Growth and characterization of ZnO nanowires and thin films

다국어 초록 (Multilingual Abstract)

ZnO was studied in this dissertation for use in one-dimensional nanoscale devices, optoelectronics, and electronic applications. The synthesis, structural, and optical properties of ZnO and MgO nanowires as nanoscale materials were investigated. The crystallinity, electrical, optical and magnetic properties of undoped and phosphorous doped ZnO thin films for p-type ZnO were examined.
The ZnO nanowires were fabricated using catalyst-driven molecular beam epitaxy. Site specific growth of ZnO and MgO nanowires was observed on Ag coated Si and Al_(2)0_(3) substrates. The structural and compositional studies indicated that the deposition of Zn and Mg resulted in two different types of radial heterostructured (Zn,Mg)O nanowires. One consists of a single crystal Zn-rich Zn_(l_x)Mg_(x)O (x < 0.02) core having a hexagonal wurtzite structure encased by an epitaxial Zn_(l_y)Mg_(y)O (y>>0.02) sheath having the same hexagonal wurtzite structure. The other consists of single crystal ZnO with a hexagonal wurzite structure surrounded by a MgO sheath with a cubic rock salt structure.
The effect of phosphorus doping on the electrical and optical properties of ZnO grown via pulsed laser beam deposition was studied. Phosphorus doping yields enhanced n-type behavior in as-deposited films, indicating the formation of shallow donor states. Annealing in 100 mTorr of oxygen led to the conversion of n-type behavior in as-deposited films to semi-insulating behavior in the annealed films. For the annealed film, these results appear to reflect phosphorus substitution on the O sites.
The characteristics of device structures that employ phosphorus-doped (Zn,Mg)O have been examined in a effort to delineate the carrier type behavior in this material. The capacitance-voltage properties of metal/insulator/P-doped (Zn,Mg)O diode structures were measured and found to exhibit a polarity consistent with the P-doped (Zn,Mg)O layer being p-type. In addition, thin-film junctions comprising n-type ZnO and P-doped (Zn,Mg)O display asymmetric I-V characteristics that are consistent with the formation of a p-n junction at the interface. Although Hall measurements of the P-doped (Zn,Mg)O thin films yielded an indeterminate Hall sign due to a small carrier mobility, these results are consistent with previous reports that phosphorus can yield an acceptor state and p-type behavior in ZnO materials.
The photo-response of ZnO doped with phosphorus was investigated. A correlation between near band-edge emission and carrier density is observed. This is similar to results found for ZnO in which the carrier density is increased via annealing in a reducing ambient. Upon annealing in an oxidizing environment, the near band-edge emission decreased for both the undoped and phosphorus doped ZnO films. This was attributed to radiative transitions involving acceptor levels deep in the gap. The phosphorus doping, combined with annealing, leads to an enhanced photoconductivity response.
The magnetic properties of phosphorus doped ZnO thin films were examined after high dose Mn implantation. Films show room temperature hysteresis in magnetization loops. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial annealing temperature.
The ratio of the near band-edge (NBE) emission to the deep-level (DL) emission of the photoluminescence spectra is related to the Zn pressure during the growth of ZnO epitaxial thin films by molecular beam epitaxy. It is suggested that green deep-level emission comes from a donor-deep acceptor (Zn vacancy, V_(Zn)^(-)) transition, and yellow emission from a donor-deep acceptor (oxygen vacancy, O_(i)) transition. The shift of deep level emission results from different concentrations of Zn vacancy, V_(Zn)^(-) and oxygen vacaney, O_(i)^(-).

목차 (Table of Contents)

• ACKNOWLEDGMENTS = iii
• TABLE OF CONTENTS = iv
• ABSTRACT = vi
• CHAPTER 1 INTRODUCTION = 1
• CHAPTER 2 LITERATURE REVIEW = 3
• 2.1 General Properties of ZnO = 3
• 2.1.1 Wide Bandgap Semiconductors = 3
• 2.1.2 Transparent Conducting Oxides = 5
• 2.1.3 Crystal Structure = 7
• 2.1.4 Epitaxial Thin Film = 8
• 2.1.5 n-type ZnO Doped with Impurities = 11
• 2.1.6 Tunable Bandgap for Heterostructure = 11
• 2.2 One-dimensional Nanoscale Materials = 14
• 2.2.1 Quantum Effect on Transport Properties = 14
• 2.2.2 Growth Mechanism = 17
• 2.2.2.1 Vapor-liquid-solid (VLS) model = 17
• 2.2.2.2 Root-growth model = 17
• 2.2.3 Synthesis Method = 20
• 2.2.3.1 Vapor-phase transport = 20
• 2.2.3.2 Chemical vapor deposition = 21
• 2.2.3.3 Laser ablation = 22
• 2.2.3.4 Electrical arc discharge = 23
• 2.2.4 Nanowires and Nanobelts of ZnO = 24
• 2.3 Doping for p-type ZnO = 27
• 2.3.1 Doping Limitation of Compound Semiconductors = 27
• 2.3.2 Dominant Donors for an Intrinsic n-type ZnO = 30
• 2.3.3 Approach to p-type Doping in Zno = 31
• CHAPTER 3 SITE-SPECIFIC GROWTH OF ZnO NANOWIRES USING CATALYSIS-DRIVEN MOLECULAR BEAM EPITAXY = 36
• 3.1 Introduction = 36
• 3.2 Experimental = 36
• 3.3 Results and Discussion = 37
• 3.3.1 Growth Condition of ZnO Nanowires = 37
• 3.3.2 Site-selective Growth of ZnO Nanowires = 41
• 3.3.3 Optical Properties = 42
• 3.3.4 Growth Mechanism = 46
• CHAPTER 4 BIMODAL GROWTH OF CORED Zn_(l_x)Mg_(x)O HETEROEPITAXIAL NANOWIRES = 50
• 4.1 Introduction = 50
• 4.2 Experimental = 51
• 4.3 Results and Discussion = 52
• 4.3.1 Bimodal Growth of Cored ZnO/Zn_(l_x)Mg_(x)O Heterostructured Nanowires = 52
• 4.3.2 Radial Heterostructure of a Core ZnO Nanowire Surrounded by a (Mg,Zn)O Sheath = 62
• 4.3.3 (Mg,Zn)O Nanowires = 69
• CHAPTER 5 EFFECT OF PHOSPHORUS DOPING ON ZnO THIN FILM GROWN BY PULSED LASER DEPOSITION = 76
• 5.1 Introduction = 76
• 5.2 Experimental = 77
• 5.3 Results and discussion = 78
• 5.3.1 Transport Properties of Phosphorus Doped ZnO = 78
• 5.3.1.1 Shallow Donor Formation = 78
• 5.3.1.2 Deep Level Acceptor Formation = 88
• 5.3.2 p-type behavior in Phosphorus-doped (Zn,Mg)O device structures = 93
• 5.3.3 Optical Properties of Phosphorus Doped ZnO = 106
• 5.3.4 Magnetic Properties of phosphorus doped ZnO after Mn implantation = 114
• CHAPTER 6 GROWTH AND CHARATERIZATION OF ZnO THIN FILM GROWN BY MOLECULAR BEAM EPITAXY = 120
• 6.1 Introduction = 120
• 6.2 Experimental = 122
• 6.3 Results & Discussion = 124
• 6.3.1 Growth and Characterization of ZnO Grown with Atomic Oxygen = 124
• 6.3.1.1 Growth and Electrical Properties = 124
• 6.3.1.2 Optical Properties = 130
• 6.3.2 Growth of ZnO Thin Film on c-plane Al_(2)O_(3) by Molecular Beam Epitaxy Using Ozone as a Oxygen Source = 140
• CHAPTER 7 SUMMARY AND CONCLUSION = 148
• REFERENCES = 153
• BIOGRAPHICAL SKETCH = 163