Zinc Selenide (ZnSe) is a II-VI compound semiconductor with a direct bandgap of approximately 2.7 eV at room temperature, making it highly suitable for optoelectronic applications in the blue-green spectral region. Its high transparency across visible and near-infrared wavelengths, combined with excellent luminescent properties, has positioned it as a key material for lasers, light-emitting diodes (LEDs), and optical windows. The material’s cubic zinc blende structure facilitates epitaxial growth on compatible substrates, while its relatively high bond strength contributes to thermal and chemical stability under operational conditions.

The wide bandgap of ZnSe enables efficient emission in the 460–530 nm range, addressing a critical need for compact blue-green light sources. Unlike narrower bandgap semiconductors, ZnSe exhibits lower intrinsic carrier concentrations at room temperature, reducing leakage currents in optoelectronic devices. Its refractive index of around 2.5 at 500 nm allows effective light confinement in waveguide structures, while a high exciton binding energy of approximately 20 meV enhances radiative recombination efficiency. These properties make ZnSe competitive with III-nitrides for certain applications, particularly where lower growth temperatures or reduced dislocation densities are advantageous.

Bulk ZnSe crystals are typically grown using melt-based techniques such as the Bridgman method or physical vapor transport. Stoichiometric control remains challenging due to selenium’s high vapor pressure, often necessitating post-growth annealing to minimize point defects. The crystals exhibit n-type conductivity in their undoped state due to selenium vacancies, with resistivities in the range of 10^3–10^5 Ω·cm. For device applications, bulk substrates are polished to atomic-level smoothness, with typical dislocation densities below 10^4 cm^-2 in high-quality material.

Thin-film growth of ZnSe predominantly employs molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE). MBE offers precise control over layer thickness and doping profiles, with growth temperatures typically between 250–350°C to prevent selenium desorption. Substrate choice significantly impacts film quality, with GaAs (001) being widely used despite a 0.27% lattice mismatch. Buffer layer engineering, such as graded ZnSe_xTe_1-x alloys, can reduce threading dislocation densities below 10^6 cm^-2. MOVPE growth utilizes precursors like dimethylzinc and diethylselenide at higher temperatures (350–500°C), achieving growth rates of 1–3 μm/hour with carbon incorporation below 10^16 cm^-3.

Doping strategies for ZnSe focus on achieving both n-type and p-type conductivity with low compensation. Chlorine serves as the most effective n-type dopant when introduced via ZnCl_2 source in MBE or HCl gas in MOVPE, yielding carrier concentrations up to 10^19 cm^-3 with mobilities exceeding 300 cm^2/V·s. Nitrogen remains the primary p-type dopant despite its deep acceptor level (150 meV), with plasma sources enabling hole concentrations near 10^18 cm^-3 in MBE-grown material. Co-doping with lithium has demonstrated reduced resistivity in MOVPE films, though at the expense of increased diffusion during device operation.

Heterostructure design leverages ZnSe’s compatibility with other II-VI compounds to form quantum wells and superlattices. ZnCdSe/ZnSe quantum wells exhibit strong carrier confinement, with composition-tunable emission wavelengths across the blue-green spectrum. Strain management becomes critical for structures exceeding the critical thickness, prompting the use of ZnMgSSe quaternary barriers with lattice-matching capabilities. These wide-gap barriers (Eg > 3.0 eV) provide both electrical and optical confinement while maintaining pseudomorphic growth on ZnSe.

Interface quality in ZnSe heterostructures depends heavily on growth initiation sequences. A selenium pre-layer on GaAs substrates reduces interface states by passivating surface arsenic dimers. For ZnSe/ZnTe interfaces, migration-enhanced epitaxy techniques suppress interdiffusion, maintaining abrupt junctions despite the 7% lattice mismatch. Characterization of these interfaces through cross-sectional TEM reveals monolayer-scale smoothness when optimized growth interrupts are employed.

Optical characterization of ZnSe layers reveals distinctive excitonic features even at room temperature due to the material’s large exciton binding energy. Near-band-edge photoluminescence spectra typically show donor-bound exciton emission at 2.796 eV and free exciton recombination at 2.802 eV in high-purity material. Deep-level emissions related to zinc vacancies (2.2 eV) or selenium interstitials (1.6 eV) serve as indicators of non-stoichiometry, with their relative intensities correlating with growth conditions. Time-resolved measurements yield radiative lifetimes around 300 ps for undoped ZnSe at 10 K, increasing with temperature due to phonon scattering.

Thermal properties influence device reliability, with ZnSe exhibiting a thermal conductivity of 18 W/m·K at 300 K – lower than GaN but superior to most organic semiconductors. The linear thermal expansion coefficient of 7.8×10^-6 K^-1 necessitates careful matching with substrates during thermal cycling. High-temperature photoluminescence studies show bandgap shrinkage following the Varshni equation with coefficients α = 5.0×10^-4 eV/K and β = 140 K, enabling accurate modeling of device performance under operational heating.

Challenges persist in ZnSe technology, particularly regarding p-type doping efficiency and contact resistance. Nitrogen incorporation above 10^18 cm^-3 induces lattice strain that promotes defect formation, while the low solubility of alternative acceptors like lithium limits their effectiveness. Ohmic contacts to p-ZnSe require multi-layer metallization schemes with annealing temperatures carefully controlled below 300°C to prevent interfacial reactions. Recent advances in delta-doping and modulation doping have shown promise for improving hole injection in laser diode structures.

The material’s radiation hardness makes it attractive for space applications, with proton irradiation studies showing minimal degradation in optical properties at fluences below 10^14 cm^-2. Environmental stability exceeds that of many organic semiconductors, though prolonged exposure to humid atmospheres necessitates protective coatings due to slow surface oxidation.

Ongoing research explores ZnSe quantum dots for wavelength-tunable emitters, with colloidal synthesis achieving quantum yields above 50% through careful surface passivation. Epitaxial quantum dots grown by strain-driven self-assembly exhibit sharper emission lines than their bulk counterparts, though size uniformity remains inferior to III-V quantum dot systems. Integration with photonic crystals and plasmonic structures has demonstrated enhanced extraction efficiency for LED applications.

In summary, ZnSe’s combination of wide bandgap, high luminescence efficiency, and versatile growth compatibility establishes it as a technologically significant material for blue-green optoelectronics. Continued refinement of doping techniques and heterostructure engineering addresses historical limitations while opening new possibilities in quantum-confined systems and hybrid device architectures. The material’s fundamental properties provide a robust platform for both conventional and emerging optoelectronic applications.