Graduation Year

2004

Document Type

Thesis

Degree

M.S.E.E.

Degree Granting Department

Electrical Engineering

Major Professor

Don Morel.

Keywords

Wideband gap semiconductors, RF Sputtering, Four point probe measurement, Hall Measurement, Absorption/Transmission Spectroscopy

Abstract

Zinc Oxide falls under the classification of transparent conductive oxides. They typical optical transmittance of Zinc Oxide is 90% in the visible wavelength region. Though stoichiometric ZnO is an insulator, due to the presence of internal defects such as Zn interstitials and Oxygen vacancies, it exists as a n-type conductor. The other important property of ZnO which could be used by the optical field is its widebandgap. ZnO has a wide bandgap of 3.2eV -3.3eV. The additional advantage of being a direct bandgap semiconductor has increased the probability of using ZnO for short wavelength applications. These practical applications are directly related to the fabrication of homostructural p-n junctions. ZnO can be readily doped n-type. Doping ZnO P-type is very difficult due to its native defects and the self-compensation that occurs during doping.

But when P-type doping is obtained in ZnO it could be used in various optical applications such as light emitting diodes and laser diodes. This provided the motivation for this research. Theoretical studies have proposed nitrogen as a suitable material to achieve p-type ZnO. Literature provides a set of conditions that could be used to improve the doping in ZnO films. In this research, a set of these conditions were used to implement p-type doping in ZnO films. A sputtering system with a setup to support two Torus - 5M guns was used to deposit the ZnO films. A codoping technique using an aluminium doped zinc oxide target was the first method. Though an improvement in the nitrogen incorporation was found in this method in the beginning, a further increase in the nitrogen pressure did not show further improvement. A co-sputtering technique of a 99.999% pure ZnO target and a 99.99% pure Zn metal target was the second method.

The ZnO target was rf sputtered while the Zn target was dc sputtered using the two guns provided in the deposition chamber. The extra Zinc obtained from sputtering the metallic Zn target was used to improve the incorporation of nitrogen. The films were later deposited in an oxygen ambient where the excess oxygen was used to suppress the oxygen vacancies that act as hole killers during the doping process. Four point probe measurement and Keithley 900 series Hall equipment were used for the electrical characterization of the films. An ORIEL monochromator was used to optically characterize the films. Hitachi S-800 T EDAX analysis system was used to measure the atomic weight % of nitrogen incorporated in the ZnO:N films. Deposition at an oxygen partial pressure of 0.3mT and 0.8mT of nitrogen produced p-type ZnO films. These films showed a carrier absorption in the short wavelength region.

The carrier concentration and the mobility obtained for these films were 4.0x10¹⁶ cm⁻³ and 0.12 cm²/V-s respectively.

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