold optical and electronic properties. Not only graphitic carbon nitride (g-C 3 N 4 ), which is considered to be a precursor for high-pressure conversion into 3D C 3 N 4 and computed as the most stable modification under ambient conditions,  but also hydrogen-richer samples seem to be promising new materials for organic semiconductor science, catalytic applications, and as photoactive materials for converting solar light into electricity. [10–17]