Composición, estructura interna y transporte electrónico de nanohilos individuales de In1-xGaxN e InN

The tunability of the direct bandgap of In1−xGaxN alloys from the UV to the IR by changing x from 0 to 1 is the basis for potential applications such us white LEDs, LDs or solar cell. However, compositional inhomogeneities and phase segregation, due not only to the lack of substrates with lattice parameter and thermal expansion coefficient matching the InN and GaN ones, but also to the low dissociation temperature of In compared to that of Ga, hamper the growth of high quality films across the entire range of compositions. In order to overcome these problems theoretical and experimental studies pointed to the growth of In1−xGaxN NWs. The first objective of this Thesis was the study of the morphological, structural and optical properties of InxGa1-xN nanowires, by catalyst free growth directly on a Si(111) substrate by plasma assisted molecular beam epitaxy (PA-MBE). The substrate temperatures (Ts) were comprised in the range between 400 and 500ºC, and the nominal In conentration [In], higher than 40%, so far, unexplored conditions. First of all, the Ts effect was studied in three series of samples grown under the following constant nominal Ga concentrations: [Ga]= 7, 13 and 27%, and Ts in the range aforementioned. The morphology and the size of the samples were studied from the pictures obtained in an scanning electron microscope (SEM). An increase in the diameter and length as Ts rises was observed, while the density of NWs by sample area diminishes. Samples grown a at lowest Ts showed hexagonal prism shape, as the Ts rises the morphology worsens. All the samples were also characterized by Raman spectroscopy, X ray diffraction (XRD) and photoluminescence (PL). The three techniques concluded that the samples are composed by an In rich region, x~5%, a region with intermediate composition, x~20%, and a third one Ga riich, x>60%. The increase of Ts gives rise to a worse Ga incorporation at the In richer regions and the opposite effect in the Ga richer ones. The effect of the variation of the Ga supply it was also studied in samples grown at constant Ts~450ºC and [Ga]=7, 13, 27 and 60%. The samples grown with [Ga]<30% showed hexagonal prism morphology. The increase of the [Ga] up to a 60% gives rise to a bimodal distribution of the NWs size and a worsening in their morphology. The three techniques proved the existence of he three compositional regions also found in the study with the Ts. With the aim of finding the origin of the inhomogeneities in the composition of the NWs, tens of single NWs were characterized by employing two techniques with nanometric spatial resolution, both based on the energy dispersive X ray spectroscopy (EDX). The first one employs as excitation source the electron beam from a transmision electron microscope (TEM), and the second one X rays generated in a synchrotron. These techniques prove the existence of a core-shell structure within each NW as origin of the compositional inhomogeneites. The core-shell structure is constituted by an In rich (almost pure InN) core covering almost the full NW volume wrapped by a thin Ga richer shell. The shell thickness diminishes from the bottom to the top of the NW and its Ga content follows the same trend. This model it was also verified by a resonant Raman spectroscopy study of single NWs, The agreement of the results from the three techniques concludes that for all the growth conditions explored spotaneous core-shell NW are formed. High quality InN NWs grown by MBE contributed to a better comprehension of the InN NWs fundamental properties, such us high type- n conductivity, despite not being intentionally doped, and the existence of an electron accumulation layer in the polar and non polar surfaces. However, a better comprehension of their electrical properties is necessary to exploit their potential applications in electronic nanodevices. So far, FET, 2 and 4 point probe configurations have been employed for electrical characterization at room temperature, but studies as temperature function only have been carried out under the influence on the contact resistances. The second objective of this Thesis was the stuy of the electronic transport in single InN NWs in four point probe geometry as a function of temperature from 0 to 300 K. in order to contact on 4 point probe single InN NWs with of about 1 micron length by electron beam lithography (EBL), therefore, the first challenge consisted on the optimization of the EBL parameters. Random potential peak destroy the NWs contacted, thus, it was necessary the design of a filter system based on opto-couplers in order to avoid that the NWs were damaged. Finally, several NWs were characterized as T function in two point probe configuration, results suggested that NWs have metal-like behavior.