Hot probe measurements on neutron irradiated , isotope enriched ZnO nanorods

We report on neutron transmutation doping (NTD) of isotopically (64Zn) enriched ZnO nanorods to produce material with holes as the majority mobile carrier. Nanorods of ZnO enriched with 64Zn were synthesised and the abundance of 64Zn in these samples is ∼ 71%, compared to the natural abundance of ∼ 49 %. The enriched material was irradiated with thermal neutrons which converts some 64Zn to 65Zn. The 65Zn decays to 65Cu with a half-life of 244 days and the Cu can act as an acceptor dopant. After 690 days, a hot probe technique was used to determine the majority charge carriers in non-irradiated and neutron irradiated nanorod samples. Non-irradiated samples were measured to be to have electrons as the majority mobile carrier and the irradiated samples were measured to have holes as the majority mobile carrier. © 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5061722 Because of its wide band-gap (3.4e V) and large exciton energy (60meV)1 ZnO is a promising material for a range of optoelectronic applications such as UV lasers and light emitting diodes. However, ZnO technology has been held back by the lack of a reliable process for reproducibly synthesizing stable p-type material. There are have been many attempts to address this outstanding problem in ZnO technology.2,3 Closest to the work reported here are studies on Neutron Transmutation Doping (NTD) carried out on n-type ZnO with a natural abundance of Zn isotopes.4–7 The distribution Zn isotopes in natural abundance material8 is 49.28% 64Zn, 27.83% 66Zn, 4.20% 67Zn; 18.3% 68Zn and 0.5% 70Zn. For NTD, 66Zn and 67Zn are not of interest because neutron irradiation results in other stable Zn isotopes. Neutron irradiation of 64Zn results in 65Zn which radioactively decays to 65Cu on a Zn lattice site. Cuzn is an acceptor dopant9 that promotes p-type doping.4 However, neutron irradiation of 68Zn and 70Zn produces 69Ga and 71Ga on a Zn site. Gazn can act as a donor dopant1 and can partially compensate p-type dopants and hence mitigate against majority carrier type conversion to holes. Thus the relative concentrations of 64Zn versus 68Zn+70Zn isotopes controls whether or not NTD will change the charge carrier type of the ZnO. In the work presented here we use 64Zn enriched10 ZnO which has relatively less 68Zn and 70Zn, and, by employing NTD, are able to demonstrate conversion to material with holes as the majority mobile carrier. It should be noted that in addition evidence that CuZn can act as acceptor2,9,11 there are also reports6,12 that CuZn site in ZnO produces a trap too deep to produce mobile holes at room temperature. There is also some evidence13 that the depth of CuZn site can be reduced by alloying with S. The synthesis and properties of the isotopically enriched ZnO nanorod samples were described previously;8,14 the 64Zn enriched sample used in this work had the following relative concentration of Zn isotopes: 71.5% 64Zn, 15.6% 66Zn, 2.3% 67Zn; 10.3% 68Zn and 0.08% 70Zn. A 64Zn enriched nanorod sample was irradiated with neutrons using the thermal reactor of the Australian Nuclear Science and Technology Organization (ANSTO) under a flux of 2.8 × 1013 cm−2s−1 for 6 hours. This means that the fluence is 6 × 1017 cm−2. After neutron irradiation the sample emitted 135 kBq of 65Zn decay radiation. In previous NTD studies4,6,12 of ZnO the samples were both annealed (under varying conditions) and unannealed. The annealing was undertaken to mitigate the effects of neutron damage to AIP Advances 9, 035223 (2019); doi: 10.1063/1.5061722 9, 035223-1

Because of its wide band-gap (3.4e V) and large exciton energy (60meV) 1 ZnO is a promising material for a range of optoelectronic applications such as UV lasers and light emitting diodes.However, ZnO technology has been held back by the lack of a reliable process for reproducibly synthesizing stable p-type material.There are have been many attempts to address this outstanding problem in ZnO technology. 2,35][6][7] The distribution Zn isotopes in natural abundance material 8 is 49.28% 64 Zn, 27.83% 66 Zn, 4.20% 67 Zn; 18.3% 68 Zn and 0.5% 70 Zn.For NTD, 66 Zn and 67 Zn are not of interest because neutron irradiation results in other stable Zn isotopes.Neutron irradiation of 64 Zn results in 65 Zn which radioactively decays to 65 Cu on a Zn lattice site. 65Cuzn is an acceptor dopant 9 that promotes p-type doping. 4However, neutron irradiation of 68 Zn and 70 Zn produces 69 Ga and 71 Ga on a Zn site.Gazn can act as a donor dopant 1 and can partially compensate p-type dopants and hence mitigate against majority carrier type conversion to holes.Thus the relative concentrations of 64 Zn versus 68 Zn+ 70 Zn isotopes controls whether or not NTD will change the charge carrier type of the ZnO.In the work presented here we use 64 Zn enriched 10 ZnO which has relatively less 68 Zn and 70 Zn, and, by employing NTD, are able to demonstrate conversion to material with holes as the majority mobile carrier.
It should be noted that in addition evidence that Cu Zn can act as acceptor 2,9,11 there are also reports 6,12 that Cu Zn site in ZnO produces a trap too deep to produce mobile holes at room temperature.There is also some evidence 13 that the depth of Cu Zn site can be reduced by alloying with S.
The synthesis and properties of the isotopically enriched ZnO nanorod samples were described previously; 8,14 the 64 Zn enriched sample used in this work had the following relative concentration of Zn isotopes: 71.5% 64 Zn, 15.6% 66 Zn, 2.3% 67 Zn; 10.3% 68 Zn and 0.08% 70 Zn.A 64 Zn enriched nanorod sample was irradiated with neutrons using the thermal reactor of the Australian Nuclear Science and Technology Organization (ANSTO) under a flux of 2.8 × 10 13 cm −2 s −1 for 6 hours.This means that the fluence is 6 × 10 17 cm −2 .After neutron irradiation the sample emitted 135 kBq of 65 Zn decay radiation.
In previous NTD studies 4,6,12 of ZnO the samples were both annealed (under varying conditions) and unannealed.The annealing was undertaken to mitigate the effects of neutron damage to the material.In the investigation reported here the samples were unannealed.
The Zn isotope nuclear reactions are well known and have been reported in detail. 12,6Neutron irradiation converts some of the 64 Zn to 65 Zn -an unstable isotope with a half-life of 244 days that converts to 65 Cu Zn by a variety of routes.After absorbing a neutron 68 Zn isotope converts to 69 Zn which converts to 69 Gazn on a time scale of around 14 hours.Because of the residual 68 Zn (10.3%) in the enriched 64 ZnO, a few days after neutron irradiation of the enriched 64 ZnO it can be expected to be more n-type because of the new 69 Gazn donor dopants that arise from the transmutation process of the 68 Zn to 69 Gazn.But as the 65 Zn converts to acceptor dopant 65 Cuzn we expect the 64 Zn enriched material to become less n-type and eventually convert to holes as the majority mobile charge carrier.The neutron irradiated 64 ZnO nanorod samples were electrical characterized more than 690 days after the neutron irradiation by which time >85% of the 65 Zn created by the neutron irradiation had converted to 65 Cu Zn .
A hot probe technique 15 adapted for nanorod samples was used to establish the majority carrier type in the irradiated and nonirradiated 64 ZnO nanorod samples.In the hot probe technique, a temperature difference, ∆T, is produced across a sample.The mobile charge carriers acquire thermal energy from the hot side of the sample and preferentially thermally diffuse from the hot to the cold part of the sample.Therefore, the mobile charge carriers accumulate at the cold side of the sample this gives rise to unbalanced charge distribution thereby an electric field and a voltage difference between the hot and cold parts of the sample.If the mobile charge carriers are electrons, then the cold part is negative with respect to the hot part and if the mobile charge carriers are holes, then the cold part is positive with respect to the hot part.
The conventional hot probe technique was adapted to characterize our ZnO nanorod samples -a technique we call the micro-hot probe.For the micro-hot probe technique, the first step was to fabricate a sample holder that could be used to generate a temperature difference, ∆T, and measure a thermal voltage across a ZnO nanorod (which typically had the morphology of a hexagonal column 1-10µm long and 0.1-0.5µmwide).The next step was to place the nanorod in the sample holder using a Focused Ion Beam-Scanning Electron Microscope (FIB-SEM).The ∆T was generated across the sample by cooling one side of the sample holder with dry (CO 2 ) ice at -78.5 degrees C (∼194 K).The sample holder has electrodes designed to allow the voltage difference across the cooled sample to be measured.Figure (1) shows a schematic of the physical layout of the sample holder and of the electrical circuit.
The sample holder has a platinum thin film pattern deposited on a glass substrate.The Pt film provides electrical contacts to a ZnO nanorod and a means of temperature measurement because the resistivity of Pt thin film is very close to linearly dependent on the temperature (an effect used in thin film Pt resistance thermometers 16 ).In our experiments we only used the measurement of the R 12 and R 34 resistances to confirm that a temperature difference was achieved; we did not make calibrated measurements of temperature difference, ∆T, as that is not required to establish the carrier type.
The gap between the two electrodes was created using the ion beam in the FIB-SEM to mill away the Pt thin film at the junction of the electrodes (see Figure (2)).The nanorod is manipulated into place using a similar technique to that described in a previous paper. 8,17Finally, ion beam deposition of a thin line of Pt was performed to electrically contact and mechanically bind the nanorod to the electrodes.Figure (2) shows a SEM micrograph of the mounted nanorod.
Both neutron irradiated and non-irradiated 64 ZnO nanorods were mounted in sample holders.Before the substrate with nanorods grown on it was neutron irradiated, a nanorod sample was extracted and was measured using the micro-hot probe to confirm that nonirradiated (as-grown) 64 ZnO nanorods were n-type (as is always the case).
To carry out the hot probe technique with these samples one side of the sample holder is cooled using a dry ice pellet at -78.8 degrees C, the dry ice pellet is located in thermal contact with one electrode and the temperature is monitored by measuring the change in the resistances R 12 and R 34 .The resistance measurements confirms the cooling of the electrode in thermal contact with the dry ice pellet but it does not give the temperature at either end of the nanorod, rather the measurement gives an average over the whole electrode.The thermally generated voltage is then measured between contact pads 1+2 and 3+4.The resistance and voltage measurements are carried out using standard (Digitech Model QM1325) multimeters.Part of the experimental procedure involves reversing the cooling by changing the electrode that is cooled with the dry ice pellet.The results are reported in Table I.
ZnO nanorod mounted across the gap that provides electrical and thermal insulation.The Pt thin film has a trench milled by the FIB and the nanorod is placed across the gap using a nanomanipulator.Electrical and thermal contact and mechanical binding are provided at either end of the nanorod by FIB deposited Pt.

FIG. 2. SEM of sample holder the insert shows a single
We consistently obtained results that show that the neutron irradiated 64 ZnO nanorods give positive voltages for the cold electrode relative to the hot electrode, indicating material with holes as the majority mobile carrier, and the non-irradiated 64 ZnO nanorods give negative voltages for the cold electrode relative to the hot electrode, indicating n-type material.
The magnitude of the thermal voltage depends on the material properties such as the magnitude of the Seebeck coefficient for isotopically enriched 64 ZnO nanorods.The Seebeck coefficient for isotopically enriched 64 ZnO nanorods has not previous been measured but values for natural abundance n-type ZnO nanorods have been measured to be −400µV/K. 18If we take this value and use the magnitude of the thermally generated voltage that we measure for the n-type nanorod then we estimate that the actual temperature difference across the nanorod is ∆T ≈ 2K.
In our experimental set-up the electrical contact between the FIB deposited Pt and the nanorod is unlikely to be a perfect ohmic contact and may well display some rectifying characteristics 19,20 and that could account for the asymmetry in the magnitude of the thermally generated voltage that is measured when the dry ice pellet is moved between the electrodes.
The evidence presented here is consistent with previous work 4 on NTD of ZnO with a natural abundance isotopic distribution which demonstrated that that Cu on a Zn lattice site, Cuzn, acts as an acceptor dopant and reduces the majority (n-type) carrier concentration.But compared to natural abundance ZnO, our enriched 64 Zn isotope material has its 64 Zn isotope concentration increased by ∼1.5 and so after neutron irradiation the concentration of Cuzn acceptor sites has increased by ∼1.5.Furthermore, comparing the natural abundance with the isotopically enriched material the concentration of the 68 Zn isotope in enriched material is reduced by a factor of 1.8.After neutron irradiation the 68 Zn isotope is converted to 69 Gazn which can act as donor dopant and mitigate against p-type doping.After neutron irradiation the isotopically enriched material thus has 1.8 times less 69 Gazn concentration compared to the natural abundance material.Compared to natural abundance ZnO, NTD of isotopically enriched ZnO material produces an increased concentration of Cuzn acceptor sites and a reduced concentration of 69 Gazn donor sites, resulting material, whereas following NTD the natural abundance ZnO material shows only a reduced concentration of mobile electrons and the material remains n-type. 4n summary, in ZnO with natural abundance of isotopes there are two important NTD processes ( 64 Zn transforming to 65 Cuzn with a half-life of 244 days, and 68 Zn transforming to 69 Gazn with a halflife of ∼ 14 hours).To induce holes as the majority charge carriers in ZnO material we have combined 64 Zn isotope enrichment of ZnO with NTD-this 64 Zn isotopic enrichment reduces the extent of the competing process of 68 Zn transformation to 69 Gazn.The microhot probe measurements presented here indicate that 690 days after thermal neutron irradiation the enriched 64 ZnO nanorods have sufficient Cuzn acceptor dopants to compensate the original as grown  The results presented in this paper tend to support the recent reports 2,9,11 that a copper atom on the zinc atom site in ZnO, Cu Zn , can act as an acceptor type dopant.

FIG. 1 .
FIG. 1. (a)shows a schematic of the physical layout of the sample holder; thin film is deposited on a glass substrate (b) shows the equivalent electrical circuit; resistances R 12 and R 34 are measured to indicate the temperature on either side of the ZnO nanorod, while the thermally generated voltage is measured between contact pad12 and contact pad34.

TABLE I .
A summary of the micro-hot probe results for 64 ZnO nanorods.The cooling of the positive electrode is reversed by moving the dry ice pellet between the positive and negative electrodes.
Table of Hot probe results for isotopically enriched 64 ZnO nanorods material and to convert to material with holes as the majority mobile carrier.