Dislocations in III-nitrides investigated by atomic force microscopy

Research on III-nitride semiconductors is achieving new heights due their high potential applications in photonics and electronics. The wide range of variation of band gap of III-N alloys allows for interesting optoelectronic applications. Low dimensional structures with high band-offsets (AlN/GaN or InN/GaN) are the basis for high efficient Light Emitting devices. Moreover, the polarization –induced high electric field, strongly localized at the interface of these heterojunctions, induces a very good confinement of 2 dimensional electron gas, which has been applied to the realization of high mobility field effect transistors [1]. However, GaN-based semiconductors are mostly grown epitaxially on sapphire, and due to the large lattice mismatch and the differences in the thermal expansion coefficients, the structures usually contain a high density of threading dislocations (TDs). While growth procedures and structures of TDs are well known [2], their electronic properties are still debated. Dislocations in GaN are known to be negatively charged and to affect mobility and electrical conduction and leakage current of GaN based devices [3,4]. Alloying of GaN with In or Al led to even a more complex scenario, as In atoms easily segregate at dislocations due to its high surface diffusivity, changing their electronic properties. In the present contribution we will show conductive AFM and phase contrast AFM studies of TDs in GaN andAl/In GaN ternary alloys to evidence the role of the strain and the composition on the electrical properties of dislocations in III-nitrides. Local I-V curves were measured at the dislocations. Metallorganic Chemical Vapor Deposition (MOCVD) grown GaN layers and heterostructures made of AlInN/AlN/GaN with different AlN thickness, InGaN/GaN with varying percentage of In, and AlGaN/GaN with varying percentage of Al were examined. The samples were obtained by different growers (AIXTRON, III-V Lab). Surface morphology, phase separation, defect structures and their effect on the electrical properties of TDs will be reported. The comparison between the results obtained in the different alloys allowed us to understand the role of In and Al on the TDs electrical properties. [1] M. Gonschorek, J.-F. Carlin, E. Feltin, M. A. Py, N. Grandjean, V. Darakchieva, B. Monemar, M. Lorenz, and G. Ramm, J. Appl. Phys. 103, 093714 (2008). [2] A. Mouti, J.-L. Rouvière, M. Cantoni, J.-F. Carlin, E. Feltin, N. Grandjean, and P. Stadelmann, Phys. Rev. B 83, 195309 (2011). [3] D. C. Look and J. R. Sizelove, Phys. Rev. Lett. 82, 1237 (1999). [4] P. J. Hansen,Y. E. Strausser, A. N. Erickson, E. J. Tarsa, P.Kozodoy, E. G. Brazel, J. P. Ibbetson, U. Mishra, V. Narayanamurti, S. P., DenBaars, and J. S. Speck, Appl. Phys. Lett. 72, 2247 (1998). Acknowledgments This work was supported by the EU under Project No. PITN-GA-2008-213238 (RAINBOW). The Project partners (AIXTRON and III-V Lab) are gratefully acknowledged for providing samples.