ingazno semiconductor thin film fabricated using pulsed laser deposition

InGaZnO semiconductor thin film fabricated using pulsed laser deposition

Jiangbo Chen, Li Wang,* Xueqiong Su, Le Kong, Guoqing Liu, Xinping Zhang

College of Applied Sciences, Beijing University of technology, Beijing 100124, China *[email protected]

Abstract: The InGaZnO thin films are fabricated on the quartz glass using pulsed laser deposition (PLD), where the target is prepared by mixing the Ga2O3, In2O3, and ZnO powders at a mol ratio of 1:1:8 before the solid-state reactions in a tube furnace at the atmospheric pressure. The product thin films were characterized comprehensively by X-ray diffraction, atomic force microscopy, Hall-effect investigation, and X-ray photoelectron spectroscopy. Thus, we demonstrate semiconductor thin-film materials with high smoothness, high transmittance in visible region, and excellent electrical properties.

©2010 Optical Society of America

OCIS codes: (160.2100) Electro-optical materials; (300.6470) Spectroscopy; (310.1860) Thin films, deposition and fabrication; (310.6860) Thin films, optical properties.

References and links

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#119246 - $15.00 USD Received 30 Oct 2009; revised 11 Dec 2009; accepted 11 Dec 2009; published 12 Jan 2010

(C) 2010 OSA 18 January 2010 / Vol. 18, No. 2 / OPTICS EXPRESS 1398

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1. Introduction

Transparent oxide semiconductors (TOS) are important optoelectronic materials because of their high transmittance in visible region and controllable carrier concentration. This kind of materials has been utilized in many optoelectronic devices [1–5], such as the P-N junction rectifier, ultraviolet light-emitting diode (UV-LED), and transparent field-effect transistors (TFETs). However, the flexible design of devices using thin films of the TOS materials are more promising and attracts more interests. For example, the TOS can be used as the channel material in the thin-film transistors (TFTs), which are important for integrated circuits and for compact displays. A variety of candidates have been exploit for the channel semiconductors, including the organic semiconductors, the hydrogenated amorphous silicon (a-Si:H), the metal-oxide semiconductors, and etc.. The ZnO-based TFT exhibits better performance due to the high charge mobility and high transmission in the visible spectral range of ZnO. However, the ZnO film is polycrystalline if produced at room temperature. As a result, the grain boundaries in the ZnO channel layer may deteriorate the TFT performance. Kenji Nomur et al have demonstrated TFTs with the amorphous In–Ga–Zn oxide (a-IGZO) channel layer deposited on an unheated glass or organic-film substrate [2]. The corresponding a-IGZO TFTs

showed a Hall effect mobility exceeding 10 cm2 V

−1 s−1

, which is comparable to those of the polycrystalline ZnO and an order of magnitude larger than that of a-Si:H.

In this paper, we reported the fabrication of InGaZnO thin films using pulsed laser deposition (PLD) with the target prepared by solid state reaction, which show excellent optoelectronic performance according to the comprehensive characterizations.

2. Experiment

The IGZO polycrystalline target was prepared by the solid state reaction method. The powders of In2O3 (99.99%),Ga2O3 (99.999%), and ZnO (99.99%) have been mixed up with a mol ratio of 1: 1: 8 and are then grinded thoroughly before being sintered in a tube furnace at 1200 °C for 6 hours. The sintered powder was crushed up and grinded, and through the course of molding by dry pressing method, it was obtained cylindrical material with 30 mm in diameter and 5 mm in thickness (Pressure: 10 MPa and holding time: 2 min). The material was sintered for 6h at 1200 °C, where after we finally got the IGZO target. All of the above courses were carried out in the normal air.

IGZO thin films were fabricated by the PLD method, where a frequency-tripled (355 nm) Nd:YAG laser (Spectra-Physics GCR-170) was used as the laser source, which has a repetition rate of 10 Hz, a pulse duration of 10 ns, and a pulse energy of 40 mJ. The substrate, on which the IGZO film is to be fabricated, is a quartz glass plate with an area of 10×10 mm and a thickness of 0.5 mm, and is mounted in the vacuum chamber, so that it is 50 mm away from the target. In the fabrication, we have used certain condition that background pressure

and oxygen partial pressure was 2.5×10−4

Pa and 5.0 Pa, with the substrate at room temperature (RT) and 200 °C, deposition time was 30 min.



In this paper, x-ray diffractometer (BRUKER D8 ADVANCE) was utilized to examine the IGZO ceramic target and thin films. What’s more, thin films’ surface morphology was observed by atomic force microscope (Veeco Multimode). The transmission spectrum and electrical properties were detected by ultraviolet spectrophotometer and Hall Effect instrument, respectively. Then, the films were tested by x-ray photoelectron spectroscopy (VG MK II), with Al kα=1486.6 eV, to characterize the films. All of the above examinations were in room temperature.

3. Result and Discussion

3.1 Preparation of the IGZO target

XRD patterns of IGZO ceramic target is shown in Fig. 1, scanning range was 10-90 °and step was 0.02 °. The acquired patterns was compared with standard cards in JCPDS database, then it concludes from the picture that the target showed typical polycrystalline structure, and the powder contains the phase of InGaZn4O7 and InGaZn5O8, doesn’t obviously have the phases of In2O3, ZnO, Ga2O3, and ZnGa2O4 etc.. The results indicated two different phases created, implying phase separation actually takes place during the process of solid state reaction in multicomponent oxides, which cannot be avoided [6]. The material similar as In2O3-Ga2O3-ZnO could be denoted by RMO3(ZnO)m (m=integer), R represents rare metal on the earth, such as Lu, Sc or In, while M represents In, Ga, Fe or Al. The IGZO material could be expressed as InGaO3(ZnO)m, so the phases of powder sample could be described InGaO3(ZnO)4 and InGaO3(ZnO)5, namely m= 4 and 5. Arun Suresh and other authors’

researches indicated that IGZO thin films had high transmittance in visible region while m ≤ 5 and they should be satisfied with the demands of oxide semiconductor TFTs [7]. For the PLD process has advantages in keeping consistent stoichiometric proportion between target and film [8], the IGZO target met the requirement for thin film growth.

Fig. 1. XRD patterns of IGZO ceramic target

In the course of target preparation, molding pressure is of great important, sintering temperature is a key element, and however, purity, granularity and particle shape are the three important factors. Raw materials’ purity determines the target’s quality and stability, and granularity also determine the surface activity, volume density, uniformity and so on, while air voids and completeness in solid state reaction rest with particle shape. Raw material powders’ purity was all above 4N, and of low impurity content in this experiment, where the effects on target purity had been suppressed. What’s more, solid state reaction mainly depends on the contact and diffusion among solid particles [9–11]. That phase separation in powder



particle may generate from the diffusion among In, Ga and Zn atoms or ions. The smaller and the more regular the particle is, the more easily the reaction can be performed. Therefore, if increasing grinding time and intensity, the existence of large powder particles will be reduced which could benefit target performance, then high quality ceramic target may also consequently suppress the generation of micro particle during PLD procedure.

3.2 Fabrication and characterization of the IGZO thin films

3.2.1 X-ray diffraction measurement

X-ray diffraction (XRD) pattern of IGZO film deposited at 200 °C with quartz glass substrate were shown in Fig. 2. Two halo peaks were observed at around 22 ° and 33 °. The first is attributed to the quartz glass, and the latter originates from the IGZO film. Because of XRD max survey depth is less than 10 µm, which is two order of magnitudes lager than the thin films, thus the diffraction peak intensity of substrate would be far greater than the thin films as deposited, and IGZO thin films’ peak was not obvious from the figure above. Well then, deducting the affection of substrate was carried out in order to get the information of IGZO films, as is shown in the inset of Fig. 2. It betrayed that there was an evident diffraction peak at 33° and no sharp peak feature is found. Therefore, it could be concluded that the IGZO thin film grown at 200 °C has an amorphous structure. The same measurement was applied in the IGZO film deposited at RT, the result was quite similar. In all, both of the films have amorphous structure.

Fig. 2. XRD pattern of IGZO film and quartz glass substrate

3.2.2 Surface morphology measurement

The demand of every layer’s roughness is very high in TFT and roughness dropping in each layer will significantly influence the whole devices function. The atomic force microscope (AFM) has been used to characterize the films. The surface morphology of IGZO thin films grown at RT and 200 °C were observed in Figs. 3(a) and 3(b), with 4×4 µm area. The RMS roughness is 2.02 nm and 2.75 nm, average height is 4.36 nm and 7.50 nm, respectively. The granular structure was smaller than 10 nm in film grown at RT, and that of 200 °C became slightly larger. So, when the substrate is in a relatively low temperature, the IGZO thin film granular structure becomes a little larger with substrate temperature increased, but not obviously affects the roughness of thin film surface. This conclusion is similar to the result from Ranjan K. Sahu [5].



Fig. 3. Surface morphology of IGZO thin film with different temperature (a) RT; (b) 200 °C

3.2.3 Optical properties

Transmission spectrum of IGZO thin films have been tested by ultraviolet spectrophotometer, as is shown in Fig. 4 and the same quartz glass acted as reference. The results show that the films have fierce absorption in ultraviolet band (200-300 nm), while in the visible region (400-700 nm), they have high transmittance, approximately 75-95%. The property of high transmittance is an important index to TOS and is one of the necessary conditions for display devices. Because IGZO thin films grown at RT and 200 °C were amorphous structure [7,12,13], thus it would reduce grain boundary scattering, Raleigh scattering would also be decreased for the flat surface; Oxygen ion could easily escape during thin films growth, so oxygen gas were put into vacuum chamber in order to suppress the formation of internal defect, finally the scattering and capturing arose from defects should be grown down. Accordingly, IGZO thin films were of high transmittance in visible region. Utilized by Tauc’

Plot method, an energy gap of Eg ≈3.2 eV was deduced, as is shown in the inset of Fig. 4, less than that of ZnO (about 3.3 eV) [14,15]. The absorption edge blue-shifted was observed with substrate temperature increased, which may attribute to the raise of the Fermi level in the conduction band in a degenerated state [7,16].

Fig. 4. Transmission spectra of IGZO thin film at different temperature

3.2.4 Electrical properties

We used Hall-effect instruments to characterize the electrical properties of IGZO films. For the sake of lowing down the contact resistance, metal In electrodes plated on the film surface. The important electrical parameters, such as resistivity, carrier mobility and carrier concentration etc., have been measured by Van Der Pauw method. The electrical properties were shown in Table 1. The results indicated that both of the films were n type, low resistance



semiconductor films, carrier concentration was over 1018

cm−3

,while carrier mobility were 8.5

and 11.7 cm2/V

−1s−1

, respectively. The carrier mobility is much more larger than a-Si:H has. Electron devices are generally applied semiconductor material with high mobility. It will

get larger drift velocity in a same electric field and have greater improvement in reaction time. The experimental results can also make out that the relation between the grown temperature and carrier mobility. In a certain range of temperature, the two electrical parameters have the characterization of definite controllable. Compared with traditional TOS material, IGZO has advantages on achieving high quality films in relatively lower temperature, as well as, carrier concentration controllability. The carrier mobility and carrier concentration in IGZO films were observed increasing with the substrate temperature. This might be due to the suppression of internal defects that result from the oxygen vacancies, well then, carrier capture arose from oxygen vacancies should be reduced.

Table 1. Electrical properties of IGZO films at different temperature

Substrate temperature °C

Resistivity Ω·cm

Carrier mobility

cm2/V−1s−1

Carrier concentration

cm−3

RH cm3/C

RT 0.148 8.5315 4.95E+18 −1.2627

200 0.0135 11.704 3.96E+19 −0.158

3.2.5 X-ray photoelectron spectroscopic analysis

Figure 5 shows the representative XPS full spectra of IGZO thin films. The C 1s, In 3d, O 1s, Zn 2p and Ga 2p peaks can be easily observed. The film surface contains five elements of C, In, O, Zn and Ga. Element C was present as contamination in the film surface [17]. The surface contains element C originated from the organic pollution adsorption, the CO2 gas, and other C pollution.

Fig. 5. XPS spectra of IGZO thin films

The In 3d, Ga 2p, Zn 2p and O 1s signal showed in Figs. 6 (a), (b), (c) and (d), respectively. From Fig. 6 (a), it betrayed that In 3d5/2 and In 3d3/2 with symmetrical peak shape was at 445.0, 452.5 eV (grown at RT) and 444.7, 452.3 eV (grown at 200 °C), and the difference value was 7.5 and 7.6 eV, which was similar to In 3d peaks of In-doped ZnO films thus there was existence of I–O bonds [18–20]. Indium in IGZO films was almost totally oxidized, existed in the form of In

3+. Figure 6 (b) showed that the Ga 2p had the same peak

values, 1118.5 eV for Ga 2p3/2 and 1145.2 eV for Ga 2p1/2, peaks position shift arose from temperature varying in Ga 2p was approaching zero. Moreover, metal gallium Ga 2p3/2



binding energy is 1116.6 eV, there was apparent chemical shift compared with metal gallium, Ga in IGZO thin film was in combination. Zn 2p3/2 symmetrical peak shape showed in Fig. 6 (c). The Zn 2p3/2 photoelectron peak was at 1022.8 and 1022.3 eV according to varied temperature, but both of the films’ binding energy was all above bulk ZnO (1021.7 eV) and metal Zn (1021.1 eV), it demonstrated Zn was in oxidation state and part of Zn was in existence of Zn

2+ with anoxic condition [21]. With increasing the temperature, a shift of the In

3d and Zn 2p binding energy in the direction of the lower energy side was observed, which was attributed to In and Zn oxidation state decrease, while Ga 2p binding energy didn’t have obvious chemical shift. Figure 6 (d) shows the O 1s peak. The typical O 1s peaks on the surface of IGZO can be divided into three types, centered at 530.35 ± 0.3, 531.31 ± 0.3, and 532.25 ± 0.3 eV, respectively [22,23]. The low, medium, and high binding energy is related to

O2-

ion in lattice, O2−

in the oxygen deficient regions, and the specific chemisorbed oxygen, respectively. Furthermore, the shift of the lower binding energy level of O 1s with increasing the temperature was attributed to the reduction in oxygen vacancies, where the surface was compensated with O atoms.

Fig. 6. IGZO thin films high resolution XPS spectra. (a) In 3d; (b) Ga 2p; (c) Zn 2p; (d) O 1s

From Table 2, the stoichiometry was described by the average composition ratio defined as the relative integrated intensity of In 3d5/2, Ga 2p3/2, Zn 2p3/2 and O1s XPS peaks corrected by the commonly used procedure with the corresponding sensitivity factors. The sensitivity factor is 3.38, 4.18, 3.73 and 0.71, respectively. The IGZO target with a mol ratio of In2O3: Ga2O3: ZnO = 1: 1: 8, therefore, the mol ratio In: Ga: Zn: O = 1: 1: 4: 7. It indicated that the element mol ratio of Ga: Zn equals 1: 4, however, the content of In and O was higher than the ideal stoichiometric composition. O atom stoichiometric ratio variance may be mainly due to the specific chemisorbed oxygen.



Table 2. XPS elemental concentrations of IGZO films

Substrate temperature °C

In at %

Ga at %

Zn at %

O at %

RT 10.4 4.3 20.0 65.3

200 10.3 4.8 20.2 64.7

4. Conclusion

In conclusion, the pulsed laser deposition technique has been employed to produce the InGaZnO thin films, where the InGaZnO ceramic target was fabricated by the solid-state reactions in the atmospheric pressure. The phase identification using X-ray diffraction indicates that the target contains InGaZn4O7 and InGaZn5O8, which are favorable chemical composition for the PLD process. The PLD experiments showed that the granular structure of the resultant InGaZnO film tends to become larger with increasing the substrate temperature from RT to 200 °C, whereas, the surface quality is almost not affected by substrate temperature. Furthermore, the InGaZnO film grown at 200 °C has higher carrier mobility and higher carrier concentration than that grown at RT. This kind of InGaZnO thin film is actually a n-type semiconductor with low resistance according to the Hall-effect tests, which shows high transmittance in the visible spectral range due to the amorphous structure, the high quality surface, and the suppression of the oxygen defects. Thus, the InGaZnO thin films with high optoelectronic performance can be obtained by the PLD technique at a relatively low temperature.

Acknowledgments

Thanks to the financial support by Beijing Municipal Education Commission Project: Km200910005019, and Ph.D. Program Foundation of Universities of China Project: 200800050013, as well as the measurements supported by the Institute of Physics Chinese Academy of Sciences, Semiconductors Chinese Academy of Sciences and Microstructure and Property of Advanced Materials of the Beijing University of Technology.



ingazno semiconductor thin film fabricated using pulsed laser deposition

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