PLD GROWTH OF HIGH QUALITY ZINC OXIDE THIN FILMS ON SI SUBSTRATES AND DEVICE DEVELOPMENT

Abstract

Growth of high quality zinc oxide thin films on silicon substrates is particularly important because it combines the unique features of zinc oxide (ZnO) with mature CMOS technology and paves the way for device developments. However, this is a challenging task due to several technical and material-related fundamental issues which exist with the growth of this highly sought after compound semiconductor. In general, metal-oxide semiconductors suffer from non-stoichiometric growth which leads to unipolar doping properties, such as ZnO grows nominally n-type while NiO grows p-type. Thus, these materials can be doped easily either n or p-type while the other polarity is hard to achieve, if not impossible. Although methods for the growth of p-type films, using extrinsic doping or exotic precursors with post growth treatments have been reported on different substrates, the problem of controlled and stable extrinsic p-type doping of ZnO films remains an open subject for research.

In the present work, we have achieved the growth of undoped p-type ZnO films on Si (100) substrates by pulsed laser deposition through the optimization of growth conditions and adjustments of growth dynamics. Currently no other reports of undoped p-type ZnO on Si substrates by PLD growth exist, showing stable p-type conductivity in a repeatable process. The structural, optical, and electrical properties of the grown films were examined using techniques such as X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), Photoluminescence spectroscopy (PL), Hall Effect four-point probe Van Der Pauw measurements, and Current-Voltage (I-V) measurements for the p-n heterojunctions. Electrical behavior of ZnO is generally attributed to intrinsic defects which include vacancies, interstitials, and anti-sites of Zn and O in addition to external contamination related defects such as hydrogen complexes. The effects of growth conditions and intrinsic defects including hydrogen contamination on the properties of the grown layers are studied. Moreover, the growth dynamics of ZnO polar planes, i.e. the stacking of O-2 and Zn+2 planes, on n and p-type Si substrates are discussed.

Once material studies and growth optimizations are completed, high quality ZnO films are used in device fabrication. Two types of optoelectronic devices, a photoresistor and a Schottky diode are fabricated on Si substrates, and the electrical behavior of the devices are investigated. The high quality ZnO films also contributed to the development of a surface acoustic wave (SAW) biosensor.