33 research outputs found

    Nanocrack-induced leakage current in AlInN/AlN/GaN

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    Here we report on the study of nano-crack formation in Al1−xInxN/AlN/GaN heterostructures, on its association with composition fluctuation and on its local electrical properties. It is shown here that indium segregation at nano-cracks and threading dislocations originating from the non-pseudomorphic AlN interlayer could be the cause of the high reverse-bias gate leakage current of Ni/Au Schottky contacts on Al1−xInxN/AlN/GaN heterostructures and significantly affects the contact rectifying behavior. Segregation of indium around crack tips in Al1−xInxN acting as conductive paths was assessed with conductive atomic force microscopy

    Nanoscale-electrical and optical properties of iii-nitrides

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    III-nitrides are wide-band gap materials that have applications in both electronics and optoelectronic devices. Because to their inherent strong polarization properties, thermal stability and higher breakdown voltage in Al(Ga,In)N/GaN heterostructures, they have emerged as strong candidates for high power high frequency transistors. Nonetheless, the use of (Al,In)GaN/GaN in solid state lighting has already proved its success by the commercialization of light-emitting diodes and lasers in blue to UV-range. However, devices based on these heterostructures suffer problems associated to structural defects. This thesis primarily focuses on the nanoscale electrical characterization and the identification of these defects, their physical origin and their effect on the electrical and optical properties of the material. Since, these defects are nano-sized, the thesis deals with the understanding of the results obtained by nano and micro-characterization techniques such as atomic force microscopy(AFM), current-AFM, scanning kelvin probe microscopy (SKPM), electron beam induced current (EBIC) and scanning tunneling microscopy (STM). This allowed us to probe individual defects (dislocations and cracks) and unveil their electrical properties. Taking further advantage of these techniques,conduction mechanism in two-dimensional electron gas heterostructures was well understood and modeled. Secondarily, origin of photoluminescence was deeply investigated. Radiative transition related to confined electrons and photoexcited holes in 2DEG heterostructures was identified and many body effects in nitrides under strong optical excitations were comprehended

    Conduction Mechanisms in Al0.84In0.16N/AlN/GaN investigated at the nanoscale

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    Nearly lattice matched Al0.84In0.16N/AlN/GaN heterostructures have a high potential to be used in HEMTs because of their ability to provide high electron mobility [1] and high 2D electron gas density [2]. However, microscopical electrical properties and morphology have not been deeply investigated up to now. Here we report the characterization of MOCVD grown Al0.84In0.16N/AlN/GaN heterostructures using Atomic force microscopy and conductive-Atomic force microscopy. We have used Atomic Force Microscopy for topography maps and conductive atomic force microscopy to obtain the current-maps at nanoscale. A bias is applied to the Atomic Force Microscopy conductive tip and the resulting current is mapped over the selected area. The analyses of these current maps allow for understanding the current flow in these structures. Figure 1 shows an example of a topography map over one of the described structure. By varying the applied bias to the tip a current-voltage characteristic is obtained which allows for the study of the electronic transport mechanism in these heterostructures. By using Fowler-Nordheim tunnelling model for the conduction mechanism in the metal (tip) oxide semiconductor (MOS) system the electronic transport mechanism has been explained. References: [1] Jinqiao Xie, Xianfeng Ni, Mo Wu, Jacob H. Leach, Ümit Özgür, and Hadis Morkoç, APPLIED PHYSICS LETTERS 91 (2007) 132116 [2] M. Gonschorek, J.F. Carlin, E. Feltin, M. A. Py, N. Grandjean, V. Darakchieva, B. Monemar, M. Lorenz, and G. Ramm, JOURNAL OF APPLIED PHYSICS 103, (2008) 09371

    Dislocations in III-nitrides investigated by atomic force microscopy

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    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 \u2013induced 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. Rouvi\ue8re, 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

    Optical and electrical characterization of ternary and quaternary gallium nitride based alloys

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    Progress in epitaxial growth techniques in the last 20 years has led to the availability of high quality ternary (InGaN) and quaternary (AlInGaN) alloys for optical devices [1], high efficient solar cells, high electron mobility transistors, microwave power applications [2] and, recently, photo-electrochemical (PEC) devices for water splitting cells [3]. Their most interesting feature is the bandgap tunable with composition, covering the whole visible spectrum. Despite extensive research on In-based alloy systems, the dependence of the bandgap, polarization properties, role of dislocations and strain relaxation mechanism on the In-content is still debated, and the effect of crystal defects on optical and electrical properties of heterostructures based on InxGa1-xN and AlxInyGa1-x-yN alloys is not yet clear. This contribution presents the results of electrical and optical characterization of InxGa1-xN/GaN and AlxInyGa1-x-yN/GaN structures by Surface Photovoltage (SPV) Spectroscopy and deep level transient spectroscopy (DLTS) in correlation with Transmission electron microscopy analysis (TEM) and light-assisted Kelvin probe force microscopy (KPFM). The influence of In content and Si-doping concentration on the misfit dislocations and electronic transitions in InxGa1-xN layers has been investigated has been discussed. Furthermore, the two-dimensional electron gas at the AlxInyGa1-x-yN/GaN heterointerfacehas been characterized by SPV and the main recombination centers in quaternary AlxInyGa1-x-yN/GaN alloys have been determined in terms of its activation enthalpy and capture cross-section. Furthermore, light-assisted KPFM reveals direct visualization of sites associated with recombination centers/traps, which includes threading dislocations, misfit dislocations and coalescence boundaries. The present analysis has allowed us to clarify the role of In content and misfit dislocations on alloy disorder and to characterize the defects acting as strong recombination center in ternary and quaternary GaN based alloys. [1] S. Nakamura et al., Proc. IEEE 101, 2211 (2013). [2] N. M. Johnson et al., Phys Today 53, 31 (2000). [3] J. Jia et al., Nat. Commun. 7, 13237 (2016)

    Electrical Properties of Quantum Wells in III-NITRIDE Alloys and the Role of Defects

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    III-nitrides (III-Ns) semiconductors and their alloys have shown in the last few years high potential for interesting applications in photonics and electronics. III-Ns based heterostructures (HS) have been under wide investigation for different applications such as high frequency transistors, ultraviolet photodetector, light emitters etc. In the present contribution a III-Ns based heterostructure, in particular the nearly lattice matched Al1-xInxN/AlN/GaN HS will be discussed. The formation of the two dimensional electron gas (2DEG), its origin, its electrical and optical properties, the confined subband states in the well and its effect on the conduction mechanisms have been studied. Moreover, extended defects and their effect on the degradation phenomena of the 2DEG have been analyzed
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