• 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