Novel optoelectronic devices based on single semiconductor nanowires (nanobelts)
© Ye et al.; licensee Springer. 2012
Received: 17 November 2011
Accepted: 3 March 2012
Published: 13 April 2012
Semiconductor nanowires (NWs) or nanobelts (NBs) have attracted more and more attention due to their potential application in novel optoelectronic devices. In this review, we present our recent work on novel NB photodetectors, where a three-terminal metal–semiconductor field-effect transistor (MESFET) device structure was exploited. In contrast to the common two-terminal NB (NW) photodetectors, the MESFET-based photodetector can make a balance among overall performance parameters, which is desired for practical device applications. We also present our recent work on graphene nanoribbon/semiconductor NW (SNW) heterojunction light-emitting diodes (LEDs). Herein, by taking advantage of both graphene and SNWs, we have fabricated, for the first time, the graphene-based nano-LEDs. This achievement opens a new avenue for developing graphene-based nano-electroluminescence devices. Moreover, the novel graphene/SNW hybrid devices can also find use in other applications, such as high-sensitivity sensor and transparent flexible devices in the future.
KeywordsSchottky junction Graphene Nanowires Nanobelts Optoelectronics
Semiconductor single-crystalline nanowires (NWs) or nanobelts (NBs) can be grown on lattice mismatched substrates and constructed into devices with the bottom-up method on basically any substrates . Hence, compared to the conventional ones, semiconductor NW- or NB-based devices have the advantage of versatility in both the material and the device structure. So far, various semiconductor NW- or NB-based nanodevices have been emerging continuously [2–4]. Developing novel high-performance nano-optoelectronic devices is not only important in diverse device applications, but also has significant meaning in exploring and realizing optoelectronic integration.
In this review, we present our research work on two types of novel optoelectronic devices based on semiconductor NWs (NBs). One is semiconductor NB metal–semiconductor field-effect transistor (MESFET)-based photodetectors . In contrast to the common two-terminal single semiconductor NB (NW) photodetectors, the three-terminal NB MESFET-based photodetector can make a balance among overall performance parameters, which is desired for practical device applications. The other is novel multicolor light-emitting diodes (LEDs) based on graphene nanoribbon (GNR)/semiconductor nanowire (SNW) heterojunctions . Herein, ZnO, CdS, and CdSe NWs were employed for demonstration. At forward biases, the GNR/SNW heterojunction LEDs emitted light from ultraviolet (380 nm) to red (705 nm), which were determined by the bandgaps of the involved SNWs. This work opens a new avenue for developing diverse graphene-based optoelectronic devices . These two works may help to promote nano-optoelectronic integration in the future.
Single CdS NB MESFET photodetector
Comparison of the key parameters for the CdS NB (NW) photodetectors with different structures
Rise; fall time
746; 794 μs
6.0 × 103
approximately 20; approximately 20 μs
7.3 × 104
-; 320 ms
137; 379 μs
approximately 2.0 × 102
approximately 2.7 × 106
Figure 2d shows the on/off photocurrent response of the CdS NB MESFET-based photodetector measured at VG = −3.8 V, which is the threshold voltage of the MESFET under light illumination. We can see that the average dark current and photocurrent are about 26 fA and 70 nA, respectively, resulting in a Ilight/Idark as high as approximately 2.7 × 106. To the best of our knowledge, this is so far among the highest reported values for single NB (NW) photodetectors [3, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]. In addition, the photoresponse processes (both rise and decay processes) are quite fast, which have exceeded the detection limit (0.3 s) of the measurement apparatus (Keithley 4200, Cleveland, OH, USA).
The Rλ, defined as the photocurrent generated per unit power of incident light on the effective illuminated area of a photoconductor, and the external quantum efficiency (EQE), defined as the number of electrons detected per incident photon, are two critical parameters for photodetectors. The Rλ and EQE can be calculated with equations and , respectively. Here, ΔΙ is the difference between the photocurrent and the dark current, Pλ is the light power density, S is the effective illuminated area, h is Planck's constant, c is the velocity of light, e is the electronic charge, and λ is the light wavelength. Using ΔΙ = 7.0 × 10−8 A (measured from Figure 2d), Pλ = 5.3 mW/cm2S = 500 nm × 13 μm (measured from the inset of Figure 1a), λ = 488 nm, the Rλ and EQE of the CdS NB MESFET photodetector can be estimated to be approximately 2.0 × 102 A/W and 5.2 × 102, respectively.
In order to further investigate the detailed photoresponse times of the single CdS NB MESFET photodetector, we employed a 200-MHz digital oscilloscope (Tektronix DPO2024, Beaverton, OR, USA) with a 10-MΩ impedance and an optical chopper working at a frequency of 1,000 Hz, as shown in Figure 2e. From the close-up of the measured result shown in Figure 2f, the rise timer, defined as the time needed for the photocurrent to increase from 10 % ipeak to 90 % ipeak, is 137 μs and the decay timed, defined analogously, is 379 μs.
We attribute the overall high performance of our CdS NB MESFET-based photodetectors to the unique advantage of the MESFET structure. Compared to two-terminal photodetectors, there are two main advantages of the MESFET-based photodetectors. First, it has a much lower dark current because the applied negative gate voltage (in our case, the threshold voltage under illumination) helps to deplete the channel carriers. Second, this gate depletion effect will also cause a fast current recovery when the light is turned off. Consequently, the decay tail, which is normally observed in a two-terminal photodetectors, is suppressed in the MESFET-based photodetectors.
Multicolor GNR/SNW heterojunction LEDs
By taking advantage of both graphene and SNWs, we have fabricated, for the first time, the graphene-based nano-LED . This achievement opens a new avenue for developing graphene-based nano-electroluminescence devices. Moreover, the novel graphene/SNW hybrid devices can also find use in other applications, such as high-sensitivity sensor and transparent flexible devices in the future.
Both the n-type NWs [20–22] and the graphene  used in this work were synthesized via the CVD method. Before device fabrication, the graphenes were transferred by the stamp method with the help of polymethyl methacrylate  to Si/300-nm SiO2 substrates for Raman and electrical property characterizations, to quartz substrates for transparency characterization, and transferred to carbon-coated grids for high-resolution transmission electron microscopy (HRTEM) characterization (Tecnai F30, FEI, Eindhoven, The Netherlands). Their electrical properties were measured by a Hall effect measurement system (Accent HL5500, York, England).
It is worth noting that the GNR/NW structure has clear advantage over the conventional Schottky structure. For comparison, we have fabricated various metal/SNW Schottky structures, where the NWs used are identical to those reported in this work. Unfortunately, no EL can be observed in these structures. We attribute this to the well-known luminescence quenching effect caused by the involved metal . Moreover, in our face-to-face contact LED, the active region, where the radiative recombination occurs, is larger and the series resistance is smaller, compared to the crossed NWs or NW/Si pad heterojunction structures [20, 30, 31]. These merits may benefit high-efficiency EL and even electrically driven laser in the future.
We review two types of novel nano-optoelectronic devices developed in our group recently. One is the photodetector, which converts light to electric signals. Our MESFET-based photodetectors have ultrahigh Ilight/Idark (approximately 2.7 × 106) and fast response (rise time, approximately 137 μs; decay time, approximately 379 μs) simultaneously. The other is LED, which converts electric power to light. At forward biases, our novel GNR/SNW heterojunction LEDs emitted light with wavelengths varying from ultraviolet (380 nm) to red (705 nm), which were determined by the bandgaps of the involved SNWs. These two types of nano-optoelectronic devices may find diverse applications in future nano-optoelectronic integration.
This work was supported by the National Natural Science Foundation of China (nos. 61125402, 51172004, 11074006, 10874011, 50732001), the National Basic Research Program of China (nos. 2012CB932703, 2007CB613402), and the Fundamental Research Funds for the Central Universities.
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