Myung Soo Huh

Improvement in the Performance of Tin Oxide Thin-Film Transistors by Alumina Doping

Thin-film transistors (TFTs) were fabricated with an aluminum oxide-doped tin oxide (SAO) channel, deposited by cosputtering SnO 2 and Al 2 O 3 targets. The effect of the Al 2 O 3 content on the device performance of the SnO x -based TFTs was investigated. The TFTs with a nondoped SnO x channel did not show a promising performance. However, the field-effect mobility and threshold voltage of the SAO TFTs with an Al concentration of about 0.9 atom % were improved to ~3.8 cm 2 /V s and ~0.6 V, respectively. This improved device performance was attributed to the greatly reduced carrier concentration induced by the carrier trapping at the Al impurity sites.

Improvement of electrical and optical properties of molybdenum oxide thin films by ultralow pressure sputtering method

In this work, we investigated the structural, electrical and optical properties of molybdenum oxide thin films deposited by the reactive dc magnetron sputtering method. The molybdenum oxide films were prepared at sputtering pressures ranging from 6.7u2009×u200910−1 to 6.7u2009×u200910−2 Pa. In order to promote their electrical conductivity, all the deposited MoOx films were annealed in Ar ambient at 450u2009°C for 8 h. The resistivity of the MoOx films varied from 10−4 to 10−2 Ωu2009cm depending on the O2 content in the sputtering ambient. The lowering of the resistivity of the MoO2 films was mainly attributed to the formation of a monoclinic MoO2 polycrystalline phase. As the sputtering pressure decreased, the content of monoclinic polycrystalline MoO2 phase increased, resulting in low resistivity films. The formation of the dominant MoO2 phase at lower sputtering pressures was attributed to the stress induced crystallization. The post-deposition annealed (PDA) MoOx film, deposited at an ultralow sputtering pressure (6.7u2009×u200910−2 Pa) and O2 content of 40%, had an atomic ratio of O to Mo ≈ 2.85 and was highly transparent and conductive: the transmittance in the visible wavelength range of 400–500u2009nm was about 73% and the resistivity was 1.05u2009×u200910−3 Ωu2009cm. This result is superior to those of MoOx films epitaxially grown by the pulse laser deposition method.In this work, we investigated the structural, electrical and optical properties of molybdenum oxide thin films deposited by the reactive dc magnetron sputtering method. The molybdenum oxide films were prepared at sputtering pressures ranging from 6.7u2009×u200910−1 to 6.7u2009×u200910−2 Pa. In order to promote their electrical conductivity, all the deposited MoOx films were annealed in Ar ambient at 450u2009°C for 8 h. The resistivity of the MoOx films varied from 10−4 to 10−2 Ωu2009cm depending on the O2 content in the sputtering ambient. The lowering of the resistivity of the MoO2 films was mainly attributed to the formation of a monoclinic MoO2 polycrystalline phase. As the sputtering pressure decreased, the content of monoclinic polycrystalline MoO2 phase increased, resulting in low resistivity films. The formation of the dominant MoO2 phase at lower sputtering pressures was attributed to the stress induced crystallization. The post-deposition annealed (PDA) MoOx film, deposited at an ultralow sputtering pressure (6.7u2009×u200910−2...

Improving the Performance of Tin Oxide Thin-Film Transistors by Using Ultralow Pressure Sputtering

Thin-film transistors (TFTs) are fabricated with a tin oxide channel deposited by using ultralow pressure sputtering (ULPS). The effect of sputtering pressure on the device performance of the tin oxide TFTs was investigated. The TFTs with tin oxide channel deposited by conventional sputtering pressure did not show a promising performance. However, the saturation mobility (μ sat ) and the threshold voltage (V th ) of the ULPS-deposited SnO x TFTs were improved to ~3.9 cm 2 /V s and ~0.6 V, respectively. The better device performance of the ULPS-deposited SnO x TFT was attributed to the reduced free electron density (~10 17 /cm 3 ) resulting from the formation of a nanocrystalline phase.

Improvement in the performance of ZnO thin film transistors by using ultralow-pressure sputtering

Thin film transistors (TFTs) were fabricated with a zinc oxide (ZnO) channel deposited by ultralow-pressure sputtering (ULPS) at a pressure less than 1.3×10−3u2002Pa. The field-effect mobility (μFE) and the subthreshold gate swing (SS) of the ULPS-ZnO TFTs were dramatically improved up to 8.5u2002cm2/Vu2009s and 0.31 V/decade, respectively, compared to 1.6u2002cm2/Vu2009s and 1.31 V/decade for the ZnO TFTs fabricated by a conventional sputtering pressure (CSP) of 6.7×10−1u2002Pa. The improved characteristics of the ULPS-ZnO TFTs compared to the CSP-ZnO one can be attributed to the greater densification of the ZnO semiconductor film at the lower deposition pressure.

Improvement in the Device Characteristics of Tin Oxide Thin-film Transistors by Adopting Ultralow-Pressure Sputtering

Recently, amorphous InGaZnO thin film transistors (TFTs) have been intensively studied because they exhibit large mobility (>10cm/Vs) and relatively good stability despite their amorphous phase. Before InGaZnO TFTs as a backplane device are adopted into the flat panel displays such as active matrix liquid crystal display or organic light-emitting diodes display in the mass-production, there are several issues to be addressed: first, the most promising candidates are based on systems including indium (over 40 % for ternary system) to achieve high channel mobility. Nevertheless, the estimated amount of indium in the earth’s crust is only 6000 tons, and its depletion is becoming a serious problem. Second, the accurate and repeatable control of such a complex material in mass production stage is not an easy task. Therefore, solutions avoiding the use of indium and finding the novel oxide system with binary oxide compositions are preferable. Here, tin-based oxide semiconductor as a channel layer of TFTs was revisited. Because tin ion [ex. Sn: 4d5s] has an orbital similarity of indium ion [In: 4d5s], the high mobility can be attainable. So far, tin oxide has been used as the main component of a transparent conductive oxide such as ITO. However, its application to active channel of TFTs was limited due to the difficulty in controlling the net electron density (Nd) (<10 cm). In this letter, we report on the fabrication of TFTs with the SnOx channel layer by using ultralow-pressure, rf magnetron sputtering (ULPS) with a sputtering pressure of less than 1.3 x 10 Pa and a substrate temperature as low as 120 C. The Nd was generally smaller in the ULPS films than in the conventional sputtering pressure (CSP) films at all O2 ratios. This is crucial to achieve the desired high on/off current (ION/OFF) ratio in the TFT devices. The Nd of the CSP films could not be decreased to lower than 10 cm even at the highest O2 % ratio, whereas it was already of the order of 10 cm for the ULPS film even at the O2 % ratio of 15%. These significant decrease in the Nd from 10 to 10 cm, which resulted in the transistor behavior having reasonable mobility and ION/OFF ratio. This was attributed to the formation of more stoichiometric SnO2 phase with the ULPS process, as shown in Fig. 1. Figure 2(a) and (b) shows the representative output characteristics for TFTs with the CSP SnOx film and ULPS SnOx film, respectively. It can be seen that the drain current (IDS) of CSP SnOx TFT is monotonously increased with increasing drain voltage (VDS) as shown in Fig. 2(a). However, there is no saturation of drain current, i.e. pinch-off phenomenon, suggesting that the channel layer near drain junction is not fully depleted presumably due to high conductivity of the channel layer itself. On the other hand, the ULPS SnOx TFT exhibited the clear pinch-off and IDS saturation, indicating that the electron transportation in active channels is totally controlled by the gate and drain voltages. Figure 2(c) and (d) show the transfer characteristics of CSP SnOx and ULPS SnOxTFT, respectively. Indeed, the CSP SnOx TFT exhibit the very high off-current of 5x10A at VGS = -40V, which is consistent with the speculation of high conductivity of active channel layer. However, the ULPS SnOx TFT exhibited the reasonable VTH of 1.2 V and μFE of 4.0 cm/Vs (in the linear region) as well as the low off-state current of 3x10 A and ION/OFF ratio of 1.4x10 (Fig.2(d)). In summary, we proposed the possibility of fabricating the SnOx TFTs by adopting ultralow-pressure sputtering. The Nd of SnOx films can be controlled by decreasing sputtering pressure during thin film growth. Thus, the ULPS SnOx TFTs are expected to find the important application in the back-planes of new emerging flexible and/or transparent display devices.