Contents lists available at ScienceDirect Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp Reheating induced atomic migration in Al-doped ZnO (AZO) films: Effect on the growth of AZO/ZnO bilayer Narendra Bandaru, Emila Panda ∗ Department of Materials Science and Engineering, Indian Institute of Technology Gandhinagar, Palaj, 382355, Gujarat, India ARTICLE INFO Keywords: Atomic migration Reheating AZO/ZnO bilayer Al-doped ZnO Sputtering Optoelectronic properties ABSTRACT In this work, reheating-induced transition in Al, Zn and O atomic positions in Al-doped ZnO (AZO) thin films is reported, and its application is shown for the growth of AZO/ZnO bilayers. Here thickening ZnO films were deposited on the AZO-coated soda lime glass (SLG) substrates by adopting two different fabrication routes. In the first process, ZnO film was deposited on the AZO-coated SLG substrate for appropriate duration immediately after the deposition of the AZO layer, whereas in the second process, the bottom AZO-coated SLG was cooled to room temperature after its deposition and then reheated again to 623 K for depositing the ZnO layer. To compare and interpret the microstructure and optoelectronic properties of these bilayers with respect to ZnO layer thickness and/or process type, the bottom AZO layer was also reheated for the same duration (as that of the second process). Atomic migration, followed by chemical reaction between these species is observed for all these reheated samples, which affected their optoelectronic properties. More precisely, some of the Al 3+ cations are found to be un-substituted from the Zn 2+ lattice sites, which then reacted with the excess oxygen present in these films to form Al x O y . These unsubstituted lattice sites are then filled by the Zn 2+ cations, which were originally positioned in the interstitial sites. This atomic rearrangement is then found to alter the optoelectronic properties of these single and bilayers. 1. Introduction Device fabrication often involves depositing various layers, many a times using different synthesis techniques and process parameters for developing these distinctive layers, that are augmented for their op- timal growth [1–4]. Out of all process parameters, substrate tempera- ture plays an important role in terms of controlling the microstructure of these layers and their thus induced optoelectronic properties. How- ever, a higher process temperature for depositing the successive layers in a device would unnecessarily heat up the bottom layers, often mul- tiple times, thereby bringing microstructural changes in these layers, which might adversely affect the microstructure and optoelectronic properties of the top layers and hence the entire device performance. To understand the effect of heating and the thus induced changes in the microstructure and optoelectronic properties of the bottom layer and its effect on the top layer(s), in this work, AZO (Al-doped ZnO)/ ZnO bilayers were used as a model system. Here, first ∼242 nm thick AZO film was deposited on cleaned soda lime glass (SLG) substrate at 623 K substrate temperature (T s ) by using the parameters optimized in a separate study [5]. Next, thickening ZnO layers were deposited on this AZO coated SLG substrate by varying the deposition time up to 15 min at the same T s of 623 K, but using the following two processes: in process I, ZnO layer was deposited on the AZO-coated SLG substrate for appropriate duration immediately after the deposition of the AZO film and in process II, the bottom AZO-coated SLG was cooled to room temperature after its deposition and then reheated again to 623 K for depositing the ZnO layer. To compare and interpret the microstructure and optoelectronic properties of these bilayers with respect to the de- position time and/or process type, the bottom AZO layer was also re- heated for the same duration (as that of process II). After deposition and/or reheating, a range of experimental techniques were used to characterize these films. Here, AZO and ZnO are used as the model bilayers as these are low cost, environment friendly optoelectronic materials and thus find ap- plications in various optoelectronic devices, like, thin film solar cells, liquid crystal displays, transparent conductors, surface acoustic wave (SAW) devices, chemical sensors, field effect transistors, light emitting devices, etc [6–12]. Though these materials are widely investigated by the researchers, a systematic understanding on the heating induced microstructural and optoelectronic change in these layers and also in the bilayers is found to be scarce. Moreover, radio frequency (RF) magnetron sputtering is used to deposit these layers, as this is a https://doi.org/10.1016/j.mssp.2019.05.011 Received 1 February 2019; Received in revised form 1 April 2019; Accepted 8 May 2019 ∗ Corresponding author. E-mail address: emila@iitgn.ac.in (E. Panda). Materials Science in Semiconductor Processing 100 (2019) 220–224 1369-8001/ © 2019 Elsevier Ltd. 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