Department of Electrical & Computer Engineering, The Ohio State University, 205 Dreese Lab, 2015

Transcription

1 Supplemental Information for Defect Manipulation to Control ZnO Micro-/Nanowire Metal Contacts Jonathan W. Cox, Geoffrey M. Foster, Alexander Jarjour, Holger von Wenckstern, Marius Grundmann, and Leonard J. Brillson, Department of Electrical & Computer Engineering, The Ohio State University, 205 Dreese Lab, 2015 Neil Ave., Columbus, OH 43210, USA Department of Physics, The Ohio State University, 191 W. Woodruff Ave., Columbus, Ohio 43210, USA Department of Physics, Cornell University, 271 Clark Hall, Ithaca, NY Institut für Experimentelle Physik II, Universität Leipzig, Linnéstr. 5, Leipzig, Germany Table of Contents I. Monte Carlo Simulations of Excitation Rate versus Depth for 1-5 kev Incident Electron Beam Energy II. Depth-Resolved Cathodoluminescence Spectra of Bare ZnO Nanowire versus Depth Between Contacts III. Native Point Defect Densities versus Excitation Depth Along Bare ZnO Nanowire IV. Depth-Resolved Spectra of SiO 2 /Si Substrate Under ZnO Wires V. Table 1. ZnO Nano-/Microwire Defect Near Band Edge Luminescence Intensity Ratio versus Wire Diameter VI. Temperature-Dependent Current-Voltage Characteristics of Milled, Unmilled, and Ga- Implanted Contacts VII. Simulations of Accelerated Ga Ion Ranges in ZnO

2 I. Monte Carlo Simulations of Excitation Rate versus Depth for kev Incident Electron Beam Energy Figure S1. CASINO simulation assuming 2D planar geometry for incident electron beam energies E B from (a) 0.5 to 3 kev and from (b) 3.5 to 5 kev. For maximum E B = 5 kev, maximum Bohr-Bethe range R B = 180 nm. Planar geometry is a reasonable approximation for the wires studied. 2

3 II. Depth-Resolved Cathodoluminescence Spectra of Bare ZnO Nanowire versus Depth Between Contacts Figure S2. DRCL spectra versus incident beam energy and excitation depth for bare ZnO (a) between Contacts 1 and 2 and (b) between Contacts 2 and 3. Besides the 3.36 ev band edge exciton, near-band edge features at ev and ev are phonon replicas of a 3.31 ev exciton and the 1.85 ev shoulder corresponds to a V Zn defect. 3

4 III. Native Point Defect Densities versus Excitation Depth Along Bare ZnO Nanowire Figure S3. Native point defect densities versus excitation depth of bare ZnO midway between (a) Contacts 1 and 2 (φ = 1000 nm), (b) Contacts 2 and 3 (φ = 600 nm), and (c) Contacts 3 and 4 (φ = 600 nm). V Zn, V Zn -R, and V O defects exhibit only slight depth variation while Cu Zn defects decrease from the bulk and increase slightly near the surface. 4

5 IV. Depth-Resolved Spectra of SiO 2 /Si Substrate Under ZnO Wires. Figure S4. DRCL spectra of SiO 2 /Si substrate supporting ZnO wires. Using our focused electron beam with nanoscale depth control, we were able to excite only the nanowire and not the underlying SiO 2 /Si substrate. Figure S4 illustrates a DRCL spectrum of a SiO 2 /Si substrate supporting these ZnO nanowires. The pronounced 1.91 ev peak is characteristic of non-bonding oxygen hole center (NBOHC) defects, 1 which are completely absent from spectra in Figures 2 and S2. Another common SiO 2 defect termed E frequently 1 Kalceff, M.A.; Phillips, M.R.; Cathodoluminescence microcharacterization of the defect structure of quartz, Phys. Rev. B, 1985, 52,

6 present in SiO 2 occurs at 2.7 ev. 2,3 but is also not present in these spectra. Pt metal overlayers also do not contribute to the observed luminescence. 2 White, B.D.; Brillson, L.J.; Bataiev, M.; Fleetwood, D.M.; Schrimpf, R.D.; Choi, B.K.; Pantelides, S.T.; Detection of trap activation by ionizing radiation in SiO 2 by spatially localized cathodoluminescence spectroscopy, J. Appl. Phys. 2002, 92, doi: / Oh, Y.M.; Lee, L.M.; Park, K.H.; Kim, Y.; Ahn, Y.H.; Park, J-Y.; Lee, S. Correlating luminescence from individual ZnO nanostructures with electronic transport characteristics, Nano Lett. 2007, 7, DOI: /nl071959o 6

7 V. Table S1. ZnO Nano-/Microwire Defect - Near Band Edge Luminescence Intensity Ratio versus Wire Diameter Nano- /MicroWire Length (µm) Diameter (nm) 5 kv Carbothermal Carbothermal Carbothermal Carbothermal Carbothermal 1 60 > Carbothermal , Carbothermal , Carbothermal (6 kv) Carbothermal (6 kv) Carbothermal (6 kv) Carbothermal (6 kv) PLD PLD PLD PLD (10 kv) PLD PLD 4 > PLD 5 > PLD 6 >100 3, CVD 1 < Table 1 shows examples of DRCLS deep level defect I(Defect) versus near band edge I(NBE) intensity ratios for 10 different nanowires grown by 3 different techniques carbothermal, pulsed laser deposition (PLD), and chemical vapor deposition (CVD). In general, carbothermal wires exhibit higher defect intensity ratios compared to PLD and CVD wires. For all three wire types, I(Defect)/I(NBE) ratios increase with increasing diameter. For the Carbothermal 3 wire, the higher 6 kv beam voltage appears to probe beyond the wire center for the lowest diameter wires, consistent with Monte Carlo simulations. Available results for 18 other PLD wires (not listed) are consistent with these results. These 28 separate wires provide statistically significant evidence for the systematic increase in defect density with wire diameter. As Table 1 shows, both carbothermal and PLD-grown wires exhibited pronounced Cu Zn and other defect features whose intensities increase with increasing wire diameter as well as radially. The lower apparent Cu Zn densities for PLD versus carbothermal may relate to the different deposition methods and source materials used. Cu is well known to be a common 7

8 impurity in mined Zn used to grow ZnO and other Zn compounds. 4 However, both methods produce nanowires with the same crystallographic orientation, which in turn can produce the piezoelectric and electrostatic fields driving the radial segregation in both wire types. As mentioned already on page 6, line 19, since native point defects such as Cu Zn and V Zn are charged species, the radial variation of the defects can be accounted for by electric fields due to piezoelectric strain as reported for IrO x contacts to ZnO 34 as well as by electric fields built into the depletion layer extending from the surface into the bulk. Nevertheless, the Cu Zn defect density in PLD-grown ZnO is significantly lower than in carbothermal-grown ZnO. So while both exhibit Cu Zn defect features and segregation toward the surface, their densities differ by orders of magnitude. The systematic variations in defect density and spatial distribution reported here differ significantly from previous studies which examined defect emissions with electron beam energies that probed the both the surface and bulk of the nanowires simultaneously, for example, Ref. S5, which along with others, noted defect intensity variations along the length of wires. Here we demonstrate a monotonic dependence of defect density on nanowire radius and at all depth scales, regardless of segregation. Similar monotonic variations in defect emission versus wire length position are found for all the other wires studied. 4 Patel, J.L.; Davies, J.J.; Nicholls, J.E. Direct optically detected magnetic resonance observation of a copper centre associated with the green emission in ZnSe, J. Phys. C: Solid State Phys., 1981, Oh, Y.M.; Lee, L.M.; Park, K.H.; Kim, Y.; Ahn, Y.H.; Park, J-Y.; Lee, S. Correlating luminescence from individual ZnO nanostructures with electronic transport characteristics, Nano Lett. 2007, 7, DOI: /nl071959o 8

9 VI. Temperature-Dependent Current-Voltage Characteristics of Milled, Unmilled, and Ga- Implanted Contacts Figure S5. Temperature-dependent I-V characteristics of milled, unmilled, and Ga-implanted contacts. Schottky-like behavior is evident between milled and Ga-implanted (ohmic) contacts whereas more ohmic behavior appears between unmilled and Ga-implanted contacts. For all three combinations, forward current is reduced by 10 2 X between T = 300 K and 80 K, consistent with conventional J 0 = A**T 2 exp (-qφ SB /kt) Schottky behavior whereas reverse current is reduced by > 10 4 X, suggesting a reduction in thermally-activation hopping conduction. Note also that the milled to Ga-implanted contacts exhibit Schottky-like behavior in contrast to the unmilled to - Ga-implanted contacts. 9

10 VII. Simulations of Accelerated Ga Ion Ranges in ZnO (a) (b) Figure S6. Stopping and Range of Ions in Matter (SRIM) simulations for (a) 5 kev and (b) 30 kev accelerated Ga ions in ZnO. We used a Helios Nanolab Focused Ion Beam (FIB) / scanning electron microscope to mill away the outer annulus of the nanowires using 5 kev Ga ions (a), versus to implant the near surface with Ga ions accelerated at 30 kev (b). Stopping and Range of Ions in Matter (SRIM) simulations demonstrate that 5 kev Ga ions have a projected range RP of 4.4 nm, and 30 kev Ga ions have an RP of 14.7 nm, shown below in Figure S4. With 5 kev Ga ions, the doping effects are limited to this narrow region, and the surface is primarily sputtered away with Ga. Ion milling conditions were: 86pA Ga-ion beam for 330 seconds to achieve a mill of the outer defect rich area. Ion implantation conditions were: 9.7pA Ga-ion beam for 9 seconds to achieve a dose of 1x10 17 cm