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Abstract
The structure and optoelectronic properties of sputtered In2MgO4 (IMO) films as the novel channel material of thin film transistors (TFTs) are investigated. The IMO films have a nano-crystalline structure with electrical resistivity decreased from 0.25 to 0.11 Ω. cm and carrier concentration increased from 2.5 × 1018 to 1.3 × 1020 cm−3 and Hall mobility increased from 5.0 to 15.0 cm2/(V.s) when deposited at an increased power which raises the crystalline of films. Besides the higher visible transmittance and wider band gap (Eg) of IMO film (>80%, 4.7 eV) than amorphous InGaZnO4 (a-IGZO) film (<80%, 4.1 eV), the IMO film has lower Urbach energy (Eu) and electron affinity (χ) and ionization potential (Ip) and larger oxygen vacancy formation energy (EVo) and conduction band offset (△Ec) at Al2O3 dielectric/channel interface (65 meV, 1.2, 5.9, 5.4 and 3.8 eV) than a-IGZO film. The IMO TFT shows better stabilities with shift of threshold voltage (△Vth = 2 V) after positive bias stress (PBS: the gate voltage, Vg = 20 V for 500 s) and |△Vth| = 0.5 V after 450-850 nm light illumination than a-IGZO TFT (|△Vth| = 7, 2 V) fabricated in the same process in our lab. The low Eu and large EVo and Ec of IMO film are the origins of improved PBS stability of IMO TFT. The wide Eg of IMO film is the factor of visible-blindness for IMO TFT. The findings reveal that IMO TFT is a stable promising alternative for the next flat panel display.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The In2MgO4 (IMO) films were formerly investigated as transparent conductive oxide with wide band gap (Eg) [1]. H+ or Li+ implantation into pulsed laser or sputter-deposited IMO films were carried out to generate carrier electrons and increase the electrical conductivity (σ) from 10−7 to 70 S/cm [2–4]. The carrier generation in IMO films were enhanced with decreasing oxygen pressure during pulsed laser deposition and the maximum σ for IMO films reached 1.3 × 103 S/cm with carrier concentration (Ne) of 3.6 × 1021 cm−3 and Hall mobility (μ) of 2.4 cm2/V.s [5]. The minimum σ of IMO film deposited by sputtering was 10−2 Ω. cm with μ~10.0 cm2/V.s and Ne~1020 cm−3 [6–8]. The work function (φ) of IMO film deposited by sputtering was about 4.65 eV [9].
However there were few reports of IMO channel thin film transistors (TFTs). The TFTs with pulsed laser deposited IMO channel exhibited on-off current ratio (Ion/Ioff) of 107 and saturation field-effect mobility (μs) of 4.6 cm2/V.s [10]. The main research focus now for oxide TFTs was to improve the stabilities of TFTs under various stress conditions [11]. The incorporation of Mg in InZnO cause the reduction of oxygen vacancy (VO) and an increase of Eg and improving TFTs' stability due to the larger formation energy of VO (EVO) and Eg of MgO than other typical oxide channel materials such as amorphous InGaZnO4 (a-IGZO), In2O3 and ZnO (EVO: MgO~9.8 eV, a-IGZO~4.1 eV, In2O3~3.1 eV, ZnO~3.8 eV; Eg: MgO~7.9 eV, a-IGZO~3.5 eV, In2O3~3.6 eV and ZnO~3.3 eV) [12]. With larger Eg of MgO than Ga2O3 (4.9 eV) the IMO TFT had be proposed with better light illumination stability than InGaOx TFT [13]. Recently with large Eg of MgO the ultraviolet (UV) photo-detecting properties of IMO [14] and a-IGZO/IMO [15] and InGaMgO photo-TFTs [16] had been reported. However there were no reports about the calculation of EVO for IMO film and the positive bias stress (PBS) and visible light illumination stabilities of IMO TFT.
The electron affinity (χ), ionization potential (Ip) and band offsets (△E) were the fundamental electronic structure parameters [17]. The χ had been employed for simulating and analyzing the operation characteristics of TFTs and understanding the contact resistances [18,19].The △E defined the potential barrier at the gate dielectric/channel interface to inhibit conduction by Schottky emission of electrons or holes into the band of dielectric. The larger △E at gate dielectric/channel interface the lower for the leakage current and the better for the PBS stability of TFTs will be gotten [20–22]. Al2O3 was a promising gate dielectric for oxide TFTs due to the low trap states and large Eg and high dielectric constant for Al2O3 dielectric [23]. However there were no reports about the χ and Ip results of IMO films and the △E values for Al2O3/IMO hetero-junction till now.
In the work, the structure, the optical and electrical properties, and the electronic structure parameters of IMO film and Al2O3/IMO hetero-junction deposited by sputtering were studied and the electrical characteristics and PBS and light illumination stabilities of IMO TFT compared with a-IGZO TFT are investigated together.
2. Experiment details
The IMO films with 50 nm thickness were deposited on glass substrates using target of MgO: In2O3 = 13: 87 wt% at room temperature (RT). The working chamber pressure was set to 0.1 Pa with Ar sputtering gas with sputtering power varied from 100,150, and 200 and 250 W. The IMO and a-IGZO TFTs were in bottom gate structures shown in Fig. 1 fabricated by photolithography patterning and wet etching process. First, Ti (100 nm) sputtered on glass substrate were used as gate electrode and patterned by photolithography and lift-off process. The Al2O3 (300 nm) gate dielectric were deposited by sputtering at RT. A 50 nm-thickness IMO or a-IGZO film were grown as the active layers by sputtering from IMO and InGaZnO4 targets respectively at RT with power of 100 W and patterned by photolithography and wet etching in diluted HCl solution. Then, Ti source/drain electrodes (300 nm) were prepared by sputtering and patterned by photolithography and lift-off process. Finally, the IMO films and TFTs were subjected to anneal in N2 ambient at 350 °C for 1 h.
The film thickness were measured by a Dektak6M step profiler. The structure and surface morphology of films were studied by a X-ray diffraction (XRD) meter and a Scanning electron microscope (SEM). The Ne and μ and resistivity (ρ) of IMO films were measured by Hall instrument. The optical spectra of films were measured with a Lambda 900 spectrometer. The electrical structural parameters of films were tested by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) with a mono-chromat Al Kα X-ray source with energy of 1486.6 eV. The electrical characteristics of TFTs were tested at RT with an Agilent 4156C precision semiconductor parameter analyzer. The PBS test was carried out at RT with a constant gate bias of 20 V for 500 s at interval of 100 s. For photo stability test the photo excitation was provided by a Xe lamp in combination with the narrow band filters. The optical wavelength (λ) range was from 800 to 450 nm with a constant power of 5 μW/cm2. For each test the light intensity was calibrated by a power meter with photodiode sensor attached and the irradiation time for each λ was 10 s. All tests were done at RT in ambient air.
3. Results and discussion
Figure 2 shows the XRD patterns of IMO films sputtered at different power and after annealing at 350 centigrade in N2 ambient for 1 h. The IMO1 to IMO4 denote to the films deposited at sputtering power from 100, 150 and 200 and 250 W, respectively. The XRD results have been compared with the standard PDF card of #40-1402 for In2MgO4. The IMO films have nano-crystalline structure with (311) main and (220), (511) and (440) minor diffraction peaks appear in all samples with peak intensity increased with sputtering power. The higher of sputtering power and the higher the energy of the sputtered particles arriving at substrates and the higher of deposition rates therefore increase the crystalline of film [24]. The surface morphology of films are shown in Fig. 3. All films have nano-crystalline structure with grain size increased with sputtering power which coincides with XRD results.
Figure 4 shows the relation of μ and ρ with Ne of IMO films. The IMO films are n-type semiconductor with Ne increase with P. The increased crystalline size of IMO films with P decreases the transport barrier at crystalline interface and therefore increases the μ and Ne and decreases the ρ. Figure 4 shows the optical properties comparison of IMO and a-IGZO films. With cut-off wavelength pushed to UV the IMO film has higher visible transmittance (>80%) than a-IGZO film (<80%) as seen in Fig. 5(a). To calculate the Eg it is assumed the absorption coefficient α∝-lnT/d where T is transmittance and d is film thickness. A plot of (αE)2 against photo energy (E) are made as shown in Fig. 5(b). The sharp absorption edges which can be accurately determined by linear fit are clearly observed. The Urbach energy (Eu) are calculated from α ∝ exp (E/Eu) [25]. The (Eg, Eu) determined are (4.1 eV, 95 meV) and (4.7 eV, 65 meV) for a-IGZO and IMO films. Here the addition of Mg widens the optical Eg and sharps the Urbach tail and thus reduces the density of sub-gap defect states in IMO film definitely.
The △E of Al2O3/IMO interface and φ, χ and Ip of IMO film were tested by XPS and UPS. The △E were calculated by core-level (CL) photoemission method [20]. Figure 6 shows the In 3d, Mg 1s, Al 2p CL and valence band maximum (VBM) spectra for samples with binding energies and VBM values summarized in Table 1. The △EV and △EC are calculated by
with values of 0.3 and 3.8 eV where the Eg of Al2O3 and IMO are taken as 8.80 [21] and 4.70 eV. The △EC of Al2O3/IMO interface is larger than Al2O3/a-IGZO interface, Fig. 7 (2.40 eV) [23]. The φ of IMO film around 4.1 eV is determined by φ = hν-△E where △E is the energy difference between Fermi level and secondly electron cut-off seen in Fig. 6(f). The VBM, φ, and χ and Ip are the energy differences between EF and the maximum of valance band (EV), EF and the energy level of vacuum (EVac), and the minimum of conduction band (EC) and EVac, and EV and EVac, respectively. The χ and Ip of IMO film are calculated to be 1.2 and 5.9 eV which are lower than a-IGZO film (4.3, 7.4 eV) [17] indicating the lower electro-negativity and easy to lose electrons of In and Mg in IMO and higher EVO of IMO. In Fig. 6(g) for O 1s spectra, the peaks at 529.5, 531.2 and 532.1 eV [26] relate to O1s in metal oxide, defect and contamination, respectively. The calculated atom ratio in IMO lattice is In: Mg: O = 2.2: 1.0: 2.3.
Fig. 6 (a) In 3d (b) Mg 1s (c) Al 2p CL (d), (e) VBM, (f) work function and (g) O 1s spectra for IMO, Al2O3 films and Al2O3/IMO interface.
Table 1. The binding energies and VBM (eV) values for samples.
The EVO of In2MgO4 is calculated by density functional theory implemented in the Vienna Ab-initio Simulation Package code (VASP) [27,28]. The projected augmented wave method [29,30] and Ceperley-Alder-type exchange correlation are used. The plane-wave cut off energy is 520 eV and the energy convergence criterion is 10−6 eV. The Gamma-centered 5 × 5 × 5 k-points grid is used throughout the calculations. The initial crystal structure is taken from reference [31] and then a total geometry optimization for lattice constant and atomic coordinates is performed in In2MgO4. The optimized lattice constant is 8.854 Å close to the value in reference [32]. Then an oxygen atom is removed in the unit cell of In2MgO4 and again the geometry optimization is done but with fixed lattice constant 8.854 Å. Finally, the EVO in In2MgO4 is calculated by
where E(O) is a half of the total energy of an oxygen molecule, E(In2MgO3) is the total energy of In2MgO4 with a VO, and the E(In2MgO4) is the total energy of In2MgO4 without VO [33]. Our calculated EVO in IMO is 5.4 eV which is larger than that of the a-IGZO (4.0 eV) reported [12] which indicates that the IMO is less apt to have VO.Figure 8(a) shows the transfer curves of IMO TFT which operates in depletion or normally on n-channel mode. The threshold voltage (Vth) is calculated by linear extrapolated from the plot of square root of Id as a function of Vg. At Vd = 0.1 V, Vth = −12 V, and the gate voltage swing S = 0.8 V/dec. The field-effect mobility in linear region (μl = 5.0 cm2/V.s) is obtained using Eq. (4) [34] when Vd = 0.1 V<<Vg. Where C is the capacitance per unit area of 300 nm-thickness Al2O3 gate insulator with the value of 2.3 × 10−8 F/cm2. At Vd = 10 V, Vth = −10 V, and S = 0.1 V/dec and Ion/Ioff ≈106. The field-effect mobility in saturation region (μs = 25.0 cm2/V.s) is obtained using Eq. (5) [33] when Vd = 10 V>Vg - Vth. The Ig are in range of 10−9-10−8 A.
Figure 8(b) shows the output curves of IMO3 TFT. At low Vd the Id increases with Vg due to the enhanced accumulated charge density at IMO/Al2O3 interface. In addition a clear pinch-off and Id saturation indicate that the electron transportation in active channel is totally controlled by the gate and drain voltages. The saturation is about 90 μA under a gate bias of 20 V. The off-resistance, R off, defined as the inverse of the slope of the I-V curve for a given gate voltage in the saturation regime, is of 0.2 MΩ. The on-resistance (Ron), defined as
is about 25 KΩ (Vg = 30V). The relation between Roff and Ron is within the range expected for fast-switch device behavior. Also the device response time, defined as being proportional to Ron × Ci, presents value of about 6 ps.μm−2, which is also highly promising for device miniaturization.The PBS and light illumination reliabilities of IMO and a-IGZO TFTs both deposited in the same process without passivation layers are compared. In Fig. 9 a gate voltage stress of Vg = 20 V was applied to samples for a period of 500 s at an interval of 100 s. The IMO TFT shows a positive shift (~2 V) in Vth after the application of bias stress superior to the a-IGZO TFT (△Vth = 7 V) after the same stress. Figure 10 shows the transfer curves of IMO and a-IGZO TFTs obtained under several energetic light irradiation. The IR light with wavelength (λ) of 800 nm generates no photo-current. A small detectable photo-current at off-state (Vg = −20 V) and negative shift of Vth are observed with the red (λ = 650 nm), green (λ = 550 nm) and blue (λ = 450 nm) light illumination. The total |△Vth| is 0.5 V for IMO TFT lower than that of the a-IGZO TFT (|△Vth = 2 V|) by the same photon energy illumination. The IMO and a-IGZO TFTs in Fig. 10 have the same S indicating the same density of trapping charges at Al2O3/channel interface which are one of the origins for the PBS reliability of TFTs. The lower Eu of IMO film than a-IGZO film is also one origin of the better PBS stability of IMO TFT than a-IGZO TFT [37]. △EC is the potential barrier for injection of electrons from channel to dielectric which is also one of the origins for the PBS instability of TFTs [21,23]. The larger △EC (3.85 eV) of Al2O3/IMO interface than Al2O3/a-IGZO interface (2.40 eV) indicates the higher PBS reliability of IMO TFT than a-IGZO TFT. The higher EVO (5.4 eV) of IMO than a-IGZO (4.0 eV) is also one of the origins for the higher PBS reliability of IMO TFT due to the ionized VO can capture electrons responsible for the positive shifts of Vth under PBS [12,35,36]. The EVO of a-IGZO tends to increase with number of neighboring Ga atoms. Thus adding Ga atoms suppresses the formation of VO and improves the PBS reliability of TFTs. Adding Mg atoms in In2O3 has more apparent role than that of the Ga atoms to increase the EVO and suppress the formation of VO and improves the PBS reliability of TFTs which is consistent with our results. The superior light illumination reliability of IMO TFT than a-IGZO TFT is due to the larger Eg of IMO than a-IGZO as well as lower density of defects presenting in IMO/Al2O3 interface and IMO buck.
Fig. 10 Transfer curves of IGZO and IMO TFTs measured under IR to blue light illuminations (dark, IR, red, green, blue).
4. Summary
In conclusion, the Eu, χ, and Ip and EVo for IMO film and △Ec at Al2O3/IMO hetero-junction are first calculated. The superior PBS and light illumination stabilities of IMO TFT than a-IGZO TFT are demonstrated. The origin of stabilities for TFTs with above materials parameters is discussed. The present results substantiated IMO was a promising oxide semiconductor active layer for TFTs.
Funding
Natural Science Foundation of China (No.61674107); Shenzhen Key Lab Fund (ZDSYS 20170228105421966); Science and Technology Plan of Shenzhen (JCYJ20170302150335518).
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