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Synthesis, Characterization and Application of Doped ZnO Nanowire

  • Kyoungwon Kim (대학원 전자전기공학과 전자전기컴퓨터공학전공)

초록/요약

In this thesis, well-aligned single-crystalline zinc oxide (ZnO) and various doped ZnO NWs (NWs) were successfully fabricated on Au film catalyzed sapphire substrate using vapor-liquid-solid (VLS) method in self-designed hot-walled pulsed laser deposition (HW-PLD). The physical and chemical properties of Ga/Ag/Ag-Al doped ZnO NWs have been investigated depending on kinds of dopants and working temperatures in ZnO NWs. In case of Ga-doped ZnO (GZO), stacking faults were observed by using FE-SEM and an exciton bound to a neutral donor (D0X) peak was clearly observed by using PL spectra. And, the morphology evolution of the GZO nanostructures from nanohorn, NW, to thin film has been clearly observed and is strongly related with different growth kinetics. After silver doping process, Ag-doped ZnO (SZO) NWs showed some change behaviors including structural, electrical and optical properties. In case of structural property, the primary growth plane of SZO NWs was switched from (002) to (103) plane. And, the electrical properties of SZO NWs were variously measured to be about 4.26 x 106, 1.34 x 106, and 3.04 x105 ？ for 1, 3, and 5 SZO NWs, respectively. In other words, the electrical properties of SZO NWs depend on different Ag ratios resulting in controlling the carrier concentration. Also, the optical properties of SZO NWs were investigated to confirm p-type semiconductor by observing the exciton bound to a neutral acceptor (A0X). In case of Ag/Al co-doped ZnO (SAZO), low temperature photoluminescence (PL) was studied experimentally in order to investigate the p-type behavior observed by the A0X. The A0X is not observed in the 1 at.% SAZO NWs by using low temperature PL because 1 at.% SAZO NWs do not have a Ag-O chemical bonding confirmed by XPS measurement. The activation energies (Ea) of the A0X were calculated to be about 18.14 meV and 19.77 meV for 3 and 5 at.% SAZO NWs, respectively, which are the lower value than those of single Ag-doped NW about 25 meV. Behavior of thermally activated free electrons (TAFEs) and trapped carriers (TCs) has been investigated by analyzing temperature stress stability test of pristine ZnO NW field effect transistor (FET). After applying temperature stress ranging from 298 to 363K, pristine ZnO NW FET device shows two different types of threshold voltage (Vth) shift. Vth in low temperature zone up to 323K shows positive shift and saturation mainly due to the increase of TCs by chemical adsorption of O2？ or H2O？ ions on pristine ZnO NW surface. On the other hand, Vth in high temperature zone up to 363K shows negative shift mainly caused by dominant TAFEs from the deep level and interface trap site of ZnO. The opposite shift of Vth in the high temperature zone has been observed mainly because the TAFEs in the high temperature range are larger in number than TCs (thermally activated free electrons > trapped carriers). For expanding sensing window of ZnO-based sensor, Ga dopants with various concentrations are doped into ZnO NWs to modulate the conductivity of NWs s and used to detect NOx gas. The diameter and length of NWs are < 200 nm and several ？m, respectively. In order to confirm the Ga-doping status in ZnO NWs, we have investigated shift direction of the near band edge (NBE) emission by PL according to the Ga concentration. Significant resistivity modulations are observed, and results are compared with cases of various materials including doped and un-doped ZnO NWs in terms of the sensitivity. The sensitivities of ZnO-based NW gas sensors with 23 ~ 372 % are easily expended by using different conditions including various Ga concentration and temperature.

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목차

1. Introduction
1.1 ZnO properties
1.1.1 Physical properties
1.1.1.1 Structure
1.1.1.2 Mechanical properties
1.1.1.3 Electronic properties
1.1.1.4 The defects of ZnO
1.1.2 Chemical properties
1.2 Nano-scale structures
1.2.1 Nanowire
1.2.2 Electronic properties of nanowire
1.2.2.1 Mechanical stability
1.2.2.2 Electronic transport

2. Synthesis and Characterization of Doped ZnO Nanowire
2.1 Doped ZnO nanowires prepared by self-designed HW-PLD
2.1.1 Gallium doing in ZnO nanowire
2.1.1.1 Introduction
2.1.1.2 Experimental procedure
2.1.1.3 Result and discussion
2.1.1.4 Summary
2.1.2 Silver doing in ZnO nanowire
2.1.2.1 Introduction
2.1.2.2 Experimental procedure
2.1.2.3 Result and discussion
2.1.2.4 Summary
2.1.3 Silver/Aluminum co-doing in ZnO nanowire
2.1.3.1 Introduction
2.1.3.2 Experimental procedure
2.1.3.3 Result and discussion
2.1.3.4 Summary

3. Applications of Doped ZnO Nanowires
3.1 Doped ZnO-based nanowire field-effect transistors
3.1.1 Application of pristine ZnO nanowire FET
3.1.1.1 Introduction
3.1.1.2 Experimental procedure
3.1.1.3 Result and discussion
3.1.1.4 Summary
3.1.2 Application of silver-doped ZnO nanowire FET
3.1.2.1 Introduction
3.1.2.2 Experimental procedure
3.1.2.3 Result and discussion
3.1.2.4 Summary
3.2 ZnO-based nanowire gas sensors
3.2.1 Application of gallium doping ZnO nanowire gas sensors
3.2.1.1 Introduction
3.2.1.2 Experimental procedure
3.2.1.3 Result and discussion
3.2.1.4 Summary

4. Conclusions

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