Epitaxial Growth and Properties of ZnO Thin Films on Si Substrates.

Abstract

ZnO is an attractive material for promising applications in short wavelength optoelectronic devices because of its wide band gap and large exciton binding energy at room temperature (RT). This dissertation is devoted to the development of high quality, single-crystalline ZnO-based light-emitting devices on Si substrates, involving thin film synthesis by pulsed laser deposition, structure-property characterization, prototype device fabrication, strain engineering of thick films, and p-type doping with antimony (Sb). ZnO epitaxy with exceptional quality was achieved on (111) Si substrates for the advantages of inexpensive large wafers, mature device technologies, and multifunctional device integration. Epitaxial bixbyite oxides M2O3 (M=Sc, Lu, Gd) were originally employed as the buffer layer between ZnO and Si. The single-crystalline ZnO films has superior structural, electrical, and optical qualities than all previous reports of ZnO on Si, such as narrow ω-rocking curves, low dislocation densities, high electron mobilities at RT, and comparable photoluminescence characteristics to those of ZnO single crystal. The epitaxial orientation relationship, intrinsic donors, microstructural defects, and residual strain of the films were investigated. Prototype n-ZnO/M2O3/p-Si devices were constructed, and ZnO near-band-edge emission was observed in electroluminescence at RT. Strain engineering of thick films by insertion of low-temperature grown ZnO interlayers was performed to improve the cracking critical thickness to ≥2 μm. Reliable ZnO p-type doping using large-size-mismatched Sb dopant was achieved for the films grown on both (0001) Al2O3 and (100) Si substrates, with a resistivity of 4.2-60 Ω cm, a Hall mobility of 0.5-7.7 cm^2/V s, and a hole concentration of 3.2×10^16-2.2×10^17 cm^-3. The origin of p-type conductivity was elucidated from conjugated effects of oxygen-rich growth condition, adequate doping concentration, and dislocation-facilitated formation of complex acceptors of Sb(Zn)-2V(Zn). The thermal activation energy and the optical ionization energy of the acceptor are estimated 115±5 meV and 158±7 meV, respectively.