Defect Reconstruction Triggered Full-Color Photodetection in Single

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Defect Reconstruction Triggered Full-color Photodetection in Single Nanowire Phototransistor Liang Hu, Qiufan Liao, Zhenyu Xu, Jun Yuan, Yuxuan Ke, Yiyue Zhang, Wenjing Zhang, Guo Ping Wang, Shuangchen Ruan, Yu-Jia Zeng, and Su-Ting Han ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01471 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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Defect Reconstruction Triggered Full-color Photodetection in Single Nanowire Phototransistor Liang Hu,† Qiufan Liao,‡ Zhenyu Xu,† Jun Yuan,† Yuxuan Ke,§ Yiyue Zhang,† Wenjing Zhang,§ Guo Ping Wang,‡ Shuangchen Ruan,† Yu-Jia Zeng,*,† Su-Ting Han*,‡

†

Shenzhen Key Laboratory of Laser Engineering, College of Optoelectronic Engineering,

Shenzhen University, Shenzhen, 518060, P. R. China

‡

College of Electronic Science and Technology, Shenzhen University, Shenzhen,

518060, P. R. China

§

International Collaborative Laboratory of 2D Materials for Optoelectronics Science and

Technology, Shenzhen University, Shenzhen, 518060, P. R. China

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ABSTRACT: The coherent developments of high performance broadband photodetection and discrimination technique are highly essential for multi-scene imaging and optical communication applications. The integration of traditional bandpass filters or stacking other spectral absorber in photodetectors often complicates the device design and leads to asymmetric photo gain for each waveband. Herein, we report on ultraviolet-visible (UV-

vis) multi-spectral photodetection based on a single ZnO nanowire (NW) phototransistor, where defect reconstructions can be reliably induced by a two-step annealing that lead to the observed broadband photodetection. Electron paramagnetic resonance and photoluminescence spectra reveal the reconstructions of zinc-atom-related defects (i.e. zinc interstitials and vacancies). Combined micro-differential reflectance and multi-mode scanning probe microscope (SPM) technique confirm the presence of a unique visiblesensitive Zn-rich ZnO shell layer and a trap-free UV-sensitive ZnO core. We achieve not only an ultrahigh carrier mobility (212.4 cm2 V-1s-1) but also a concurrent improvement for UV-vis photodetection with superior responsivities and detectivities on the orders of 105 AW-1 and 1015 Jones at 100 mV, respectively, and response speeds less than one second. Moreover, photocurrents under blue, green and red stimuli can be selectively switched


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on/off by tuning the gate stress. These high performances in all figures of merit have opened new route to tailor intrinsic properties of a single NW for optoelectronic applications.

KEYWORDS.

defect

engineering;

ZnO

nanowire;

phototransistor;

broadband

photodetection; spectral selectivity


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Free-standing nanowires (NWs), deemed as a crystalline cylinder with diameters of 2–200 nm and lengths up to several micrometers, have shown unique electrical, optical and magnetic properties.15

These superiorities are strongly associated with the sub-wavelength space confinement of

carrier/photon in one dimension (1D),4 which make NWs ideal building blocks for next-generation device architectures that feature miniaturization, flexibility, low cost, low power consumption and high performance.2, 6-7 Among various NWs, much attentions have been paid to zinc oxide (ZnO), in particular to light-matter interactions. The wide direct bandgap of 3.37 eV and a large exciton binding energy of 60 meV of ZnO allow optically pumped ultraviolet (UV) lasing at room temperature.8 Thanks to their easy tailoring for intrinsic optoelectronic attribute,7, 9-11 as well as the hybridization potentials with diverse functional materials,12-14 ZnO NWs have been configured as key components in the fields of spectral detection,15-16 photo-inverter,17-18 solar cell,19 electrically pumped lasing,20 and piezo-phototronics.21 Till now, much more studies have focused on ensemble of NWs (e.g. array type) as compared to those on single NW, which is simply due to the intrinsic difficulties for fabricating and characterizing individual NWs. However, single NW usually gives rise to better performances than ensemble, in particular on photodetection.7, 9 which has not yet been fully understood. Therefore, in-depth understanding of intrinsic properties of single NW is crucial for device applications. Because of high demand for both extended light harvesting and multifunctional applications, highperformance broadband photodetectors are highly desired.14, 22 Currently, inorganic broadband photodetectors mainly refer to silicon-based ones that can detect light from UV to visible.23 The integration of complex bandpass filters along with a low UV photo gain limit device miniaturization and diverse applications in multi-scene imaging, day- and night-time surveillance and broadband optical communication.23 ZnO NWs intrinsically respond to UV radiation. The


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introduction of visible-sensitive impurities or point defects in ZnO can effectively enhance the absorption and photoelectric conversion in visible spectrum.24, 25 However, these defects are fairly uncontrollable and often unevenly distributed, hindering the reliability and reproducibility of detection performances.26,27 On the other aspect, hybrid photodetectors that integrate ZnO NWs and other photoactive materials to expand the spectrum range have been proven to be feasible.1314, 28-29

Unfortunately, suffering from the high density of defects as well as interface disorders,

hybrid photodetectors can hardly lead to a concurrent improvement for each waveband, which often increase the visible detectivity at the expense of UV or vice versa (asymmetric response).24, 28

Furthermore, the presence of rich surface states/traps also intends to chemically adsorb

oxygen/water molecules and induce surface carrier depletion. The emerging photogating effect make devices relax slowly and result in the so-called persistent photocurrent (PPC).15,

30

To

increase the response speed and maintain the high photo gain (subject to gain-bandwidth tradeoff),15 it is of great importance to minimize the influence of surface defects/traps. Previous reports have only focused on achieving considerable UV detection performances of ZnO NWs.31-38 More methodologies on optimizing broadband response, in particular visible photodetection, and further discriminating different wavebands are fairly limited. In this study, to resolve the aforementioned two key issues, i.e. defect tailoring and concurrent improvement for each waveband on a single ZnO NW, we have demonstrated defect-reconstructed core-shell ZnO NWs that are synthesized by a two-step annealing approach, where the ZnO core is singlecrystalline and the shell is composed of visible photosensitive Zn-rich ZnO (ZnO:Zn). This unique interface structure can not only deplete the defects/traps induced carriers (reducing the dark current and the surface band bending) but also trigger generation and separation process of multispectral


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photocarriers (improving the photo gain and the response speed) in single NW photodetectors. To the best of our knowledge, we report, for the first time, concurrent improvements on UV and visible photodetection of a single ZnO NW photodetectors. These ZnO NWs have ultrahigh carrier mobility above 200 cm2 V-1s-1 and the capability in broadband spectral absorption and gate-tunable selective response. High-performance detection characteristics from UV to visible spectrum have been extracted based on silicon-industry-compatible back-gating modulated phototransistors, which includes superior responsivities, detectivities and photo gains on the orders of 105 AW-1, 1015 Jones and 106 at 100 mV, respectively, fast response speeds, as well as the high signal-tonoise ratio that allows weak light detections down to tens of nW cm-2.

■ EXPERIMENTAL SECTION Preparation of NWs. Pristine ZnO NW powders were collected on downstream cold terminal of horizontal tube furnace via a catalyst-free thermal evaporation approach. Raw material details and experimental procedure can be referred to Reference 5. All NW samples have been processed by a two-step annealing, namely first vacuum annealing at 500℃ for an hour followed by zinc vapor annealing at 600℃ for another hour. Device Fabrication. Single ZnO NW was configured as photosensitive field-effect transistors (phototransistors/photo-FETs) on prepatterned p++-Si/SiO2 substrates (SiO2 of 300 nm). Ti (5 nm) /Au (45 nm) contacts were defined with e-beam lithography (EBL, Raith PIONEER Two) and deposited by magnetron sputtering.


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Characterizations. The feature sizes and surface morphology of NWs were measured by scanning electron microscope (SEM, Hitachi S-4800). And the crystalline quality and structural properties were characterized by high resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30). The presence of singly-ionized defect states was confirmed by using an electron paramagnetic resonance (EPR) spectrometer (Bruker ESRA-300) operating at 9.86 GHz (X-band). X-ray photoelectron spectroscopy (XPS) studies were carried out on an ESCALAB 250XI (Thermo Scientific) using Al Kα monochromatic source (1486.6 eV). Micro-photoluminescence (PL) measurements excited by 325 nm laser were conducted at fluorescence spectrophotometer (HORIBA LabRAM HR Evolution). Micro-differentialreflection spectra (DRS) were captured based on a WITec alpha300R spectrophotometer equipped with a charge-coupled device (CCD) and spectrometer. And the adopted light source is continuouswave (CW) white light laser. The information of height, conductivity and contact potential difference (CPD) of NWs were visualized in multi-mode combination of atomic force microscopy (AFM), conductive AFM (CAFM), Kelvin probe force microscopy (KPFM) in AIST scanning probe microscope (SPM). Photodetection Performance Measurements. The laser diodes (LDs) with peak wavelengths of 365 nm, 405 nm, 450 nm, 520 nm and 635 nm were adopted to illuminate ZnO-NW-based phototransistors. Their incident power densities are calibrated as 1.05, 0.57, 1.17, 0.11 and 0.12 mW cm-2 by optical power meter, respectively. All current-voltage (I-V) measurements including


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transfer and output characteristics, and temporal photocurrent tests were conducted in Keysight B2902A semiconductor analyzer in ambient atmosphere.

■ RESULTS AND DISCUSSION Figure 1a-c depict SEM morphological evolution of single ZnO NW after the two-step annealing procedure (three samples are denoted as pristine, vacuum and zinc, respectively, with feature sizes of length~20 μm and diameter~200 nm). Smooth surface topology (Figure 1a) and corresponding intact lattice fringe (Figure 1d) reveal a perfect single crystalline nature of pristine ZnO NW. It is observable from SEM image that the vacuum treatment induces a wrinkled surface texture of ZnO

Figure 1. SEM images (a-c), HRTEM images (d-f) and EPR spectra (g-i) of pristine (a, d, g), vacuum-annealed (b, e, h), and zinc-annealed (c, f, i) ZnO NW samples. (j) Ball-and-stick model


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of Zni and VZn defect configuration and schematic demonstration of the formation process of ZnO/ ZnO:Zn core-shell NW structure.

NW, which is further confirmed by an enlarged view near the surface zone by HRTEM as shown in Figure 1e. From defect-sensitive EPR spectra in Figure 1g and h, pristine samples possess a pair of Frenkel defect species,27 i.e. singly-charged zinc vacancy (VZn, g-factor~2.05) and interstitial zinc (Zni, g-factor~1.96).39-40 Generally, g~1.96 can also be associated with electrons bound to shallow donor levels (e.g. AlZn, GaZn, InZn or Hi).40 Zni has a low ionization energy (30~51 meV) and thus can be considered as the origin of unintentional n-type conductivity.41 However, unlike other donors, Zni can easily diffuse out from lattice by a so-called kick-out mechanism.40 After vacuum annealing at 500℃, a dramatic reduction of donor defect density is found, accompanying with a simultaneous reduction of dark current (see later) and unchanged VZn signal. This phenomenon well coincides with theoretical prediction of out-diffusion temperature of Zni from ZnO lattice.40, 42 It is conjectured that the coarse surface might be correlated with stress relaxation after Zni atoms out-diffuse towards surface. To further eliminate vacancy-type defects, zinc vapor annealing was adopted. Unsurprisingly, VZn defects are largely passivated in this process as shown in Figure 1i. Due to the exotic zinc atom penetration, HRTEM image in Figure 1f reveals that rough surface texture of NW is replaced by a conducting ZnO:Zn layer with a thickness of 8~10 nm, where ZnO is the main phase with some embedded Zn nanocrystals circled by the dashed lines. The above deduction about Zn atoms motion highly coincides with the variation of local vibrational mode of Zn atoms (see Figure S1 in the Supporting Information). Possible defect configurations and two-step formation process of ZnO(core)/ZnO:Zn(shell) NWs are sketched in Figure 1j.


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Insights towards surface micro-environment, defect population and spectral absorption for three different samples have been delicately extracted using XPS, micro-PL and DRS, respectively. As seen in Figure 2a, the Zn 2p double peaks of the zinc-annealed sample redshifts to lower binding energies (~0.3 eV), indicating the formation of metallic Zn or Zn-rich structure near the surface region.43 From Figure 2b, it is observable that vacuum annealing does remove almost all Zni donor defects, which manifests itself by a quenching of defect emission at ~510 nm and an enhancement of band edge emission at ~380 nm that is consistent with the above deduction. After the second step of the zinc in-diffusion, the defect band becomes broader and stronger with reduced band-edge transition, revealing the significance of surface engineering induced defect reconstruction. In Figure 2c-f, we have measured the change in reflectance (ΔR/R) induced by s

u

r

f

a

c

e

e

n

g

i

n

e

e

r

i

n

g

Figure 2. (a-b) XPS in Zn 2p core-level regions (a) and PL (b) spectral comparison of pristine, vacuum-annealed, and zinc-annealed ZnO NWs. (c) Schematic diagram of DRS setup. (d) DRS


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mapping scan corresponding to 405nm (Violet/V), 450nm (Blue/B), 520nm (Green/G), and 635nm (Red/G) light illumination. (e) A typical ΔR/R difference between pristine and zinc-annealed ZnO NWs collected from the whole NWs. All scale bars in (d-e) are 5 μm. (f) Single point extracted ΔR/R spectra along axis direction of single NW (inset of (e)).

on a single core-shell NW. Using DRS technique (see Figure 2c), the difference of absorption capability from 400 to 650 nm in core and shell regions can be clearly captured by the real-time mapping. As depicted in Figure 2d, the absorption in the shell layer is more intense than that in the core for violet, blue, green and red bands. And NW shows peak absorption for green illumination among all bands. We have further plotted the dependence of ΔR/R on wavelength in Figure 2e (integrated acquisition for the whole NW) and Figure 2f (single point acquisition). Obviously, the visible absorption of defect-reconstructed NW is significantly higher than that of pristine ZnO NW. Similar broadband absorption peak shapes reveal a high uniformity of core-shell structure. A SEM image and 3D mechanistic diagram of device operation are given in Figure 3a-b. Before I-V measurements, we have again checked the status of core-shell NW with patterned electrodes to verify structural stability of shell layer after the EBL procedure. In Figure 3c, through multimode combination of AFM, CAFM and KPFM in the same area, conductive and visible-sensitive shell layer are again clearly observed, confirming the robustness of shell layer structure. It should be noted that the possibility of sub-bandgap photoresponse from photon-assisted molecule desorption can be excluded because in that case the surface region is highly depleted (highlyresistive) which is not our case.30 As shown in Figure 3d, we compare the dark state transfer curves of three typical samples. After two-step thermal treatment, transistor characteristics of the zinc-annealed NW show a significant


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improvement. By extrapolating the linear region of transfer characteristic curve to the current axis, a positive threshold voltage (Vth) of +29.4 V is obtained for the zinc-annealed NW, revealing an n-channel enhancement mode. By contrast, the pristine NW displays a depletion type (Vth= -11.5 V). Enhancement mode is commonly observed in thinner ZnO NWs (diameters below 100 nm), while depletion mode is more often observed in thicker ones (diameters above 100 nm), which is correlated with diameter-dependent surface band bending.44 For our core-shell NW transistor without working, since the shell layer is more conductive, the deficiency of electrons in core region is mainly due to the defect/trap removal by a two-step zinc atoms diffusion process rather than the surface band bending, which therefore can switch the depletion mode to the enhancement mode in such a thick NW-based phototransistor. The on/off current ratio of the optimized zinc-annealed


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Figure 3. (a) A SEM image of typical core-shell ZnO NW phototransistor. Feature sizes include NW diameter of ~200 nm and effective channel length of ~3 μm. (b) Electrical measurement demonstration of phototransistor. (c) A group of stacked AFM, CAFM and KPFM mapping pictures collected from the same area. (d-g) Transfer characteristic curves of off-states (d) and onstates under illumination from pristine (e), vacuum-annealed (f), zinc-annealed (g) NW samples. (h-i) Double logarithmic plot fitting of current-voltage characteristics for pristine NW samples (h) and zinc annealed ones (i). (j) The dependence of ΔR/R and Rλ on incident light wavelength.

device can be up to ~105, which is one order of magnitude larger than pristine ZnO. The maximum transconductance gm ≡ dIDS/dVGS at a source-drain voltage of VDS=100 mV is extracted to be 24.8 gmL2

nS μm-1. A field-effect electron mobility is thus calculated as 212.4 cm2 V-1s-1 by 𝜇 = VDSCSiO , where 2

L =3 μm is effective channel length and gate capacitance CSiO2 can be estimated as 3.15×10-16 F through a cylinder-on-plate model CSiO2 = 12 F

2πε0εrL -1

,45 where vacuum permittivity ε0=8.85×10-

cosh [(d + 2t) d]

m-1, relative permittivity of SiO2 εr=3.9, d = 200 nm is diameter of core-shell NWs and t = 300

nm is the thickness of back-gated dielectric layer. The obtained mobility is even larger than that of a bulk ZnO single crystal grown by vapor-phase transport (205 cm2 V-1s-1),46 rendering a highquality electrical characteristic of single core-shell ZnO NW. Sub-threshold swing (SS) is given by the equation of SS = dVgs/dlogIds, i.e. ~250 mV/dec. When VGS = 40V is applied, the enhanced electron concentration (ne) can be calculated to be 2.21×1017 cm-3 using ne =

4(VGS - Vth) CSiO2 eπd2L

. Note

that for pristine and vacuum-annealing ones, carrier mobilities are likewise deduced as only 25.9 and 0.18 cm2 V-1s-1, respectively.


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Under multi-spectral illuminations (Figure 3e-g), the Vth values of all ZnO NW phototransistors shift to the negative direction, suggesting a remarkable increase in photogenerated carrier concentration. The zinc-annealed sample shows better performance in photocurrent multiplication (microampere level) than other NWs and maintain highly stable transconductance and mobility. Such excellent properties result from ideal symmetric Ohmic contacts regardless of incident wavelengths (see Figure S2 in the Supporting Information), where the conductive shell induced by zinc annealing is believed to play a critical role. Figure 3h-i show a comparison of photoconduction mechanism (I~V α ) based on double logarithmic plot fitting of I-V c

h

a

r

a

c

t

e

r

i

s

t

i

c

s

.

Table 1. Summary of detection performances measured at Vg =0 V and Vds = 100 mV.

Samples

Pristine

λ (nm)

On/off Ratio

Gλ

Rλ (A W-1)

Dλ* (Jones)

LDR (dB)

365

40.95

4.31×105

1.27×105

2.81×1013

32.25

405

5.05

7.26×104

2.37×104

5.24×1012

14.07

450

3.99

2.34×104

8.50×103

1.88×1012

12.01

520

2.00

7.23×104

3.03×104

6.71×1012

6.02

635

1.40

1.54×104

7.92×103

2.07×1012

2.91

365

1.89×104

3.21×105

9.43×104

5.26×1014

85.51

405

3.24×103

9.13×104

2.98×104

1.66×1014

70.21

450

1.01×103

1.25×104

4.54×103

2.53×1013

60.11

520

4.03×102

4.58×104

1.92×104

1.07×1014

52.11

Vacuum


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Zinc

635

38.73

3.23×103

1.65×103

9.21×1012

31.76

365

1.60×105

8.63×105

2.54×105

2.51×1015

104.08

405

8.66×104

7.76×105

2.53×105

2.50×1015

98.75

450

6.43×104

2.53×105

9.16×104

9.07×1014

96.16

520

2.98×104

1.08×106

4.52×105

4.47×1015

89.48

635

2.96×103

8.03×104

4.11×104

4.07×1014

69.43


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Figure 4. (a) Responsivity and specific detectivity of zinc-annealed single ZnO NW versus gate bias stress measured at VDS = 1 V. (b) Comparison of temporal photocurrent during switching on/off UV illumination of three kinds of NW samples. (c-e) Gate-tunable blue (c), green (d) and red (e) response with time when applied bias VDS = 1 V. (f) Temporal photocurrent tests when device illuminated by weak UV light (30.9 nW cm-2). (g-h) Multi-spectral response schematic diagram in terms of energy band bending when gate voltage is set as negative (g) and positive (h).


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Vacuum-annealed ones can be found in Figure S3 and the extracted power exponent values and the corresponding conduction mechanism are given in Table S1 (Supporting Information). For zinc-annealed sample, all exponents are near unity, which unambiguously demonstrate that photoconductions in the core and shell region are dominated by Ohmic transport rather than traprelated space charge limited (SCL) one, revealing an underlying trap-free photoconductivity behavior. Furthermore, we calculate all detection performance parameters (on/off current ratio, i.e. sensitivity, photo gain Gλ, responsivity Rλ, specific detectivity Dλ* and linear dynamic range LDR) of detectors as listed in Table 1. Detailed calculation methods can be found in Supporting Information. These characteristics are overall far better than the existed reports on single ZnONW-based photodetectors (see Table S2 in the Supporting Information) and exhibit an overall performance superiority (broadband Rλ and Dλ* on the orders of 105 AW-1 and 1015 Jones at 100 mV, respectively). Interestingly, in Figure 3j, we can also observe that Rλ values highly coincide with DRS spectral evolution, i.e. a more intense visible absorption corresponds to a larger responsivity, indicating a harmonious photoelectric conversion capability over the whole spectrum. We have further measured the response performance at a higher bias of 1V (see Figure S4 in the Supporting Information) and plotted Rλ and Dλ* values as a function of the gate voltage (Figure 4a). For 365 nm (UV), 405 nm (V), 450 nm (B), 520 nm (G) and 635 nm (R), the maximum Rλ (Dλ* ) at VDS= 1V can be up to 3.67×106 A W-1 (1.17×1016 Jones), 3.08×106 A W-1 (9.85×1015 Jones), 7.51×105 A W-1 (2.40×1015 Jones), 5.40×106 A W-1 (1.73×1016 Jones) and 7.94×106 A W-1 (2.54×1016 Jones), respectively, which are both improved by an order of magnitude without sacrificing any detection performance in a certain frequency. When sweeping the gate voltage, the cut-off current ratios of blue, green and red response range from two to five orders of magnitude,


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which demonstrate the potential of gate-modulated visible detection performance. Furthermore, the UV decay time of device (Figure 4b) are also significantly shortened from tens of seconds (pristine) to 100 ms with visible decay time of 330 ms (zinc-annealing), which should benefit from a high carrier mobility. To verify if field effect can modulate visible photocurrent, different gate voltage VGS are applied for the temporal photocurrent on/off tests. Figure 4c-e clearly give successful examples of gate-tuned photocurrents under blue-, green-, and red-light stimuli, demonstrating the possibility of spectrally selective response only by applied VGS rather than commercial color filters, which can to some extent simplify the current complex bandpass filter structure. This is the first systematic demonstration for single ZnO nanowire full-color phototransistor, which displays a spectral-specific response selectivity for red, green and blue color illumination by the gate control. Similar full-color response performance has also been observed in other materials but few can show improvements in both responsivity and detectivity.13, 17, 23, 28, 47

It is worth noting that our devices can response to weak light down to tens of nW cm-2 (Figure

4f) and shows a surprising faster response speed (millisecond level), providing a promising application prospect for ultrasensitive high-speed light detection. Figure 4g-h depict gatemodulated multispectral detection mechanism. First, by zinc atoms related defect reconstructions, visible-sensitive ZnO:Zn shell layer can be formed between the gate electrode and the trap-free ZnO core. Due to the visible transitions between sp and 3d bands in metallic Zn,43, 48 along with the UV transitions between the conduction and valence bands in ZnO core, broadband absorption and the generation of photocarriers will synchronously take place. Second, thanks to a superior charge transport performance (corresponding to high carrier mobility and trap-free circumstances), the separation of photocarrier can be efficiently driven by the electric field and ultimately contribute to substantial photoconductivity. Third, in the dark state, when the gate stress is applied,


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the Fermi level (EF) of the core-shell nanowire will be tuned downwards/upwards, which induces the depletion/accumulation of electrons in the shell layer. Therefore, the electron transport channel between the core and the shell can be opened up when a positive gate stress (VGS>0) is applied. A high electron mobility in the core region results in a high on/off ratio and a good response recovery performance. When the phototransistor is illuminated, a negative gate stress (VGS0, the accumulation of electrons can easily overcome this electronic barrier and induce more probabilities of excess photocarriers transferred to the core and shell regions, ultimately leading to a distinct detection performance under B/G/R band stimuli. We can understand such a high photoresponsivity/photogain in our work from four aspects. First, First, assuming the photogenerated carriers are not spatially uniform, high gains (106~108) in NWbased photodetectors should come from the excess majority carrier accumulation on conduction channel and the excess minority carrier localization on defect/trap regions, which result in the long lifetime/response time.49 Due to the removal of defect/trap states in the core region, a high electron mobility above 200 cm2 V-1s-1 is obtained. Hence, the first way to achieve high gain values is to decrease the carrier transit time.50 A high mobility can ensure that those separated photocarriers are quickly transferred to both electrodes. Compared with the highest photogain record (~108),15 our gain of ~106 is smaller (but still fairly substantial). On the other hand, the response time of device is also shortened by two orders of magnitude, which results in an overall improvement of the device performance. Second, defect reconstruction induced surface coarsening (metallic zinc clusters) is beneficial to increase the light absorption cross-section. It is reported that Zn particles formed on ZnO surface can largely enhance the wide-band absorption from UV to visible


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spectrum.51 Third, since the conductive shell layer is rich in defect/trap states (the origin of visible response), this layer can act as a more stable photogating layer than the ambient oxygen, which plays a role of governing charge injection into the core region. Similar physical processes have been observed in several nanowire phototransistors.52-53 Finally, by applying different gate voltages in NW phototransistor, photocarrier concentration can be readily increased to the higher level or decreased to the dark state, resulting in higher on/off ratios.

■ CONCLUSION Single trap-free ZnO/ZnO:Zn core−shell NW has been synthesized by a two-step annealing approach. Based on this unique structure, a full-color (UV-visible) phototransistor has been demonstrated, which shows excellent detection performances with responsivities and detectivities on the orders of 105 A/W and 1015 Jones, respectively and fast response speeds of 100 ms (UV) and 330 ms (visible) under 100 mV bias. In contrast to doping induced photoconductivity, this defect reconstruction triggered photodetection reveals a gate-tunable spectral selectivity. Moreover, the key detection performances of such a phototransistor in both UV and visible bands are superior to that of commercial silicon-based detectors. Improved carrier mobility as well as robust multi-spectral detection capability suggest that this core-shell design route is highly suitable for filter-free, costless and low-power-consumption NW-based broadband photodetection applications.

■ ASSOCIATED CONTENT


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Supporting Information. The following files are available free of charge. Raman spectra, linear and logarithmic I-V characteristics, fitted parameter table, calculation details of photodetection performance, output characteristic curves, and performance comparison of 1D ZnO-based photodetectors (PDF)

■ AUTHOR INFORMATION Corresponding Author

* E-mail: [email protected] (Y.J. Zeng); [email protected] (S.T. Han)

Author Contributions L. Hu and Q.F. Liao contributed equally to this work.

Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China (Grant No. 61804098),

the

Shenzhen

Science

and

Technology

Project

(Grant

No.

JCYJ20170302153853962 and JCYJ20170412105400428), the Shenzhen Peacock Technological Innovation Project under Grant No. KQJSCX20170727101208249 and the Natural Science Foundation of Guangdong Province (Grant No. 2017A030310072)

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Defect Reconstruction Triggered Full-Color Photodetection in Single

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