[Paper Review] Defect related photoluminescence and EPR study of sintered polycrystalline ZnO

Sanjiv Kumar Tiwari|arXiv (Cornell University)|Feb 28, 2012

ZnO doping and properties4 references3 citations

TL;DR

This study investigates defect-related photoluminescence and electron paramagnetic resonance (EPR) in sintered polycrystalline ZnO to identify paramagnetic centers and their electronic interactions. Using low-temperature PL and EPR, the authors attribute EPR signals at g = 1.985 and g = 1.956 to Zn interstitials and oxygen vacancies, respectively, and correlate crystal field splitting and spin-orbit coupling with excitonic emission shifts, revealing insights into defect-induced electronic behavior in polycrystalline ZnO systems.

ABSTRACT

We report on low temperature photoluminescence (PL) and EPR study of bulk polycrystalline ZnO . Variation of peak position of donor bound exciton transition (D1XA) and FXAn=1-1LO with pump intensity show red shift and blue shift respectively due to exciton-exciton scattering. EPR spectra reveals three peaks at g value of 1.985, 1.956 and 1.939 respectively, g=1.956 and g=1.985 is due to shallow donors Zn interstitial and oxygen vacancy respectively An EPR spectrum at 100 K reveals higher degree of asymmetry and hyperfine splitting due to crystal field and inhomogeneous relaxation of paramagnetic centers. Strength of crystal field splitting (CFS) and spin orbit coupling (SOC) in sample is discussed using PL peak position of various excitonic emissions. Whereas, inhomogeneous relaxation of paramagnetic centers is discussed in terms of their activation energy during thermal quenching process.

Motivation & Objective

• To identify and characterize paramagnetic defects in sintered polycrystalline ZnO using electron paramagnetic resonance (EPR) spectroscopy.
• To correlate photoluminescence (PL) features with specific defect states, particularly donor-bound exciton transitions.
• To investigate the influence of crystal field splitting (CFS) and spin-orbit coupling (SOC) on excitonic emission energies.
• To analyze inhomogeneous relaxation of paramagnetic centers through thermal quenching behavior and activation energy estimation.
• To understand the role of defect-related electronic states in determining optical and magnetic properties of polycrystalline ZnO.

Proposed method

• Low-temperature photoluminescence (PL) spectroscopy was used to measure excitonic emission peaks, including donor-bound exciton (D1XA) and free-exciton (FXAn=1-1LO) transitions.
• EPR spectroscopy at 100 K revealed three distinct signals at g = 1.985, 1.956, and 1.939, indicating multiple paramagnetic centers.
• The g-values were assigned to specific defects: g = 1.985 to Zn interstitials (shallow donors), and g = 1.956 to oxygen vacancies.
• Crystal field splitting (CFS) and spin-orbit coupling (SOC) were estimated from the energy shifts of excitonic PL peaks.
• Thermal quenching behavior was analyzed to extract activation energies for inhomogeneous relaxation of paramagnetic centers.
• Pump intensity-dependent PL measurements revealed red shift in D1XA and blue shift in FXAn=1-1LO, indicating exciton-exciton scattering effects.

Experimental results

Research questions

• RQ1 What are the nature and origin of the paramagnetic centers observed in sintered polycrystalline ZnO via EPR?
• RQ2 How do crystal field splitting and spin-orbit coupling influence the energy positions of excitonic PL emissions?
• RQ3 What is the role of exciton-exciton scattering in the intensity-dependent shift of D1XA and FXAn=1-1LO PL peaks?
• RQ4 How does thermal quenching behavior reflect the inhomogeneous relaxation of paramagnetic centers in the sample?
• RQ5 What is the assignment of the three EPR signals at g = 1.985, 1.956, and 1.939 in terms of specific defect species?

Key findings

• The EPR signal at g = 1.985 is attributed to Zn interstitials, identified as shallow donors in the ZnO lattice.
• The EPR signal at g = 1.956 is assigned to oxygen vacancies, which act as shallow donors in polycrystalline ZnO.
• The EPR signal at g = 1.939 is associated with a third paramagnetic center, possibly related to complex defect structures or surface states.
• Crystal field splitting (CFS) and spin-orbit coupling (SOC) were quantified using the energy shifts of excitonic PL peaks, indicating strong electronic interactions in the defect states.
• Thermal quenching analysis revealed an activation energy for inhomogeneous relaxation of paramagnetic centers, reflecting energetic disorder in the defect distribution.
• Pump intensity-dependent PL showed a red shift in D1XA and a blue shift in FXAn=1-1LO, confirming the presence of exciton-exciton scattering in the system.