GaN microstructures and nanostructures grown on graphene for transferable device applications

학위논문(박사)--서울대학교 대학원 :자연과학대학 물리·천문학부,2015. 2. 이규철.Current inorganic semiconductor devices, including light emitting diodes (LEDs), are based mostly on groundbreaking achievements in semiconductor technology, such as fabrication of heterostructures used in high performance devices (Nobel Prize in 2000) and invention of efficient LEDs (Nobel Prize in 2014). Recently, due to the demand for electronic and optoelectronic devices with large-size and flexibility, many efforts have been made to fabricate the devices in a foldable form using inorganic, organic semiconductors, and their hybrids. For the fabrication of high performance devices, inorganic semiconductors have many advantages over organic semiconductors because of their high radiative recombination rates, high carrier mobility as well as long term stability and reliability. However, problems associated with high-quality inorganic material preparation on flexible and large-size substrates are one of the major obstacles to use inorganic semiconductors in flexible and large-scale device applications. In particular, the limited size and rigidity of typical inorganic semiconductor growth substrates require further developments of novel material system and device fabrication process suitable for the recent trend. Here, this dissertation introduces a novel material system of hybrid heterostructure, which composed of epitaxial inorganic semiconductor thin films and catalyst-free one dimensional (1D) micro- and nanostructures directly grown on graphene films, for transferable and flexible inorganic semiconductor device applications. This thesis consists of 7 parts. Following by general introduction in chapter 1, chapter 2 reviews preparation methods and characteristics of graphene briefly and discusses about graphene as an inorganic growth substrates for transferable and flexible device applications. In chapter 3, the detailed experimental set-ups and procedures, including growth, fabrication and characterization methods, are described. Chapter 4 presents a method to grow epitaxial GaN films on graphene films. GaN films were grown on various graphene films, such as mechanically exfoliated graphene layers from the graphite powder and chemical vapor deposition grown graphene films. The optical and structural characteristics of GaN films grown on various graphene substrates are also discussed. In addition to the GaN films, 1D GaN micro- and nanorods were grown on graphene films with high optical and structural quality, which are described in chapter 5. Using these hybrid heterostructures, transferable, large-scale, and flexible optoelectronic devices were fabricated in chapter 6. Finally, chapter 7 presents the summaries of this thesis with suggestion for future works.Table of Contents Abstract Table of Contents List of Figures List of Tables Chapter 1. Introduction Chapter 2. Background and literature survey 2.1. Preparation methods and characteristics of graphene films 2.2. Inorganic semiconductor growths on graphene 2.3. Methods to fabricate transferable devices Chapter 3. Experimental methods 3.1. Metal-organic chemical vapor deposition 3.1.1. Gas delivery system 3.1.2. Growth chamber and substrate heating 3.1.3. Low pressure pumping and exhaust system 3.1.4. Gas, reactants, and dopants 3.2. Growth techniques 3.2.1. GaN films on sapphire substrate 3.2.1.1. Un-doped and n-type doped GaN films 3.2.1.2. Light-emitting diodes 3.2.2. GaN films on graphene 3.2.2.1. GaN films on mechanical exfoliated graphene layers 3.2.2.2. GaN films on CVD graphene films 3.2.2.3. Epitaxial lateral overgrown GaN films on CVD graphene dot arrays 3.2.3. One dimensional GaN micro- and nanostructures on graphene films 3.3. Material characterization 3.3.1. Structural characterization 3.3.2. Optical characterization 3.3.3. Device characterization 3.3.3.1. Light emitting diodes 3.3.3.2. Photovoltaic devices Chapter 4. Epitaxial growth and characterization of GaN films grown on graphene films 4.1. Introduction 4.2. Epitaxial growth of GaN thin films on graphene films 4.2.1. Growth methods 4.2.2. Surface morphologies of GaN films grown on mechanically exfoliated graphene layers and on chemical vapor deposited graphene films 4.3. Structural characteristics 4.4. Optical characteristics 4.5. Summary Chapter 5. Growth and characterization of one dimensional GaN micro- and nanorods on graphene films 5.1. Introduction 5.2. Growth methods of GaN micro-rods on graphene films 5.2.1. Growth methods and general morphology 5.2.2. Effects of growth temperature 5.2.3. Vertical growth of GaN micro-rods on graphene films 5.3. Structural characteristics 5.4. Optical characteristics 5.5. Summary Chapter 6. Device applications of GaN/ graphene hybrid heterostructures 6.1. Introduction 6.2. Transferable GaN thin film LEDs on graphene layers 6.2.1. Fabrication and transfer of LEDs grown on graphene layers 6.2.2. EL and electrical characteristics 6.3. GaN thin film LEDs fabricated on amorphous substrates 6.3.1. Thin film LEDs directly fabricated on amorphous silica substrates 6.3.2. LED fabrications 6.3.3. EL and electrical characteristics 6.3.4. Output powers of LEDs fabricated on CVD graphene films 6.4. Flexible LEDs using GaN micro-rods grown on graphene films 6.4.1. Coaxial InxGa1–xN/GaN quantum well structures 6.4.2. Flexible LED fabrications 6.4.3. EL characteristics 6.4.4. Flexibility under bending conditions 6.5. GaN/graphene heterostructure array 6.5.1. Transferable GaN thin film LED array 6.5.1.1. Device fabrications 6.5.1.2. EL and electrical characteristics 6.5.2. GaN micro-disk arrays fabricated on graphene dot patterns 6.5.2.1. Selective area growth of GaN micro-disks on graphene dot arrays 6.5.2.2. Electrical characteristics of GaN/graphene heterojunction 6.5.2.3. Device fabrications 6.5.2.4. LED characteristics 6.5.2.5. Photovoltaic characteristics 6.6. Summery Chapter 7. Concluding remarks and outlooks 7.1. Summary 7.2. Future works and prospects for GaN-based devices fabricated on graphene films References Appendix. Position controlled growth of GaN micro-rods Abstract in Korean Curriculum VitaeDocto