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Effect of Silicon Doping on GaN Nanorods during Vapor--Liquid--Solid
Growth
Article in Japanese Journal of Applied Physics · August 2013
DOI: 10.7567/JJAP.52.08JE08
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REGULAR PAPER
Effect of Silicon Doping on GaN Nanorods during Vapor–Liquid–Solid Growth
Mohamed Ebaid, Jin-Ho Kang, June Key Lee, and Sang-Wan Ryu
Jpn. J. Appl. Phys. 52 (2013) 08JE08
# 2013 The Japan Society of Applied Physics
Person-to-person distribution (up to 10 persons) by the author only. Not permitted for publication for institutional repositories or on personal Web sites.
REGULAR PAPER
Japanese Journal of Applied Physics 52 (2013) 08JE08
http://dx.doi.org/10.7567/JJAP.52.08JE08
Effect of Silicon Doping on GaN Nanorods during Vapor–Liquid–Solid Growth
Mohamed Ebaid1 , Jin-Ho Kang1 , June Key Lee2 , and Sang-Wan Ryu1
1
Department of Physics, Chonnam National University, Gwangju 500-757, Korea
School of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, Korea
E-mail: sangwan@chonnam.ac.kr
2
Received October 13, 2012; revised November 26, 2012; accepted December 11, 2012; published online May 20, 2013
High optical quality and reproducible Si-doped GaN nanorods (NRs) were grown by low pressure metal–organic chemical vapor deposition.
Different fluxes of SiH4 were introduced and at 2:7 10 9 mol/min the optimized morphology was obtained. Clear improvement of GaN NRs
morphology characterized by smooth side walls and straightness was achieved after Si-doping. Near-band edge (NBE) emission at 356.88 nm
(3.474 eV) and small full width at half maximum (FWHM) of 27.3 meV, measured by room temperature photoluminescence, confirmed the high
optical and crystalline qualities. For Si-doped GaN NRs, a small blue-shift of NBE emission peak was observed, which was attributed to the bandfilling effects. # 2013 The Japan Society of Applied Physics
1. Introduction
GaN is a vital material for UV and visible optoelectronic
devices. GaN nanorods (NRs) showed several advantages
over the conventional thin films for the application on these
devices. Reducing the dislocation density,1) minimizing the
strain-induced defects and improving the light extraction
efficiency2,3) are all valuable reasons for GaN NRs to be
carefully studied. In order to realize GaN NRs-based
devices, n- and p-type doping of GaN NRs by Si and Mg
is considered indispensable.4) Despite the need, n-type
doping of GaN NRs is not well studied yet.3,5) Several
approaches have been employed for the growth of Si-doped
GaN NRs, including chemical vapor deposition (CVD),5)
molecular beam epitaxy (MBE),6) plasma-assisted MBE
(PAMBE),4,7) thermal evaporation,8) and metal–organic
CVD (MOCVD).9,10) Si-doped GaN NRs grown by CVD
were characterized by wool-like structures and poor
emission properties.5,8) Satisfied morphology of Si-doped
GaN NRs was obtained by MBE and PAMBE, but the used
Si-flux was higher than the regular doping levels used in
optoelectronic devices.3,5,6) Furthermore, self-assembled Sidoped GaN NRs were grown by MOCVD on a pre-deposited
SiNx seed layer, but also strong effects was observed at high
Si-fluxes.9) The effect of Si-doping were compared for GaN
NRs and thin films grown by MOCVD, which showed that
the incorporation of Si during growth is not advantageous for
GaN NRs.10)
In this work, we studied the effect of Si-doping on the
morphological and optical properties of GaN NRs during
vapor–liquid–solid (VLS) growth. Effects of Si-doping on
the morphological properties have been studied by using
field-emission scanning electron microscopy (FE-SEM),
whereas optical qualities have been characterized by using
room temperature photoluminescence (RT-PL).
2. Experiments
GaN NRs were grown by the widely employed VLS
technique in a low pressure vertical MOCVD reactor on
c-plane sapphire substrates. First, about 1 nm Ni–metal film
was deposited by using e-beam evaporator on sapphire
substrate. In order to make nano-dispersed Ni-particles,
sapphire substrate coated with Ni was loaded into the reactor
and annealed at 830 C for 5 min under high nitrogen flow.
After annealing, the Ni–metal film produced nano-dispersed
Fig. 1. SEM image of nano-dispersed Ni-particles after 5 min annealing
in MOCVD reactor.
particles with sizes of about 30 nm, as shown in Fig. 1. The
growth of GaN NRs was performed at a temperature range
from 800 to 700 C and a low pressure of 30 Torr for 20 min
in hydrogen-environment. Trimethylgallium (TMGa) and
NH3 were used as the Ga- and N-precursors, respectively.
The NH3 flow rate was 2:23 10 2 mol/min, while that of
TMGa was about 1:26 10 5 mol/min. After the optimization of undoped GaN NRs, n-type doping was achieved by
the injection of different fluxes of diluted SiH4 gas during
the growth. The total flow of SiH4 was varied and
morphological changes of GaN NRs were observed by
using FE-SEM. RT-PL measurements were used to evaluate
the optical qualities of undoped and Si-doped GaN NRs.
3. Results and Discussion
Figure 2 shows typical FE-SEM images of undoped GaN
NRs grown at different growth temperatures from 800 to
700 C. Dense GaN NRs with irregular surfaces were
produced only at 700 C. It has been found that, increasing
the growth temperature will enhance the pyrolysis of
precursors causing the effective V/III ratio to increase
during the growth, which favors the vapor–solid growth
mode over VLS growth.11) Consequently, GaN NRs growth
were suspended and only rough surfaces were formed at
higher growth temperatures than 700 C. Ni-nanoparticles
appeared at the apex of GaN NRs grown at 700 C confirmed
the VLS growth mode, as shown in Fig. 2(d).
Figure 3 demonstrates the effect of Si-doping on the
morphological properties of GaN NRs. The main features
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# 2013 The Japan Society of Applied Physics
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Jpn. J. Appl. Phys. 52 (2013) 08JE08
(a)
(b)
(c)
(d)
M. Ebaid et al.
(a)
(b)
(c)
Fig. 2. SEM images of undoped GaN NRs at various growth
temperatures, (a) 800 C, (b) 750 C, (c) 700 C and (d) magnified view of
(c) showing the formation of Ni-nanoparticles at the apices of GaN NRs.
Fig. 4. Morphological changes of GaN NRs as a function of SiH4 flow:
(a) 1:3 10 9 , (b) 2:0 10 9 , and (c) 4:5 10 9 mol/min.
Undoped GaN NR
Si-doped GaN NR
Undoped GaN thin film
2000
(b)
Relative Intensity (arb. unit)
(a)
Fig. 3. SEM images of Si-doped GaN NRs: (a) morphological changes
after the injection of SiH4 gas and (b) magnified view showing the needlelike shapes after Si-doping.
acquired by Si-doping are: (i) the enhanced morphology,
which was characterized by smooth surfaces and straightness [Fig. 3(a)], (ii) the broadening of the nanorod base
combined with sharp tips that led to the formation of needlelike shapes [Fig. 3(b)], (iii) the decrease of nanorods length
and density. These effects are much more pronounced for
Si-doped GaN NRs.4,8,9) Morphological variations after Sidoping showed large dependency on the flow of SiH4 gas.
Based on the morphological changes as a function of
different SiH4 fluxes, we optimized the SiH4 flow rate at
2:7 10 9 mol/min; Fig. 3, which may equivalent to bulk
Si-doping concentrations in the range of 1018 to 1019 cm 3 .
With lower SiH4 flow rate, GaN NRs morphology was also
enhanced, but the density was reduced, as shown in
Figs. 4(a) and 4(b). In contrast, when the SiH4 flow rate
was increased to 4:5 10 9 mol/min, which is about two
times higher than the optimized SiH4 flow, GaN NRs
were not observed, as given by Fig. 4(c). It was attributed to
the formation of the amorphous silicon nitride layer under
Si- and N-rich conditions,4) which may hinder the nucleation
of GaN.
It has been reported that the incorporation of Si-atoms
during the growth of GaN will affect the surface kinetics.3,4)
Theoretical studies showed that the formation energy of a
1600
1200
800
400
0
350
360
370
380
390
400
410
420
Wavelength (nm)
Fig. 5. PL spectra of undoped and Si-doped GaN NRs compared to that of
bulk GaN layer.
Ga-vacancy in GaN is about 0.7 eV, which is much smaller
than the energy needed for a N-vacancy (4 eV)12) and also
the formation energy needed for the Si-atom to reside on the
original Ga-atom site is about 1 eV.13) Additionally, it has
been showed that under nitrogen-rich conditions and the
existence of Si, the (0001) surface of GaN is reconstructed
by the replacement of Ga-adatoms with the dopant Siatoms.14) Based on the above observations, we suggest that
Si-atoms were resided at the original Ga-sites on the bare
(0001) surface, which may reduce the axial growth and
taper the Si-doped GaN NRs. Furthermore, the substituted
Ga-atoms may possibly diffuse to be incorporated on the
side walls, which could increase the radial growth rate and
enlarge the bases of GaN NRs.
The optical quality of both undoped and Si-doped GaN
NRs was measured by RT-PL, as shown in Fig. 5. The
absence of defects-related yellow luminescence and small
full width at half maximum (FWHM) of about 19.5 and
27.3 meV for the undoped and Si-doped GaN NRs, respectively, has confirmed their high optical quality. Virtually, the
luminescence obtained from GaN NRs was compared with
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Jpn. J. Appl. Phys. 52 (2013) 08JE08
M. Ebaid et al.
that of the bulk GaN layer grown by the same MOCVD,
which showed a substantial improvement for both undoped
and Si-doped GaN NRs; however the emission intensity was
increased by twice for the later. Undoped GaN NRs showed
NBE emission peak centered at 357.12 nm (3.472 eV), while
that of Si-doped GaN NRs was relatively blue shifted at
356.88 nm (3.474 eV), which has combined with a shoulder
at 355.04 nm (3.492 eV). The blue-shift after Si-doping
may be attributed to the band filling effects due to the
improvement of carrier concentrations, which correspondingly shifted the quasi-Fermi level. This phenomenon is
known as Burstein–Moss shift.15) Results obtained from RTPL reflected the high optical and crystalline qualities of GaN
NRs after Si-doping.
control by Si-doping, which will be critical for nanorodbased GaN devices.
Acknowledgement
This work was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and
Technology (2011-0017190).
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this work to produce the n-type doping of GaN NRs, i.e.,
VLS growth technique has been employed. Undoped GaN
NRs were formed with high density but irregular surfaces.
Si-doped GaN NRs were characterized by straightness and
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We attributed these modifications to the surface reconstruction caused by Si-dopants as well as Ga-atoms diffusion on
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