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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.31 No.6 pp.320-324
DOI : https://doi.org/10.3740/MRSK.2021.31.6.320

Effects of Growth Ambient, Process Pressure, and Heat Treatments on the Properties of RF Magnetron Sputtered GaMgZnO UV-Range Transparent Conductive Films

Vijay Patil, Chesin Lee, Byung-Teak Lee†
Photonic and Electronic Thin Film Laboratory, Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Gwangju 500-757, Republic of Korea
Corresponding author E-Mail : btlee@jnu.ac.kr (B.-T. Lee, Chonnam Nat'l Univ.)
March 31, 2021 May 7, 2021 May 13, 2021

Abstract


Effects of growth variables and post-growth annealing on the optical, structural and electrical properties of magnetron-sputtered Ga0.04Mg0.10Zn0.86O films are characterized in detail. It is observed that films grown from pure oxygen plasma showed high resistivity, ~102 Ω·cm, whereas films grown in Ar plasma showed much lower resistivity, 2.0 × 10− 2 ~ 1.0 × 10−1 Ω·cm. Post-growth annealing significantly improved the electrical resistivity, to 4.3 ~ 9.0 × 10−3 Ω·cm for the vacuum annealed samples and to 1.3 ~ 3.0 × 10−3 Ω·cm for the films annealed in Zn vapor. It is proposed that these phenomena may be attributed to the improved crystalline quality and to changes in the defect chemistry. It is suggested that growth within oxygen environments leads to suppression of oxygen vacancy (Vo) donors and formation of Zn vacancy (VZn) acceptors, resulting in highly resistive films. After annealing treatment, the activation of Ga donors is enhanced, Vo donors are annihilated, and crystalline quality is improved, increasing the electron mobility and the concentration. After annealing in Zn vapor, Zn interstitial donors are introduced, further increasing the electron concentration.



초록


    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    The MgZnO films (ZnO doped with Mg), with the large energy bandgap (Eg) from 3.3 eV to 7.8 eV, have been attracting great attention as the base material for the optoelectronic devices operating in the ultraviolet (UV) range, such as UV light emission diodes and UV photodetectors. 1-5) It has been reported that the MgZnO films with high Mg contents are too resistive to be utilized for the devices, with a resistivity of ≥ 102 Wcm, but their electrical properties may be significantly improved by doping group III elements, such as Ga and Al.6,7)

    It has been reported that the properties of semiconducting films grown with a variety of deposition methods are strongly affected by the growth conditions and the annealing treatments,8-11) and understanding of the correlations between the growth conditions and film properties are essential to obtain high-quality films with proper characteristics. In fact, for the ZnO-related films, it was proposed that the growth parameters affected the microstructures and the chemistry of various point defects,9-11) resulting in significantly different optical, electrical and/ or structural properties.

    In this work, results of detailed investigation on the effects of growth ambient, process pressure, and postgrowth heat treatment on the optical, electrical and structural properties of the magnetron sputter grown Ga-doped MgZnO (GaMgZnO) films are reported. The chemical states of the related elements and native defects within the films were also proposed, as well as the operating mechanisms responsible for the observed results.

    2. Experimental

    The Ga0.04Mg0.10Zn0.86O films were deposited on sapphire substrates via RF magnetron sputtering using 2 inch ceramic targets, prepared by a conventional solid-state reaction method using the cold isostatic press and the furnace. The sputtering chamber was initially pumped down to a pressure of less than 1 × 10−4 Pa and then the working pressure during deposition was changed between 0.3 Pa and 2.0 Pa. The RF powder, gas flow rate, substrate temperature were maintained at 125W, 40 sccm, and room temperature, respectively.

    After deposition, annealing treatments were performed in two types of forming atmospheres. To guarantee the comparability, as-grown films were cut into 4 pieces, and two pieces of samples were annealed in Zn vapor and the others in the vacuum. For the Zn vapor annealing, samples were placed in quartz tubes with metallic Zn powder (99.999 %) and then vacuum-sealed at a pressure of less than 5 × 10−2 Torr. Annealing was performed in a box furnace at 600 oC for 12 h. To check reproducibility, experiments were repeated three times and the results were averaged.

    The microstructures of obtained films were characterized by an X-ray diffraction (XRD, Philip 4-crystal triple axis system) operated at 40 kV and 30 mA using Ni-filtered Cu Kα radiation and a field-emission scanning electron microscopy (JSM-6710F JEOL). The electrical properties of the MgGaZnO films were measured by a Hall measurement system (M/N #7707_LVWR, Lake Shore Cryotronic Inc.) using a Van der Pauw geometry at room temperature. Optical transmittance spectra were recorded using an UV-Visible spectrophotometer (VARIAN Technologies) and the calibration of the spectrophotometer was pre-performed to exclude a contribution from the substrate before measurement.

    3. Results and Discussions

    Fig. 1 shows the resistivity (ρ), carrier concentration (n), and Hall mobility (μ) of the Ga0.04Mg0.10Zn0.86O films sputtered at different working pressures, (a) in the Ar plasma and (b) in the O2 plasma. It is observed that all the films exhibit n-type conductivity and the electrical properties are strongly dependent on the growth ambient. The resistivity of the films sputtered at 0.3 Pa increases from 2.0 × 10−2 Ω·cm for Ar to 1.6 × 102 Ω·cm for O2, with an abrupt reduction in the carrier concentration and mobility from 1.2 × 102 cm−3 and 2.7 cm2/Vs for Ar to 5.3 × 1016 cm−3 and 0.75 cm2/Vs for O2.

    It is also observed that the mobility slightly increases and, as a result, the resistivity decreases with the decreasing pressure. The electron mobility and the resistivity changes from 0.6 cm2/Vs and 1.0 × 10−1 Ω·cm at 2 Pa to 2.7 cm2/ Vs and 2.0 × 10−2 Ω·cm at 0.3 Pa for the films grown in the Ar plasma [Fig. 1(a)] and from 0.2 cm2/Vs and 3.4 × 102 Ω·cm at 2 Pa to 0.7 cm2/Vs and 1.6 × 102 Ω·cm at 0.3 Pa for the GaMgZnO films grown in the O2 plasma [Fig. 1(b)]. The carrier concentration is not critically affected by the pressure.

    It is believed that the observed change would be related to the variation of the species and concentration of the points defects during the sputter process. It has been reported that the oxygen vacancy (Vo, donor) and the Zn vacancy (VZn, acceptor) are the dominant defects within the sputter grown ZnO-related thin films. 10-14) It is believed that the GaMgZnO films grown in Ar plasma would have a sizable amount of free electrons, emitted from the Ga dopant donors (GaZn) and the Vo donors. The carrier concentration would be substantially reduced by adding O2 to the sputter plasma, as the formation of the Vo donors would be suppressed and the VZn acceptors would be formed to compensate the GaZn donors. The mobility slightly decreases with the O2 addition due to the formation of the defects such as VZn and the GaZn+ ionized donors.

    As to the pressure effects, it has been reported in the case of the undoped ZnO and the Ga-doped ZnO films that the films with an improved microstructure with larger grain sizes and reduced amount of loosely bound oxygen atoms are obtained at low working pressures, increasing the electron mobility.12)

    Fig. 2 displays optical transmittance spectra of the GaMgZnO films grown at various working pressures within (a) Ar plasma and (b) O2 plasma. It is observed that all the films are highly transparent in the visible region of 370 ~ 800 nm and the multi-absorption edge is not found in all spectra, denoting that the films are composed of a single wurtzite phase. It is also noticed in Fig. 2 that the films grown in O2 plasma shows lower cut-off energy, which is related to the lower carrier concentration. This point will be further discussed later.

    Fig. 3 shows electrical properties of the GaMgZnO films annealed in the vacuum and in the Zn vapor after sputtered at 0.3 Pa (a) in pure Ar and (b) in pure O2. It is observed in Fig. 3a that the electrical resistivity and the electron mobility of the films grown in pure Ar improved by the post-growth annealing treament, from 2.0 × 10−2 Ω·cm and 2.7 cm2/Vs for the as-grown films to 9.1 × 10−3 Ω·cm and 7.8 cm2/Vs for the vacuum annealed films, and 3.0 × 10−3 Ω·cm and 9.8 cm2/Vs for the annealed film in Zn vapor. It is further noticed that the electrical properties of the film grown in pure O2 drastically improved after the annealing, from 1.6 × 102 Ω·cm resistivity, 5.3 × 1016 cm−3 carrier concentration, and 0.75 cm2/Vs mobility for the as-deposit film to 4.3 × 10−3 Ω·cm, 1.7 × 1020 cm−3, and 8.6 cm2/Vs for the vacuum annealed film, and 1.3 × 10−3 Ω·cm, 4.1 × 1020 cm−3, and 11.4 cm2/Vs for the annealed film in Zn vapor

    It is believed that the improvement of electrical properties of the GMZO film by the thermal treatment would be mainly relatd to the chemical equilibrium of participating defects. It has been reported that the Vo donor and the VZn acceptor are the dominant defects within the as-grown ZnO thin films and the Zn interstitials (Zni, donor) mainly forms when GaZnO the films are annealed in the Zn vapor.10-12)

    It is suspected that, for the GaMgZnO films sputtered in O2 plasma, the vacuum annealing annihilated VZn acceptors (presumably due to an out-diffusion of the oxygen atoms) and promoted the substitution of Ga in Zn site (activation of GaZn donors), resulting in the increased carrier concentration. The Zni donor defects form during the annealing within the Zn vapor, additionally enhancing the carrier concentration. The mobility of the films is substantially improved by the annealing, as the VZn defects are annihilated, more Ga dopants substitute regular Zn site, and the crystalline quality is improved by the enhanced diffusion at high temperature.

    For the films sputtered in Ar plasma, annealing would enhance the substitution of Ga in Zn site and and improve the crystalline quality to increase the electron mobility. The carrier concentration is not critically affected, as Vo donors are reduced or removed but the GaZn donors and Zni donors are activated by the annealing.

    Fig. 4 shows the optical transmittance spectra of the as-grown and the post-annealed GaMgZnO films, which show that the films are highly transparent in the visible range of 380 ~ 780 nm wavelength. Sharp absorption edges are clear at the UV-region in all the spectra, indicating no additional phase.

    From the Tauc’s relationship15) between the absorption coefficient (α)2 and the photon energy (hν), the optical band gaps (Eg) were estimated and are illustrated in boxes inside the figures. In the case of the GaMgZnO films grown in O2 ambient, the Eg is remarkably enhanced from 3.34 eV for the as-grown film to 3.58 eV for the vacuum annealed film and 3.60 eV for the Zn vapor annealed film, whereas Eg of the films grown in Ar was slightly enhanced from 3.64 eV for to 3.65 eV for the vacuum annealed film and 3.72 eV for the Zn vapor annealed film.

    It is well established that the Eg of degenerate semiconductors widenes due to the Burstein-Moss (B-M) band filling effect.16) Basically, the conduction band becomes filled at high doping concentration and the lowest energy states in the conduction band are blocked, resulting in the increase of apparent optical Eg. Thus, it is suggested that the optical band-gap behavior observed in the present work would be closely related to the B-M effect.

    Fig. 5 shows the x-ray diffraction (XRD) patterns of the MgGaZnO films, (a) as-grown in Ar plasma at 0.3 Pa, (b) as-grown in O2 plasma at 0.3 Pa, and (c) as-grown in Ar plasma at 2.0 Pa, and (d) annealed in Zn vapor after deposit in O2 plasma. All the XRD peaks can be assigned to wurtzite ZnO phase, which indicates that any secondary phase, such as MgO, MgGa2O4, and ZnGa2O4, has not been formed due to the Ga and/or Mg doping.

    In Fig. 5(a), the XRD pattern of the film sputtered in pure Ar, it is observed that an intense ZnO (002) diffraction peak is dominant with a strong c-axis preferred orientation, whereas in Fig. 5(b), the XRD pattern of the film sputtered in pure O2, represent a typical poly-crystal characteristics with various peaks. It is suspected that the energy balance between the atomic planes were affected by the growth environment, resulting in the observed orientation change. It is also seen in Fig. 5 that the process pressure [Fig. 5(c)] and the annealing treatment [Fig. 5(d)] does not have significant effects on the XRD patterns of the GaMgZnO films.

    Fig. 6 shows the diagonal-view SEM images of the GMZO films, (a) as-grown in Ar plasma at 0.3 Pa, (b) as-grown in O2 plasma at 0.3 Pa, and (c) as-grown in Ar plasma at 2.0 Pa, and (d) annealed in Zn vapor after grown in O2 plasma at 0.3 Pa. The film thickness was controlled to be ~ 1 um by adjusting the sputter time. All the films show the columnar microstructure and have a smooth surface morphology without visible voids. It is mentioned that no discernable difference in the film microstructure was found, regardless of the growth conditions and the heat treatment.

    4. Summary

    Effects of growth variables and post-growth annealing on the optical, structural and electrical properties of magnetron-sputtered Ga0.04Mg0.10Zn0.86O films were characterized in detail. It was observed that films grown from pure oxygen plasma showed high resistivity, ~ 102 Ωcm, whereas films grown in Ar plasma showed much lower resistivity, 2.0 × 10−2 ~ 1.0 × 10−1 Ωcm. Resistivity of the films slightly decreased as the working pressure decreases, from 1.0 × 10−1 Ω·cm at 2 Pa to 2.0 × 10−2 Ωcm at 0.3 Pa for the films grown in Ar. It was also observed that the post-growth annealing significantly improved that electrical properties of the GaMgZnO films. The resistivity of the films sputtered in O2 plasma at 0.3 Pa decreased to 4.3 ~ 9.0 × 10−3 Ωcm for the vacuum annealed samples and to 1.3 ~ 3.0 × 10−3 Ωcm for the films annealed in Zn vapor.

    It was proposed that the observed phenomena may be attributed to the improved crystalline quality and the variation of the species and concentration of point defects within the film. It was suggested that the GaMgZnO films grown in Ar plasma would have a sizable amount of free electrons, emitted from the Ga dopant donors (GaZn) and the Vo donors. The carrier concentration would be substantially reduced by adding O2 to the growh plasma, as Vo donors would be suppressed and the VZn acceptors be formed. The mobility slightly decreases due to the formation of the defects such as VZn and GaZn+ ionized donors.

    It is suspected that, for the GaMgZnO films sputtered in O2 plasma, the vacuum annealing annihilates VZn acceptors and promotes activation of the GaZn donors, resulting in the increased carrier concentration. The Zni donor defects form during the annealing in the Zn vapor, additionally enhancing the carrier concentration. The mobility of the films is substantially improved by the annealing, as the VZn defects are annihilated, more Ga dopants substitute regular Zn site, and the crystalline quality is improved.

    Acknowledgement

    This study was financially supported by Chonnam National University (Grant number: 2019-3902).

    Figure

    MRSK-31-6-320_F1.gif

    Resistivity, carrier concentration, and Hall mobility, of the Ga0.04Mg0.10Zn0.86O films grown at different working pressures, (a) in the Ar plasma and (b) in the O2 plasma.

    MRSK-31-6-320_F2.gif

    Optical transmittance spectra of the GaMgZnO films grown at various working pressures within (a) Ar plasma and (b) O2 plasma.

    MRSK-31-6-320_F3.gif

    Electrical properties of the Ga0.04Mg0.10Zn0.86O films annealed in the vacuum and in the Zn vapor after sputtered at 0.3 Pa (a) in pure Ar and (b) in pure O2.

    MRSK-31-6-320_F4.gif

    Optical transmittance spectra of (a) the as-grown and (b) the post-annealed GaMgZnO films. Optical band gaps estimated from the Tauc’s relationship are illustrated in the boxes.

    MRSK-31-6-320_F5.gif

    XRD patterns of the MgGaZnO films, (a) as-grown in Ar plasma at 0.3 Pa, (b) as-grown in O2 plasma at 0.3 P, and (c) asgrown in Ar plasma at 2.0 Pa, and (d) annealed in Zn vapor after deposit in O2 plasma.

    MRSK-31-6-320_F6.gif

    Diagonal-view SEM images of the GMZO films, (a) asgrown in Ar plasma at 0.3 Pa, (b) as-grown in O2 plasma at 0.3 Pa, (c) as-grown in Ar plasma at 2.0 Pa, and (d) annealed in Zn vapor after grown in O2 plasma at 0.3 Pa.

    Table

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