I. INTRODUCTION

Currently, MgZnO is a semiconductor alloy that has attracted significant attention in the field of optoelectronics due to its tunable electrical and optical properties. The optical band gap of $\text{Mg}_x\text{Zn}_{1-x}\text{O}$ can be modulated from approximately 3.3 eV (characteristic of ZnO) to 7.8 eV (characteristic of MgO) by varying the Mg content in the alloy ^{[1- 3]}. Moreover, $\text{Mg}_x\text{Zn}_{1-x}\text{O}$ can exist in two main crystal phases: a hexagonal wurtzite structure (typical of ZnO) when $x < 0.4$ (band gap < 4 eV), and a cubic rocksalt structure (similar to MgO) when $x > 0.5$ (band gap ranging from 5 to 7.5 eV) ^[4]. Notably, the similarity in ionic radii between $\text{Mg}^{2+}$ (0.57 Å) and $\text{Zn}^{2+}$ (0.60 Å) allows Mg atoms to substitute Zn in the crystal lattice without causing significant lattice distortion during doping ^[5].

In addition, MgZnO can be synthesized in various nanostructured forms, including nanorods for Schottky diodes, nanowires for gas sensing applications, and nanoparticles for piezo-photocatalytic processes ^{[6- 8]}. Especially, owing to its band gap tunability, MgZnO thin films can be utilized to construct quantum well structures, such as ZnO/MgZnO or MgZnO/MgO, offering promising applications in deep ultraviolet (UV) light-emitting diodes (LEDs) and UV laser diodes ^{[9, 10]}. High-quality $\text{Mg}_x\text{Zn}_{1-x}\text{O}$ thin films can be synthesized using a variety of deposition techniques, including the sol-gel method, magnetron sputtering, pulsed laser deposition (PLD), molecular beam epitaxy (MBE), and metal-organic chemical vapor deposition (MOCVD) ^{[11- 15]}. Among these, RF-reactive magnetron sputtering is widely adopted due to its advantages, such as precise control of film thickness and uniformity, high deposition rate, and compatibility with industrial-scale production ^{[16, 17]}. The thin film quality can be effectively tuned by adjusting key parameters such as oxygen gas flow rate, working pressure, annealing temperature, and sputtering power. In particular, substrate temperature plays a critical role in promoting Mg incorporation into the ZnO lattice, thereby enabling effective alloy formation and improving crystallinity ^[18].

In Yonghong Hu’s study, hexagonal MgZnO alloys with Mg concentrations ranging from 0% to 50% were investigated through both simulation and experiment. The results showed a reduction in lattice constants and a widening of the band gap with increasing Mg content. Notably, the valence band maximum narrowed with Mg concentration, reaching its minimum at 37.5%, indicating the potential for tuning not only the band gap but also the electronic properties of MgZnO ^[19]. Keyun Gu et al. fabricated MgZnO thin films on glass substrates via RF-magnetron sputtering with an Mg:Zn ratio of 1:9. The thin films were deposited at substrate temperatures ranging from room temperature to 500 $^\circ\text{C}$. Crystallinity improved with increasing temperature up to 300 $^\circ\text{C}$, but degraded at higher temperatures. The optical band gap also increased slightly from 3.29 to 3.32 eV as substrate temperature increased ^[20]. In contrast, Hongbin Wang et al. deposited wurtzite $\text{Mg}_{0.3}\text{Zn}_{0.7}\text{O}$ thin films on quartz substrates using the sol-gel method at temperatures ranging from 700 to 1000 $^\circ\text{C}$. The films exhibited strong (002) orientation and c-axis growth, particularly at 900 $^\circ\text{C}$. The optical band gap increased from 3.46 to 3.50 eV with substrate temperature, while the highest resistivity reached $5.25\times 10^7\ \Omega\cdot\text{cm}$ and the absorption coefficient remained very low in the visible region. Wurtzite structures are typically preferred due to their advantages in enhancing UV transparency, improving device signal-to-noise ratios, and minimizing lattice mismatch when integrated with ZnO-based materials ^[21].

Our work provides a systematic experimental investigation into the growth and optical property of $\text{Mg}_{0.35}\text{Zn}_{0.65}\text{O}$, a composition near the challenging wurtzite-rocksalt transition. In contrast to previous studies on sputtered MgZnO thin films, we employed RF-reactive magnetron sputtering with a Zn-Mg alloy target to successfully obtain highly crystalline, strongly (002)-oriented thin films, an average visible transmittance of 95%, at substrate temperatures below 300 $^\circ\text{C}$. The alloy-target approach promotes superior Mg incorporation compared to oxide targets, enabling low-temperature growth with enhanced optical quality. These findings provide a detailed clarification of the effects of low-temperature growth using an alloy-target approach, highlighting the technological potential for scalable, energy-efficient deposition of transparent MgZnO thin films.

II. EXPERIMENTS

MgZnO thin films were deposited using RF magnetron sputtering with Zn/Mg alloy targets prepared by casting high-purity Zn (99.95%) and Mg (99.95%). The Mg doping level was set to 35 at.%, defined by the atomic ratio $\text{Mg}/(\text{Mg} + \text{Zn})$. Glass substrates were cleaned sequentially with acetone, ethanol, and isopropyl alcohol, followed by ultrasonic treatment in deionized water and 2-propanol for 40 seconds each. Prior to deposition, the substrates were preheated at 150 $^\circ\text{C}$ for 15 minutes in a vacuum chamber at a base pressure of $10^{-8}\text{ Torr}$. Deposition parameters were fixed: sputtering power at 90 W, working pressure at $5 \times 10^{-3}\text{ Torr}$. The deposition was performed at substrate temperatures of 150 $^\circ\text{C}$, 200 $^\circ\text{C}$, 250 $^\circ\text{C}$, and 300 $^\circ\text{C}$, labelled as MgZnO-150, MgZnO-200, MgZnO-250, and MgZnO-300, respectively. The structural properties were analyzed by X-ray diffraction (XRD, D8-ADVANCE). Film thickness ($\sim$150 nm) was measured using a Taylor-step profilometer. Surface morphology was observed via SEM (S-4800, Hitachi). The element composition, quantification, and distribution of thin film were quantified by EDX and EDX mapping. The optical properties were characterized by UV-Vis spectroscopy (JASCO V-550, Japan).

III. RESULTS AND DISCUSSION

The X-ray diffraction (XRD) patterns of MgZnO thin films deposited at various substrate temperatures are presented in detail in Fig. 1. The main diffraction peaks, specifically the (100) and (002) peaks, were observed at $2\theta$ angles of $30.9^\circ$ and $34.0^\circ$, respectively. These peaks are characteristic of the hexagonal wurtzite structure of the material, consistent with previous reports ^{[22, 23]}. Notably, the (002) peak exhibited very high crystallization intensity, clearly demonstrating that the MgZnO films preferentially crystallize along the c-axis, perpendicular to the substrate surface. Another important observation was the absence of cubic MgO phases or any other impurity phases such as metallic Mg. This indicates that Mg atoms tend to occupy substitutional sites within the ZnO lattice rather than interstitial positions.

Fig. 1. XRD pattern of MgZnO thin films at various substrate temperatures.

Upon examining the MgZnO thin films deposited in the substrate temperature range from 150 $^\circ\text{C}$ to 300 $^\circ\text{C}$, the diffraction angle of the (002) plane was determined to be within the range of $34.17^\circ$ to $34.35^\circ$. It is noteworthy that all these values are smaller than that of bulk ZnO ($2\theta = 34.4^\circ$, according to Joint Committee on Powder Diffraction Standards card No. 36-1451). This shift can primarily be attributed to the difference in ionic radii between $\text{Mg}^{2+}$ and $\text{Zn}^{2+}$ cations, as well as the difference in the thermal expansion coefficients between the MgZnO film and the glass substrate ^{[17, 24]}.

The maintenance of a single-phase hexagonal structure at these temperatures can be attributed to a kinetic trapping mechanism. While thermodynamic equilibrium might favor phase separation into wurtzite and rocksalt phases at higher Mg concentrations, the low substrate temperature (150-300 $^\circ\text{C}$) limits the diffusion length of Mg adatoms, preventing the nucleation of a secondary cubic MgO phase. In the study by Ohtomo et al., single-phase hexagonal MgZnO films were deposited using the PLD method with an Mg-content of up to 0.33 at a substrate temperature of 600 $^\circ\text{C}$ ^[25]. Similarly, Takeuchi reported that single-phase MgZnO thin films with Mg concentrations ranging from 0.37 to 0.6 could also be achieved at the same temperature ^[26]. In contrast, when using the RF-magnetron sputtering method, a single-phase hexagonal structure was obtained at temperatures below 300 $^\circ\text{C}$ with an Mg content of approximately 0.35.

The crystallographic parameters, including crystallite size ($D$), dislocation density ($\delta$), and microstrain ($\epsilon$) of the MgZnO thin films along the (002) plane, were calculated using standard equations based on X-ray diffraction (XRD) data ^{[16, 27]}:

Fig. 2. Variation of crystallite size (D), full width at half maximum (FWHM), dislocation density (δ), and microstrain (ϵ) with substrate temperature (TS).

The crystallite sizes of the MgZnO thin films ranged from 7.97 to 12.98 nm at substrate temperatures between 150 and 300 $^\circ\text{C}$. As the substrate temperature increases, the full width at half maximum (FWHM) of the diffraction peaks decreases, indicating improved crystallinity as a result of increased crystallite size. At lower temperatures, atoms do not possess sufficient energy to effectively diffuse across the substrate surface, leading to the formation of crystal defects in the MgZnO films. In contrast, higher substrate temperatures provide atoms with adequate kinetic energy to migrate to energetically favorable lattice positions, resulting in more orderly atomic arrangements, reduced defect density, and enhanced crystal quality.

Simultaneously, the energetic nature of RF-reactive sputtering provides sufficient adatom mobility to ensure the formation of a crystalline wurtzite lattice rather than an amorphous one. This balance between energetic deposition and low-temperature quenching allows for the growth of a metastable, highly (002)-oriented MgZnO solid solution. However, when the substrate temperature exceeds 300 $^\circ\text{C}$, the crystallite size decreases significantly. The reduction may be attributed to the decomposition or re-evaporation of atoms from the film surface, leading to thermodynamic instability and adversely affecting crystal growth. The degradation in the crystal quality of the film may be attributed either to random substitution of Mg atoms into the ZnO lattice, which causes structural distortion ^{[17, 28]}.

The values of microstrain and dislocation density in the MgZnO thin films were found to range from $9.05 \times 10^{-3}$ to $14.80 \times 10^{-3}$, and $5.94 \times 10^{-3}$ to $15.74 \times 10^{-3}\text{ nm}^{-2}$, respectively. Both parameters exhibit a significant decreasing trend with increasing substrate temperature, reaching their lowest values in the MgZnO-250 sample, as shown in Fig. 2. The improvement in crystal quality contributes to the reduction in total grain boundary area, crystal mismatch, and defect density, resulting in a more uniform and structurally stable film. The decrease in microstrain and dislocation density with elevated substrate temperature can be explained by the structural zone model ^[29]. As $T_{\text{substrate}}/T_{\text{melt}}$ rises, adatom mobility enhances, facilitating the rearrangement of atoms into more stable lattice structures. It improves granular coalescence and reduces lattice mismatch strain and dislocation formation. At around 250 $^\circ\text{C}$, the thin film is in the transition zone ($0.2 < T_{\text{substrate}}/T_{\text{melt}} < 0.4$), where competitive grain growth leads to minimized structural defects. These findings indicate that substrate temperature is a key factor in enhancing crystal structure, with 250 $^\circ\text{C}$ being identified as the optimal condition. The crystallographic parameters of the MgZnO thin films are summarized in Table 1.

Fig. 3. SEM image of MgZnO thin film deposition at substrate temperature of 250 ◦C.

Fig. 4. EDX and EDX mapping of MgZnO at substrate temperature of 250 ◦C.

The surface morphology of the MgZnO thin film deposited at a substrate temperature of 250 $^\circ\text{C}$ is shown in Fig. 3. The film exhibits a nanograin structure with polycrystalline crystallinity, forming a continuous and uniform layer that fully covers the substrate surface. These features indicate that the deposition conditions at 250 $^\circ\text{C}$ promote the formation of a high-quality film with good surface coverage and stable crystal structure. Fig. 4 presents the EDX spectra and elemental mapping of MgZnO thin films deposited at a substrate temperature of 250 $^\circ\text{C}$. The atomic percentages of Zn, Mg, and O were determined to be 19.98%, 10.02%, and 70.00%, respectively. From these values, the Mg composition ($x$) in $\text{Mg}_x\text{Zn}_{1-x}\text{O}$ was calculated to be 0.33 for the MgZnO-250 sample, which shows good consistency with the Mg:Zn molar ratio of the deposition target.

The optical properties of the MgZnO thin films on glass substrates were investigated using UV-Vis transmittance spectroscopy in the wavelength range of 300 to 1000 nm, as shown in Fig. 5(a). The films exhibit strong absorption in the ultraviolet region below 400 nm, corresponding to electron transitions from the valence band to the conduction band, which is a characteristic feature of ZnO-based materials. In the visible light region, the average transmittance values of the MgZnO thin films were determined to be 95.98%, 94.69%, 95.83%, and 95.62%, corresponding to substrate temperatures of 150 $^\circ\text{C}$, 200 $^\circ\text{C}$, 250 $^\circ\text{C}$, and 300 $^\circ\text{C}$, respectively. These values suggest that the transmittance remains nearly unchanged with increasing substrate temperatures, indicating that the films possess uniform surfaces, low surface roughness, and are not significantly affected by thermal variation. Moreover, the transmittance is influenced by various factors such as crystallite size, impurity centers, defects, band gap energy, and lattice strain.

Fig. 5. Optical properties of MgZnO thin films deposited at different substrate temperatures: (a) UV-Vis transmittance spectra, (b) Tauc plots for optical band gap determination, (c) refractive index (n), and (d) extinction coefficient (k) as a function of wavelength.

From the transmittance spectra, the absorption coefficient ($\alpha$) was determined in the strong absorption region based on the Beer-Lambert law ^{[16, 29]}:

where $T$ is the normalized transmittance and $d$ is the film thickness. The optical band gap energy of the films was determined using Tauc’s relation ^{[16, 30]}:

The absorption coefficient ($\alpha$, defined as absorbance divided by film thickness), the optical band edge constant ($A$), the photon energy ($h\nu$), and the optical band gap ($E_g$) are related through Tauc’s equation. For ZnO with a wurtzite hexagonal structure, the exponent $k$ is equal to 1/2, corresponding to a direct allowed transition.

The Tauc plots of the MgZnO thin films are shown in Fig. 5(b), from which the optical band gap values were extracted as 3.80 eV, 3.82 eV, 3.84 eV, and 3.86 eV, corresponding to substrate temperatures of 150 $^\circ\text{C}$, 200 $^\circ\text{C}$, 250 $^\circ\text{C}$, and 300 $^\circ\text{C}$, respectively ^[4]. Although all thin films retain a stable hexagonal wurtzite structure, the slight variation in $E_g$ does not arise from any major geometrical distortion. The Mg composition remains nearly constant at $\sim$35 at.%, which lies near the critical threshold for the wurtzite-to-cubic transition reported in earlier studies ^{[4, 31, 32]}, but no phase change is observed here. Instead, the small band-gap change is attributed to temperature-dependent microstructural effects, including variations in residual strain, crystallinity, and defect-related localized states. In the MgZnO alloy system, local compositional inhomogeneity may also introduce minor spatial fluctuations in the band edges, where Mg-rich regions exhibit slightly wider band gaps and Mg-poor regions show narrower ones ^[11]. However, since the overall Mg content is essentially unchanged, compositional bowing plays a minimal role. The observed $E_g$ shift is therefore primarily linked to subtle structural refinements-consistent with the shift and FWHM evolution of the (002) XRD peak-rather than any alteration of the main crystal phase.

The refractive index is one of the fundamental optical properties of a material. It reflects the electronic polarizability of the constituent ions and the local electric field within the material. The refractive index ($n$) and film thickness ($d$) were extracted from the transmission spectra using the Swanepoel envelope method ^[33]. The upper ($T_M$) and lower ($T_m$) envelopes were generated using a cubic spline interpolation of the interference extrema. To ensure the credibility of the results, the thickness was first estimated in the transparent region where the film’s absorption is negligible ($k \simeq 0$). The reliability of the extraction was confirmed by the consistency of the calculated thickness across multiple fringe orders, with a standard deviation of less than 3 nm. Potential uncertainties in the optical constants may arise from the spectrophotometer’s transmittance precision ($\pm 0.3\%$) and the envelope-fitting process, leading to an estimated cumulative uncertainty of $\pm 1.2\%$ for the refractive index values. This precision level is in good agreement with established literature for sputtered oxide thin films. Correlation between $n$ and $k$ can be determined:

Here, $k$ is the extinction coefficient (calculated by $k = \alpha\lambda/4\pi$) and $T$ is the transmittance. The values of the refractive index ($n$) of MgZnO thin films at different wavelengths ($\lambda$) are presented in Fig. 5(c). In the visible region, the refractive index ($n$) of the MgZnO thin films decreases with increasing wavelength and shows lower values at higher substrate temperatures, varying approximately from 2.58 to 2.15. The result indicates a reduction in optical density and packing density of the films. Fig. 5(d) shows that the extinction coefficient ($k$) decreases rapidly with increasing wavelength and remains very low in the visible region, indicating weak optical absorption and high transparency of the MgZnO thin films. This behavior may be attributed to the non-uniform diffusion of Mg atoms or the formation of undesired secondary phases in the MgZnO crystal structure, consistent with the XRD analysis. These findings highlight the potential of MgZnO thin films in fabricating ZnO/MgZnO quantum wells, particularly for applications in deep ultraviolet (UV) light-emitting diodes (LEDs) and laser diodes.

IV. CONCLUSION

In conclusion, this study provides a systematic clarification of the structural and optical evolution of MgZnO films grown by RF reactive magnetron sputtering. MgZnO samples with up to 35% Mg maintain the hexagonal crystal structure of zinc oxide (ZnO) at 150-300 $^\circ\text{C}$. The elements Zn, Mg, and O were clearly identified, and the Mg composition was quantified as $x \approx 0.33$ in $\text{Mg}_x\text{Zn}_{1-x}\text{O}$. Accordingly, the doping efficiency was evaluated to be approximately 94.3% relative to the nominal target composition of $x \approx 0.35$. The preferred growth along (002) is confirmed. As substrate temperatures increase, film crystal quality improves, peaking at 250 $^\circ\text{C}$. MgZnO has a nano-sized granular surface, exhibiting polycrystalline properties. In visible light, MgZnO thin films have an average transmittance of 95% and an optical band gap energy of 3.8 eV. The MgZnO thin film’s refractive index is 2.58-2.15, indicating exceptional optical quality. These findings suggest that MgZnO films could be used in ZnO/MgZnO heterostructures.