Transport-property tailored thin films for thermoelectrics through atomic/molecular layer deposition

Abstract

Atomic layer deposition (ALD) creates a unique opportunity for effective materials nanostructuring. In this thesis, ALD, along with its other form, molecular layer deposition (MLD) and spin coating (SC), are utilized to alter the electrical, thermal, and structural aspects of thin films. Such alterations can be advantageous in various applications; however, thermoelectrics has been the subject of this study as a proof of concept. Thermoelectric energy harvesters are an intriguing group of materials capable of transforming thermal gradient to electrical potential and vice versa. The sequential nature of ALD and its independent deposition parameters provide tools for modifying the deposited films. Higher electrical and lower thermal conductivities are needed to achieve a better thermoelectric material. However, achieving this goal can be challenging due to the thermal conduction via electrons. The only option left is to suppress the thermal conductivity via phonons. The current research has illustrated that even a minimal change in deposition parameters, such as purge time after the metal precursor pulse, can improve thermoelectric performance through defect formation. Consequently, increase the carrier conduction via electrons and decrease the thermal conduction via phonons. Another approach to improving thermoelectric performance is to create interfaces inside the films. In the current study, this was carried out by growing superlattice films using a combination of ALD, MLD, and SC. Specifically, superlattice films of ZnO with different organics, i.e., p-phenylenediamine (PPD), hydroquinone (HQ), terephthalic acid (TPA), 4,4'-oxydianiline (ODA), and cellulose nanocrystals (CNCs) were prepared and studied. The results indicate that different organic compounds can have distinct effects on the inorganic matrix. This thesis focuses primarily on ZnO as an ideal n-type thermoelectric material. However, a p-type equivalent must be coupled with the n-type ZnO to complete the thermoelectric module. To address this issue, here, a versatile ALD process for SnO was developed in this study. The obtained films were analyzed and confirmed to be pure SnO films.