Optical characteristics of GaInP/GaP double-heterostructure core-shell nanowires embedded in polydimethylsiloxane membranes

Article abstract of DOI:10.1063/1.3455340

The authors report on the optical properties of GaInP/GaP double-heterostructure (DH) core-shell nanowires (NWs) embedded in polydimethylsiloxane (PDMS) membranes. Self-catalyzed NW structures are grown on Si (111) substrates by initiating with the formation of Ga droplets as a catalyst which is followed by the growth of GaP core and GaInP DH shells. Optical characteristics of GaInP/GaP DH core-shell NWs transferred from Si substrates into PDMS membranes show enhanced 77 K light emission at 630 nm. The signal at 775 nm from the surface states of NWs can be mitigated by embedding the NWs in a PDMS membrane that acts as a surface state passivant.

Full text of DOI:10.1063/1.3455340

Optical characteristics of GaInP/GaP double-heterostructure core-shell nanowires
embedded in polydimethylsiloxane membranes
J. Tatebayashi, G. Mariani, A. Lin, R. F. Hicks, and D. L. Huffaker
Citation: Applied Physics Letters 96, 253101 (2010); doi: 10.1063/1.3455340
View online: http://dx.doi.org/10.1063/1.3455340
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APPLIED PHYSICS LETTERS 96, 253101 ͑2010͒
Optical characteristics of GaInP/GaP double-heterostructure core-shell
nanowires embedded in polydimethylsiloxane membranes
J. Tatebayashi,1,a͒ G. Mariani,1 A. Lin,1 R. F. Hicks,2 and D. L. Huffaker1,b͒
1Department of Electrical Engineering, University of California, Los Angeles, 420 Westwood Plaza,
Los Angeles, California 90095, USA
2Department of Chemical and Biomolecular Engineering, University of California, Los Angeles,
405 Hilgard Avenue, Los Angeles, California 90095, USA
͑Received 19 April 2010; accepted 27 May 2010; published online 21 June 2010͒
The authors report on the optical properties of GaInP/GaP double-heterostructure ͑DH͒ core-shell
nanowires ͑NWs͒ embedded in polydimethylsiloxane ͑PDMS͒ membranes. Self-catalyzed NW
structures are grown on Si ͑111͒ substrates by initiating with the formation of Ga droplets as a
catalyst which is followed by the growth of GaP core and GaInP DH shells. Optical characteristics
of GaInP/GaP DH core-shell NWs transferred from Si substrates into PDMS membranes show
enhanced 77 K light emission at 630 nm. The signal at 775 nm from the surface states of NWs can
be mitigated by embedding the NWs in a PDMS membrane that acts as a surface state passivant.
© 2010 American Institute of Physics. ͓doi:10.1063/1.3455340͔
Recent technologies on organic light-emitting diodes
͑OLEDs͒ have been attracting growing attention for the re-
alization of low-cost, low-weight, full-color, and flexible
flat-panel display as well as other emissive products. Since
the first demonstration of organic thin-film electrolumines-
cence in 1987,1 many researchers have been devoted to the
realization and development of high-efficiency OLEDs and
flexible displays.2–4 Furthermore, the integration of organic/
inorganic materials into hybrid optoelectronic nanostructures
enables active devices that combine the diversity of organic
materials with the high-performance and well-established
optical/electronic properties of inorganic nanostructures. So
far, several groups have demonstrated hybrid light-emitting
diodes ͑LEDs͒, solar cells, or photovoltaics that combine
organic materials with semiconductor nanomaterials such
as nanocrystals, colloidal quantum dots, or Si nanowires
͑NWs͒.5–7
Compound semiconductor NWs, which are crystalline,
vertically aligned, and highly oriented crystalline nanostruc-
tures utilizing either the vapor-liquid-solid method8 or the
“catalyst-free” method using patterned substrates,9 have
been intensively exploited as a key component for diverse
applications in nanophotonics and nanoelectronics such
as nanolasers,10–12 LEDs,13–16 single-photon emitters,17
transistors,18 photodetector,19 and solar cells.20 Furthermore,
NWs on Si substrates have recently attracted practical inter-
est because of their compatibility with existing Si-based
electronic devices using mature and “low-cost” Si technolo-
gies. Therefore, hybrid combination of these well-
established, intriguing NW technologies with organic mate-
rials which includes polymers would enable the realization
of flexible organic/inorganic hybrid devices utilizing com-
pound semiconductor NWs such as flexible, low-cost LEDs/
displays, detectors, or solar cells which might be able to
circumvent the substantial life-time problems of the existing
all-organic devices by using semiconductor materials as an
active.5
In this study, we focus on the development of “self-
catalyzed” GaP NWs growth on Si to prevent the contami-
nation from foreign metal catalysts which causes the deep-
level trap in III–V NWs and in turn deteriorates device
characteristics.21 GaP has the smallest lattice mismatch to Si
substrates ͑Х0.4%͒ at room temperature ͑RT͒, and GaP-
based material systems have been of practical interests for
their application to LEDs in visible regimes such as red,
yellow, or green. All samples are grown by low-pressure
metal-organic chemical vapor deposition on Sb-doped Si
͑111͒ substrates at a total pressure of 60 Torr. After sequen-
tial cleaning processes with acetone, isopropanol, and de-
ionized water in an ultrasonic bath, the Si wafers are im-
mersed into diluted hydrochloric acid–hydrogen peroxide
mixture ͑HCl:H2O2:H2O=1:1:1͒ to remove the residual
microparticles on the surface. Next the wafers are etched in
5% hydrofluoric acid solution to remove a native oxide on
the surface and loaded immediately into the reactor. The
growth temperature for all the growth including Ga droplets
and GaP NWs is 480 °C. First, Ga droplets are deposited by
supplying trimethylgallium ͑TMG͒ with a mole flow of 2.2
ϫ10−5 mol/min for 30 s to seed NWs on the Si substrate
͓Fig. 1͑a͔͒. Next, the GaP core for the growth of GaInP/GaP
double-heterostructure ͑DH͒ NWs is formed by supplying
tertiarybutylphosphine and TMG with a TMG mole flow of
2.9ϫ10−5 mol/min at a V/III ratio of 27 for 10 min imme-
diately after the formation of Ga droplets ͓Fig. 1͑b͔͒. In this
growth process, Ga droplets act as a catalyst which allow for
vertical NW formation along the ͗111͘ direction. After the
growth of GaP core, GaInP DH layers are grown at a V/III
ratio of 27 which are comprised of thin GaInP layer with a
Ga:In flux ratio of 50:50 ͑GaInP-2͒ cladded by GaInP layers
with a Ga:In flux ratio of 70:30 ͑GaInP-1͒. The mole flow of
TMG is kept constant at 2.2ϫ10−5 mol/min and the growth
duration for the growth of GaInP-1 and GaInP-2 are 1 min
and 6 s, respectively. The Ga compositions of each layer,
GaInP-1 and GaInP-2, can be inferred from the x-ray diffrac-
tion analyses of the bulk material growth to be approxi-
mately 77% and 61%, respectively. After the growth of NWs,
polydimethylsiloxane ͑PDMS͒ base, which is a transparent,
a͒
Electronic mail: tatebaya@ee.ucla.edu.
Electronic mail: huffaker@ee.ucla.edu.
b͒
0003-6951/2010/96͑25͒/253101/3/$30.00
96, 253101-1
© 2010 American Institute of Physics
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253101-2
Tatebayashi et al.
Appl. Phys. Lett. 96, 253101 ͑2010͒
FIG. 1. ͑Color online͒ Schematic diagrams of the whole processes which
involve ͑a͒ the formation of Ga droplets on Si substrates, ͑b͒ the growth of
GaInP/GaP DH core-shell NWs, ͑c͒ PDMS application and curing pro-
cesses, ͑d͒ peeling-off process, and ͑e͒ turnover for structural analyses and
optical characterization. ͑f͒ Schematic and digital camera images of the
fabricated NWs embedded in PDMS membranes.
polymeric, and viscous organosilicon compound, is mixed
together with a curing agent in a 10:1 ratio and dropped on
the samples ͓Fig. 1͑c͔͒. Next, the sample is left at RT for at
least half a day until the polymer penetrates into the grooves
among the pillars and incorporates them completely.22 Then,
the sample is cured on a hot plate at 120 °C for 10 min to
change the phase of the PDMS from viscous/liquid to solid.
Once the sample is cooled down at RT, the PDMS membrane
is gently peeled off with tweezers so that it remains intact
and does not break ͓Fig. 1͑d͔͒. The peeled-off sample is then
turned over for structural analyses and optical characteriza-
tion ͓Fig. 1͑e͔͒. As can be seen in Fig. 1͑f͒, the final structure
peeled off from the Si substrate can be easily folded. Struc-
tural characterization of the as-grown GaInP/GaP DH NWs
and the NWs embedded in PDMS membranes is carried out
using transmission electron microscopy ͑TEM͒ and high-
resolution scanning electron microscopy ͑SEM͒, respec-
tively. Optical characterizations of the NW structures are car-
ried out by conventional photoluminescence ͑PL͒ setup at 77
K excited by a 6 mW blue semiconductor laser ͑402 nm͒
with a beam spot size of approximately 1.5 mm.
FIG. 2. ͑a͒ Top-view and ͑b͒ bird-eye-view SEM images of GaInP/GaP DH
core-shell NWs on Si ͑111͒ substrates. ͑c͒ Cross-sectional side-view TEM
image of the NWs. ͑d͒ Bird-eye-view SEM images of the NWs embedded in
PDMS membranes. An inset is the SEM image of the PDMS membranes
without NWs.
three peaks in the PL spectrum of the NWs on Si at 565, 625,
and around 775 nm. From our detailed investigation which is
discussed in Ref. 21, the PL peak at 625 nm originates from
GaInP DH core-shell structure in the NWs, and the light
emission at around 775 nm originates from surface states of
GaP ͑or GaInP͒.23 At lower pump power densities, only the
peak from GaInP-2 shell is observed at 625 nm. The PL peak
at 565 nm as well as the PL peak from the surface state at
775 nm emerges at higher pump power densities. This might
be due to an excess of photopumped carriers that diffused
from the GaInP-2 shell to the surface of NWs where the
The NW density is approximated to be 1.6ϫ108/cm2
from the top-view SEM image shown in Fig. 2͑a͒. The SEM
image in Fig. 2͑b͒ shows the formation of vertical GaP NWs
with an average height and diameter of 900 nm and 100 nm,
respectively. A TEM image of the GaInP/GaP DH NW
shown in Fig. 2͑c͒ clearly proves the existence of core-shell
interfaces in the GaInP/GaP DH NW among GaInP-1/
GaInP-2 shells and GaP core, along with the formation of Ga
droplets on the top of the NW. The thicknesses of GaInP-1
and GaInP-2 are approximated to be 8 nm and 7 nm, respec-
tively, from the high-resolution cross-sectional TEM ͑not
shown͒. Figure 2͑d͒ shows the SEM image of the NWs em-
bedded in PDMS membranes. The NWs which are con-
trasted in the white dots can be observed on the surface of
the PDMS membrane, while the surface of the PDMS with-
out NWs shows no white contrasts.
Figure 3 shows the PL spectra at 77 K of GaInP/GaP DH
core-shell NWs on Si with different incident pump power
FIG. 3. ͑Color online͒ PL spectra at 77 K of GaInP/GaP DH core-shell NWs
on Si substrates with different incident pump power densities ranging from
densities ranging from 390 ␮W to 6 mW. There are mainly
390 ␮W to 6 mW.
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253101-3
Tatebayashi et al.
Appl. Phys. Lett. 96, 253101 ͑2010͒
branes. Self-catalyzed NWs are grown on Si ͑111͒ substrates
by initiating with the formation of Ga droplets as a catalyst
which is followed by the growth of GaP core and GaInP DH
shells, and are then transferred into PDMS membranes. The
optical characteristics of the NWs in PDMS membranes
show enhanced light emission at 635 nm whose PL intensity
is 3.5 times stronger than that of the NWs on Si substrates.
The signal at 775 nm from the surface states of NWs is
mitigated by embedding NWs in PDMS membranes that acts
as a surface state passivant. These results would enable the
realization of organic/inorganic hybrid nanophotonic devices
such as flexible LEDs/displays, detectors, and solar cells.
This work was financially supported by the National Sci-
ence Foundation ͑through Grant No. ECCS-0824273͒ and
the U.S. Department of Defense ͑through Grant No. NSSEFF
N00244-09-1-0091͒.
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In summary, we report on the optical properties of
GaInP/GaP DH core-shell NWs embedded in PDMS mem-
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