mapping strain by micro-raman and micro-photoluminescence ... · crystalline quality in those two...
TRANSCRIPT
• Dislocation density in post: 109 cm-2
• Side of wing is atomically flat
• Four-to-five order of magnitude decrease
of dislocation density in wings.
Pendeo GaN on SiC
Experimental Setup
x
y
He
Janis microscope cryostat
CCD, nitrogen cooled
1m monochromator
HeCd (325 nm)Ar-Ion (333 nm, 488 nm)
Zeiss, 65x
100%
Low-passor 50/50
notch¼ m monochromator
150, 600, 1200 g/mm
2400 g/mm
50 µmpinhole
CCD
C. Kisielowski et al. PRB 54, 17745 (1996).
E2 (high) amplitude
•E2(high) peak amplitude large in wing, weak in post – consistent with TEM measurements of crystalline quality in those two regions
•Position/shape of LO phonon peaks imply carrier concentrations < 1017 cm-3
-No phonon-plasmon coupling is evident
scan
6H-SiC
µ-PL on non-coalesced pendeo GaN
0 6 12 180
6
12
18
x (microns)
y (m
icro
ns)
0 6 12 180
6
12
18
x (microns)
y (m
icro
ns)
postwing winggap
wing postwing winggap
wing
SiC
A1(TO)E1(TO)
E2(high)
A1(LO)
Ram
an-amplitude (a.u.)
gap wing post wing gap
700 nm
0.5 cm-1
Ram
an f
requ
ency
(cm
-1)
(rel
ativ
e to
566
cm
-1)
Micrometer
Combination plot: Raman-Shift and Amplitude
Ram
an A
mpl
itude
(a.
u.)
biaxial Strain ∆c/c
x 10-4
gap wing post wing gap
2D Stress Map
Raman Strain Quantification
µ-Raman on non-coalesced pendeo GaN
E2(high) Line-cut
Raman Spectrum
0 6 12 180
6
12
18
x (microns)y
(mic
ron
s)
• Raman line shift E2(high) between wing & post ~ 0.5 cm-1
• spatial resolution 700 nm• ∆∆c/c ~ 1.8 x 10-4
D0X, FWHM ~ 400 µeV
A0X
(D0X)
FEA polaritonSplitting 1 meV
FEB
FECFEAn=2
?
?
?
Due to the high quality and low defect concentration in the wings of pendeo GaN on SiC extremely narrow
bound and free exciton lines can be observed in photoluminescence, which are comparable only to Gan on
freestanding GaN [Kornitzer (1999), Miskys (2000)]
•Line width of donor bound exciton DX is less than 300 µeV (limited by instrumental resolution)
•Donor bound exciton D0X shows fine structure: three lines spaced by ~ 300 µeV
•1 meV polariton splitting of free exciton A measured
•D0X and Free excitonic emission stronger in wings; A0X emission stronger in post
A0X emission D0X emission
• A0X emission strongest in post region • D0X emission strongest in wing region
D0X-maximum
wingwing
gap
PL Strain QuantificationStrain in two different wings
Strain within wing
• Strain differs between wings associated with neighboring posts
• Line shift of 1.3 meV corresponds tochange in biaxial strain of ∆c/c ~ 0.7 x 10-4
wing
winggap
post
wing
FEADX
0
AX0
1.3 meV
Biaxial strain: wing vs. post
• Wings contain domains of constant strain• (separated by defects/cracks ?)
• High crystalline quality within domain• strong PL, narrow linewidths, fine structure (see Poster P2.2)
• Sampling adjacent domains simultaneously results in multi-peak spectral features
µ-PL spectrum:boundary between two strain domains
D0XD0X
A0X
A0X
free standing GaN
GaN on SiC
GaN on SiC
free standing GaN
Bia
xial
str
ain
Bia
xial
str
ain
Biaxial Strain Measured Optically
• Strain is measured by tracking the frequencies ofthe E2(high) Raman mode and near-band-edgephotoluminescence (PL) peaks
• Create strain maps with high spatial resolution:~ 700 nm for Raman~ 300 nm for PL
• E2(high) Raman mode:shifts by 4.2 cm-1 per GPa
• Photoluminescence lines:Shift by 27 meV per GPa
Raman
Photoluminescence
Coalesced pendeo GaN
Summary
Comparison: Raman PL
Raman Amplitude Image
PL Image: 2-photon
post
wing
coalescedregion
In coalesced sample, wings from adjacent posts are grown until they join together
•Both PL and Raman intensities are stronger in thewings and weaker in the post and coalesced regions
•Higher quality material in wings
•PL spectra broader than in non-coalesced
•Coalesced GaN under greater tensile strain than non-coalesced GaN
550 555 560 565 570 575 5800
10000
20000
30000
Non-coalescedCoalesced
E2 Raman spectra
Inte
nsity
(a.
u.)
Frequency (cm-1)
~ 2 cm-1
GaN on free-standing GaN∆c/c < 4 x 10-5
non-coalescedGaN on SiC∆c/c ~ 2.5 x 10-4
CoalescedGaN on SiC∆c/c ~ 1.8 x 10-4relative to GaN/GaN∆c/c ~ -5.2...-7 x 10-4
relative to GaN/GaN∆c/c ~ -1...-3.5 x 10-4
Stra
in ∆
c/c
GaN on GaN
non-coalescedGaN on SiC
coalescedGaN on SiC
PL s
igna
l (a.
u.)
wingpost
post wing
Increasing tensile strain
Raman Line cuts
Incr
easi
ng te
nsile
str
ain
High spatial resolution allowed us to optically map strain in pendeo GaN
films grown on SiC
• Dislocation density in wings five orders of magnitude less than in posts
• Strain larger in posts than in wings for non-coalesced (∆c/c ~ 2x10-4)
• Strain differs from wing to wing
• Domains of constant strain exist in wings
• Strain larger in coalesced sample than in non-coalesced sample (∆c/c ~ 5x10-4)
• Agreement between PL and Raman strain measurements
GaN films have been grown on 6H-SiC substrates employing a new form of selective laterepitaxy, namely pendeo epitaxy (see Thomson (1999), Zheleva (1999), Linthicum (1999) andPoster P2.6)
Non-coalescedpendeo-GaN
Coalescedpendeo-GaN
Side view (SEM) Top view (light microscope)
10 µm
11.7 µm
• Versatile system for single photon andtwo-photon photoluminescence, and spontaneous Raman scattering.
• Excitation with HeCd (325 nm) or Ar-ion(333 nm: PL, 488 nm: two-photon, Raman)
• Spatial resolution with Zeiss 65x, 0.75 NA objective (confocal): ~ 300 nm (PL), ~700 nm (Raman).
• Spectral resolution 300 µeV with 1m,2400 lines/mm monochromator.
• 1/4 monochromator for wide range spectra,
• Detection with nitrogen cooled CCD.
• Stabilized temperature T > 8 K in Janis microscope cryostat.
position
freq
uenc
y
Post
Post
Wing
gap
Wing
E1(LO)
Mapping Strain by Micro-Raman and Micro-Photoluminescence Spectroscopy in Pendeo-Epitaxial GaNP. James Schuck, Robert M. Grober, Department of Applied Physics, Yale University
Ulrich T. Schwarz, Department of Experimental and Applied Physics, Regensburg UniversityA. M. Roskowski, P. Q. Miraglia, R. F. Davis, Department of Material Science and Engineering, North Carolina State University
• Raman line width: approx 3.5 cm-1 (limited by monochromator resolution)
• Data taken in Z(X,-)Z (backscattered) configuration, Z parallel to [0001]
-Only E2(high) and A1(LO) phonons active in this symmetry-Observe A1(TO), E1(TO), and E1(LO) phonons at wing edges