Investigation and Fabrication of High Efficiency Nitride Based LED Devices

Abstract

In the dissertation we approached the high efficiency LED via both epitaxy growth and chip process Meanwhile we investigated the efficiency drop of LED at high current injection and the influence on LED output power while raising temperature And then during the aging experiment we found a potential failure mode for LED chip We proposed out our experiment results for reference in the design of LED chip In our study introducing Si delta-doping (Si-DD) layers on specified positions the interface of N layer and the first strain releasing multiple quantum wells the interface of the first strain releasing multiple quantum wells and the second strain releasing multiple quantum wells based on a well optimized LED grown on a planar substrate can greatly change the LED electric characteristics many times improvement of reverse bias was observed and improve the output power Paper research told us that remove the laser debris by high temperature solution and introducing the defect selective etching can result in truncate sidewalls surrounded the chip mesa to improve the extraction efficiency The author experimented with it on a standard sapphire substrate Instead of standard substrate we applied the defect selective etching on LEDs grown on the pattern sapphire substrate PSS We found the great improvement of film quality due to grow on PSS causes the failure of defect selective etching So we intended to make the buffer a little inferior via a high temperature AlN layer as the buffer layer to enable the defect selective etching Our experiment indicated that LEDs with HT AlN as buffer has a little poor ability against high temperature etching than LEDs with LT GaN as buffer Undercut sidewalls of mesa for LEDs with high temperature AlN buffer layer HT AlN were seen after dipping them in 250℃ H3PO4 solution for 10 minutes The change of mesa shape and the uncovered pattern sapphire cones play as the media to lead out more light from the chip Our data indicated a 12% output power enhancement at one ampere operation Besides we also investigate the influence of varying the barrier thickness on the efficiency of nitride LED operating at high current density The existing studies used to focus on the discussion of electron transportation With a simulation result the holes distribution within the multiple quantum wells might play more important role at high current injection We designed experiments with different barrier thickness Our experiment results suggest thinner the barrier better the efficiency at high current injection However it’s trade-off the lower efficiency at lower current density We thought the change of holes distribution in multiple quantum wells leads the efficiency poor at lower current density We also discussed on the mechanism of Hot-Cold Factor for LEDs running at different current density Different electron blocking layer thickness and different last barrier thickness were designed for our experiment The experimental results suggest both electron blocking layer and last barrier have significant influence on Hot-Cold Factor And we found a finite effect of EBL thickness to Hot-Cold Factor Instead our data suggest more significant effect the last barrier thickness is An extreme case the sample with six times LB thickness delivered a very superior Hot-Cold Factor the values is 1 049 at 120 mA In additions samples with thicker LB also keep such advantage at two times current density Our result indicated an improvement on Hot-Cold Factor can be achieved via design of LED structure Narrowing the metal finger width and using a reflector structure under metal fingers and pads were used to increase output power in the fabrication of LED chip And Al has been demonstrated as one of the best candidates for the reflector In our study we found both measures may cause failure due to the Al electro-migration under tough aging condition such as 80℃ 125 A/cm2 We found the Al whiskers emerge out usually at certain position of metal fingers The whisker length will increase with aging time Eventually the whiskers touch other parts of chip leading the chip short Al migration such as a failure mechanism that has to be taken into account in the design of HV-LEDs

Date of Award	2014 Aug 5
Original language	English
Supervisor	Shoou-Jinn Chang (Supervisor)