Marco Moreira

In contemporary optoelectronic devices, the transparent conductive oxides commonly demonstrate n-type conduction characteristics, with indium-doped tin oxide emerging as a prominent example. However, in applications involving fully invisible electronics that necessitate p-type conductive oxides, there exists a demand for a quintessential material possessing properties akin to its n-type counterpart. CuCrO₂, a delafossite semiconductor based on copper, presently represents a notable compromise between optical and electrical attributes within the realm of p-type semiconductors. Despite numerous studies focusing on this material, the charge carrier transport regime within the material remains unclear. The commonly reported hole transport mechanism in CuCrO₂ is the small polaron model. However, this work evidences several contradictions when this transport mechanism is assumed. Using the same methodology as previous studies, we investigated the holes’ transport mechanism by the means of the measurement of electrical conductivity and the Seebeck coefficient at varying temperatures. Different charge transport models in high intrinsic defect doped CuCrO₂ thin films are explored: small polarons, grain boundaries scattering in degenerate semiconductors, and variable range hopping with nearest neighbor hopping. The small polaron model does not provide conclusive results within the temperature range analyzed. Interestingly, no specific hole transport mechanism can be undoubtedly selected. The limitations of the models highlight the influence of peculiar defects within CuCrO₂ thin films on the hole transport mechanism, particularly the adoption of well-ordered copper vacancies columns.

Off-stoichiometric Cu–Cr–O delafossite thin films with different thicknesses were grown by metal organic chemical vapor deposition on substrates with different coefficients of thermal expansion. Seebeck thermoelectric coefficient and resistivity measurements were performed on the range of 300–850 K. A qualitative change in the temperature-dependence of the resistivity is observed at the temperature corresponding to the deposition process, where the transition from tensile to compressive strain takes place. Arrhenius plots reveal different slopes in these two thermal ranges. The fact that the shift is more pronounced for the thinner films might indicate the induced strain plays a role in changing electrical behaviour. Furthermore, changes below 0.1% in electrical mobility were measured when the strain is induced by mechanical bending.

Transparent conductive oxides (TCOs) constitute a class of materials that combine high electrical conductivity and optical transparency. These features led to the development of the transparent electronics applications, such as flat panel displays, “smart” windows or functional glasses. N-type TCOs dominate the applications market, and the lack of a suitable p-type counterpart limits the fabrication of a completely transparent active device, which might be considered as a technological breakthrough. Among the wide range of p-type candidates, delafossite CuCrO₂ (and its out-of-stoichiometry derivatives) is a promising material to achieve the desired p-type TCO properties as, up to date, it is presenting the foremost trade-off between optical and electrical properties. The present paper covers the research work and the major achievements related to copper chromium delafossite. A comprehensive overview of fabrication methods and opto-electronic properties is presented. The source of doping and the charge carriers transport mechanism are also thoroughly discussed. Graphical abstract

Solution-processed metal oxides have been investigated as an alternative to vacuum-based oxides to implement low-cost, high-performance electronic devices on flexible transparent substrates. However, their electrical properties need to be enhanced to apply at industrial scale. Amorphous indium-gallium-zinc oxide (a-IGZO) is the most-used transparent semiconductor metal oxide as an active channel layer in thin-film transistors (TFTs), due to its superior electrical properties. The present work evaluates the influence of composition, thickness and ageing on the electrical properties of solution a-IGZO TFTs, using solution combustion synthesis method, with urea as fuel. After optimizing the semiconductor properties, low-voltage TFTs were obtained by implementing a back-surface passivated 3-layer In:Ga:Zn 3:1:1 with a solution-processed high-к dielectric; AlO_x. The devices show saturation mobility of 3.2 cm² V⁻¹ s⁻¹, I_On/I_Off of 10⁶, SS of 73 mV dec⁻¹ and V_On of 0.18 V, thus demonstrating promising features for low-cost circuit applications.