Thin-film speed-up
The new approach spin coats a 50Angstrom thick layer of crystalline and continuous metal chalcogenide films. Chalcogens are the four chemical elements - oxygen, sulfur, selenium and tellurium – that make up the sixth column of the Periodic Table. The term is derived from two Greek words meaning "ore formers". Some metal chalcogenides - such as cadmium sulfide, tin selenide and zinc telluride - are known as having high-performance semiconductor properties.
The spin-coat deposition is based on the low-temperature decomposition of highly soluble hydrazinium precursors. Hydrazine is a molecule made up of two nitrogen and two hydrogen atoms. To dissolve the semiconducting material, the researchers combined a very strong hydrazine solvent with equal numbers of chalcogen atoms and semiconducting metal chalcogenide molecules (such as sulphur and tin sulphide, respectively). While hydrazine is generally not a good solvent for metal chalcogenides, the presence of the extra chalcogen atoms both improves solubility and enables control over the film's final composition and grain structure.
Heating the resulting film causes both the hydrazine and extra sulphur to dissociate and evaporate, leaving just a very thin layer of solid metal chalcogenide with a uniform thickness as small as 5nm. When the team optimised the molecular proportions, spin-coating conditions and heat/annealing procedures, the films exhibited charge mobilities approaching that of polycrystalline silicon and 10 times that of any previously spin-coated material or amorphous silicon.
The team further fabricated thin-film field effect transistors based on semiconducting SnS(2-x)Se(x) films. These layers exhibit n-type transport with current densities greater than 1E5A/cm2. Mobilities exceed 10cm2/V/s.
The spin coating technique is expected to apply to a range of metal chalcogenides – particularly those based on main-group metals. Applications for solution-processed electronics include advanced displays, flexible devices, high-function smartcards, RFID tags, photovoltaic solar cells, thermoelectrics and phase-change solid-state memories.
"These types of easily processed semiconducting films could eventually be used to make circuitry for very-low-cost or flexible displays, high-performance smartcards, sensors and solar cells or for flexible electronics coated onto a wide variety of moulded or plastic shapes," says David Mitzi, the IBM Research team leader.
A spin-coated film's thickness is usually determined by the solution's viscosity (its resistance to flow) and the rate and duration of spinning. The liquid is then cured into a solid thin film upon which transistors and other various electronic devices can be made.
Mitzi's next step is to reduce or replace the use of hydrazine, a highly energetic molecule also used as rocket fuel, with a more benign but still effective solvent.
"Now that I understand how this new process dissolves the metal chalcogenide, I'm confident that new solvents can be substituted," Mitzi says.
Until now, the only semiconducting materials that could be made using spin coating had limited usefulness due to low charge mobility. Better semiconductors could not be dissolved in any liquid that would result in a thin film that retained the desired charge mobility. IBM researchers believe the new technique will significantly accelerate progress toward the widespread use of thin-film electronics made by the family of fast, inexpensive, high-throughput solution processes such as spin coating, printing, stamping, nanoimprinting, inkjet printing and dipping.
Picture: Thin-film transistor structure
