Institute of Physics Conference Series 184 (2005) 393 - 398

Photoluminescence from Single Hexagonal Nano-Wire
Grown by Selective Area MOVPE
Shinjiroh Hara*, Junichi Motohisa, Jinichiro Noborisaka, Junichiro
Takeda and Takashi Fukui
Graduate School of Information Science and Technology, and ResearchCenter for Integrated Quantum Electronics, Hokkaido University, North
14 West 9, Sapporo 060-0814, Japan * hara@rciqe.hokudai.ac.jp
Abstract We demonstrate formation of nano-wires with GaInAs/GaAs
double hetero-structures using selective area metal-organic vapor phase
epitaxy (SA-MOVPE) on SiO2 masked GaAs (111) B substrates withperiodic circular openings. Hexagonal nano-wires with six vertical{110} sidewall facets and a lateral size of around 70 to 260 nm aresuccessfully fabricated on the masked GaAs (111) B substrates. Using
microscopic area photoluminescence (µ-PL) measurements, PL emission
from single GaInAs/GaAs double hetero-structure nano-wires is clearly
identified at energy of around 1.28 eV. Full width at half maximum(FWHM) of the PL emission spectra from nano-wires is as narrow asaround 10 meV.
1. Introduction
Nano-structures, such as carbon nano-tubes and semiconductor nano-wires, have been
intensively investigated because of their enormous potential for new functional
electronic and photonic device applications in the next generation. As a new techniqueto realize ultra small logic circuits [1], lasers [2], photo-detectors [3] and field effecttransistors [4], semiconductor nano-wires have been intensively studied in recent years,
and nano-wire superlattice (or hetero-) structures have been demonstrated for further
electronic and photonic device applications. [5, 6] So far, almost all of thesemiconductor nano-wires have been fabricated by vapor-liquid-solid (VLS) growth [7]or chemical vapor deposition (CVD) with metal catalysts, such as gold (Au) particles.
However, these growth techniques with metal catalysts substantially have a poor
controllability of positioning and size uniformity of nano-wires because it depends on
the position and size of metal particles randomly deposited on semiconductor substrates.
In previous studies, we have proposed and demonstrated a novel fabrication technique
for semiconductor nano-wire arrays by using selective area metal-organic vapor phase
epitaxy (SA-MOVPE) on partially masked semiconductor substrates without any small
particles of metal catalysts. [8 - 10] SA-MOVPE is very promising epitaxial growthtechniques for fabricating not only nano-wires but also other nano-structures, e.g.
quantum dots [11], photonic crystals [12, 13] and two-dimensional artificial Kagome
lattices [14], because we can expect better controllability for crystal quality, abruptnessof hetero-structure interfaces, doping as well as positioning and size uniformity ofnano-structures. In this paper, we report on successful fabrication of GaInAs/GaAs

Institute of Physics Conference Series 184 (2005) 393 - 398

double hetero-structure nano-wires with well defined vertical facets by utilizing
SA-MOVPE growth. We also show strong photoluminescence emissions from single
nano-wire, which indicate good crystal qualities of nano-wires. These characteristics ofdouble hetero-structure nano-wires are quite important for future applications to ultra
small nano-wire light emitters and single photon emitters.
2. Experimental Procedures
SA-MOVPE growth was carried out in horizontal low pressure MOVPE systems at aworking reactor pressure of 76 Torr. Arsine (AsH3), tri-methyl-gallium (TMGa) andtri-methyl-indium (TMIn) were used as group V and III source materials. Schematic
cartoons of a fabrication process for nano-wire arrays were depicted in Fig. 1(a). SiO2
masked GaAs substrates with periodic circular openings were formed using electronbeam (EB) lithography and wet chemical etching techniques. The shape of the openingsof SiO2 mask pattern is circular because of the resolution limit in EB lithography andwet chemical etching process, although the shape of the designed ones was hexagonal.
Diameter of initial circular openings, d0, was around 100 nm. These circular openings
were arranged to form triangular lattice with a lattice constant (period of the holes), a,
as defined in Fig. 1(b). We changed a period of the holes, a, from 0.65 to 3 µm.
GaInAs/GaAs double hetero-structures were grown on partially SiO2 masked GaAs(111) B substrates. Planar GaAs (001) and (111) B substrates were also used as areference at the same growth runs for nano-wire fabrications. Figure 1(c) shows aschematic of layer sequence and estimated layer thickness of GaInAs/GaAs double
hetero-structures on planar GaAs (001) surfaces. Indium contents and layer thickness ofGaInAs layers on planar GaAs (001) substrates were 2.4 % and 10 nm, respectively.
Typical growth temperatures, Tg, for nano-wires were 750 and 600 °C for GaAs andGaInAs layers, respectively. AsH3 partial pressures during GaAs and GaInAs layer
growth were 2.5 × 10-4 and 6.3 × 10-5 atm, respectively. Estimated growth rates ofFigure 1. (a) Cartoons of SiO2 mask pattern and SA-MOVPE on GaAs
(111) B substrates. (b) Definition of diameter, d0, and period, a, of
initial openings on SiO2 masked GaAs substrates. (c) Schematic oflayer sequence and nominal layer thickness estimated on planar GaAs
(001) substrates.
2Institute of Physics Conference Series 184 (2005) 393 - 398

GaAs layers on reference planar GaAs (111) B surfaces were 0.16 and 0.08 nm/s.
Growth rate on masked and reference planar (111) B surfaces strongly depends on
growth conditions, e.g. V/III ratios and Tg, because of a well-known surface
reconstruction of arsenic trimers on arsenic-terminated GaAs (111) B surfaces.
Scanning electron microscope (SEM) observations and microscopic area
photoluminescence (µ-PL) measurements were carried out as structural and opticalcharacterizations, respectively. He-Ne lasers operating at the wavelength of 632.8 nm
was used as an excitation light source for µ-PL measurements. Spot size of the focused
excitation light was around 2 µm. The experimental set-up for µ-PL measurements wasdescribed in detail in Ref. 8.
3. Results and Discussion
Figure 2(a) shows a bird’s eye view of SEM images of GaInAs/GaAs nano-wire arrayswith a = 0.65 µm. A highly magnified image of a typical nano-wire was also observed,
as shown in Fig. 2(b). Hexagonal wires consisting of six vertical {110} sidewall facets
were successfully formed. Typical lateral size of the nano-wires was around 200 nm.
Some of the nano-wires had larger lateral size and lower height. That is presumably dueto inhomogeneous lateral over-growth of GaAs and GaInAs layers grown underrelatively low Tg conditions (600 °C) and/or due to diameter size fluctuations of initial
circular openings, d0, caused by unintentional fluctuation of EB lithography conditions.
The µ-PL spectra for the nano-wire arrays with a = 0.65 µm show several peaks, which
probably come from different size of nano-wires. Figure 2(c) shows dependence of PL
emission spectra on excited areas of the nano-wire samples shown in Figs. 2(a) and (b).
PL peaks at the energies of around 1.48 eV were attributable to shallow acceptor levels
in GaAs layers. Therefore, peak energies below 1.42 eV were originated from GaInAs
layers of the nano-wires. PL emission peaks observed at various energies for thenano-wires with a = 0.65 µm were probably caused by different incorporation
efficiencies of indium atoms into GaInAs layers in the nano-wires with different size
shown in Fig. 2(a).
We also formed GaInAs/GaAs nano-wires with larger period of the initial openings
under various growth conditions. Figure 3 shows typical SEM bird’s eye view ofGaInAs/GaAs nano-wire arrays with a = 3 µm. Typical lateral size of the hexagonal

Figure 2. (a) Bird’s eye view of SEM images of GaInAs/ GaAs
hexagonal nano-wire arrays with a = 0.65 µm, and (b) highly
magnified images of a nano-wire. (c) Typical PL spectra fromnano-wire arrays. Spot size of a focused excitation light was around 2µm.
3Institute of Physics Conference Series 184 (2005) 393 - 398

Figure 3. Bird’s eye view of typical SEM images of GaInAs/GaAshexagonal nano-wire arrays with a = 3 µm, and highly magnified
images of the nano-wires. Nano-wires were grown under the growth
conditions for GaAs layers of (a) V/III = 173 and (b) 86, respectively.
Typical lateral size of nano-wires is around (a) 260 nm and (b) 70 nm,respectively.
nano-wires observed in Fig. 3(a) was around 260 nm. In our previous studies [8 - 10], it
was revealed that the shape of the wires was independent of initial shape of theopenings on partially masked substrates, but depended on growth conditions, such as
growth temperatures and V/III ratios (or AsH3 partial pressures) during SA-MOVPE
growth. The lateral size of the wires depends on not only diameter of the initial circular
openings, d0, on partially masked substrates but also the growth conditions. These facts
mean that lateral size of the nano-wires can be controlled by changing the growth
conditions and/or EB lithography conditions. As shown in Fig. 3(b), much narrower
GaInAs/GaAs hexagonal nano-wires with typical lateral size of 70 nm were formed.
For the nano-wires shown in Figs. 3(a) and (b), the V/III ratios during SA-MOVPE
growth of GaAs nano-wires were 173 and 86, respectively, and those of GaInAs layers
were 42 and 21, respectively. In the present growth conditions, lateral growth on six
vertical {110} surfaces was supposed to be higher for the nano-wires shown in Fig. 3(a)than for the ones shown in Fig. 3(b). Therefore, nano-wires with much smaller lateralsize were formed under lower V/III ratio conditions during SA-MOVPE growth. Here,it should be noted that the deference in the lateral size of nano-wires involved someamount of the deference in diameters of the initial circular openings, d0, between these
two samples because of unintentional fluctuation in EB lithography conditions.
In order to clarify PL emission from single nano-wire, µ-PL measurements werecarried out at 4.2 K for the nano-wires shown in Fig. 3(a). Figure 4(a) shows an optical
microscope image through objective lens of the µ-PL measurement systems. Solidblack circles (b) to (d) in the image represent the areas actually excited by He-Ne lasersin µ-PL measurements. At positions of (b) and (d), nano-wires are observed, while
there are no wires at a position of (c). PL spectra obtained for each of the excited areas
were shown in Figs. 4(b) to (d), respectively. PL peaks at energies of around 1.48 eV
were originated from shallow acceptor levels in GaAs layers, and the ones observed atenergies of around 1.52 eV were due to the band edge emissions of GaAs. In Figs. 4(b)and (d), PL emission spectra from GaInAs/GaAs nano-wires were observed at theenergies of around 1.28 eV. Full width at half maximum (FWHM) of the spectra was as
narrow as around 10 meV. In the case of Fig. 4(c), however, no additional PL emission
spectra were detected. These results clearly show that relatively strong PL emission
from single GaInAs/GaAs nano-wires can be identified by using µ-PL measurements,
and the nano-wires with good crystal qualities were successfully formed by
SA-MOVPE growth. From PL peak energies of GaInAs layers at 1.28 eV, incorporation
4

1.20

Acceptors
in GaAsGaAs
GaInAs(1.28 eV)
8 meV

1.30 1.40 1.50
Energy (eV)

(c)
4.2 K

1.20

Acceptors
in GaAs

GaAs

1.30 1.40 1.50
Energy (eV)PL Intensity (arb. units)

(b)
4.2 K

PL Intensity (arb. units)

PL Intensity (arb. units)

Institute of Physics Conference Series 184 (2005) 393 - 398

(d)
4.2 K

1.20

Acceptors
in GaAsGaAs
GaInAs(1.28 eV)
11 meV

1.30 1.40 1.50
Energy (eV)Figure 4. (a) Optical microscope image observed for µ-PL
measurement sample of the nano-wires shown in Fig. 3 (a). The brightspot in the image is a focused excitation light (< 2 µm) of He-Ne laser.
Solid black circles (b), (c) and (d) in the image represent the areas
measured by µ-PL, and PL spectra from each of the measured areas areshown in Figs. 4 (b), (c) and (d), respectively.
of indium atoms seemed to be drastically enhanced in GaInAs layers of the nano-wires.
Indium contents in the nano-wires were about 16 %, which were roughly estimatedexcluding strain effects in the GaInAs layers, although indium contents in the GaInAs
layers on the reference planar GaAs (001) surfaces were only 2.4 %. In order to discuss
indium contents and layer thickness of GaInAs layers in the nano-wires in detail,further experiments to investigate piezoelectric field effects for GaInAs/GaAs quantum
wells on GaAs (111) B surfaces are needed. [15]For the nano-wires with a lateral size of 70 nm shown in Fig. 3(b), µ-PL
measurements have also been carried out at 4.2 K. In area with nano-wires on thesamples, PL emissions were observed at the energies of around 1.30 eV as well as PLemissions from GaAs layers. The emissions at the energies of 1.30 eV were probably
originated from single GaInAs/GaAs nano-wires with a lateral size of 70 nm. Detailed
studies combing structural characterizations and µ-PL measurement techniques areunderway to investigate growth condition dependences of nano-wire formation for thesamples with GaInAs/GaAs double hetero-structures in order to fabricate ultra small
nano-wire light emitters and single photon emitters using SA-MOVPE.
4. Summary
Using SA-MOVPE growth on SiO2 masked GaAs (111) B substrates with periodic
circular openings, we demonstrated formation of hexagonal nano-wires with six
vertical {110} sidewall facets and GaInAs/GaAs double hetero-structures. Nano-wires
with a lateral size from 70 to 260 nm were successfully fabricated. Using µ-PL
measurements, relatively strong PL emissions from single GaInAs/GaAs nano-wires
were identified. These results indicated that GaInAs/GaAs nano-wires had good crystal
quality, and that SA-MOVPE growth techniques are very promising for realizing ultra
small nano-wire devices. To investigate characteristics of single nano-wire in detail,5Institute of Physics Conference Series 184 (2005) 393 - 398

controllability of positioning and density uniformity of nano-wires is a crucial issue tobe solved. Therefore, these results here suggest that SA-MOVPE growth techniques
combining µ-PL measurements are quite powerful and useful methods to form andcharacterize nano-wires.
Acknowledgements
The authors acknowledge Prof. H. Hasegawa for his stimulating discussions, and thankS. Akamatsu for supporting MOVPE growth of nano-wires. This work was financially
supported in part by the Grant-in-aid for Scientific Research of the Japan Society for
the Promotion of Science (JSPS).
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