1.  Field of the Invention
Aspects of the present invention relate to a method of fabricating a polycrystalline silicon (poly-Si) layer, a thin film transistor (TFT) fabricated using the same, a method of fabricating the TFT, and an organic lighting emitting diode (OLED) display device including the TFT.  More particularly, aspects of the present invention relate to a method of fabricating a poly-Si layer, in which a thermal oxide layer is formed to a thickness of about 10 to 50 Å on an amorphous silicon (a-Si) layer so as to crystallize the a-Si layer into a poly-Si layer using a super grain silicon (SGS) crystallization method, a TFT fabricated using the same, a method of fabricating the TFT, and an OLED display device including the TFT.  In the method, an additional process of forming a capping layer typically required for the SGS crystallization method may be omitted. 
2.  Description of the Related Art
In general, a polycrystalline silicon (poly-Si) layer is widely used as a semiconductor layer for a thin film transistor (TFT) because the poly-Si has a high field-effect mobility, can be applied to a high-speed operating circuit, and can used to configure a complementary-metal-oxide-semiconductor (CMOS) circuit.  A TFT using a poly-Si layer is typically used as an active device of an active-matrix liquid crystal display (AMLCD) or a switching device or a driving device of an organic light emitting diode (OLED). 
Methods of crystallizing an a-Si layer into a poly-Si layer may include a solid phase crystallization (SPC) method, an excimer laser crystallization (ELC) method, a metal induced crystallization (MIC) method, and a metal induced lateral crystallization (MILC) method.  In the SPC method, an a-Si layer is annealed for several to several tens of hours at temperatures below 700° C., the temperature at which the glass substrate typically used in a TFT for a display device is transformed.  In the ELC method, excimer laser beams are irradiated onto an a-Si layer so that the a-Si layer is partially heated to a high temperature in a very short amount of time.  In the MIC method, a metal such as nickel (Ni), palladium (Pd), gold (Au), or aluminum (Al) is brought into contact with or doped into an a-Si layer to induce a phase change of the a-Si layer into a poly-Si layer.  In the MILC method, silicide formed by reaction of metal with silicon laterally diffuses so as to sequentially induce crystallization of an a-Si layer. 
However, the SPC method takes too much time and may lead to deformation of the substrate because the substrate is annealed at a high temperature for a long time.  The ELC method requires expensive laser apparatuses and results in the formation of protrusions on the poly-Si surface, thereby degrading interfacial characteristics between a semiconductor layer and a gate insulating layer.  When the MIC or MILC method is employed, a large amount of metal catalyst may remain in a crystallized poly-Si layer, thereby increasing leakage current of the semiconductor layer of a TFT. 
Recently, a vast amount of research has been conducted on methods of crystallizing an a-Si layer using a metal catalyst in order to crystallize the a-Si layer at a lower temperature and in a shorter amount of time than in an SPC method.  Typical methods of crystallizing an a-Si layer using a metal catalyst are the MIC method and the MILC method mentioned above.  In these methods, however, the device characteristics of a TFT may be degraded due to contamination caused by the metal catalyst. 
In order to prevent the contamination caused by the metal catalyst, a super grain silicon (SGS) crystallization method has been developed.  In the SGS crystallization method, the amount of metal catalyst that diffuses into the a-Si layer is controlled to provide a low concentration of the metal catalyst in the a-Si layer.  Because the metal catalyst is spaced apart at a low concentration in the a-Si layer, the size of crystal grains that are catalyzed by the metal catalyst ranges from several to several hundred μm.  Typically, the SGS crystallization method may include forming a capping layer to control diffusion of the metal catalyst into the a-Si layer, forming a metal catalyst layer on the capping layer, and annealing the metal catalyst layer so that a low concentration of the metal catalyst diffuses through the capping layer and into the a-Si layer, and then crystallizing the a-Si layer into a poly-Si layer. 
However, the above-described SGS crystallization method involves an additional process of forming a capping layer, which is typically carried out using a chemical vapor deposition (CVD) technique or a physical vapor deposition (PVD) technique, thereby complicating the fabrication process of the poly-Si layer.