Black Phosphorus Photodetector

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Functional Inorganic Materials and Devices

High-Performance Hybrid InP QDs/Black Phosphorus Photodetector Dohyun Kwak, Parthiban Ramasamy, Yang Soo Lee, Min-Hye Jeong, and Jong-Soo Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07910 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Applied Materials & Interfaces

High-Performance Hybrid InP QDs/Black Phosphorus Photodetector Do-Hyun Kwak, Parthiban Ramasamy, Yang-Soo Lee, Min-Hye Jeong, Jong-Soo Lee* Department of Energy Science & Engineering, DGIST, Daegu, 42988, Republic of Korea

Abstract

0D-2D hybrid optoelectronic devices have demonstrated high sensitivity and high performance due to high absorption coefficient of 0D materials with a tunable detection range and high carrier transport property of 2D materials. However, the reported 0D-2D hybrid devices employ toxic nanomaterials as sensitizing layers, which can limit the practical applications. In this study, we first fabricated 0D-2D hybrid photodetector using non-toxic InP quantum dots (QDs) as a light-absorbing layer and black phosphorus (BP) as a transport layer. The surface treatment using 1, 2 ethanedithiol (EDT) and thermal treatment were carried out to remove the surface long ligands of colloidal QDs, which can

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accelerate the charge injection of the photo-generated carriers through the interfaces between InP QDs and BP. The InP QDs/BP hybrid photodetector demonstrates a high responsivity of 1 × 109 A/W and detectivity of 4.5 × 1016 Jones at 0.05 μW/cm2 under 405 nm illumination. The results show that 0D-2D hybrid photodetectors based on III-V semiconducting QD materials can be optimized for high-performance photodetectors.

KEYWORDS: Indium phosphide, black phosphorus, 0D-2D hybrid device, photodetector, surface ligands.

INTRODUCTION

Over the few decades, zero-dimensional (0-D) semiconductor quantum dots (QDs) have demonstrated potentials for electronic and optoelectronic devices due to their sizedependent bandgap tunability, low-cost solution process, high absorption properties, and large-area manufacturability.1-2 QD photodetectors have exhibited high photosensitivity and photo-selectivity through a tunable absorption spectrum from visible up to midinfrared (IR) range.3-4 However, their low charge carrier mobility remains a bottleneck for high-performance photodetector.5-6 In recent years, to overcome the transport issue of


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QD devices, hybrid devices combining QDs with high photosensitivity and twodimensional (2D) materials having excellent charge transfer characteristics are attracting attention.7-10 Graphene based hybrid phototransistors have exhibited a high responsivity compared with other van der Waals families owing to its high carrier mobility.7, 9 However, graphene with a zero band gap leads to a high dark current, which can restrict a high sensitive photo-detection.9, 11-12 One prominent candidate to replace graphene is black phosphorus (BP), which has a direct band gap of 2 eV in monolayer and 0.4 eV in bulk flakes.13-14 Few-layer BP has demonstrated a high hole carrier mobility up to 1000 cm2/V1s-1

and a high on-off ratio up to 105.13 The presence of bandgap in BP offers an

opportunity to be highly sensitive photodetector.15 The BP photodetectors have been demonstrated a responsivity of 82 A/W at a mid-infrared range of 3.32 μm and 9 × 104 A/W at UV range.16-17 More recently, mid-infrared BP photodetector with broad optical tunability has been demonstated. The waveguide-integrated BP photodetector with a responsivity of 23 A/W at 3.68 μm and 2 A/W at 4 μm has been realized. Also, the detection limit of BP has been extended to 7.7 um by leveraging the Stark effect.18-19 Moreover, the BP-QD hybrid phototransistors have exhibited highly enhanced


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responsivity of 1.16 × 109 A/W with n-type CdSe QDs and 5.36 × 108 A/W with p-type PbS QDs, respectively.20 The cadmium and lead-based QDs have been well established for the production of high-quality and employed in various 0D-2D hybrid devices.9, 20-21 However, the toxicity of QDs based on cadmium or lead has limited the practical applications. Recently, III-V semiconducting QD materials (i.e., GaAs, InP, and InAs) have attracted much attention as channel materials in electronic and optoelectronic devices due to their direct bandgap transient characteristics, low toxicity, and high mobility.22-23 As an attractive alternative to replacing Cd-based nanomaterials, ecofriendly InP QDs without heavy metals (Cd, Pb, Hg, etc.) have been well developed by showing significant optical properties.24-25 In a colloidal synthesis of InP QDs, however, hydrocarbon tails are used to passivate their surface as surface ligands, which can provide a negative effect on the charge carrier transport.22

In this article, to remove the long surface ligands of colloidal InP QDs, the fabricated hybrid devices were chemically treated by 1, 2‐ethanedithiol (EDT) via layer-by-layer deposition. However, the InP QD/BP hybrid photodetector remained an unstable device


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operation due to completely unremoved long surface ligands, resulting in poor charge carrier transport. The remained surface long ligands were removed by an additional thermal treatment at high temperature. The employment of BP as a transport layer and InP QDs as a light-absorbing layer in the hybrid InP QD/BP photodetector resulted in a high photo-response of ~109 A/W and a high detectivity of 4.5 × 1016 Jones under a 405 nm laser diode. Also, we also confirmed the origin of photocurrent generation in the entire active layer of InP QD/BP hybrid photodetector using a photocurrent mapping technique. The photocurrent of hybrid photodetector is predominantly generated in the region of InP QDs deposited on BP layer. Under illumination, the photo-generated electrons from the InP QD layers are quickly transferred to the BP transport layer and shown n-type doping behavior.

RESULTS AND DISCUSSION

The schematic of InP QD/BP hybrid photodetectors is illustrated in Figure 1a. The hybrid photodetectors were prepared by spin-coating InP QDs over the BP field-effect transistor (FET). For the hybrid photodetector, we used a few-layer BP flake of ~8 nm thickness as


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measured in the atomic force microscopy (AFM) in Figure S1. The BP FET with channel length/width (2/10 μm) was fabricated by electron beam lithography shown in Figure 1b, and the source-drain electrodes (Ti/Au) were deposited with Ti/Au (5/45nm), respectively. The back gate electrode was used by degenerated p-type doped Si wafer. InP QDs were synthesized by using optimized method in our group.26 A TEM image in Figure 1c shows the typical morphology of InP QDs of tetrahedral shape with ~12 nm size, resulting from the employment of both the oleylamine and chloride ligands which completely passivate cation-rich (111) facets of InP QDs.27 The co-passivation method with halide and amine benefits the synthesis of high-quality InP QDs by stabilizing their surfaces.27 The absorption spectra of purified InP QDs is illustrated in Figure S2a. X-ray diffraction (XRD) pattern of the InP QDs corresponds to the peaks of the InP bulk [JCPDF: 32-0452]. The XRD peaks of our InP QDs in Figure 1d clearly indicate (111) plane at 27o, (220) plane at 31o, (220) plane at 45o, and (311) plane at 53o, respectively.

The synthesis of InP QDs requires the use of long surface ligands due to their high growth temperature. However, the long ligands hamper the charge carrier transport


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between InP QDs and a BP layer, and it is necessary to exchange them to short ligands.5 In general, EDT molecules were used as an alternative short ligand in various 0D-2D hybrid devices because of its short length and strong reducing agent, which can reduce the BP surface oxides formed during device fabrication.7,

21, 28

By using the EDT

molecules, layer by layer process was carried out during InP QDs deposition on BP device, but the long ligands were completely unremoved. Unlike acid-based ligands, amine- or phosphine-based ligands are not easily removed from QDs during the ligand exchange method.22 To completely remove remaining long ligands after the EDT treatment, the InP QD/BP hybrid photodetectors were annealed at high temperatures of 100, 200, and 300 °C for one hour in N2 atmosphere, respectively. Thermal annealing at a high temperature can decompose the carbon-based ligands as well as QDs.29 Figure 1d clearly shows XRD peaks and the thermal stability of the InP QD films at 100, 200, and 300 °C without any decomposition. Until the annealing temperature of 200 °C, the XRD peaks of the InP QD films showed no changes after annealing. The XRD patterns of the InP QD film annealed at 300 °C showed a slight lattice reduction at an angle as high as 0.5° even at high annealing temperature, but no diffraction peaks change were


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observed to interpret that the QDs were sintered. Figure 1e shows the transmittance peaks for C-H stretch on the InP QD film before and after thermal annealing by using Fourier-transform infrared (FT-IR) spectroscopy. After the EDT treatment without thermal annealing, the InP QD film still exhibits the strong vibrations for C-H stretch showing two peaks at 2852 and 2923 cm-1.30-31 The peaks for C-H stretch show reduction with increasing annealing temperature due to the decomposition of the long hydrocarbon tails. The reduction of long ligands results in the enhancement of photo-generated carrier transport. In Figure S2b, as increasing annealing temperature up to 300 °C, the InP QD/BP hybrid photodetector reveals clear photoresponse suggesting the decrease of the long hydrocarbon tails.

We also confirmed the vertical structure of the InP QD/BP hybrid photodetector to characterize the cross-section morphology of InP QD and BP layers by using transmission electron microscopy (TEM). Figure 2a reveals a vertical cross-section TEM image of the hybrid photodetector in which two layers of InP QDs are deposited on a fewlayer BP without sintering effect at even 300 °C annealing. The thickness of the InP QD


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film is ~11 nm in the TEM image in the agreement of the AFM result in Figure S1b. The TEM image also demonstrates the thermal stability of InP QDs annealed at 300 °C in agreement with the XRD data in Figure 1d. BP under InP QDs clearly shows the layered structure without any degradation. Figure 2b indicates the intensity of atomic elements in the InP QD/BP hybrid photodetector by using energy-dispersive X-ray spectroscopy (EDX). The positions of In and P atoms correspond to the positions of InP QDs and BP in the TEM image. Figure 2c exhibits the closely packed InP QD layers and the amorphous layer of 2~3 nm thickness between BP layers and InP QD layers in the magnified cross-section TEM image. The BP thickness is approximate ~5 nm under InP QD layers in the TEM image of Figure 2c, while its initial thickness under the Ti/Au electrode in the TEM image of Figure S3 is ~8 nm in the agreement of the AFM result in Figure S1a. The formation of the amorphous layer is attributed to the deformation of BP layers after the deposition of InP QDs. When InP QDs are deposited on the BP layer under ambient condition, the water molecules and moisture are adsorbed on the BP surface to form phosphoric acid (P4O10 + 6H2O → 4H3PO4) on the BP surface32, which


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causes the surface degradation and accelerates the destruction of the BP structure. However, the thermal annealing of BP sheet can evaporate the phosphoric acid on the surface of BP sheet, so that a native oxide is formed on the surface of BP sheets, as shown in Figure 2C.32

Figure 3a shows the I-V transfer curves of the BP device before and after the deposition of InP QDs. The pristine BP device shows a typical hole-rich ambipolar transport behavior (blue line) and the calculated electron and hole mobility are 39 cm2V-1s-1 and 222 cm2V1s-1,

respectively. The hole and electron mobility of the BP device are calculated by the

definition of which the mobility in the linear regime is μlin = Lgm/ (WCiVDS), where L is the channel length, W is the channel width, gm = transconductance, Ci is the capacitance of 300 nm SiO2.13 We illustrated the mechanism of charge carrier injection between InP QDs and BP at non-contact state, contact state under dark, and contact state under illumination in Figures 3b-d, respectively. The conduction level (valence level) of InP QDs is higher (lower) than that of BP layer, by showing type-I semiconductor in Figure 3b. The band diagrams are considered by the reports for band gap of InP QD and BP.14, 27 In the


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contact state of Figure 3c, the electrons of InP QDs migrate to the BP layer to satisfy the equilibrium condition at the interface of the hybrid materials and induce the n-type doping behavior by shifting the I-V curve toward the negative gate bias as shown in Figure 3a. As n-type doping by InP QD deposition, the electron and hole mobility of the BP device are 72 and 194 cm2V-1s-1, respectively. Under the illumination of 405 nm laser with beam size of 100 m, photo-generated electrons in InP QD layers quickly transfer to BP layer as shown in Figure 3d. Thus, photo-generated electrons from InP QDs recombine with holes in BP layer, leading to negative photocurrent under Vg = 16 V, which is the neutral point to determine charge polarity of the hybrid device. In contrast, the positive photocurrent is generated due to injection of electrons from InP QDs to BP layer above the neutral point.

The optical power density can affect the n-type doping level of the InP QD/BP hybrid photodetectors. Figure 4a demonstrates the I-V transfer curves of the InP QD/BP hybrid photodetector measured at 1 Vds (source-drain bias) as a function of gate bias with different optical power densities. The photocurrent of the hybrid photodetector was


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decreased under the neutral point and increased current above the neutral point by increasing the optical power density up to 32 μWcm2 under 405 nm illumination. In addition, the n-type doping behavior of the hybrid photodetector becomes strong at the increased power density in the inset of Figure 4a. Responsivity, as a figure of merit in photodetector, is defined by photocurrent at given optical power, following the equation R=Ip/P, where Ip is the photocurrent, and P is optical power density.10 The photocurrent linearly depends on optical power density. In contrast, the responsivity inversely proportional to the optical power density. Figure 4b shows the photocurrent and responsivity of the hybrid photodetector as a function of gate bias with different optical power densities. Photocurrent in the hole region of the hybrid photodetector is higher than that in the electron region, attributing to the high conduction from hole-rich ambipolar behavior of BP. Figure 5a shows the output curves and photocurrent of the InP QD/BP hybrid photodetector at Vg= -60 V (at Vg =60 V in Figure S4) as a function of Vds with different optical power densities up to 32 μWcm2 under 405 nm illumination. The hybrid photodetector shows the ohmic contact behavior and the negative photocurrent which depends linearly on Vds as well as optical power density. The responsivity of the hybrid


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photodetector reduces with an increasing optical power density as shown in Figure 5b. We extracted the responsivity of the hybrid photodetector at each optical power density and 1 Vds from Figure 5b, and denote them as a function of optical power density in Figure 5c. The highest responsivity shows 1 × 109 A/W at the optical power density of 0.05 μ W/cm2 under 405 nm illumination. As another figure of merit of the photodetector, detectivity indicates the sensitivity of the photo-detection under the noise level floor of its dark current. To understand the sensitivity of the hybrid photodetector, the detectivity is calculated by following equation D* = (RA1/2) / iSN, where R is the responsivity, A is an active area of the device, q is the electron charge, and iSN is shot noise current of the device.12 Shot noise current (iSN) can be extracted from the dark current as following equation, 𝑖𝑆𝑁 = 2𝑞𝐼𝑑𝑎𝑟𝑘. Here, we assumed that the shot noise from direct current (DC) mainly contributes to the noise.21 Generally, the shot noise has a tendency to increase with increasing dark current. Our InP QD/BP hybrid photodetector exhibits a high detectivity of 4.5 × 1016 Jones at 0.05 μW/cm2 under 405 nm illumination. The responsivity and detectivity in the hybrid photodetector are inversely proportional to optical power density. The high performance of the hybrid photodetector is attributed to high absorption


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of QDs and high carrier mobility of BP. Figure 5d shows the photo-response at 1 Vds and Vg = -60 V as switching on-off light with different optical power densities. The photocurrent of the hybrid device increases with higher optical power density by clearly showing on-off switching behavior. In Figure S5a, the rise and decay time for the hybrid photodetector are 5 ms and 120 ms, respectively. The photo-response time are calculated by the variation of 90 % photocurrent. In order to understand the ligand effect on photo-response time, we compared the photo-response time of the hybrid photodetector with and without EDT treatment in Figure S5. Without EDT treatment, the hybrid photodetector shows unstable photodetector operation of which the current is not saturated under illumination due to the bottleneck of photo-generated carrier transport by the long ligands, in addition slower photo-response decay time of 280 ms. EDT treatment enhances the performance of the hybrid photodetector, but further improvement of photo-response time is necessary for highly sensitive photodetector.

To investigate the source region of photocurrent in the InP QD/BP hybrid photodetector, we carried out its spatial photocurrent mapping. Figure 6a shows the optical image of the


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hybrid photodetector and mapping area by the red box. The photocurrent of the hybrid photodetector was scanned across the entire active area with ~1 μm spot size under 405 nm illumination. Figure 6b exhibits the photocurrent map of the hybrid photodetector at 1 Vds and Vg = -60 V under a 405 nm illumination. In the previous reports, the source of photocurrent in the pristine 2D materials is the region contacted with electrodes resulting from the formation Schottky or p-n junction at the interface between metal and semiconductor.33-34 The photocurrent in p-n diodes have been revealed at the p-n junction region. Interestingly, the photocurrent in the hybrid photodetector is generated on the BP region deposited by InP QDs. In the hybrid photodetector, the junction barrier between InP QDs and BP as shown in Figure 3c seems to play a role of charge carrier separation under illumination.

CONCLUSIONS

We demonstrated highly photosensitive InP QDs based hybrid photodetector, employing BP as a transport layer. To remove the long ligands of QDs, layer-by-layer method was


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carried out by using EDT treatment. After the EDT treatment, the remaining the long ligands further are decomposed by the thermal annealing process at 300 °C. The InP QD/BP hybrid photodetectors exhibited high responsivity of 1 × 109 A/W and detectivity 4.5 × 1016 Jones at 0.05 μW/cm2 under 405 nm illumination. The high performance of the hybrid photodetector is attributed to the injection of photo-generated electrons in InP QDs to BP. Also, we found that the source of photocurrent in the hybrid photodetector is the junction between QDs and BP. However, the slow time of few hundreds of the InP QD/BP hybrid photodetector remains the severe bottleneck in photodetector application. In order to improve photo-response time, further studies will focus on the understanding of interface between QDs and 2D layers.

Experimental Section

Synthesis of InP QDs.


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InP QDs used in this study were synthesized by slightly modifying the previous results reported in our group.24, 26 Detailed experimental methods can be found in the supporting information.

Characterization : Structural analysis and optical characterization of InP QDs and BP sheet were performed using XRD (Rigaku MiniFlex 600 diffractometer), TEM (Hitachi HF3300), absorption spectroscopy (Cary 5000 UV−vis−NIR ) and AFM (PSIA Xe-150). Detailed sample preparation and experimental methods can be found in the supporting information.

Device Fabrication ; Few-layer BP exfoliated from bulk BP flakes is deposited on a Si/SiO2 substrate. Source and drain electrodes were patterned by e-beam lithography. Ti/Au electrodes with 5/45 nm thickness were deposited by the e-beam evaporator. The back gate electrode was used by degenerated p-type doped Si wafer. To improve the contact resistance, all devices were annealed at 150 °C for an hour in an N2 atmosphere glovebox

QD Film Deposition. InP QDs dispersed in toluene were spin-coated with 4000 rpm on the fabricated BP device. To remove the surface ligands of QDs, 2 vol % EDT diluted in


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acetonitrile was dropped on the hybrid device during the spin coating. After deposition of InP QDs on the BP device, the thermal treatment was performed at 100, 200 and 300 °C for 1 hour in a glovebox filled in nitrogen, respectively.

Device measurement. The FET and photocurrent characteristics were measured by using Keithley 2636A controlled by homemade LabVIEW program in an N2 glovebox.

Scanning photocurrent mapping of the devices was measured by scanning photocurrent mapping system combined with Keithley 2636B. All mapping data was precisely colored in the origin plot program with interval 110 steps. Detailed experimental methods can be found in the supporting information.

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FIGURES

Figure 1. (a) A schematic of the InP QD/BP hybrid photodetector. (b) Optical image of the BP FET. Scale bar indicates 10 μm. (c) TEM image of InP QDs. Scale bar indicates 50 nm. (d) XRD patterns of the InP QD films with different annealing temperatures of 100, 200, and 300 oC. (e) FT-IR spectra of the InP QD films with different annealing temperatures of 100, 200, and 300 oC after the EDT treatment.


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Figure 2. (a) A cross-section TEM image of the InP QD/BP hybrid photodetector. (b) EDX Analysis of In and P atoms in Fig. 1(a). (c) The magnified cross-section TEM image of the InP QD/BP hybrid photodetector.


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Figure 3. (a) I-V characteristic of the BP device and InP QD/BP hybrid photodetector under dark, and under light. Inset indicates the logarithm data of I-V curves of the InP QD/BP hybrid photodetector. (b-d) Mechanism of charge carrier injection between InP QDs and BP at noncontact state, contact state under dark, and contact state under illumination.


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Figure 4. (a) Transfer curves of the InP QD/BP hybrid photodetector as a function of gate bias with different optical power densities up to 32 μW/cm2 at 1 Vds. Inset indicates the logarithm scale of the transfer curves at 1 Vds. (b) Responsivity and photocurrent of the InP QD/BP hybrid photodetector as a function of gate bias with different optical power densities at 1 Vds.


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Figure 5. (a) Photocurrent of the InP QD/BP hybrid photodetector measured at Vg = -60 V. (b) Responsivity and photocurrent of the InP QD/BP hybrid photodetector as a function of sourcedrain bias with different optical power densities at Vg = -60 V. (c) Responsivity and detectivity of the hybrid photodetector at different optical power densities. (d) Photo-response of the hybrid photodetector with different optical power densities at 1 Vds and Vg = -60 V.


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Figure 6. (a) Optical image of the InP QD/hybrid photodetector and the photocurrent mapping area by a red box. (b) Photocurrent map of the hybrid photodetector at 1 Vds and Vg = -60 V with 0.05 W/cm2 under 405 nm illumination.


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ASSOCIATED CONTENT

Supporting Information.

Materials; Synthesis of InP QDs, Device Fabrication and Measurement; AFM images of BP and InP QD film; Absorption spectra of InP QDs and photo-response of the hybrid photodetector with different annealing temperatures; TEM image of the hybrid photodetector under electrodes; Output characteristic of the hybrid photodetector at 1 Vds and Vg = 60 V.; The photo-response by EDT effect on the hybrid photodetector.

AUTHOR INFORMATION Corresponding Author

* [email protected]

ACKNOWLEDGMENT


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This

work

was

supported

by

Basic

Science

Research

Program

(2019R1A2B5B02004441) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning. We also thank H. S. Jang, S. K. Jeon, and J. B. Bang (CCRF DGIST) for discussions of Electron beam and Photolithography system. Nano-device fabrication was carried out in CCRF of DGIST.


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Table of contents Graphic


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Black Phosphorus Photodetector

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