Light-Induced Positive and Negative Photoconductances of InAs

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Light Induced Positive and Negative Photoconductances of InAs Nanowires Toward Rewritable Nonvolatile Memory Xutao Zhang, Ziyuan Li, Xiaomei Yao, Hai Huang, Dongdong Wei, Chen Zhou, Zhou Tang, Xiaoming Yuan, Pingping Chen, Weida Hu, Jin Zou, Wei Lu, and Lan Fu ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00368 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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ACS Applied Electronic Materials

Light Induced Positive and Negative Photoconductances of InAs Nanowires Toward Rewritable Nonvolatile Memory Xutao Zhang†ξ#, Ziyuan Liξ, Xiaomei Yao†#, Hai Huang†#, Dongdong Wei†, Chen Zhou∥, Zhou Tang†, Xiaoming Yuan&ξ, Pingping Chen†*, Weida Hu†, Jin Zou ∥ ‡, Wei Lu†* and Lan Fuξ †State

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

Chinese Academy of Sciences, 500 Yutian Road, Shanghai 200083, China ξDepartment

of Electronic Materials Engineering, Research School of Physics and

Engineering, The Australian National University, Canberra, ACT 2601, Australia #University

‡Centre

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

for Microscopy and Microanalysis,

∥ Materials

Engineering, The University of

Queensland, St. Lucia, Queensland 4072, Australia &School

of Physics and Electronics, Hunan Key Laboratory for Supermicrostructure and

Ultrafast Process, Central South University, 932 South Lushan Road, Changsha, Hunan 410083, China

ACS Paragon Plus Environment

ACS Applied Electronic Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Here, we demonstrate that single InAs nanowire (NW)-based field-effect transistor exhibits wavelength-selective switching between positive and negative photoresponses caused by abundant defect states on the NW surface. Base on this, by modifying the NW carrier transport behavior through the strong local electric field formed by polarized ferroelectric polymer P(VDF-TrFE), a rewritable NW memory device can be achieved with electrical bistability and low-power consumption when writing with different wavelength light pulses without any gate voltage. Our study clearly reveals the significant role of the NW surface states in not only photodetecting devices, but also promising novel nonvolatile light-assisted memory devices, for numerous future applications. 1 Introduction Owing to the high carrier mobility, direct narrow bandgap, unique and excellent optoelectronic properties, indium arsenide (InAs) nanowires (NWs) have attracted tremendous attention, and have achieved great success for high photoconductive gain, fast response, efficient light-to-current conversion, and broad-spectrum photodetection.[1-19] In addition, due to the huge surface-area-to-volume ratio and the abundant defect states onto the surface, the surface states of InAs NW play a crucial role in the photosensitive properties of the device.[5-7] Different from other NW systems where positive photoresponse has normally been observed,[20-22] two distinctively different photoresponse


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behaviors have been reported for single InAs NW photodetectors: positive photoresponse (PPR)[8, 9, 11-13] and negative photoresponse (NPR).[5-7] The surface states of InAs NWs can be regulated by the incident light, which in turn causes the device to have different output currents under different illumination conditions. Interestingly, under certain conditions, the output current of a device can be maintained even after the incident light is removed.[5, 23] Thus, understanding and manipulating the nature of the outside surface states offers further opportunities to explore new physics and functionalities. For example, an optical-memory device can be achieved by taking the advantage of the carriers depletion caused by highenergy light irradiation, and the current remains almost unchanged after illumination.[5] The "write" function of this device can be achieved by light irradiation, however, the "read" function requires an additional voltage pulse to be implemented. This makes the usability of the device worse and the energy consumption higher, making it difficult for their practical applications. In this study, we fabricated single molecular beam epitaxy (MBE)-grown InAs NW transistors with a ferroelectric dielectric layer. By tuning electron trapping states on NWs’ surface through the strong local electric field formed by polarized dielectric layer P(VDFTrFE), the photoconductive behavior of the InAs NW device transforms from NPR to PPR when the wavelength of incident light is increased from visible to short wavelengthinfrared without applying any gate votage. Furthermore, different stable levels of current



output can be obtained and durably maintained when illuminated with different wavelength light pulses. Thus the "read" and "write" functions of the device can be achieved with light assistance, which can be potentially employed as an optically modulated nonvolatile memory device. 2 Results and Discussion In this study, InAs NWs were grown by ultrahigh vacuum MBE using Au-assisted vapor–liquid–solid (VLS) technique (see Methods for more details) with an average diameter of 80 nm and length of ~ 11 μm in the middle, as shown in the scanning electron microscopy (SEM) image in Figure 1a. Our transmission electron microscopy (TEM) characterization confirms that the as-grown NWs are single crystals and have the wurtzite (WZ) structure with the growth direction along the direction. Figure 1b and the inset are high-resolution TEM (HRTEM) image as well as the corresponding selected area electron diffraction (SAED) pattern taken from a typical NW, respectively. As can be seen from Figure 1b, the NW is covered by a thin outside oxide layer (~ 2 nm), as marked by a red dashed rectangle. After transferred to the SiO2 /Si substrate, single InAs NW field effect transistor (FET) was designed and fabricated (see Methods for more details). Prior to metallization, the outside oxide layer of the electron beam lithography (EBL)-exposed NW regions were removed to undertake good Ohmic contacts. The SEM image of the InAs NW device indicates that the channel length is ~ 4.4 μm, as shown in Figure 1c. Figure 1d


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presents the typical dark current-voltage (Ids-Vds) output characteristic curves measured from a single InAs NW FET at room temperature (RT) and 77 K under the vacuum (~ 103

Pa), respectively. The quasi-linear output curves Ids - Vds show the formation of good

Ohmic contacts.

Figure 1. (a) SEM image (45° tilted from the surface normal) showing as-grown InAs NWs. (b) HRTEM image and corresponding SAED pattern (inset) of a section of a typical InAs NW showing high crystalline WZ structure. (c) SEM image of a fabricated InAs NW FET. (d) Output characteristic curves of the FET at a gate voltage of 0 V at different temperatures under dark condition.



To obtain an ultra-high electric field to tune the electron trapping states on NWs’ surface, a layer of polymer P(VDF-TrFE) (about 300 nm thickness) was spun and coated onto the InAs NW devices, followed by baking at 135 °C for 4 h to improve the crystallinity of the P(VDF-TrFE) layer. After adding an ∼ 10 nm ultrathin semitransparent aluminium electrode on the dielectric layer (P(VDF-TrFE)) as the top-gated electrode, an additional 45 V of gate voltage pulse was applied for poling the ferroelectric dielectric layer, resulting in an upward polarization direction of the P(VDF-TrFE).[24] Then a remnant polarization value at about 7 µC cm−2 and an ultrahigh electric field at about 109 V m−1 can be obtained, which have been characterized in our previous works.[25, 26] In this situation, the surface of the nanowire is surrounded by negative charges from the dielectric layer, leading a great deal of electrons to be electrostatically driven from NW outside layer to the core. Accordingly, a local electric field near the NW surface layer is formed, causing electrons to accumulate in the core of InAs NW (as the outside oxidation layer can be considered as the NW shell) which are involved in the transport, as shown in the inset of Figure 2a.


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Figure.2 (a) Photocurrent response of the device under different light illuminations. The inset is a schematic of polarization up. (b) Light pulses are used to study the storage capabilities of the device. Pulse with different wavelength light (450 nm or 2000 nm) can modulate devices with differently stable current output levels. The power densities of 450 and 2000 nm are ∼ 0.078 and 0.9 mW/mm2, T = 77 K and Vds = 1 V, respectively.

Figure 2a shows the observation of a time-resolved on/off photo-switching behavior that is tuned under different excitation wavelengths after modifying the surface states of



NW through the strong local electric field from remnant polarization of dielectric layer (P(VDF-TrFE)). It clearly shows an unusual illumination wavelength dependent photoresponse (positive or negative) without applying any gate voltage. For the case of 450 nm light illumination, before light illumination, the highest current (state S0) of the device is obtained which then drops rapidly to a moderately state S2 during the light exposure, showing an NPR behavior. After the light is turned off, the current quickly decreases to the lowest current state S3. When the light is turned on again, the current of device rises from S3 to S2, showing a PPR behavior. When the light is turned off and on again, the current only switches between S2 and S3. During the first few cycles of the 450 nm light switching, electron trapping and de-trapping processes have not reached dynamic equilibrium under the action of photoexcitation and remnant electrostatic field generated by polarized P(VDFTrFE), resulting in a small decrease in dark current (refer to S3 of Figure 2a). For the case of 2000 nm light illumination, the device current rises from S3 to S1 at the first pulse and then toggles between S2 to S1 showing PPR. The photoelectric response is repeatable and stable after several switching cycles without any deterioration. When the 2000 nm light is turned off, the device dark current rises from S3 to S2. This is because that the photogenerated holes under 2000 nm light were combined with the electrons trapped in the trapping level, resulting in a reduction in the surface local electric field and an increase in current.


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To study the storage function under different wavelength light pulses, we measured the conductance of the device showing easiness to be tuned for achieving the high performance rewritable nonvolatile light-assisted memory devices with electrical bistability by employing the P(VDF-TrFE) polymer’s polarization property without any gate-voltage. Figure 2b shows the switch between two stable current outputs (S2 and S3) obtained with different wavelength light pulses. Compared with previous reports,[5] whose switch requires not only the light but also the gate voltage pulses, the advantage of "read" and "write" functions of our devices is that they can be switched just by different wavelengths light pulses. This greatly enhances the usability of the device and reduces power consumption. Simultaneously, owing to their single transistor realization, such a device structure is easy to be integrated in large-scale circuits. Therefore, this study demonstrates a new approach to fabricate a novel light-assisted memory.



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Figure 3. Photoresponse measurements of the InAs NW phototransistors with P(VDFTrFE) coating. Time-dependent photoresponse of the InAs photodetector under different wavelengths of the incident light of (a) 450 nm, (b) 940 nm, (c) 1060 nm, (d) 1310 nm, (e) 1550 nm, and (f) 2000 nm, which were tested under 77 K, Vds = 5 V and Vbg = 10 V.

To better understand the complex carrier transport property and its impact on the photoresponse of our InAs NW devices, the light-dependent time-resolved on-and-off Ids was obtained before poling the P(VDF-TrFE) polymer, respectively. Figure 3 presents wavelength-dependent time-resolved photoresponse under back-gate voltage (Vbg) = 10 V. As noted by the curve in Figure S1, clear ambipolar transport characteristics can be observed, showing an impressive Ion/Ioff ratio of 106 (n-type) and 102 (p-type).

[27]

The

device shows an increase in Ids as the gate voltage increases from 0 V, suggesting the conduction of majority carrier is electron when Vbg ＞ 0 V; while Ids increases as the gate voltage decreases from 0 V, exhibiting the hole transport when Vbg < 0 V. Therefore, the choice of the gate voltage of 10 V is to ensure that the device is electronically conductive (n-type). It should be noted that, when the light wavelength is no more than 940 nm, the photoresponse performance of the device is consistent with that when irradiated with a 450 nm light (the photoresponse performance under 637 nm and 830 nm light illumination has also been tested, respectively, showing similar behavior). However, when we apply a


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longer wavelength (≥ 1060 nm), the photoelectric response mechanism of the device undergoes a significant change from the NPR to PPR (Figure 3c-f). The photocurrent observed in Figure 3b-d is not as stable as that under 450 nm illumination, which is expected due to the intensity of the infrared lights are relatively sensitive to the testing conditions such as temperature.[13] Meanwhile, when the device is p-type (Vbg = -20 V), it always exhibits the PPR when the wavelength of the incident light is varied between 450 nm and 2000 nm (Figure S2).

Figure 4. Time-dependent on-and-off Ids measurements of the InAs NW photodetector with P(VDF-TrFE) coating under varied bias voltages (Vds) and different back-gate voltages of (a) -40 V, (b) 0 V, (c) 10 V and (d) 20 V, which were tested under 450 nm light illumination at a power density ∼ 0.078 mW/mm2, T = 77 K. The normalized time-resolved photoresponse for the varied wavelengths of the incident light of (e) 450 nm and (f) 2000



nm. (e) The red and magenta lines fit the falling (response time tresp1) and rising (response time tresp2) edges, and the blue lines fit the falling (recovery time trec) edge, respectively. (f) The red and blue lines fit the rising (response time tresp) and falling (recovery time trec) edges, respectively.

To investigate the changes in photoconductivity under different carrier transport scenarios, the gate-voltage-dependent time-resolved on-and-off Ids was obtained at different source-drain voltages under 450 nm light illumination (Figure 4a-d), respectively. An increased dark current with decreased Vbg can be observed at Vbg = -40 V (Figure 4a) and Vbg = 0 V (Figure 4b), indicating that holes have been injected into the NW. Upon light exposure, Figure 4a-b show a PPR, with an increase in the S/D voltage resulting in an increased net photocurrent. When the gate voltage increases to 10 V, the device changes to n-type with a much more complex time-resolved behavior. Before illumination, at t (time) ~ 0, the dark current is at a relatively high state of I0. When the device is exposed to the light for the first time, the current increases rapidly at the initial moment of light exposure and then slowly decays to another relatively low state I1, showing an NPR behavior. When the incident light is turned off, the device’s current quickly decreases to a stable low current state I2 and then maintains. When the incident light is turned on again, the device’s current increases rapidly to the previous state I1, showing a PPR behavior. Figure 4d shows similar


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results as Figure 4c, however, as Vbg increases from 10 to 20 V, there are approximately two orders of magnitude reduction (I0-I1) in the photocurrent, meaning that the more free electrons in the device channel, the more pronounced the negative photoresponse after light illumination. To understand above experimental phenomena of the single InAs NW devices, we propose a mechanism closely related to the defect levels caused by the NW surface states. The outside surface layer of InAs NWs has been well understood to have a thin In2O3 layer which may cause the surface defect states.[5, 28] Previous studies have shown that the surface defect state density on InAs NW surfaces is about 1012 ~ 1013 cm-2, which is approximately 0.1 % of the atomic surface density.[5, 29] The existence of these defect states leads to the formation of the trapping levels above the InA NW conduction band.[23,

30]

as

schematically shown in Figure 5a. When the incident light is on, the photogenerated electrons with much higher energy than the conduction band edge can transfer over the barrier to reach a higher level and in turn be trapped by the electron-trapping states. The presence of electron-trapping level (Et) is the key to the transformation of photoresponse between NPR and PPR.



Figure 5. Schematic diagrams of the formation of single InAs NW device and working principle. (a) The band diagram of InAs NWs under different wavelengths of light illumination. (b) A schematic of the single InAs NW-based device under illumination. Working principle of single InAs NW phototransistor (c) before high energy light illumination, (d) the dynamic process under the illumination, and (e) the steady state after illumination.

To further understand the NPR behavior under high energy light illumination (the photon energy is greater than ~ 1.32 eV), we consider three different scenarios (Figure 5ce). 1) When the light is off, the free electrons in the NW core flow in the channel to form the dark current under the external source drain voltage (Figure 5c). 2) Upon short wavelength light exposure, the excited photogenerated electrons with higher energy are trapped in the electron-trapping level above the conduction band, leaving photogenerated


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holes to recombine with the free electrons in NW core (Figure 5d). 3) In return, the trapped electrons further electrostatically deplete the electrons in the NW channel and in turn lower the current, leading to the so-called photogating effect.[6] The combined effect of these factors results in a photocurrent decrease (see Figure 5e). The electron's detrapping process can be greatly quenched at 77 K because it is primarily a thermal activation process. Therefore, when the light illuminates the NW device at 77 K (with incident light wavelength shorter than 940 nm), the device's current is rapidly reduced from a high level (I0) to a lower level and gradually stabilized at I1 as the photogenerated electrons are trapped onto surface states and then more free electron carriers are depleted by the ensuing electrostatic field, showing a clear NPR behavior. After the light is turned off, the current quickly decreases to a stable low current state I2 as photogenerated carriers are no longer generated. When the light is turned on again, because of the saturation of the high energy trapping states, the generation of photogenerated electrons are not trapped anymore and thus contribute to the conductance back to I1 showing the PPR behavior (Figure 3a-b and Figure 4c-d). Compared with the high gate-voltage of 20 V (Figure 4d), the dark current is relatively weak when the Vbg = 10 V, thus the photogenerated electrons and holes make the current increase evidently at the initial moment of the light exposure (Figures 3a-b and 4c). As Vbg increases, dark current increases dramatically and dominates, causing the current increase to become less noticeable at the moment of the first light exposure. Simultaneously, when the concentration of free electrons continues to increase with



increasing of the gate voltage, the probability for electron to be captured by the surface state increases, causing more electrons to be trapped and thus enhanced electrostatic field. Accordingly, the depletion of electrons by the electrostatic field becomes more prominent with increasing the electron concentration. As shown in Figure 4d, a much larger reduction (of about two orders of magnitude) in the photocurrent occurs during the first light exposure. When the light is turned off, the current slowly increases after the quick drop because a small amount of electrons continually escape from the surface state. When the wavelength of incident light is greater than 1060 nm, the excited electrons do not have competent energy to transfer from conduction band to the high energy trapping state, and thus remain in the conduction band showing PPR (Figure 3c-f and Figure S3). On the other hand, when the device is p-type, its photoresponse is always PPR (see Figure S2) and does not change regardless of the wavelengths of the incident light, which demonstrates that the hole-trapping level is absent. When the temperature is increased from 77 K to RT, strengthened impurity ionization and intrinsic excitation lead to an increase in the electron concentration. The majority carrier type involved in the transport behavior of the device changes from holes to electrons when the gate voltage becomes negative, the photoresponse of the device changes from PPR to NPR even under negative gate voltage, as shown in Figure S4 and Figure S5. Meanwhile, as shown in Figure 4 and Figure S5 the NPR is valid both InAs NW FETs with and without P(VDF-TrFE) coating.


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Therefore, as clearly demonstrated in Figure 2b, with employing the two stable levels of current output by writing with different wavelength light pulses, a memory device can be achieved. Furthermore, Figure 3 shows that the dark currents are different after different light illuminations. This also suggest that advanced, multi-level memory functions may be implemented due to the variation in the surface states under different energies photon illumination. Meanwhile, there are reports that the thermal detrapping process can be suppressed by surface engineering, allowing the device to operate at RT.[5] It is of interest to note that the response-speed is a vital parameter that can reflect the working principle of photoelectric response, and determines the performance of a device to detect a fast-varying light signal. Figure 4e-f show the single normalized fitting curve taken from one cycle in Figure 4c and Figure S3 under the gate voltage of 10 V, respectively. Furthermore, by respectively fitting the rising and falling edges using both equations of I = I0[1 + Aexp(– t/tresp)] and I =I0[1 – Bexp(– t/trec)] (where I0 is the dark current, tresp, and trec are the time constants), we can obtain three time-related constants: tresp1 (154.3 ms), tresp2 (5.5 ms) and trec (7.7 ms) from Figure 4e, which correspond to the slow and fast response processes and the fast recovery process of the device under 450 nm light illumination conditions, respectively. tresp1 in the slow response process indicates the process by which the photogenerated electrons are captured by the electron-trapping level and the enhancement of photogating effect with light excitation, and trec in the fast recovery



process indicates the thermally activated is greatly quenched at low temperature after the light was blocked, and tresp2 in the fast response process indicates the process of rapid generation of photogenerated electron-hole pairs and then be quickly collected by the electrode when the light is turned on again. From Figure 4f, we can obtain two time-related constants: tresp (0.9 ms) and trec (0.7 ms), which correspond to the response process and the recovery processes of the device under 2000 nm light illumination conditions, respectively. The tresp represents the process of fast photogenerated carriers, and trec indicates the fast recombination process after the light was blocked. It should be noted that, when the incident light wavelength is 2000 nm, the photogenerated electrons are not sufficient to be excited to trapping level, resulting in a more rapid response tresp process and recovery process trec. Overall, the device exhibits an extremely fast response under light pulse signals. 3 Conclusion In this study, we reveal that electron-binding energy levels above the conduction band may be formed due to the surface states in our single InAs NW FET devices. The number of electrons being trapped in these energy levels vary with light illuminated under different wavelengths, resulting in a wavelength-selective switching photoresponsive. By tuning the carrier transport behavior of our NW devices through the strong local electric field from remnant polarization of dielectric layer P(VDF-TrFE), we achieved different stable levels of current output when our devices are illuminated with different wavelength light pulses


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without any gate voltage. This work shows a new perspective for understanding and modulating the surface states in NW and could also lead to new type light-assisted memory nano-devices with fast switching speed and low power consumption. 4 Experimental Section Nanowire Growth: The InAs NWs were grown in a Riber 32R@D MBE system though Au-assisted VLS method. The GaAs (111)B substrate was subjected to a degassing process in preparation chamber and then a deoxidizing process in growth chamber to clean adsorbed gas and organic pollutants, respectively. Subsequently, a thin GaAs layer was grown for 15 min at 580 ℃. Then, thermal evaporation was used to deposit a 10 nm gold film in the preparation chamber. Next, this Au thin film was annealed and then separated into nano-particles in the growth chamber at 550 °C under As atmosphere for 5 min. Last, InAs NWs was grown under the In vapor pressure ∼2.5 × 10−7 Torr and As vapor pressure ∼ 4.4 × 10−6 Torr at 230 °C for 2 h. Device Characterization: The single InAs NWs FET were fabricated through the EBL and the e-beam evaporation processes. The metal of S/D electrodes are 10 nm Ti and 60 nm Au. Then, a Philips Tecnai F20 system was used to measure the microstructure properties of NWs and a KEITHLEY 4200-SCS Parameter Analyzer was used to characterize the optoelectronic performances of the NW device. Supporting Information



(1) Transfer curves of the FET device with P(VDF-TrFE) coating at 77 K. (2) Timedependent photoresponse under varied wavelength of the incident light. (3) The family of the time-resolved current rise and decay curves for various back-gate voltage under 2000 nm light illumination with P(VDF-TrFE) coating. (4) Transfer curves under various temperature. (5) Photo-electric characteristics under 450 nm illumination for various temperature without P(VDF-TrFE) coating. Author information Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Xutao Zhang: 0000-0001-5259-5745 Pingping Chen: 0000-0002-7898-9117 Jin Zou: 0000-0001-9435-8043 Author Contributions X.T. Zhang and Z.Y. Li contributed equally to this work. Conflict of Interest


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The authors declare no conflict of interest. Acknowledgments

This work was supported by the National Natural Science Foundation of China (No.11634009 and 11774016), the Shanghai Science and Technology Foundation (No. 18JC1420401), the National Key R&D Program of China (2016YFB0402401 and No. 2016YFB0402404), and the Australian Research Council. Australian Microscopy& Microanalysis Research Facility is acknowledged for providing TEM characterization facilities.

KEYWORDS: InAs nanowires, MBE, field-effect transistor, surface trapping states, rewritable nonvolatile memory

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82x44mm (300 x 300 DPI)


Light-Induced Positive and Negative Photoconductances of InAs

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