Defect dynamics evolution in Zn1-xAlxO nanocrystals: Interplay of structural, optical, and electrical properties

Abstract

Defect engineering is essential for tuning structural, optical, and electronic properties of semiconductors. This study presents a comprehensive investigation of defect evolution in Zn₁₋ₓAlₓO nanocrystals, showing how aluminum incorporation dynamically modifies the defect configuration within the host matrix. The doping process leads to a reduction of the crystallite radius (from ∼ 20 to ∼8 nm) while increasing the residual strain (from ∼ 0.2 to ∼ 1.1 %). UV-Vis spectroscopy, modeled using Elliott's approach, reveals defect dynamics through variations in band gap and Urbach energy, with these parameters reaching extrema at low doping levels (up to x = 0.02). The broadening parameter supports formation of the Znᵢ, reflecting increased lattice disorder and pronounced electronic effects. Raman spectroscopy revealed charge carrier concentration (n) linked to the Al-content, with longitudinal optical phonon–plasmon coupled mode yielding an evolution of n given a maximum value of ∼10²² cm⁻³ at x = 0.015 of Al-content. Electron paramagnetic resonance of undoped ZnO showed prominent signals at g₁ = 1.9000 and g₂ = 2.0024, attributed to singly ionized interstitial zinc (Znᵢ) and singly ionized zinc vacancies (VZn), respectively. These signals reflect a high density of paramagnetic defects, consistent with the small crystallite size and high surface-to-volume ratio. Upon Al incorporation, the g₂ signal intensity decreases, indicating passivation or annihilation of VZn centers. In contrast, the g₁ signal intensity shows a transient increase at x = 0.005, suggesting an initial rise in Znᵢ concentration before its suppression at higher doping levels. Impedance spectroscopy demonstrated decreasing resistivity with rising Al-content, particularly from x = 0.005–0.020, corresponding to higher carrier concentrations. This trend aligns well with the carrier density evolution inferred from Raman spectroscopy via longitudinal optical phonon–plasmon coupled.