Arbitrary cross-section SEM-cathodoluminescence imaging of growth sectors and local carrier concentrations within micro-sampled semiconductor nanorods

Future one-dimensional electronics require single-crystalline semiconductor free-standing nanorods grown with uniform electrical properties. However, this is currently unrealistic as each crystallographic plane of a nanorod grows at unique incorporation rates of environmental dopants, which forms axial and lateral growth sectors with different carrier concentrations. Here we propose a series of techniques that micro-sample a free-standing nanorod of interest, fabricate its arbitrary cross-sections by controlling focused ion beam incidence orientation, and visualize its internal carrier concentration map. ZnO nanorods are grown by selective area homoepitaxy in precursor aqueous solution, each of which has a (0001):+c top-plane and six {1–100}:m side-planes. Near-band-edge cathodoluminescence nanospectroscopy evaluates carrier concentration map within a nanorod at high spatial resolution (60 nm) and high sensitivity. It also visualizes +c and m growth sectors at arbitrary nanorod cross-section and history of local transient growth events within each growth sector. Our technique paves the way for well-defined bottom-up nanoelectronics.

top-plane), at position 2-6 (at nanocolumn side-plane) and position 7 (on PMMA-covered ZnO (0001) substrate). This nanocolumn also exhibits a redshifted 3.25 eV NBE CL emission at its side-plane as well as the ordinary 3.28 eV NBE CL emission, which becomes dominant as the measurement position goes from 1 to 6. ZnO (0001) substrate exhibits a 3.29 eV NBE CL emission only.
In general, stacking faults or dislocations may be generated by the coalescence of ZnO nuclei which may modulates CL properties locally. However, such minor CL contrasts are not clearly observed. Instead, ZnO nanocolumn exhibits red-shifted NBE CL emission on its side-plane from that on its top-plane, in the same manner as the ZnO nanorod in Fig. 1. This demonstrates that each nucleus grows its +c top-plane and m side-planes biaxially at different donor incorporation rates.
NBE CL emission energy of ZnO nanocolumn is also attributed to the local carrier concentration: 3.28 eV emission to n +c = 2×10 17 cm -3 in the axial +c growth sector and 3.25 eV emission to n m = 2×10 18 cm -3 in the lateral m growth sector.

Supplementary Figure 4 | "Differential" I-V measurements of an individual ZnO
free-standing nanorod. (a) A schematic illustration of the "differential" I-V measurement of ZnO free-standing nanorods before or after the nanorod is removed by W-probe micro-manipulation.
The ZnO nanorod (D w = 500 nm) has a height of L 12 = 3.0 µm and diameters of D 1 = 1.0 µm and D 2 = 0.8 µm. A series resistance between a W-probe and a ZnO (0001) substrate back contact is measured where its equivalent circuit consists of series connections of R S (resistance between the nanorod root and back contact), R R (resistance of the nanorod), and R PR (resistance at a W-probe contact on the nanorod top-plane). The R R of our interest is deduced by subtracting the series resistance when the probe contacts to the nanorod root from that when the probe contacts to the  Note that the electron beam is blanked during the I-V measurements. (d) Comparison of I-V curves and resulting differential series resistances with respect to the bias voltage V S . The differential series resistance in each case decreases with increasing |V S | and converges at |V S | = 5 V, due to the bias voltage concentration to Schottky W-probe contact and resulting R PR convergence to 0 Ω. Note that the R PR on the nanorod root is lower than R PR on the +c top-plane. This may originates from the hexagonal nanocolumn containing both +c and m growth sectors, which has net carrier concentration higher than pure +c growth sector. The gap between differential series resistances in two cases yields R R = 6×10 4 Ω. Considering the slightly tapered nanorod shape, net electrical conductivity within a nanorod is estimated to be σ e = 0.7 Ω -1 cm -1 .  (2)], which realizes stationary growths of ZnO nanorods. 7,8 Zn Reported speciation diagram shows that a large portion of zinc present in solution phase is of the