Letter pubs.acs.org/NanoLett
An Electrically Driven, Ultrahigh-Speed, on-Chip Light Emitter Based on Carbon Nanotubes Tatsuya Mori,† Yohei Yamauchi,† Satoshi Honda, and Hideyuki Maki* Department of Applied Physics and Physico-Informatics, Keio University, Yokohama 223-8522, Japan S Supporting Information *
ABSTRACT: The integration of high-speed light emitters on silicon chips is an important issue that must be resolved in order to realize on-chip or interchip optical interconnects. Here, we demonstrate the first electrically driven ultrafast carbon nanotube (CNT) light emitter based on blackbody radiation with a response speed (1−10 Gbps) that is more than 106 times higher than that of conventional incandescent emitters and is either higher than or comparable to that of light-emitting diodes or laser diodes. This high-speed response is explained by the extremely fast temperature response of the CNT film, which is dominated by the small heat capacity of the CNT film and its high heat dissipation to the substrate. Moreover, we experimentally demonstrate 140 ps width pulsed light generation and real-time optical communication. This CNT-based emitter with the advantages of ultrafast response speeds, a small footprint, and integration on silicon can enable novel architectures for optical interconnects, photonic, and optoelectronic integrated circuits. KEYWORDS: Carbon nanotubes, high-speed light emitter, blackbody radiation, optical communication
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Electrically driven CNT emitters, which can be based on electron−hole recombination4−8 or blackbody radiation,9−17 possess several distinctive advantages. For example, (i) a small footprint emitter can easily be obtained due to its simple fabrication process in which two electrodes are deposited and where complex bandgap engineering by carrier doping and composition control is not necessary. Additionally, (ii) CNTs can be prepared directly on a Si wafer, unlike compound semiconductor-based light emitters. These advantages could open new routes to photonics or optoelectronics integrated with silicon-based electronics;18,19 for example, the CNT emitters are advantageous over the compound-semiconductor emitters for high-speed and high-density optical interconnects because the extremely small CNT emitters can be directly connected to optical waveguides without waveguide coupler and can enable direct signal encoding without optical modulators, which are required in the current interconnect technology. However, for such applications the question remains as to whether an electrically driven CNT emitter can be modulated at high frequency as well as compoundsemiconductor LEDs and LDs, which have modulation speeds on the order of megahertz to gigahertz. In this study, we report the first electrically driven, ultrahigh-speed CNT light emitter based on blackbody emission. Blackbody emitters based on CNTs have been previously demonstrated by passing a current through bundled ropes,9,10 films,11−13 vertically aligned
lectrically driven high-speed light emitters based on compound semiconductors, such as light-emitting diodes (LEDs) and laser diodes (LDs), are widely used in the areas of optical communication, sensing, time-resolved spectroscopy, digitally controlled light sources, and so forth. In addition, the continuous-wave compound-semiconductor LDs are also used for higher-speed optical communication with the optical modulators as external E/O converters. However, because of their large footprints, the low crystallinity of the compound semiconductors grown directly on Si wafers, and the sophisticated device structures associated with p−n junctions and bandgap engineering,1 these emitters face significant challenges with respect to their integration with silicon-based electronics, photonics, and micromechanical platforms. In comparison, the blackbody emitter, which is associated with Joule heating, is another type of electrically driven emitter. This emitter has a broad temperature-dependent spectrum that is given by Planck’s law2 in contrast to LDs and LEDs, which have sharp emission peaks that are associated with the bandgap. Although blackbody emitters have been employed in many steady-state or low-speed illumination and heating applications, such as incandescent bulbs, conventional blackbody emitters based on metal wires have not been used for high-speed applications, such as in optical communication devices, due to their slow response time (on the order of milliseconds), large footprint, and low electromigration immunity. Single-walled carbon nanotubes (CNTs) are an attractive material for optical and optoelectronic applications, such as optically3 and electrically4−7 excited light sources, because of their unique optical, electrical, and thermal properties. © 2014 American Chemical Society
Received: February 24, 2014 Revised: April 12, 2014 Published: May 5, 2014 3277
dx.doi.org/10.1021/nl500693x | Nano Lett. 2014, 14, 3277−3283
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Figure 1. (a) A schematic illustration of the device. A CNT film is lying on a SiO2/Si substrate between two Pd electrodes. Modulated blackbody emission is obtained by applying either a rectangular or pulsed bias voltage Vds. (b) SEM image of the device. The low-density CNT film is in direct contact with the substrate. (c) DC Vds dependence of the current I at a gate voltage Vg = 0 V. In the high Vds region, the current shows slight saturation due to Joule heating and electron scattering with hot optical phonons. Inset, the Vg dependence of I at Vds = 10 mV. (d) An optical microscope image of a CNT emitter for high-frequency measurements with a coplanar transmission lines. A CNT film is formed at the termination of the transmission lines. (e) An NIR camera image of CNT emission at Vds = 4 V. Bright NIR emission is observed between the electrodes. (f) Emission spectra for Vds = 0−4 V with steps of 0.2 V. The blue lines are fitting curves based on Planck’s law. Inset, the bias voltage dependence of the CNT temperatures obtained by fitting to Planck’s law.
films. Under a rectangular bias voltage, both the rise and fall response times of the emission are approximately 100 ps due to high heat dissipation to the substrate. Moreover, we demonstrate 140 ps width pulsed light generation, experimental 1 Gbps and theoretical 10 Gbps modulation, and experimental 1 Mbps optical communication using this blackbody emitter. The mechanism of the ultrafast response in this emitter is elucidated by performing analytical calculations of a twodimensional heat conduction equation in which thermal transport in the substrate is taken into account. A schematic structure and SEM image of the fabricated device are illustrated in Figure 1a,b. Details regarding device fabrication are given in the Supporting Information. Although CVD-grown CNTs can yield both metallic and semiconducting CNTs, the DC bias voltage (Vds) dependence of the current exhibits Ohmic behavior at a low bias voltage because metallic CNTs are the primary contributors to the electrical conduction due to their lower contact resistance and higher carrier density (Figure 1c).19 This fact also explains the approximately constant current measured in the gate voltage (Vg) dependence (Figure 1c inset) in which a slight current change (approximately 1%) occurs due to the presence of p-type conduction through semiconducting CNTs. In a high Vds region, the current shows slight saturation due to Joule heating and electron scattering with hot optical phonons.19−23,27,28 For time-resolved emission measurements under high-frequency bias voltage, the electrodes were designed as 50 Ω coplanar transmission lines on an undoped silicon substrate (Figure 1d). Emission images from the CNT blackbody emitter under a DC bias voltage were observed with a near-infrared (NIR) camera
arrays,14 and individual wires15−17 of single-, double-, or multiwalled CNTs. Although these emission properties have been studied under steady-state conditions, the transient properties of these emitters have not been reported to date. To realize a high-speed CNT emitter, we focus on an unsuspended single-walled CNT thin film, in which the bottoms of the majority of the CNTs are in direct contact with the substrate. In comparison to an emitter with an individual CNT, bright emission can be obtained from a CNT film. Additionally, a blackbody emitter with an unsuspended thin CNT-film presents distinctive advantages for high-speed modulation: (i) the Joule heat can be quickly dissipated from the CNTs to the substrate due to the direct CNT−substrate contact,13,20−29 and (ii) the very thin CNT film has a comparatively small heat capacity per unit area because the heat capacity is proportional to the thickness of the CNT film. These advantages enable fast temperature modulation properties that are synchronized with the applied electric power, and the intensity of the blackbody emission can be quickly modulated. Furthermore, the CNT emitter presents advantages related to the intrinsic properties of CNTs: (iii) a comparatively small electrical capacitance (for example, the p−n junction capacitance in an LED limits its response speed due to the large time constants of the charge and discharge phases), (iv) extremely high thermal conductivity (∼3000 Wm−1 K−1),30,31 and (v) a high current density (∼109 A/cm) with low electromigration.32 Here, we report an electrically driven, ultrahigh-speed, small footprint, on-chip CNT blackbody emitter based on the distinctive thermal properties of very thin unsuspended CNT 3278
dx.doi.org/10.1021/nl500693x | Nano Lett. 2014, 14, 3277−3283
Nano Letters
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(Figure 1e). The emission was highly localized at the position of the nanotube film. The emission intensity was nonuniform along the width of the electrodes because the emission intensity of each CNT depends on its electrical contact resistance.19 The light emission spectra exhibit very broad emission patterns, the intensity of which increases with increasing applied bias voltage (Figure 1f). Because these spectra can be fitted with Planck’s law (the blue line in Figure 1f),2,9,11,14,15,19 these emission curves can be explained by the blackbody radiation generated by Joule heating. From these fittings, the temperature of emitted CNTs under a DC bias can be estimated (inset of Figure 1f). The observed temperatures are linearly dependent on the bias voltage in the high-voltage region.19 Theoretically, because heat conduction along a CNT axis is negligible for a long (≫0.2 μm) CNT,24 the steady-state temperature at the center of a CNT under a DC bias between two electrodes is given by p′ = gtot(T − T0), where p′ is the Joule heating rate per unit length, gtot is the total net heat loss rate to the substrate through the SiO2 insulator per unit length,13,20,23 and T and T0 are the CNT and ambient temperatures, respectively. By fitting the observed temperature with this equation, gtot is estimated to be 0.09 W K−1 m−1, which is in reasonable agreement with the reported values of gtot, ranging from 0.007 to 0.2 W K−1 m−1.22,24,26,27 Using the estimated gtot and the Stefan− Boltzmann law, the efficiencies of this emitter at T = 600 and 1400 K are estimated to be one the order of 10−6 and 10−5, respectively (see the Supporting Information for estimation details). To investigate the dynamic response of this emitter, we carried out time-resolved emission measurements based on a single-photon counting method using a Geiger-mode avalanche photodiode (APD) under applied rectangular bias voltages of 0.8 to 10 ns in width (Figure 2a) (see the Supporting Information for measurement details). As shown in Figure 2b, the measured emission intensities (thin curves) quickly respond to the applied bias voltage. The rise and fall times defined as the response time of 10−90% intensity are
