Download Properties of HfO 2 Deposited on AlGaN/GaN Structures Using e

Properties of HfO2 Deposited on AlGaN/GaN Structures
Using e-beam Technique
V. Tokranov a, S. Oktyabrsky a, S.L. Rumyantsev b, M.S. Shur b, N. Pala
b,c,
R. Jain c, J. Yang c and R. Gaska c,*
a) Coll. of Nanoscale Sci. & Eng., Univ. at Albany-SUNY, NY
b) Rensselaer Polytechnic Institute, Troy NY 12180-3590
c) Sensor Electronic Technology, Inc. 1195 Atlas Road Columbia, SC 29209
*Corresponding author: [email protected]
Motivation
Leakage Current
 GaN/AlGaN HEMTs suffer from poor long-term stability and the
-2
gate leakage current of the Schottky gate devices.
10
 Gate leakage increases the sub threshold current and shunts the
gate to channel capacitance, which decrease the maximum output
power.
10
-3
-4
10
-5
2
10
|I|, A/cm
-6
 Insulated gate devices have reduced leakage current and can
operate at positive gate voltage, yielding higher saturation current
and higher output RF power.
10
-7
10
-8
10
-9
10
 However, current gate insulation techniques cause decrease of
the device transconductance and increase of the threshold voltage.
-10
10
-10
-5
0
C-V Characteristics
-7
5x10
As deposited HfO2
o
HfO2 annealed @ 650 C
Control sample
-7
C, F/cm
2
Small leakage current in both directions should allow for the
transistor operation both at negative and positive gate voltages [1].
Capacitance and Conductivity
-7
-2
4x10
10
6
10 Hz
5
5x10 Hz
-3
3
10
10 Hz
-7
-4
10
G, S
C, F/cm
2
3x10
-7
2x10
-5
10
6
10 Hz AnnelaedHfO2
-7
5
1x10
5x10 HzAnnelaedHfO
-6
10
6
10 Hz Control sample
Annealed HfO2
5
5x10 Hz Control sample
-7
0
10
-8
-6
-4
V, V
-2
0
-8
-6
-4
V, V
-2
0
The conductivity above threshold is practically the same for both
samples [1].
Conclusion
The dielectric constant of the HfO2 on AlGaN was found εHfO>23-24,
which is close to the highest reported values for this material. No
dielectric losses comparable with losses related to the 2D gas and
small leakage current were found. The conductance measurements
indicate low concentration of the interface traps in comparison with
electron concentration on the AlGaN/GaN interface.
-7
6x10
10
V, V
 The solution of the problem might be using the high-k dielectrics.
Experimental Details
The AlGaN/GaN heterostructures were grown by MOCVD on
sapphire. They consisted of a 50-nm-thick AlN buffer layer, 0.4-mmthick undoped GaN layer, followed by Al0.2Ga0.8N barrier layer, which
was doped with silicon to approximately 21018 cm-3. The Hall
mobility and concentration of the 2d gas on the interface were
μ=1600cm2Vs and n=1.33cm-2, respectively. The HfO2 thin film was
deposited by a reactive e-beam evaporation of Hf with O2 flow
directed from manual leak valve to a substrate. The SPECS EBE-1
(a mini e-beam evaporator from SPECS GmbH) with flux stabilization
was used for the Hf evaporation. A custom design process chamber
was pumped down to 5x10-11 Torr. The process pressure was defined
by flowing pure O2 and maintained at 10-6 Torr. The distance between
the Hf source and the substrate was about 7 cm. The thickness of
the deposited HfO2 was dHfO10nm.
5
4x10
-7
3x10
-7
2x10
-7
1x10
0
-10
-5
0
5
V, V
10
15
Control Sample:
dAlGaN20nm (from C at V=0)
Vt= -5.4V
n=C0Vt/q=1.251013cm-2 (in agreement with the Hall measurements.)
As deposited HfO:
εHfO23-24
After [1].
Annealed HfO:
εHfO26 (or higher)
20- 30% thickness reduction
Acknowledgements
The work at CNSE, SUNY at Albany has been supported by the
Materials Structures and Devices Focus Center (MSD Focus Center).
The work at RPI has been supported by ONR (Project Monitor Dr.
Colin Wood) and by the National Science Foundation under
“Connection one” I/UCR center. The AlGaN/GaN transistors
development at SET, Inc. has been supported under SBIR Phase II
contracts from DARPA (M. Rosker) and ARL (P. Shah).
References
1. V. Tokranov, S.L. Rumyantsev, M.S. Shur, R.Gaska, S. Oktyabrsky, R. Jain,
N. Pala, “The HfO2/AlGaN/GaN structures with HfO2 deposited at ultra low
pressure using e-beam” phys.stat.sol. (RRL) 1, No. 5, 199-201 (2007)