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Self-assembly High-Performance UV-vis-NIR Broadband #In2Se3/Si Photodetector Array for Weak Signal Detection Zhaoqiang Zheng, Jiandong Yao, Bing Wang, Yibin Yang, Guowei Yang, and Jingbo Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16329 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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ACS Applied Materials & Interfaces

Self-assembly High-Performance UV-vis-NIR Broadband β-In2Se3/Si Photodetector Array for Weak Signal Detection

Zhaoqiang Zheng1, Jiandong Yao2, Bing Wang3*, Yibin Yang1, Guowei Yang2*, Jingbo Li1, 4*

1

School of Materials and Energy, Guangdong University of Technology, Guangzhou,

510006, Guangdong, P. R. China. 2

State

Key

Laboratory

of

Optoelectronic

Materials

and

Technologies,

Nanotechnology Research Center, School of Materials Science & Engineering, Sun Yat-sen University, Guangzhou, 510275, Guangdong, P. R. China. 3

Institute of Micro-nano Optoelectronic Technology, Shenzhen Key Lab of

Micro-nano Photonic Information Technology, College of Electronic Science and Technology, Shenzhen University, Shenzhen, 518060, Guangdong, P. R. China. 4

State Key Laboratory for Superlattices and Microstructures, Institute of

Semiconductors, Chinese Academy of Sciences, Beijing, 100083, P. R. China. *Corresponding authors: [email protected]; [email protected] and [email protected]

Keywords: β-In2Se3/Si heterojunction; layered materials; photodetector array; p-n junction; weak signal detection.

1

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Abstract The emergence of a rich variety of layered materials has attracted considerable attention in recent years due to their exciting properties. However, the applications of layered materials in optoelectronic devices are hampered by the low light absorption for mono-/few-layers, the lack of p-n junction, and the challenges for large-scale production. Here, we report a scalable production of β-In2Se3/Si heterojunction array using pulsed-laser deposition. Photodetectors based on the as-produced heterojunction array are sensitive to a broadband wavelength from ultraviolet (370 nm) to near-infrared (808 nm), showing a high responsivity (5.9 A/W), a decent current on/off ratio (~600), and a superior detectivity (4.9*1012 Jones) simultaneously. These figures-of-merits are among the best values of the reported heterojunction-based photodetectors. In addition, these devices can further enable the detection of weak signals, as successfully demonstrated with weak light sources including a flashlight, lighter and fluorescent light. Device physics modeling shows that their high performance is attributed to the strong light absorption of the relatively thick β-In2Se3 film (20.3 nm) and the rational energy band structures of β-In2Se3 and Si, which allows efficient separation of photoexcited electron-hole pairs. These results offer a new insight into the rational design of optoelectronic devices from the synergetic effect of layered materials as well as mature semiconductor technology.

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Introduction In recent years, layered materials such as graphene (Gr)1, borophene2, transition metal dichalcogenides (TMDs) (MoS23, WS24, MoSe25 and WSe26), group IIIA-IVA (GaS7, InSe8 and In2Se39) and IVA-VIA compounds (SnS210, SnSe211 and SnSe12), have attracted widespread attention due to their unique 2D structure with large specific surface area13. These materials have many appealing properties such as prominent light-matter interaction14, wonderful flexibility15, excellent carrier mobility16-17, wide bandgap coverage18, and layer-dependent electronic and optical properties19-20, which render them the promising candidates for future optoelectronics applications. Layered indium selenide (In2Se3) is an intriguing member of the group IIIA-VIA semiconductors21. Specifically, it exhibits a direct bandgap whether in monolayer or bulk form, allowing both a high absorption coefficient and efficient generation of electron-hole pairs under photoexcitation22-23. Moreover, the light absorption of In2Se3 spans from ultraviolet to near-infrared spectral region24, suggesting a broadband spectral response. Motivated by these appealing properties, layered In2Se3 (α-In2Se3 and β-In2Se3) has proven to be the desirable material for optoelectronics devices, such as photodetectors. For instance, monolayer α-In2Se3 flakes have been fabricated by a physical vapor deposition (PVD) method, which exhibited excellent photoresponse in terms of high responsivity25. However, pristine In2Se3-based photodetectors always suffer from low detectivity (~1011 Jones) and poor current on/off ratio (less than 10) because of the absence of type-II staggered p-n junctions, which allows efficient separation of photoexcited electron-hole pairs21, 26-27. On the other hand, owing to 3



their small thickness, monolayer or few-layer In2Se3 can only absorb a small portion of incident light, restricting photocurrent improvement9. Furthermore, differing from graphene, which can be attained in large-scale through catalytic growth on metal substrates, scalable production of intact layered In2Se3 film has not yet been widely demonstrated, making it difficult for large-scale device applications. In comparison to the formation of heterojunctions from two different layered materials, combining layered materials with the mature silicon (Si) technology opens up a feasible and easier scenario to construct innovative optoelectronic devices by harnessing the advantages of both materials28. However, up to now, there are such few reports on the β-In2Se3/Si heterojunctions due to it is challenging to produce large-area intact β-In2Se3 film. Hence, the optoelectronic property of β-In2Se3/Si heterojunction is still a virgin land waiting for exploration. Herein, for the first time, we report that scalable photodetector array integrated with β-In2Se3/Si heterojunctions are self-assembled via a facile pulsed-laser deposition (PLD) technique. Significantly, these devices demonstrate excellent photoresponse properties from ultraviolet (UV, 370 nm) to near-infrare (NIR, 808 nm) region. Specifically, the photodetector array demonstrates a high responsivity of 5.9 A/W and a decent current on/off ratio up to 600, combined with a superior detectivity reaching 4.9*1012 Jones. Besides, we also successfully demonstrated the detection of very weak signals by employing flashlight, lighter and fluorescent light as the weak light sources. This study suggests a new scenario for designing and constructing next generation high performance optoelectronic devices. 4


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Experimental Section Self-assembly of the β-In2Se3/Si photodetector array. PLD was exploited to self-assemble the β-In2Se3/Si heterojunctions in a large scale. Firstly, we covered the backside of a SiO2/p-Si substrate with an aluminum (Al) layer by sputtering (Figure 1a). Subsequently, photolithography was used to pattern 84 windows with the size of 100 * 100 µm on the SiO2/p-Si substrate. Then, SiO2 in the windows were selectively etched by using a buffered oxide etchant (BOE), and expose the fresh surface of the p-type Si wafer (resistivity of ~10 Ω·cm), as shown in Figure 1b. Thereupon, Au contact pads were patterned around the windows using a standard photolithography process, followed by electron beam evaporation (Figure 1c). Then, we mounted the substrate in the deposition chamber, evacuated the base pressure of the chamber to 10-4 Pa, and heated the substrate temperature to 360 °C. In the deposition process, a pulsed KrF excimer laser (248 nm) with pulse duration of 20 ns was focused to ablate the In2Se3 target (99.99%). The working pressure was set at 20 Pa with flowing highly pure Ar2 (99.999%) as the working gas at a rate of 50 sccm. The β-In2Se3 film was deposited onto the exposed windows of the SiO2/p-Si substrate (Figure 1d), and the device structure of an individual β-In2Se3/Si photodetector was schematically depicted in Figure 1e. Characterization of the β-In2Se3/Si heterojunction. The surface morphology was characterized using a FEI scanning electron microscope (SEM, Quanta-400). The thickness profile and the Kelvin probe force microscopy (KPFM) measurements were conducted on an atomic force microscopy (AFM, Bruker Dimension Fastscan). 5



Crystallinity and compositions of the β-In2Se3 film were characterized by using X-ray diffraction (XRD, Rigaku D-MAX 2200 VPC), Raman spectroscopy (Renishaw), and X-ray photoelectron spectroscopy (XPS, ESCALAB 250). The band structure of the β-In2Se3 film was evaluated by using ultraviolet photoelectron spectroscopy (UPS, ESCALAB 250) with He I line (21.22 eV) as the radiation source. The absorption spectrum was recorded by a UV-vis-NIR spectrophotometer (Lambda950, PerkinElmer). Optoelectronic measurements of the photodetector array. Optoelectronic measurements were carried out in a probe station (Lakeshore) equipped with a semiconductor characterization system (Keithley 4200). Incident illumination with wavelength of 370 was generated from the CrystaLaser Class IIIb laser, while the illuminations with wavelengths of 447, 532, and 808 nm were generated from Viasho semiconductor lasers. All of the measurements were performed under ambient condition at room temperature.

Results and Discussion Morphology and structure of the β-In2Se3/Si photodetector array. Figure 2a shows a typical SEM image of the β-In2Se3/Si photodetector array, which actually spreads out over an area of 10 mm2. A high-magnification SEM image of the β-In2Se3 film is presented in Figure 2b, which clearly reveals the continuous and compact morphology. AFM measurement across the edge of the β-In2Se3 film is displayed in Figure 2c. The thickness of the β-In2Se3 film is calculated to be ca. 20.3 nm, and the 6


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root mean square (RMS) roughness is as small as 2.1 nm. Then, XRD and Raman characterizations are performed to further investigate the crystalline quality of the PLD-grown β-In2Se3 film. As shown the XRD pattern in Figure 1c, all the diffraction peaks can be indexed to β-phase In2Se3 (JCPDS 35-1056) and Au electrodes (JCPDS 65-8601). As marked in the corresponding labels, main peaks of the β-In2Se3 can be attributed to the (003) family lattice planes, which reveal its highly c-axis oriented nature. Then, to further assess the crystal structure, Raman spectrum of the prepared β-In2Se3 film is presented in Figure 2d. The rather strong characteristic peak centering at 110 cm-1 can be regarded as the dominant β-In2Se3 phase lattice phonon mode29-30. In addition, the peaks located at 175 and 208 cm-1 can be assigned to the In-Se vibrations, and A1(LO + TO) phonon mode of β-In2Se3, respectively31-32. Additionally, the weak peaks at 62 and 304 cm-1 have also been detected in previous results32-33. Finally, XPS measurement is conducted to verify the elemental composition of the PLD-grown β-In2Se3 film, as shown in Figure S1. The full spectrum in Figure S1a indicates the mainly presence of In and Se elements, while the C element is from a reference for normalization. Therefore, the prepared β-In2Se3 film is of relatively high purity, which is benefit from the clean nature of PLD growth method. Figure S1b and c present the high resolution XPS spectra of In 3d and Se 3d, respectively. The peaks appear at 444.7 and 452.3 eV are in accordance with the In 3d5/2 and In 3d3/2 doublet binding energies, respectively33-34. The peaks located at 53.6 and 54.4 eV are in accordance with the Se 3d5/2 and Se 3d3/2 binding energies, respectively. The 7



stoichiometric ratio of In : Se is calculated to be 2:2.8, which is approximately consistent with the theoretical value (2:3), implying the formation of high quality β-In2Se3 film. Optoelectronic performance of the β-In2Se3/Si photodetectors. Photoresponse properties of the β-In2Se3/Si photodetectors are systematically investigated upon various light irradiations. Figure 3a plots the typical current-voltage (I-V) characteristic curves of the β-In2Se3/Si heterojunction device in the dark. As can be seen, the device shows an obvious rectifying behavior with a rectification ratio of 5.3, indicating the interaction between β-In2Se3 and p-type Si35. Then, typical I-V characteristics of the as-fabricated device measured with and without 532 nm light irradiation is depicted in Figure 3b. It shows clearly that the current of the device increases significantly when it is illuminated. The current on/off ratio is more than 600 (16.5 mW/cm2), indicating that the as-prepared device showed good light-switching behavior. Thereupon, the corresponding light intensities dependent photocurrent (Iph = Ilight - Idark) is summarized in Figure 3c (blue squares). Fitting the plot as a simple power law equation of Iph ~ Pα, we achieve the value of α = 0.51. Here, both defects and charge impurities may account for this sub-linear response, and similar phenomenon has also been previously observed in other layered materials-based photodetectors11, 36-37. It should be pointed out that we have prepared β-In2Se3 films with vary thickness. Figure S2 shows the photocurrent evolved with the thickness of the β-In2Se3 film under the same incident intensity (7 µW/cm2). It is clear that the phototocurrent increases first and then decreases with the increasing of 8


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β-In2Se3 thickness. Hence, the thickness of 20.3 nm is demonstrated to be superior for photodetection application. To have a deeper insight into the device performance, several important parameters, including responsivity (R), external quantum efficiency (EQE), and detectivity (D*), are quantificationally calculated. Responsivity is defined as the ratio of the generated photocurrent in response to incident power, which can be calculated according to the equation: R=

I ph

(1)

PS

Where Iph is the photocurrent, P is the incident power density, S is effective area of the photosensitive region38. EQE is defined as the ratio of electrons flowing out of the device in response to impinging photons. It can be estimated according to the equation:

EQE =

hcR eλ

(2)

Where h is the Planck constant, c is the light velocity, λ is the incident light wavelength, and e is the electronic charge. Detectivity is a parameter reflecting the photodetector’s capability to detect weak optical signal. Considering a dominating shot noise in the dark state current Idark, we have:

D* = RS

1

2

/ (2eI dark )

1

2

(3)

Figure 3c (magenta triangles) and Figure 4 shows the light intensities dependent responsivity, external quantum efficiency, and detectivity at Vds = -4 V. Clearly, all of them decrease with the increase of incident light intensity, which is consistent with previously reported layered material-based photodetectors11, 9


38

. Remarkably, our


β-In2Se3/Si device achieves a high responsivity of 5.9 A/W, along with EQE of 1376% and detectivity of 4.9*1012 Jones. These figures-of-merits are much higher than many other recently reported heterojunction-based photodetectors39-41. Then, as shown in Figure 3e, the cycling behavior of the photodetector is also checked with a series of periodical light stimulations at Vds = 0 V (magenta) and -4 V (blue). It is notable that our β-In2Se3/Si device exhibits definite switching characteristics and maintains good repeatability without severe deviation after multiple cycles, suggesting the great photodetection stability and fast response time. To estimate the response rate of our β-In2Se3/Si device, we also record a complete photoswitching cycle with a better temporal resolution. As presented in Figure S3, defining the response time as the photocurrent increases from 10% to 90% of the stable state, we find a fast response time less than 8.3 ms. This value is comparable to other layered material-based photodetectors42-43. For clear comparison, Table S1 summarizes the relevant figures-of-merits of recently developed heterojunction-based photodetectors, and commercial ones. In general, our device stands out in the comprehensive consideration of the overall performances, which reveals the superiority of our β-In2Se3/Si heterojunction in the photodetection applications. Sequentially, the measured photocurrents of the photodetector array are statistically shown in Figure 3f. There are 84 devices in a chip, and the variation of device to device is small, which indicates the uniformity of the as-grown β-In2Se3-Si heterojunction array. It can be expected that if the manufactured photodetector array are irradiated with a projected light source with a fixed intensity distribution, the 10


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obtained contrast map with spatial intensity variation will match the intensity distribution of the light source. Therefore, this β-In2Se3/Si photodetector array could be able to function as an image sensor. Then, broadband photodetection characteristics of the constructed photodetector are also investigated. Figure 4a shows the absorption spectrum of the β-In2Se3/Si heterojunction and pristine Si substrate. It is to be noted that heterojunction exhibits a broadband absorption ranging from UV to NIR region, manifesting the potential for broadband photodetection. Moreover, the light adsorption of the β-In2Se3/Si heterojunction is much higher than that of the pristine Si substrate, suggesting a positive effect on the high photodetection performances. Figure 4b shows the I-V characteristics of the photodetector in the dark and under UV-vis-NIR light illumination with the same light intensity (1.58 mW/cm2). Notably, the device exhibits a pronounced response to the UV-vis-NIR light. Figure 4c-d present the light intensities dependent responsivity and detectivity of the photodetector under illumination with various wavelengths. In all case, the responsivity and detectivity also exhibit a negative dependence on the incident light intensity. Meanwhile, the device achieves high performance under all light illumination, revealing its broadband photodetection capability. Then, given the high detectivity achieved in our β-In2Se3/Si photodetector, it may be suitable for weak signal detection. As a demonstrated, we use a flashlight, fluorescent light, and lighter as the weak signal sources. Figure 5 records the I-V characteristics of the heterojunction photodetector in the dark and excited by the weak 11



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signal sources. As can be seen, the device exhibits an obvious response to these weak light sources, confirming its weak signal detection capability. Heterojunction mediated photodetection mechanism. To unveil origin of superior photodetection properties of the β-In2Se3/Si photodetector, synergistic effect between n-type β-In2Se3 and p-type Si is systematically investigated. At first, energy band structure of the heterojunction is estimated. Figure S4 presents the UPS spectrum of the PLD-grown β-In2Se3 film. By linear extrapolating the low energy cutoff part of the UPS spectrum and ﬁnding the interset with the background signal (Figure 6a), the work function (Φ) of our β-In2Se3 film can be calculated to be 4.72 eV according to the formula44:

φ = hv − Ecutoff

(4)

Where hv is the incident photon energy of 21.22 eV. Then, by performing a linear fit of the onset part of the UPS spectrum and ﬁnding the interset with the background signal (Figure 6b), we can acquire the difference between the Fermi energy and the valence band to be 1.13 eV. Thereupon, UV-vis-NIR absorption spectrum of the β-In2Se3 film and the corresponding Tauc plot is presented in Figure 6c. The bandgap of the β-In2Se3 is determined to be 1.21 eV. Then, given that the band structure of p-type Si was determined in the previous reports28, 45, we sketch the energy band structure diagram of the β-In2Se3/Si heterojunction in Figure 6d. This type-II staggered band alignment is in favor of the separation of photoexcited electrons-hole pairs, and subsequently reduces their recombination, bringing about a high photocurrent at the same bias voltage46-47. 12


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In order to further verify the carriers’ separation in the β-In2Se3/Si heterojunction, KPFM measurements were exploited to quantitatively analyze the changes of surface potential around the heterojunction. Figure S5 demonstrates the synchronous measurements for AFM topography and their corresponding surface potential difference (SPD) distribution around the β-In2Se3/Si heterojunction in the dark and under white light illumination. The SPD between the AFM tip and the β-In2Se3 or Si surface can be defined as:

SPDIn2 Se3 = φtip − φIn2 Se3 SPDSi = φtip − φSi

(5) (6)

Where Φtip, ΦIn2Se3, and ΦSi are work functions of the AFM tip, β-In2Se3, and Si, respectively48-49. Due to the work function difference between β-In2Se3 and Si, the surface potential interface can be observed from the SPD images (Figure S5b and e). Moreover, the Fermi level difference between β-In2Se3 and Si (∆Ef) can be estimated through the formula: ∆E f = φIn2 Se3 − φSi = SPDSi − SPDIn2 Se3

(7)

As shown in Figure S5c, the ∆Ef is determined to be about 54.7 mV in the dark. Under light illumination, the ∆Ef increases to about 78.5 mV. This significant change of ∆Ef between dark and illumination conditions confirms an obvious charge transfer in the β-In2Se3/Si heterojunction under light illumination. To clearly illustrate the transport dynamics of the photogenerated carriers, we demonstrate a schematic diagram of the system in Figure 6e. Under illumination, a large amount of electron-hole pairs are generated in the β-In2Se3 and Si (blue arrows 13



in Figure 6e). Subsequently, driven by the built-in electric field in the heterojunction, the photoexcited electrons tend to move towards the conduction band of n-type β-In2Se3, while the photoexcited holes are directed to the valence band of the p-type Si (green arrows in Figure 6e). As a result, the quasi-Fermi levels of these two materials shift in opposite directions, which are schematically presented Figure S6. Therefore, the Fermi level difference between β-In2Se3 and Si widens upon light illumination, which is consists with the increasing in ∆Ef in Figure S4. Considering the above results, we propose that the synergistic effect of β-In2Se3 and Si, which allows efficient separation of photoexcited electron-hole pairs, bringing about high photodetection properties of the device.

Conclusion In summary, we have demonstrated the scalable fabrication of the β-In2Se3/Si heterojunction array via a facile PLD method. Photodetectors integrated with the as-prepared heterojunction array exhibit a broadband photoresponse from UV to NIR region. Remarkably, they show a high responsivity of 5.9 A/W, decent current on/off ratio than 600, and a superior detectivity reaches 4.9*1012 Jones. Taking advantage of the high detectivity of the β-In2Se3/Si photodetector, we successfully demonstrated the detection of weak optical signals. We attributed the superior device performance to the strong light absorption of the relatively thick β-In2Se3 film and the rational energy band structures of β-In2Se3 and Si, which allows efficient separation of photoexcited electron-hole pairs. This work opens up an opportunity for designing and constructing 14


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novel optoelectronic devices.

Supporting

Information.

Summary

of

reported

photodetectors,

XPS

characterizations and UPS spectrum of the PLD-grown β-In2Se3 film, photocurrent evolved with the thickness of the β-In2Se3 film, response time of the β-In2Se3/Si photodetector, KPFM measurements at the interface of the β-In2Se3/Si heterojunction.

Acknowledgements. This work was supported by National Natural Science Foundation of China (50902097, 11674310), State Scholarship Fund of China Scholarship Council (201708440013), Basic Research Project of Shenzhen (JCYJ20160308091322373), ‘‘One Hundred Talents Program’’ of the Guangdong University of Technology (GDUT), and State Key Laboratory of Optoelectronic Materials and Technologies.

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Figure Captions

Figure 1. (a-d) Schematic diagrams showing the fabrication process of the β-In2Se3/Si heterojunction photodetector array. (e) Schematic device structure of a single β-In2Se3/Si photodetector.

Figure 2. Morphology and structure of the prepared heterojunction photodetector array. (a) Typical SEM image of the β-In2Se3/Si photodetector array. (b) Zoom-in SEM image of the β-In2Se3 film. (c) AFM height profile across the edge of the β-In2Se3 film. The inset is the AFM image of the scan area. The thickness of the β-In2Se3 film is 20.3 nm. (d) XRD pattern and (e) Raman spectrum of the prepared heterojunction photodetector.

Figure

3.

Optoelectronic

characteristics

of

the

β-In2Se3/Si

heterojunction

photodetector. (a) Dark I-V characteristics of the photodetector on logarithmic scale (blue) and linear scale (magenta). (b) I-V curves of the β-In2Se3/Si photoconductor measured in the dark and 532 nm light illumination with various light intensities. (c) Light intensities dependent photocurrent (blue squares) and responsivity (R, magenta triangles) of the photodetector at Vds = -4 V. (d) Light intensities dependent external quantum efficiency (EQE, blue) and specific detectivity (D*, magenta) at Vds = -4 V. (e) I-t curves of the β-In2Se3/Si photodetector with periodical on/off switching upon 532 nm light illumination under biases of -4 V (blue) and 0 V (magenta). (f) 23



Normalized photocurrent distribution of 84 devices recorded at P = 7 µW/cm2 and Vds = -4 V. Inset: optical image of the photodetector chip.

Figure 4. Broadband photodetection of the β-In2Se3/Si photodetector. (a) Absorption curve of the β-In2Se3/Si heterojunction. (b) I-V characteristics of the photodetector in the dark and under UV-vis-NIR light illumination with the same light intensity (1.58 mW/cm2). Light intensities dependent (c) responsivity and (d) detectivity of the photodetector at Vds = -4 V.

Figure 5. Weak signal detection of the β-In2Se3/Si photoconductor. I-V characteristics of the heterojunction photodetector in the dark and excited by (a) flashlight; (b) fluorescent light; and (c) lighter.

Figure 6. (a) UPS spectra near the low energy cutoff part of the β-In2Se3. (b) UPS spectra near the onset part of the β-In2Se3. (c) UV-vis-NIR absorption spectrum of the β-In2Se3 film. Inset shows the corresponding Tauc plot. (d) Schematic illustration of energy band structure of β-In2Se3/Si heterojunction. (e) Schematic illustration of charge-transfer process in the β-In2Se3/Si heterojunction under light irradiation.

24


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Figure 1

(b)

(a)

(c)

Fabricate electrodes

Remove SiO2

β-In2Se3 Deposition

(d)

(e) Au

Al

25



Page 26 of 31

Figure 2

(a)

100 μm

(c)

(b)

20.3 nm

300 nm

(e)

(d)

26


Si

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Figure 3

(a)

(b)

In dark

(c)

(d)

(e)

(f)

27



Figure 4

(a)

(b)

(c)

(d)

28


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Figure 5

(a)

(c)

(b)

29



Figure 6

(a)

(b)

(c)

(d)

(e)

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