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Avalanche Photoemission in Suspended Carbon Nanotubes: Light Without Heat Bo Wang, Fatemeh Rezaeifar, JIhan Chen, Sisi Yang, Rehan Kapadia, and Stephen B. Cronin ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00455 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on October 8, 2017
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ACS Photonics
Avalanche Photoemission in Suspended Carbon Nanotubes: Light Without Heat Bo Wang1, Fatemeh Rezaeifar2, Jihan Chen2, Sisi Yang1, Rehan Kapadia2, and Stephen B. Cronin1, 2 1
Department of Physics and Astronomy, 2Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA
We observe bright electroluminescence from suspended carbon nanotube (CNT) field effect transistors (FETs) under extremely low applied electrical powers (~nW). Here, light emission occurs under positive applied gate voltages, with the FET in its “off” state. This enables us to apply high bias voltages (4V) without heating the CNT. Under these conditions, we observe light emission at currents as small as 1nA, and corresponding electrical powers of 4nW, which is three orders of magnitude lower than previous studies. Thermal emission is ruled out by monitoring the G band Raman frequency, which shows no evidence of heating under these small electrical currents. The mechanism of light emission is understood on the basis of steep band bending that occurs in the conduction and valence band profiles at the contacts, which produces a peak electric field of 500kV/cm, enabling the acceleration of carriers beyond the threshold of exciton emission. The exciton-generated electrons and holes are then accelerated in this field and emit excitons in an avalanche process. This is evidenced by an extremely sharp increase in the current with bias voltage (45mV/dec). We also observe light emission at negative applied gate voltages when the FET is in its “on” state at comparable electrical powers to those reported previously (~5µW). However, substantial Joule heating (T>1000K) is also observed under these conditions, and it is difficult to separate the mechanisms of thermal emission from hot carrier photoemission in this regime.
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Keywords: ballistic, avalanche, high-field, band-to-band, photoemission TOC Figure
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The one-dimensional nature of carbon nanotubes gives rise to several interesting optical and optoelectronic phenomena including excitons, trions, and single photon emission at room temperature.1-4 Electroluminescence (EL) was first observed from substrate-supported individual carbon nanotube (CNT) pn-junctions in 2003 by the IBM group, who reported an electroluminescence efficiency of ~10-4 photons per injected electron–hole pair.5, 6 In addition to pn-junction devices, there have been several papers reporting hot carrier electroluminescence from unipolar CNT devices under high bias voltages (up to 60V).7-11 These hot carriers, in turn, create excitons through impact ionization under unipolar conditions. These excitons recombine radiatively producing bright electroluminescence locally at the CNT-contact region. This excitation mechanism is estimated to be ~1000 times more efficient than recombination of independently injected electrons and holes, and it results from weak electron-phonon scattering and strong electron-hole binding caused by the one-dimensional confinement. From photoluminescence studies, it is known that the luminescence efficiency of suspended CNTs is several orders of magnitude higher than substrate-supported CNTs.12-14 In the work of Lefebrve et al., PL emission was only observed from the suspended segments of carbon nanotubes.13 Based on their signal-to-noise ratio, they estimate the PL enhancement factor of suspended CNTs to be at least 100X. The IBM group also studied electroluminescence from partially-suspended CNTs under high bias. Here, they used the high local fields (E≈50kV/cm) produced at the junction between the suspended and supported parts of a single carbon nanotube to accelerate carriers beyond the threshold for photon emission.11 In this previous work, the threshold of detectible light emission occurred under applied electrical powers of >10µW. In this regime, however, it is hard to rule out the effects of thermal emission caused by Joule heating. Previously, our group studied electrical heating of suspended carbon nanotubes under high bias
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conditions using Raman spectroscopy and thermal emission spectroscopy, and found that that the onset of heating typically occurred around electrical powers of 1µW.15-18 In the work presented here, we demonstrate light emission at extremely low powers (~4nW) from individual CNTs suspended between two metal contacts. Raman spectroscopy is used to measure the temperature of the CNTs under high bias voltages in order to rule out thermal emission caused by Joule heating of the nanotubes. By gating the FET into the “off” state, we are able to apply high electric fields (>500kV/cm) without heating the device. In addition, the ballistic nature of the electron (and hole) transport in these suspended CNTs provides an ideal system for studying hot carrier phenomena. In order to verify the proposed mechanism of light emission, we calculate the band and electric field profiles along the length of the nanotube using a self-consistent Poisson solver. The CNT devices are fabricated by first etching a 500nm deep, 1µm wide trench in a Si/SiO2/Si3N4 wafer. Platinum source and drain electrodes are then deposited perpendicular to the trench, and a Pt gate electrode is deposited in the bottom of the trench, as shown in Figure 1.19-21 Ferric
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lithographically-defined windows patterned on the drain and source electrodes, as can be seen in Figure 1a. CNTs are then grown by chemical vapor deposition (CVD) using a mixture of argon gas bubbled through ethanol and hydrogen gas at 825°C, yielding suspended single wall carbon nanotubes in a field effect transistor (FET) geometry, as depicted in Figures 1c.22, 23 The nanotube growth is the final processing step in the sample fabrication, which ensures that these nanotubes are not contaminated by any chemical residues from lithographic processes. Also, since these nanotubes are suspended, there are no effects introduced by the underlying substrate. In our measurements, the drain is kept at 0V (i.e., ground) and the gate and bias voltages are applied with respect to the drain electrode. Near infrared images were collected with a 4 Environment ACS Paragon Plus
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thermoelectrically-cooled InGaAs 2D array (Xenics, Inc.) sensitive over the wavelength range from 1000-1600nm. Figures 1b and 1d show IR illuminated and electroluminescence images of this device, respectively. Here, bright electroluminescence can be observed under an applied gate voltage of Vg=+10, a bias voltage of Vbias=5.1V, and a current of I=4nA. Figure 2a shows the low bias I-Vg characteristics of a suspended CNT FET device with a threshold voltage around Vg=+1.5V. Figure 2b shows the electric current (linear) and electroluminescence (EL) intensity (log) plotted as a function of bias voltage with a gate voltage of Vg=+10V. Here, we see the onset of EL at a bias voltage of Vbias=4V and a current of Ibias
