Magnesium zinc oxide (MgₓZn₁₋ₓO) alloys represent a critical class of ultra-wide bandgap semiconductors, offering tunable bandgap energies ranging from 3.3 eV (ZnO) to 7.8 eV (MgO). This broad range enables applications in ultraviolet (UV) optoelectronics, including LEDs, photodetectors, and high-power electronic devices. However, the alloy system faces significant challenges, particularly phase separation at Mg compositions exceeding 50%, which limits its practical utility. This article examines the growth techniques, material properties, and device applications of MgₓZn₁₋ₓO, alongside key challenges such as p-type doping and contact resistance.

Bandgap engineering in MgₓZn₁₋ₓO is achieved by adjusting the Mg composition, which directly influences the alloy’s electronic and optical properties. At low Mg concentrations (x < 0.5), the wurtzite structure of ZnO remains stable, and the bandgap increases nearly linearly with Mg content. Experimental studies confirm that for x = 0.2, the bandgap reaches approximately 3.7 eV, while x = 0.4 yields a bandgap near 4.2 eV. Beyond x = 0.5, phase separation becomes a dominant issue, as the thermodynamic stability of the wurtzite phase diminishes, leading to the formation of undesirable cubic MgO-rich regions. This phase segregation degrades material quality, increasing defect densities and reducing carrier mobility. Advanced growth techniques are required to mitigate these effects.

Radio-frequency (RF) sputtering and molecular beam epitaxy (MBE) are the most widely used methods for depositing high-quality MgₓZn₁₋ₓO thin films. RF sputtering offers a cost-effective and scalable approach, with precise control over composition through target doping and sputtering parameters. Films grown via RF sputtering typically exhibit smooth morphologies and uniform Mg distribution at lower concentrations. However, achieving high Mg content without phase separation demands careful optimization of substrate temperature and oxygen partial pressure. MBE, on the other hand, provides superior control at the atomic level, enabling the growth of metastable wurtzite phases even at Mg concentrations exceeding 50%. The use of plasma-assisted MBE further enhances Mg incorporation while minimizing defects. Despite these advantages, MBE systems are expensive and less suited for large-scale production.

The primary application of MgₓZn₁₋ₓO lies in UV optoelectronic devices. UV LEDs based on MgₓZn₁₋ₓO/ZnO heterostructures have demonstrated emission wavelengths tunable between 280 nm and 380 nm, covering the UVA and UVB spectral ranges. These devices rely on n-type MgₓZn₁₋ₓO layers paired with p-type ZnO or other p-type oxides, though efficiency remains limited by poor hole injection and high contact resistance. Similarly, MgₓZn₁₋ₓO photodetectors exhibit high responsivity in the solar-blind region (240–280 nm), with reported detectivities exceeding 10¹² Jones for optimized structures. The alloy’s wide bandgap also makes it suitable for high-voltage power electronics, where its high critical electric field (estimated at 8–10 MV/cm for x = 0.5) outperforms conventional semiconductors like SiC and GaN.

A major obstacle in MgₓZn₁₋ₓO technology is the difficulty of achieving reliable p-type doping. While ZnO can be doped p-type using nitrogen or phosphorus acceptors, the incorporation of Mg further exacerbates compensation effects due to increased defect formation. As a result, hole concentrations in p-type MgₓZn₁ₓO rarely exceed 10¹⁷ cm⁻³, leading to high series resistance in devices. Contact resistance is another critical issue, as most metal contacts form Schottky barriers on high-Mg-content alloys. Recent studies have explored the use of graded contact layers and alternative metallization schemes, such as nickel/gold bilayers, to reduce contact resistivity below 10⁻³ Ω·cm².

Recent advances in heterojunction design have improved device performance by leveraging the band alignment between MgₓZn₁₋ₓO and ZnO. Type-I band alignments facilitate efficient carrier confinement in quantum well structures, enhancing radiative recombination in UV LEDs. Meanwhile, the introduction of superlattices and modulation-doped layers has reduced dislocation densities and improved electron mobility in high-Mg-content films. Another promising approach involves the integration of MgₓZn₁₋ₓO with other wide-bandgap materials, such as Ga₂O₃ or AlN, to create hybrid heterostructures with tailored electronic properties.

Despite progress, several challenges remain unresolved. The thermal stability of MgₓZn₁₋ₓO at elevated temperatures is a concern for high-power applications, as Mg tends to diffuse and segregate under thermal stress. Additionally, the lack of a lattice-matched substrate for high-Mg-content films results in high threading dislocation densities, impairing device longevity. Future research directions include the development of novel doping strategies, such as co-doping with hydrogen or lithium, and the exploration of metastable alloy compositions through non-equilibrium growth techniques.

In summary, MgₓZn₁₋ₓO alloys offer a versatile platform for UV optoelectronics and high-power devices, but their full potential is hindered by phase instability, doping challenges, and contact resistance. Continued advancements in growth techniques and heterojunction engineering are essential to overcome these limitations and unlock new applications in emerging technologies.