||This thesis investigates thin-film P-N homojunction diodes made of c-textured ZnO deposited on a-oriented ZnO (a-ZnO) using r-Al2O3 as substrates. First, an a-ZnO epitaxial layer is grown on the r-oriented Al2O3 (r-Al2O3) substrate by RF sputtering. X-ray diffraction (XRD) was used to analyze the crystal structure and general qualities of the films via 2ɵ-ω, rocking curve, GIXRD and Φ- scans. Carrier concentration and mobility were determined by measuring the electrical and Hall resistivity using a Physical Property Measurement System (PPMS) made by Quantum Design, Inc., U.S.A. The charge carries of the epitaxial a-ZnO are p-type, while those of the textured c-ZnO are n-type.|
As ZnO tends to grow at room temperature (RT) along its c-axis without in-plane alignment, the n-layer was first grown at RT for 30s~120s, after which the growth temperature was raised to the desired levels while the deposition continued without interruption. The growth time at RT, final deposited temperature, and sputter-gun power were varied systematically in seeking optimal conditions for the diode performance. This includes the attempts, albeit unsuccessful, to make the c-textured films epitaxial.
The textured nature of the n-type layer turns out to contain electrically leaky path, most likely along the boundaries among the single-crystalline domains. In order to incapacitate or remove these possible conducting channels, HF was used to etch the c-textured surface. The resulted devices were analyzed according to their I-V characteristics for samples obtained using the aforementioned material growth conditions and HF etching times.