Während über hexagonales (alpha) GaN zum ersten Mal 1932 berichtet wurde, gelang erst 1989 die Synthese einer mit Molekularstrahlepitaxie (MBE) auf 3C-SiC epitaktisch gewachsenen, metastabilen kubischen (eta) GaN Schicht. Die vorliegende Arbeit befaßt sich mit der Herstellung der Verbindungen eta-(In,Ga,Al)N mittels RF-Plasma unterstützter MBE auf GaAs(001) und den mikrostrukturellen sowie optischen Eigenschaften dieses neuartigen Materialsystems. Im Vergleich zur hexagonalen bietet die kubische Kristallstruktur auf Grund ihrer höheren Symmetrie potentielle Vorteile für die Anwendung in optischen und elektronischen Bauelementen. Viele wichtige Materialgrößen der kubischen Nitride sind jedoch noch gänzlich unbekannt, da sich die Synthese einkristalliner Schichten als sehr schwierig erweist. Das Ziel dieser Arbeit ist es daher erstens, die technologischen Grenzen der Herstellung von bauelementrelevanten kubischen (In,Ga,Al)N Heterostrukturen auszuweiten und zweitens, einen Beitrag zur Aufklärung der bis dato wenig bekannten optischen und elektronischen Eigenschaften des GaN und der Mischkristalle In GaN zu leisten. Zunächst wird ein optimierter MBE Prozess unter Einsatz einer Plasmaquelle hohen Stickstofflusses vorgestellt, welcher nicht nur die reproduzierbare Epitaxie glatter, einphasiger GaN Nukleationsschichten auf GaAs ermöglicht. Vielmehr können damit auch dicke GaN. Schichten mit glatter Oberflächenmorphologie hergestellt werden, welche die Grundlage komplizierterer eta-(In,Ga,Al)N Strukturen bilden. An einer solchen GaN Schicht mit einer mittleren Rauhigkeit von nur 1.5 nm werden dann temperaturabhängige Reflexions- und Transmissionsmessungen durchgeführt. Zur Auswertung der Daten wird ein numerisches Verfahren entwickelt, welches die Berechnung des kompletten Satzes von optischen Konstanten im Spektralgebiet 2.0 = 0.4 wären grün-gelbe Laserdioden. Zusammenfassung in PostScript / While the earliest report on wurtzite (alpha) GaN dates back to 1932, it was not until 1989 that the first epitaxial layer of metastable zincblende (eta) GaN has been synthesized by molecular beam epitaxy (MBE) on a 3C-SiC substrate. The present work focuses on radio frequency (RF) plasma-assisted MBE growth, microstructure, and optical properties of the eta-(In,Ga,Al)N material system on GaAs(001). Due to their higher crystal symmetry, these cubic nitrides are expected to be intrinsically superior for (opto-) electronic applications than the widely employed wurtzite counterparts. Owing to the difficulties of obtaining single-phase crystals, many important material constants are essentially unknown for the cubic nitrides. The aim of this work is therefore, first, to push the technological limits of synthesizing device-relevant zincblende (In,Ga,Al)N heterostructures and, second, to determine the basic optical and electronic properties of GaN as well as to investigate the hardly explored alloy InGaN. An optimized MBE growth process is presented which allows not only the reproducible nucleation of smooth, monocrystalline GaN layers on GaAs using a high-nitrogen-flow RF plasma source. In particular, thick single-phase GaN layers with smooth surface morphology are obtained being a prerequisite for the synthesis of ternary eta-(Ga,In,Al)N structures. Temperature dependent reflectance and transmittance measurements are carried out on such a GaN film having a RMS surface roughness as little as 1.5 nm. A numerical method is developed which allows to extract from these data the complete set of optical constants for photon energies covering the transparent as well as the strongly absorbing spectral range (2.0 -- 3.8 eV). Inhomogeneities in the refractive index leading to finite coherence effects are quantitatively analyzed by means of Monte Carlo simulations. The fundamental band gap EG(T) of GaN is determined for 5 < T < 300 K and the room temperature density of states is investigated. Systematic studies of the band edge photoluminescence (PL) in terms of transition energies, lineshapes, linewidths, and intensities are carried out for both alpha- and GaN as a function of temperature. Average phonon energies and coupling constants, activation energies for thermal broadening and quenching are determined. Excitation density dependent PL measurements are carried out for both phases in order to study the impact of nonradiative recombination processes at 300 K. A recombination model is applied to estimate the internal quantum efficiency, the (non)radiative lifetimes, as well as the ratio of the electron to hole capture coefficients for both polytypes. It is seen that the dominant nonradiative centers in the n-type material investigated act as hole traps which, however, can be saturated at already modest carrier injection rates. In summary, despite large defect densities in GaN due to highly mismatched heteroepitaxy on GaAs, band edge luminescence is observed up to 500 K with intensities comparable to those of state-of-the-art alpha-GaN. For the first time, thick InGaN films are fabricated on which blue and green luminescence can be observed up to 400 K for x=0.17 and x=0.4, respectively. Apart from bulk-like InGaN films, the first coherently strained InGaN/GaN (multi) quantum wells with In contents as high as 50 % and abrupt interfaces are grown. This achievement shows that a ternary alloy can be synthesized in a metastable crystal structure far beyond the miscibility limit of its binary constituents despite the handicap of highly lattice mismatched heteroepitaxy. The well widths of these structures range between 4 and 7 nm and are thus beyond the theoretically expected critical thickness for the strain values observed. It is to be expected that even higher In contents can be reached for film thicknesses below 5 nm. The potential application of such InGaN/GaN multi quantum wells with x >= 0.4 would thus be diode lasers operating in the green-yellow range. abstract in PostScript