@article {12196, title = {Pressure Controlled GaN MBE Growth Using a Hollow Anode Nitrogen Ion Source}, journal = {Materials Research Society Proceedings}, volume = {449}, year = {1997}, note = {

1996 MRS Fall Meeting

}, abstract = {

GaN films were grown on sapphire substrates at temperatures below 1000 K utilizing a Hollow Anode nitrogen ion source. A Ga flux limited growth rate of ~0.5 μm/h is demonstrated. Active utilization of strain and the assistance of a nitrogen partial pressure during buffer layer growth are found to be crucial issues that can improve the film quality. The best films exhibit a full width at half maximum of the x-ray rocking curves of 80 arcsec and 1.85 meV for the excitonic photoluminescence measured at 4 K. A Volmer-Weber three dimensional growth mode and the spontaneous formation of cubic GaN inclusions in the hexagonal matrix are observed in the investigated growth temperature range. It is argued that this growth mode contributes to a limitation of the carrier mobility in these films that did not exceed 120 cm2/Vs through a minimum canier concentration of ~1015 cm-3 was achieved.

}, doi = {10.1557/PROC-449-221}, author = {Michael S.H. Leung and Ralf Klockenbrink and Christian F. Kisielowski and Hiroaki Fujii and Joachim Kr{\"u}ger and Sudhir G. Subramanya and Andr{\'e} Anders and Zuzanna Liliental-Weber and Michael D. Rubin and Eicke R. Weber}, editor = {Joachim Kr{\"u}ger} } @proceedings {11942, title = {Impact of Growth Temperature, Pressure and Strain on the Morphology of GaN Films}, journal = {Materials Research Society Symposium N {\textendash} III-V Nitrides}, volume = {449}, year = {1996}, pages = {227}, abstract = {

GaN films grown on sapphire at different temperatures are investigated. A Volmer-Weber growth mode is observed at temperatures below 1000K that leads to thin films composed of oriented grains with finite size. Their size is temperature dependent and can actively be influenced by strain. Largest grains are observed in compressed films. It is argued that diffusing Ga ad-atoms dominate the observed effects with an activation energy of 2.3 {\textpm} 0.5 eV. Comparably large grain sizes are observed in films grown on off-axes sapphire substrates and on bulk GaN. This assures that the observed size limitation is a consequence of the 3D growth mode and not dependent on the choice of the substrate. In addition, the grain size and the surface roughness of the films depend on the nitrogen partial pressure in the molecular beam epitaxy (MBE) chamber,most likely due to collisions between the reactive species and the background gas molecules. This effect is utilized to grow improved nucleation layers on sapphire.

}, doi = {10.1557/PROC-449-227}, author = {Hiroaki Fujii and Christian F. Kisielowski and Joachim Kr{\"u}ger and Michael S.H. Leung and Ralf Klockenbrink and Michael D. Rubin and Eicke R. Weber}, editor = {Joachim Kr{\"u}ger} } @conference {12157, title = {Origin of Strain in GaN Thin Films}, booktitle = {23rd International Conference on the Physics of Semiconductors}, volume = {4}, year = {1996}, pages = {513}, address = {Singapore}, abstract = {

Photoluminescence measurements are used to determine the strain in GaN thin films grown by Molecular Beam Epitaxy. The strain which originates from growth on lattice mismatched substrates and from differences in thermal expansion coefficients is found to be greatly relaxed. Residual strains are shown to depend on the thickness of GaN buffer layers and the III/V flux ration during main layer growth. The results strongly suggest that the residual biaxial strain caused by the post-growth cooling can be modified by the incorporation of point defects during the main layer growth which introduce an additional hydrostatic strain field. The effect allows for strain engineering of GaN crystals.

}, author = {Christian F. Kisielowski and Joachim Kr{\"u}ger and Michael S.H. Leung and Ralf Klockenbrink and Hiroaki Fujii and Tadeusz Suski and Sudhir G. Subramanya and Joel W. Ager III and Michael D. Rubin and Eicke R. Weber}, editor = {Joachim Kr{\"u}ger} } @article {1849, title = {Strain Related Phenomena in GaN Thin Films}, journal = {Physical Review B}, volume = {54}, number = {24}, year = {1996}, month = {12/1996}, pages = {17745-17753}, chapter = {17745}, abstract = {

Photoluminescence (PL), Raman spectroscopy, and x-ray diffraction are employed to demonstrate the co-existence of a biaxial and a hydrostatic strain that can be present in GaN thin films. The biaxial strain originates from growth on lattice-mismatched substrates and from post-growth cooling. An additional hydrostatic strain is shown to be introduced by the presence of point defects. A consistent description of the experimental results is derived within the limits of the linear and isotropic elastic theory using a Poisson ratio nu =0.23+/-0.06 and a bulk modulus B=200+/-20 GPa. These isotropic elastic constants help to judge the validity of published anisotropic elastic constants that vary greatly. Calibration constants for strain-induced shifts of the near-band-edge PL lines with respect to the E2 Raman mode are given for strain-free, biaxially strained, and hydrostatically contracted or expanded thin films. They allow us to extract differences between hydrostatic and biaxial stress components if present. In particular, we determine that a biaxial stress of one GPa would shift the near-band-edge PL lines by 27+/-2 meV and the E2 Raman mode by 4.2+/-0.3 cm-1 by use of the listed isotropic elastic constants. It is expected from the analyses that stoichiometric variations in the GaN thin films together with the design of specific buffer layers can be utilized to strain engineer the material to an extent that greatly exceeds the possibilities known from other semiconductor systems because of the largely different covalent radii of the Ga and the N atom.

}, doi = {10.1103/PhysRevB.54.17745}, author = {Christian F. Kisielowski and Joachim Kr{\"u}ger and Sergei Ruvimov and Tadeusz Suski and Joel W. Ager III and Erin C. Jones and Zuzanna Liliental-Weber and Michael D. Rubin and Eicke R. Weber and Michael D. Bremser and Robert F. Davis}, editor = {Joachim Kr{\"u}ger} }