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Document Sample
Effects of thermal annealing in ion-implanted
Gallium Nitride
F. Iucolano, F. Giannazzo, F. Roccaforte, V. Puglisi, M. G. Grimaldi, V. Raineri
National Research Council - Istitute for Microelectronics
and Microsystems (CNR-IMM), Catania-Italy
ST Microelectronics, Catania-Italy
University of Catania - Department of Physics and Astronomy
October, 4th 2007
IMM 1
Outline
•Introduction
•Rapid thermal annealing:
- for selective n-type doping on GaN
- for p-type compensation on GaN
- for device isolation
•Conclusion
IMM 2
Properties and applications of GaN
Property Si 4H-SiC GaN
Bandgap(eV) 1.12 3.2 3.4
Breakdown field EB(MV/cm) 0.25 3 4
Electron mobility µ(cm2/Vs) 1350 800 1300
Saturation velocity 1 2 3
vs(107cm/s)
Thermal conductivity χ 1.5 4.9 1.3
(W/cmK)
Dielectric constant ε 11.8 9.7 9.0
Direct band gap Optoelectronics devices
High critical field High power and high frequency devices
IMM 3
Motivation
Improvement of GaN crystalline quality in the last years Power and Frequency devices
Power Schottky Diodes High Electron Mobility
Transistor (HEMT)
Schottky barrier Ohmic contacts
S G D
AlGaN barrier
n- i-GaN Channel
GaN 2-DEG AlN Buffer
n+
Al2O3 Al2O3
P-type compensation Device Isolation
Selective n-type doping
IMM 4
Selective n-type doping
Commonly, Si-ions implantation in GaN is used for n-type doping.
Selective n-type doping permits to increase the carrier concentration under the contact
=>Formation of low resistance ohmic contact
Annealing at temperatures >1200°C required for the electrical activation, but they lead to a
degradation of the surface morphology (roughness, GaN decomposition).
Use of appropriate cap layers (AlN or SiO2 or Si3N4) on surface to prevent the degradation of the
surface .
IMM 5
Quantitative determination of the electrical active dopant
profile in GaN
Secondary ions mass spectrometry (SIMS) and Hall measurements are used to determine the
Implanted dopant profile and Dopant electrical activation respectively.
However, no assessed profiling method for carrier concentration with suitable spatial
resolution (10 nm) and wide dynamic range (1015-1020 cm-3) is presently available for GaN.
Scanning capacitance microscopy (SCM)
Calibration Curve
1.5
SCM amplitude (a.u.)
SCM sensor Tip
SiO2 cap layer 1.0 Ga2O3
GaN
GaN 0.5
Al2O3 0.0
Metal Metal ~ 16 17 18 19 20 21
10 10 10 10 10 10
-3
Concentration (cm )
F. Giannazzo, D. Goghero, and V. Raineri, J. Vac. Sci. Technol. B 22 (2004) 2391.
IMM 6
Si Implantation
Si ions
n- GaN ~ 4.2×1016cm-3
(C-V measurements)
n- GaN n+ GaN Al2O3 n+ GaN ~ 1.7×1018 cm-3
(Hall measurements)
80keV 7x1013 ioni/cm2
180keV 2x1014 ioni/cm2 Annealing process
13
80 keV (7x10 cm )
-2
Trim simulation 1 µm SiO2 capping layer was deposited to prevent
14
180 keV (2x10 cm )
13 -2
-2
14 -2
surface degradation during annealing
80 keV (7x10 cm )+ 180 keV (2x10 cm )
19
1.6x10
1100°C, 1hr, furnace annealing (ramp rate 5°C/min), N2
Concentration (ions/cm )
19
2
1.4x10
19
environment
1.2x10
19
1.0x10 1200°C, 1hr, furnace annealing (ramp rate 5°C/min), N2
18
8.0x10 environment
18
6.0x10
1100°C, 60 s, RTA (ramp rate 100°C/s), N2
18
4.0x10
environment+1200 °C, 1 hr, furnace annealing (ramp
18
2.0x10 rate 5°C/min), N2 environment
0.0
0 50 100 150 200 250 300 350
IMM Depth (nm) 7
Surface Morphology
As-grown RMS = 0.2nm
5.0nm Implanted + 1200°C 1h not protected
20.0nm
5.0nm 0.0µm 5.0µm
0.0µm 5.0µm
Implanted + 1200°C 1h protected
with SiO2 RMS = 0.4nm
0.0µm 5.0µm
The surface morphology almost completely preserved using a SiO2 capping layer
IMM 8
SCM on as-implanted samples
2.8µm 0.9µm
n- GaN n+
Al2O3
GaN
Implant profile
n- GaN n+ GaN Al2O 3 As-grown
10 ~2.8µm thick region with dC/dV~ 1 ⇒ n- doped GaN epilayer
5 ~0.9µm thick region with dC/dV~ 0.5 ⇒ n+ doped GaN buffer layer
(a) region deeper than 3.8 µm with |dC/dV|~0 ⇒ insulating Al2O3
0
Width (µm)
10 As-implanted (3° tilt angle)
⇒
5 ~1.9µm thick region with dC/dV~ 0
(b) implantation induced electrically inactive GaN
0
0 1 2 3 4
Depth (µm)
0 -0.5 -1
10 angle)
As-implanted (10° tilt ⇒
5 ~1.5µm thick region with dC/dV~ 0
0 implantation induced electrically inactive GaN
0 1 2 3 4
The deactivated region is much larger then the calculated implantation profile (TRIM simulation,
~240 nm). Occurrence of ion-channelling effects
IMM 9
Rapid Thermal Annealing for Si electrical activation in GaN
Implanted
region n- GaN n+ GaN Al2O3
0.0
-0.5 1100°C, 1hr (furnace)
SCM signal (a.u.)
(a)
-1.0
0.0 In all three samples the electrically
-0.5 1200°C, 1hr (furnace) active Si profile extends within 0.5µm.
(b)
-1.0
0.0
-0.5 1100°C, 60s (RTA) + 1200°C, 1hr (furnace)
(c)
-1.0
0 1 2 3 4
Depth ( µm)
1100°C 1h
19
10 13 -2
(4.8x10 cm , 18%)
Concentration (cm )
-3
1200°C 1h
13 -2
(9.7x10 cm , 36%) The 1100°C RTA pre-annealed sample
1100°C 60s + 1200°C 1h
10
18 14 -2
(1.7x10 cm , 63%)
shows a significantly higher activation
(63%) than the sample annealed at 1200°C
only (36%).
17
10
0.0 0.2 0.4 0.6
Depth (µm)
IMM 10
Rapid pre-annealing process effect
2.8µm 0.9µm
n- GaN n+
Al2O3
GaN
Implant profile
10
5 The surface electrically inactive
As-implanted
region (|dC/dV|=0) was reduced from
0
∼1.9 µm to ∼0.3 µm (removal of
n- GaN n+ GaN Al2O3 defects).
Width (µm)
10
5 1100°C RTA
0
0 1 2 3 4
Depth (µm)
0 -0.5 -1
SCM signal (a.u.)
Ion beam damage is reduced by the rapid thermal annealing.
IMM 11
P-type Compensation
Commonly, Mg-ions implantation in GaN is used for p-type doping.
Fabrication of p-n junction, edge termination in power rectifier (guard ring).
Compensation of the donors in the difficult to obtain a p-type compensation
GaN epilayer (substitutional Si
and some implant related defects) Rapid annealing at high temperatures
by the Mg acceptors
Trim simulation
19
Mg ions 8.0x10
+ 14 -2
50 keV Mg Φ=5x10 cm
Concentration (at/cm )
3
19
6.0x10
n- GaN n+ GaN Al2O3 19
4.0x10
19
2.0x10
50keV 5x1014 ioni/cm2
0.0
0 500 1000 1500 2000
Depth (A)
IMM 12
Rapid Thermal Annealing for p-type compensation
1150°C 30s
Mg implant 1150°C 60s
1200°C 30s
1.0 SCM signal < 0 n-type doping
Depletion
region
0.5 SCM signal > 0 p-type doping
SCM signal (a.u.)
0.0
By increasing the annealing
-0.5 EJ n- type temperature and/or time, an increasing
GaN
electrical activation of Mg dopants is
-1.0 obtained.
0 1 2
Depth (µm)
Only after the rapid thermal annealing at 1200°C compensation is achieved.
IMM 13
Device Isolation
Commonly, ions implantation in GaN is used to obtain isolation.
S G D
AlGaN barrier
i-GaN Channel
2-DEG AlN Buffer It is necessary to fabricate the device isolation
Trim simulation
Al2O3
18
6x10
N-concentration (at/cm )
3
N-ions
18
5x10
18
4x10
AlGaN
i-GaN
Al2O3
18
3x10
18
2x10
18
1x10
20, 40, 80 keV 0
0 25 50 75 100 125 150 175 200 225 250
1×1013, 2×1013, 5×1013 cm-2 Depth (nm)
Rapid annealing
It is necessary to obtain a good and thermally stable isolation at 700-800°C
IMM 14
Conclusion
RTA for selective n-type doping on GaN
Rapid pre-annealing process at 1100°C and the 1200°C furnace annealing process could be
significantly improved the dopant activation to 63% of the Si implanted fluence because RTA
removes the defects causing the electrical deactivation.
RTA for p-type compensation on GaN
Mg-ions implantation and rapid thermal annealing at 1200°C permit to obtain a
compensation, which is applied to decrease the leakage current in the Power Schottky diodes.
RTA for device isolation on GaN
N-ions implantation and rapid thermal annealing at 750°C permits to obtain a good and
thermally stable isolation, which is applied in the HEMT device.
IMM 15
Acknowledgment
CNR-IMM University
F. Roccaforte M. G. Grimaldi
V. Raineri L. Romano
F. Giannazzo
S. Di Franco
A. Marino
A. Alberti ST Microelectronics
P. Fiorenza
F. Ruffino V. Puglisi
M. Camalleri
IMM 16
