Crossed Ga2O3/SnO2 multiwire architecture: a study of local structure with nanometre resolution

27-10-2014

Crossed nanowire structures are the basis for the high-density integration of a variety of nanodevices. The intersections of the nanowires play a critical role in creating hybrid architectures, therefore a nanometre-scale investigation of the local structure within them is of great interest. In this work, the compositional uniformity and symmetry of point contacts between single crossed Cr-doped Ga2O3/SnO2 nanowires grown by thermal evaporation have been investigated using a hard X-ray nanoprobe.

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Nanowires have been proposed as an ideal system for the assembly of a wide range of applications such as memory, sensing, logic, light emission and waveguide devices. Large-scale integration of nanowires into functional circuits requires practical interconnections for nanoscale devices. Much work has already gone towards the development of crossed semiconducting nanowires [1]. However, the formation of such individual structures remains challenging and requires a systematic study of the structural and chemical properties of point contacts between different nanowires. There are many unknowns requiring investigation such as the role of impurities during coupling formation and the local atomic site configuration, the best route to achieve full control over local composition, diffusion paths, and/or structural modifications, as well as phase separation problems [2].

Recently, using the hard X-ray nanoprobe at beamline ID22NI (today ID16B), we have addressed such issues by characterising a nanowire junction formed by Ga-doped SnO2 nanowires that lie across one Cr-doped Ga2O3 nanowire obtained in a one-step thermal evaporation method. Five polymorphs of Ga2O3 (α, β, γ, δ, and ε) have been reported so far, but only two phases (α and β) are well-known structures. In particular, β-Ga2O3 (monoclinic structure) is the thermodynamically stable phase, which consists of tetrahedral GaO4 and octahedral GaO6 units in a 1:1 ratio, whereas Ga atoms in α-Ga2O3 (trigonal structure) only exist in octahedral coordination. Spinel γ-Ga2O3 has both tetrahedral and octahedral cation sites but with different ratios compared to β-Ga2O3, whereas, the stable phase of SnO2 is the rutile structure, in which Sn occupies octahedral sites in the crystalline lattice.

Average XRF spectrum of the crossed multiwire structure at 12 keV.

Figure 1. (a) Average XRF spectrum of the crossed multiwire structure at 12 keV. (b) Scanning electron microscopy image of the structure. (c) XRF map in RGB visualisation that depicts the XRF intensities of Sn (green), Ga (red), and Cr (blue). Brightness (light represents high counts, dark low counts) indicates the intensity ranges. The highlighted region is magnified in Figure 2.

Figure 1 shows the X-ray nanoimaging results of the single multiwire structure: the average X-ray fluorescence (XRF) of the probed area (a), and the geometry of the structure by scanning electron microscopy (SEM) and X-ray fluorescence imaging (b). The XRF map clearly exhibits high contrast between Sn and Ga, as well as for Cr, the dopant element present in the Ga2O3 wire. There are small features associated with the morphological heterogeneities observed by SEM. The highlighted region (dashed circle) is shown magnified in Figure 2. The estimation of the concentration of the dopants is (0.72 ± 0.01) atom % Ga in SnO2 [(6.07 ± 0.01) atom % Sn and (0.014 ± 0.004) atom % Cr in Ga2O3]. It has been reported that the occurrence of interdiffusion via the formation of mixed spinel structures at heterointerfaces plays a crucial role on the resulting physical properties such as the magnetic response. Our XRF findings point to a rather uniform crossed multiwire structure, without relevant signatures of junction-induced defects, elemental diffusion, and/or agglomeration effects.

Imaging the nanowire junction

Figure 2. (a) Scanning electron microscopy image of the nanowire junction. (b) X-ray transmission image of the intersection region obtained at 12 keV. (c) Magnified view of the highlighted area of the XRF map in RGB visualisation. (d) Normalised XRF line profiles for Sn (green symbols), Ga (red symbols), and Cr (blue symbols), respectively, collected along the white dotted line shown in (c).

To get deeper insight into the local structure at the crossing point, we probed the gallium partial density of states in the conduction band both inside and outside of the nanowire junction using nano-X-ray absorption spectroscopy (Figure 3a). XANES data around the Ga K-edge showed that there is no significant structural disorder induced by the intersection region. Within the sensitivity of our experimental techniques, XANES spectra exhibited distinguishable peaks that were assigned to the tetrahedral and octahedral Ga sites of β-Ga2O3 and α-Ga2O3 environments on the basis of a comparison with reference polymorphs [4].

XANES data recorded around the Ga K-edge: spectra acquired at the junction

Figure 3. (a) XANES data recorded around the Ga K-edge: spectra acquired at the junction and outside a multiwire intersection. The spectra were shifted vertically for clarity. (b) Magnitude of the FTs of the EXAFS functions (open symbols) around the Ga K-edge and their best fits (solid lines) recorded at the junction and outside.

To further investigate the local structure at the wire intersection, we collected EXAFS data around the Ga K-edge (Figure 3b). Theoretical backscattering amplitudes and phase shifts for all single and multiple scattering paths were calculated using a mixed environment of α-Ga2O3 and monoclinic β-Ga2O3 model clusters. A fixed coordination of oxygen nearest-neighbour atoms and Ga second-nearest-neighbour atoms was applied to both α- and β-Ga2O3 polymorphs. The analysis yields a value of 1.83 Å for the Ga−O distance in the wire, which agrees well with the distance for tetrahedral sites in the β-Ga2O3 phase. However, for the junction, the EXAFS fits yield a Ga−O distance of 1.86 Å. The preliminary EXAFS curve-fitting analysis suggests that another phase or defects are most likely present in the crossing point, giving rise to the small changes in the interatomic distances of the first two neighbour shells.

In summary, the self-assembly of crossed multiwires during a single step thermal growth appears to be a viable strategy for organising individual nanowires. The formation mechanisms of interconnected Ga2O3/SnO2 wires could be extended to other semiconductor oxide systems. The experimental technique demonstrated here opens new avenues for the study of local structures with nanometre resolution.

 

Principal publication and authors
Crossed Ga2O3/SnO2 multiwire architecture: a local structure study with nanometer resolution, G.Martinez-Criado (a), J. Segura-Ruiz (a), M.-H. Chu (a), R. Tucoulou (a), I. López (c), E. Nogales (c), B. Mendez (c), J. Piqueras (c), Nano Letters 14, 5479 (2014).
(a) ESRF
(b) Universidad Complutense de Madrid (Spain)

 

References
[1] Y. Wu, J. Xiang, C. Yang, W. Lu, C.M. Lieber, Nature 430, 61 (2004).
[2] Y.-C. Chou, K. Hillerich, J. Tersoff, M.C. Reuter, K.A. Dick, F.M. Ross, Science 343, 281 (2014).
[3] K. Nagashima, T. Yanagida, K. Oka, M. Kanai, A. Klamchuen, S. Rahong, G. Meng, M. Horprathum, B. Xu, F. Zhuge, Y. He, B.H. Park, T. Kawai, Nano Letters 12, 5684 (2012).
[4] T. Oshima, T. Okuno, S. Fujita, Jpn. J. Appl. Phys. 46, 7217 (2007).

 

Top image: A crossed Ga2O3/SnO2 multiwire structure viewed with scanning electron microscopy and X-ray fluorescence nanoimaging.