On the optical and microstrain analysis of graded InGaN/GaN MQWs based on plasma assisted molecular beam epitaxy

Pawan Mishra,Tien Khee Ng,Ahmed Y Alyamani,Dalaver H Anjum,Bilal Janjua,Munir M El-Desouki,Chao Shen,Rami T Elafandy,Aditya Prabaswara,Abdelmajid Salhi,Boon S Ooi

doi:10.1364/ome.6.002052

Abstract

In this paper, c-plane stepped- and graded- InGaN/GaN multiple quantum wells (MQWs) are grown using plasma assisted molecular beam epitaxy (PAMBE) by in situ surface stoichiometry monitoring (i-SSM). Such a technique considerably reduces the strain build-up due to indium clustering within and across graded-MQWs; especially for QW closer to the top which results in mitigation of the quantum-confined Stark effect (QCSE). This is validated by a reduced power dependent photoluminescence blueshift of 10 meV in graded-MQWs as compared to a blueshift of 17 meV for stepped-MQWs. We further analyze microstrain within the MQWs, using Raman spectroscopy and geometrical phase analysis (GPA) on high-angle annular dark-field (HAADF)-scanning transmission electron microscope (STEM) images of stepped- and graded-MQWs, highlighting the reduction of ~1% strain in graded-MQWs over stepped-MQWs. Our analysis provides direct evidence of the advantage of graded-MQWs for the commercially viable c-plane light-emitting and laser diodes.

Highlights

Due to their vital importance in electronics and optoelectronics, the field of group-III nitrides, has gone through spectacular developments ranging from material development, all the way to device fabrication
C-plane stepped- and graded- InGaN/GaN multiple quantum wells (MQWs) are grown using plasma assisted molecular beam epitaxy (PAMBE) by in situ surface stoichiometry monitoring (i-SSM). Such a technique considerably reduces the strain build-up due to indium clustering within and across graded-MQWs; especially for QW closer to the top which results in mitigation of the quantum-confined Stark effect (QCSE)
This is validated by a reduced power dependent photoluminescence blueshift of 10 meV in graded-MQWs as compared to a blueshift of 17 meV for stepped-MQWs

Summary

Introduction

Due to their vital importance in electronics and optoelectronics, the field of group-III nitrides, has gone through spectacular developments ranging from material development, all the way to device fabrication. The performance of InGaN based LEDs at high injection current density is limited by efficiency droop To address this issue, it is important to understand the fundamental properties associated with the group-III nitride material system and InGaN/GaN multiple quantum wells (MQWs). Due to their wurtzite crystal structure, which lacks centrosymmetry, group-III nitrides have built-in spontaneous polarization fields These LEDs suffer from piezoelectric fields induced by the heteroepitaxial growth on conventional lattice-mismatched substrates such as c-plane sapphire and silicon carbide (SiC) and the large strain gradients caused by the lattice mismatched InGaN/GaN layers within the active region [4]. Optical characteristics and strain maps of the grown samples were further examined using power and temperature dependent photoluminescence, Raman spectroscopy, HR-STEM techniques, HAADF-STEM intensity profile, and GPA

Experimental

Optical characterizations

Geometrical phase analysis

Conclusion