Lithium and electrical properties of ZnO

Abstract

Hydrothermally grown n-type ZnO samples have been investigated by deep level transient spectroscopy (DLTS), thermal admittance spectroscopy (TAS), temperature dependent Hall effect (TDH) temperature, and secondary ion mass spectrometry (SIMS) after thermal treatments up to 1500◦C, in order to study the electrical properties of samples with different lithium content. The SIMS results showed that the most pronounced impurities were Li, Al, Si, Mg, Ni and Fe with concentrations up to ∼ 5 × 1017cm−3. The Li concentration was reduced from ∼ 1017 cm−3 in as-grown samples to ∼ 1015 cm−3 for samples treated at 1500◦C, while the concentration of all the other major impurities appeared stable. The results from DLTS and TAS displayed at least five different levels having energy positions of Ec − 20 meV , Ec − 55 meV , Ec − 0.22 eV , Ec − 0.30 eV , and Ec − 0.57 eV (Ec denotes the conduction band edge), where the Ec − 55meV level is the dominant freeze out level for conduction electrons in samples treated at temperatures < 1300◦C, while higher annealing temperatures revealed the shallower (Ec − 20 meV ) level. The TDH measurements showed a pronounced increase in the electron mobility for the heat treated samples, where a peak mobility of 1180 cm2/V s was reached for a sample treated at 1300◦C. The results provide strong evidence that Li in hydrothermal ZnO is almost exclusively in the substitutional configuration (LiZn), supporting theoretical predictions that the formation of LiZn prevails over Li on the interstitial site for Fermi level positions at and above the middle of the band gap.