Strains and stresses in GaN heteroepitaxy – sources and ... - Laytec

EpiCurve at production line
TSSEL reactor for GaN on Si
Armin Dadgar

Motivation

Motivation
- The lack of homo substrates requires GaN growth on
hetero substrates as sapphire, SiC or Si.
- This introduces lattice and thermal mismatch and thereby
stress in the layers.
- Stress can lead to the formation of dislocations and cracks.
- For the growth on cheap Si substrates thermal mismatch is
too large to achieve device-relevant crack-free GaN layer
thicknesses.

Motivation
Monitoring curvature and stress gives information on:
- crystallite size � tensile with decreasing size
- lattice mismatch and composition � tensile or compressive

Curvature (m -1 )
Motivation – determining composition
0.5
0.4
0.3
0.2
0.1
GaN Al ~0.24 Ga ~0.76 N
d AlGaN ~300 nm
r~4.0 m
σ~0.41 GPa
cracking
r~15 m, σ~0.12 GPa d AlGaN ~200 nm
0.0
2000 3000 4000 5000 6000
Time (s)
r~4.4 m, σ~0.39 GPa
From change in curvature: ε = 0.0185 � [Al] = 23.4%
60
50
40
30
20
10
Intensity (arb. u.)

Motivation
Monitoring curvature and stress gives information on
- crystallite size � tensile with decreasing size
- lattice mismatch and composition � tensile or compressive
- residual stress � thin film devices, processing
-curvature � optimizing (reducing) for processing

Homebuilt in-situ curvature sensor
on an AIX 200/4 RF-S
Curvature measurements
Distance (pixels)
285
280
275
270
265
260
255
250
245
240
heat up
AlN
GaN
raw data averaged over 1 s
AlN
GaN
AlN
GaN
InGaN
0 2000 4000 6000 8000 10000
Time (s)
GaN

Homebuilt in-situ curvature sensor
on an AIX 200/4 RF-S
Curvature measurements
Curvature (m -1 )
0.2
0.1
0.0
-0.1
-0.2
-0.3
curvature after averaging over 2s
0 2000 4000 6000 8000 10000
Time (s)
tensile
compressive

Homebuilt in-situ curvature sensor
on an AIX 200/4 RF-S
Curvature measurements
Intensity (arb. u.)
45
40
35
30
25
20
AlN
heat up
GaN
AlN
reflected intensity
GaN
AlN
0 2000 4000 6000 8000 10000
Time (s)
GaN
InGaN
GaN

Homebuilt in-situ curvature sensor
on an AIX 200/4 RF-S
Curvature measurements
film stress: Stoney equation
σ
f
=
6
κE
s
h
2
s
( 1 −ν
s ) h f
κ : curvature
E S:
Young´s modulus of the substrate
h s:
substrate thickness
h f:
layer thickness
ν : Poisson ratio of the substrate
G. Stoney, Proc. R. Soc. Lond. A 82, 172 (1909)

Curvature measurements in TSSEL reactors
First EpiCurve TT @TSSEL CCS reactor

Stress x thickness (GPa µm)
0.00
-0.25
-0.50
-0.75
-1.00
-1.25
-1.50
3.5 µm thick GaN-FET on Si
seed layer
cooling
0 25 50 75 100 125 150
Time (min)
LT-AlN
Crack-free, low stress FET structure on 2 inch Si

Curvature measurements in TSSEL reactors
Challenge: In TSSEL reactors optical access is limited
� slightly modified window with conical light path and larger
top window is advantageous (but standard view-port work).
Showerhead hole is unchanged! No influence on growth!
First EpiCurve set-up used at TSSEL:
single window used for multiple wafer curvature monitoring:
The two beams are closely spaced
� resolution limitted to: ± 10km -1
High rotation speed compared to Aix 200/4
� short time to get data, large jitter
� enhanced position determination needed (triggering to wafer centre)

Curvature measurements in TSSEL reactors
EpiCurve HighRes
@TSSEL CCS reactor
Resolution enhancement
by using
two view-ports

Stress x thickness (GPa µm)
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
150 mm GaN-LED on Si
concave bowing from heating
seed layer
LT-AlN interlayers
cooling
0 50 100 150 200 250
Time (min)
MQW
Crack-free LED structure on 150 mm Si
σ~0.15 GPa

Stress x thickness (GPa µm)
1.00
0.75
0.50
0.25
0.00
-0.25
-0.50
-0.75
150 mm GaN-LED on Si
GaN
compression
LT-AlN interlayers
-1.00
100 120 140
Time (min)
GaN
successful strain compensation

Stress x thickness (GPa µm)
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
150 mm GaN-LED on Si
concave bowing from heating
seed layer
LT-AlN interlayers
cooling
0 50 100 150 200 250
Time (min)
MQW
Cracked LED structure on 150 mm Si
σ~0.23 GPa

Stress x thickness (GPa µm)
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
150 mm GaN-LED on Si
GaN
LT-AlN interlayers
0.0
100 120 140
Time (min)
GaN
failure of strain compensation

Summary
• EpiCurve gives new insight into the growth of GaN
• It is suited for optimising strain and to locate
difficulties in growth
• For strained ternary alloys the concentrations and
critical layer thicknesses can be determined
• A higher resolution can be achieved even on vertical
TSSEL reactors by larger beam separation