APPLIED PHYSICS LETTERS 110, 191101 (2017)

Cold exciton electroluminescence from air-suspended carbon nanotube
split-gate devices
N. Higashide,1 M. Yoshida,2,3 T. Uda,2,4 A. Ishii,1,2 and Y. K. Kato2,3,a)1

Department of Electrical Engineering, The University of Tokyo, Tokyo 113-8656, Japan
Nanoscale Quantum Photonics Laboratory, RIKEN, Saitama 351-0198, Japan
3
Quantum Optoelectronics Research Team, RIKEN Center for Advanced Photonics, Saitama 351-0198, Japan
4
Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan
2(Received 7 February 2017; accepted 22 April 2017; published online 8 May 2017)
Electroluminescence from individual carbon nanotubes within split-gate devices is investigated. By
characterizing the air-suspended nanotubes with photoluminescence spectroscopy, chirality is identified and electroluminescence peaks are assigned. We observe electroluminescence linewidths
comparable to photoluminescence, indicating negligible heating and state-mixing effects. Splitgate and bias voltage dependences are consistent with emission from an electrostatically formed
C 2017 Author(s). All article content, except where otherwise noted, is licensed underpn-junction. V
a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
[http://dx.doi.org/10.1063/1.4983278]

Single-walled carbon nanotubes (CNTs) offer uniquecharacteristics for applications in nanoscale optoelectronics,
being a one-dimensional semiconductor that can be directlygrown on silicon. With their diameters of a few nm or less,they can be utilized as near field emitters, while their lengths
can be over many microns, allowing for straightforwardfabrication of electrically driven devices. Consequently,
electroluminescence (EL) from CNTs has been investigatedin various device structures,1–8 and numerous mechanisms
have been proposed to interpret the experimental results.
Nevertheless, typical EL spectra exhibit broad linewidthsof over 100 meV,2,3,5,7,9 indicative of significant heating
caused by bias voltages required to drive the devices. Such aspectral broadening is undesirable for coupling to highquality-factor cavities10–15 and it is also known that quantum
efficiencies decrease at higher temperatures.16 In this context,
a promising result has been reported for electrostaticallyformed pn-junctions using split-gate devices, where a linewidth as narrow as 35 meV was observed from a nanotube
embedded between a substrate and a gate dielectric.4 Further
investigation of split-gate devices, in particular those utilizing
air-suspended tubes, has revealed ideal diode behavior andsuperior photoconductivity performance.17–22 Considering
these experimental efforts on electrostatically formed pnjunctions in air-suspended CNTs, improved EL performanceis expected as well.
Here, we report on EL from air-suspended carbon nanotube split-gate devices and show that EL can be obtained
without any spectral broadening in comparison to photoluminescence (PL). By characterizing the nanotubes with PL
excitation spectroscopy, nanotube chirality is identified and
EL is assigned to E11 exciton recombination. Split-gate andbias voltage dependences of EL intensity are both consistent
with emission originating from an electrostatically formed
pn-junction, and the reconfigurable nature of split-gate devices is also demonstrated.
a)Author to whom correspondence should be addressed: yuichiro.kato@riken.jp

0003-6951/2017/110(19)/191101/4

A schematic of our split-gate field-effect device isshown in Fig. 1(a). Air-suspended CNTs are contacted on
both sides of a trench, and two local gates are used for electrostatic doping. The devices are fabricated on silicon-oninsulator substrates with a 260-nm-thick top Si layer and1-lm-thick buried oxide.23 The top Si layer is boron dopedwith a resistivity of 18.0 6 4.5 X cm, allowing for use as

FIG. 1. (a) Schematic of a carbon nanotube field-effect split-gate device. (b)Scanning electron micrograph of a typical device. (c) and (d) PL and reflectivity images, respectively, in a device with a nanotube showing EL. Data
are taken with an excitation power P ¼ 30 lW and an excitation wavelengthof 836 nm. For (c), the spectral integration window for the PL image is from1304 to 1330 nm. (e) PL excitation map of the nanotube measured in (c),and P ¼ 50 lW is used. In (b)–(d), the scale bars are 1 lm. For (c)–(e), thelaser polarization is perpendicular to the trench.

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split gates. We begin by performing electron beam lithography to define trenches with a width of 1 lm for suspending
the nanotubes and etch 1000 nm deep through the top Si
layer and into the buried oxide with an inductively coupledplasma etcher with CHF3 gas. Thermal oxidation is then performed at 1050  C for 30 min to form a 55-nm-thick SiO2
layer on the top Si layer, and another electron beam lithography step patterns the split-gate electrodes. A wet etching process removes 40 nm of thermal oxide, and we deposit Ti(2 nm)/Pt (20 nm) by electron beam evaporation for contacting the top Si layer. Following a lift-off process of the gateelectrodes, source and drain contacts are patterned right next
to the trenches by a third electron beam lithography step. We
evaporate SiO2 (80 nm)/Ti (2 nm)/Pt (20 nm) and perform
lift-off to form the contacts. A fourth lithography step defines
the catalyst areas overlapping small windows within thesource and drain contacts, and Fe/silica dissolved in ethanol
is spin-coated and lifted-off.24 Finally, CNTs are synthesizedby ethanol chemical vapor deposition at 750  C for 1 min.
Figure 1(b) shows a scanning electron micrograph of a typical device.
The devices are characterized with a home-built microspectroscopy system, where an automated three-dimensional
stage is utilized for sample scanning.23–25 For EL measurements, the source contact and the substrate are grounded,
and we apply voltages VG1 to gate 1, VG2 to gate 2, and VDS
to the drain contact. As an excitation source for PL measurements, a wavelength-tunable continuous wave Ti:sapphirelaser is used, and an objective lens with a numerical apertureof 0.8 and a focal length of 1.8 mm focuses the laser on thesample. Light emission from the sample is collected with thesame objective lens, and a 50-mm focal length lens is used tocouple to a 300-mm spectrometer which houses a 150-lines/
mm grating for spectroscopy and a mirror for imaging. The
emission signals are detected by a 25.6-mm-long 512-pixel
InGaAs photodiode linear array. Background subtraction is
performed with data taken at all gate and bias voltages atzero for EL measurements, while a laser beam shutter is utilized for PL spectroscopy. Samples are kept in the N2 atmosphere to reduce gate hysteresis,26 and all measurements are
performed at room temperature.
In order to find suspended nanotubes that show EL, we
perform one-dimensional imaging measurements by using themirror in the spectrometer. Linear images with a field-of-viewof 50 lm are acquired at voltages of VG1 ¼ V, VG2 ¼ –V, andVDS ¼ V/2, and each 610-lm-long trench is scanned by collecting such images in 50-lm steps. If no emission isobserved, we repeat the scan with an increased V until we finda signal or V reaches 30 V. About 2% of the tubes identified
by PL are found to exhibit EL.
Once a light-emitting nanotube is identified, we turn off
the voltage and characterize the tube by PL spectroscopy. A
confocal pinhole with a diameter of 150 lm is placed at theentrance of the spectrometer, and the mirror in the spectrometer is switched to the grating. In Fig. 1(c), a PL image
obtained by mapping out the integrated PL intensity IPL isshown. By comparing to a reflectivity image [Fig. 1(d)], we
confirm that the nanotube is fully suspended over the trench.
In order to identify the chirality, PL excitation spectroscopy
is performed with a laser power P ¼ 50 lW [Fig. 1(e)], andAppl. Phys. Lett. 110, 191101 (2017)

we determine the tube chirality to be (12,4) using tabulated
data.24 The laser polarization dependence of PL is also measured to determine the angle of the tube.27 These measurements ensure that EL spectroscopy is performed on asemiconducting nanotube that is fully suspended and individual. We note that some nanotubes do not show PL but exhibit
electrically induced emission with broad linewidths on theorder of 100 meV (supplementary material Figure). We attribute such emission to thermal light emission from metallic
CNTs.3On the (12,4) tube characterized in Figs. 1(c)–1(e), we
have obtained an EL spectrum under an application of voltages VG1 ¼ 16 V, VG2 ¼ –16 V, and VDS ¼ 5 V [Fig. 2(a)].
The peak emission wavelength and the linewidth are identical to those for a PL spectrum taken in the absence of thevoltages [Fig. 2(b)], showing that the EL peak originatesfrom the E11 excitons. The observed full-width at halfmaximum of the emission is about 8 meV, which is significantly smaller than previous reports on EL linewidths
ranging from 35 meV to 100 meV.2,4–9 Broadening ofemission has been attributed to high carrier temperatures aswell as field-induced mixing of excitonic states with the continuum,4 and in particular, heating-induced line broadening
has been well characterized by PL spectroscopy.28 The sharp
peak observed in Fig. 2(a) indicates that excitons remain
cold, with negligible Joule heating effects keeping the nanotube in thermal equilibrium with the substrate. In addition,
the data also demonstrate that EL can be obtained at modest
bias and internal fields where state mixing effects do notplay a role. The observation of such a narrow linewidth spectrum is likely due to higher emission efficiencies in as-grown
air-suspended nanotubes that allow for detection of EL atmuch lower excitation currents. EL spectra obtained fromtypical devices on the same chip have also exhibited muchnarrower linewidth compared to the previous reports (supplementary material Table).
We note that EL intensity [Fig. 2(a)] is considerably
smaller compared to PL intensity [Fig. 2(b)], which may becaused by differences in the length of luminescent regions.
Emission intensity quenches with an application of gate voltage,23,29,30 and only a limited length would remain active
in a pn-junction configuration. In comparison, the entire

FIG. 2. Light emission from the nanotube characterized in Figs. 1(c)–1(e).
(a) EL spectrum taken at VG1 ¼ 16 V, VG2 ¼ –16 V, and VDS ¼ 5 V. (b) PL
spectrum taken with VG1 ¼ VG2 ¼ VDS ¼ 0 V and P ¼ 5 lW. The laser polarization angle is adjusted to maximize the PL signal, and a resonant excitation
wavelength of 836 nm is used. (c) Integrated EL as a function of VG1 and
VG2 taken with VDS ¼ 7 V.

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Appl. Phys. Lett. 110, 191101 (2017)that EL can be obtained without heating up the nanotube andwith negligible field-induced state mixing. Split-gate voltage
and bias voltage dependences show that emission occurs forvoltage combinations corresponding to forward-biased pnjunctions, confirming the flexible configuration of the device
utilizing electrostatic carrier doping.

FIG. 3. (a) Drain voltage dependence of integrated EL intensity for a (9,8)
nanotube with Vpn ¼ 10 V (orange open circles) and –10 V (blue filled triangles). (b) Integrated EL as a function of VDS and Vpn.

suspended length of the nanotube is luminescent for PL
measurements. Another likely reason is the difference in the
excitation powers for EL and PL. Unfortunately, we are notable to determine the current through this nanotube as ourdevice structure has multiple tubes, making it difficult toquantify the electrical excitation power and the external
quantum efficiency. In addition, there exists uncertainties forabsorption cross section of nanotubes,24,31–35 causing complications in quantifying optical excitation rates as well.
Another possible reason for the differences in the emission
intensity is the existence of triplet excitons which may be
more efficiently generated in the case of EL.
In order to confirm the role of electrostatically formed
pn-junction, we investigate EL dependence on VG1 and VG2
at a constant bias VDS ¼ 7 V [Fig. 2(c)], where spectrally
integrated EL intensity IEL is collected by switching the grating to a mirror. We observe bright emission only in thebottom-right corner of the map, corresponding to the region
with VG1 > 0 V and VG2 < 0 V where a forward biased pnjunction should be formed. In the top-left corner of the map,a pn-junction should also exist but it would be reverse
biased, and in the top-right and the bottom-left regions of themap, there should not be any pn-junction as VG1 and VG2have the same polarity. The experimental results are mostly
consistent with such a picture, and slight deviations may
arise from residual hysteretic effects of the gates. We notethat EL linewidth did not show any detectable change fordifferent gate voltage combinations.
Next, we examine the VDS dependence for another tube,
where we define Vpn ¼ VG1 ¼ –VG2 to be the symmetricallyapplied gate voltage that forms the pn-junction. The bias
dependence of EL [Fig. 3(a)] resembles a current-voltage
curve for a pn-junction diode, which is reasonable as emission should be proportional to the current. EL is observed forboth polarities of VDS for this device, if the sign of Vpn isalso inverted to form a forward biased pn-junction. We notethat not all tubes show EL for both polarities of VDS, presumably due to asymmetries in the contact resistance. A detailed
map of gate and bias dependence [Fig. 3(b)] shows that thisdevice can be switched from a pn-junction to an np-junction
configuration with a change of few volts, demonstrating thereconfigurability of the split-gate devices.
In summary, we have investigated EL from suspended
carbon nanotubes within split-gate devices. By characterizing
the same nanotube by PL spectroscopy, emission has been
attributed to E11 exciton recombination. We observe ELemission linewidth comparable to those for PL, indicating

See supplementary material for the emission spectrumof a suspended carbon nanotube with a broad linewidth and atable listing linewidths for other nanotubes.
This work was supported by JSPS (KAKENHI
JP16H05962, JP26610080), MEXT (Photon Frontier Network
Program, Nanotechnology Platform), Canon Foundation, andAsahi Glass Foundation. T.U. was supported by ALPS and
A.I. was supported by MERIT and JSPS Research Fellowship.
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