Bandgap Alteration of Transparent Zinc Oxide Thin Film with Mg Dopant

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  • ABSTRACT

    We have successfully demonstrated a bandgap alteration of transparent zinc oxide (ZnO) thin film with Mg dopant by using sol-gel spin coating technique. By increasing the dopant from 0 to 30 atomic percent (at.%), a decrement value in the cutoff is observed, where the absorption edge shifts continuously to the shorter wavelength side, towards 300 nm. This resulted in a significant bandgap increment from 3.28 to 3.57 eV. However, the transmittance of the thin film at 350-800 nm gradually downgraded, from 93 to 80 % which is most probably due to the grain size that becomes bigger, and it also affected the electrical properties. The decrement from 45 to 0.05 mA at +10 V was observed in the I-V characteristics, concluding the significant relationship; where higher optical bandgap materials will exhibit lower conductivity. These findings may be useful in optoelectronics devices.

  • KEYWORD

    ZnO , MgZnO , Optical band gap , Thin film

  • 1. INTRODUCTION

    Recently, many researchers gained the interest to study transparent zinc oxide (ZnO) thin film semiconducting nanostructures. They were interested to study their potential properties and applications especially in field effect transistor (FET), light emitting diode, laser diode and other optoelectronic devices [1-5]. Owing to a direct and wide bandgap of 3.3 eV at room temperature and its ability to alter or tailor the bandgap makes ZnO to become a potential candidate for further exploration and study. Tailored materials are necessary to build many technologically important devices including field effect transistor in transparent electronics based [2-3]. Tailoring and altering the bandgap will affect the electrical behavior as well as other properties. This can be done through doping, alloying and heterostructures with some materials such as Al, Mn and Mg [6-8]. Till date, Mg is the most popular dopant material as the ionic radii of Mg2+ and Zn2+ are nearly equal and so, this resulted in a very little lattice distortion when Zn ion is replaced by Mg ion [8,9]. However, there is a difficulty of tailoring the bandgap due to the immiscibility of the different crystal structures between ZnO (wurtzite) and MgO (rocksalt) [10]. Hence, many reports have come out with lots of technique in order to deposit MgZnO thin film and overcome that problem such as metal-organic vapour phase epitaxy (MOVPE) [11], molecular beam epitaxy (MBE) [12], RF magnetron co-sputtering [13], sol-gel [14], etc. Among all the deposition techniques, it is believed that sol-gel has got certain distinct advantages in the making of alloy oxides and doping. Besides being an easy and cheap operation, low processing temperatures lead to the easiness in the kinetics control and also in the minimization of chemical interaction between the material and the container wall. It also leads to an excellent compositional control. To be more significant, it also leads to an ability to achieve atomic scale mixing of individual components [14]. The growth of MgZnO thin film at high temperature and its physical properties, the electrical properties that are to be used as high-k dielectric material, and the effect of post annealing on a thin film has been reported earlier by Kim et al., Liang et al., and Meher et al., respectively [15-17]. However, none of them have emphasized the effect of those processes on the bandgap, which more or less reflects the electrical properties of MgZnO thin film as well as other properties.

    Hence, in this paper, we present the bandgap alteration of transparent ZnO thin film by using Mg dopant. Moreover, we also present the growth of MgxZn1-xO (0.0[17]. However, structural transition from hexagonal wurtzite to a cubic structure could pose limitations near the mixed phase region [17]. Due to this reason, we limit our Mg content to only 30 at.% in order to retain the wurtzite structure while altering the bandgap of the ZnO thin film.

    2. EXPERIMENTAL DETAILS

    The MgxZn1-xO (02 quartz substrate by using sol-gel method with a spin coating technique. Methanol and de-ionized water have been used for the purpose of cleaning before being rinsed with de-stilled water and dried with a nitrogen gun. Then, the substrate is ready to be used.

    The starting materials that were used were zinc acetate dehydrate [Zn(CH3CO)2.2H2O], together with solvent, 2-methoxyethanol [C3H8O2], mono-ethanolamine [C2H7NO] as a stabilizer and magnesium nitrate hexahydrate [Mg(NO3)2.6H2O] as a dopant. For 0.4 M solution, with x varies from 0.0 to 0.3, the weightage of each chemical have been calculated before being mixed and stirred together in one solution respectively, as has been discussed elsewhere [1].

    Before the deposition process, the solution was placed in a sonicator for about 30 minutes before being stirred and left at room temperature for 2 h. The deposition of a thin film was done in a spin coater with Ar gas at 4 mbar, 0.5 s/lit. The spin coater was programmed to rotate at 3000 rpm for 60 sec. During rotation, 10 drops sol was dropped onto the substrate. Then, each layer was pre heated at 150℃ for 10 min. Annealing process took place after the completion of all those 5 layers at 500℃ for 1 h. This process was repeated for x = 0.1, 0.2, and 0.3.

    The surface morphology of the thin film has been studied by using field emission scanning electron microscopy (FESEM, ZEISS Supra 40VP). The optical properties were observed by UV-VIS Nir spectrophotometer (Varian Cary 5000) and photoluminescence (PL) (Horiba Jobin Yvon-79 DU420A-OE-325) spectrometer in order to calculate the band gap. Finally, the electrical properties of each sample were analyzed by using Keithley 2400 I-V measurement system.

    3. RESULTS AND DISCUSSION

    The surface morphologies of MgxZn1-xO (0.0Fig. 1(a), the small particles of zinc

    oxide can be seen with an average diameter of 20 nm. This grain size becomes bigger and the surface becomes rougher but more uniform when the atomic percentage of magnesium increased as depicted in Fig. 1(b), (c), and (d). This is due to the substitution of magnesium ion with zinc during the sol-gel process. The increment in the diameter size of MgxZn1-xO (0.0Fig. 2

    Figure 3 shows the optical transmittance spectra of MgxZn1-xO (0.0

    Conversely, Fig. 4 shows the absorption coefficient, α of MgxZn1-xO (0.0

    for various Mg contents. The absorption coefficient was calculated using the transmittance data, which has been presented previously. Lambert’s Law has been applied to obtain the value of absorption coefficient at a respective wavelength with the Eq.(1) as follows:

    where, t is the thickness and T is the transmittance spectra of thin films. The result indicates that all films exhibit high absorption in the ultra violet (UV) range and low absorption in the visible and near the infra red (NIR) range. The absorption properties of MgxZn1-xO (0.0[18]. Absorption coefficients in the UV region significantly increased with Mg progress. The result suggests improvement in the optical absorption in the UV region with Mg progress, which provides useful information especially in the optoelectronic devices and device fabrication.

    Approximately, the bandgap alteration of the thin film can be deduced from Fig. 3. Here, it evidently shows that changes in the absorption edges are in parallel with the Mg progressed in the thin film. In order to appropriately estimate the optical bandgap,

    the Tauc`s plot is used, as the following equation:

    where, h is the Plank constant, v is the frequency of the incident photon, A is a constant depending on the electron-hole mobility and E_g is the optical bandgap energy. Fig. 5 shows the measured optical bandgap energy versus Mg progressed and it also shows a blue shift in the thin film. By extrapolating the linear part of the curve that intersects at the x axis, it will give the optical bandgap energy value. For 0 at.% of Mg content which identical to ZnO thin film, the optical bandgap energy was 3.28 eV, which is slightly higher compared to other reports [19,20]. With an increment in the Mg content up to 10 at.%, the bandgap obtained increased to 3.34 eV. This was followed by 3.42 and 3.57 eV for 20 and 30 at.% of Mg content, respectively. An increment of at least 0.3 eV in the optical energy bandgap was obtained from 0 to 30 at.% of Mg content in ZnO thin film. This evidently shows that the bandgap of ZnO thin film can be altered. To support this finding, the room temperature PL of the thin film in the range of 350-800 nm is presented in Fig. 6. The overall PL spectra shows an emission band with two obvious peaks, where the first peak, the UV peak which also called the emission or near band

    edge emission contributed to the free exciton recombination [18]. The second broad peak, also known as the green emission corresponds to the recombination of a photon generated hole with an electron in singly ionized [18]. From the first peak, the optical bandgap can be simply calculated by using the relation,

    image

    where, h is the plank constant and λ is the wavelength. With this relation, the optical bandgap can simply be calculated by referring to the emission peak in the PL spectra. For 0 at.% of Mg, the emission peak falls at 380.50 nm and the optical badgap calculated was 3.26 eV. The increment of 0.02 eV was observed when the Mg increased to 10 at.% where the emission peak falls at 378.06 nm. This was further increased, by reaching to 3.29 eV(377 nm) for 20 at.%, and it stops at 3.33 eV (372.95 nm) when the Mg content reached 30 at.%. When compared to the previous value calculated by using Tauc`s plot, the values were slightly lower, but the increment patent was significantly noticed and it was identical. Thus, it can be concluded that the optical bandgap energy can be tuned, tailored and altered by using Mg dopant to the ZnO thin film.

    Further investigation on electrical properties was carried out using Palladium metal contact. Fig. 8 shows the current-voltage (I-V) characteristics of MgxZn1-xO (0.0[16,21].

    When the four-point probe measurement took place, the reading shows an increasing value of sheet resistance with an increasing number of magnesium ions in the thin film. This is probably due to the widening of the energy bandgap with more magnesium ions to substitute the zinc oxide [16].

    4. CONCLUSIONS

    In summary, the physical, optical and electrical properties of MgxZnO1-x (0.0xZnO1-x (0.0

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  • [Fig. 1.] Surface morphology of (a) ZnO, (b) Mg0.1Zn0.9O, (c) Mg0.2Zn0.80, (d) Mg0.3Zn0.70 thin film.
    Surface morphology of (a) ZnO, (b) Mg0.1Zn0.9O, (c) Mg0.2Zn0.80, (d) Mg0.3Zn0.70 thin film.
  • [Fig. 2.] Diameter size of MgxZn1-xO (0.0<x<0.3) nanoparticles.
    Diameter size of MgxZn1-xO (0.0<x<0.3) nanoparticles.
  • [Fig. 3.] Optical transmittance spectra of MgxZn1-xO (0.0<x<0.3) thin film .
    Optical transmittance spectra of MgxZn1-xO (0.0<x<0.3) thin film .
  • [Fig. 4.] Absorption coefficient of MgxZn1-xO (0.0<x<0.3) thin film .
    Absorption coefficient of MgxZn1-xO (0.0<x<0.3) thin film .
  • [Fig. 5.] Optical band gap estimation of MgxZn1-xO (0.0<x<0.3) thin film using Tauc`s plot.
    Optical band gap estimation of MgxZn1-xO (0.0<x<0.3) thin film using Tauc`s plot.
  • [Fig. 6.] PL spectra of MgxZn1-xO (0.0<x<0.3) thin film.
    PL spectra of MgxZn1-xO (0.0<x<0.3) thin film.
  • [Fig. 7.] Optical band gap energy of MgxZn1-xO (0.0<x<0.3) thin film calculated by using Tauc`s plot.
    Optical band gap energy of MgxZn1-xO (0.0<x<0.3) thin film calculated by using Tauc`s plot.
  • [Fig. 8.] Electrical properties of MgxZn1-xO (0.0<x<0.3) thin film on Pd metal contact.
    Electrical properties of MgxZn1-xO (0.0<x<0.3) thin film on Pd metal contact.