SUBJECT: M.S. Thesis Presentation
   
BY: Shweta Natarajan
   
TIME: Thursday, October 18, 2012, 11:00 a.m.
   
PLACE: MARC Building, 201
   
TITLE: Thermal Metrology Techniques for Ultraviolet Light Emitting Diodes
   
COMMITTEE: Dr. Samuel Graham, Chair (ME)
Dr. Andrei Fedorov (ME)
Dr. Shyh-Chiang Shen (ECE)
 

SUMMARY

AlxGa1-xN (x>0.6) based Ultraviolet Light Emitting Diodes (UV LEDs) emit in the UV C range of 200 290 nm and suffer from low external quantum efficiencies (EQEs) of less than 3%. This low EQE is representative of a large number of non-radiative recombination events in the multiple quantum well layers, which leads to high device temperatures due to self-heating at the device junction. Knowledge of the device temperature is essential in order to mitigate optical degradation and lifetime reduction due to thermal overstress. The micro-scale nature of these devices and the presence of large temperature gradients in the multilayered device structure merit the use of several indirect temperature measurement techniques to resolve device temperatures. In this work, UV LEDs with AlxGa1-xN active layers, grown on sapphire or AlN growth substrates, were studied. Thermal metrology results will be presented for devices with interdigitated and micropixel electrode geometries. A comparative study of optical techniques such as Infrared (IR), micro-Raman and Electroluminescence (EL) spectroscopy for the thermal metrology of UV LEDs, will be presented. The presence of vertical temperature gradients within the device layers will be established using micro-Raman spectroscopy, while the occurrence of thermal anomalies such as hotspots and shorting paths will be studied using IR spectroscopy. The Forward Voltage method, an electrical junction temperature measurement technique, will also be investigated. The modified Thermal Resistance Analysis by Induced Transient (TRAIT) procedure, whereby electrical data at short time scales from an operational device is used to extract the package thermal resistance from the total junction- to-ambient heat path, will be discussed. The results of the modified TRAIT procedure applied to visible and UV LEDs will be presented. The scope and applicability of each thermal metrology and analysis technique will be examined.