Ultrasensitive Mid-wavelength Infrared Photodetection Based on a

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Ultrasensitive Mid-Wavelength Infrared Photodetection Based on Single InAs Nanowire Xutao Zhang, Hai Huang, Xiaomei Yao, Ziyuan Li, Chen Zhou, Xu Zhang, Pingping Chen, Lan Fu, Xiaohao Zhou, Jianlu Wang, Weida Hu, Wei Lu, Jin Zou, Hark Hoe Tan, and Chennupati Jagadish ACS Nano, Just Accepted Manuscript • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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Ultrasensitive Mid-Wavelength Infrared Photodetection Based on Single InAs Nanowire Xutao Zhang†#ξ, Hai Huang†#, Xiaomei Yao†#, Ziyuan Liξ, Chen Zhou ∥ , Xu Zhangξ§, Pingping Chen†*, Lan Fuξ, Xiaohao Zhou†, Jianlu Wang†, Weida Hu†, Wei Lu†*, Jin Zou ∥‡, Hark

†State

Hoe Tanξ, and Chennupati Jagadishξ

Key Laboratory for Infrared Physics, Shanghai Institute of Technical Physics,

Chinese Academy of Sciences, 500 Yutian Road, Shanghai 200083, China #University

of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China

ξDepartment

of Electronic Materials Engineering, Research School of Physics and

Engineering, The Australian National University, Canberra, ACT 2601, Australia ∥ Materials

Engineering, ‡Centre for Microscopy and Microanalysis, The University of

Queensland, St. Lucia, QLD 4072, Australia §School

of Information Engineering, Zhengzhou University, Zhengzhou, 450052, China


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ABSTRACT: One-dimensional InAs nanowire (NW) based photodetectors have been widely studied due to their potential application in mid-wavelength infrared (MWIR) photon detection. However, the limited performance and complicated photoresponse mechanism of InAs NW based photodetectors have held back their true potential for real application. In this study, we developed ferroelectric polymer P(VDF-TrFE) coated InAs NW-based photodetectors and demonstrated that the electrostatic field caused by polarized ferroelectric materials modifies the surface electron-hole distribution as well as the band structure of InAs NW, resulting in ultrasensitive photoresponse and wide photodetection spectral range. Our single InAs NW photodetectors exhibit a high responsivity (R) of 1.6 × 104 A W−1 as well as a corresponding detectivity (D*) of 1.4 × 1012 cm·Hz1/2W-1 at a light wavelength of 3.5 μm without an applied gate voltage, ~ 3 to 4 orders higher than the maximum value of photo-responsivity reported or commercially used mid-wavelength infrared (MWIR) photodetectors. Moreover, our device shows below band gap photoresponse for 4.3 μm MWIR light with R of 9.6 × 102 A W−1 as well as a corresponding D* of ~ 8.5 ×1010 cm·Hz1/2W-1 at 77 K. Our study shows that this approach is promising for fabrication of high-performance NW-based photodetectors for MWIR photon detection.


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KEYWORDS:

InAs

nanowires,

mid-wavelength

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infrared

photodetection,

photoresponsivity, electrostatic field, Franz-Keldysh effect As the vital atmospheric window, mid-wavelength infrared (MWIR) 3 - 5 μm radiation has be extensively used in industrial, scientific, law enforcement, medical, military and civilian applications, including target acquisition, surveillance, fingerprint recognition, night vision, homing, infrared tracking, guidance, thermal imaging, reconnaissance, warning and astronomical observation.1-5 Therefore, there is an urgent need to develop highly sensitive MWIR detectors. The promising and prominent avalanche photodetector is a very good choice because less photons or even single photons are demanded to trigger the avalanche multiplication process and cause macroscopic current changes.5,

6

However, avalanche

photodetectors are often accompanied by increased noise when the signal is enhanced, resulting in a low signal-to-noise ratio of the device, which has restricted the wider applications. On the other hand, the photodetectors in photoconductive or photovoltatic mode may offer an option to realize high response and signal-to-noise ratio infrared detection without the influence of increased noise. InSb and HgCdTe (MCT), by far the paradigmatic candidates for MWIR applications, demonstrate vigoroso performance and have been widely used in commercial products with high detectivity (D*) of ~ 1011 cm·Hz1/2W-1 and responsivity (R) of 3 A W−1 at 77 K.7-12 Some emerging materials, such as bP/MoS2 heterojunction,13 HgTe CQDs/As2S3,14 and hBN/b-AsP/hBN heterostructures15 also show a very attractive application prospect in MWIR detection. However, although


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these detectors exhibit good detectivity (about 1011 cm·Hz1/2W-1), their R values are quite low (no more than 3 A W−1). Due to its huge surface-area-to-volume ratio, direct narrow bandgap and high carrier mobility, InAs nanowire (NW) photodetectors are expected to have an excellent response in the MWIR range. The huge surface-area-to-volume ratio of NWs leads to a great increase in the NW light absorption while rich surface defect states are introduced as well. These surface defect states in InAs NWs enhance ionization scattering and in turn decrease electron mobility, resulting in low photo gain and holding back its true potential for real applications.16-21 Recently, the InAs NW device has extended the response wavelength to 3.1 μm.22 Therefore, developing a device structure that can shield ionization scattering of surface defects and thus enhance the photodetection capability and signal-to-noise ratio of single InAs NW detectors operating in the MWIR range is of great interest. In this study, we designed a single InAs NW-based phototransistor with a ferroelectric dielectric layer, where InAs NW is used as the photosensitive material and the ferroelectric polymer P(VDF-TrFE) is utilized to modulate carrier concentration in the NW core. By applying a short pulse of external bias voltage, a very high local electrostatic field of about 109 V m-1 can be generated in the surface of ferroelectric polymer. The non-volatile remnant polarization could suppress the dark current at a fairly low level without applying any gate voltage. Meanwhile, with such a high ferroelectric polarization field, the defect states onto the NW surfaces can be pre-adjusted and filled with electrons to completely


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shield the ionization scattering, resulting in a large increase in carrier mobility and lifetime in the NW core. These greatly increase the detection sensitivity of our InAs NW photodetector: not only the D* is greater than that current reported and commercial devices, but also R is ~ 4 orders higher than that reported at λ = 3.5 μm.1-3, 8-12, 14, 15, 23-25 In addition, the strong band bending and thus the Franz-Keldysh effect caused by such high polarization field further enable sensitive sub-bandgap photodetection through InAs NWs, extending the detector’s photoresponse beyond the InAs bandgap and exhibiting high performance. RESULTS AND DISCUSSION The growth of InAs NWs was carried out in an ultrahigh vacuum molecular beam epitaxy (MBE) system on a GaAs (111)B substrate based on the vapor-liquid-solid (VLS) growth mechanism (Figure S1 and Figure 1, see Methods for more detailed information). Figure 1a shows a 30°-tilted scanning electron microscopy (SEM) image of one single InAs NW, the length of a typical NW is ~ 20 m. The NWs grew along the direction were demonstrated to have a high-quality wurtzite (WZ) structure through the highresolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) measurements (Figure 1c-d and Figure S1). Figure 1b is cross-section bright-field (BF) TEM image, showing it has a hexagonal shape and the diameter is ~ 80 nm. In particular, according to the SAED from the [0001] zone-axis (Figure 1c), it can be determined that the NW has six {1120} side facets. The HRTEM image was taken from the apex of the hexagon (refer to Figure 1d), indicating potentially the formation of a native


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oxide layer upon exposure to the atmosphere after growth. Indeed, Figure 1e is the energy dispersive spectroscopy (EDS) analysis of points A and B in Figure 1d, confirms this outer layer has a In:As:O ratio of 46:31:23. Single InAs NW was transfer to SiO2/Si substrate and followed by electron beam lithography (EBL) process to make it into a field effect transistor (FET). The oxidation layer at the surface were removed by hydrofluoric acid prior to metallization to ensure good Ohmic contacts. As shown in the inset of Figure 2a, the length of source/drain (S/D) channel was fixed as 4.4 μm. Figure 2a shows the linear output characteristics (Ids-Vds) of InAs NW FET under dark condition, indicating the good Ohmic contact at the metalsemiconductor junction. The InAs NW FET was then spin-coated with a 300 nm-thick P(VDF-TrFE) polymer film, and annealed at 135 °C for 4 hours. Finally, an ultrathin semitransparent Al (~ 10 nm) film was deposited on the top of ferroelectric layer as an electrode. Figure 1f shows the schematic structure of a single InAs NW FET device. Subsequently characterization measurements in the device were carried out under vacuum at 77 K.


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Figure 1. Material characteristics of the InAs NW and device structure. (a) SEM image with a 30° tilt angle showing a typical single InAs NW. (b-d) BF-TEM images of a typical NW cross section and corresponding SAED pattern. (e) EDS spectra measured from points A and B, marked in figure 1d. (f) A schematic diagram of the single InAs NW FET. The left half is the section-view.

The ferroelectric characteristics of P(VDF-TrFE) thin film has been well studied in our previous studies.26, 27 Figure 2b depicts the transfer curves of the top-gated InAs NW phototransistor. Notably, in comparison to the SiO2/InAs NW hybrid device (inset of Figure 2b), our top-gated NW FET displays a higher Ion/Ioff ratio up to 107 and extremely low off current of ~ 4×10−13 A. In addition, the transfer curve shows a large current


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hysteresis which has been related with the NW surface states.28, 29 It is of interest to note that the hysteresis loop direction of the P(VDF-TrFE)-polarized NW device is not reversed but in a clockwise direction, the same with the hysteresis of a SiO2 backgated InAs NW device.26, 27 This property demonstrates that not only the polarized ferroelectric dielectric layer but also the NW surface states have a great influence on the transfer characteristics of the device. Details of these characteristics is discussed below.

Figure 2. (a) Ids - Vds curves of a InAs NW device measured under dark without any gatevoltage. The inset showing the SEM image of a single InAs NW device. (b) Ids - Vtg curves of the ferroelectric polymer/InAs NW top-gate device in dark. The inset showing the transfer characteristics of SiO2/InAs NW back-gate device. (c-f) Vds - Ids characteristics with three different states of P(VDF-TrFE) film at Vgate = 0 V, which are: fresh state (P(VDF-TrFE) film before polarization), Polarized upward state (“Pup,” polarized by a −


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45 V top-gate voltage pulse with a width of 2 s), and polarized downward state (“Pdown,” polarized by a + 45 V top-gate voltage pulse with a width of 2 s). (g-i) Energy band diagram of three P(VDF-TrFE) polarization states at Vds = 0 V. EC, EV, and EF representing respectively the lowest conduction band energy, the highest valence band energy, and the Fermi level, in which ΦB represents the small Schottky barrier height between electrodes and photosensitive material, δ is the height from EC to EF, and δ1, δ2, and δ3 correspond to different polarization directions, respectively.

Using the ferroelectric polymer/InAs NW top-gate structure, three different states can be achieved: the fresh state before polarization, polarized upward state (Pup), and polarized downward state (Pdown), in which Pup and Pdown can be obtained by applying top-gate voltage pulse of - 45 V and + 45 V, respectively. Figure 2b is the Ids - Vtg curves of the ferroelectric polymer/InAs NW top-gate device under dark condition, indicating that the InAs NW channel can be entirely accumulated or depleted of electrons under Vtg = - 45 V and + 45 V, respectively. Figure 2c shows the Vds - Ids characteristics (at Vtg = 0 V and dark condition) of these three states. Among them, the current in the Pdown state is the lowest, which is completely different from previous studies.26, 27 When the polarization direction of the ferroelectric dielectric layer is down after Vtg = + 45 V is applied, as shown in Figure 2f, positive charges surround the NW surface, causing a large amount of electrons captured on the surface defect states of the InAs NW. These trapped electrons form a built-in


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electrostatic field in the NW, resulting in depletion of the carriers in the NW core which in turn contribute to transport. It shows that the electrostatic field formed by the polarized ferroelectric film causes the carriers depletion state in the InAs NW channel. Accordingly, electrons accumulate in the photosensitive material under the Pup state. Figures 2d-f and 2g-i show respectively the schematic views of cross-sectional photodetector structures and the corresponding equilibrium band diagrams in three different polarization directions. In addition, after removing Vtg, the detector can still maintain the polarization operation caused by the polarized ferroelectric film, revealing that our photodetectors can work without any further application of gate voltage and thus leading to low power consumption. The photodetection performance of the InAs NW detector was characterized at Vgate = 0 V, and the results are shown in Figure 3. Firstly, we investigate the case under the Pup state, in which accumulation of electrons results in an enhanced channel current (green curve) in comparison to the fresh state (not poled, black curve, refer to Figure 3a). Figure 3a displays the Vds - Ids characteristics under two illumination wavelengths: Ids drops under 0.45 μm illumination (blue curve) while remains virtually unchanged under 2 μm illumination (red curve), in comparison to the dark measurement. This effect is due to the presence of electron-trapping states presenting in the outside surface oxides. The free electrons in the NW core may be trapped into these states after excitation with a photon energy observably above the InAs band gap.18 Thus, the device shows negative photoresponse (NPR) when applying the incident light wavelength of 0.45 μm. On the


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other hand, due to the low photon energy (  2 μm), the photogenerated electrons do not gain sufficient energy captured by the surface trapping states. They can contribute to the photocurrent, and therefore a positive photoresponse (PPR) can be observed. However, when the incident light wavelength is 2 μm, a large background noise was formed by tunnelling and thermionic currents, resulting in the photocurrent (Ilight) being hardly distinguished from the large dark current (Idark). Figure 3b clearly shows the phenomena of NPR and PPR under illumination above and below the energy barrier between the electrontrapping level and the valence band. Meanwhile, the threshold voltage on the up sweeps shift toward positive value under the 0.45 μm illumination and negative value under the 2 μm illumination, indicating a decrease and an increase in the mobile carrier density under different light illuminations, respectively.18

Figure 3. Photodetection performance of the single InAs NW photodetector with ferroelectric top-gate under the polarized upward state without any additional top-gate voltage. (a) Vds - Ids output characteristic curves of the detector at Vds = 1 V under different


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lighting conditions. (b) Vtg - Ids transfer curves of InAs NW FET under different lighting conditions.

To achieve sensitive detection of infrared light, low dark current, high carrier mobility, long carrier lifetime are essential for InAs NW photodetectors. Therefore, the polarization direction the ferroelectric dielectric layer was pre-set at the Pdown state by a +45 V top-gate voltage pulse. It is expected that our device operates with a relatively low dark current in this state. Figure 4a shows that the channel current (green curve) in this state is significantly lower than that of the fresh state (not poled, black curve), indicating that the electrons in the channel are completely depleted. Meanwhile, when it is illuminated with either 0.45 or 2 μm light, the current is increased as a result of PPR, as shown in Fig. 4a. This clearly indicates that the NW surface states are saturated with electrons under the action of the polarized ferroelectric dielectric layer. Therefore, under the 0.45 μm illumination, the photogenerated electrons cannot be trapped by the surface states anymore, and instead participate in the transport. By pre-filling electrons into the defect states, the ionization scattering effect of the surface state on carriers can be eliminated, resulting in a large increase in carrier mobility and lifetime of the NW core. It is worth pointing out that, in the Pdown state, due to the electron depletion under the ferroelectric field, thermionic current and the tunnelling current are suppressed (refer to Figure 2i). Figure 4b shows the Vds - Ids output curves measured at the MWIR wavelength


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of 3.5 μm for different illumination intensities, in which photogenerated current dominates the channel current, resulting in an increased infrared photoresponse. The photocurrent is increased significantly with increasing the excitation power at a fixed bias voltage Vds, thereby achieving a highly sensitive photoelectric response at the band edge (~ 3.5 μm) of InAs NW photodetector. The red curve in Figure 4c shows the excitation power dependence of the net photocurrent (Iph) of the detector, showing that the photoelectric response is closely related with the power of incident light. The corresponding sublinear relationship of Iph and incident power (P) can be obtained with the power law as Iph ∝ P0.64, where 0.64 is an empirical value extracted from Figure S2.30

Figure 4. Enhanced photoelectric response of the ferroelectric polymer/InAs NW hybrid photodetector under polarized downward state without any additional top-gate voltage. (a) Vds - Ids output curves of the device under different lighting conditions. (b) Vds - Ids output curves of the device for 3.5 μm exciting light at different power densities. (c) Dependence


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of Iph and R on light power densities at Vds = 1 V, showing high sensitivity, defined as Iph = Ilight – Idark. (d) Vds - Ids output curves of the device under different MWIR lighting and dark conditions. Photocurrents of the detector under (e) 3.5 μm and (f) 4.3 μm illuminations.

To evaluate the photodetection performance of devices, photoresponsivity R is a key parameter, defined as: R = Iph / (PA), where A and P represent the effective illuminated region on the semiconductor channel and the light power density, respectively. The effective illuminated area can be estimated by the cross-sectional area of the InAs NW, which can be expressed as A = L × d, where L and d respectively represent the length of the semiconductor channel and diameter of the NW.27 The blue curve in Figure 4c presents the dependence of the calculated R of the device on incident power density. A maximum R of ~ 1.6 × 104 A W−1 is achieved for 3.5 μm exciting light with P ~ 8.5 mW cm−2. In addition, the photo gain (G) and specific photoelectric detectivity (D*) can also be obtained. The photo gain of the device is presented as G = (Iph/e) / (PA/ (hν)), where e and hν respectively represent the electronic charge and energy of an incident photon.16, 17, 27, 31 The specific photoelectric detectivity is a vital figure-of-merit to evaluate a photodetector’s capability of the faintest detectable signal. The photo detectivity can be presented as D* = RA1/2 / (2eIdark)1/2. 16, 17, 27, 31 Under the effect of remnant polarization from the ferroelectric polymer, the dark current can be supressed to an extremely low level. Thus, G and D* of the photodetector reaches as high as 5.7 × 103 and 1.4 × 1012 cm·Hz1/2W-1 under the 3.5


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μm illumination, respectively. Compared with other reported values of InAs NW detectors at MWIR wavelengths,20, 22, 32-34 the values of G, R and D* obtained in this study show the best photodetection performance. We note that, an even higher responsivity for the device could be obtained by replacing the Al top-gate electrode with a transparent one. The range of the detection wavelength is also an important factor for photodetectors. The typical photoelectric detection range of the InAs NW-based detector is from visible to MWIR (2.6 or 3.5 µm) depending on the crystal phase of InAs NWs with a band gap of 0.477 eV for the wurtzite phase and 0.35 eV for zinc-blende phase at room temperature.18, 37

A decrease in temperature causes an increase in the band gap,38 resulting in a blue shift

in the detection range of the InAs detectors. In Pdown state, Figure 4d shows the photocurrent of the device under different MWIR lighting conditions. Time-resolved on-and-off Ids measurements were also measured under different MWIR lighting conditions (refer to Figure 4e-f and Figure S3). Excitingly, our device still has significant detection capabilities at illumination conditions up to 4.3 μm. At this wavelength, G = 2.8 × 102, R = 9.6 ×102 A W-1 and the corresponding D* of ~ 8.5 ×1010 cm·Hz1/2W-1 can be obtained for P ~ 62.3 mW cm−2. The decrease in device detection performance is due to the reduced light absorption of the device in this sub-wavelength photoexcitation mode.39 In particular, this study extends the photodetection of an InAs NW device from 3.5 to 4.3 µm. Compared with other recently reported MWIR detectors, the obtained D* and R of our photodetectors exhibit excellent performance at excitation wavelength of 3.5 μm and also comparable


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performance at excitation wavelength of 4.3 μm (refer to Table 1). It is worth noting that, 4.3 μm is not its detection cutoff wavelength and our detectors may have good photoresponse in even longer wavelength region, which is not demonstrated here due to our equipment limitation. In this study, our device photoelectric response performance under the fresh state has also been investigated at 77 K, where the device can hardly distinguish the Iph signal from the large Idark under MWIR light illumination.

Table 1. Comparison of photoresponse Detectivity and Responsivity of Various Photodetectors. Photodetectors

Detectivity (cm·Hz1/2W-1)

Responsivity (A W-1)

Temperature

Reference

In0.1Ga0.9As/Al0.4Ga0.6As (QWIPs)

1 × 107 at 5 μm

0.1 at 5 μm

300 K

35

HgTe CQD

3.3 × 1011 at 5 μm

1.3 at 5 μm

85 K

36

PtSe2

7 ×108 at 10 μm

4.5 at 10 μm

300 K

3

bP/MoS2

1.1 × 1010 at 3.8 μm

0.9 at 3.6 μm

77 K

13

HgTe/As2S3

3.5 × 1010 at 3.5 μm

0.5 at 3.5 μm

230 K

14

1.2 × 1011 at 4 μm

0.5 at 4 μm

77 K

24

InSb p-i-n

3.08 × 109 at 5.3 µm

0.475 at 5.3 µm

77 K

9

InSb p-i-n

8.8 × 109 at 5.3 μm

0.7 at 5.3 μm

80 K

10

Commercial HgCdTe detector

6.8 × 1010 at 5 µm

2.2 at 5 µm

180 K

7

Commercial InSb detector

1 × 1011 at 5 μm

3 at 5 μm

77 K

11, 12

InAs/GaSb

superlattice


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InAs NW

1.4 × 1012 at 3.5 μm

1.6 × 104 at 3.5 μm

77 K

this work

InAs NW

8.5 ×1010 at 4.3 μm

9.6 × 102 at 4.3 μm

77 K

this work

It should be noted that the above outstanding properties of the InAs NW photodetectors benefit from the high electrostatic field caused by polarized ferroelectric dielectric layer. This remnant electrostatic field in-between the ferroelectric film and InAs NW can be calculated from the equation: σ = εε0Ε, where ε0，ε and E represent the vacuum permittivity, permittivity of InAs (~ 15.15), and electric field strength, respectively. In fact, σ represents the charge density of the ferroelectric film surface, which is corresponding to the remnant polarization (Pr ~ 7.0 µC cm−2) of P(VDF-TrFE).26,

30

The estimated

electrostatic field applied to the InAs NW surface is ~ 0.52 × 109 V m−1. In conventional devices reported so far,26, 29, 32, 40, 41 this ultra-high electric field is relatively difficult to obtain. In the Pdown state, under the effect of this ultrahigh electric field, a large number of electrons are accumulated at the NW surface in contact with the P(VDF-TrFE), causing the NW energy band to bend strongly downward and in turn forming an ultrahigh built-in electrostatic field. This results in the Franz-Keldysh effect with the electron/hole wavefunctions “tunnel” into the bandgap region, causing the overlap of the electron and hole wavefunctions, a similar phenomenon observed in GaN NW.42-44 The detailed working principle diagram is shown in Figure 5. Therefore, it is expected for a photon of


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lower energy than the band gap to excite an electron, allowing the detector to achieve subbandgap photodetection beyond InAs band edge, as shown in the dotted line b of Figure 5. The bandgap variations under ultrahigh electrostatic field induced by polarized P(VDFTrFE) polymer (as previously demonstrated in MoS2 phototransistors26) may also contribute to the ultra-broad photoelectric response. Further investigations are necessary to fully understand the working mechanisms of ferroelectric polarization effect on the InAs NW bandgap. Nevertheless, our study shows the great promise of these ferroelectric polymer/InAs NW hybrid photodetectors for the MWIR photodetection.

Figure 5. Schematic electron transition band diagram of the effect of band bending in the Pdown state. The line a showing the typical electronic transition in the InAs NW under photoexcitation with the incident light wavelength < 3.5 µm. Under the Pdown state, the


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ultrahigh electric field leading to electron accumulation on the NW surface and strong band bending, allowing overlap of electron and hole wave functions within the bandgap and thus a low-energy electron transition (dotted line b) when the incident light wavelength > 3.5 µm.

CONCLUSIONS In conclusion, we investigated single InAs NW FET devices with designed ferroelectric polymer film top-gate, and demonstrate that by controlling the remnant polarization state of the polymer film, the device exhibits completely different photoelectric behaviour. In the Pup state, the device exhibits either a positive or negative photoresponse depending on the illumination wavelength. In the Pdown state, this ferroelectric polymer/InAs NW hybrid device demonstrates exceptional photoresponse performance. Comparing with the previously reported InAs NW detectors, it reaches a maximum attainable G of 5.7 × 103, R of 1.6 × 104 A W−1, and corresponding D* of 1.4 × 1012 cm·Hz1/2W-1 at λ = 3.5 µm, which are the record values of InAs NW photodetector operating in the MWIR range. More significantly, the detection range could be extended below the band gap of InAs to λ = 4.3 µm due to the Franz-Keldysh effect and the strong band banding under the ultrahigh electric field induced by the ferroelectric polymer film. The ultra-broad and highly-sensitive MWIR photoresponse of this ferroelectric


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polarization gated InAs NW photodetector indicates the promising applications in infrared imaging and free-space communications. METHODS Growth information: The InAs NWs were epitaxially grown via the VLS (Au as the catalysts) growth approach in a Riber 32 MBE system. After degassing the GaAs substrate in the preparation chamber and deoxidizing the substrate in the growth chamber to remove any contaminants, a buffer layer (GaAs) was deposited at 580°C for 15 min. Subsequently, a 10 nm Au thin film was deposited in the preparation chamber through thermal evaporation. The Au-coated GaAs substrate was then annealed under As atmosphere in the growth chamber at 550 °C for 5 min, and in this process this 10 nm Au film will aggregate into isolated nano-particles for catalyzing the NW growth. After that, the temperature was decreased 230 °C for the NW growth while the In and As vapor pressures were maintained at ~ 2.5 × 10−7 and ~ 4.4 × 10−6 Torr, respectively. Device Fabrication and Characterization: InAs NWs were transferred to Si (p+)/ SiO2 (285 nm) substrates mechanically. The S/D electrodes pattern were made through the EBL, and 10 nm Ti/60 nm Au were deposited as metal electrode with the e-beam evaporation. All the photoelectronic performances of the NW detector were measured using a KEITHLEY 4200-SCS Parameter Analyzer. The microstructures of grown InAs NWs were characterized by SEM (JEOL7800) and TEM (Philips Tecnai F20). Individual NW cross-sections were sectioned by an ultramicrotome (Leica EM UC6).


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ASSOCIATED CONTENT Supporting Information Microstructural features of InAs NWs (TEM and SAED pattern); the relationship of photocurrent and incident power; Time-resolved on-and-off current of the phototransistor under different MWIR lighting conditions. AUTHOR INFORMATION Corresponding Authors *E-mail: (P. Chen) [email protected] *E-mail: (W. Lu) [email protected] Author Contributions (X. Zhang and H. Huang) These authors contributed equally to this work. ORCID Xutao Zhang: 0000-0001-5259-5745 Jin Zou: 0000-0001-9435-8043 Hark Hoe Tan: 0000-0002-7816-537X Lan Fu: 0000-0002-9070-8373 Conflict of Interest: The authors declare no competing financial interest. Acknowledgments


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This study was supported by the National Key R&D Program of China (No. 2016YFB0402401 and 2016YFB0402404), the Royal Society-Newton Advanced Fellowship (Grant No. NA170214), the National Natural Science Foundation of China (No.11634009, 61835012 and 61722408), the Basic Research Programs of Shanghai Science and Technology Commission (No.16JC1400404), the Key Programs of Frontier Science of the Chinese Academy of Sciences (No.QYZDJ-SSW-JSC007), and the Australian Research Council. The Australian National Fabrication Facility and Australian Microscopy & Microanalysis Research Facility are acknowledged for accessing the device fabrication facilities and characterization facilities. REFERENCES 1.

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Ultrasensitive Mid-wavelength Infrared Photodetection Based on a

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