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Properties of metal oxides for electronic and optoelectronic devices

Periodic Reporting for period 1 - Pro-Oxides (Properties of metal oxides for electronic and optoelectronic devices)

Reporting period: 2015-04-01 to 2017-03-31

Metal oxides are an essential building block of thin film electronics (a multi-billion dollar industry) and are incorporated in a range of commercially successful applications including photovoltaics, transparent transistors, random access memory and many more. Since the operation of thin film devices is dictated by the band alignment of the constituent layers, the strength of metal oxides is the diversity in the optoelectronic properties. The work function for an oxide can even be tuned due to the sensitivity of metal oxide properties to oxygen stoichiometry, while other band energetics can be modulated through cation doping. This sensitivity to stoichiometry is unique and offers unrealised opportunities for accurate energy level alignment with oxides but also makes electronic properties of oxides strongly dependent upon fabrication methods and parameters.

The growth of the thin film electronics industry depends on its ability to compete with already established silicon-based electronics, it is, therefore, necessary to improve the economic viability of this technology. To this end, there is a growing demand for a shift from expensive vacuum based to a low-cost solution based fabrication process which is expected to lower the cost by about 64%. [J. Phys. D: Appl. Phys., (2016) 49, 433001] Unfortunately, at this stage there is no clear understanding of how adopting new fabrication methods will affect the electronic properties of oxides, although recently Chen et al. [J. Mater. Chem., (2012) 22, 24202-24212] highlighted that for a few specific oxides the work function of air-exposed and solution processed films are 0.5-1.6 eV lower than vacuum deposited films.

The objective of the present proposal is to address the ambiguity in metal oxide properties and to establish principles for energy-level alignment between metal oxide layers. The specific goals of this project are to:
• Perform comprehensive electrical, optical, structural and chemical characterisations of a collection of metal oxide compounds each with a controlled variation in the chemical composition
• Correlate the electronic and optical properties of compounds to the stoichiometry
• Provide insight into the energy-level alignment at metal oxide/metal oxide interfaces
Understanding metal oxide/metal oxide interfaces
We developed a novel methodology for directly mapping the depth profile of the band alignment, in air, by growing a sample with an intentional thickness gradient and correlating the surface electrical potential, measured using a scanning Kelvin probe across the gradient. Although we often measured on the order of 30 points, we interpolated to 13 data-points (to coordinate with data from other instruments); we attain more points with a single deposition than typically reported when using the conventional approaches. To analyse the experimental results, we developed a straightforward fitting algorithm that solves the 1D Poisson’s equation in order to calculate the amount and width of the band bending and the carrier concentration. This allows a universal description of the energy-level scheme that can be applied to complete thin film devices and is compatible with conventional semiconductor physics. Our approach is non-destructive and rapid, can be performed in air (or vacuum) and provides superior statistics while enjoying high depth (2 nm) and energy resolution (5 meV).

The main findings have been disseminated at two international conferences as posters and one conference as a presented and at two invited laboratory visits. Two publications have been prepared and submitted to peer-review journals.

Figure 1. Top: Schematic depicting the lateral measurement of the Kelvin probe across substrate with a metal oxide of uniform thickness and an adlayer of a second metal oxide. Bottom: the corresponding evolution of the measured work function as the adlayer thickness is increased.

Characterisation of the band alignment of metal oxide surfaces in air
To investigate the effects of air on the band alignment of metal oxide surfaces we performed an extensive study on collection of almost 20 different oxides. Each oxide was grown as a film in high vacuum and the initial work function of the surface was measured immediately upon removing the sample from the growth chamber, for some oxides the ionisation energy was also measured. The work function and ionisation energies were remeasured after a week, once it had stabilised in air. In addition, we performed optical measurements to establish the band gap. As summarised in Figure 2.

Figure 2: Summary of the energy level alignment for metal oxides measured in air immediately after removal from the deposition chamber (initial) and remeasured after a week or more storage in air (air-stable).

For every oxide we measured we observed a reduction in the work function and ionisation energy upon exposure to air.There is a greater reduction for oxides with a high initial work function. By using such a large set of oxides we can conclude that the work function reduction upon exposure to air is universal. We also found that the reduction of the work function occurs exponentially with time in air and in all cases saturates within a week. These finding are very exciting and will be vital in developing new non-vacuum fabrication methods for oxides. A manuscript detailing this research is in preparation for publication in a high-quality journal given the great relevance of this work and exhaustive number of oxides analysed.
The Pro-oxide project made breakthroughs in three key aspects of the implementation of metal oxides in thin film devices:

1. We devised a novel depth profiling methodology that requires relatively inexpensive instrumentation that is commonly found in research laboratories and is performed in air, avoiding the need of vacuum equipment. Finally, the relevant theory used to analyse the results is well-established and applicable to other systems. We propose that this method will be adopted by others and will promote further advances in low-cost depth profiling and ultimately the characterisation of thin film interfaces.

2. The systematic study on the influence of air on the band alignment of metal oxides undertaken during this project was the first of its kind and revealed the impact of air exposure on the electronic properties of metal oxides. This information is critical for developing new, non-vacuum fabrication techniques and understanding how the exposure to air during growth will affect the operation of thin film devices. Furthermore, this study is the perfect foundation for studying the effects of other contaminants.

3. During this project we characterised the band alignment of a large selection of metal oxides, many of which have research and commercial applications. This information provides insights into the electronic properties of oxides but also into how oxides will interact with other materials and is therefore crucial to countless researchers designing new thin-film devices. The advantage of the large study undertaken during this project is that it permits direct comparison of the band alignment values for the reported oxides without the issues of system errors that arise from using values from various sources.
Figure 1. Schematic of Kelvin probe measurements and measured work function
Figure 2: Summary of the energy level alignment for metal oxides measured in air