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Abstract
InGaZnON (IGZON) thin films are attracting considerable research interest for their role as the active layers in thin-film transistors (TFTs). Investigating the temperature-dependent optical and electrical properties of IGZON thin films is important for understanding the mechanisms underlying the temperature stabilities of IGZON TFTs in display applications and for developing new applications such as TFT-based temperature sensors and optical temperature sensors. This study is the first to investigate the temperature-dependent optical and electrical properties of IGZON thin films, using transmittance spectra and Hall measurements. Transmittance spectra were obtained between room temperature (RT) and 423 K. The absorption edge shifts to longer wavelengths (red-shift) from 16 to 25 nm as the temperature increases, while the sensitivity changes from 0.12 to 0.17 nm/℃. Free-carrier absorption increases with temperature and shows a linear dependence on the electrical conductivity (σ) and the free-carrier concentration (n). The optical band gap displays a negative linear dependence on temperature, with a coefficient ranging from -0.0007 to -0.001 eV/K. The results highlight the potential for applying IGZON thin films to optical temperature sensing. The carrier-transport properties were studied between 103 K and RT. Thermally-activated behavior in σ is apparent when n is less than 2×1019 cm-3 under non-degenerate conditions, described as σ = σ0 exp[-A/T1/4], characteristic of variable-range-hopping conduction. A linear σ-T relation is also visible, arising from weak-localization dominated by electron-electron interactions. At larger n, the behavior evolves toward degenerate conduction. Weak thermally activated behavior is displayed by the n and Hall mobility over the entire temperature range.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The InGaZnON (IGZON) thin films are frequently used as the active layer in oxide-semiconductor thin-film transistors (TFTs) [1–6]. The temperature stabilities [5–11] and temperature-dependent characteristics of TFTs [12–15] are important for applications, including hot display dashboards in cars and TFT-based temperature sensing, respectively. The temperature stabilities of channel layers in oxide-semiconductor TFTs is generally assessed in terms of the thermal excitation of carriers, which may be generated by intrinsic excitation from oxygen vacancies [7] and other point defects [8], or by hole generation at high temperatures [8–11]. Oxide-semiconductor TFTs can also be used as temperature sensors by exploiting the increase in off-currents with temperature [12–14]. The temperature dependence of the optical transmittance of ZnO thin films can also be exploited to fabricate optical temperature sensors [15]. It is therefore important and necessary to fill the gaps in the present understanding of the temperature-dependent optical and electrical properties of IGZON thin films.
Carrier-transport properties are known to affect TFT performance significantly. Whereas the carrier-transport properties of InGaZnO4 (IGZO) thin films [16–19] have been extensively studied, the corresponding mechanisms in IGZON thin films have so far not been established. It is also well known that the optical band gap (Eg) of semiconductors changes with temperature [15,20–22]. Despite the importance of this effect, very few studies to date have considered for IGZON thin films.
The present study is therefore the first to investigate the temperature-dependent optical transmittance, free-carrier absorption (FCA), and Eg behaviors for IGZON thin films between room temperature (RT) and 423 K. The temperature-dependent carrier-transport properties of these films are then studied between 103 and 673 K.
2. Experiment details
The IGZON thin films were deposited onto K9 glass substrates by RF sputtering from an IGZO target with base and working pressures of 9.0×10−4 and 0.5 Pa, respectively, with an Ar: N2 flow-rate ratio of 40:10 SCCM, and with a power of 100 W. The substrate temperature was increased from RT to 100 ℃, and from 200 ℃ to 300 ℃ for the IGZON1 to IGZON4 thin films, respectively. Moreover, the IGZON4 thin film was co-sputtered from IGZO and Zn targets, together with DC sputtering from a Zn target with a power of 50 W. All the thin films had a thickness of 200 nm. The thin-film structure and surface morphology were studied using an X-ray diffraction (XRD) meter and a scanning electron microscope (SEM) attached with an energy-dispersion spectrometer (EDS) to analyze the thin-film composition. A UV-visible spectrometer was used to measure the thin-film transmittance between RT and 423 K. The carrier-transport properties were measured using a Hall system in the van der Pauw configuration in an N2 atmosphere between 103 and 673 K.
3. Results and discussion
Figure 1 shows the XRD patterns for the IGZON thin films. Each thin film can be described as an essentially amorphous structured matrix, with the minor c-axis aligned with the (009) [23] diffraction peaks of crystalline IGZO, and with a crystal size at the order of a few tens of nanometers. The (009) diffraction peak refers to InGaZnO4 with hexagonal crystalline phase and R-3m (166) space-group with unit cell 3.295 × 3.295 × 26.07 < 90.0° × 90.0° × 120.0°> referred from PDF#38-1104 crystallographic card. The (009) peak for the crystalline thin films corresponds to the high degree of crystallinity exhibited by domains oriented perpendicular to the substrate. These lattice planes correspond to the alternating layers of indium oxide and gallium/zinc oxide in the unit cell. As the deposition temperature increases, the surface diffusion and transfer of atoms increase, resulting in an increase of the crystal size, as confirmed by the SEM results in Fig. 2. Following the co-sputtering of Zn and IGZO, the IGZON4 thin film forms with a ZnN-like crystal structure [24]. Moreover, with increasing deposition temperature, the concentration of O in the thin films decreases and that of N (CN) increases, especially in the IGZON4 thin film.
Figure 3 shows the variation in the ultraviolet-visible transmittance spectra of the IGZON thin films as the temperature increases from RT to 423 K. All the samples exhibit a sharp absorption edge in the wavelength range 300∼400 nm, resulting from the direct transition of electrons from the valence to the conduction bands. Notably, the absorption edge shifts to longer wavelengths (red-shift) as the temperature is increased. The wavelength shift and sensitivity change from 25 nm and 0.17 nm/℃, and 16 nm and 0.11 nm/℃, and 18 nm and 0.12 nm/℃, and 22 nm and 0.15 nm/℃, respectively. The temperature sensitivity is higher than that reported for ZnO thin films [15], suggesting a suitability for potential applications of IGZON thin films to optical temperature sensing. In the visible-light region, the deposited thin films display a high transmittance of almost above 80%, owing to the superior transmission capability of the IGZON thin films. Figure 4 shows the temperature-dependent transmittance of the IGZON thin films in the infrared region. The temperature-dependent free-carrier concentration (n) in the thin films was measured using the Hall effect. For all the thin films, with increasing temperature, the n increases owing to the thermal activation of the free carriers. The transmittance also decreases because of the FCA.
Fig. 3. Temperature-dependent transmittance for the IGZON thin films (ultraviolet to visible wavelengths).
Figure 5 shows the temperature-dependent optical-absorption coefficient (α) of the IGZON thin films. This coefficient is defined via the equation I = I0 exp(αd), where I and I0 are the intensities of the transmitted and incident lights, and d is the thin-film thickness. Since the transmittance T is defined as I/I0, α can be calculated as α = -ln (T/d). FCA is observed for a photon energy of approximately 0.7 eV (corresponding to an optical wavelength near 1800 nm). The FCA increases with temperature. The temperature-dependent FCA coefficient at a photon energy of 0.6 eV is extracted and the temperature-dependent electrical conductivity (σ) is measured using the Hall effect. These quantities are related as shown in Fig. 6. There is almost a positive linear proportionality between α and σ, which both increase with temperature. The degree of linearity increases qualitatively with decreasing temperature, as the relative contribution of electron-hole scattering to the charge-carrier mobility increases, owing to the relative decrease in phonon scattering.
Fig. 6. Dependent of the FCA on the temperature and electrical conductivity in the IGZON thin films.
The thin-film band gap (Eg) depends on the temperature and is expressed [15] as Eg(T) = Eg(0) + γT2/(T+β), where Eg(T) and Eg(0) denote the values at temperatures T and 0 K, respectively, and γ and β are two constant parameters. Assuming the parabolic densities for the band states within IGZON, the Eg values were extracted by the Tauc method [25], where (Eα)1/2 = B(E-Eg), E is the photon energy, and B is the slope of curve. Eg values were obtained by fitting the plots of (αE)1/2 as functions of E in Fig. 7, extrapolating the linear regions to the energy axis. The relationship between T and Eg were thus determined. Figure 8 shows the experimental data and linear fits to the plots of Eg versus T. The fits to the experimental data reveal an essentially linear relationship between Eg and T, expressed as Eg(T) = Eg(0) + rT, with the temperature coefficient taking negative values r = -0.001, -0.0008, -0.0007, and -0.0007 eV/K for the thin films, from IGZON1 to IGZON4, respectively. The corresponding r value for ZnO thin films is -0.0003 eV/K [15]. The temperature sensitivity is therefore greater in the IGZON films because of the larger absolute r values. There are two main explanations for the shift in Eg with temperature in semiconductors. Firstly, lattice thermal expansion is related to the dependence of electron energies on the volume. In other words, the variation of Eg with temperature may be attributed to a shift in the relative position of the valance and conduction bands that is due to the temperature dependent of lattice dilation. Secondly, Eg is directly renormalized by the temperature dependence of electron-phonon interactions. However, this shift accounts for only a small fraction of the total variation of Eg. Also, as seen in Figs. 7 and 8, the Eg values from the thin films IGZON1 to IGZON4 decrease with increasing CN because of the N states in the valence band.
The carrier-transport mechanisms in the IGZON thin films were studied using temperature-dependent electrical measurements over the temperature range from 103 K to RT. Figure 9 plots the temperature-dependent σ, which displays the thermally activated behavior when the n is less than 2×1019 cm-3, as shown later under the non-degeneration condition for the IGZON1 and IGZON2 thin films. For the IGZON3 and IGZON4 thin films, the behavior changes to the degenerate conduction at the greater n values, where σ becomes almost temperature-independent. As seen in Fig. 9(b), σ for the IGZON1 and IGZON2 thin films obeys σ = σ0 exp[-A/T1/4], suggestive of variable-range-hopping conduction. In addition, the same data appear to display a linear σ-T relation, as plotted in Fig. 9 (c). This behavior is often interpreted as indicating weak-localization dominated by electron-electron interactions. Figure 10 plots the temperature-dependent n and the Hall mobility (µ). Both n and µ exhibit weak thermal-activation behavior over the entire temperature range, indicating that the Fermi level is below the mobility edge and that potential barriers exist above the mobility edge. The IGZON thin films are mainly IGZO-based with structural randomness in the crystal structure, whereby Ga3+ and Zn2+ ions randomly share the same ionic sites. The non-localized tail states are formed in the vicinity of the conduction-band minimum with potential barriers. Therefore, carrier conduction is limited by the potential barriers when the Fermi level is located in the tail states, while carrier transport is no longer affected by potential barriers and large mobilities are obtained if the Fermi level exceeds the potential barriers.
Fig. 9. Temperature-dependent electrical conductivity (σ) from 103 K to RT for the IGZON thin films, plotted as log σversus (a) 1000/T, (b) 1/T-1/4, and (c) T.
Fig. 10. Temperature-dependent (a) free-carrier concentration (n) and (b) Hall mobility (µ) in the IGZON thin films, from 103 K to RT.
4. Summary
In conclusion, this study is the first to report the temperature-dependent optical properties of IGZON thin films. The absorption edge shifts to longer wavelengths (from 16 to 25 nm) as the temperature is increased, while the sensitivity increases from 0.12 to 0.17 nm/℃. The FCA increases with the temperature, displaying a linear dependence for σ and n. The Eg shows a negative linear dependence on temperature, with a coefficient ranging from -0.0007 to -0.001 eV/K. These results show much promise for the development of novel applications of IGZON thin films for optical temperature sensing. We are also the first to report on the carrier-transport properties of IGZON thin films. Thermally activated behavior in σ is apparent when n is less than 2×1019 cm-3, obeying σ = σ0 exp[-A/T1/4], as expected for variable-range-hopping conduction. A linear σ-T relation, associated with weak localization dominated by electron-electron interactions, is also observed. The behavior changes to that of degenerate conduction at the larger n. Weak thermally activated behaviors are also displayed by n and µ over the entire temperature range.
Funding
National Natural Science Foundation of China (61674107); Shenzhen Science and Technology Innovation Commission (JCYJ20170302150335518); Shenzhen Key Lab Fund (ZDSYS 20170228105421966).
Acknowledgments
This work was financially supported by the Natural Science Foundation of China (No. 61674107), the Shenzhen Key Lab Fund (ZDSYS 20170228105421966), and the Science and Technology Plan of Shenzhen (JCYJ20170302150335518).
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