High Performance Amorphous Multilayered ZnO-SnO<sub>2</sub> Heterostructure Thin-Film Transistors: Fabrication and Characteristics
2015; Electronics and Telecommunications Research Institute; Volume: 37; Issue: 6 Linguagem: Inglês
10.4218/etrij.15.0114.0743
ISSN2233-7326
AutoresSu‐Jae Lee, Chi‐Sun Hwang, Jae‐Eun Pi, Jong‐Heon Yang, Chun‐Won Byun, Hye Yong Chu, Kyoung-Ik Cho, Sung Haeng Cho,
Tópico(s)Electronic and Structural Properties of Oxides
ResumoETRI JournalVolume 37, Issue 6 p. 1135-1142 ArticleFree Access High-Performance Amorphous Multilayered ZnO-SnO2 Heterostructure Thin-Film Transistors: Fabrication and Characteristics Su-Jae Lee, Corresponding Author Su-Jae Lee leesujae@etri.re.kr Corresponding Authorleesujae@etri.re.krSearch for more papers by this authorChi-Sun Hwang, Chi-Sun Hwang hwang-cs@etri.re.kr Search for more papers by this authorJae-Eun Pi, Jae-Eun Pi jepi@etri.re.kr Search for more papers by this authorJong-Heon Yang, Jong-Heon Yang delmo@etri.re.kr Search for more papers by this authorChun-Won Byun, Chun-Won Byun cwbyun@etri.re.kr Search for more papers by this authorHye Yong Chu, Hye Yong Chu hychu@etri.re.kr Search for more papers by this authorKyoung-Ik Cho, Kyoung-Ik Cho kicho@etri.re.kr Search for more papers by this authorSung Haeng Cho, Sung Haeng Cho helloshcho@etri.re.kr Search for more papers by this author Su-Jae Lee, Corresponding Author Su-Jae Lee leesujae@etri.re.kr Corresponding Authorleesujae@etri.re.krSearch for more papers by this authorChi-Sun Hwang, Chi-Sun Hwang hwang-cs@etri.re.kr Search for more papers by this authorJae-Eun Pi, Jae-Eun Pi jepi@etri.re.kr Search for more papers by this authorJong-Heon Yang, Jong-Heon Yang delmo@etri.re.kr Search for more papers by this authorChun-Won Byun, Chun-Won Byun cwbyun@etri.re.kr Search for more papers by this authorHye Yong Chu, Hye Yong Chu hychu@etri.re.kr Search for more papers by this authorKyoung-Ik Cho, Kyoung-Ik Cho kicho@etri.re.kr Search for more papers by this authorSung Haeng Cho, Sung Haeng Cho helloshcho@etri.re.kr Search for more papers by this author First published: 01 December 2015 https://doi.org/10.4218/etrij.15.0114.0743Citations: 22 Su-Jae Lee (corresponding author, leesujae@etri.re.kr), Chi-Sun Hwang (hwang-cs@etri.re.kr), Jae-Eun Pi (jepi@etri.re.kr), Jong-Heon Yang (delmo@etri.re.kr), Chun-Won Byun (cwbyun@etri.re.kr), Hye Yong Chu (hychu@etri.re.kr), Kyoung-Ik Cho (kicho@etri.re.kr), and Sung Haeng Cho (helloshcho@etri.re.kr) are with the Information & Communications, Core Technology Research Laboratory, ETRI, Daejeon, Rep. of Korea. This work was supported by the ICT R&D Program of MSIP/IITP (10041416, the core technology development of light and space adaptable energy-saving I/O platform for future advertising service). AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Multilayered ZnO-SnO2 heterostructure thin films consisting of ZnO and SnO2 layers are produced by alternating the pulsed laser ablation of ZnO and SnO2 targets, and their structural and field-effect electronic transport properties are investigated as a function of the thickness of the ZnO and SnO2 layers. The performance parameters of amorphous multilayered ZnO-SnO2 heterostructure thin-film transistors (TFTs) are highly dependent on the thickness of the ZnO and SnO2 layers. A highest electron mobility of 43 cm2/V·s, a low subthreshold swing of a 0.22 V/dec, a threshold voltage of 1 V, and a high drain current on-to-off ratio of 1010 are obtained for the amorphous multilayered ZnO(1.5 nm)-SnO2(1.5 nm) heterostructure TFTs, which is adequate for the operation of next-generation microelectronic devices. These results are presumed to be due to the unique electronic structure of amorphous multilayered ZnO-SnO2 heterostructure film consisting of ZnO, SnO2, and ZnO-SnO2 interface layers. I. Introduction Thin-film transistors (TFTs) based on wide bandgap amorphous oxide semiconductors, including zinc tin oxide (ZTO), indium gallium zinc oxide (IGZO), indium zinc oxide, and indium zinc tin oxide have been intensively studied in the field of thin-film electronics during the past decades [1]–[7], and are rapidly approaching commercialization as a replacement for amorphous Si in high-performance electronics applications, such as active-matrix organic light-emitting diodes, active-matrix liquid-crystal displays, and flexible displays [7]. In particular, IGZO is one of the most promising channel materials because of its high carrier mobility, good TFT performance, low processing temperature, excellent environmental stability, and high transparency. Thus far, there have been significant attempts to find a new oxide semiconductor as an alternative to indium/gallium-based semiconducting oxides, owing to their rarity, high cost, and toxicity. Still, for TFTs and next-generation microelectronic device applications with low power consumption and higher performance, it is necessary to develop new eco-friendly semiconducting oxide materials exhibiting better transistor properties; that is, higher mobility, a smaller subthreshold swing (ss), a higher ratio of on-to-off drain current (Ion/Ioff), and longer stability. Multilayer thin films have been widely studied because their properties are different from those of conventional thin films and bulk materials owing to the two-dimensional film and high-density interface. Since the recent observation of high-mobility two-dimensional electron gas (2DEG) at the interface between the two insulating oxides in a LaTiO3/SrTiO3 heterostructure [8], a high mobility 2DEG behavior has been seen in semiconductor ZnO-based ZnO/Zn1−xMgxO heterostructures. Thus far, several studies regarding the formation of 2DEG and the performance of field-effect transistors (FETs) with a single crystalline or polycrystalline semiconducting ZnO/MgZnO heterostruture have been carried out [9]–[12]. K. Koike and others reported high-performance ZnO-based FETs that were designed to take advantage of a high-mobility electron channel formed in the ZnO/ZnMgO heterostructure, and demonstrated a transconductance of 0.7 mS/mm with a very high field-effect mobility of 140 cm2/V·s at room temperature [9]. Such interfacial electron gases in a two-oxide heterostructure can be used to design innovative oxide electronic devices. Thus, the electronic structure of a two-oxide heterointerface is important for its stability, function, and improved performance in many devices. Among the ZnO-based semiconducting oxides, the binary ZnO-SnO2 systems that possess high electron conductivity and high electron mobility have currently attracted significant attention as low-cost indium/gallium-free alternative transparent conducting and amorphous semiconducting oxide materials for applications in many devices, such as transparent electrodes for solar cells, electronics, flat panel displays (FPDs), sensors, photocatalysts, and the active channel layer of TFTs [3]–[4], [13]. ZnO and SnO2 are generic n-type semiconductors with wide band gaps of 3.2 eV and 3.6 eV, respectively. ZnO and SnO2 have different crystal structures; namely, wurtzite and rutile structures, respectively. In addition, they also have different semiconducting properties. Multilayered ZnO-SnO2 heterostructure thin films consisting of two ZnO and SnO2 layers that are expected to have an unusual property owing to the formation of unusual charge states at the interface, which are inaccessible in conventional thin films and bulk materials, may be realized, which will enhance the efficiency and performance of many electronic and optical devices. Thus, one motive of this study is to observe the effects of a high-mobility two-dimensional electron channel in multilayer ZnO-SnO2 heterostructure TFTs such as a 2DEG in a heterointerface. In this work, multilayered ZnO-SnO2 heterostructure thin films were produced by alternating the pulsed laser ablation of ZnO and SnO2 targets, and their structural and field-effect electronic transport properties were investigated as a function of the thickness of the ZnO and SnO2 layers. II. Experiments Multilayered ZnO-SnO2 heterostructure films were grown by the alternating deposition of ZnO and SnO2 layers using pulsed laser deposition (PLD) on SiO2/Si and pre-patterned source-drain (ITO)/gate insulator (Al2O3)/gate electrode (ITO)/glass substrates. After a base pressure of lower than Torr was achieved, the ZnO and SnO2 targets were ablated with a KrF excimer laser at a laser energy density of 2 J/cm2 and a pulsed laser frequency of 2 Hz. The deposition process was carried out at room temperature under an oxygen pressure of 30 mTorr, and then annealed at 350°C in air. The crystallographic structure, microstructures, and elemental composition analysis of the multilayered ZnO-SnO2 heterostructure films were investigated using glancing-angle X-ray diffraction (GXRD), a Cs-corrected scanning transmission electron microscope (STEM), and an electron dispersive spectroscopy (EDS) embedded in the STEM, respectively. To investigate the field-effect electronic transport properties of multilayered ZnO-SnO2 heterostructure thin films, field-effect TFTs were fabricated, which have a bottom gate TFT configuration. A lithographically patterned 150 nm-thick indium tin oxide (ITO) film on a glass substrate was used as the bottom gate electrode (BG). A 176 nm-thick Al2O3 film as a gate insulator (GI) was deposited at a temperature of 150°C by means of atomic layer deposition (ALD) with trimethylaluminum [TMA, Al(CH3)3] as precursors and H2O as the oxidant. A 150 nm-thick ITO as a source–drain (S/D) electrode was deposited by DC sputtering at room temperature, and then annealed at 200°C. After ITO (S/D) patterning, 30 nm-thick multilayered ZnO-SnO2 heterostructure films, as a channel layer, were deposited using a shadow mask by PLD. The channel width (W) and length (L) were 40 μm and 20 μm, respectively. The field-effect transport properties were measured at room temperature in air using a Keithley 4200 semiconductor parameter analyzer. III. Results and Discussion The crystalline phase formation of multilayered ZnO-SnO2 heterostructure thin films was analyzed using GXRD. Figure 1 shows the GXRD pattern of a 350°C annealed multilayered ZnO-SnO2 heterostructure film stacked onto 1.5 nm-thick ZnO and 1.5 nm-thick SnO2 layers grown on a SiO2/Si substrate. The total thickness of the film is 114 nm. The film exhibited no diffraction peaks related to the hexagonal wurtzite ZnO or tetragonal rutile SnO2 phases, and exhibited only a broad diffraction peak, which indicates that the film is in an amorphous phase. These results indicate that ZnO-SnO2 film has an amorphous multilayered heterostructure, consisting of amorphous ZnO and SnO2 layers. Figure 1Open in figure viewerPowerPoint GXRD patterns of multilayered ZnO(1.5 nm)-SnO2(1.5 nm) heterostructure thin film grown on SiO2/Si substrate. Figure 2(a) shows a high-resolution cross-sectional STEM image of a multilayered ZnO-SnO2 heterostructure film grown on a SiO2/Si substrate and annealed at 350°C. The film has a multilayer heterostructure in which the ZnO and SnO2 layers are stacked periodically, and do not show any crystallinity. The thickness of the ZnO and SnO2 layers was observed to be about 2.0 nm. The interface between the ZnO layer and SnO2 layer cannot be clearly discerned. The inset of Fig. 2(a) shows the selected area electron diffraction (SAED) patterns of the film, which exhibited only broad diffuse diffraction rings, indicating the characteristics of an amorphous phase. Based on the XRD, STEM, and SAED results, we can confirm that multilayered ZnO-SnO2 films annealed at 350°C have an amorphous heterostructure consisting of amorphous ZnO and SnO2 layers. The chemical composition of the amorphous multilayered ZnO-SnO2 heterostructure film was analyzed using an elemental line profile of EDS embedded in the STEM. Figure 2(b) shows the EDS results obtained for the direction normal to the surface. The inset in Fig. 2(b) is a bright field STEM image. It shows only peaks for oxygen, zinc, and tin elements, which show periodic peaks corresponding to ZnO and SnO2 layers. Figure 2Open in figure viewerPowerPoint (a) High-resolution cross-sectional STEM images and (inset) SAED patterns, and (b) elemental line profile of EDS for multilayered ZnO-SnO2 heterostructure film grown on SiO2/Si substrate. To investigate the field-effect electronic transport properties of amorphous multilayered ZnO-SnO2 heterostructure thin films, the bottom-gate TFTs were fabricated and electrically characterized at room temperature. To study how the thickness of the ZnO and SnO2 layers influences the transistor properties, the amorphous multilayered ZnO-SnO2 heterostructure films acting as channel layers were deposited by varying the thickness of the ZnO and SnO2 layers. The following films were prepared: ZnO(0.5 nm)-SnO2(0.5 nm), ZnO(1.0 nm)-SnO2(1.0 nm), ZnO(1.5 nm)-SnO2(1.5 nm), ZnO(2.0 nm)-SnO2(2.0 nm), ZnO(2.5 nm)-SnO2(2.5 nm), and ZnO(3.0 nm)-SnO2(3.0 nm). For example, in the multilayered ZnO(1.5 nm)-SnO2(1.5 nm) heterostructure film, the thickness of the ZnO and SnO2 layers was 1.5 nm. To obtain an active channel layer with a total thickness of 30 nm, 1.5 nm-thick ZnO and SnO2 layers were alternately deposited ten times, and stacked to a total of 20 layers. Figure 3 shows the representative drain current versus drain-to-source voltage (Ids–Vd) output curves for a transistor with an amorphous multilayered ZnO(1.5 nm)-SnO2(1.5 nm) heterostructure film channel at various gate voltages (Vg). The inset in Fig. 3 shows a schematic layout of the fabricated bottom-gate TFT structure. The n-type transistor behavior with a pinch-off and drain current saturation can clearly be seen from the output curve, which indicates that the electron transportation in the active channel is totally controlled by the gate and drain voltages as in conventional metal–oxide–semiconductor FETs. Figure 3Open in figure viewerPowerPoint Representative drain current (Ids) vs. drain–source voltage (Vd) output curves of amorphous multilayered ZnO (1.5 nm)-SnO2(1.5 nm) heterostructure TFTs, and (inset) schematic layout of fabricated TFT structure. The field-effect electronic transport properties of the amorphous multilayered ZnO-SnO2 heterostructure thin films were investigated as a function of the ZnO and SnO2 layer thickness. Figure 4(a) shows the transfer characteristics of the drain current versus gate voltage (Ids–Vg) at drain voltages (Vd) of 0.1 V and 15 V of amorphous multilayered ZnO-SnO2 nanostructure TFTs for various ZnO and SnO2 layer thicknesses. The total thickness of the ZnO-SnO2 channel layer is 30 nm. All of the TFTs show typical transfer curves of well-behaved transistors. The TFT with the ZnO(1.5 nm)-SnO2(1.5 nm) channel layer exhibits the highest drain on-current (Ion) at Vd of 0.1 V and 15 V, and at Vg of 30 V. The drain on-current (Ion) shows a clearly decreasing trend with increasing ZnO and SnO2 layer thicknesses. The drain off-current (Ioff) is in the range of 10−13 A to 10−15 A for all TFTs, which is below the maximum level of 10−12 A for an FPD [5]. The drain current on-off ratio (Ion/Ioff) derived from the transfer curve for a Vd of 15 V was as high as approximately 1010 for all TFTs, which is a superior property compared with those of the other reports for amorphous ZTO TFTs (approximately 108) [3]–[4], [13]–[15]. These very low off currents and high on-off ratio may result from a small number of channel interface traps and mobile ions in an amorphous multilayered ZnO-SnO2 heterostructure film. In addition, the turn-on voltage, which is the gate voltage at the onset of the initial sharp increase in current in the transfer curve, was in the range of 0 V to −1.5 V except for the ZnO(3.0 nm)-SnO2(3.0 nm) transistor, which was negatively shifted (−3 V). The negative Von shift is attributed to the increasing of free carrier concentration at the thicker ZnO and SnO2 layers, which contributes to the channel conductivity. The performance parameters of the TFTs, including the field-effect saturation mobility (μsat) and the threshold voltage (Vth), were calculated from the transfer curves in the saturation region using , where W and L are the channel width and length, respectively; Ci is the capacitance per unit area of the Al2O3 gate insulator, and Vth is the threshold voltage. The saturation mobility (μsat) was obtained through the transconductance in the saturation region of the Ids–Vg curves for a Vd of 15 V. The threshold voltage (Vth) was determined from the interception with the Vg axis of the slope of the Ids1/2 versus Vg plot. In addition, the subthreshold swing was extracted from the linear part of the log (Ids) versus Vg plot. Figure 4(b) shows the extracted saturation mobility (μsat), threshold voltage (Vth), and subthreshold swing (ss) as a function of the ZnO and SnO2 layer thicknesses. The thickness of the ZnO and SnO2 layers clearly influences the mobility of the transistors. It can be seen that the mobility is largely dependent on the ZnO and SnO2 layer thickness in the channel layer. The mobility increased with the increased thickness of the ZnO and SnO2 layers at up to 1.5 nm, and then significantly decreased with further thickness increases. The highest saturation mobility of 43 cm2/V·s was observed for the amorphous multilayered ZnO(1.5 nm)-SnO2(1.5 nm) heterostructure TFTs. This value is larger than those reported for conventional TFTs with an amorphous ZTO channel layer [3]–[4], [13]–[15]. It is speculated that the high mobility of the TFTs is due to the effect of the interface layer related to a high-mobility electron channel formed in a ZnO-SnO2 heterostructure. Three ZnO, SnO2, and ZnO/SnO2 interface channels in amorphous multilayered ZnO-SnO2 heterostructure TFTs can be formed. The cause of the decrease in mobility at the thicker ZnO and SnO2 layers might be understood as the ZnO and SnO2 layers playing a role as the dominant active channel in the TFTs, and that the number of ZnO-SnO2 interface layers decreased with an increase in the ZnO and SnO2 layer thicknesses at the same channel thickness of 30 nm. These results indicate that the mobility of the TFTs can be controlled by optimizing the thickness of the ZnO and SnO2 layers. The subthreshold swing (ss) is an important parameter for determining the quality of a TFT. As shown in Fig. 4(b), the ss did not show a specific trend according to the thickness of the ZnO and SnO2 layers, and showed a value ranging from 0.18 V/dec. to 0.27 V/dec. for all transistors. These values are comparable or better than those reported for ZTO transistors [3]–[4], [13]–[15]. The ss value is related to the total density of the trap states (Nt) in the bulk channel layer (Nbulk) and at the interface between the channel/dielectric layer (Nit). From the ss, the density of the trap states (Nt) can be calculated using , where q, kB, T, and Ci are the electron charge, the Boltzmann constant, absolute temperature, and capacitance of the dielectric layer, respectively [16]–[18]. The calculated trap-charge density (Nt) of the TFTs has a small value of cm−2 to cm−2. In spite of the high mobility of the TFTs, a low subthreshold swing for all transistors is attributed to the low density of the trap states in the ZnO, SnO2, and ZnO-SnO2 interface channel layer and at the interface between the channel and dielectric layer. In addition, the threshold voltage (Vth), corresponding to the Vg for which an accumulation layer is formed did not show a specific dependence on the thickness of the ZnO and SnO2 layers. The threshold voltage Vth has positive values between 0.35 V and 1.1 V for all transistors, indicating that the amorphous multilayered ZnO-SnO2 heterostructure TFTs operate in enhancement mode on a positive bias. Figure 4Open in figure viewerPowerPoint (a) Transfer Ids–Vg characteristics of amorphous multilayered ZnO–SnO2 heterostructure TFTs with various ZnO and SnO2 layer thicknesses, and (b) performance parameters including saturation mobility (μsat), subthreshold swing (ss), and threshold voltage (Vth) of TFTs as function of ZnO and SnO2 layer thicknesses. Figure 5(a) shows the transfer Ids–Vg characteristics of amorphous multilayered ZnO-SnO2 heterostructure TFTs with various ZnO layer thicknesses for a fixed SnO2 layer thickness (1.5 nm). The Ion showed a decreasing trend with increasing ZnO layer thickness. The minimum Ioff was generally in the range of 10−13 A to 10−14 A, which is below the maximum level of 10−12 A. The Ion/Ioff ratio derived from the transfer curves for Vd of 15 V was as high as approximately 1010 for all transistors. The turn-on voltage (Von) was in the range of 0 V to −1 V except for the ZnO(1.0 nm)-SnO2(1.5 nm) transistor, which was negatively shifted (−2.4 V). Figure 5(b) shows the extracted field-effect saturation mobility (μsat), subthreshold swing (ss), and threshold voltage (Vth) as a function of the ZnO layer thickness with a fixed SnO2 layer thickness. The μsat increased to a peak value of 43 cm2/V·s at ZnO(1.5 nm)-SnO2(1.5 nm) TFTs, and then significantly decreased at a higher thickness of the ZnO layer. It can be seen that the mobility is largely dependent on the ZnO layer thickness in the channel layer. The subthreshold gate swing (ss) did not show a specific trend according to the thickness of ZnO for a fixed SnO2 layer thickness, and showed a value ranging from 0.17 V/decade to 0.27 V/decade for all transistors. In addition, the threshold voltage (Vth) did not show a specific dependence on the thickness of ZnO for a fixed SnO2 layer thickness, which has positive values between 0.36 V and 1.35 V for all transistors. Variation in the ZnO layer thickness with a fixed SnO2 layer thickness clearly influenced the electrical parameters of the transistors. Several investigations have demonstrated that the mobility of ZTO TFTs depends on the Zn/Sn composition ratio in active materials. M.G. McDowell and others reported a decreasing trend of mobility with a decrease in the Zn/Sn ratio in a film formed using a sputtering method [15]. Recently, J.H. Heo and others reported on ZTO TFTs grown by repeating a deposition cycle of ZnO and SnO2 layers using ALD [4]. It was shown that the mobility of ALD-based ZTO TFTs can be increased by increasing the number of ZnO layer deposition cycles with a constant SnO2 layer thickness; it was concluded that a higher Zn content leads to a higher mobility when the Zn/Sn ratio is larger than one. Our results show that mobility decreases rapidly with an increase in the thickness of the ZnO layer, which contradicts the conclusions of previous works [4], [15] reporting that the mobility was found to increase with an increase in the Zn/Sn ratio. The thickness of the ZnO layer in a ZnO-SnO2 heterostructure channel significantly impacts on the electronic transport properties of the TFT. Based on these observations, it was concluded that a higher ZnO layer thickness leads to a lower mobility. Figure 5Open in figure viewerPowerPoint (a) Transfer Ids–Vg characteristics of amorphous multilayered ZnO-SnO2 heterostructure TFTs with various ZnO layer thicknesses for fixed SnO2 layer thickness (1.5 nm), and (b) performance parameters including saturation mobility (μsat), subthreshold swing (ss), and threshold voltage (Vth) of TFTs as function of ZnO layer thickness for 1.5 nm thick SnO2 layer. Figure 6(a) shows the transfer Ids–Vg characteristics of amorphous multilayered ZnO-SnO2 heterostructure TFTs with various SnO2 layer thicknesses for a fixed ZnO layer thickness (1.5 nm). As shown in Fig. 6(a), the Ion/Ioff ratio derived from the transfer curve for a Vd of 15 V was as high as approximately 1010 for all transistors. The turn-on voltage (Von) was in the range of 0 V to −1 V except for the ZnO(1.5 nm)-SnO2(2.5 nm) and ZnO(1.5 nm)-SnO2(3.0 nm) transistors, which were largely negatively shifted (−4.8 V and −9.2 V, respectively), dependent on the thickness of the SnO2 layer, and shifted toward to the negative voltage with an increase in the thickness of the SnO2 layer. Generally, the Von shift of oxide semiconducting TFTs can be explained by various factors, such as defects, carrier concentration, and charge trapping in oxide channel layers and interface between channel and gate dielectrics. The Von shifts of TFTs with thicker SnO2 layer are attributed to the increase in the carrier concentration with an increase in the thickness of the SnO2 layer, which contributes to the channel conductivity. Figure 6(b) shows the extracted saturation mobility (μsat), subthreshold swing (ss), and threshold voltage (Vth) as a function of SnO2 thickness for a fixed ZnO layer thickness (1.5 nm). The μsat increased to a peak value of 43 cm2/V·s at ZnO(1.5 nm)-SnO2(1.5 nm) TFTs, and then decreased at a higher thickness of the SnO2 layer. The ss did not show a specific trend according to the thickness of the SnO2 layer for a fixed ZnO layer thickness, and showed a value ranging from 0.12 V/dec. to 0.27 V/dec. for all transistors. However, the threshold voltage has a negative value for a high thickness of the SnO2 layer (2.5 nm and 3.0 nm). The thickness of the SnO2 layer in a ZnO-SnO2 heterostructure channel significantly impacts on the performance parameters of the TFT, such as mobility and threshold voltage. Based on these results, it was concluded that a higher SnO2 layer thickness leads to a lower mobility and a negative Vth shift. Figure 6Open in figure viewerPowerPoint (a) Transfer Ids–Vg characteristics of amorphous multilayered ZnO-SnO2 heterostructure TFTs with various SnO2 layer thicknesses for fixed ZnO layer thickness (1.5 nm), and (b) performance parameters including saturation mobility (μsat), subthreshold swing (ss), and threshold voltage (Vth) of TFTs as function of SnO2 layer thickness for 1.5 nm thick ZnO layer. IV. Conclusion In conclusion, amorphous multilayered ZnO-SnO2 heterostructure thin films were produced using a pulsed laser ablation of ZnO and SnO2 oxides targets, and their field-effect electronic transport properties were investigated as a function of the thickness of the ZnO and SnO2 layers. The films have an amorphous multilayered heterostructure consisting of ZnO and SnO2 layers. The thicknesses of the ZnO and SnO2 layers have a significant impact on the electronic transport properties of the TFTs. Based on our observations, it was concluded that the mobility of the TFTs can be controlled by optimizing the thickness of the ZnO and SnO2 layers. The highest field-effect mobility of 43 cm2/V·s, a subthreshold swing of 0.22 V/dec., a threshold voltage of 1 V, and a drain current on-to-off ratio of 1010 were obtained for the amorphous multilayered ZnO (1.5 nm)-SnO2(1.5 nm) heterostructure TFTs. These results reflect the fact that the high mobility of a TFT is attributed to a unique electronic structure owing to the advantageous combination of three ZnO, SnO2, and ZnO-SnO2 interface layers related to a high-mobility electron channel formed in a ZnO-SnO2 heterostructure film. Our results suggest that an amorphous multilayered ZnO-SnO2 heterostructure system as an oxide semiconductor can be a potential candidate for the fabrication of high-performance field-effect transistors. Future work is needed to investigate the proper device stability. Biographies Su-Jae Lee received his BS degree in physics from Kyungsung University, Busan, Rep. of Korea, in 1986 and his MS and PhD degrees in physics from Pusan National University, Rep. of Korea, in 1988 and 1997, respectively. Since he joined ETRI in 1997, he has been involved in the development of ferroelectric thin-film/microwave tunable devices and oxide nanofiber electronics. His research interests include the development of new functional superlattice oxides, high mobility semiconducting oxides, and oxide TFTs for next-generation nano-oxide electronics and displays. Chi-Sun Hwang received his BS degree from Seoul National University, Rep. of Korea, in 1991 and his PhD degree from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 1996, both in physics. From 1996 to 2000, he worked to make DRAM devices with 0.18 μm technology at Hyundai Semiconductor Inc., Incheon, Rep. of Korea. In the year 2000, he joined ETRI. Since then, his research has focused on display technology based on active-matrix flat-panel displays using TFTs, especially oxide TFTs. His recent research interests are environment adaptive displays, digital holography, and novel switching devices. Jae-Eun Pi received his BS and MS degrees in electronic engineering from Konkuk University, Seoul, Rep. of Korea, in 2009 and 2011, respectively. In 2011, he joined the Smart I/O Control Device Research Section of ETRI. His main research interests include transparent and flexible electronics and the design of driving circuits for flat-panel and digital holographic displays. Jong-Heon Yang received his BS degree in electrical engineering from the Korea Advanced Institute of Science And Technology, Daejeon, Rep. of Korea, in 2000 and his MS degree in electronic engineering from Pohang University of Science and Technology, Rep. of Korea, in 2002. He is currently a senior engineer with the Convergence Technology Research Division of ETRI. Chun-Won Byun received his BS and MS degrees in electrical and computer engineering from Hanyang University, Seoul, Rep. of Korea, in 2002 and 2007, respectively. From 2007 to 2010, he worked for ETRI, and from 2011 to 2013, he worked for Samsung Display, Giheung, Rep. of Korea. Since 2013, he has returned to ETRI and is now working for their Smart I/O Control Device Research Section. His research interests include non-volatile memory devices and their applications; driving methods for new displays; and circuits for flat-panel displays. Hye Yong Chu received her BS and MS degrees in physics from Kyung Hee University, Seoul, Rep. of Korea, in 1987 and 1989, respectively. She joined ETRI in 1989. She earned her PhD degree in information displays from Kyung-Hee University, in 2008. Her current research interests include novel device architectures in organic light-emitting devices and next-generation displays. Kyoung-Ik Cho received his BS degree in materials science from the Ulsan Institute of Technology, Rep. of Korea, in 1979 and his MS and PhD degrees in material science and engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 1981 and 1991, respectively. He joined ETRI in 1981. He has been working on the development of advanced display devices, and new electronic devices and materials. His current research interests include oxide TFTs and transparent displays; environment adaptable displays; and flexible electronic devices. Sung Haeng Cho received his BS, MS, and PhD degrees from Seoul National University, Rep. of Korea, in 1996, 1998, and 2003, respectively, all in physical chemistry. From 2005 to 2012, he worked to develop LTPS TFTs on a Si backplane for gate driver integration and oxide semiconductor TFTs for large-sized high-resolution high-speed displays at Samsung Display, Giheung, Rep. of Korea. In 2012, he joined ETRI. His current research focusses on high-mobility and high-speed oxide TFT backplane technology for super high-resolution displays. References 1K. Nomura et al., "Room-Temperature Fabrication of Transparent Flexible Thin-Film Transistors Using Amorphous Oxide Semiconductors," Nature, vol. 432, Nov. 2004, pp. 488– 492. 2E. Fortunato et al., "Amorphous IZO TTFTs with Saturation Mobility Exceeding 100 cm2/V·s," Physica Status Solidi (RRL), vol. 1, no. 1, Jan. 2007, pp. R34– R37. 3H.Q Chiang et al., "High Mobility Transparent Thin-Film Transistors with Amorphous Zinc Tin Oxide Channel Layer," Appl. Physics Lett., vol. 86, no. 1, 2005, pp. 013503–1– 013503–3. 4J. Heo, S.B. Kim, and R.G. Gordon, "Atomic Layer Deposited Zinc Tin Oxide Channel for Amorophous Oxide Thin Film Transistors," Appl. Physics Lett, vol. 101, 2012, pp. 113507–1– 113507–5. 5T. Kamiya, K. Nomura, and H. Sosono, "Present Status of Amorphous In-Ga-Zn-O Thin Film Transistors," Sci. Technol. Adv. Mater., vol. 11, no. 4, Sept. 2010, pp. 044305–1– 044305–23. 6W.S. Cheong, J. Park, and J.-H. Shin, "Effect of Oxygen Binding Energy on the Stability of Indium-Gallium-Zinc-Oxide Thin-Film Transistors," ETRI J., vol. 34, no. 6, Dec. 2012, pp. 966– 969. 7J.F. Wager, "Flat-Panel-Display Backplanes: LTPS or IGZO for AMLCDs or AMOLED Displays?" Inf. Display, vol. 30, no. 2, Mar. 2014, pp. 26– 29. 8A. Ohtomo and H.Y. Hwang, "A High-Mobility Electron Gas at the LaAlO3/SrTiO3 Heterointerface," Nature, vol. 427, Jan. 2004, pp. 423– 426. 9K. Koike et al., "Characteristics of a Zn0.7Mg0.3O/ZnO Heterostructure Field-Effect Transistor Grown on Sapphire Substrate by Molecular-Beam Epitaxy," Appl. Physics Lett., vol. 87, no. 11, 2005, pp. 112106–1– 112106–3. 10H. Tampo et al., "Polarization-Induced Two-Dimensional Electron Gases in ZnMgO/ZnO Heterostructures," Appl. Physics Lett., vol. 93, no. 20, 2008, pp. 202104–1–202104–3. 11M. Nakano et al., "Electronic-Field Control of Two-Dimensional Electrons in Polymer-Gated–Oxide Semiconductor Heterostructures," Adv. Mater., vol. 22, no. 8, Apr. 2010, pp. 876– 879. 12H.A Chin et al., "Two-Dimensional Electron Gases in Polycrystalline MgZnO/ZnO Heterostructures Grown by rf-Sputtering Processes," J. Appl. Physics, vol. 108, no. 5, 2010, pp. 054503–1–054503–4. 13S.J Lee et al., "Characterization of ZnO-SnO2 Nanocomposite Thin Films Deposited by Pulsed Laser Ablation and their Field Effect Electronic Properties," Mater. Lett., vol. 122, May 2014, pp. 94– 97. 14W.B. Jackson, R.L. Hoffman, and G.S. Herman, "High-Performance Flexible Zinc Tin Oxide Field-Effect Transistors," J. Appl. Physics Lett., vol. 87, 2005, pp. 193503–1–193503–3. 15M.G. McDowell, R.J. Sanderson, and I.G. Hill, "Combinational Study of Zinc Tin Oxide Thin-Film Transistors," J. Appl. Physics Lett., vol. 92, 2008, pp. 013502–1–013502–3. 16D.W. Greve, "Field Effect Devices and Application: Devices for Portable, Low Power, and Imaging Systems," 1st ed. Englewood Cliffs, NJ, USA: Prentice-Hall, 1998. 17R. Martins et al., "Role of Order and Disorder on the Electronic Performance of Oxide Semiconductor Thin Film Transistors," J. Appl. Physics, vol. 101, no. 4, 2007, pp. 044505–1– 044505–7. 18M.G Yun et al., "Effects of Channel Thickness on Electrical Properties and Stability of Zinc Tin Oxide Thin-Film Transistors," J. Physics D: Appl. Physics, vol. 46, no. 47, Oct. 2013, pp. 475106–1– 475106–5. Citing Literature Volume37, Issue6December 2015Pages 1135-1142 FiguresReferencesRelatedInformation
Referência(s)