The present invention relates to the formation of thin film semiconductors from organic metallic compounds.
Considerable research efforts in both industry and academia have been directed towards the development of new methodologies for fabricating ultra-thin organic or polymeric films of controlled architecture, high processability and robustness. Incorporation of these films in optoelectronic applications such as light emitting diodes impose additional requirements including semiconductor-purity and precise dimensional control.
Thermal evaporation of small molecules and oligomers has led to significant advances towards uniform films of controlled architecture, but morphological changes (such as crystallization) alter the structure of these films during operation of these devices. The natural ability of polymers to suppress such phase transitions requires purity and controlled multilayer design, easily attained on the other hand with low molecular weight evaporated organics.
Self-assembly has been explored as a way to control both the supramolecular and local structure. Polymers as well as small molecules have been self-assembled to yield uniform thin films for semiconductor applications. Poly(anions) with poly(cations) encompass current polymer self-assembly techniques. On the other hand, supramolecular film growth from small molecules has been achieved by self-assembly of zirconium organo-phosphonates, cobalt-diisocyanobenzenes, ruthenium pyrazines, and w-mercaptoalkanoic acids with copper or gold. Semiconductor applications including NLOs, dielectrics, photoluminescent, and photocharge generation have been demonstrated by both organo-phosphonates and polymers, although only the latter have produced electroluminescent devices.
Recent advances in organic light emitting diodes have established molecular-based light emitting diodes in the forefront for commercialization. 8-Hydroxyquinoline (8-Hq) chelates have been shown to be among the most promising of these materials for electron injection, in conjunction with triphenylamine or phenylenevinylene derivatives as hole injectors. The recent discoveries of white light emission, enhanced electroluminescence efficiency through molecular doping, and color-tuning through microcavity-based devices justify the increased interest towards commercialization of this technology. However, their performance and lifetime is significantly limited in applications demanding elevated temperatures or high brightness.
Considerable research efforts from both industry and academia have been directed towards the identification and prevention of the various failure mechanisms. Localized heating from non-ohmic contacts and film or ITO imperfections result in a number of physical and chemical transformations contributing to device degradation. Among the most prominent is the heat-activated crystallization of these organics, causing densification and large film thickness variations that lead to short-circuit failures. Polymers have been proposed as a natural avenue to overcome this problem. However, the inability to purify long molecules with incorporated defects within the chain, poses an insurmountable difficulty in attaining semiconductor purity.
Although vital advantages exist from the near-semiconductor purity of sublimed molecular organic materials as opposed to their polymeric counterparts, the cost benefits are not far superior to traditional chemical vapor deposited inorganics. The large vacuum chambers and uniform molecular beams required for both inorganics and sublimable organics, increase the costs exponentially with increasing device surface area. This presents an insurmountable barrier to the fabrication of large area electroluminescent (EL) displays and illuminators. Polymers, on the other hand, could be easily deposited from solutions to form uniform thin films (although the pin-hole density increases substantially as the film thickness decreases). This benefit is negated by the inherently low purity due to defects incorporated within the polymer chain. Optimization of carrier transport and radiative recombination is usually accomplished by successive n-type and p-type polymer layers. Unless the prior layer has been converted to an insoluble state, conventional spin- and dip-coating techniques are usually ineffective for depositing successive layers. The necessity for additional layers (such as hole or electron blocking and doped emissive layers) to achieve further performance enhancement quickly renders the polymer approach impractical.
The alternating spontaneous adsorption of monolayers of oppositely charged polymers, first introduced by Decher et al. in Thin Solids Films 1992, 210/211, 831 has been successfully utilized in fabricating complex superstructures of insulating, conducting and semiconducting multilayers as described by Fou et al in Macromolecules 1995, 28, 7115 and in Appl. Phys. 1996, 79, 7501 and by Cao et al in Acc. Chem. Res. 1992, 25, 420. Devices made from this self-assembly technique have shown remarkable film-forming uniformity and ability to manipulate a variety of alternatively charged polymers, molecular dyes and fullerenes. Although operational devices from as thin as 300 .ANG. have been constructed, device efficiency and lifetime are still limited by purity issues associated with polymeric semiconductors.
The quest for polymer analogues of 8-hydroxyquinoline based metal-chelates (such as aluminum quinolate, etc.) has been a challenging task for development of electroluminescent structures. These metal chelate polymers are non-traditional polymers and usually entail considerable difficulties in handling. The principal intricacy arises from the complexation-decomplexation dynamics, which are very sensitive to the pH, ionic strength and solvating power of the solvent. Usually for linear metal chelate polymers, solublization (if any at all) can only occur in polar aprotic solvents. These solvents are difficult to remove from spun films. The insoluble and intractable nature of these polymers makes them amenable to a self-assembly growth that would be the subject of this paper.
It is an object of the present invention to provide a novel self-assembly method for generating ultra thin film semiconductor devices based upon a metallo-organic chelate.
It is also an object to provide such a method in which the thin film devices exhibit high purity and long life.
Another object is to provide such a method which can be conducted in either organic solvents or aqueous environments.
A further object is to provide novel ultra thin film semiconductor devices with complex exhibiting dimensional and thermal stability and relatively long life.