Plasma-Enhanced Atomic Layer Deposition of Molybdenum Oxide Thin Films at Low Temperatures for Hydrogen Gas Sensing

Abstract

Molybdenum oxide thin films are very appealing for gas sensing applications due to their tunable material characteristics. Particularly, the growing demand for developing hydrogen sensors has triggered the exploration of functional materials such as molybdenum oxides (MoO x ). Strategies to enhance the performance of MoO x -based gas sensors include nanostructured growth accompanied by precise control of composition and crystallinity. These features can be delivered by using atomic layer deposition (ALD) processing of thin films, where precursor chemistry plays an important role. Herein, we report a new plasma-enhanced ALD process for molybdenum oxide employing the molybdenum precursor [Mo(N t Bu) 2 ( t Bu 2 DAD)] (DAD = diazadienyl) and oxygen plasma. Analysis of the film thickness reveals typical ALD characteristics such as linearity and surface saturation with a growth rate of 0.75 Å/cycle in a broad temperature window between 100 and 240 °C. While the films are amorphous at 100 °C, crystalline β-MoO 3 is obtained at 240 °C. Compositional analysis reveals nearly stoichiometric and pure MoO 3 films with oxygen vacancies present at the surface. Subsequently, hydrogen gas sensitivity of the molybdenum oxide thin films is demonstrated in a laboratory-scale chemiresistive hydrogen sensor setup at an operation temperature of 120 °C. Sensitivities of up to 18% are achieved for the film deposited at 240 °C, showing a strong correlation between crystallinity, oxygen vacancies at the surface, and hydrogen gas sensitivity.

Thermionic Emission of Atomic Layer Deposited MoO3/Si UV Photodetectors

“…The results show that the samples showed n -type semiconducting behavior. This is may have come from the n -type nature of MoO 3 and the presence of oxygen vacancies in suboxides [ 42 ]. The stoichiometric form of α-MoO 3 is an insulator, but it is commonly found as an n -type semiconductor because of the presence of oxygen vacancies [ 43 ].…”

Amorphous Bimetallic Cobalt-Based Transition-Metal Oxides for Gas Sensing Application with Regulated Selectivity

“…It is well-known that the gas sensing performances are tightly related to the chemical component and nanostructure of sensing materials. − In recent years, several strategies have been developed to improve the sensing properties by modifying MOS with various materials such as metal nanoparticles, graphene, two-dimensional materials, and conductive polymers or the formation of a MOS–MOS heterojunction . However, crystalline materials usually have tightly packed structures and pronounced grain boundaries, which lead to uneven distribution of exogenous phases on the surface of transition-metal oxides and high energy consumption due to unexpectedly high operating temperatures in gas sensing .…”

Electrochemistry of Thin Films and Nanostructured Materials

“…The applications of thin films are very extensive and cover metallic conductive layers in silicon integrated circuits and integrated electronics [ 24 , 35 ], thin layers covering glasses in smart windows that transmit visible light, and reflect UV and infrared light [ 21 , 36 ], thin foils for magnetic recording, data storage, and logic circuits [ 22 , 37 , 38 ], transparent semiconductor layers in solar cells and photovoltaics [ 15 , 39 ], thin films in Li-ion batteries and supercapacitors [ 40 , 41 ], thin catalytic and electrocatalytic films for water electrolysis, wastewater treatment, and detecting chemical compounds [ 42 , 43 ], thin films protecting against corrosion, friction, and wear on parts of car and aircraft engines (pistons, cylinders, turbine blades, etc.) [ 44 , 45 ], and many others [ 46 , 47 ].…”