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Understanding the origin of the strong difference of electrical parameters between as grown and annealed undoped ZnO films prepared at a temperature range of 100–200 °C by thermal atomic layer deposition is essential for their future applications. In this paper, we show that the conductivity drop by up to 4 orders of magnitude as effect of post-growth annealing is accompanied by multiple simultaneous effects like a two orders of magnitude decrease of hydrogen impurity content, a strong width reduction of the luminescence peaks, and an increase of crystallite sizes influencing the carrier scattering. We show that a level of structural and optical improvement as well as the final electrical parameters of annealed films strongly depend on the previously employed growth temperature, which is related to transition from oxygen- to zinc-rich conditions influencing a type and concentration of native point defects. The growth temperature does not only influence the bandgap energy but also the binding energies of existing donors and the relative ratio between the number of donors and acceptors; hence, it determines the final electrical characteristics of the films. This means that electrical properties of undoped ZnO-atomic layer deposition films can be tuned by native defects engineering. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pd Schottky contacts to H 2 O 2 functionalized ALD grown ZnO were realized. • Due to the H 2 O 2 functionalization a ~ 200 nm thick modified oxygen rich layer forms. • Low series resistance devices were obtained by using AZO as a back contact. • Leakage current analysis indicates defective channels as main leakage sources. • Oxygen rich layer fingerprints are also observed electrically. Vertical structures based on Pd Schottky contacts to a ZnO/AZO bilayer have been realized and electrically characterized. Following a preliminary annealing study on the separated ZnO and AZO layers the ZnO/AZO bilayer was first annealed at 450 °C for 30 min to reduce the carrier concentration and then functionalized with a H 2 O 2 dip prior to Pd deposition. On the basis of Atomic Force Microscopy, Transmission electron Microscopy and Capacitance vs Voltage measurements it has been found that the H 2 O 2 treatment lowers the surface roughness and creates a ~ 200 nm thick layer formed by ZnO crystallites with grain size ≲ 10 nm embedded into an amorphous matrix with reduced carrier concentration. However, contrary to what previously reported no formation of ZnO 2 has been observed. The obtained Schottky contacts showed a rectification ratio ≳ 10 2 at −2 V/+2 V with a detailed analysis based on the differential approach pointing to field enhanced emission from defective channels as the main leaking mechanisms. [ABSTRACT FROM AUTHOR]