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Institute for Frontier Materials, Deakin University, 75 Pigdons Road, Waurn Ponds, Geelong,. Victoria 3216, Australia. KEYWORDS: phase-engineering, In...
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Phase-engineering Driven Enhanced Electronic and Optoelectronic Performance of Multilayer In2Se3 Nanosheets Wei Feng, Feng Gao, Yunxia Hu, Mingjin Dai, He Liu, Lifeng Wang, and PingAn Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10194 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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Phase-engineering Driven Enhanced Electronic and Optoelectronic Performance of Multilayer In2Se3 Nanosheets Wei Feng, *,† Feng Gao,‡ Yunxia Hu, ‡ Mingjin Dai,‡ He Liu,† Lifeng Wang,§ PingAn Hu*,‡ †

Department of Chemistry and Chemical Engineering, College of Science, Northeast Forestry

University, Harbin, 150040, China ‡

Key Lab of Microsystem and Microstructure of Ministry of Education, Harbin Institute of

Technology, Harbin, 150080, China §

Institute for Frontier Materials, Deakin University, 75 Pigdons Road, Waurn Ponds, Geelong,

Victoria 3216, Australia KEYWORDS: phase-engineering, In2Se3, field-effect transistors, photodetectors, thermalannealing

ABSTRACT: Here, we report electronic and optoelectronic performance of multilayer In2Se3 are effectively regulated by phase-engineering. The electron mobility is increased to 22.8 cm2V1 -1

s

for β-In2Se3 FETs, which is 18 times higher than 1.26 cm2V-1s-1 of α-In2Se3 FETs. The

enhanced electronic performance is attributed to larger carrier sheet density and lower contact resistance. Multilayer β-In2Se3 photodetector exhibits an ultrahigh responsivity of 8.8×104 A/W

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under 800 nm illumination, which is 574 times larger than 154.4 A/W of α-In2Se3 photodetector. Our results demonstrate phase-engineering is an valid way to tune and further optimize electronic and optoelectronic performance of multilayer In2Se3 nano-devices.

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Recently, two-dimensional (2D) semiconductors have attracted a great deal of attentions because of their unique optical, electrical and magnetic properties1, 2. A great deal of 2D semiconductors have been successfully obtained and studied, such as transition metals dichalcogenides (TMDs)35

, black phosphorus6 and III-VI group materials7-9. Many electronic and optoelectronic devices

have been demonstrated by 2D semiconductors, including field effect transistor (FETs)4, photodetector3, diode5, solar cell10 and so on. FETs based on black phosphorus6 and InSe11 show an excellent electron transport performance with a high mobility of 103 cm2V-1s-1 at room temperature, which is comparable to Si-based devices. Monolayer MoS2 photodetector exhibits a ultrahigh responsivity of 880 A/W due to direct bandgap structure allowing high absorption coefficient and efficient electron–hole pair generation12. The solar cell fabricated from largescale monolayer n-MoS2/p-Si heterojunction achieves a high power conversion efficiency of 5.23%, demonstrating that 2D semiconductors hold great promise for integration with commercial Si-based electronics in highly efficient devices10. Theory simulation has demonstrated perovskites show good electronic and optoelectronic performance13-16, which is compare to those of 2D semiconductors. Though 2D semiconductors are promising materials for high performance electronic and optoelectronic devices, electrical and optical properties of 2D semiconductors should be further modulated for satisfying various application requirements17. In2Se3 is a typical III- VI semiconductor with multiple crystalline phases at various temperature, including α18-20, β21, γ22 and κ23, 24. The layered α-phase, defect wurtzite γ-phase and hexagonal structured κ-phase are stable at room temperature, while layered β-phase is stable at relatively high temperatures25. The temperature-dependent phase transitions of bulk In2Se3 have been widely studied: α → β phase transition at 473 K, the β → γ phase transition at 793 K and κ → α phase transition at 673 K.24, 25 It has been demonstrated that electronic and optoelectronic

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performance of In2Se3 nanowires are strongly dependent on crystalline phases24. The electrical conductivity and spectral responsivities of α-phase In2Se3 nanowires are 1000 and 200 times larger than those of κ-phase In2Se3 nanowires, respectively24. Moreover, 2D In2Se3 is a potential material for optoelectronic devices applications due to its narrow and tunable direct bandgap, high absorption coefficient, broad response range and high sensitivity9. For 2D In2Se3 nanosheets, the α-phase can transfers to β-phase by thermal annealing and the β-phase is stable at room temperature21, which is totally different with bulk In2Se3. The electrical resistivity of β phase 2D In2Se3 is 1−2 orders of magnitude lower than that of the α phase, suggesting that β phase 2D In2Se3 has more potential application in electronic devices. Electronic and optoelectronic properties of 2D α-In2Se3 have been studied9, 19, 20, and high performance 2D βIn2Se3 photodetectors also have been demonstrated26,

27

. However, the electronic transport

properties of 2D β-In2Se3 have been limited. Moreover, the performance of 2D α-In2Se3 and βIn2Se3 strongly depends on the fabrication methods and processes of materials, making it difficult to directly compare their performance and confirm the pivotal factors for different performance of 2D α-In2Se3 and β-In2Se3. For practical applications in electronic and optoelectronic devices, the phase related electronic transport and photoresponse performance of 2D In2Se3 are further needed to systematic investigated. In this letter, we demonstrate that electronic and optoelectronic performance of multilayer In2Se3 can be effectively tuned by phase-engineering, which is induced by thermal annealing. Multilayer β-In2Se3 FETs show a heavily degenerate n-doping conductance behavior with a low current on/off ratio of 1.6. The electron mobility of β-In2Se3 FETs is calculated to be 22.8 cm2V1 -1

s , which is 18 times higher than 1.26 cm2V-1s-1 of α-In2Se3 FETs. Multilayer β-In2Se3

nanosheets show a higher carrier sheet density of 1.02 ×1013 cm-2 and lower contact resistance of

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31.5 KΩ, which increases 18.6 times and reduces 338 times than those of α-In2Se3, respectively. Multilayer β-In2Se3 photodetector shows an ultrahigh responsivity and detectivity of 8.8×104 A/W and 2.9 × 1013 Jones under 800 nm light illumination, which is 574 times and 7 times larger than those of α-In2Se3 photodetector. Our results suggest that phase-engineering is an effective tool to tune electronic and optoelectronic performance of multilayer In2Se3 nano-devices. Figure 1a is crystal structure of α-In2Se3 and β-In2Se3, respectively. Though each quintuple (Se−In−Se−In−Se) layer is bonded to each other through weak van der Waals interactions in both types of crystal structure, it is obvious that they display slightly different crystal structures. Figure 1b depicts the Raman spectrums of as-exfoliated α-In2Se3 nanosheets and annealed samples. There are three Raman peaks of 91cm-1, 104 cm-1 and 181 cm-1 for as-exfoliated sample, which are assigned to be α phase In2Se3.21 The dominated peak of 104 cm-1 is attributed to A1 phonon mode in α phase In2Se3. Another two peaks at 91cm-1 and 181 cm-1 are attributed to E symmetry mode and A1 (TO) symmetry mode of α phase In2Se3, respectively. For α→β phase transformation of multilayer In2Se3, as-exfoliated α-In2Se3 nanosheets were thermal annealed at 623 K.21 After thermal annealing at 623 K in Ar/H2 and cooling to room temperature, Raman spectrum of annealed sample is changed. The dominant peak at 104 cm-1 takes a blue-shifted to 110 cm-1, which is attributed to β phase lattice phonon mode In2Se3,21, 28 definitely demonstrating that successful α→β phase transformation is induced by thermal annealing. This result is consistent with latest studies21, 28.

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Figure 1. Phase transformation characterizations of multilayer In2Se3 nanosheets: (a) Crystal structure of αIn2Se3 and β-In2Se3, respectively. (b) Raman spectra of as-exfoliated α-In2Se3 nanosheets and annealed In2Se3 nanosheet at 623 K.

Electronic and optoelectronic properties of semiconductors are strongly dependent on their crystal structures. Here, to investigate phase engineering effects on electronic transport properties of 2D In2Se3, back-gated FETs based on multilayer In2Se3 nanosheets were fabricated (fabrication process in Experiment section). All electronic performance of devices were conducted before and after annealing in ambient condition. Figure 2a is the corresponding 3D schematic structure of back-gated multilayer In2Se3 FETs in this study. Metal electrodes (5 nm/30 nm Cr/Au) were fabricated by thermal evaporation deposition with copper shadow masks. Multilayer In2Se3 nanosheet is the channel materials, 300 nm SiO2 layer is dielectric material and p-type Si layer serves as back gate. Figure 2b is a typical optical image of multilayer In2Se3 FETs, the channel length is 10 µm and channel width is 15 µm, respectively. The thickness of multilayer In2Se3 channel is determined to be 15 nm by AFM as shown in Figure S1.

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Figure 2. 2D In2Se3 FETs: (a) 3D schematic structure of In2Se3 FETs. (b) A typical optical image of multilayer 2D In2Se3 FETs (channel length is 10 µm, channel width is 15 µm and the thickness is 15 nm).

Figure 3a and 3b show logarithmic scale and linear scale transfer curves of as-exfoliated and annealed multilayer In2Se3 FETs, respectively. As-exfoliated multilayer α-In2Se3 exhibits a typical n-type semiconducting transport behavior with current increasing as gate bias sweeping from negative to positive field and an obvious current switching behavior with a current on/off ratio of 500. After thermal annealing, multilayer α-In2Se3 nanosheets transfer to β-In2Se3 and annealed devices still show n-type conductance behavior as shown in Figure 3a and 3b. However, the current on/off ratio of β-In2Se3 FETs is 1.6 and current is almost independent of gate voltage, which is similar to semimetal properties, such as graphene29. The on/off ratio is 1.9 for β-In2Se3 FETs measured in a larger gate voltage range of -40~40 V as shown in Figure S2, which demonstrates the on/off ratio of β-In2Se3 FETs slightly depends on gate voltage range. Field-effect mobility is an important parameter to assess electronic properties of a FETs, which can be determined by the following equation: µ = [L/(W×(ε0εr/d)×Vds)]×dIds/dVg, where L is length of channel, W is width of channel, ε0 = 8.854 × 10-12 Fm-1 is vacuum permittivity, εr is 3.9 for SiO2 and d = 300 nm is the thickness of SiO2. From linear scale of transfer curves in Figure 3b, the field-effect mobility values of as-exfoliated and annealed device are calculated to be 1.26 cm2V-1s-1 and 22.8 cm2V-1s-1, respectively. After phase transformation, the mobility value of β-

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In2Se3 FETs is 18 times larger than that of α-In2Se3 FETs. All those results demonstrates that electronic properties of multilayer In2Se3 is effectively tuned via phase engineering.

Figure 3. Electronic characterizations of as-exfoliated and annealed In2Se3 FETs: (a) Logarithmic scale and (b) linear scale transfer curves of as-exfoliated and annealed In2Se3 FETs at Vds = 1 V. The corresponding output curves of (c) as-exfoliated and (d) annealed In2Se3 FETs. Measurements are conducted under ambient environment.

Figure 3c and 3d are corresponding output curves of as-exfoliated and annealed multilayer In2Se3 FETs, respectively. The output curves of as-exfoliated multilayer α-In2Se3 FETs show a linear regime at low Vds and a saturation regime at high Vds. This phenomenon is due to the conducting channel converting to “pinch-off” condition at high Vds. The annealed FETs show an entirely different Ids-Vds behavior: Ids shows a good linear dependence on Vds and unsaturated curves are observed at high Vds, suggesting a higher ON current at higher Vds. The output current

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of β-In2Se3 FETs is larger than that of as-exfoliated α-In2Se3 FETs, which is in agreement with corresponding transfer curves. Moreover, the Ids at on state (large positive gate voltage) is improved by 1000 times after annealing, which can attributed to two factors: 1) lower contact resistance of Cr/β-In2Se3 for more electron injection than that of α-In2Se3 and 2) higher channel conductance in β-In2Se3 arising from higher electron concentration. To clarify electronic performance improvement of β-In2Se3, the carrier sheet density (n2d), contact resistance and Schottky barrier (SB) values of α-In2Se3 and β-In2Se3 samples are needed to explore in details. The carrier densities of multilayer α-In2Se3 and β-In2Se3 nanosheets can be calculated by the following equation: n2d = Ids × Ci/(e × dIds/dVg), where Ids is drain-source current at Vg = 0 V, Ci is capacitance of 300 nm SiO2 dielectric layer of 1.15×10-8 F/cm2, e is electron charge of 1.60 ×10-19 C and dIds/dVg is the slope of transfer curve at Vg = 0 V. The calculated n2d values are 5.49 ×1011 cm-2 and 1.02 ×1013 cm-2 for pristine α-In2Se3 and β-In2Se3 FETs, respectively. The n2d value of β-In2Se3 nanosheet is 18.6 times larger than that of pristine α-In2Se3 nanosheet. The contact resistance (Rc) of Cr-In2Se3 can be roughly evaluated by the linear region of output curves at high positive gate regions with the following equation: Ron = 2Rc, where the channel is highly conductive and the source-drain current is mainly limited by source and drain contact resistances30. The Rc values of α-In2Se3 and β-In2Se3 are estimated to be 11 MΩ and 31.5 KΩ respectively. Compared with as-exfoliated α-In2Se3 nanosheet, the contact resistance of β-In2Se3 is drastically reduced by 338 times. According to thermionic emission theory, the SB value can be calculated by the following equation: Is→m = AA*T2exp(-qФSB/kT), where Is→m is the current at V = 0 V, A is device area, A* is Richardson’s constant, T is Temperature, q is the electronic charge, ФSB is SB and K is Boltzmann constant. The SB values are 0.71 eV and 0.56 eV for α-In2Se3 and β-In2Se3, respectively. The SB value of In2Se3 and Cr is

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decreased by 0.15 eV after thermal annealing. All above results declare that electronic performance improvement of β-In2Se3 FETs is due to higher carrier density, lower contact resistance and lower Schottky barrier. To compare photodetection performance of α-In2Se3 and β-In2Se3 nanosheets, various lights from 500 nm to 800 nm were vertically illuminated onto α-In2Se3 and β-In2Se3 photodetectors. The α-In2Se3 and β-In2Se3 photodetectors show an obvious response to visible-near illumination light as shown in Figure 4a and 4b, respectively. The photocurrent (Iph) is defined as Iph = Iillumination - Idark, which is important parameter to evaluate a photodetector. The Iph- λ curves of αIn2Se3 and β-In2Se3 nanosheets illuminated under various lights at V = 2 V are shown in Figure S3. It clearly shows that β-In2Se3 photodetector generates larger Iph than that of α-In2Se3 photodetector, which is mainly attributed to lower Schottky barrier for more photogenerated carrier injection. For example, the generated Iph is 4.58 µA for β-In2Se3 photodetector illuminated by 800 nm light at applied bias of 2 V, which is approximately 572 times higher than 8 nA for αIn2Se3 photodetector. Responsivity (R) and detectivity (D*) are another two key parameters to evaluate a photodetector. The R is defined as R = Iph/PS, where Iph is the generated photocurrent at a specific illumination light, P is illuminated light intensity, S is effective illuminated area. Figure 4c exhibits the R values of α-In2Se3 and β-In2Se3 photodetectors illuminated by various lights at V = 2 V. For as-exfoliated α-In2Se3 photodetector, the R values are 59.0 A/W, 55.8 A/W, 52.4 A/W, 51.5 A/W, 63.2 A/W and 154.4 A/W for 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm and 800 nm, respectively, which are consistent with early report11. After thermal annealing, the R values of β-In2Se3 photodetector are 3.6 × 104 A/W, 3.5 × 104A/W, 3.0 × 104 A/W, 2.7 × 104 A/W, 3.3 × 104 A/W, 3.7 × 104 A/W and 8.8 × 104 A/W for 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm and 800 nm, respectively. Obviously, the responsivity of β-In2Se3

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photodetector is much larger than those of α-In2Se3 photodetector. The detectivity can be calculated by the following equation: D* = RS1/2/ (2eId) 1/2, where R is responsivity, S is the area of photodetector channel of 150 µm2, e is the electron charge of 1.6 × 1019 C and Id is the dark current. It is clear that β-In2Se3 photodetector show higher D* values than those of α-In2Se3 photodetector. For example, the calculated D* value of β-In2Se3 photodetector is 2.9 × 1013 Jones for 800 nm light at V = 2 V, which is 7 times higher than 4.1 × 1012 of α-In2Se3 photodetector. Those results demonstrate that photodetection performance of In2Se3 photodetector can be dramatically enhanced by phase-engineering. For practical application, stability and response speed are also important and necessary to study in detail. The stabilities of α-In2Se3 and β-In2Se3 photodetectors were investigated by 10 s of periodic 700 nm light at applied bias of 2 V with a light intensity of 126.1 µW/cm2. Under switching the illumination light on and off, the on/off state of α-In2Se3 photodetector for each cycle show the almost same level in Figure S4, suggesting a high reproducibility and stability of optical switching response. For β-In2Se3 photodetector, the optical response turns to a relatively stable value with illumination time increasing. Figure S5a and S5b are speed time of α-In2Se3 and β-In2Se3 photodetectors, respectively. As-exfoliated α-In2Se3 photodetector show a rapid rise and decay response time of 20 ms, which agrees with early report of 18 ms. 9 Compared with αIn2Se3 photodetector, β-In2Se3 photodetector show low response time, which are 1 s and 5 s for rise time and decay time, respectively. This may be attributed to defects rising from phase transform, which requires more research in future.

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Figure 4. Photoresponse characteristics of α-In2Se3 and β-In2Se3 photodetectors. (a) I-V curves of α-In2Se3 photodetector in the dark and illuminated by various lights (The light intensity is 180.9 µW/cm2, 191.1 µW/cm2, 196.2 µW/cm2, 179.6 µW/cm2, 126.1 µW/cm2, 101.9 µW/cm2 and 34.4 µW/cm2 for 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm and 800 nm, respectively). (b) Corresponding I-V curves of β-In2Se3 photodetector. c) The calculated responsivity values and d) calculated detectivity values of α-In2Se3 and βIn2Se3 photodetector.

In summary, electronic and optoelectronic performance of multilayer In2Se3 are effectively tuned by thermal annealing induced phase-engineering. Multilayer β-In2Se3 FETs show a heavily degenerate n-doping transport behavior with a low current on/off ratio of 1.6. The field effect electron mobility is increased to 22.8 cm2V-1s-1 for β-In2Se3 FETs, which is 18 times higher than 1.26 cm2V-1s-1 of α-In2Se3 FETs. Multilayer β-In2Se3 nanosheets show a higher carrier sheet density of 1.02 ×1013 cm-2 and lower contact resistance of 31.5 KΩ, which increases 18.6 times

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and reduces 338 times than those of α-In2Se3, respectively. Multilayer β-In2Se3 photodetector shows an ultrahigh responsivity and detectivity of 8.8×104 A/W and 2.9 × 1013 Jones under 800 nm light illumination, which is 574 times and 7 times larger than those of α-In2Se3 photodetector. Our results demonstrate that phase-engineering is an effective way to improve electronic and optoelectronic performance of In2Se3 nano-devices. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Description of experimental methods, AFM image of In2Se3 FETs, transfer curve of β-In2Se3 FETs measured at a larger gate voltage range, Iph, time-dependent photoresponse characteristics and speed time of α-In2Se3 and β-In2Se3 photodetectors. AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no any competing financial interest. ACKNOWLEDGMENT

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This work is supported by China Postdoctoral Science Foundation funded project (No. 2018M630329), Fundamental Research Funds for the Central Universities (No. 2572018BC14), National Natural Science Foundation of China (NSFC, No. 61172001, 61390502 and 21373068), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51521003), and by Self-Planned Task (Grant No. SKLRS201607B) of State Key Laboratory of Robotics and System (HIT). REFERENCES (1)

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