Electronic transport through individual ZnO nanowires

Li, Q. H.; Wan, Qing; Yu, Liang; Wang, T. H.

doi:10.1063/1.1759071

Cited by 279 publications

(182 citation statements)

References 12 publications

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“…Schematics of the NW energy band diagrams in dark and under illumination are displayed in parts b and c of Figure 2, respectively, illustrating the charge separation process of photogenerated electrons and holes under the intrinsic NW electric field and the occupation of surface states by photogenerated holes. In ZnO, it has been previously shown that the following trapping mechanism is governing the photoconduction in thin films 25 and NWs: 8,9,13,18,[26][27][28] in the dark (Figure 2b), oxygen molecules are adsorbed on the oxide surface and capture the free electrons present in the n-type oxide semiconductor [O 2 …”

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confidence: 99%

ZnO Nanowire UV Photodetectors with High Internal Gain

et al. 2007

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ZnO nanowire (NW) visible-blind UV photodetectors with internal photoconductive gain as high as G ∼ 10 8 have been fabricated and characterized. The photoconduction mechanism in these devices has been elucidated by means of time-resolved measurements spanning a wide temporal domain, from 10 -9 to 10 2 s, revealing the coexistence of fast (τ ∼ 20 ns) and slow (τ ∼ 10 s) components of the carrier relaxation dynamics. The extremely high photoconductive gain is attributed to the presence of oxygen-related hole-trap states at the NW surface, which prevents charge-carrier recombination and prolongs the photocarrier lifetime, as evidenced by the sensitivity of the photocurrrent to ambient conditions. Surprisingly, this mechanism appears to be effective even at the shortest time scale investigated of t < 1 ns. Despite the slow relaxation time, the extremely high internal gain of ZnO NW photodetectors results in gain-bandwidth products (GB) higher than ∼10 GHz. The high gain and low power consumption of NW photodetectors promise a new generation of phototransistors for applications such as sensing, imaging, and intrachip optical interconnects.Because of its wide band gap (E g ) 3.4 eV), low cost, and ease of manufacturing, ZnO is emerging as a potential alternative to GaN in optoelectronic applications, 1 including light-emitting diodes, laser diodes, and photodetectors for the UV spectral range. In the past decade, the demonstration of a large variety of functional ZnO nanowire (NW) devices such as field effect transistors, 2,3 optically pumped lasers, 4,5 and chemical and biological sensors 6 have aroused growing interest in this material. 7 In particular, ZnO NW photodetectors and optical switches have been the subject of extensive investigations. [8][9][10][11][12][13][14][15][16][17][18] Despite the abundant research on NW photoconduction, 19 the two main factors contributing to the high photosensitivity of such nanostructures have been scarcely recognized: (1) the large surface-to-volume ratio and the presence of deep level surface trap states in NWs greatly prolongs the photocarrier lifetime; (2) the reduced dimensionality of the active area in NW devices shortens the carrier transit time. Indeed, the combination of long lifetime and short transit time of charge carriers can result in substantial photoconductive gain. [20][21][22] In this letter, we present ZnO NW photodetectors with large photoresponse; upon UV illumination at relatively low light intensities (I ∼ 10 µW/cm 2 ), the current in ZnO NWs increases by several orders of magnitude, which translates to a photoconductive gain of G > 10 8 . To elucidate the photoconduction mechanism that involves fast carrier thermalization and trapping at the NW surface and electronhole recombination at extended and localized states, we have studied the photoconductivity of ZnO NWs by time-resolved measurements and in different ambient conditions (e.g., in air or under vacuum). A physical model was developed to illustrate the origin of the photoconductive gain in ...

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ZnO Nanowire UV Photodetectors with High Internal Gain

et al. 2007

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“…An alternative strategy is, first, to form the conductive metal electrodes on the insulating substrate of choice, and then to place the NRs/NWs on top of the electrodes so as to realize good ohmic contact between the ZnO and the electrodes. [15] Many prototype devices based on NRs/NWs have been reported, but the transfer and positioning of the NRs is too arduous for adoption in industrial applications. Harnack et al [16] demonstrated an electrical method of aligning semiconducting ZnO NRs onto pre-defined electrodes.…”

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confidence: 99%

Hydrothermal Growth of ZnO Nanorods Aligned Parallel to the Substrate Surface

et al. 2008

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Hydrothermal growth of ZnO nanorods with their c-axis aligned parallel to the surface of a Si (100) wafer substrate is demonstrated. With appropriate choice of process conditions (precursor concentrations, solution temperature, growth time), the (002) face of such nanorods is seen to support the emergence of as many as 10-20 aligned ZnO nanowires, with diameters ~50 nm and lengths that can extend to several microns. Similar growth of parallel nanorods/nanowires is observed on an amorphous carbon film surface. The influence of the substrate surface on the formation of such nanostructures is discussed. Such demonstrations of nanorod/nanowire growth parallel to a substrate surface could prove helpful in future nanodevice fabrication strategies.2

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“…This forms a low-conductivity depletion layer near the surface, where the screening of piezoelectric potential by free carriers effect is suppressed. [30,[32][33][34][35][36] This effect has been used to neglect free carriers in most simulation studies dealing with the piezoelectric response of semiconducting nanowirebased devices. A few papers have accounted for free carriers, although without account for SFLP.…”

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Unveiling the Influence of Surface Fermi Level Pinning on the Piezoelectric Response of Semiconducting Nanowires

Tao

Mouis

Ardila

2017

Adv Elect Materials

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environment ever since they were first demonstrated. [4] Experimentally, it has been proved that ZnO NW based piezoelectric devices can generate a potential of a few volts. [15] This can be retrieved theoretically as long as there are no free carriers. However, experimentally prepared ZnO NWs are spontaneously n-type doped by impurities during their synthesis [16,17] and in such a case, screening by free carriers is expected to reduce drastically the piezoelectric response. There remain many such contradictions between experimental results and the present theoretical understanding of ZnO NW-based devices when realistic doping levels are considered. Chief among them are (1) decent and length-dependent performance of ZnO NWs, [18] while anticipations based on analytical and computational study showed that the output of NWs under compression were reduced to a few millivolts and presenting length-independent performance due to the screening effect; [19,20] (2) enhanced piezoelectric coefficients, [21] which were measured for ZnO NWs with diameter beyond the size effects anticipated by ab initio method; [22] and (3) dissymmetric piezoelectric response of bent nanogenerators (NGs) under tensile and compressive strain. [23] Relatively high potentials (>0.2 V) can be repeatedly obtained from NGs integrating long and large ZnO NWs [24][25][26][27] with presumably nonintentional doping. Increasing the doping concentration will reduce the generated piezopotential. [28] The nature of matrix material, as well as processing conditions, has been shown to play an important role. [24] (Table S1, Supporting Information).In this paper, we solve these contradictions by accounting for surface Fermi level pinning (SFLP). This effect commonly exists at the surface of III-V and II-VI semiconductor compounds. [29][30][31] At the surface of n-type ZnO NWs, oxygen molecules get negatively charged by capturing the free electrons from NW core. This forms a low-conductivity depletion layer near the surface, where the screening of piezoelectric potential by free carriers effect is suppressed. [30,[32][33][34][35][36] This effect has been used to neglect free carriers in most simulation studies dealing with the piezoelectric response of semiconducting nanowirebased devices. A few papers have accounted for free carriers, although without account for SFLP. [20,37] Here instead, full coupling between piezoelectric effect and semiconducting physics, including free carriers and SFLP, was considered.ZnO nanowires (NWs) are excellent candidates for the integration of energy harvesters, mechanical sensors, piezotronic and piezophototronic devices. However, ZnO NWs are usually nonintentionally n-doped during their growth. Thus, it can be expected theoretically that their piezoelectric response be degraded and mostly geometry-independent as a result of strong screening effects by free carriers, while experimentally many NW-based piezoelectric transducers demonstrate quite reasonable performance. In this paper, this apparent contradiction is explained by...

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Electronic transport through individual ZnO nanowires

Cited by 279 publications

References 12 publications

ZnO Nanowire UV Photodetectors with High Internal Gain

ZnO Nanowire UV Photodetectors with High Internal Gain

Hydrothermal Growth of ZnO Nanorods Aligned Parallel to the Substrate Surface

Unveiling the Influence of Surface Fermi Level Pinning on the Piezoelectric Response of Semiconducting Nanowires

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