Study of the Metal-Semiconductor Contact to Zno Films

Vacuum 155 (2018) 210–213

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Vacuum

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Study of the metal-semiconductor contact to ZnO ﬁlms T ∗ Yu Yana, Wei Mia, , Jinshi Zhaoa, Zhengchun Yanga, Kailiang Zhanga, Chongbiao Luanb a School of Electrical and Electronic Engineering, Tianjin Key Laboratory of Film Electronic & Communication Devices, Tianjin University of Technology, 391 West Binshui Road, Tianjin, 300384, China b Institute of Fluid Physics, China Academy of Engineering Physics, Mianshan Road 64, Mianyang, Sichuan, 621999, China

ARTICLE INFO ABSTRACT

Keywords: High resistance Zinc oxide (ZnO) films have been prepared on Si (100) substrates using magnetron sputtering ZnO method. Structure analysis revealed a clear out-of-plane orientation of ZnO (001) || Si (100). The metallic Thin films composition of the contact is a critically important parameter for making ohmic contacts to ZnO films. Al/Ti Ohmic contact metal contacts show linear I-V characteristics indicative of ohmic behavior, while other metal contacts such as Al and Ti show nonlinear characteristics with rectification, that reveal the presence of schottky barriers.

1. Introduction 2. Experimental details

In recent years, zinc oxide (ZnO) films have attracted extensive at- The ZnO films were grown on Si (100) substrates (thickness: tention because of the abundant and inexpensive raw materials [1–3]. 0.5 mm) using a high vacuum magnetron sputtering system. The Si ZnO is a direct wide band gap material with the band gap of 3.37 eV substrates were cleaned in organic cleaner and deionized water with and exciton of binding energy of 60 meV at room temperature [4]. ZnO ultrasonic irradiation for 20 min, respectively. Then the substrates were is one of the promising ultraviolet photoelectric materials with high dried with compressed nitrogen and placed into the reaction chamber. chemical and thermal stability. So it has the application in optoelec- The ZnO film was sputter deposited at room 100°C using a 99.99% ZnO tronics, solar cells, optical devices, UV lasers, LEDs and sensors [5,6]. target and a 15% O2/85% Ar mixture gas with a sputtering power of The realization of these devices depends on the electrode of ZnO ma- 60 W. The background vacuum of the equipment is better than − terial. Then it is important to prepare high quality electrode for ZnO. 2×10 8 Torr, and the deposition pressure is 8 mTorr. To fabricate the Lee et al. prepared Al doped ZnO thin films by a sol-gel method and electrode area, blanket metal layers were deposited through the elec- investigated the structural, electrical and optical properties of the films. tron-beam-evaporation process on the ZnO film. The photolithography − A minimum resistivity of 4.2 × 10 3 Ω cm was obtained for the 650 °C and lift-off processes were performed to achieve a strip electrode. In the annealed films doped with 1.0 at.% Al [7]. Kim et al. reported the Ti/Au process of deposition, the final thicknesses of the metal layer were electrode for Al doped ZnO film. The carrier concentration of the film controlled by the INFICON SQC-310. In order to improve the interface − was about 2 × 1017 cm 3. The annealed samples show good ohmic stability and reduce the defects of ZnO thin films, the samples were contact [8]. Ryu et al. reported the Ni/Au electrode for As doped ZnO annealed at 200°C in N2 protection atmosphere for 2 min. film, the electrode display linear contact [9]. However, most reports on The crystal structure was analyzed by X-ray diffractometer (Rigaku the contact characteristics of semiconducting ZnO films with metals Ultima IV) with Cu Kα1 radiation (λ = 1.5406 Å). The cross-section focus on the low resistance ZnO, and the electrode materials involved and surface micrograph were obtained using Zeissmerlin compact precious metals such as Au, Pt. High resistance ZnO films can be used in scanning electron microscope (SEM). The chemical composition of the the power devices and some switch devices. In this paper, high re- films was measured by the ESCALAB MK II X-ray photoelectron spec- sistance ZnO films were grown on silicon substrate by magnetron troscopy (XPS). XPS source gun type is Al Kα, and spot size is 500 μm sputtering method which has many advantages such as high deposition with 30 eV pass energy. The film resistance was detected by RTS-9 four- rate, low working pressure, easy control and suitable for large area film point probe and the resistance value of the film was beyond measure- preparation, the structural characterization, components of ZnO films ment range (greater than 2.5 × 104 Ω/□). Therefore, the resistance and the electrical properties of metal-semiconductor contact were in- value of ZnO thin films measured by ZC-36 ultra-high resistance tester. vestigated in detail. The resistance of the film is 1.2 × 106 Ω/□. The current-voltage

∗ Corresponding author. E-mail address: [email protected] (W. Mi). https://doi.org/10.1016/j.vacuum.2018.06.017 Received 25 April 2018; Received in revised form 10 May 2018; Accepted 5 June 2018 Available online 06 June 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved. Y. Yan et al. Vacuum 155 (2018) 210–213

Fig. 1. XRD θ-2θ scans of the ZnO ﬁlms prepared on Si substrate. characteristic was measured by an Agilent B1500A semiconductor parameter analyzer.

3. Results and discussion

Fig. 1 shows the XRD θ-2θ scans of the ZnO films prepared on Si substrate. Besides the Si (400) diffraction peak located at about 69.3°, a clear diffraction peak at around 34.2° (PDF#65–3411) corresponds to hexagonal ZnO (002) can be observed. There is no other diffraction peak. So the structure of the sample can be presumed as single crystal with a clear out-of-plane orientation of ZnO (001) || Si (100). To determine the chemical composition and atomic ratio of the film, the sample was analyzed by XPS. The binding energy was calibrated with the C–C binding energy of the adventitious C signal (284.5 eV). To avoid excessive etching of ZnO and leading to a reduction reaction, the sample was not cleaned before testing. Fig. 2 (a) shows a survey scan in the energy ranging from 0 to 1200 eV. The Zn 2p, C 1s and O 1s peaks can be detected. The C 1s peak comes from the adsorbed C on the surface of the sample. In Fig. 2 (b), two symmetrical peaks of Zn 2p3/2 located at about 1021.7 eV and Zn 2p1/2 located at 1144.7 eV. The separation distance between these two peaks is about 23.0 eV, which is in good agreement with the binding energy of the Zn 2p [10]. Fig. 2 (c) shows the O1s spectrum of the sample at the binding energy of about 530.3 eV. The O1s peak is usually in the range of 529–535 eV, the O1s signal originates from the oxygen in the lattice is usually located in the range of 529–530 eV. The peak pattern in the curve (c) presents obvious asymmetry. Binding energy at about 530.3 eV attributes to the Zn-O bonding in ZnO lattice, the other peak at 532.0 eV can be regard as the − C/O or OH adsorbed species on the surface [11,12]. The composi- tional ratio of Zn/O is about 1.09 (At.%), Zn element is slightly more than that of O because of the existence of oxygen vacancies and the concentration of oxygen vacancies is low [17]. The XPS result illustrates that the obtained sample is ZnO with less defects, and higher square resistance can also prove this fact. Fig. 3 (a) was the cross-section area of the ZnO film. The obvious boundary between the substrate and film can be observed. The thickness of the film is calculated to be about 130 nm. From Fig. 3 (b), an orderly surface with regular shaped crystals is obtained. The boundaries of each crystalline particle can be seen clearly. The silicon tip scanning range of the AFM image is 3 × 3 μm. The analogous surface topography can be observed, the results are in accord with the SEM. The surface Fig. 2. XPS spectra of the sample. roughness can be represented by root-mean-square average (RMS) va- lues, which are given by [13]: of the relative vertical height and the mean height of the surface. The

n RRMS of the film is about 0.642 nm. The AFM results are in accord with 1 2 RRMS = ∑ zi the SEM results. n i1= The I-V characteristics for the metal contacts are shown in Fig. 4 (a). Curves a, b and c corresponds to Al (100 nm), Ti (100 nm) and Al Where n is the number of data points of the profile, zi are the difference

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Fig. 4. (a) I-V characteristics of Al (100 nm), Ti (100 nm) and Al (20 nm)/Ti Fig. 3. (a) The SEM images of the cross-section area of the sample between ZnO (80 nm) metal contacts on ZnO films. (b) The cross-section SEM image of Al/Ti film and substrate. (b) The plain-view SEM images of ZnO films, with the 2D electrode sample. AFM results of film in the inset.

because of the protection of Ti, the sample of Al/Ti electrode, compared (20 nm)/Ti (80 nm) electrode. From curves a and b, the curve has a with the sample of single Al electrode, captured a proper amount of local bend. This means that there are no good ohmic contact formed oxygen ions, which resulted in the accumulation of oxygen vacancy on fi between the electrode and the ZnO lms. The curve of Al (20 nm)/Ti the surface of ZnO as donor and increased the carrier concentration on fi (80 nm) electrode show a ne linear relationship, which means that the the surface [8]. A better ohmic contact is achieved (as shown in the c fi Al/Ti metal bilayer forms an ohmic contact to the ZnO lm. The results curve of Fig. 4). For the sample of single Ti electrode, the trend is si- illustrate that metallic composition of the contact is a critically im- milar to that of the sample of single Al, such as the b curve in Fig. 4. fi portant parameter for making ohmic contacts to ZnO lms. This is also explained as TiOx (x < 2), which increases the contact The cross-section SEM image of Al/Ti electrode sample was shown barrier. in the inset of Fig. 4 (b). It can be seen from the image that the thickness of the Al layer is approximately 17 nm (the dark area indicated by an 4. Conclusions arrow), Ti and ZnO layer are corresponding to 80 nm and 130 nm. The designed thickness of the Al layer is 20 nm, the causes of the thickness High resistance Zinc oxide (ZnO) films have been prepared on Si reduction of the Al layer will be analyzed in detail below. (100) substrates by RF magnetron sputtering method. Structure analysis The result is that standard Gibbs energy of formation of oxides of Al showed a clear out-of-plane orientation of ZnO (001) || Si (100). A (−1054.9 kJ/mol O ) and Ti (−888.8 kJ/mol O ) forms a much larger 2 2 orderly surface with regular shaped crystals were obtained, the R of than Zn (−350.5 kJ/mol O )[16], which proves that Al and Ti have the rms 2 the film was about 0.642 nm. The resistance of the film is 1.2 × 106 Ω/ ability to capture oxygen ions in ZnO. For Al and Al/Ti electrode □. Compared with previous researches, the contact characteristics of samples, due to the thermal budget of the growth process, this in- zinc oxide with high resistance are rare. This research provides a pos- evitably leads to an Al-O-Zn interface overlapping layer between Al and sible solution for high resistance zinc oxide ohmic contact by com- ZnO films [14]. The difference between Al and Al/Ti electrode samples paring different electrodes, and it makes a meaningful analyze. Al/Ti is owing to the sample of single Al electrode is not protected by Ti layer, metal contacts show linear I-V characteristics indicative of ohmic be- and the Al metal will tend to lead to a higher degree of oxidation havior, while other metal contacts such as Al and Ti show nonlinear (compared to Al/Ti electrode samples) in both growth and annealing characteristics with rectification, that reveal the presence of schottky process, thus the Schottky barrier height of the interface was increased, barriers. The high resistance ZnO films can be used in the power devices leading to a curve as shown in Fig. 4 curve a. On the other hand, and switch devices.

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