In situ monitoring atomic layer doping processes for Al-doped ZnO layers: Competitive nature of surface reactions between metal precursors

Abstract

In this work, surface reactions during the atomic layer doping (ALDp) process of aluminum-doped zinc oxide (AZO) films have been studied. Conventional supercycle and alternative quasi-simultaneous codosing methods are analyzed within the 100–200 °C substrate temperature range. Two quasi-simultaneous codosing cases are investigated: (1) diethylzinc (DEZ) followed by trimethylaluminum (TMA) and (ii) TMA followed by DEZ. Quasi-simultaneous codosing experiments featured back-to-back DEZ/TMA or TMA/DEZ precursor and H2O pulses separated by nitrogen (N2) purge cycles. The grown films were characterized via (i) real-time in situ ellipsometry to monitor the individual surface ligand exchange reactions via variations in the film thickness in each half-cycle; (ii) ex situ ellipsometry to determine the film optical constants; (iii) x-ray photoelectron spectroscopy to measure the elemental composition and chemical bonding structure, and (iv) x-ray diffraction to evaluate the crystal properties. The most significant finding of the study is the dominance of TMA over DEZ: for all of the quasi-simultaneous codosing samples, no matter which precursor is pulsed first and whether there is a time delay between TMA and DEZ pulses or not, zinc (Zn) incorporation within the AZO films is substantially lower than aluminum (Al). This result demonstrates the competitive nature of surface reactions between TMA and DEZ, where the winning side is TMA. Al is effectively incorporating into the film while severely limiting Zn-incorporation and even replacing chemisorbed Zn-groups via conversion surface reactions. As a result, the quasi-simultaneous codosing approach for AZO films using DEZ and TMA precursors leads to minimally (less than 2%) Zn-doped Al2O3 films (ZAO), depicting the advantages of controlled ALDp process via the conventional supercycle method.