Zinc oxide (ZnO) fabricated by atomic layer deposition (ALD) is a promising material to replace tin‐doped indium oxide (ITO) as standard transparent conductive oxide (TCO). However, its conductivity is still often about one order of magnitude below that of ITO. Variations in precursor choice, deposition process parameters, and post‐processing of ALD‐ZnO are tested concerning their impact in improvement of the TCO performance. Replacing the conventional diethylzinc (DEZ) with dimethylzinc (DMZ) as well as post‐deposition annealing in H2 yields no improvement for the highly conductive films (<10 mΩ cm) deposited at around 200 °C. However, reducing the purge time of the ALD process from 10 s down to 1 s and simultaneously increasing the DEZ pulse duration leads to a reduction in resistivity from 7.6 mΩ cm to 2.8 mΩcm. Furthermore, this process optimization involves a reduction of the total deposition time by almost one order of magnitude.
Zinc oxide (ZnO) fabricated by atomic layer deposition (ALD) is intrinsically well-conductive (∼5 mΩ cm), in contrast to the single-crystalline bulk material or sputtered ZnO thin films. There are generally three groups of candidates for the intrinsic n-type conductivity: intrinsic point defects, elemental impurities other than hydrogen, and incorporated hydrogen itself. In this study, we assess the different candidates concerning their impact on conductivity. In the presence of free electron densities of up to 5 × 1019 cm−3, impurities other than hydrogen are ruled out due to their ultra-low concentrations in the ppm range. Intrinsic point defects are also considered unlikely since the evolution of conductivity with deposition temperature is not reproduced in the Zn/O ratio as measured by Rutherford backscattering spectrometry. Hence, the most promising candidate is hydrogen with a concentration of ∼1 at. %, i.e., more than sufficient to account for the free electron density. In addition, we find a correlation between the deposition-temperature dependence of the carrier concentration and the hydrogen concentration. The formation energy of the conductive, hydrogen-related state is determined to be ∼40 meV. Hall measurements down to liquid helium temperatures revealed that the electron densities are constant over the whole temperature range. This constitutes a quasi-metallic behavior of ALD-ZnO for deposition temperatures of ≥150 °C. We propose that the very high concentration of hydrogen-induced donor states causes a vanishing ionization energy so that the donor band merges energetically with the ZnO conduction band. This model is supported by ultraviolet photoelectron spectroscopy measurements.
Understanding the stability and deposition parameter dependence of intrinsically conductive undoped ZnO prepared by thermal atomic layer deposition is mandatory for future applications. The authors investigate the conductivity of ZnO films deposited at temperatures between 100 and 200 °C as well as its evolution over a period of 160 days under different storing conditions. Most importantly, the conductivity increases by about 1 order of magnitude when the deposition temperature is increased from 100 to 150 °C. Highest conductivities of up to 170 S/cm are reached for ≥175 °C, and these samples do not show any aging effects of the conductivity under ambient storing conditions. In contrast, for deposition temperatures ≤150 °C, accelerated aging led to a significant decrease in conductivity. The best trade-off between the low deposition temperature and good long-term stable conductivity is found to be at 175 °C. A correlation between the intensity of the well-known defect photoluminescence peak (∼1.9 eV) and the conductivity was observed, which indicates that both are related to the same physical origin.
Zinc oxide (ZnO) thin films deposited by atomic layer deposition (ALD) and a (0001)-oriented bulk-ZnO single-crystal are compared by ultrafast time-resolved and spectral photoluminescence spectroscopy as well as by luminescence quantum yield (QY) measurements. While the ALD-ZnO is intrinsically conductive, bulk-ZnO is electrically rather insulating. Nevertheless, PL spectra of both materials reveal similarities: A peak in the near-UV originating from inter-band transitions and a defect peak in the green-yellow spectral region associated to deep trap states. We investigate the dynamics and efficiency of these luminescence emissions to understand the interdependency of defects and the yet insufficiently understood conductivity mechanism of ALD-ZnO. Spectrally, a PL blueshift of the trap-mediated peak is observed for the transition from poor to good conductivity, which 2 occurs with increasing ALD-deposition temperature. The quantum yield of ALD-ZnO is shown to be ~18-times lower and the luminescence lifetime of the UV-peak is ~14-times shorter compared to bulk-ZnO. We conclude that these properties are related to a significantly higher density of non-radiative defects, which might also represent electrically active scattering centers. These results indicate a new direction for further optimization of ALD-ZnO towards indium tin oxide (ITO)-compatible electrical conductivities by reducing or passivating these defects.
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