2021 年 62 巻 7 号 p. 915-920
The effects of post-annealing treatment temperature and ambient gas on the properties of ZnO thin films were investigated. The results showed that the properties of the ZnO thin film depended on the crystal quality, the type and density of the intrinsic defects. Treating the ZnO thin films at 300°C, regardless of the ambient gas, affected only on the crystal quality of the thin films. When the samples were treated at 500°C and 600°C, the structural properties were significantly improved, accompanied by changes in the optical and electrical properties. The extent of improvements varied with the treatment ambient. These variations in the optical and electrical properties of the thin films were attributed to the different reaction mechanisms when ZnO was treated under different ambient conditions. The thin films, treated at 600°C under N2O ambient, exhibited the best structural, optical, and electrical properties for metal-semiconductor-metal photodetector (MSM-PD) applications.
Zinc oxide (ZnO) is a widely used material. ZnO and its alloys possess many unique physical properties such as high exciton binding energy (∼60 meV), high saturation velocity, and extreme resistance to high-energy particle irradiation. In addition to the nontoxicity, ZnO has the advantages of ease of processing; it can be grown on inexpensive substrates via simple, low-cost fabrication techniques along with wet-etch availability. High-quality ZnO films can be grown at relatively low temperatures (less than 700°C). These attributes make ZnO a promising candidate for optoelectronic devices, especially for ultraviolet (UV) photodetectors (PDs).1,2) However, owing to the difficulties in obtaining a stable and reproducible p-type ZnO material, the metal-semiconductor-metal (MSM) is the most attractive device structure for ZnO-based PD applications. The performance of MSM-PDs is evaluated mainly by their responsivity, response speed, and dark current. The material properties of the ZnO thin film determine mostly the responsivity of the detectors. The response speed is usually limited by the optically generated carrier transition time, which is governed fundamentally by the electron and hole mobilities. The dark current depends on the metal–semiconductor Schottky barrier, the doping level of the material, and operating temperature. Therefore, to realize ZnO-based MSM-PD with superior performance, good material quality, high carrier mobility, and low doping and surface defects are imperative.3)
The characteristics of the film such as the structural, optical, and electrical properties not only depend strongly on the growth conditions and fabrication technique but also rely on the post-growth treatment.3,4) Magnetron radio-frequency (RF) sputtering is one of the good growth techniques to achieve a homogeneous ZnO thin film on a large area with reproducibility.5,6)
Post-growth thermal treatments are processes in which a film, previously grown on a substrate, is exposed at a given temperature under certain conditions for a specified time. The thermal treatment is intended to improve the structural properties of crystals and also to adjust the stoichiometry of compound materials. These alterations, generally, result in considerable enhancements of most semiconductor properties.7) Post-growth thermal treatments are considered as an effective technique to modify the properties of ZnO thin films to meet several desired requirements. Such enhancements include improvement of the material crystallinity and stability,8) decrease in carrier concentration,9) decrease in ZnO–metal contact resistance,10,11) enhancement of the luminescence properties,12) and improvement of the electrical characteristics.13)
In this work, we investigated the influence of the post-annealing treatment temperature and the ambient gas on the structural, morphological, optical, and electrical properties of ZnO thin films. All the thin films, used in this study, were grown on glass substrates via RF sputtering.
The ZnO thin films were deposited on glass substrates by using a ZnO target (99.999% purity, 2 inch diameter, 3 mm thickness, SWI Inc) in an AJA RF sputtering system. Prior to deposition, the substrates were first cleaned by soaking in acetone and then in isopropanol by using an ultrasonic bath for five minutes each. Thereafter, the substrates were rinsed for one minute in running distilled water, and then, blown-dry with a N2 gun. To ensure that the substrates were completely dried before deposition, they were mounted on a preheated (200°C) hotplate for five minutes. The ZnO thin films were deposited at 150 W of power at a chamber-base pressure of 2 × 10−6 Torr and under an argon (Ar) pressure of 3 mTorr at room temperature. The average thickness of all the deposited thin films was around 500 nm, as measured using Bruker Dektak XT surface profiler. To investigate the effects of the post-annealing treatment temperature and ambient gases, the as-deposited samples were treated for 3 h in a quartz tube furnace (OTF-1200X-S) in different ambient gases (vacuum, N2, N2O, and He) and at various temperatures, including 300°C, 500°C, and 600°C. The films were heated at a rate of 5°C per min and then cooled to room temperature inside the furnace under the same ambient conditions.
Different characterization techniques were employed to investigate the structural, optical, and electrical properties of the thin films. The morphology of all the prepared ZnO thin films was examined using a JSM-7600F field-emission scanning electron microscope (FESEM). The structural properties of the films were examined using a MiniFlex 600 X-ray diffractometer (XRD) (Rigaku, Japan, Cu K-α radiation = 1.544 Å). The optical properties of the thin films were determined at room temperature using a UV-1800 spectrophotometer (Shimadzu, Japan) and FP-1800 spectrofluorometer (Jasco, Japan). The electrical properties were evaluated using a Keithly 4200-SCS parameter analyzer (Tektronix, USA) and an Ecopia’s Hall-effect measurement setup.
Figure 1 shows the FESEM surface morphology of the as-deposited ZnO film and those of the films treated at 600°C in an N2 ambient and under vacuum. The grains were compact, dense, and uniformly distributed on the surfaces of all the samples. The grains of the thermally treated samples were larger than those of the as-deposited sample. In addition, there was no significant difference in the surface morphologies of the ZnO samples treated at the same temperature but in different ambient (images are not shown here).
FESEM images of ZnO thin films deposited on glass: (a) as-deposited (b) treated at 600°C in N2 ambient (c) treated at 600°C under vacuum.
The phase identifications and crystal structures were determined via XRD 2θ-θ scan in the angle range of 20–80° with a step of 0.02°, and the scanning time per step was kept at 0.5 s. Figures 2(a) and (b) show the XRD patterns of the as-deposited ZnO thin film and those of the samples treated at different temperatures in N2O ambient and under vacuum, respectively. The XRD spectra of all the ZnO thin film samples have the same trend. The samples show a single strong crystal orientation peak around 34°, corresponding to the (002) plane of wurtzite hexagonal crystal structure of ZnO with a growth orientation along the c-axis perpendicular to the substrate surface. This result confirms that good-quality biaxial texture thin films were formed during the deposition and improved with the different thermal treatments. The peak orientation of the as-deposited sample shifted slightly toward higher 2θ values as the annealing temperature increased; the extent of the shift was dependent on the ambient gas. This expected shift is attributed to the presence of lattice defects in the treated thin films. These defects are attributed to the microstrain between the thin film and the substrate owing to the difference in their thermal expansion coefficients (TEC).14)
XRD 2θ-θ patterns of as-deposited ZnO thin films and those of the samples treated (a) in N2O and (b) under vacuum.
The crystal quality of all the samples was monotonically enhanced by annealing treatments regardless of the annealing ambient. The improvement is evidenced by the increase in the peak intensity and narrowing of the full-width at half maximum (FWHM). The crystallite size of the ZnO thin film samples was calculated from the XRD spectra using the Debye-Scherrer formula: $D = \text{k}\lambda /\beta \cos \theta $, where k is a constant, λ the Cu Kα radiation wavelength, and β the full-width at half maxima.15) The average crystallite sizes of the various samples are listed in Table 1.
Figure 3 shows the optical images of the as-deposited ZnO thin film and those of the samples treated at 600°C in different ambient gases. All the thin films were transparent and colorless. The appearance of the samples is consistent with the transmittance spectrum shown in Fig. 4 for the same samples. All prepared films exhibited high optical transmittance fluctuating between 100 and 85% in the IR and visible light range and a strong absorption in the UV region with a sharp absorption edge. The transmittance fluctuations is caused by the interference of the multiple reflections of light at the ZnO thin film interfaces with air and the glass substrate. The high transmittance of the films is attributed to the highly crystalline nature of the films,16) and the strong absorption in the UV range is due to the band to band absorption. The treated samples revealed a clear blueshift of the absorption edge. Similar blueshift is seen in the transmittance spectrum of the 500°C-treated samples, whereas a smaller shift is seen with the 300°C-treated samples.
Optical images of ZnO thin films treated under different ambient conditions.
Transmittance spectra of as-deposited ZnO thin film and those treated at 600°C in N2, N2O, He, and under vacuum.
The absorption coefficient spectrum (α) of the samples was calculated from the optical transmittance (T) and ZnO thin film thickness (t) using the Beer-Lambert’s Law expressed as α = ln(1/T)/t.17) Figure 5 shows the absorption coefficient spectra of the as-deposited sample and those treated at 600°C under different ambient gases. It can be seen that all samples have a high absorption coefficient in the UV region ranging from 1.25 × 105 to 1.5 × 105 (cm−1). The optical energy bandgaps of the samples were obtained from the absorption coefficient and the transmittance spectrum using the Tauc plot.18) The bandgap of the as-deposited ZnO thin film was 3.29 eV and all other treated samples at 600°C have bandgaps around 3.32 eV, and it was not affected much by the treatment conditions. The reasons for the apparent bandgap increase, seen after annealing at high temperature, have been discussed in details in Refs. 19–22).
Absorption coefficient spectra of as-deposited ZnO thin film and those of the samples treated at 600°C in N2, N2O, He, and vacuum.
Photoluminescence (PL) spectrometry measures the radiative recombination of photoexcited electrons with the holes left in the lower-energy states. It is generally used to investigate the crystal quality of semiconductors. The PL measurement system used in this study has a continuous-wave excitation Xe arc lamp (150 W), and 350 nm was set as the excitation wavelength. All the measurements were performed at room temperature, and the measurement step size for scanning the wavelength was set to 0.5 nm. Figures 6(a)–(d) show the PL spectra of the samples treated in N2O, N2, He, and vacuum, respectively. The spectra of the as-deposited sample and those of the samples treated at two different temperatures for each of the ambient environments are presented. All the samples showed a strong free exciton emission peak in the UV band around 407 nm, which is attributed to the near-band-edge emission from ZnO.23) However, the samples treated at 600°C in N2, He, or vacuum showed a minor broad peak in the visible band. The minor emission peak is centered at 440, 509, and 513 nm for the samples treated at 600°C in N2, vacuum, and He, respectively. This visible emission is attributed to radiative recombination at intrinsic defects. According to Zeng et al.,24) the blue emission for the N2-treated sample is a result of an increase in the Zinc interstitial (Zni) defect density, whereas the green emissions of the He- and vacuum-treated samples are attributed to an increase in the oxygen vacancies (VO) defect density.25,26)
Room-temperature PL spectra of the as-deposited ZnO thin film and the samples treated in (a) N2O, (b) N2, (c) He, and (d) vacuum.
The resistivity and Hall coefficients (carrier concentration and mobility) of the thin films were measured using the Van Der Pauw technique, in which multiple current-density–voltage (I–V) measurements are performed with an applied magnetic field of 0.5 T and under different applied biases. Table 2 lists the measured carrier concentration, mobility, and resistivity of the as-deposited and treated ZnO thin films. Although all the measurable samples exhibited the n-type conductivity, the carrier concentration and mobility of the ZnO thin films varied with the post-annealing temperature as well as the ambient.
The as-deposited ZnO thin film and all the samples treated at 300°C regardless of the ambient gas exhibited such high resistances that could be measured by our system. This high resistivity is attributed to the grain boundary potentials of the thin films, which act as a trap for moving carriers.27,28) However, after annealing the samples at 500°C, the resistance decreased due to the increase in the grains size, and measurements became possible and reliable. The properties of the treated thin films varied with the annealing ambient. For the samples annealed in N2O and He, increasing the annealing temperature from 500°C to 600°C resulted in a decrease in the electron concentration and an increase the electron mobility as expected. These occurred due to the decrease in the intrinsic donor defects in the ZnO thin films mainly represented by Zni.25,26) On the other hand, increasing the annealing temperature from 500°C to 600°C had only a small effect on the electron mobility and concentration of the ZnO thin films treated in vacuum and N2.
The samples treated at 300°C exhibited improved structural properties. However, this temperature was not enough to cause a noticeable change in the optical and electrical properties. The samples treated at 500°C and 600°C showed significant improvements in the structural properties, accompanied by changes in the optical and electrical properties. The extent of improvements differed with the treatment ambient. These effects on the optical and electrical properties of the films are attributed to the different reaction mechanisms when ZnO is treated in different ambient conditions. When the treatment temperature is increased, some of the Zni are expected to evaporate,25) resulting in oxygen loss from the ZnO thin films.29) This oxygen deficiency is the main cause of Zni and Vo formation. Therefore, the treatment ambient condition plays an important role in specifying the dominant mechanism. The as-deposited ZnO film had a small grain size. Grain boundaries trap free electrons, resulting in the high resistivity of the as-deposited films. Annealing the films at 300°C increased the grain size, which was expected to be accompanied by a decrease in resistivity, but the resistivity of the films remained too high to be measured by our system. The resistivity became measurable after treating at temperatures of 500°C and above. Increasing the annealing temperature from 500°C to 600°C in N2O ambient, the carrier concentration decreased and the carrier mobility increased without a noticeable effect on the PL intensity. This can be attributed to the domination of the Zn evaporation process because the presence of oxygen in N2O gas could compensate for the oxygen loss. The same effect was obtained in the electrical properties of the He-treated samples, but the sample treated at 600°C showed a green emission in the PL intensity, which implies that the density of Zni was decreased, besides the increase in the Vo density, as the annealing temperature increased. This is consistent with the results reported in Ref. 30). On the other hand, increasing the annealing temperature from 500°C to 600°C, there was no significant effect on the electrical properties of the samples treated in vacuum and N2, but the PL intensities were remarkably affected, especially the N2-treated sample. Therefore, oxygen loss was predominant in the N2 and vacuum ambient. These results show that the carrier concentration of ZnO thin films is controlled more by Zni defects than Vo defects. The ZnO thin films treated at 600°C in the N2O ambient exhibited the best structural and optical properties with the lowest carrier concentration and the highest mobility, which make the treatment conditions promising for MSM-PD applications. Also, the samples treated in N2, N2O, and He exhibited high stability with time compared with those treated under vacuum.
The effects of post-annealing treatment temperature and ambient gas on the structural, morphological, optical, and electrical properties of ZnO thin films grown on glass substrates via RF sputtering were investigated. The results show that the properties of the thin films depend on the crystal quality as well as the type and density of the intrinsic defects. When the ZnO thin films were treated at 300°C, the structural properties of the films were improved. However, this temperature was not enough to cause a noticeable change in the optical and electrical properties. At 500°C and 600°C, the structural properties were significantly improved, accompanied by changes in the optical and electrical properties. The degree of change was dependent on the treatment ambient. In the N2O ambient, increasing the temperature caused a decrease in the Zni defects, resulting in lower carrier concentration and higher mobility without a noticeable change in the PL spectra. In the He ambient, as the temperature increased, the Zni defects density decreased and that of Vo increased, resulting in green emissions in the PL spectra. In the N2 ambient, an increase in temperature had an obvious effect on the PL spectra, which is attributed to the increase in the Zni defects with a small increase in the carrier concentration. However, in the vacuum ambient, an increase in the treatment temperature had no significant effect on the electrical properties and PL spectra of the films. Among all, the ZnO thin films treated at 600°C in the N2O ambient exhibited the best structural and optical properties with the lowest carrier concentration and highest mobility, which make the treatment condition promising for MSM-PD applications. In addition, the samples treated in N2, N2O, and He exhibited high stability with time compared with those treated under vacuum.
This Work was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdul-Aziz City for Science and Technology, Kingdom of Saudi Arabia, grant Number (11-NAN1923-02).
