Hydrothermal growth of ZnO nanostructures and crystalline/quasi-monocrystal films and their applications on related optoelectronic devices

Abstract

The dissertation aims at the preparation of ZnO nanostructures and crystalline/quasi-monocrystal films and theirs applications on related optoelectronic devices such as light emitting diodes (LEDs) solar cells (SCs) and ultraviolet photodetectors (UV-PDs) A surface roughening scheme with ZnO-based nanostructures is proposed to improve the light extraction efficiency of GaN-based LED and the energy conversion efficiency of crystalline Si-based SCs The growth of crystalline (or quasi-monocrystal) ZnO film on p-GaN (or ITO) using the hydrothermal growth (HTG) methods also is proposed A thermal annealing process for ZnO-based optoelectronic devices is developed together to reduce the amount of oxygen vacancies in the grown ZnO films for improving both the optical and electrical properties In the present study ZnO nanotapers (ZnO-NTs) were grown on the regular horizontal LED (HLED) surface by HTG method The surface roughening technology using ZnO-NTs offers advantages of low cost and low temperature processing which facilitates a surface roughening structure with a graded refractive index Through the use of ZnO-NTs a considerable improvement in light output power (LOP) of LED by 36 15% at 350 mA as compared to regular HLED has been obtained A surface roughening scheme with SiNx-coated GaN nanowire (NW) arrays is also proposed in this work to further improve the LOP of GaN-based vertical LEDs (VLEDs) At first ZnO NW arrays were grown on the VLEDs surface using HTG method With ZnO NW as a mask a dry etching using an inductively coupled plasma reactive ion etching process was conducted on the n-GaN surface with GaN NW arrays Finally a SiNx layer was coating on the GaN NW using a plasma-enhanced CVD system and the VLEDs with graded refractive index was obtained Compared with that of regular VLEDs the use of SiNx-coated GaN NW arrays leads to an improvement in LOP by 28 7% at 350 mA ZnO NWs were also grown on the regular SCs with a SiNx/micropyramid surface by HTG method and then the ZnO nanotubes were formed via the etching process at room temperature atmosphere Then a SiON layer was coating on the ZnO nanotubes using a plasma-enhanced CVD system and the SCs with graded refractive index were fabricated Note that the use of SiON-coated ZnO nanotubes increases the energy conversion efficiency by 39 2% under AM1 5G (100 mW/cm2) as compared to that of regular SCs with a SiNx/micropyramid surface The preparation of quasi-monocrystal ZnO film and deposition p-Cu2O-film heterojunction (HJ) for UV-PDs are studied and demonstrated The HJ was formed via the sputtering deposition of p-type copper oxide onto HTG-ZnO-film that offers advantages of low cost low temperature processing effective prevention of the p-Cu2O from reaching the substrate during HJ formation and avoidance the possible short-circuit problem usually encountered for ZnO-NW-based UV-PDs The prepared UV-PDs with different Cu2O thicknesses (250~750 nm) with thermal annealing at 600oC in nitrogen exhibit well-defined rectifying current-voltage characteristics superior response to UV light illumination with UV light sensitivity as high as 55 6 and rise/fall time of ~20/26 s were obtained The growth of single crystalline ZnO film on p-GaN substrate which has small lattice mismatch of 1 8% with ZnO is proposed to further improve the quality of ZnO film The proposed technology has advantages of low-pressure and low-temperature processing ease of fabrication and a relative low cost A good rectifying diode behavior and 150 % improvement in leakage current at -4 V of n-ZnO/p-GaN UV-LED after thermal annealing was obtained Strong ultraviolet lights emission from the annealed n-ZnO/p-GaN UV-LED at around 375 nm without defect-related emissions in the visible region are observed from electroluminescence (EL) spectra Porous p-ZnO films were grown on the n-GaN epilayer by hydrothermal growth method and the fabrication and characterization of UV-PDs based on p-ZnO/n-GaN HJ are presented The prepared UV-PDs with different pores sizes of 300~600 nm exhibit well-defined rectifying current-voltage characteristics superior response to UV light illumination with UV light sensitivity (IUV/Idark) as high as 80 and rise/fall time of ~6/7 s were obtained In this study the surface roughening technology using ZnO-based nanostructures shows a considerable improvement in LOP of GaN-based LEDs by 36 15% and 28 7% as well as in energy conversion efficiency of Si-based SCs by 39 2% Furthermore the fabricated UV-LED with thermal annealing has the main lights emissions at around 375 nm and a 150 % improvement in leakage current Moreover the prepared p-Cu2O/n-ZnO and p-ZnO/n-GaN UV-PDs exhibit well-defined rectifying current-voltage characteristics superior response to UV light illumination with UV light sensitivity as high as 55 6 and 80 respectively According to these results it is expected that the proposed ZnO nanostructure and crystalline/ quasi-monocrystal films based optoelectronic devices with a suitable fabrication processes tuning could be an effective vehicle for future LED SCs and UV-PDs applications

Date of Award	2015 Aug 12
Original language	English
Supervisor	Shui-Jinn Wang (Supervisor)