Due to its exceptional electronic and optical properties, including a substantial exciton binding energy of 60 meV and a wide band gap energy of 3.37 eV at room temperature, zinc oxide (ZnO) has garnered significant interest as a transparent conductor oxide (TCO). This versatile material finds applications in various fields, such as gas sensors [1], catalysts [2–6], photovoltaic solar cells [7], supercapacitors [8], and flexible piezoelectric nanogenerators [9]. Numerous researchers have dedicated their efforts to exploring the effect of doping on ZnO, aiming to enhance its optical, electrical, and magnetic characteristics. Incorporating minute quantities of dopant metal oxide materials, such as Mn2+, Mg2+, Sb3+, In3+, Cu2+, Co3+, Al3+, and others [10–18], represents the primary approach to synthesizing ZnO alloys with widened band gaps in various nanostructures, including nanofibers, nanocônes, nanowires, and more [19–21].
Transition metal (TM) ion-doped zinc oxide (ZnO) has gained significant scientific interest due to its remarkable properties and potential for spintronic applications, where the coexistence of electron spin and charge plays a crucial role.The strong correlation between the “3D” orbital of transition metals and the “s” and “p” orbitals of the anion profoundly alters the electronic structure of the ZnO host lattice. Consequently, ZnO-based dilute magnetic semiconductors (DMS) have emerged as a promising class of materials, surpassing conventional semiconductors in their potential applications [22, 23]. Extensive research efforts have focused on incorporating transition metals like Mn, Fe, Co, Cu, Ni, Cr, and V as substitutional ions in the ZnO lattice to achieve ferromagnetic properties at or above room temperature.
Manganese (Mn2+) is distinguished among the various TM elements because of its notable thermal solubility in ZnO [24]. With its half-filled 3D orbitals housing five spins, Mn2+ exhibits a maximum moment value of 5μB/Mn (where μB represents the Bohr magneton) [25]. Despite the difference in ionic radii between Mn2+ (0.083 nm) and Zn2+ (0.074 nm) and the limited solubility of approximately 13% in the ZnO matrix [25], extensive studies confirm that manganese ions can effectively substitute for zinc ions without inducing distortion in the ZnO lattice. At low concentrations (1%–5%), this substitution process provides favorable properties such as homogenous grain shape, solid texture, and improved crystallinity [6, 26–28]. Consequently, ZnMnO thin films prove to be an ideal material for short-wavelength magneto-optical applications.Currently, a variety of techniques are employed for the synthesis of ZnO nanomaterials, including hydrothermal methods [29–31], solution processing [32], pulsed laser deposition [33], and pyrolysis sputtering [10, 13, 34]. Among these approaches, pyrolysis is favored in this study because of its convenience of use, nonvacuum, good homogeneity, and cost-effectiveness. Moreover, as a critical phase in the development of reliable devices, the ZnMnO samples undergo Ar+ ion sputtering [35], followed by a precisely controlled thermal annealing process within the ultra-high vacuum (UHV) system [36]. Prior to treatment, the ZnO samples are investigated using XRD, XPS, AFM, and PL techniques, while the ZnMnO samples are examined both before and after treatment.
The aim of this research is to explore the impact of a 4% Mn dopant and investigate how cleaning and thermal annealing processes (Ar+ sputtering followed by high-temperature UHV annealing) affect the structure, morphology, and optoelectronic properties of ZnMnO film. Furthermore, establish a correlation between the electronic band gaps, band transitions, and the structural and morphological characteristics of these thin films.
On a silicon (Si) substrate, thin films of pure ZnO and ZnMnO (4at% Mn) were prepared using the ultrasonic spray pyrolysis technique. Zinc acetate dehydrates (ZnC4H6O4_2H2O) (99%, Sigma Aldrich) were dissolved in 100 ml of methanol at a molar concentration of 0.4 mol/l to produce ZnO. Manganese chloride tetrahydrate (MnCl2 4H2O) (Sigma Aldrich, 98%) is used as a dopant to create ZnMnO. The precursor solution contains 4% of Mn as the molar ratio concentration |Mn|/(|Mn|+|Zn|). To make sure the two solutions are homogeneous, they are each stirred for one hour at 40°C. The elaboration procedure was succeeded by spraying the precursor solutions over the heated substrates for 60 min at a temperature of 400°C.
Using a Scientaomicron UHV system with a preparation chamber and an analysis chamber, thin films of manganese-doped ZnO at 4% Mn were created. The samples were transported into the preparation or analysis chamber by a magnetically coupled probe transporter, and each chamber was pumped separately using manual or electropneumatic gate valves. When it comes to cleaning, an ion gun with a pulverized source is used to bombard the samples’ surfaces in order to get rid of different contaminants like carbon and oxygen that are on the surface layer. The cathodic sputtering ion source’s low energy is sufficient to produce flawless cleaning. It should be noted that the pressure of Ar+ ion gas released by a leaky valve becomes 2.10−6 mbar. With a low ion current of 3 A, the energy is fixed at 1.2 keV for 15 min. Using a standard manipulator and a pyrolytic boron nitride (PBN)-resistive device, the heating process was conducted in steps of increasing temperature (from 350°C up to 800°C) for 20 min at a time.
To study the structural properties, the EMPYREAN Diffractometer System was employed for conducting X-ray diffraction (XRD) analysis on ZnO and ZnMnO thin films. Operating at 45 kV and 40 mA, the system utilizes CuKα radiation (λ = 1.54060 Å) to capture the XRD spectrum within the angular range of 10° to 66°, employing a step size of 2
Figure 1 exhibits the line profiles of ZnO and ZnMnO, accompanied by the 2D and 3D AFM images, denoting pure ZnO (a), untreated ZnMnO (b), and treated ZnMnO (c), respectively. The surface morphology of ZnMnO differs significantly from that of ZnO, necessitating a precisely controlled treatment procedure involving Argon ion (Ar+) beam sputtering [37], followed by thermal annealing [38]. The purpose of this treatment is to enhance surface roughness and diminish grain size. Notably, Figure 1c illustrates the outcome of this process. Roughness, a parameter associated with geometric defects on the actual surface, serves as an informative indicator of the adhesion and surface conditions of the deposited layers. ArzuÇolak, Zandvliet, and Poelsema [39] previously conducted comprehensive assessments of adhesion force in relation to surface morphology and power spectral density on both smooth and etched surfaces. Additionally, this technique facilitates the determination of particle size and the distance between different grain boundaries [40]. The AFM images are substantiated quantitatively in terms of Ra (average roughness), Rms (root mean square), maximum height (
The main statistical parameters calculated through the AFM images in Figure 1 using Gwyddion software
Sample | Maximum height (Zmax) (nm) | Median height (Zmed) (nm) | Mean roughness (Sa) (nm) | Number of grains (N) |
---|---|---|---|---|
ZnO | 98.84 | 60.22 | 13.50 | 115 |
Untreated |
198.0 | 106.46 | 27.05 | 152 |
Treated |
288.84 | 132.52 | 43.03 | 126 |
Figure 2 illustrates the XRD patterns obtained for the two samples. The diffraction peaks of pure ZnO and treated ZnMnO exhibit well-matched indexing with the würtzite crystal structure (space group P63mc) [JCPDF card 36-1451], with preferable orientation along (002). This suggests that the synthesized ZnO crystals are predominantly aligned along the c-axis. Moreover, it is worth mentioning that the c/a values closely resemble those of the ideal würtzite structure
Figure 2 shows fluctuating disturbance halos appearing on the diffraction spectrum above (hump) and below (hollow) the line representing the noise contained in the diffraction. The spectra of the two ZnO and ZnMnO samples are located in the two regions: 2
The XRD analysis reveals that the ZnMnO crystal is free from any additional phases or impurities. Interestingly, the major peaks observed in the XRD pattern of ZnMnO show a slight shift towards lower angles compared to pure ZnO. This shift suggests a minor expansion of the unit cell in the ZnMnO sample. Consequently, the calculated lattice parameters for ZnMnO are slightly larger than those for undoped ZnO. These findings strongly suggest the successful incorporation of Mn2+ ions (with an ionic radius of 0.83 Å) into the Zn2+ lattice sites (with an ionic radius of 0.74 Å). The findings of the XRD analysis are summarized in Table 2, which provides the position peaks (2
Structural parameters of ZnO and treated
Sample | Peak position (2 |
hkl | FWHM (2 |
d-spacing (Å) | Average grain size D (nm) | Cell parameters (Å) | Strain ( |
||||
---|---|---|---|---|---|---|---|---|---|---|---|
Scherrer equation | W-H Plot | Scherrer equation | W-H Plot | ||||||||
ZnO(0%) | 31.84 | 1 00 | 0.58 | 2.80 | 25.20 | 28.88 | 3.242 | 5.195 | 1.602 | 0.0014 | 0.00085 |
34.50 | 002 | 0.33 | 2.60 | ||||||||
36.32 | 1 0 1 | 0.30 | 2.47 | ||||||||
47.65 | 1 02 | 0.33 | 1.90 | ||||||||
56.68 | 1 1 0 | 0.36 | 1.62 | ||||||||
63.02 | 1 03 | 0.83 | 1.47 | ||||||||
ZnMnO(4%) | 31.70 | 1 00 | 0.56 | 2.82 | 12.41 | 12.38 | 3.256 | 5.212 | 1.600 | 0.0025 | 0.00122 |
34.38 | 002 | 0.67 | 2.60 | ||||||||
36.17 | 1 0 1 | 0.36 | 2.48 | ||||||||
47.56 | 1 02 | 0.29 | 1.91 | ||||||||
56.44 | 1 1 0 | 0.21 | 1.63 | ||||||||
62.87 | 1 03 | 0.30 | 1.47 |
Moreover, the average grain size (D) is determined through the XRD spectroscopy study by calculating it as a function of the broadening of the highest intensity peak corresponding to the diffraction plane (002). This calculation utilizes Debye Scherer’s formula, as given in Eq. 1 [50].
The particle size of pure ZnO measures 25.20 nm, whereas the crystallite size of ZnMnO demonstrates a significant reduction to 12.41 nm. This decrease can be attributed to the enhanced solubility of Mn in ZnO, as observed in the study by Staumal et al. [51]. Additionally, it is important to note that the material experiences an increase in strain. This change arises from multiple factors, including the exchange of ions with different ionic radii, shifts in lattice constants, the introduction of impurities or defects, and the thermal extinction coefficient between the substrate and the film [52].
Figure 3 depicts the variance in
The W-H analysis for grain size look comparable; however, the results for strain appear to be widely off. These findings are consistent with the results obtained from the AFM study, providing further support for the notion that the diminished crystallite size predominantly stems from the distortion of the ZnO host lattice caused by the presence of Mn impurities. Consequently, this distortion facilitates accelerated crystal growth and the amalgamation of crystal grains.
This study utilizes X-ray photoelectron spectra (XPS) to investigate the impact of Mn incorporation and UHV treatment on the chemical composition of ZnO thin films. Representative XPS results of clean ZnO and ZnMnO(untreated, sputtered, and annealed) are showcased in Figure 4(a). Notably, all photoelectron peaks (Zn 2p, Mn 2p, O 1s, C 1s, Zn 3s, Zn 3p, and Zn 3d) and Auger transition peaks (C-KLL, O KLL, and Zn LMM) associated with ZnO or ZnMnO materials are accurately identified. For data correction, the C 1s peak, located at 284.6 eV, is utilized as a reference. The atomic percentages (at%) of oxygen, manganese, and zinc elements are derived from the O 1s, Mn 2p3/2, and Zn 2p3/2 signals and are presented in Table 3. Figure 4(b) reveals a noteworthy observation that the C1s carbon signal is more pronounced in ZnMnO-compared to clean ZnO. Furthermore, the untreated ZnMnO exhibits a more significant C1s carbon signal than the annealed ZnMnO.
Atomic percentages of Zn, O, and Mn elements corresponding to pure ZnO and treated
Sample | at(%) | Comments | |||
---|---|---|---|---|---|
Zn | O | Mn | (a): |
(b): |
|
34.00 | 66.00 | 0.00 | 0.00 | 1.94 | |
22.30 | 74.04 | 3.65 | 14.07 | 2.85 |
In Figure 4(c), the presence of the Zn 2p core level, which represents the divalent state of zinc ions in ZnO, is confirmed. The Zn 2p3/2 (1021.90 eV) and Zn 2p1/2 (1044.91 eV) photoelectron peaks exhibit a double peak energy separation of 23.01 eV, consistent with previous studies [54, 55]. However, when 4% Mn doping is introduced, the ZnMnO signals of Zn 2p3/2 and Zn 2p1/2 become less resolved due to the presence of a carbon contamination layer on the surface [56, 57]. Notably, following UHV treatment of ZnMnO, there is a substantial increase in the intensity of the Zn 2p signals, accompanied by a shift towards lower binding energies (approximately 0.45 eV) [58]. This observed shift could be attributed to morphological changes, as illustrated by the AFM images.
Figure 4(e) presents the high-resolution Mn 2p spectra, revealing prominent peaks at binding energy values of 641.8 eV and 653.9 eV, which correspond to Mn 2p3/2 and Mn 2p1/2, respectively. Interestingly, the Mn 2p signals exhibit negligible intensity in the contaminated state. However, upon subjecting the samples to UHV treatment, a notable improvement in peak shapes was observed.
Gaussian deconvolution was employed to explore the distribution of the Mn element within the thin films. As shown in Figure 5(a), the presence of distinct peaks at 641.10 eV and 653.0 eV provides unequivocal evidence for the existence of Mn2+ ions. These findings not only corroborate the Mn2+ oxidation state but also affirm the successful substitution of Mn2+ ions into the Zn site positions within the ZnO lattice. Furthermore, higher binding energy peaks at 642.55 eV and 654.60 eV are unequivocally assigned to Mn3+ ions present in the ZnMnO thin film. The coexistence of two charge states, Mn+2 and Mn+3, in Mn-doped ZnO has also been reported in previous studies [59, 60], further validating the observed Mn3+ peaks. Additionally, the presence of two satellite peaks, located around 645.32 eV and 657.15 eV, can be attributed to the electron shake-up of Mn2+.
Figure 4(d) presents the high-resolution oxygen 1s signal, showcasing a binding energy of 530.65 eV for the annealed ZnMnO sample. Notably, the O 1s peak undergoes asymmetry upon Mn doping but regains its symmetrical shape following UHV treatment. Deconvolution of the O 1s peak in untreated ZnMnO, as illustrated in Figure 5(b), reveals three distinct Gaussian peaks labeled as a, b, and c. The low binding energy peak O
In this study, photoluminescence (PL) spectroscopy is utilized to investigate the impact of Mn incorporation and UHV treatment on the presence of defects. These defects subsequently alter the band gap and luminescence characteristics of zinc oxide [10, 19, 63].
Figure 6 shows the spectrum (i) related to pure ZnO. It exhibits two distinct emission bands. Firstly, a sharp ultraviolet (UV) emission at 3.27 eV (
Furthermore, Figure 6 (ii) illustrates three significant changes resulting from the incorporation of 4% Mn into the ZnO matrix: (1) The UV emission peak becomes more intense and broader compared to the undoped film. (2) The visible emission peak decreases. (3) There is a notable increase in the NIR (near infrared) emission peak at 1.65 eV. It is worth mentioning that the broadening of the NBE peak observed in spectrum (ii) aligns with the XRD findings, which indicate a broader FWHM for Mn-doped ZnO due to its inferior crystalline nature. The analysis of the PL spectrum further confirms the successful incorporation of Mn into the lattice.
Moreover, upon comparing spectra (ii) and (iii), it is evident that the UHV treatment provides several changes. Firstly, it facilitates the removal of the primary visible peak by completely eliminating the structural defects associated with O
For more clarification of modifications due to Mn doping and UHV treatment, Figure 7 gives a Gaussian deconvolution of the PL spectra corresponding to pure ZnO (i), untreated Zn-MnO(ii), and treated ZnMnO(iii), accompanied by schematic band diagrams with the approximate positions of defect levels and luminescence mechanisms as suggested by other authors.
The NBE peak has been reported to include three sub-peaks. The first one at 3.275 eV (378 nm) concerns the free excitons (FX) from the state energy of 0.06 eV under the bottom of the conduction band minimum to the valence band maximum
The broad visible emission was found to be fitted with four Gaussian subpeaks positioned at 2.50, 2.28, 2.05, and 1.80 eV for pure ZnO. Figure 7 shows a noticeable shift in the position peaks and clear variations in the relative intensities when Mn substitutes Zn in the ZnO film. The blue luminescence centered at 2.50 eV (496 nm) originates from the transition of electrons between V
The near-IR luminescence shows two sub-peaks at 1.65 and 1.5 eV, respectively; the peak at 1.5 eV due to the hydrogen impurity (Hi) (Hi → V
The objective of this paper was to investigate the effects of low-level Mn substitution and the treatment process on the chemical, structural, morphological, and optoelectronic properties of ZnO and ZnMnO thin films with 3.6 at% Mn. The thin films were synthesized using the ultrasonic pyrolysis technique on Si (100) substrates, following a standardized deposition procedure. Analysis of the AFM images revealed significant changes in surface morphology attributed to the incorporation of Mn, leading to noticeable decreases in grain size, and increases in height,and average roughness. XRD analysis confirmed the presence of the würtzite crystal structure of ZnO, with the dominant (002) plane governing the growth process across all films. Furthermore, slight reductions and shifts towards lower diffraction angles in the XRD peaks, accompanied by minor variations in lattice parameters and an increase in micro strain, provided compelling evidence supporting the successful integration of Mn2+ ions through Zn2+ substitution.
To gain deeper insights into the optical properties, XPS and PL spectroscopy were employed. The observed near band-edge emission (NBE) at 3.27 eV within the ultraviolet (UV) range, suppression of the yellow-orange, green emission (~2.6-1.8 eV), and the emergence of a distinct emission peak at 1.65 eV were identified as characteristic signatures associated with the presence of Mn atoms. Therefore, the above-mentioned results, in combination with the reduced grain size, improved micro-stain deformations, and decreased ionization potential, facilitated the accurate reproduction of the diagrams representing both ZnO and ZnMnO compounds. This is both before and after UHV treatment and annealing. It is worth noting that the structural defects responsible for visible emissions were effectively eliminated in ZnMnO, resulting in well-defined emissions confined solely to the nearinfrared and ultraviolet ranges.