The Control Of Polarity In ?-nitride Materials And Its Application In UV-LED Devices

Ultra-Violet Light-emitting diodes based on AlGaN ?-nitride semiconductors offer many advantages including miniaturization,reliability,reduced costs,low power consumption,and ultimately a choice of wavelength of operation between 365 and 200 nm.? nitride compounds are widely used for LED structures because of their recombination.However,UV-LED has relatively low external efficiency because of poor Ohmic contact and also low light extraction efficiency between n-AlGaN and metal Ohmic contact is very important in realizing high efficiency LEDs because Ohmic contact provides a single direction current flow without barrier heights.In order to solve these two problems,novel design of LED structure is necessary.It is found that lateral polarity structures(LPS)can realize better Ohmic contact.And with proper control of polarity,we can also fabricate damage-free nanopillar structures to increase light extraction efficiency.This project is divided to 4 parts:A background of this research including LPS and schottky barrrier diode(SBD),Metal organic Chemical Vapor Deposition(MOCVD)the latest development of optoelectronics and electronic devices,Polarity control in?-nitride and its applications are given in Chapter 1LPS Structure GaN was grown on sapphire substrate with patterned AIN buffer layers.Surface morphologies and chemical etching susceptibility of Ga-polar and N-polar GaN were compared.Schottky barrier diodes have been fabricated based on LPS GaN utilizing high n-type conductivity N-polar for Ohmic contact formation and relatively higher resistivity Ga-polar GaN for Schottky contact formation.Compared to SBD fabricated on conventional undoped GaN,SBD fabricated on LPS-GaN demonstrated both increased forward current and reduced barrier height.With oxide passivation on the surface,reverse leakage current reduced by 2 orders of magnitude,demonstrating the annihilation of surface traps on the surface of LPS.The application of LPS in design of SBD provides an alternative way in realizing lateral n+/n-devices without intentionally doping,the results are shown in chapter 2We used nanostructures to improve light extraction of LED devices.The nanoscale ? polar and N-polar AlGaN/GaN multiple quantum wells were grown simultaneously on patterned AIN buffers using MOCVD.RIE damage-free nanostructures were achieved by wet chemical etching of the LPS.PL data indicates that there is an enhancement of emission intensity in wet etched LPS nanostructure compared to conventional RIE nanostructure.So,it is concluded that that LPS can be a promising technique in realizing high efficiency nanostructure UV-LEDs,the results are given in chapter 3The light extraction of LED devices was improved using networking Ni/Au contact on p-GaN contact layer.It is demonstrated that the Ohmic contact and characteristics of current-voltage was improved by using network structure.It was showed that networked Ni/Au contact LED structures improved the light extraction Though,more experiments are required to find the optimum networking conditions and performing of this technique in LED devices,the results are given in chapter 4.