The growth of wide bandgap III-nitrides on diamond substrates is very interesting since it would provide the highest thermal conductivity for efficient extraction of the large amounts of heat produced in the active region of high power light emitters or microwave transistors. This paper reports on the growth and properties of high quality single crystalline (0001) GaN films on diamond substrates by nitrogen RF plasma source molecular beam epitaxy (RFMBE). A range of diamond surface orientations have been investigated including (100), (110) and (111) to determine the dependence of GaN crystal quality. FE-SEM and AFM observations of the GaN-on-diamond surfaces revealed the growth of compact GaN films with adequately smooth surfaces. Micro-cracks appeared for the 2.0mm GaN films. A low rms roughness value of 3.5nm was determined for 10x10mm² AFM scans. XRD (q-2q) scans covering an extensive range of diffraction angles revealed that single crystalline (0001) GaN films were grown for all the different orientations of diamond substrates. However, the crystalline quality of the GaN films, as evidenced by the FWHM of XRD rocking curves (RC) around the symmetric (0002) and asymmetric (10-15) reflections of GaN exhibited a dependence on the diamond orientation, with the (100) orientation clearly resulting to inferior structural quality. The c lattice constant was also measured by XRD using the extended Bond method. The 1.4mm GaN films were under tensile strain of 1.74 x 10^-3, which dropped to 0.96 x 10^-3 in the 2.0mm GaN film with formation of micro-cracks. Low temperature PL spectra exhibited excitonic emission as well as defect or impurities related emission peaks. Bandedge luminescence was recorded at room temperature, exhibiting a red-shift of the PL peak as expected for the present tensile stress. The

Acknowledgment: This work was supported by MORGAN European project FP7 NMP IP 214610.

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