Characterization of the relaxation by misfit dislocations confined at the interface of GaN/Al2O3(0001) studied by TEM

A transmission electron microscopy (TEM) investigation of the relaxation process by misfit dislocations confined at the interface of the highly mismatched system GaN / Al2O3(0001) has been carried out. Wurtzite type GaN was grown epitaxially by plasma induced molecular beam epitaxy (PIMBE) on the (0001) basal plane of Al2O3. We determined the orientation between epilayer and substrate by selected area electron diffraction (SAED). The diffraction images show that the [11 (2) over bar 0]-direction in GaN is parallel to the [10 (1) over bar 0]-direction in Al2O3 whereas the [0001]-direction is the same in both materials. In this orientation a high lattice mismatch of f=-13,9% appears and therefore the critical thickness of dislocation formation is reached when the first monolayer of GaN is grown. An expected interfacial relaxation process is observed and characterized by the results of high resolution transmission electron microscopy (HRTEM) which reveals a relaxation process with misfit dislocations confined at the interface: HRTEM images show {11 (2) over bar 0}(Al2O3)-lattice fringes terminating at the interface in regular intervals of about 8 lattice spacings, whereas no bending of the (0001) lattice planes is obtained in the interface region. Thus misfit dislocations have Burgers vectors parallel to the (0001)-interface plane, and therefore the extension of these dislocations into the GaN-layer is obviously suppressed. The quantitative evaluation of the HRTEM images shows the effectiveness of the observed relaxation process by confined misfit dislocations: In this work a degree of relaxation delta=(-11.8+/-1.1)% was measured. A residual strain of epsilon(r)=(-2.1+/-1.1)% causes a typical threading dislocation density of 10(10)cm(-2) in the GaN epilayer as reported by many authors.