Carrier Lifetimes of Atomic-Layer-Deposition-Grown Zinc Oxide Nano Thin Films
Date Issued
2012
Date
2012
Author(s)
Liu, Chao-Hsun
Abstract
When we mention the material “zinc oxide,” people always take it as a potential stock of near-ultraviolet light emitting devices in the future. Actually it is not a surprise for us: Because the bandgap energy of zinc oxide is 3.4 eV, and it is similar to the most popular material of LED growing, gallium nitride. Shown by the reports in the past, the carrier lifetime of zinc oxide thin film measured at room temperature is short, and it is about several tens of picosecond in average. However, we discovered that the carrier lifetime of zinc oxide nano thin film grown by atomic layer deposition, which has better thickness control than traditional deposition technique, is about hundreds of picosecond.
In this thesis, for the purpose of understanding the carrier lifetime of zinc oxide nano thin films grown by atomic layer deposition, and further estimating the radiative recombination carrier time related to the luminescence efficiency, we prepared four kinds of zinc oxide thin films which grown on sapphire substrates: First, annealed zinc oxide thin films; Second, zinc oxide thin films without annealing; Third, annealed zinc oxide thin films covered by magnesium cap layers; and the last, zinc oxide thin films without annealing and covered by magnesium cap layers. We found that we cannot do further discussion about the last sample due to the carrier lifetime is compatible to laser period, and the carrier lifetimes of other three types are all hundreds of picosecond. Among all the samples, annealed samples have more uniform performance, and their carrier lifetimes are obviously longer than the samples without annealing. Samples which covered by magnesium cap layers have slightly shorter lifetime than samples without magnesium cap layers. In the aspect of the photon energy of probe lasers, we can observe the phenomenon that carrier lifetimes decrease when the photon energy of probe beams increases in all of the samples. These observations are reasonable because of the relation between the photon energy of probe beams and energy loss mechanism
If we observe the carrier lifetimes of samples by same magnitude of energy above the bandgap energy, and under the conditions of different optically induced carrier densities, we can find that the carrier lifetimes almost maintain constant for annealed zinc oxide thin films. For the annealed zinc oxide thin films with magnesium cap layers, the carrier lifetimes slightly decrease when optically induced carrier densities increase. The carrier lifetimes of zinc oxide thin films without annealing obviously decrease with increasing optically induced carrier densities. The results above may be related to the mechanisms of carrier recombination, and the weight of each mechanism presents in the process. By observing the optically induced carrier densities and their lifetimes, we estimate that the radiative recombination carrier lifetimes of zinc oxide nano thin films grown by atomic layer deposition are about 600 ps.
Subjects
ultrafast laser
atomic layer deposition
zinc oxide
carrier lifetime
Type
thesis
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