Combined use of in-situ curvature and full- wafer ... - Laytec

Combined use of in-situ curvature and fullwafer
temperature profile measurements for
GaN growth on 6-inch Si substrates
D. Zhu, C. McAleese, M.J. Kappers and C.J. Humphreys
Department of Materials Science and Metallurgy
University of Cambridge, UK
dz218@cam.ac.uk
1

Motivation
A potential low-cost manufacturing route for GaN based solid-state lighting
Advantages:
Issues:
� Availability of large size substrates at lower cost
� Much lower fabrication costs of the devices
� Compatibility with standard Si processing equipment
� Higher thermal conductivity than sapphire
� Possibility of vertical contact LEDs on conducting silicon
� Possibility of integrating electronics and optical devices
� Large thermal expansion mismatch (Si: 54% that of GaN)
� Large lattice mismatch (lattice constant Si: 17% less than GaN)
Resulting in:
� Tensile strain and high dislocation density in epitaxial GaN
� Wafer bowing and/or cracking, making processing difficult
2

LED structure on Si substrate
Schematic LED structures
on Si substrates
Mg-doped GaN
InGaN/GaN MQW
(Si-doped) GaN
AlGaN buffer layer
AlN nucleation layer
Si (111) substrate
MOVPE growth:
� Stress control: AlGaN buffer
In-situ monitoring:
� LayTec Epicurve ® TT: wafer curvature, true
temperature and reflectance
� Aixtron Argus: temperature profiler
Ex-situ characterisation:
� XRD: strain state and quality, quantum well structure
� TEM: TD analysis
� AFM: Surface morphology and TD analysis
� PL & EL: MQW and LED performance
� Graphic-EYE: whole wafer optical imaging
� KSA Multi-Beam optical sensor (MOS) ultra-scan TM :
ex-situ wafer curvature
3

In-situ curvature measurement - EpiCurve
EpiCurve measurement principle
(after LayTec Epicurve manual)
Etalon
Beam
splitter
Laser
Susceptor
Detector
Wafer
4

Curvature evolution during growth of an LED structure on Si
Process temperature (Tc)
Curvature/km -1
1200
1000
800
600
400
200
50
0
-50
-100
-150
0
0.00
0 5000 10000 15000 20000
0 5000 10000 15000 20000
Time (s)
Concave
Convex
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
Reflectance (a.u.)
LayTec Epicurve ® TT
After cooling:
Mg-doped GaN
Si-doped GaN
AlGaN
AlN
Si substrate
5

More about stress control
Curvature/km -1
50
0
-50
-100
-150
in-situ annealing
AlN nucleation
layer (300 nm)
AlN nucleation
layer (200 nm)
AlGaN buffer (680 nm)
Si-doped GaN (750 nm)
AlGaN buffer (1.3 µm)
InGaN-GaN
MQW
0 5000 10000 15000 20000
Si-doped GaN (1.5 µm)
Time (s)
Cooling
InGaN-GaN
MQW
LED1
LED2
Concave
Convex
The optimised buffer layer thickness will depend on the thickness of GaN layer required
QW active region was grown on a convex bowed wafer.
Cooling
6

Graphic-EYE whole wafer optical imaging
Badly-cracked sample with extended
cracks across the wafer
150 mm
Cracks show up as bright lines in the image with very strong contrast
Crack-free GaN wafers have been achieved
Generally crack-free sample with cracks
only at extreme edges of wafer
150 mm
6 mm
Courtesy of QinetiQ
7

Additional growth issues: slip in Si (111) wafers
Two sets of thermal stresses considered, arising from:
Silicon wafer
Through-thickness temperature variations
Radial temperature variations
Stresses are resolved onto the various slip systems: {111}
Radial temperature variations
Through-thickness
temperature
variations
At 1000 ºC, with a temperature difference of 10 ºC across the wafer thickness, the
maximum stress and resulting curvature are calculated to be:
0.4 MPa, 16 m for 2-inch wafer, 275 µm thick
0.6 MPa, 38 m for 6-inch wafer, 650 µm thick
not sufficient by itself to cause slip – although it may enhance radial T variations.
8

Wafer curvature change
Temperature ( o C)
caused by through-thickness temperature variations
1100
1000
900
800
700
600
500
400
0 1000 2000 3000
Time (s)
Radius of wafer curvature changes from flat to ~30 m (concave) during temperature
ramp from room temperature to 1100 ºC
40
20
0
Curvature/km -1
RT 1100 ºC
9

Shear stress on the (1-11)[10-1] and (1-11)[0-1-1] slip system
10 ºC Radial temperature variations
Temperature ( o C)
Temperature ( o C)
1005
1000
995
990
1005
1000
995
990
Temperature profile 1
-10 0 10 20 30 40 50 60 70 80
Radial distance (mm)
Temperature profile 2
0 10 20 30 40 50 60 70 80
Radial distance (mm)
Wafer centre Wafer edge
Max stress ~2 MPa
Max stress ~1 MPa
Stress calculations: courtesy of
Kirsten McLaughlin, Cambridge
10

Monitoring the temperature profile across the whole wafer
Temperature ( o C)
Aixtron Argus temperature profiler
1200
1000
800
600
400
Wafer centre
Wafer edge
200
0 2000 4000 6000 8000
Time (s)
(d)
Susceptor
Si slips due to the temperature nonuniformity
across the wafer, resulting in
very bowed wafer.
Susceptor C B A B C
Wafer
1062
1052
1042
1032
1022
1012
1002
992
982
972
962
Temperature ( o C)
After Si slip
1060
1040
1020
1000
980
960
940
Centre
Edge
11

Maintaining flat temperature profile
LayTec Epicurve ® TT
Temperature ( o C)
Temperature ( o C)
1100
1000
900
800
700
600
500
Aixtron Argus
400
0 5000 10000 15000
1200
1000
800
600
400
200
Wafer centre
Wafer edge
Time (s)
0 1 2 3 4
Time (s)
80
60
40
20
0
-20
-40
-60
-80
-100
-120
Curvature/km -1
Susceptor
concave
convex
C B A B C
6-inch Si wafer
775
770
765
760
755
750
745
740
735
730
725
Temperature ( o C)
770
760
750
740
730
Centre
Edge
12

GaN-based LED looks good on 6-inch Si
Picture of an InGaN-GaN blue-emitting LED structure grown
on 6-inch Si, this wafer is flat and completely crack-free.
AFM image of p-GaN surface
PL peak emission wavelength:
Average: 448 nm;
Std. Dev. 7 nm
1 µm
PL intensity: Std. Dev. 12%
EL: YES, over whole wafer
Wafer bow:

Electroluminescence from 6-inch LED wafer
D. Zhu et al., “GaN-based LEDs grown on 6-inch diameter Si (111)
substrates by MOVPE”, Proc. SPIE 7231, 723118 (2009)
Processed at QinetiQ
Current (mA)
Photo diode current (a.u.)
30
25
20
15
10
5
0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
EL intensity (a.u.)
2500
2000
1500
1000
500
0
350 400 450 500 550 600 650
Wavelength (nm)
0 1 2 3 4
Voltage (V)
Photo diode current (a.u.)
0.3
0.2
0.1
0.0
0 10 20 30 40 50 60 70
Drive current (mA)
0 100 200 300 400 500
Drive current (mA)
14

Summary
� Flat and crack-free GaN-based LED structures have been grown on 6-inch Si (111)
substrates by MOVPE.
� LayTec Epicurve ® TT was used to measure the wafer curvature evolution during the
growth, and allows rapid optimisation of the AlGaN buffer layer for stress management.
� Temperature profiles over full 6-inch Si wafer were measured in real-time using Aixtron
Argus instrument, showing the changes across the wafer due to curvature. A flat
temperature profile can be maintained over the full LED run by adjusting the heater
zone settings accordingly.
� Combined use of in-situ curvature and full-wafer temperature profile measurements is
very useful for successful development of GaN growth on 6-inch Si substrates.
15

Acknowledgements
� Project partners
� Funding:
PARSEM
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