Atmospheric pressure spatial atomic layer deposition of metal oxides for different applications

Abstract

Atmospheric-pressure spatial atomic layer deposition (AP-SALD) is an emerging technique for the rapid, open-air deposition of metal-oxide thin films. In this thesis, I study two metal oxides, namely silicon oxide and zinc oxide, for use in two different applications.

Silicon oxide (SiOₓ) is first studied. It is a highly versatile material used in different applications. However, its conventional growth and deposition methods often require very high temperature or the use of plasma. In this thesis, I present a plasma-free, low-temperature process for depositing high quality SiOₓ thin films using AP-SALD. An aminodisilane precursor, diisopropylaminosilane (SiH₃N(C₃H₇)₂, DIPAS), was synthesized and tested with different oxidants such as ozone and 30% hydrogen peroxide aqueous solution. Initial attempts with hydrogen peroxide solution resulted in precursor condensation and the formation of nano crystallite SiOₓ contaminated with organic molecules, indicating that the deposition process is oxidant limited. In contrast, using ozone as the oxidant facilitated the deposition of high quality amorphous SiOₓ films. The microstructure was highly dependent on the deposition temperature, transitioning from nano crystallites at lower temperatures to amorphous films at temperatures of 70°C to 100°C. X-ray photoelectron spectroscopy (XPS) confirmed the deposition of continuous SiOₓ films at 70°C or above using ozone, and the growth per cycle was ~1 Å/cycle, consistent with atomic layer deposition (ALD) of SiOₓ. This work shows that high-quality SiOₓ films can be produced by AP-SALD using DIPAS and ozone, without the aid of plasma or any surface functionalization, at low growth temperatures (T >= 70°C).

For the second study, I investigate the nucleation and growth mechanism of ZnO on different 2D materials. Uniform deposition of metal oxides on 2D materials is a crucial step to realize the integration of 2D materials in practical devices. In this study, I investigate the nucleation and growth of ZnO on 2D transition metal dichalcogenides, specifically MoS₂ and WS₂. The ZnO depositions were carried out using an atmospheric-pressure spatial atomic layer deposition system operated in atmospheric pressure spatial chemical vapor deposition (AP-SCVD) mode at 100°C. The nucleation of ZnO was found to be different on MoS₂ and WS₂, whereby the ZnO nuclei formed larger clusters and nanoribbon on MoS₂ as compared to WS₂. AP-SCVD led to rapid ZnO nucleation on both CVD-grown MoS₂ and WS₂ in as little as 5 AP-SCVD oscillations and complete film closure was achieved on CVD-grown WS₂ flakes in less than 60 AP-SCVD oscillations. This was attributed to the higher precursor partial pressures and uniform precursor delivery afforded by the AP-SCVD process. Raman and photoluminescence (PL) spectroscopy revealed that AP-SCVD is a benign process that doesn’t damage the underlying 2D materials and rather helps to passivate defects via oxygen/water adsorption from the air, when performed in the appropriate temperature window. Deposition of the ZnO was found to impact the optical and structural properties of CVD-grown MoS₂ and WS₂ differently. For the MoS₂ - ZnO heterostructure, electron doping and strain dominate, resulting in a reduction in the PL of MoS₂, whereas for the WS₂ - ZnO, strain and dielectric screening have a larger impact, resulting in an enhanced PL.