THE INFLUENCE OF INDUCED STRAIN ON ELECTRIC PROPERTIES OF NONSTOICHIOMETRIC COPPER CHROMIUM OXIDE

Transparent p-type semiconductors with optoelectronic properties comparable to those of n-type semiconductors are essential for advancing transparent electronics applications. The development of a completely transparent p-n junction remains a significant challenge due to the lack of high-performing p-type transparent (semi)conducting materials. These materials often exhibit low carrier mobilities, making them unsuitable for industrial-scale applications in modern "invisible" circuitry. Enhancing mobility is essential for improving the speed, efficiency, and overall performance of transparent electronic devices, which are increasingly relevant as technology evolves. Achieving a high figure of merit for p-type materials is regarded as the 'holy grail' for fabricating technologically relevant, fully invisible electronic devices.
One promising approach to enhance carrier mobility is strain engineering, which can significantly improve mobility by altering the band structure due to interatomic displacement. This thesis focuses on the effect of strain on the hole mobility of delafossite copper chromium oxide (CCO). Various mechanisms to induce strain were reviewed and applied, including mechanical bending, magnetostriction, thermal expansion and extrinsic doping.
We synthesised thin films using metal-organic chemical vapour deposition. Thermal strain was induced by growing CCO on substrates with different coefficients of thermal expansion. Mechanical bending resulted in minimal changes in mobility for very small strain values, as higher strains were unattainable due to substrate breakage. No changes were observed in the electrical properties of CCO grown on a magnetostrictive nickel substrate when strain was induced. For thermally induced strain, the electrical conductivity exhibited semiconductive behaviour, while the temperature-dependent Seebeck coefficient indicated energy dispersion within the polaronic states in these polycrystalline films. A distinct feature in the electrical conductivity curve emerged at the deposition temperature, indicating the shift between tensile and compressive strain regimes in the thin films. This feature is more pronounced in thinner films and nearly vanishes in thicker films due to reduced average strain. This observation suggests that strain could influence mobility, although definitive quantitative conclusions cannot yet be drawn.
This study also explored the effect of doping CCO with trivalent (Al, Mn, Sc, Y) and divalent (Mg, Zn) cations. Although doped films exhibited off-stoichiometry characterised by excess oxygen, all doped films exhibited similar room-temperature electrical conductivities. X-ray diffraction sin2Ψ profile analysis revealed compressive strain in all doped films, but the differences were too negligible to conclude a linear variation of strain with dopant cation radii. The sin2Ψ method also indicated compressive strain in both (006) and (012) reflections at room temperature, suggesting that the first epitaxially grown layers altered the effect of thermal strain.
The findings indicate that strain engineering in polycrystalline CCO can influence mobility, but not significantly enough to achieve the high values required for technological applications. The results confirm that the hole effective mass in polaronic materials is inherently high and challenging to manipulate. The peculiar behaviour of the Seebeck coefficient was investigated, and a method to extract accurate values of optoelectrical parameters may provide insights that could resolve ongoing debates regarding mobility values in small polaronic materials.