Yana Gaoa,
Jianguo Lub,
Jianhua Zhanga and
Xifeng Li*a
aKey Laboratory of Advanced Display and System Application, Ministry of Education, Shanghai University, Shanghai 200072, China. E-mail: lixifeng@shu.edu.cn
bKey Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
First published on 17th April 2015
The energy band tailoring of indium zinc oxide (IZO) through Al incorporation was studied and measured directly by ultraviolet photoelectron spectroscopy (UPS). Al doped IZO (AIZO) thin films have lower work function (3.90 eV) and wider bandgap (3.75 eV) compared with IZO (4.15 and 3.5 eV, respectively). These changes induced by Al incorporation would result in higher activation energy and higher flat voltage for AIZO TFTs, which may imply the origin of the AIZO TFTs electrical properties such as threshold voltage shift and off-state current decrease.
Nowadays, most studies on AIZO semiconductors were focused on the fluctuation of oxygen vacancy amount and carrier concentration of those.7,8 However, development of emerging and future applications in the area of thin film transistors with oxide semiconductors requires a detailed control of Fermi level position and a depth study on carrier conduction mechanism.11 In this work, work functions, energy band gap as well as energy band alignment at interfaces influencing carrier conduction are described and discussed. It provides a fundamental understanding of AIZO thin film transistors performance in terms of the energy band tailored by Al incorporation. An intuitional measurement of AIZO work function and valance band energy level was conducted by ultraviolet photoelectron spectroscopy (UPS) along with the UV-visible spectroscopy analysis. The results show that AIZO films have wider energy bandgap and lower work function compared with IZO films. In this paper, we investigated the difference of IZO and AIZO in terms of band bending and carrier conduction mechanism which can explain the reduced off-state current and positive shifted threshold voltage. In addition, chemical solution deposition processes are the most promising for direct deposition, low cost manufacturing, and various compositions of oxide thin films.2,6 Hence, solution processed both oxide semiconductor and gate dielectric TFTs were fabricated in our paper.
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Fig. 1 Transfer (IDS − VGS) characteristics of solution processed AIZO TFTs as a function of the Al amounts from 0 to 15 at%. The AIZO TFTs were annealed at 300 °C. |
Al (at%) | usat (cm2 V−1 s−1) | Ion/off | VT (V) | S.S (V dec−1) |
---|---|---|---|---|
0 | 16.99 | 1.4 × 103 | — | — |
5 | 8.03 | 5.7 × 105 | 0.49 | 0.76 |
10 | 6.03 | 1.0 × 106 | 0.89 | 0.73 |
15 | 1.3 | 3.0 × 106 | 1.56 | 0.51 |
The saturation mobility (usat) extracted from the transfer curves were listed in Table 1. It can be seen that the addition of Al reduces the saturation mobility which is coincident with the results reported in previous reports.7,8 Furthermore, the subthreshold swing (S.S) was decreased with the increase of Al amounts indicating the decrease of the interface defects densities. This result was mainly originated from surface morphology of AIZO films. Fig. 2 shown the root mean square (RMS) values of IZO and AIZO (10 at%) films annealed at different temperatures. The RMS values of AIZO films were lower than that of IZO films regardless of the annealing temperature. The smooth surface may be original to the amorphous phase of AIZO films.13 XRD results indicated that the AIZO films stay in an amorphous phase at annealing temperatures range from 300 °C to 500 °C. These indicated that the incorporation of Al can attribute to a smoother surface for IZO films which could reduce the defects density and suppress the charge trapping in the interface between the channel and gate dielectric.12 Thus, the S.S was decreased with the increase of Al addition.
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Fig. 2 RMS values of IZO and AIZO (10 at%) films annealed at different temperatures. Inset is the AFM image of AIZO films annealing at 300 °C. |
Overall TFT transfer characteristics were improved and optimized with the Al mole ratio of 10 at%. The results revealed that Al played an important role in suppressing the carrier concentration which was further confirmed by Hall measurement. Because Al atom had a relatively lower SEP than other In and Zn, it was more easily ionized and then strongly combined with oxygen atom, which reduced carrier concentration in the film due to the decrease of the oxygen vacancy serving as a source of carriers.3 The detail effects of Al on the carrier concentration and carrier conduction were analyzed in the following sections, and we focused on the comparison between the AIZO (10 at%) and IZO solutions/films.
To further verify the effects of Al on the electrical properties IZO and AIZO films, Hall measurement was performed. Fig. 3 shows the dependence of carrier concentration and resistivity on the annealing temperature of IZO and AIZO films. For AIZO films annealed at 300 °C, the Hall mobility which calculated through carrier concentration and resistivity decreased from 2.5 to 1.5 cm2 V−1 s−1 as doping of Al. The results were coincided with the changes in mobility of TFTs as shown in Table 1.
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Fig. 3 Carrier concentration and resistivity of the IZO and AIZO films with varying temperatures (300, 400 and 500 °C). |
Moreover, the decreasing carrier concentration resulted in the increasing resistivity. Considering the results of Hall measurement, Al can suppress carrier concentration due to its low SEP (−1.66 V). The higher chemical bonding energy of Al with oxygen, rather than In or Zn, rarely provides oxygen vacancies and therefore lowers the carrier concentration.14 Thus, Al is an effective suppressor of oxygen deficiencies generation originating from the shallow donor level in the sub gap density of states of the amorphous oxide thin film. Consequently, AIZO thin films can be used as active layer due to the decrease in shallow donor level and lowering Fermi level, which were further confirmed in the following sections. Furthermore, as the annealing temperature increased (from 300 °C to 500 °C), the carrier concentration increased and resistivity decreased. A quite similar temperature dependence of electrical behavior of other oxide semiconductors was observed.15
XPS measurements were performed to better understand the chemical and structural differences among AIZO and IZO films. Fig. 4a and (b) shows the XPS spectra of In and Zn ions for IZO and AIZO films annealed at 300 °C. Compared the XPS spectra, it is estimated that the added Al3+ ions were bonded and attracted oxygen atoms connected primarily with Zn2+ and In3+ ions in the AIZO compound. As a result, the additive Al3+ led to the deoxidization of In–O and Zn–O bonds and a shift in the lower binding energy of Zn 2p3/2 and In 3d5/2 peaks from 1021.8 and 444.9 eV to 1021.1 and 444.5 eV.16
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Fig. 4 Zn 2p (a) and In 3d (b) spectra of IZO and AIZO films annealed at 300 °C. O 1s XPS spectra of IZO (c) and AIZO (d) films annealed at 300 °C. |
Fig. 4c and (d) shows oxygen 1s (O 1s) XPS spectra for IZO and AIZO films annealed at 300 °C, which can be deconvoluted into three peaks, 529.9 ± 0.2 eV (peak 1), 530.8 ± 0.2 eV (peak 2), 531.7 ± 0.2 eV (peak 3), respectively. Prior studies indicate that the peak (peak 1) at 530 eV corresponds to lattice oxygen in a fully coordinated environment, while the peak (peak 2) at higher binding energies near 531 eV arise from oxygen-deficient environments, most likely to be related to oxygen vacancies.17 And the highest binding energy peak (peak 3) is related to the oxygen in adsorbed water molecules or O–H bonding, since H is more electronegative than the metals, the M–OH oxygen atoms are less negatively charged than those in oxides, resulting in a shift toward to higher binding energy.18 Compared the O 1s XPS spectra of IZO with that of AIZO films, the relative area of peak 2 relating to the oxygen vacancy changed from 26% to 17%. This may attribute to the oxygen–aluminum ionic bonds formed in AIZO. Since the oxygen vacancy is known in general as source of the free electrons, the usat of IZO TFTs was higher than AIZO TFTs.
A suppression of carrier through incorporation of Al was observed from all above, however, the results is insufficient in explaining the mechanism of controlling the VT and off-state current. Thus, we carried out the IZO and AIZO energy band alignment by UPS measurement. Moreover, the optical bandgap could be extracted directly by absorption coefficient as shown in Fig. 5.
Fig. 5 shows the transmittance spectra of the IZO films and AIZO films on glass substrate through UV-visible spectroscopy. And optical bandgap of IZO and AIZO films are shown in the inset of Fig. 5. Compared the transmittance of films annealed at 300 °C, a blue shift in the transmittance of AIZO films from that of the IZO films was observed. The optical bandgap (Eg) can be expressed by the Tauc relation:19
(αhν)2 = A(hν − Eg) | (1) |
![]() | (2) |
The work function and valence band energy of IZO and AIZO were measured by UPS (Fig. 6). He I (21.22 eV) was utilized as a photon source for the UPS measurement. Binding energies were calibrated by measuring the Fermi step position and the Au 4f7/2 core level of a clean gold film. The Fig. 7 shows the scale of binding energy with the Fermi level (EF) set at 0 V. The vacuum level (EVAC) should be located 21.22 eV above the cut off energy of the spectrum. According to the UPS spectrum, the value of the valence band maximum (EVB) is located at 2.85 eV below the EF for AIZO. And the work functions of IZO, AIZO based on this definition were estimated to be 4.15 and 3.90 eV, respectively. The work function of IZO was higher than AIZO, and the Fermi level of IZO was located much closer to the conduction band minimum (CBM) compared with AIZO. The distance from the CBM to the Fermi level of IZO and AIZO was 0.70 and 0.90 eV, respectively. This was in accordance with the results of Hall measurement. The higher carrier concentration of IZO induced a lower activation energy (EC − EF) compared to AIZO. And the bandgap of IZO was smaller than that of AIZO (Fig. 5). Based on these values of IZO and AIZO films, energy-band diagrams of the AIZO and IZO films as shown in Fig. 7a and b could be estimated.
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Fig. 6 UPS photoemission spectra of IZO and AIZO films. The inset was UPS spectra in the valence band region. |
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Fig. 7 Energy-band diagrams of (a) IZO and (b) AIZO films. Schematic band diagram of IZO (c and e) and AIZO (d and f) TFTs under no bias stress and positive bias stress. |
With regard to the work function (Φ) calculated by UPS measurement, band bending occurred between the gate and semiconductor when no gate bias stress as shown in Fig. 7c and d. The work function of ITO was 4.7 eV.20 We ignore the influence of interface charge (Qint) and the flat band voltage (VFB) was determined by the work function difference between ITO and semiconductor (Φm − Φs) according to the eqn (3):21
VFB = Φm − Φs − C−1oxQint | (3) |
In general the threshold voltage is very close to the flat band voltage.22 For AIZO TFTs, the flat voltage was larger than IZO TFTs. When gate bias increased, the separation between EF and EC decreased, but the separation for AIZO TFTs was larger (Fig. 7e and f). Thus, the AIZO TFTs was more difficult to operate. This may explain the positive shift of threshold voltage with different content of Al addition as shown in Fig. 1 and Table 1. Also, the high activation energy of AIZO films would suppress the carrier generation in TFTs, and then result in a lower off-state current. All these results can verify that the addition of Al into IZO can decrease the carrier concentration and influence the carrier conduction in TFT. TFT performance can be controlled using AIZO active layer.
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