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The photoluminescence of GaN films grown by molecular-beam epitaxy on 6H-SiC(0001) substrates was studied. Owing to the high structural perfection of the films, a series of narrow emission lines related to donor–acceptor recombination was detected in the blue spectral region. This finding lends support to the donor–acceptor origin of the blue emission in GaN. In the spectra of less perfect films, these lines overlap and merge into a broad band.

The phase composition and luminescent properties of GaN films grown by molecular beam epitaxy on (0001) sapphire and 6H-SiC substrates were studied. The films grown on SiC were found to consist only of the hexagonal phase and contain a lower concentration of impurities. Grains of cubic GaN, as well as donor and acceptor impurities, were found in the GaN film grown on sapphire. The formation of impurity centers is caused by the diffusion of oxygen and aluminum from the sapphire substrate during crystal growth.

The effects of substrate cleaning, nitridation time, and substrate temperature in the range 800–1000 °C on the microstructure of AlN/Si(111) films grown by simultaneous plasma-assisted molecular beam epitaxy have been investigated. It has been demonstrated, using a combination of conventional and high-resolution transmission electron microscopy, that the interface structure, the film defect structure, and the film surface roughness are strongly related. The formation of single crystal 2H–AlN films with atomically flat surfaces occurs at 800 °C for conditions of 2.5 nm/min growth rate on very pure, atomically flat Si substrates.

Epitaxial growth of SiC on hexagonal (or α)-SiC(0001) has been performed by means of solid-source molecular beam epitaxy (MBE). The solid-source MBE growth conditions have been analyzed concerning the supersaturation and the excess phase formation of silicon and carbon. In general, our results demonstrate that control of the Si/C ratio and the supersaturation (S) is essential for the growth mode and the kind of polytype grown. Low temperature (T 1450K under near surface equilibrium conditions, different growth modes, and conditions stabilizing the growth of certain polytypes have been found. With a step decrease of S, a step-flow growth mode of both 4H-and 6H-SiC was obtained and, for the first time in case of epitaxial SiC growth, a homogeneous nucleation of α-SiC at more C-rich conditions has been realized. Conditions stabilizing the growth of certain polytypes have been estimated by thermodynamic calculations considering the influence of polytype structure on the supersaturation and the surface energy. Based on these results, we have demonstrated the growth of a double-heterostructure by firstly growing a 3C-SiC layer on 4H-SiC(0001) at low temperature and a subsequent growth of 4H-SiC under near surface equilibrium conditions on a C-stabilized surface on top of this layer.

Molecular beam epitaxial (MBE) experiments demonstrate that it is possible to achieve a controlled layer-by-layer growth of SiC polytypes at temperatures below 1000°C under surface-stabilized conditions. It is of substantial importance to stabilize characteristic Si-rich superstructures by an alternate supply of Si and C, switching periodically between more Si-rich and less Si-rich surface superstructures. The MBE growth of SiC happens in two basic steps: Firstly, Si overlayers are grown on SiC. Secondly, the Si overlayers are carbonized under supply of C, leading to growth of SiC. On the basis of first principles calculations within the framework of density functional theory in the local density approximation, we derive an equilibrium phase diagram for various structural models which provide a theoretical framework to rationalize some of the experimental findings qualitatively.

Second-harmonic (SH) generation is a versatile method applicable to in-situ characterization of even non-centrosymmetric media like silicon carbide (SiC). In particular, the azimuthal rotational anisotropy of the SH response from SiC observed in reflection allows identification of various polytypes. The nonlinear-optical results are compared to X-ray diffraction data. The abundance of information obtained through the SH studies makes characteristic fingerprinting of the 6H, 15R, and 3C polytypes of SiC is possible. The spatial resolution of the optical sampling was about 50 μm in the lateral direction with a typical penetration depth of 100 nm for the fundamental radiation. Defect regions of different crystallographic structures in large SiC samples were identified by observing the spatially resolved dependence of the SH intensity.

Epitaxial wurtzite aluminum nitride (AIN) films have been grown on (111) oriented silicon substrates by plasma-assisted solid source molecular beam epitaxy (MBE). The growth has been performed in the two-dimensional (2D) growth mode. Occuring surface reconstructions have been monitored by reflection of high-energy electron diffraction (RHEED). The films have been characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). 2D grown films exhibit atomically smooth surfaces, are singlecrystalline and void of oriented domains. High-resolution (HR) TEM micrographs of the heterointerface show the presence of an interfacial atomic arrangement where (5:4) unit cells coincide along the AIN[2110]/Si[110] direction.Surface acoustic wave (SAW) properties of MBE-grown AIN(0001)/Si(ll1) have been studied for the first time. The AIN film exhibits very good piezoelectricity. The SAW time response indicates low scattering of the waves during their propagation. An electromechanical coupling coefficient (EMC) of 0.07% has been measured for the present interdigital transducer (IDT) geometry.

We investigated the effect of doping CuGeO3 with Zn, Ni, Mn and Mg on the Cu-site, and Al, Si, Ga, As, In, Sn, Sb and Ti on the Ge-site. The lattice parameters show a linear dependence on doping. The spin-Peierls (SP) transition temperature TSP decreases linearly with increasing substitution, the slope depending on the dopant. For increasing Zn-content, a Neel-like groundstate which exhibits spin-flop-behaviour appears after the disappearence of the SP-ground-state. Si-and Ti-doped samples exhibit a similar phase in co-existence with the SP-groundstate.

The substitution of Zn on the Cu-site and Si on the Ge-site of CuGeO3 quenches the spin-Peierls state. All substitutions lead to a suppression of the spin-Peierls gap seen below T SP =14K at 30 cm−1 and to a change of the temperature dependence of the spinon continuum at 200–350 cm−1 attributed to a frustration induced Heisenberg exchange scattering. A substitution of Zn on the Cu-site has the strongest effect. The change of the continuum results from a local depression of the spin-Peierls gap near the impurity.