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Title Page
Abstract
국문요약
Contents
Chapter 1. INTRODUCTION 18
1.1. Graphene 18
1.2. Thesis overview 19
1.3. References 20
Chapter 2. Graphene interlayer for current spreading enhancement 22
2.1. Lateral-type GaN-based light-emitting diodes 22
2.2. Fabrication 24
2.3. Results and discussion 26
2.4. Conclusion 35
2.5. References 36
Chapter 3. Effect of p-GaN hole concentration on a graphene current spreading layer 41
3.1. Graphene current spreading layer 41
3.2. Fabrication 42
3.3. Results and discussion 43
3.4. Conclusion 54
3.5. References 54
Chapter 4. Ag-mesh-combined graphene for an indium-free current spreading layer 58
4.1. Ag-mesh-combined graphene 58
4.2. Fabrication 60
4.3. Results and discussion 61
4.4. Conclusion 72
4.5. References 73
Chapter 5. Graphene/metal mesh hybrid films through prime location and metal doping 77
5.1. Graphene/metal mesh hybrid films 77
5.2. Fabrication 78
5.2.1. Fabrication of LED devices with a variety of transparent conductive layers 78
5.2.2. Preparation for transfer length method on p-GaN layers 79
5.3. Results and discussion 81
5.3.1. Device properties fabricated with various types of TCLs 81
5.3.2. Depending on graphene location 83
5.3.3. Metal-doped graphene 88
5.4. Conclusion 93
5.5. References 93
Chapter 6. Summary 96
Curriculum vitae 99
Table 5.1. Turn on voltage and series resistance values with respect to the NUV LEDs. 83
Table 5.2. Specific contact resistance and contact resistance values with respect to the... 87
Table 5.3. Raman shifts with respect to G peak and 2D peak. 90
Table 5.4. Specific contact resistance and contact resistance values with respect to metal-... 92
Figure 1.1. Schematic illustrations showing various graphene applications. 18
Figure 2.1. Fabrication process of the three types of LEDs with different current spreading... 24
Figure 2.2. Results of Raman spectroscopy. '1' through '6' represent the Raman spectra with... 26
Figure 2.3. (a) Transmittance of 'GR', 'ITO', and 'ITO on GR'. (b) Sheet resistance of each... 27
Figure 2.4. (a) Work functions of 'GR' and 'ITO' measured by ultraviolet photoelectron... 28
Figure 2.5. (a) Light output power versus input current for 'GR', 'ITO', and 'ITO on GR'... 28
Figure 2.6. The electroluminescence (EL) image of the 'GR'by changing current injection. 29
Figure 2.7. (a) An optical micro scope image of a probe-tip used in EL measurement. (b) An... 30
Figure 2.8. (a), (b), and (c) represent the band diagram of CSLs and p-type GaN 32
Figure 2.9. (a) and (b) show a schematic of the current flow from the p-type metal pad to the... 33
Figure 2.10. (a) and (b) are the optical microscope images of luminescence for 'ITO' and... 34
Figure 2.11. (a) and (b) are simulation results for current injection into the active layer... 35
Figure 3.1. Fabrication process of LED devices with different p-GaN doping concentrations. 42
Figure 3.2. (a) SEM image of a pristine graphene sheet (GR). (b)AFM image of the GR. 43
Figure 3.3. (a) Results of Raman spectroscopy for GR and a bare sapphire substrate. (b)... 44
Figure 3.4. (a) Current-voltage characteristics of LED devices consisting of lightly doped p-... 45
Figure 3.5. (a) Electroluminescence spectra at 20 ㎃. (b) Level of current injection at low... 46
Figure 3.6. (a) Light output power versus current. (b) Light emission images of LED devices... 46
Figure 3.7. Light output of an NUV LED with LD p-GaN layer and HD p-GaN formed by... 48
Figure 3.8. (a) SEM images of the NUV LEDs after current injection with respect to LD p-... 49
Figure 3.9. (a) Capacitance-voltage characteristics of LD p-GaN and HD p-GaN. (b)... 49
Figure 3.10. (a) Energy band diagram between GR and the LD p-GaN. (b) Energy band... 51
Figure 3.11. (a) and (b) show current-voltage characteristics of LD p-GaN and HD p-GaN... 52
Figure 3.12. (a) Optical microscope images of rectangular patterns of the graphene sheet for... 53
Figure 4.1. Process of fabricating the NUV LEDs with different current spreading layers: 200... 60
Figure 4.2. (a) Optical microscope images of ITO, GR, 300 ㎛-gap Ag mesh on graphene... 62
Figure 4.3. (a) Sheet resistances of each material obtained by Hall measurements. The inset... 63
Figure 4.4. (a) Current-voltage characteristics of ITO, GR, RAM 300, RAM 150, and RAM... 64
Figure 4.5. (a) Light output power versus input current. (b) Light output power versus input... 65
Figure 4.6. (a) Optical microscope images of each device before current injection. Scale bar is... 66
Figure 4.7. Results of Raman spectroscopy with respect to GR, ITO, RAM 300, RAM 150,... 67
Figure 4.8. Luminous intensity of each device versus distance between p-pad and n-pad,... 68
Figure 4.9. Schematic of Ag-mesh-combined graphene CSL on DPS with the increase of Ag... 69
Figure 4.10. Results of Hall measurements related to sheet resistance, surface charge density,... 69
Figure 4.11. Lateral current flow through ITO, GR, and RAM 150 CSL. 70
Figure 4.12. I-V curves for extracting contact resistances. 71
Figure 4.13. Simulation results for current flow using ITO, GR, and RAM 150 CSL. 72
Figure 5.1. Schematic illustration of each LED fabrication process using five types of... 78
Figure 5.2. Electrical and optical properties of the LED devices fabricated with each TCL 81
Figure 5.3. (a) Transmittance values with respect to each TCL formed on double polished... 84
Figure 5.4. SEM images of different graphene locations 85
Figure 5.5. (a) Current-voltage characteristics extracted from transfer length method (TLM)... 88
Figure 5.6. (a) Schematic illustration showing sample preparations for Raman analysis. (b)... 89
Figure 5.7. (a) Results of measured work functions through UPS. (b) Band diagrams between... 90
Figure 5.8. Current-voltage characteristics extracted from transfer length method (TLM) at a... 91
Figure 5.9. (a) Transmittance values with respect to the metal-doped graphene sheets formed... 92
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