Enhanced Thermoelectric Transport and Stability in Atomic Layer Deposited-HfO₂/ZnO and TiO₂/ZnO-Sandwiched Multilayer Thin Films

Abstract

Herein, enhancements in thermoelectric performance, both power factor and thermal stability, are exhibited by sandwiching HfO 2 and TiO 2 layers onto atomic layer deposited ZnO thin films. High temperature thermoelectric measurements from 300 to 450 K revealed an almost two-fold improvement in electrical conductivity for TiO 2 /ZnO (TZO) samples, primarily owing to increase in carrier concentration by Ti doping. On the other hand, HfO 2 /ZnO (HZO) achieved the highest power factor values owing to maintaining Seebeck coefficients comparable to pure ZnO. HZO also exhibited excellent stability after multiple thermal cycles, which has not been previously observed for pure or doped ZnO thin films. Such improvement in both thermoelectric properties and thermal stability of HZO can be attributed to a shift in crystalline orientation from a-axis to c-axis, as well as the high bond dissociation energy of HfO, stabilizing the ZnO structure. These unique properties exhibited by ALD HZO and TZO thin films pave the way for next generation transparent thermoelectric devices.

Influence of the Substrate, Process Conditions, and Postannealing Temperature on the Properties of ZnO Thin Films Grown by the Successive Ionic Layer Adsorption and Reaction Method

“…These potential applications have boosted the research related to the development of betterquality ZnO thin films over the span of ongoing decades. Both physical and chemical methods have been used for the synthesis of ZnO thin films for instances, successive ionic layer adsorption and reaction (SILAR), 27,28 chemical bath deposition, 29 pulsed laser deposition, 30 radio frequency magnetron sputtering, 31 metal−organic chemical vapor deposition, 32 sol− gel-derived dip coating, 33 spray pyrolysis, 16 hydrothermal process, 34 molecular beam epitaxy, 35 drop casting, 36 water oxidation, 37 electrodeposition, 38 atomic layer deposition (ALD), 39 different sol−gel-derived spin coating 40 techniques, and so forth. Some of the abovementioned deposition techniques have disadvantages such as usage of surfactants, high processing temperatures, hazardous chemicals, and expensive as well as sophisticated instruments.…”

“…Some of the abovementioned deposition techniques have disadvantages such as usage of surfactants, high processing temperatures, hazardous chemicals, and expensive as well as sophisticated instruments. 39 Specifically, the major demerits of ALD are the rate of speed and deposition of fractional monolayer per cycle and the chance of staying residues from precursor solution to the chamber. 41 Similarly, the main cons of electrodeposition method are the requirement of reasonably conducting substrates and slow rate of film deposition.…”

“…Both physical and chemical methods have been used for the synthesis of ZnO thin films for instances, successive ionic layer adsorption and reaction (SILAR), , chemical bath deposition, pulsed laser deposition, radio frequency magnetron sputtering, metal–organic chemical vapor deposition, sol–gel-derived dip coating, spray pyrolysis, hydrothermal process, molecular beam epitaxy, drop casting, water oxidation, electrodeposition, atomic layer deposition (ALD), different sol–gel-derived spin coating techniques, and so forth. Some of the abovementioned deposition techniques have disadvantages such as usage of surfactants, high processing temperatures, hazardous chemicals, and expensive as well as sophisticated instruments . Specifically, the major demerits of ALD are the rate of speed and deposition of fractional monolayer per cycle and the chance of staying residues from precursor solution to the chamber .…”

Robust and Electrically Conductive ZnO Thin Films and Nanostructures: Their Applications in Thermally and Chemically Harsh Environments

“…The doping of group IV elements (Ti, Hf, Zr, etc.) has been investigated to increase the conductivity and improve the thermal stability of ZnO thin films owing to their higher electronegativity and weaker ionic characteristics than conventional dopant elements (Al, Ga, and In). − These group IV elements have been suggested to suppress V O formation because of their relatively high metal–oxygen binding energy. , As shown in Figure , a previous study reported that Zr-doped ZnO thin films exhibited stable optical and electrical properties in the temperature range of 350–550 °C . In this sample, the resistivity increased significantly under thermal stresses above 550 °C, owing to the significant degradation in both carrier concentration and mobility (Figure b).…”

“…The desorption spectra of these samples indicate that the thin Al capping layer suppressed Zn desorption, which occurred at approximately 320 °C in the AZO thin films without a capping layer. Felizco et al investigated ZnO thin films sandwiched between Hf-doped or Ti-doped ZnO, demonstrating that the Hf-doped ZnO capping layer enhanced the electrical conductivity thermal stability during a thermal cycle test at 178 °C . Hüpkes et al.…”