Vol 18 No 7, July 2009
c
° 2009 Chin. Phys. Soc.
1674-1056/2009/18(07)/3014-04
Chinese Physics B
and IOP Publishing Ltd
The improvement of Al
2
O
3
/AlGaN/GaN MISHEMT
performance by N
2
plasma pretreatment
∗
Feng Qian(冯 倩)
†
, Tian Yuan(田 园), Bi Zhi-Wei(毕志伟), Yue Yuan-Zheng(岳远征),
Ni Jin-Yu(倪金玉), Zhang Jin-Cheng(张进成), Hao Yue(郝 跃)
‡
, and Yang Lin-An(杨林安)
School of Microelectronics, Xidian University, Xi’an 710071, China
Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices, Xidian University, Xi’an 710071, China
(Received 31 October 2008; revised manuscript received 25 December 2008)
This paper discusses the effect of N
2
plasma treatment before dielectric deposition on the electrical performance
of a Al
2
O
3
/AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor (MISHEMT), with Al
2
O
3
deposited by atomic layer deposition. The results indicated that the gate leakage was decreased two orders of magnitude
after the Al
2
O
3
/AlGaN interface was pretreated by N
2
plasma. Furthermore, effects of N
2
plasma pretreatment on the
electrical properties of the AlGaN/Al
2
O
3
interface were investigated by x-ray photoelectron spectroscopy measurements
and the interface quality between Al
2
O
3
and AlGaN film was improved.
Keywords: Al
2
O
3
/AlGaN/GaN MISHEMT, atomic layer deposition, N
2
plasma pretreatment
PACC: 7340N, 7340Q
1. Introduction
The gate leakage current is an important factor
that limits the performance and reliability of conven-
tional high-power AlGaN/GaN high electron mobility
transistors (HEMTs).
[1]
Thus, a thin dielectric oxide
layer is often inserted between the gate metal and Al-
GaN barrier layer leading to the approach of the MOS
HEMT.
[1−7]
Among them, Al
2
O
3
is considered to be
one of the potential candidates because of its relatively
high dielectric (high-k) constant and wide band gap.
However, due to the difficulty in preparing an
atomically cleaned surface before the deposition of
the dielectric layer, surface pretreatment is required.
Many works have reported how to improve the in-
terface properties using ammonia (NH
3
) pretreatment
before atomic layer deposition (ALD) of high-k mate-
rials on Si or Ge substrates.
[8,9]
However, there is no
report on atomic-layer-deposited Al
2
O
3
on a plasma
pretreated AlGaN substrate.
In this work, the electrical performance of an
Al
2
O
3
/AlGaN/GaN metal-insulator-semiconductor
(MIS) HEMT with Al
2
O
3
deposited on acid-cleaned
and N
2
plasma-treated AlGaN surface is presented.
Then, in this work we also take C–V measurements
and an x-ray photoelectron spectroscopy (XPS) mea-
surement to study the impact of this technological
procedure on the interface quality between the Al-
GaN epilayer and the Al
2
O
3
film.
2. Device fabrication
The schematic illustration of a cross-sectional
view of the fabricated device is shown in Fig.1. The
AlGaN/GaN heterojunction structure was grown on a
5 cm c-face sapphire substrate by metal-organic chem-
ical vapour deposition (MOCVD). This layer structure
consisted of a 1 µm thick undoped-GaN layer and a
25-nm Al
0.3
Ga
0.7
N barrier layer. Its measured room-
temperature Hall mobility and sheet carrier concentra-
tion, they were 1230 cm
2
/(V·s) and 1.0×10
13
cm
−2
,
respectively.
Electrical isolation was made by forming a mesa
structure with inductively coupled plasma etching
with chlorine gas. In order to form the source and
drain ohmic electrodes, Ti/Al/Ni/Au were deposited
by electron beam evaporation and annealed by rapid
thermal annealing at 830
◦
C for 30s in nitrogen gas
ambient. Prior to the Al
2
O
3
deposition, one of the
wafers was etched in HF : HCl : H
2
O = 1 : 1 : 8 solution
∗
Project supported by National Advanced Research Program (Grant No 51308030102) and Xi’an Applied Materials Innovation
Fund (Grant No XA-AM-200616) and National Natural Science Foundation of China (Grant Nos 60506020 and 60676048).
†
E-mail: qfeng@mail.xidian.edu.cn
‡
Corresponding author. E-mail: yhao@xidian.edu.cn
http://www.iop.org/journals/cpb
http://cpb.iphy.ac.cn