Molecular Precursors for the CVD of Gallium and Indium Oxides

Lead Research Organisation: University College London
Department Name: Chemistry

Abstract

The goal of this study is to develop new highly volatile CVD precursors to deposit gallium oxide and indium oxide films free from contamination (e.g. C, F) and for a detailed investigation of the gas sensing and TCO (thermally conductive oxide) properties of the resulting films. Gallium oxide (Ga2O3) is considered to be one of the most ideal materials for application as thin-film gas sensors at high temperature. It is thermally stable and an electrical insulator at room temperature but semiconducting above 400 oC. At temperatures above 900 oC the electric conductivity changes depend on the concentration of oxygen, hence the oxygen concentration can be detected. Oxygen gas sensors have practical use in monitoring and controlling oxygen concentrations in exhaust gases of automobiles, as well as waste gases and chemical processes. Above 400 oC Ga2O3 thin-film operates as a surface-control-type sensor to reducing gases, e.g. CO and EtOH. Therefore, it is possible to switch the function of the sensor with temperature. Indium oxide films are both transparent to visible light and conductive (TCO). Dopants (e.g. Sn) can be used to increase the conductivity of the films and to make them more suitable for applications such as in solid-state optoelectronic devices. Group 13 hydrido species possess several notable characteristics that result in them being attractive as precursors to solid-state materials. Firstly, the lack of metal-carbon bonds has the potential to reduce the amount of carbon impurities in the final material and processing temperatures can potentially be reduced due to the thermally frail metal-hydride bonds. Secondly, group 13 hydrides are attractive as precursors as they are considerably more volatile than alkyl derivatives. Thus, a range of novel volatile hydrido-gallium and indium alkoxide complexes as well as heteroleptic alkoxides will be developed. The deposition of Ga2O3 and In2O3 thin-films from the novel precursors synthesised in this programme via low pressure chemical vapour deposition (LP)CVD and aerosol assisted (AA)CVD will be investigated and the gas sensor properties of the films will be assessed. By utilising a wide range of precursors and deposition techniques we will be able to produce different microstructures and develop a correlation landscape between microstructure and gas sensing response. Indium gallium oxide (GaxInyO3) is an exceptional material for TCO applications with absolute transparency that exceed all other oxides / coupled with extremely high charge mobility. Thin-films of GaxInyO3 will be grown using combinatorial atmospheric pressure (AP)CVD and mixed nanoparticulate Ga2O3 inside host In2O3 by AACVD/APCVD from the novel precursors. We have the ability to lay down thin films using a new combinatorial APCVD reactor to make films of graded composition. This new reactor enables upto 400 different compositions to be made on a single plate in one CVD experiment. This is important as it will enable us to rapidly screen composition space in the gallium-indium oxide system and make idealised and optimised compositions for gas sensing and TCO applications. The ability to optimise composition and hence performance in a single CVD experiment would demonstrate the power of the combinatorial technique. Further we have a new reactor design for making indium oxide with embedded nanoparticles- such as gallium oxide. In this system the aerosol flow enters the deposition chamber below the APCVD gas flow, this has the benefit of allowing composite films to be made in which nanoparticles either present or generated in the aerosol droplet are embedded in the APCVD host film. This combined approach will enable us to investigate different nanoparticle densities, sizes and forms and how these effect the gas sensing properties.

Publications

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Description The goal of this study was to develop new highly volatile chemical vapour deposition (CVD) precursors to deposit gallium oxide and indium oxide films free from contamination (e.g. C, F) and for a detailed investigation of the gas sensing and TCO (transparent conductive oxide) properties of the resulting films. Gallium oxide (Ga2O3) is considered to be one of the most ideal materials for application as thin-film gas sensors at high temperature. It is thermally stable and an electrical insulator at room temperature but semiconducting above 400 oC. At temperatures above 900 oC the electric conductivity changes depend on the concentration of oxygen, hence the oxygen concentration can be detected. Oxygen gas sensors have practical use in monitoring and controlling oxygen concentrations in exhaust gases of automobiles, as well as waste gases and chemical processes. Indium oxide films are both transparent to visible light and conductive and so find application as the TCO material in a range of optoelectronic devices. Dopants, such as Sn, can be used to increase the conductivity of the films and to make them more suitable for applications such as in solid-state optoelectronic devices and gas sensors. During this project we have successfully synthesised a number of novel compounds to use as precursors for the deposition or gallium oxide, indium oxide, doped-indium oxide and gallium-indium oxide films. Group 13 hydrido species possess several notable characteristics that result in them being attractive as precursors to solid-state materials. Firstly, the lack of metal-carbon bonds has the potential to reduce the amount of carbon impurities in the final material and processing temperatures can potentially be reduced due to the thermally frail metal-hydride bonds. Secondly, group 13 hydrides are attractive as precursors as they are considerably more volatile than alkyl derivatives. A range of novel volatile hydrido-gallium alkoxide complexes have been prepared and these precursors deposited crystalline Ga2O3 at the lowest temperature observed for the CVD of gallium oxide to date. However, the synthesis of these compounds was challenging so alternative precursors were also developed including gallium and indium beta-ketonates. The deposition of Ga2O3 and In2O3 thin-films from the novel precursors synthesised was achieved using aerosol assisted (AA)CVD and the gas sensor properties of the films were assessed. Doping indium oxide with tantalum resulted in the formation of films which showed an enhanced and selective response to reducing gases. Indium gallium oxide (GaxInyO3) is an exceptional material for TCO applications with absolute transparency that exceeds all other oxides, coupled with an extremely high charge mobility. Thin-films of GaxInyO3 were grown using a new technique, namely combinatorial AACVD which resulted in films of graded composition. These films enabled us to rapidly screen composition space in the gallium-indium oxide system and assess the TCO properties. Finally, indium oxide nanoparticles were also deposited onto gas sensor substrates and these films showed an excellent response to reducing gases.
Exploitation Route Gas sensors and TCO materials have a range of uses in many applications. Gas sensors show great promise for use in electronic noses and can also be used for all sorts of monitoring applications for example in food production or fire safety. One issue with metal oxide semi conductor devices is that they tend to be indiscriminate, i.e. they provide a response to a wide variety of gases, however, the indium oxide gas sesnors we produced showed a selectivity to ethanol. TCO's are utlised in a range of applications from heat-mirror window-coatings, which control the transmission of infrared energy into and out-of buildings, to their use as the transparent electrode materials in photovoltaic cells, touch-screen technology and flat panel displays. The primary beneficiaries of this work are the gas sensor community. Also inorganic chemists interested in materials synthesis, materials scientist and engineers will benefit. The work benefits a cross section of UK industry, from SAFC Epichem, a company interested in the development and utilisation of molecular precursors in CVD, City Technology (one of the worlds leading suppliers of gas/sensors) to large companies such as Pilkington Glass who are the worlds leading supplier of TCO coated glass. To enable films of industrial standard to be deposited and used in applications further research is required which is currently being supported by Pilkington.
Sectors Chemicals,Energy

 
Description The results from this award have resulted in 13 publications in high impact journals and interest in the further development of precursors to be utilised in thin film deposition has been gained by further support from Pilkington NSG. The results also led to further funding by EPSRC and Johnson Matthey. Although to date precursors developed have not been further commercialised further work is continuing based on the results generated here.
First Year Of Impact 2012
Sector Chemicals
Impact Types Policy & public services

 
Description Combinatorial CVD
Amount £814,511 (GBP)
Funding ID EP/H00064X/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 11/2009 
End 02/2013
 
Description Johnson Matthey
Amount £25,500 (GBP)
Funding ID CASE award - 57801 
Organisation Johnson Matthey 
Sector Private
Country United Kingdom
Start 10/2011 
End 04/2015
 
Description Metal Oxide Precursors
Amount £60,000 (GBP)
Organisation University College London 
Sector Academic/University
Country United Kingdom
Start 09/2010 
End 09/2013
 
Description Photocatalytic Films
Amount £60,000 (GBP)
Organisation University College London 
Sector Academic/University
Country United Kingdom
Start 09/2012 
End 09/2015
 
Description Precursor Chemistry and the CVD of Transparent Conducting Oxides
Amount £461,645 (GBP)
Funding ID EP/K001515/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 12/2012 
End 05/2015