Low-temperature processed InGaZnO MES-FET for flexible device applications

Amorphous oxide semiconductor (AOSs) of an In-Ga-Zn-O (IGZO)1) is expected to be used as a channel material for thin-film transistors (TFTs) because the IGZO TFTs exhibit field-effect motility (μFE) of over 10 cm2/Vs and good uniformity even fabricate at room temperature. The oxide TFTs with metal-insulator-semiconductor (MIS) structure have been employed widely; however, maximum processing temperature of 300-400 °C is required to guarantee the performance and reliability of the TFTs. In contrast, metal-semiconductor field effect transistor (MES-FET) has several advantages especially for flexible devices since a Schottky gate can be formed at low temperature with AOSs. There are a few reports of AOSs based MES-FET2, 3); however, it has remained an issue to form stable and good Schottky contact on the AOSs. We reported the top-gated MES-FET with the IGZO channel, which was deposited by mist chemical vapour deposition at 350 °C, and sputtered silver oxide (AgOx) Schottky gate4). The μFE of 3.2 cm2/Vs and subthreshold swing (SS) of 356 mV/decade were achieved. However, a maximum processing temperature of the MES-FET was 350 °C, which was not suitable for flexible device applications. In this presentation, the IGZO MES-FET with AgOx Schottky gate was fabricated at a maximum processing temperature of 150 °C. We investigated the influences of deposition conditions and post-deposition annealing on electrical properties of the low-temperature processed IGZO MES-FET. Figure 1 shows a cross sectional view of the IGZO MES-FET. First, a 100 nm-thick IGZO film was deposited on glass substrate by DC magnetron sputtering without intentional substrate heating from InGaZnO (In:Ga:Zn=1:1:1 mol.%) target. Deposition pressure was kept at 1.0 Pa, while the O2 gas ratio [R(O2)=O2/(Ar+O2)] was varied at 0.66, 0.80, and 1.00%. The IGZO film was patterned into an active channel by conventional photolithography and wet etching. The IGZO channel was then annealed at 100 or 150 ºC for 1h in ambient air. A 120 nm-thick AgOx was deposited by DC reactive sputtering, and Au was deposited on the AgOx by thermal evaporation. The AgOx/Au stacked Schottky gate was patterned by lift-off. Finally, Mo source and drain electrodes was formed by lift-off. Channel width/length of the MES-FET was 100/10 μm. Figure 2 shows the (a) forward and reverse currents of the IGZO/AgOx Schottky diode and (b) on and off current of the IGZO MES-FET, as a function of the Hall carrier concentration (NHall) in the IGZO channel. The diode properties were well correlated with the NHall; however, on-current of the MES-FET depended on not only NHall but also the R[O2] of the IGZO deposition. Carrier transport mechanism of the IGZO MES-FET and control methods of electrical properties will be discussed at the conference. Please click Additional Files below to see the full abstract

Base pressure controlled fabrication of high-mobility In2O3 thin film transistors

Transparent amorphous oxide semiconductors (TAOSs) have been extensively studied as active channel layers of thin-film transistors (TFTs) for next-generation flat-panel displays. Among TAOSs, amorphous In–Ga–Zn–O (a-IGZO) TFTs have now become the backplane standard for active-matrix liquid-crystal displays and activematrix organic light-emitting diode displays because of their reasonable field-effect mobility (μFE) of over 10 cm2 V−1 s−1, extremely low leakage current, low process temperature (\u3c350 °C), and large-area scalability [1]. Please click Download on the upper right corner to see the full abstract

Defect Passivation and Carrier Reduction Mechanisms in Hydrogen-Doped In-Ga-Zn-O (IGZO:H) Films upon Low-Temperature Annealing for Flexible Device Applications

Low-temperature activation of oxide semiconductor materials such as In-Ga-Zn-O (IGZO) is a key approach for their utilization in flexible devices. We previously reported that the activation temperature can be reduced to 150 °C by hydrogen-doped IGZO (IGZO:H), demonstrating a strong potential of this approach. In this paper, we investigated the mechanism for reducing the activation temperature of the IGZO:H films. In situ Hall measurements revealed that oxygen diffusion from annealing ambient into the conventional Ar/O2-sputtered IGZO film was observed at >240 °C. Moreover, the temperature at which the oxygen diffusion starts into the film significantly decreased to 100 °C for the IGZO:H film deposited at hydrogen gas flow ratio (R[H2]) of 8%. Hard X-ray photoelectron spectroscopy indicated that the near Fermi level (EF) defects in the IGZO:H film after the 150 °C annealing decreased in comparison to that in the conventional IGZO film after 300 °C annealing. The oxygen diffusion into the film during annealing plays an important role for reducing oxygen vacancies and subgap states especially for near EF. X-ray reflectometry analysis revealed that the film density of the IGZO:H decreased with an increase in R[H2] which would be the possible cause for facilitating the O diffusion at low temperature

Quantum Confinement Effect in Amorphous In–Ga–Zn–O Heterojunction Channels for Thin-Film Transistors

Electrical and carrier transport properties in In–Ga–Zn–O thin-film transistors (IGZO TFTs) with a heterojunction channel were investigated. For the heterojunction IGZO channel, a high-In composition IGZO layer (IGZO-high-In) was deposited on a typical compositions IGZO layer (IGZO-111). From the optical properties and photoelectron yield spectroscopy measurements, the heterojunction channel was expected to have the type–II energy band diagram which possesses a conduction band offset (ΔEc) of ~0.4 eV. A depth profile of background charge density indicated that a steep ΔEc is formed even in the amorphous IGZO heterojunction interface deposited by sputtering. A field effect mobility (μFE) of bottom gate structured IGZO TFTs with the heterojunction channel (hetero-IGZO TFTs) improved to ~20 cm2 V−1 s−1, although a channel/gate insulator interface was formed by an IGZO−111 (μFE = ~12 cm2 V−1 s−1). Device simulation analysis revealed that the improvement of μFE in the hetero-IGZO TFTs was originated by a quantum confinement effect for electrons at the heterojunction interface owing to a formation of steep ΔEc. Thus, we believe that heterojunction IGZO channel is an effective method to improve electrical properties of the TFTs

Influence of Grain Boundary Scattering on the Field-Effect Mobility of Solid-Phase Crystallized Hydrogenated Polycrystalline In2O3 (In2O3:H)

Hydrogenated polycrystalline In2O3 (In2O3:H) thin-film transistors (TFTs) fabricated via the low-temperature solid-phase crystallization (SPC) process with a field-effect mobility (μFE) exceeding 100 cm2 V−1 s−1 are promising candidates for future electronics applications. In this study, we investigated the effects of the SPC temperature of Ar + O2 + H2-sputtered In2O3:H films on the electron transport properties of In2O3:H TFTs. The In2O3:H TFT with an SPC temperature of 300 °C exhibited the best performance, having the largest µFE of 139.2 cm2 V−1 s−1. In contrast, the µFE was slightly degraded with increasing SPC temperature (400 °C and higher). Extended X-ray absorption fine structure analysis revealed that the medium-range ordering in the In2O3:H network was further improved by annealing up to 600 °C, while a large amount of H2O was desorbed from the In2O3:H films at SPC temperatures above 400 °C, resulting in the creation of defects at grain boundaries. The threshold temperature of H2O desorption corresponded well with the carrier transport properties; the µFE of the TFTs started to deteriorate at SPC temperatures of 400 °C and higher. Thus, it was suggested that the hydrogen remaining in the film after SPC plays an important role in the passivation of electron traps, especially for grain boundaries, resulting in an enhancement of the µFE of In2O3:H TFTs

Thermopower Modulation Analyses of High-Mobility Transparent Amorphous Oxide Semiconductor Thin-Film Transistors

Transparent amorphous oxide semiconductor InSnZnOx (ITZO)-based thin-film transistors (TFTs) exhibit a high field-effect mobility (μFE). Although ITZO-TFTs have attracted increasing attention as a next-generation backplane of flat panel displays, the origin of the high μFE remains unclear due to the lack of systematic quantitative analyses using thermopower (S) as the measure. Here, we show that the high μFE originates from an extremely light carrier effective mass (m*) and a long carrier relaxation time (τ). The S measurements of several ITZO films with different carrier concentrations clarified that m* of ITZO films is ∼0.11 m0, which is ∼70% of that of a commercial oxide semiconductor, amorphous InGaZnO4 (∼0.16 m0). We then fabricated bottom-gate-top-contact ITZO-TFTs displaying excellent transistor characteristics (μFE ∼ 58 cm2 V–1 s–1) using amorphous AlOx as the gate insulator and demonstrated that the effective thickness increases with the gate voltage. This suggests that the bulk predominantly contributes to the drain current, which results in τ as long as ∼3.6 fs, which is quadruple that of amorphous InGaZnO4-TFTs (∼0.9 fs). The present results are useful to further improve the mobility of ITZO-TFTs