Subsolidus phase relationships and TFT properties of the transparent semiconducting compositions in the Al2O3-ZnO-SnO2 and GeO2-ZnO-SnO2 ternary systems

Abstract

Active matrix organic light emitting diode (AMOLED) and active matrix liquid crystal diode (AMLCD) require high performance thin film transistors (TFTs) to fulfill the demands of modern display technology. To achieve this goal, transparent oxide semiconductors (TOSs) have been proved to be one of the best candidates among semiconductors because of high mobility and transparency. Due to the high cost and low availability of Indium, an Indium-free low-cost TOS is desirable which can compete with In-Ga-Zn-O (IGZO) in electrical performance. In this study, we investigated subsolidus phase relationships in the Al2O3-ZnO-SnO2 and GeO2-ZnO-SnO2 ternary systems to discover semiconducting compositions which can be used as efficient TOSs in TFT devices. First, we investigated subsolidus phase relationships in the Al2O3–SnO2–ZnO ternary system at 1200ºC in air. The phases and lattice parameters of samples were analyzed by powder X-ray diffraction (XRD). Only one type of solid solutions was found in this ternary system, which is Al-substituted zinc stannate spinel ternary compounds of Zn2-xSn1-xAl2xO4-type solid solutions with the solubility limit of 2x ≈ 0.045 at 1200ºC in air. We compared solubility limit of Al in Zn2SnO4 with other group IIIA elements such as In and Ga in the periodic table. In comparison with In2O3–SnO2–ZnO and Ga2O3–SnO2–ZnO ternary systems previously reported, the solubility limit of Al3+ (≈ 4.5 mol %) in the Zn2-xSn1-xAl2xO4-type solid solution is much smaller than those of In3+ and Ga3+ in Zn2SnO4 for the substitution of the Sn4+ site, where In3+ and Ga3+ solubility limits are 90 and 100 mol % at 1275ºC and 1250ºC, respectively, in air. The reason for this limited solubility of Al is described in detail. In addition, unlike previous report, no solid solution was found to exist more than 0.5 mol% of Zn2+ in Al2O3 at 1200°C in air. On the basis of our experimental data, we were able to construct the subsolidus phase diagram of the Al2O3–SnO2–ZnO ternary system at 1200°C in air. Second, we selected few compositions (T1-6) from this ternary system near Zn2SnO4 to prepare RF sputtering target. Thin films were prepared using these targets. Structural, electrical and optical properties of the thin films were investigated in detail. On the basis of these properties, films prepared using target T1, T2 and T3 were selected for active layer in TFT devices. Thin films prepared with these compositions showed amorphous structure with good TFT properties, where µSat = 5.446 (cm2/Vs), SS =1.097V/dec, Vth =3.95V and Ion/Ioff = 5.3x105. These properties are comparable with other Zn-Sn-O based TFTs (Table 1.2). Third, the subsolidus phase diagram of the GeO2–ZnO–SnO2at 1100ºC in air was constructed by carefully investigating the phase compatibilities in the GeO2–ZnO–SnO2 ternary system. For this study, numerous samples of various nominal compositions were prepared by conventional solid state reaction. Two different solid solutions were found in this ternary system at 1100ºC in air. One was a new ternary compounds of Zn2Sn1-xGexO4–type solid solutions, and the other was the binary compounds of Sn1-yGeyO2-type solid solutions, with the solubility limits of x ≈ 0.08 and y ≈ 0.06, respectively. Fourth, we selected two compositions (Zn2Sn0.95Ge0.05O4 and Zn2SnO4) from the Ge-Zn-Sn-O ternary system for the RF sputtering target. Structural, electrical and optical characterizations were performed on the thin films prepared with these targets. On the basis of those properties, two films were selected for the active layer in TFT devices, one is Ge4 (Ge-doped ZnSn-O) and other ZTO4 (undoped Zn-Sn-O). Enhancement of the TFT characteristics of Zn-Sn-O thin films by trap density reduction due to Ge doping observed. Thin film samples were prepared using RF magnetron sputtering of the single targets composed of Zn2Ge0.05Sn0.95O4 and Zn2SnO4, showed amorphous structure. Ge-doped Zn-Sn-O thin films exhibit much lower carrier concentration (1.02x1013 cm-3), high resistivity (35260 Ωcm) with high Hall mobility (17.99 cm2/Vs) compared to Zn-Sn-O films. The Ge-doped Zn-Sn-O TFT show a superior performance with subthreshold swing of 1.39 V/decade, threshold voltage (Vth) of +5 V and Ion/Ioff ratio of 2.5 x106. The bulk trap density of Ge-doped Zn-Sn-O film and the trap density at the active layer/gate oxide (SiO2) interface decreased one order of magnitude to 7.047x1018 eV−1cm−3 and 3.52 x1011 eV−1cm−2, respectively. Thus, Ge was confirmed as an effective carrier suppressor and trap reducer in the Zn-Sn-O system with an improved TFT performance.