Infrared lasing in InN nanobelts

Article abstract of DOI:10.1063/1.2714291

Infrared lasing from single-crystalline InN nanobelts grown by metal organic chemical vapor deposition was demonstrated. Transmission electron microscopy studies revealed that the InN nanobelts of rectangular cross section grew along [110] direction and w

Full text of DOI:10.1063/1.2714291

Infrared lasing in InN nanobelts
Ming-Shien Hu, Geng-Ming Hsu, Kuei-Hsien Chen, Chia-Ju Yu, Hsu-Cheng Hsu, Li-Chyong Chen, Jih-Shang
Hwang, Lu-Sheng Hong, and Yang-Fang Chen
Citation: Applied Physics Letters 90, 123109 (2007); doi: 10.1063/1.2714291
View online: http://dx.doi.org/10.1063/1.2714291
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APPLIED PHYSICS LETTERS 90, 123109 ͑2007͒
Infrared lasing in InN nanobelts
Ming-Shien Hu, Geng-Ming Hsu, and Kuei-Hsien Chena͒,b͒
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
Chia-Ju Yu, Hsu-Cheng Hsu, and Li-Chyong Chena͒,c͒
Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
Jih-Shang Hwang
Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan
Lu-Sheng Hong
Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607,
Taiwan
Yang-Fang Chen
Department of Physics, National Taiwan University, Taipei 10617, Taiwan
͑Received 28 October 2006; accepted 10 February 2007; published online 21 March 2007͒
Infrared lasing from single-crystalline InN nanobelts grown by metal organic chemical vapor
deposition was demonstrated. Transmission electron microscopy studies revealed that the InN
nanobelts of rectangular cross section grew along ͓110͔ direction and were enclosed by ͑001͒ and
¯
͑110͒ planes. The infrared lasing action was observed at 20 K in the InN nanobelts grown on an
amorphous silicon nitride coated silicon substrate by continuous wave laser pumping. © 2007
American Institute of Physics. ͓DOI: 10.1063/1.2714291͔
Indium nitride ͑InN͒ is a promising semiconductor
which can be used in many potential applications, such as
high frequency electronic devices,1 light emitting diodes,2,3
and solar cells.4 Recently, InN has received more attention
due to the growing debate over the revision of its fundamen-
tal band gap from visible ͑1.8–2.1 eV͒ to infrared
͑0.7–0.8 eV͒ spectral range,5 although some minor contro-
versy remained.6 Actually, resolving the band gap contro-
versy as well as realizing the device fabrication strongly rely
on the growth of high quality crystal of InN. Nevertheless, it
is still a challenging task due to the extremely high equilib-
rium vapor pressure of nitrogen and the low decomposition
temperature of InN during growth.7
In contrast to its thin film counterpart, freestanding one-
dimensional ͑1D͒ nanostructures are easier to grow in single-
crystal forms without defects, and hence lasing in the crys-
tals could be more expected. Currently, various InN
nanostructures, such as nanocrystals,8 nanocolumns,9
nanorods,10 nanowires,11 nanotubes,12 nanotips,13 and
nanobelts,14 have been extensively reported. Noticeably
among these, InN nanobelts grown by a guided-stream ther-
mal chemical vapor deposition ͑GSCVD͒ technique have
shown a sharp infrared photoluminescence ͑PL͒ emission
with a full width at half maximum ͑FWHM͒ of ϳ14 meV at
20 K, along with a linewidth narrowing at high excitation
power, which was attributed to amplified spontaneous emis-
sion ͑ASE͒.14 The onset of the ASE in the GSCVD-derived
InN nanobelts sheds some light on the possibility of lasing if
a proper cavity is present. Similar lasing behaviors in a va-
riety of 1D nanostructures including ZnO, GaN, CdS, and
ZnS have been observed.15 In light of the above, in this letter,
we report the observation of infrared lasing in high quality
single-crystalline InN nanobelts fabricated by metal organic
chemical vapor deposition ͑MOCVD͒. The realization of in-
frared lasing in InN nanobelts enables miniaturized IR laser
light sources and opens up opportunities in the area of opti-
cal communication, gas sensors, and life sciences.
InN nanobelts were grown on Si ͑100͒ substrates in a
homemade MOCVD reactor using trimethylindium and am-
monia ͑NH3͒ as the source materials of indium and nitrogen,
respectively, while purified nitrogen was used as the carrier
gas. For better optical confinement, the same growth was
also conducted later on Si ͑100͒ substrates but coated with a
thin amorphous silicon nitride ͑a-SiNx͒ layer ͑ϳ200 nm͒ us-
ing low-pressure chemical vapor deposition. Prior to growth,
both two kinds of substrates were deposited a thin layer of
gold film ͑2 nm͒, as the catalytic layer, with a direct current
sputter ͑Emitech, K550X͒. The synthesis of InN nanobelts
was all performed at the temperature of 540 °C and the pres-
sure of 1 Torr for 4 h.
After growth, blackish products with a beltlike morphol-
ogy were found to cover the entire substrates ͑2ϫ2 cm2͒,
regardless of the presence of the a-SiNx layer on the silicon
substrates. The beltlike morphology of the products was con-
firmed using field emission scanning electron microscopy
͑FESEM͒ on a JEOL 6700 system. X-ray diffraction ͑XRD͒
on a Toshiba model A-40-Cu and transmission electron mi-
croscopy ͑TEM͒ on a high resolution JEOL model JEM-
4000EX were employed to investigate the microstructure of
the nanobelts. PL studies were performed at 20 K in a
closed-cycle refrigerator system by using the 488 nm line of
an Ar+ laser as the excitation source and an InGaAs detector
was used to record the spectra in the IR range.
Figure 1͑a͒ shows typical FESEM images of the InN
nanobelts grown on Si͑100͒, which basically have the same
morphology as that grown on a-SiNx coated silicon. Clearly,
high density nanobelts with lengths in the range of
15–62 ␮m can be observed. As shown in the high-
a͒
Authors to whom the correspondence should be addressed.
Electronic mail: chenkh@pub.iams.sinica.edu.tw
b͒
c͒
Electronic mail: chenlc@ntu.edu.tw
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123109-2
Hu et al.
Appl. Phys. Lett. 90, 123109 ͑2007͒
FIG. 1. ͑a͒ Typical XRD spectrum of MOCVD-grown InN nanobelts. The
inset shows low-magnification SEM image of InN nanobelts. ͑b͒ High-
magnification SEM image of a single InN nanobelt with well-defined facets
and the observation of metallic nanoparticles attached at the growth front
ends of InN nanobelts ͑inset.͒
FIG. 2. ͑a͒ Low-magnification TEM image of a single InN nanobelt. ͑b͒
HRTEM image of InN nanobelt and corresponding selected area diffraction
pattern ͑inset͒ of taken from the ͓001͔ zone axis. ͑c͒ Schematic diagram of
the nanobelt crystallographic directions.
uted to the effect of ASE similar to the case observed in the
GSCVD grown nanobelts.14 Note that further increasing the
pump power would only lead to structural damage of the
nanobelts and no lasing behavior was observed. We have
ascribed the reason to the poor light confinement of the
nanobelts at the InN/Si͑100͒ interface, since the refractive
index of silicon ͑nsi=3.47͒ ͑Ref. 17͒ was much larger than
that of InN ͑nInN=2.9͒ ͑Ref. 18͒. Please refer to the report of
Maslov and Ning19 for a detailed discussion about the reflec-
tivities, gain threshold, and lasing modes of nanowires.
For better light confinement, the a-SiNx coated Si͑100͒
magnification FESEM image of Fig. 1͑b͒, the nanobelts have
well-faceted morphologies and atomically smooth surfaces,
and have widths ranging from 40 to 250 nm and thicknesses
from 10 to 35 nm. The observation of the catalytic nanopar-
ticles attached at the growth front ends, as shown in Fig.
1͑c͒, strongly supports that the growth of nanobelts is domi-
nated by the vapor-liquid-solid ͑VLS͒ growth mechanism.
XRD spectrum of the nanobelts was shown in Fig. 1͑d͒. The
diffraction peaks can be well indexed based on the hexagonal
structure of InN, with lattice constants of a=0.353 nm and
c=0.570 nm, which are in agreement with the standard lit-
erature values of a=0.354 nm and c=0.571 nm.16
substrate, where na-SiN =1.9,20 was employed. Eventually,
x
the lasing behavior was observed, as summarized in Fig. 4.
At low excitation intensity ͑Ͻ50 kW/cm2͒, as shown in Fig.
4͑a͒, the spectrum consisted of a single broad spontaneous
emission band with a FWHM of ϳ30 nm at 1594 nm. As the
pumping intensity increases up to 73 kW/cm2, several sharp
peaks representing multiple laser modes emerged in the spec-
tra between 1559 and 1644 nm. The typical linewidth of the
observed sharp emission peaks at the highest pumping inten-
sity of 75 kW/cm2 is ϳ2.3 nm, which is nearly 14 times
narrower than that ͑ϳ33 nm͒ of the spontaneous emission
below the threshold. The pump-power-dependent PL inten-
sity was summarized in the inset of Fig. 4͑a͒, which showed
clearly that above a threshold excitation intensity ͑Ip͒ of
ϳ70 kW/cm2, a strong superlinear dependence of the emis-
sion intensity ͑Ist͒ with Ist=I4p.9 was present, while the depen-
Microstructure of the MOCVD-derived InN nanobelts
was further characterized by TEM. Figure 2͑a͒ shows a typi-
cal TEM bright-field image of the InN nanobelts. The dark
fringes on the nanobelts result from bending contours, which
are normally observed in bent thin crystals by TEM. The
corresponding high resolution TEM ͑HRTEM͒ image of the
nanobelt is shown in Fig. 2͑b͒, which reveals the hexagonal
structure of InN nanobelts with interplanar spacings of 0.177
and 0.308 nm along the ͓110͔ and ͓100͔ directions, respec-
tively. Shown in the inset of Fig. 2͑b͒ is the typical selected
area electron diffraction pattern recorded along the ͓001͔
zone axis of the hexagonal InN. The clear spotty pattern
indicates the single-crystal nature of the nanobelts. The
above TEM studies concluded that the InN nanobelts were
grown along the ͓110͔ direction and were enclosed by
¯
͑001͒ and ͑110͒ facets, as depicted in Fig. 2͑c͒. Further-
more, it should be emphasized that InN nanobelts grown on
bare Si substrates and a-SiNx coated Si substrates both ex-
hibit identical crystallographic properties, including growth
direction and side facets.
To explore the possibility of stimulated emission from
InN nanobelts, power-dependent photoluminescences of InN
nanobelts grown on a Si͑100͒ substrate and an a-SiNx coated
Si͑100͒ substrate were both carried out. Figure 3 shows the
20 K power-dependent PL spectra of InN nanobelts grown
on
a Si substrate. At low excitation intensity, i.e.,
10 kW/cm2, a single broad spontaneous emission band with
a FWHM of ϳ33 nm at 1600 nm was observed, which cor-
responded to the near band edge emission of InN. Appar-
ently, a progressive narrowing of the peak width from
ϳ33 to 17 nm associated with an enhancement of PL inten-
sity can be clearly seen as the excitation power increases
FIG. 3. Excitation power dependence of the emission spectra of InN nano-
belts grown on Si substrates taken at 20 K. Excitation power was increased
from 10, 19, 29, 39, and 77 kW/cm2 for spectra ͑I͒, II, III, IV, and ͑V͒,
respectively. The inset shows the integrated intensity as a function of exci-
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from 10 to 77 kW/cm2. The peak narrowing could be attrib-
tation intensity.
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123109-3
Hu et al.
Appl. Phys. Lett. 90, 123109 ͑2007͒
dence of the spontaneous peak intensity was sublinear with
Ist=I0p.83. The simultaneous superlinear dependence of the
emission intensity and the linewidth narrowing at high
pumping power ͓shown in Fig. 4͑b͔͒ suggests a transition
from spontaneous emission to stimulated emission in the
MOCVD-grown InN nanobelts.
Note here that for lasing in the InN nanobelts, a nanobelt
cavity with effective reflectance on both ends is required.
Though InN nanobelts were not vertically aligned with re-
spect to the substrate, they were grown from the substrate,
which enables good contact between the bottom end and the
substrate. The observation of the lasing behavior from the
InN nanobelts grown on the a-SiNx coated silicon confirmed
the fact that the waveguide structure based on the refractive
index difference at the interface of air ͑nair=1͒/InN ͑nInN
FIG. 4. ͑a͒ Power-dependent PL spectra of InN nanobelts grown on SiN
coated Si substrates recorded at 20 K with excitation intensities of 39, 50,
62, 73, and 75 kW/cm2 for PL spectra ͑I͒, II, III, IV, and ͑V͒, respectively.
The inset shows the integrated intensity under different excitation intensi-
ties. ͑b͒ Linewidth of the emission peak as a function of excitation intensity.
͑c͒ Lasing spectrum obtained at different sites of the same sample as ͑a͒ at
the excitation intensity of 77.5 kW/cm2.
ϳ2.9͒/SiNx ͑nSiN ϳ1.9͒ facilitates better confinement and
x
amplification of the light within the waveguide structure of
air/InN/SiNx for lasing action. This effect may be similar to
the UV lasing action of ZnO nanoneedles grown on SiO2
coated Si substrates, as reported by Lau et al.21
Assuming that the measured lasing modes were resulted
from a Fabry-Pérot cavity, the cavity length ͑L͒ can be de-
termined by L=␭2/2n⌬␭, where n is the refractive index and
⌬␭ is the mode spacing. Taking n=2.9 for InN and ⌬␭
=7 nm, a cavity length of ϳ60 ␮m is required to sustain the
observed lasing modes. This calculated cavity length falls
well in the range of the length of nanowires measured by
SEM. Figure 4͑c͒ depicts another Fabry-Pérot cavity mode
taken at a different site of the same sample, wherein an av-
erage mode spacing of 11 nm was observed, corresponding
to a cavity length of 41 ␮m. According to a first
approximation,22 the gain threshold is proportional to 1/L,
thus leading to a lower lasing threshold for longer cavity
length. In the present case, more than one cavity length was
observed, suggesting that lasing takes place from InN nano-
belts of different lengths. This fact also reflects the nonuni-
formity of nanobelt length. Furthermore, as shown in Fig.
4͑a͒, a clear redshift of the gain profile was observed from
39 to 75 kW/cm−2. The redshift of the gain profile could be
induced by the band gap renormalization due to Coulomb
interactions among amplified free carriers at the band edge.
Similar redshift phenomena have been reported in 1D GaN
͑Ref. 23͒ or ZnO ͑Ref. 24͒ nanostructures. Another possibil-
ity of the redshift could be ascribed to the local heating of
the nanowires, as high power densities of continue wave ex-
citation were used in our study.
Nano Science and Technology is gratefully acknowledged.
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