(A) study on high mobility indium oxide thin films and thin-film transistors by means of plasma-enhanced atomic layer deposition

Abstract

After over a decade of huge effort, oxide TFTs have been adopted in the backplane of high resolution TFT-LCD and AMOLED displays. However, as displays evolve to next generation, high resolution over 2000 ppi and high driving speed are required. To meet these demands, realization of high mobility switching and driving TFTs are crucial. Indium oxide (IO) has been suggested as a potential high mobility materials, but its high carrier density and polycrystalline phase often limit the electrical performance. To address this limitation, we propose ultra-thin IO films by means of plasma-enhanced atomic layer deposition. By applying PEALD, uniform and well-defined films can be obtained with precisely fcontrolled thickness. The film was successfully grown by the reaction between $Et_2InN(SiMe_3)_2$ precursor and oxygen plasma. Plasma reaction facilitate film growth rate and change properties of films. A variety of tools were used to measure chemical and electrical characteristics of IO films with deposition temperature ranged from $100^\circ C$ to $250^\circ C$. Depending on substrate temperatures, each film contained different amount of impurities and Si, resulting in chemical composition difference. To estimate the IO film as a channel layer, a bottom-gate coplanar structured TFTs were fabricated with 5-nm thick IO layer. Even though it had polycrystalline structure, not fully amorphous phase, it exhibited quite high mobility. The resultant device fabricated at substrate temperature of $250^\circ C$ showed the field-effect mobility of as high as $39.2 cm^[2}/V \cdot s$, acceptable turn-on voltage value of -1.18 V, subthreshold voltage of 0.27 V/dec. in linear region. Furthermore, this PEALD-IO layer was adopted to vertical channel TFT which has the smallest footprint among the TFTs. Although its field-effect mobility was degraded, it showed a possibility of high performance TFT with good step coverage.