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Non-coalescedpendeo GaN
Coalescedpendeo GaN
postwing
GaN
GaN
SiC
wing gap
postwing wing
Dislocation density in overgrowth region approximately . This is a four-to-five order of magnitude decrease of dislocation density in wings [Zheleva (1999)].
5 -210 cm
Sides of wings in non-coalesced pendeo GaN are atomically flat.
11.7 mm
10 mm
xy
He
Janis-mikroskope-cryostate
1m spectrometer
HeCd (325 nm)
Ar-Ionen (333 nm, 488 nm)
Zeiss, 65x
100%
Low-passor 50/50
notch¼ m spectrometer 150, 600, 1200 g/mm
2400 g/mm
Abstract
Pendeo GaN on SiC
Experimental Setup
50 mm pinhole
CCD
mPL of non-coalesced pendeo GaN on SiC
0D X, FWHM ~ 400 meV
0A X
0(D X)2e-
FEALO/TO
FEB
FECn=2FEA
?
1meV
~ B
an
dg
ap
Side view (SEM) Top view (light microscope)
SiC
scan
Trace mPL along pendeo-structure
Trace mPL perpendicular to pendeo-structure
FEA
FEA
0D X
0D X
0A X
0A X
3x0.6 meV 3x0.6 meVpost
wing
wing
wing
gap
post
wing
FEA
FEA
0D X
0D X
0A X
0A X
-41.3 meV, Dc/c ~ 0.7 x 10
-46 meV, Dc/c ~ 3 x 10
scan
0Line width of donor bound exciton D X is less than 400 meV (limited by instrumental resolution).
0Donor bound exciton D X shows fine structure: four linesspaced by 600 meV, 360 meV, and 870 meV. [compare tofive subpeaks observed in Kornitzer (1999)].
Measured 1 meV LO/TO splitting of free exciton A hasthe expected order of magnitude.
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. Bound exciton lines of comparable widths were observed only in GaN on freestanding GaN [Kornitzer (1999), Miskys (2000)].
Discrete shifts of exciton peaks are observed.
Double peaks are observed,which are probably caused by limited spatial resolution: two regions of different strain are observed simultaneously.
0 0Frequency shifts of A X, D X and FEA are strongly correlated.
Frequency shift is caused by strain, not by different donors or acceptors.
0Fourfold splitting of donor bound D X is real.
-4Residual uniaxial strain in wings Dc/c = 10 , compared toGaN on freestanding GaN is confirmed by m-Raman experiments [ ].see poster P19.15
-5Strain across wing is approximately constant (Dc/c < 4x10 ).This small strain variation still causes shifts larger than theextremely narrow linewidth of the donor bound exciton.
0Because of this the D X photoluminescence appears as bright spots of approximately 5 mm extension, when measured with high spatial and spectral resolution.
Photoluminescence in post is down-shifted and broadened: this could be caused by higher and inhomogeneous tensile strain and higher dislocation densitycausing hydrostatic strain.
Versatile system for photoluminescence, two-photon photoluminescence, andspontaneous Raman scattering.
Excitation with HeCd (325 nm) or Ar-ion(333 nm: PL; 488 nm: two-photon, Raman).
Spatial resolution with Zeiss 65x, 0.7 NA objective: ~ 400 nm (PL), ~700 nm (Raman).
Stabilized temperature T >= 8 K in Janis microscope cryostat.
Detection with nitrogen cooled CCD.
Spectral resolution 300 meV with 1m, 2400 lines/mm spectrometer.
1/4 spectrometer for wide range spectra.
We report on micro-Photoluminescence spectroscopy in GaN stripes grown by pendeo--epitaxy (PE). These stripes are grown vertically and horizontally from etched posts of GaN on SiC. The free-standing GaN wings are relaxed, as can be shown by the spectral position of the photoluminescence peak of the free exciton A.
0Due to the high crystalline quality and low defect concentration in the wing area the donor bound exciton D Xshows a fine structure with four different peaks of sub meV width and spaced by a few meV. The relative intensity of these different D0X peaks varies spatially.
An intensity distribution of the individual peaks can be used to map impurity distribution with high (< 700 nm) spatialresolution. We correlate this data with a strain map obtained from the shift of the free exciton A frequency to
0check the influence of local varying strain on the position and nature of the D X bound exciton subpeaks.
In the same sample we observed a pronounced increase of the broad yellow luminescence (1.8 to 2.4 eV) and decrease of the blue luminescence (2.7 to 3.2 eV) during illumination with UV radiation (HeCd laser at 325 nmand Ar--Ion laser at 333 nm) on a time scale of seconds at low temperature (T < 10 K), but not at room temperature. This connection of yellow and blue luminescence and its temperature dependency gives stringent constrictions to an explanation of the luminescence of deep level impurities in GaN.
This indicates that the different peaks are not due to multiple interactions within one donor bound exciton complex, but likely due to different donors types.
Transformation of blue luminescence (BL) to yellow luminecence (YL) during UV irradiation
PL
sig
na
l (a
rb. u
ntis
)
PL
sig
na
l (a
rb. u
ntis
)
Energy (eV) Time (s)
blue luminescenceblue
luminescence 1 - yellow luminescence(increase) yellow
luminescence
boundexciton
boundexciton
Conclusions
References
T= 0 s
50 s
300 s
Initially pendeo GaN samples show a strong blue luminescence, which is strongest along the interfaces between post and wings. This indicates that the BL centers might be caused by the inductively coupled (ICP) etching of the posts [see Brown (1999)] .
During irradiation with a HeCd laser (325 nm) the blue luminescence decreases and a increase of the yellow luminescence increases on the time scale of seconds. The change in color is so drastic that it can be observed by eye or a CCD camera. The yellow post on the right side has been exposed earlier.
This transformation is efficient only at low temperatures (T =10 K). But the induced YL is stable up to room temperature.
We used a 1/4 m spectometer to measure the time dependency and spectral shape of the BL and YL. The whole spectra from yellow to UV was measured simultaneously, which allowed to measure fast photoluminescence with 0.5 s time resolution. High spatial allowed us to measure selectively region of high blue luminescence only (interface between post and wing).
Long time decrease of the maximum of BL and increase of the maximum of YL show an exponential time dependency with similar time constants.
2 2.25 2.5 2.75 3 3.25 3.5
0.25
0.5
0.75
1
1.25
1.5
1.75
0 50 100 150 200 250 300
0.02
0.05
0.1
0.2
0.5
S. A. Brown, E. J. Reeves, C. S. Haase, R. Cheung, C. Kirchner, and M. Kamp, Appl. Phys. Lett. 75, 3285 (1999).
0A fast decay of the UV luminescence (mainly D X) to approximately 50% of the initial value was observed. This fast decay of bound exciton PL correlates with inital fast decay of blue luminescence.
Fast decay of (donor) bound exciton and blue luminescence during UV excitation indicates metastable donor which is not involved in yellow luminescence.
On a longer time scale a slow decay of blue luminescence and increase of yellow luminescence are linked byidentical time constants. The induced yellow luminescence was stable up to room temperature.
Decay of (donor) bound exciton and blue luminescence and increase of yellow luminescence during 0325 nm (HeC) illumination). Left: Spectra at different times. Right; Logarithmic plot of decay (D X, BL)
and increase (const. - YL).
GaN films have been grown on 6H-SiC substrates employing a new form of selective lateralepitaxy, namely pendeo-epitaxy [see Thomson (1999), Zheleva (1999), Linthicum (1999) and
in this session].Poster P2.6
- GaN seed layer on AlN/6H-SiC substrate is grown (MOVPE).
- selective etching through Ni mask to form GaN posts.
- growth of GaN from post side walls and top.
K. J: Linthicum et al., MRS Internet J. Nitride Semicond, Res. 4S1, G 4.9 (1999).
D. B. Thomason et al., MRS Internet J. Nitride Semicond, Res. 4S1, G 3.37 (1999).
T. S. Zheleva et al., MRS Internet J. Nitride Semicond, Res. 4S1, G 3.38 (1999).
GaN epilayer were grown in 6H-SiC with the pendeo technique, which is a maskless form of lateral epitaxy.
These samples show extremely narrow (< 400 meV FWHM) photoluminescence peaks of donor and acceptor bound excitons, which indicate the low defect concentration of the material.
03.463 eV~ A X03.4654 eV~ D X
03.4664 eV~ D X 03.4678 eV~ D X
post
post
post
post
wing
wing
wing
wing
wing
wing
wing
wing
ga
pg
ap
ga
pg
ap
wing
wing
wing
wing
NO!
C. R. Misky, M. K. Kelly, O. Ambacher et al., Appl. Phys. Lett. 77, 1858 (2000).
K. Kornitzer, T. Ebner, M. Grehl et al., Phys. Stat. Sol. (b) 216, 5 (1999).
Pos
t
Win
g
Win
g
T=0 s T=26 s T=93 s
FEA frequency (eV)
fre
qu
en
cy (
eV
)
FEA
0D X
0A X
Correlation plot of freeand bound excitons.
2d map of photoluminescence intensity in narrow frequencyintervals at the energy of acceptor and donor bound excitons.
Possible other causes of this downshift in the posts are:
0 0- increase of A X and decrease of D X photoluminescence.
- photoluminescence of free carriers.
A four-fold substructure of the donor bound exciton was observed.
-5Residual strain (Dc/c < 4 10^ ) in the wings can be tracked by the shift of the donor bound exciton.
P2.2MICRO-PHOTOLUMINESCENCE STUDIES of PENDEO- EPITAXY GaN on SiCU.T. Schwarz, Univ. Regensburg, Dept. of Exp. and Appl. Physics, Regensburg, Germany
P.J. Schuck and R.D. Grober, Yale Univ., Dept. of Applied Physics, New Haven, CT A.M. Roskowski, P.Q. Miraglia, R.F. Davis, North Carolina State Univ., Dept. of Material Science and Engineering, Raleigh, NC.
