Reactive
plasma deposition (RPD) method is one of the techniques for depositing thin
films on a substrate. This method offers the advantages for low-ion damage,
low deposition temperature, large area deposition and high growth rates.
In this work, undoped and Ga-doped (3 wt.%) n-type ZnO thin films were grown
by the RPD method on glass substrates at 200 °C under oxygen flow rate of
0 -- 50 sccm. Piezoelectric photothermal spectroscopy (PPTS) was measured
for characterization of nonradiative recombination processes directly, in
addition to the high sensitivity for very small optical absorption. Therefore,
the PPT technique is complement the photoluminescence (PL) technique. In
the room temperature PPT spectra, strong peak due to the band edge transition
was observed around 3.3 eV in all the samples. Furthermore, a broad band
of the PPT signal at 2.5 eV was observed for the undoped ZnO samples grown
under low oxygen flow rate. This signal disappeared with increasing the
oxygen flow rates and did not appear in the Ga-doped samples. Therefore,
it is considered that this PPT signal at 2.5 eV was due to the electron
transition through the oxygen vacancies acting as a non-radiative center.
(EMRS2004)
Un-doped and Ga-doped (3 wt%) n-type ZnO thin films were grown by a Reactive Plasma Deposition (RPD) method on glass substrates at 200 °C under oxygen flow rate from 0 to 50 sccm. In this paper, we report on the defect and band edge related signals in the optical absorption spectra for ZnO thin film by using a piezoelectric photo-thermal (PPT) spectroscopy, which is effective to observe a non-radiative transition process. The PPT peak around 2.5 eV was observed only for the un-doped ZnO samples grown under low oxy
gen flow rate. This signal is considered to be related to the oxygen vacancies, because it disappears with increasing the oxygen flow rates. No corresponding peak was found for Ga-doped samples. This result indicates that Ga doping inhibits the generation of the oxygen vacancies. This agreed with that from the first-principle electronic band structure calculations. We have also carried out the theoretical calculation for the optical absorption edge of degenerated ZnO as a function of the carrier concentration. Burstein-Moss effect and Band-Gap-Narrowing effect in ZnO should be considered in the case of high carrier concentration. Comparing the experimental results with the theoretical predictions, we found that the proposed PPT edge energy coincides well with the Fermi level EF. American Institute of Physics. [DOI:10,1063/1.2173040] (J. Appl. Phys., 2006)
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