DESIGN AND DEVELOPMENT OF GAN BASED MONOLITHIC TUNABLE MULTI-WAVELENGTH LEDS

Abstract

Thesis work presents the design of two novel monolithic LED device structures and the deposition of Gallium Nitride (GaN) on various substrates. Recent achievements of 40% Indium concentrations in processing of InGaN layers made it possible for researchers to design and develop superior semiconductor devices. Current LED technologies for multicolor and white light emissions suffer from complex device structures, shorter lifetime and higher cost. In order to overcome these issues, monolithic multi junction LEDs structures are actively under research and development stage. Y. D. Qi et al. have fabricated dual wavelength LED by MOVPE which emits blue and green photons of constant intensities. David B. Nicol has also reported a First Generation two terminal multi-junction LED structure of blue (λ=460nm) and UV (λ=400 nm) emissions. However, this device was destroyed within its operating range. Therefore the multi-junction LED structures are required further investigations and optimization in order to produce more reliable monolithic multi-color LEDs. Tunable Dual Emission Layers (DEL) LED structures are studied in this work. The emission layers are based on InGaN and GaN semiconductors which are separated by a barrier layer. With appropriate Indium contents, the emission layers can be designed for a wide range of colors. If the structure has been carefully designed then the individual emission can be mixed and generate desired color of the emitting light. Emitted colors can also be controlled externally with applied potentials. Under a first forward biasing condition (VFB1), carriers are trapped within the Top Emission Layer (TEL). The barrier layer blocks the direct flow of the carriers through the remaining part of the device. Further increase of external bias reduces the barrier height which allows the carriers to flow towards the Bottom Emission Layer (BEL). For the safe and reliable operation device structure has to be design carefully before it gets fabricated. In this work, white and multicolor LED structures are designed and simulated with the industrial standard SILVACO Technology Computer Aided Design (TCAD) simulation software. Where the simulation studies of the optical devices has already been validated with experimental measurements. vi Deposition of polycrystalline GaN on a low cost substrate is the active area of research. Highly doped and Low cost GaN can play an important role to cut down cost of wide range of domestic and industrial products. As GaN is a direct band gap material, it is also a best candidate for the development of highly efficient opto-electronic devices. In this work, silicon doped GaN is grown different substrates. These substrates are glass, silicon and sapphire. The deposition process is carried out using “e-beam assisted thermal evaporation technique”. The evaporation process is carried out NILOP (PAEC). A very thin GaN buffer layer (20 nm) is grown prior to the deposition of GaN layer. The buffer layer mitigates lattice mismatch. It also reduces thermal strain in the film on post-growth cooling of the structure. GaN is evaporated under N2 environment using thermal resistance evaporation. Nitrogen environment suppresses decomposition of GaN target material into Ga and N. Silicon is used as a doping material in this evaporation process to achieve n-type doped GaN. The covalent radii of Ga and Si match with each other. The activation energy required for Si to obtain n-type doped GaN is very low. Melting point of Si is very high. So e-beam evaporation system is employed to thermally evaporate Si material. Microscopy, AFM, XRD characterization, and Hall Effect Measurement characterization of the as grown samples is carried out at SCME, NUST.