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Ultrahigh sensitivity and gain white light photodetector based on GaTe/Sn : CdS nanoflake/nanowire heterostructures

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 445202 (http://iopscience.iop.org/0957-4484/25/44/445202) View the table of contents for this issue, or go to the journal homepage for more

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Nanotechnology Nanotechnology 25 (2014) 445202 (9pp)

doi:10.1088/0957-4484/25/44/445202

Ultrahigh sensitivity and gain white light photodetector based on GaTe/Sn:CdS nanoflake/nanowire heterostructures Weichang Zhou, Yong Zhou, Yuehua Peng, Yong Zhang, Yanling Yin and Dongsheng Tang Key Laboratory of Low-dimensional Quantum Structures and Quantum Control of Ministry of Education, College of Physics and Information Science, Hunan Normal University, Changsha 410081, People’s Republic of China E-mail: [email protected] and [email protected] Received 8 July 2014, revised 22 August 2014 Accepted for publication 4 September 2014 Published 14 October 2014 Abstract

Optoelectronic diode based on PN heterostructure is one of the most fundamental device building blocks with extensive applications. Here we reported the fabrication and optoelectronic properties of GaTe/Sn : CdS nanoflake/nanowire PN heterojunction photodetectors. With high quality contacts between metal electrodes and Sn : CdS or GaTe, the electrical measurement of GaTe/Sn : CdS hybrid heterojunction under dark condition demonstrates an excellent diode characteristic with well-defined current rectification behavior. The photocurrent increases drastically under LED white light as well as red, green, UV illumination. The on-off ratio of current is about 100 for forward bias and 3000 for reverse bias, which clearly indicates the ultrahigh sensitivity of the heterostructure photodetector to white light. The responsivity and optical gain are determined to be 607 A W−1 and (1.06–2.16) × 105%, which is higher than previous reports of single GaTe or CdS nanostructures. Combination the Ids–Vds curves under different illumination power with energy band diagrams, we assign that both the light modulation effect under forward and reverse bias and the surface molecular oxygen adsorption/ desorption mechanism are dominant to the electrical transport behavior of GaTe/Sn : CdS heterojunction. This heterostructure photodetector also shows good stability and fast response speed. Both the high photosensibility and fast response time described in the present study suggest strongly that the GaTe/Sn : CdS hybrid heterostructure is a promising candidate for photodetection, optical sensing and switching devices. S Online supplementary data available from stacks.iop.org/NANO/25/445202/mmedia Keywords: nano-heterostructures, photodetector, high sensitivity, GaTe/Sn : CdS, 2D semiconductor (Some figures may appear in colour only in the online journal) 1. Introduction

response speed, high photosensitivity, good stability, high reliability, low cost and long lifetime are highly desired [3]. Because of the large surface-to-volume ratio and quantum confinement effect, low-dimensional nanostructures show huge advantage over the corresponding bulk counterpart [4]. Up to date, various inorganic semiconductor nanostructures, such as ZnO, CdS, InAs, MoS2, GaX (X = Te, Se, S), InSe, In2Se3 and In2Te3, have been used to fabricate photodetectors

Photoconductors or photodiodes, which can convert optical signal into electrical signal, are essential element in many fields, such as binary switch, optical communication, environmental monitoring, flame sensing, missile plume detection, and optoelectronic memristor [1, 2]. For practical applications, photodetectors with high photocurrent gain, fast 0957-4484/14/445202+09$33.00

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© 2014 IOP Publishing Ltd Printed in the UK

Nanotechnology 25 (2014) 445202

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2. Experimental details

[1, 5–12]. It has been demonstrated that the photoresponse properties of these detectors are determined critically by a variety of parameters, including contact type of device, selected material, as well as the crystalline quality and surface adsorption. Single crystal two-dimensional (2D) structures, such as graphene, have attracted great attention in recent years and show great potential applications in next-generation electronics [5–7, 13]. Due to the zero band gap, weak light absorbance and short exciton lifetime, nanodevices based on graphene may find limit applications in optoelectronics. However, layered semiconductors with finite band gap and relative high carrier mobility, for example, MoS2 and black phosphorus, are excellent candidates for optoelectronics devices [14]. Many kinds of layered semiconductors, including layered transition metal dichalcogenides (MX2, M = Mo, W; X = S, Se, Te), group-III metal chalcogenides (MX, M = In, Ga; X = S, Se, Te), group-IV metal chalcogenides (MX2, M = Sn, Ge; X = S, Se, Te), have been peeled off from bulk materials and used to explore the corresponding optoelectronics properties [15]. Compared with the indirect layered semiconductor (the indirect can transfer to direct when the thickness decreases to single layer), GaTe is a typical p-type direct material with band gap of 1.7 eV, which act as efficient light absorber and emitter. Recently, few-layer GaTe has been fabricated as photodetector or phototransistor and shows interesting properties [16]. However, the photoresponse gain of GaTe photodetector is low due to the large dark current. Cadmium sulfide (CdS), as an important n-type semiconductor with direct band gap of 2.49 eV (∼499 nm) at room temperature, has demonstrated interesting physical and chemical properties. Due to the highly insulative feature of 1D pure CdS micro/nano-structures, appropriate doping is much desired to tailor the energy band structure and modulate the transport property [17]. Since heterojunction photodetectors usually have advantages of low dark current, high sensitivity, fast response speed and high stability [18], we fabricate GaTe/Sn : CdS nanoflake/nanowire hybrid photodetectors successfully in the present work. In this nanodevice, the photo-generated electron–hole pairs (EHPs) can be separated quickly to free charged carriers by the build-in electric field at the heterostructure interface. The different drift of the photo-generated electrons and holes in GaTe and Sn : CdS under forward and reverse bias dominate the photo-response behaviors of GaTe/ Sn : CdS heterostructures. Moreover, the surface molecular oxygen adsorption/desorption also play an important role to the high photo-response current. The responsivity and optical gain of the hybrid photodetector are calculated to be 607 A W−1 and (1.06–2.16) × 105%, which are much larger than those of individual GaTe or CdS based photodetectors. In addition, this heterostructure photodetector shows good stability and fast response speed. These results demonstrate that the GaTe/Sn : CdS hybrid nano-heterostructure can be applied to nano-optoelectronic fields.

2.1. Materials

The GaTe nanoflakes were exfoliated on SiO2 (300 nm)/Si substrate from bulk GaTe single crystal (6–8 mm in size) by using scotch-tape based mechanical exfoliation method. The Sn : CdS nanowires were prepared by a simple thermal evaporation method. The more details about the synthesis and characterization of Sn : CdS nanowires are described in [19]. Raman scattering spectrum, which was performed in a confocal microscope (LABRAM-010) using He–Ne laser (632.8 nm) as the excitation light source, was used to confirm the crystal structure of exfoliated GaTe nanoflakes and the effective doping of tin in the CdS nanowires. The thickness of GaTe nanoflake was determined by atomic force microscopy (AFM). 2.2. Fabrication and measurement of photodetector

The as-prepared Sn : CdS nanowires were dispersed into ethanol by sonication. Then, the suspension was dropped on the GaTe nanoflake/SiO2/n-Si wafer to form GaTe/Sn : CdS PN heterostructures. The single GaTe nanoflake or Sn : CdS nanowire and hybrid GaTe/Sn : CdS nanodevice were fabricated by following UV lithography procedure, thermal evaporation and lift-off process. The contact electrode Ti/Au was about 2 nm/120 nm. The space between the two Ti/Au electrodes was about 10 μm. The room-temperature electrical transport measurement was carried out with keithley source meter 2602 (Keithley Instruments) in air. To measure the photoresponse properties, a commercial white light LED with different power was used to illumination on the nanodevices.

3. Results and discussion Figure 1(a) is a typical optical microscopy image of GaTe nanoflake. The varied color represent different thicknesses. According to the AFM image, the thickness of the GaTe nanoflake ranges from 16 nm to 72 nm (figure (S1)). Figure 1(b) is the optical microscopy image of Sn : CdS nanowire, which has a diameter of about 200 nm. Raman spectroscopy is a powerful tool to characterize the crystal structure of layered and doped material. Figure 1(c) shows the Raman spectra of GaTe nanoflake with different thicknesses. Both the 128.1 cm−1 and 142.5 cm−1 peaks are assigned to be Ag mode. There are no obvious frequency shifts corresponding to different thicknesses, which is same with the recent results [20] while is different from others 2D semiconductor material, such as MoS2 and GaS [21, 22]. Figure 1(d) shows the micro-Raman scattering spectrum of a single tin-doped CdS nanowire at room temperature. The two strong peaks at 296 and 592 cm−1 are designated to be oneLO and twoLO modes of CdS crystal, respectively. The other more four phonon modes (208, 320, 337, 357 cm−1) in the Raman scattering spectrum are designated to be Eg, A1g, A2u(TO), A2u (LO) impurity vibration modes of doped SnS2, respectively, 2

Nanotechnology 25 (2014) 445202

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Figure 1. (a), (b) optical microscopy image of single GaTe nanoflake and Sn : CdS nanowire, respectively. (c) Raman spectra of GaTe nanoflake with different thicknesses. Curves 1–6 correspond to the positions 1–6 marked in (a). (d) Raman spectrum of single Sn : CdS nanowire.

Figure 2. (a), (c), (d) Ids–Vds curves under dark condition as well as light illumination of UV (405 nm), green (532 nm) and red (650 nm) of single Sn : CdS nanowire, GaTe nanoflake and GaTe/Sn : CdS nano-heterostructure, respectively. (b) Far-field PL spectra of Sn : CdS nanowire under different excitation power.

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Figure 3. (a), (c) Ids–Vds curves in the dark and under white light illumination of single Sn : CdS nanowire, GaTe nanoflake, respectively. (b),

(d) Drain-source current Ids as a function of back gate voltage Vg at fixed drain-source bias voltage Vds = 2 V of Sn : CdS nanowire, GaTe nanoflake, respectively. (e, f) Ids–Vds curves of GaTe/Sn : CdS heterostructure nanodevice under the dark condition and white light illumination, respectively. Inset of (f) is the logarithm plot of Ids–Vds.

light with wavelength longer than 520 nm. The present broad spectral response is due to the increased concentration and strong response of deep trapped levels and optical whispering gallery mode microcavity effect induced by the doped tin in CdS nanowire [19]. The micro-photoluminescence was used to confirm the whispering gallery mode effect and deep trapped state in the doped CdS nanowire (figure 2(b)). Figure 2(c) is the Ids–Vds curves of GaTe nanoflake under dark condition and illumination. Inset is the corresponding SEM image. The GaTe nanoflake can response to red, green and UV due to the intrinsic band gap (1.7 eV). It seen that both Sn : CdS nanowires and GaTe nanoflakes can response to the visible light. Figure 2(d) is the Ids–Vds curves of GaTe/ Sn : CdS nanoflake/nanowire heterostructure under dark condition and illumination, which demonstrate that this

which confirmed the effective doping of tin into the CdS nanowires [19]. Figure 2(a) shows the current–voltage (Ids–Vds) curves of tin-doped CdS nanowire under dark condition as well as illumination with different wavelength. Inset is the SEM image of corresponding nanodevice. We can observe an increasing of photocurrent in the Sn : CdS nanowire photodetector when it was illuminated by UV light (405 nm), green light (532 nm), red light (650 nm). Generally, the photoconductive sensitivity is due to the EHPs excited by the incident light with energy larger than the band gap. That is to say, only light with enough photon energy to excite electron from the valence band to the conduction band is able to induce a significant increase in conductance. Therefore, the sensitivity of CdS photodetector is rather low (