Transport Property of II-VI Organic-Inorganic Hybrid Materials

β-ZnTe(en)0.5, an organic-inorganic hybrid material, demonstrates exceptional long-term stability exceeding 15 years in ambient conditions, surpassing materials like perovskites. This stability arises from strong covalent-like bonds within its quasi-2D layered structure, where ZnTe inorganic layers are bonded with ethylenediamine. The material exhibits quantum confinement effects, resulting in a significant bandgap blueshift compared to bulk ZnTe, and anisotropic thermal expansion with a low uniaxial thermal expansion coefficient. High crystallinity, evidenced by narrow Raman line widths, further highlights its quality, making β-ZnTe(en)0.5 a promising candidate for optoelectronic applications.
β-ZnTe(en)0.5 crystals were synthesized via a solvothermal method, leveraging high autogenous pressures and controlled crystallization in sealed reaction vessels at elevated temperatures. This technique, using ethylenediamine as a solvent, facilitated the formation of colorless flake-like crystals through controlled reaction parameters and purification. For device fabrication, a shadow mask approach was chosen over conventional lithography like UV photolithography and electron beam lithography (EBL), offering a resist-free, versatile, and less invasive patterning method that avoids chemical exposure and preserves material integrity. This allowed for high-quality devices with minimal artifacts, enabling reliable transport property investigation.
To explore charge transport, Space-Charge-Limited Current (SCLC) analysis, based on the Mott-Gurney law (J ∝ V²), was employed. While real materials deviate from ideal behavior, this analysis established a baseline understanding of charge carrier mobility. Two-probe electrical measurements on both vertical and lateral device configurations were conducted. Initial vertical measurements on pristine samples with metal electrodes revealed SCLC behavior, with mobilities of 8.8 × 10-3 cm2/(Vs) and 2.5 10-3 cm2/(Vs) for samples synthesized in 2007 and 2019, respectively. Lateral measurements, conducted along the a- and c-axes, revealed significantly higher mobilities, with values of 1.787 × 102 cm2/(Vs) along the a-axis and 1.696 × 101 cm2/(Vs) along the c-axis, demonstrating anisotropic charge transport. These results underscore the importance of SCLC measurements in characterizing the anisotropic transport properties of materials.
Complementary electronic structure analysis using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), Kelvin probe force microscopy (KPFM), and hot probe measurements were performed. XPS revealed a valence band maximum (Ev) of 0.80 eV below the Fermi level (EF), indicating p-type conductivity, corroborated by a 3.55 eV bandgap from photoluminescence (PL). KPFM yielded a work function of 4.59 ± 0.03 eV, consistent with UPS data. Integrating these results produced a reliable energy band diagram, confirming p-type conductivity and establishing band alignment. This multi-technique approach emphasizes its importance for accurate electronic structure determination.