Increasing the hole energy by grading the alloy
composition of the p-type electron blocking layer
for very high-performance deep ultraviolet
light-emitting diodes
ZI-HUI ZHANG,
1,4
JIANQUAN KOU,
1
SUNG-WEN HUANG CHEN,
2
HUA SHAO,
1
JIAMANG CHE,
1
CHUNSHUANG CHU,
1
KANGKAI TIAN,
1
YONGHUI ZHANG,
1
WENGANG BI,
1
AND HAO-CHUNG KUO
2,3,
*
1
Institute of Micro-Nano Photoelectron and Electromagnetic Technology Innovation, School of Electronics and Information Engineering,
Hebei University of Technology, Key Laboratory of Electronic Materials and Devices of Tianjin, Tianjin 300401, China
2
Department of Photonics and Institute of Electro-Optical Engineering, Taiwan Chiao Tung University, Hsinchu 30010, China
3
Department of Electrical Engineering and Computer Sciences and TBSI, University of California at Berkeley, Berkeley, California 94720, USA
4
e-mail: zh.zhang@hebut.edu.cn
*Corresponding author: hckuo@faculty.nctu.edu.tw
Received 12 December 2018; revised 20 February 2019; accepted 20 February 2019; posted 21 February 2019 (Doc. ID 354950);
published 22 March 2019
It is well known that the p-type AlGaN electron blocking layer (p-EBL) can block hole injection for deep ultra-
violet light-emitting diodes (DUV LEDs). The polarization induced electric field in the p-EBL for [0001] oriented
DUV LEDs makes the holes less mobile and thus further decreases the hole injection capability. Fortunately,
enhanced hole injection is doable by making holes lose less energy, and this is enabled by a specifically designed
p-EBL structure that has a graded AlN composition. The proposed p-EBL can screen the polarization induced
electric field in the p-EBL. As a result, holes will lose less energy after going through the proposed p-EBL, which
correspondingly leads to the enhanced hole injection. Thus, an exte rnal quantum efficiency of 7.6% for the
275 nm DUV LED structure is obtained.
© 2019 Chinese Laser Press
https://doi.org/10.1364/PRJ.7.0000B1
1. INTRODUCTION
A deep ultraviolet (DUV) light source possesses wide applica-
tions in, e.g., air purification, water sterilization, and biosensing.
At the current stage, DUV photons are generated by mercury-
based fluorescent light emitters. Mercury threatens the health of
human organs, such as the nervous system and kidneys. The
Minamata Convention on Mercury was initiated in 2017, which
requires the elimination of the usage of mercury by the end of
2020 [1]. Therefore, the AlGaN-based DUV light-emitting
diode (LED) is emerging as an alternative. One of the very im-
portant benchmarks for evaluating DUV LED performance is
external quantum efficiency (EQE), which is currently still low
[1–4]. The low EQE partly arises from the low hole injection
efficiency for DUV LEDs. On one hand, the p-type AlGaN
layer has a Mg activation energy from 170 up to 630 meV de-
pending on the AlN composition [5]. The Mg doping efficiency
and the corresponding free hole concentration can be remark-
ably reduced in the Al-rich p-AlGaN layer, i.e., the Mg ioniza-
tion efficiency can be as low as ∼10
−9
[6]. The low Mg doping
efficiency furth er increases the surface depletion width for the
p-type contact nitride layer, and this gives rise to the very low
hole tunneling efficiency and hinders the hole injection capabil-
ity [7]. On the other hand, the energy band discontinuity be-
tween the p-AlGaN electron blocking layer (p-EBL) and the
p-AlGaN layer generates another barrier for hole injection
[8]. Note that the AlN composition for the p-EBL is larger than
that for the p-AlGaN layer. Accordingly, hole injection can be
improved by enhancing the hole tunneling efficiency at the in-
terface of the p-contact/p-type nitride layer, which is doable by
optimizing the alloyed metal and/or utilizing the metal/insula-
tor/semiconductor structure [7,9,10]. When the holes arrive at
the p-type hole injection layer, hole injection can be improved
by enhancing the hole injection, and this can be enabled by
adopting the three-dimensional hole gas (3DHG) [11–15].
The hole injection across the p-type hole injection layer can also
be promoted, as long as the holes are accelerated such that “hot”
holes are generated [8]. The next obstacle for hole transport orig-
inates from the p-EBL, and therefore, the valence band barrier
height shall be reduced for reducing the hole blocking effect
[16–19]. We also report that the [0001] oriented last quantum
Research Article
Vol. 7, No. 4 / April 2019 / Photonics Research B1
2327-9125/19/0400B1-06 Journal © 2019 Chinese Laser Press