II-VI semiconductors, particularly zinc oxide (ZnO) and cadmium telluride (CdTe), have garnered significant interest for space applications due to their unique optoelectronic properties, direct bandgap transitions, and potential for high radiation tolerance. In the harsh environment of space, materials must withstand extreme conditions, including high-energy particle radiation and wide temperature fluctuations. Understanding the intrinsic radiation hardness and thermal stability of these materials is critical for their deployment in satellites, space-based sensors, and photovoltaic systems.

Radiation hardness in semiconductors refers to the ability of a material to maintain its electronic and structural integrity when exposed to ionizing radiation, such as protons, electrons, and heavy ions prevalent in space. ZnO, with its wide bandgap (~3.37 eV) and high exciton binding energy (60 meV), exhibits intrinsic resistance to radiation-induced damage. Studies have shown that ZnO demonstrates remarkable stability under proton irradiation at fluences up to 10^15 cm^-2, with minimal degradation in photoluminescence intensity. This resilience is attributed to its strong ionic bonding and the ability to annihilate point defects through dynamic annealing processes. However, high-energy particles can still introduce oxygen vacancies (V_O) and zinc interstitials (Zn_i), which act as recombination centers, subtly altering carrier lifetimes. Post-irradiation annealing at moderate temperatures (300–500°C) has been observed to partially recover optical and electrical properties by reducing defect concentrations.

CdTe, another prominent II-VI semiconductor, possesses a direct bandgap (~1.5 eV) ideal for space photovoltaics. Its radiation hardness is influenced by its defect chemistry, particularly the role of cadmium vacancies (V_Cd) and tellurium antisites (Te_Cd). Under electron irradiation, CdTe exhibits displacement damage thresholds higher than silicon, with carrier mobility remaining stable up to fluences of 10^14 cm^-2. However, the formation of Te precipitates and extended defects at higher fluences can degrade performance. Thermal annealing at 400–600°C has been shown to mitigate these effects by promoting defect recombination. Notably, CdTe solar cells have demonstrated less power degradation compared to traditional silicon cells in low-Earth orbit environments, making them attractive for long-duration missions.

Thermal stability is equally critical, as space applications subject materials to temperature cycles ranging from cryogenic conditions to over 150°C. ZnO maintains structural integrity across this range due to its high melting point (~1975°C) and low thermal expansion coefficient. However, at elevated temperatures, oxygen desorption from the surface can increase n-type conductivity, which may affect device performance in optoelectronic applications. Encapsulation or surface passivation techniques are often explored to mitigate this effect, though intrinsic solutions such as doping with magnesium (Mg) have shown promise in stabilizing electrical properties.

CdTe faces challenges related to tellurium sublimation at temperatures above 500°C, which can lead to stoichiometric imbalances and increased defect densities. Doping with chlorine (Cl) or copper (Cu) has been employed to improve thermal stability by reducing vacancy concentrations. Additionally, the thermal conductivity of CdTe (~6 W/m·K) is lower than that of many conventional semiconductors, necessitating careful thermal management in high-power applications. Despite this, CdTe’s ability to operate efficiently at elevated temperatures makes it suitable for near-sun missions where radiative heating is significant.

Comparative analysis of ZnO and CdTe reveals trade-offs between radiation hardness and thermal stability. ZnO excels in high-radiation environments due to its robust defect tolerance but requires careful thermal management to prevent surface degradation. CdTe offers superior performance in thermal cycling scenarios but is more susceptible to displacement damage at extremely high fluences. The choice between these materials depends on the specific mission requirements, balancing radiation exposure levels with operational temperature ranges.

Recent advances in defect engineering have further enhanced the suitability of II-VI semiconductors for space applications. In ZnO, intentional doping with transition metals (e.g., cobalt or manganese) has been shown to introduce deep-level traps that capture radiation-induced carriers, reducing their impact on device performance. Similarly, in CdTe, controlled incorporation of selenium (Se) to form CdTeSe alloys has improved radiation resistance by altering defect energetics. These strategies highlight the potential for tailoring II-VI materials to meet the demands of extreme environments.

The intrinsic behavior of these materials under radiation and thermal stress is governed by fundamental mechanisms such as defect diffusion, recombination kinetics, and phase stability. For instance, in ZnO, the migration energy of zinc vacancies is approximately 1.6 eV, influencing how quickly radiation-induced defects can be annihilated at operating temperatures. In CdTe, the activation energy for tellurium diffusion is around 2.0 eV, dictating the material’s propensity for stoichiometric changes under thermal load. Understanding these parameters allows for predictive modeling of material performance in space conditions.

Future research directions include exploring the role of nanostructuring in enhancing radiation hardness. Nanocrystalline ZnO films have demonstrated reduced defect accumulation compared to bulk counterparts due to grain boundary-assisted defect annihilation. Similarly, CdTe quantum dot systems exhibit altered defect dynamics under irradiation, presenting opportunities for novel radiation-tolerant designs. Advances in computational modeling, such as density functional theory (DFT) simulations of defect formation energies, are also providing deeper insights into material behavior under extreme conditions.

In conclusion, ZnO and CdTe exhibit compelling intrinsic properties for space applications, with distinct advantages in radiation hardness and thermal stability. While challenges remain in defect management and thermal degradation, ongoing advancements in material science are steadily improving their viability for next-generation space technologies. The continued study of these II-VI semiconductors will be essential for unlocking their full potential in the demanding environment of space.