High photoresponsive backward diode by two dimensional SnS2

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High photoresponsive backward diode by two dimensional SnS2/Silicon heterostructure Seyed Ali Hosseini, Ali Esfandiar, Azam Iraji Zad, Seyed Hossein Hosseini-Shokouh, and Seyed Mohammad Mahdavi ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01626 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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High photoresponsive backward diode by two dimensional SnS2/Silicon heterostructure Seyed Ali Hosseini,a Ali Esfandiar*b , Azam Iraji zad* ab, Seyed Hossein Hosseini-Shokouh b, Seyed Mohammad Mahdaviab , a Institute

for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588-89694, Iran

b Department

of Physics, Sharif University of Technology, Tehran 11155-9161, Iran.

*Corresponding authors: [email protected] , [email protected]

KEYWORDS

Two dimensional hetero-junction; SnS2; Photodetector; Band to band tunnelling

ABSTRACT

Two dimensional semiconductor materials can be combined with conventional silicon based technology and sort out part of future challenges in semiconductor technologies due to their novel electrical and optical properties. Here, we exploit the optoelectronics property of the Silicon/SnS2 heterojunction and present a new class of backward diodes using a straightforward fabrication method. The results indicate an efficient device with fast photoresponse time (5-10 µs), high photoresponsivity (3740 AW-1) and high quantum efficiency (490%). We discuss device behavior by considering band to band tunneling model and band bending characteristics of the heterostructure. This device structure is fully compatible to semiconductor industry process and allows direct integration with commercial silicon-based technology for novel applications.

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INTRODUCTION The 21st century is the era of Information Technology (IT) and its growth has been planned based on continuous reduction size of silicon metal oxide field effect transistors (MOSFETs). Nowadays, this trend has faced some serious challenges. First, with the reduction of the transistor size in the bulk materials, the gate does not have a good control over the carriers in the channel. The second challenge is that, increasing power density in device by scale down of the transistor due to limitations of the thermionic mechanism at decreasing supply voltage [1]. These two challenges indicate that new devices, materials and approaches are needed to continue the Moore's law. As one of the favorable solutions, tunneling field effect transistor (TFET), can be used to figure out the problem of reducing the supply voltage. In a typical TFET device, the switching mechanism, instead of thermionic emission, is a main mechanism as quantum mechanical bandto-band tunneling (BTBT) through a barrier to inject carriers to the channel. Due to the quantum tunneling process, off-state leakage current and subthreshold swing (SS) are less than conventional metal oxide semiconductor field effect transistors (MOSFETs) [2]. Recently, two-dimensional semiconductors have been emerged as a promising candidate for replacement/complementary with silicon. These materials have enough electrostatic control in very thin channels [3]. Metal dichalcogenides are the most famous members of two-dimensional semiconductor with the common formula as MX2, where M is a metal (M = Mo, W, Sn, and so on), while X is a chalcogen (X = S, Se, Te). In single layer form of the Metal dichalcogenides, the M atoms are sandwiched between the two X atoms and each layer is bonded together by van der Waals forces [4].


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MX2 layered materials are new candidates to utilize as the TFETs channel. In these type of channels because of thin thickness enough electrostatic control over the channel, and lack of dangling bonds, low SS can be achieved. As far we know a few studies have been done on fabrication of the two-dimensional semiconductors based TFET channel [5-10]. Apart from above mentioned difficulties in transistor minimization process, another essential daily demand, is fast and safe optical communication and imaging that requires new and efficient optoelectronic devices with high sensitivity and fast response speed. Two-dimensional semiconductors with energies ranging from a fraction of the electron volt to a few electron volts are suitable materials to use in optoelectronic devices as well. So far, many studies have been carried out on application of these materials in devices such as photodetectors, solar cells, light emitting diodes (LEDs), etc [11]. To commercialize this type of devices in technical point of view, studying and designing devices well matched with current configuration of silicon-based semiconductor industry is required. One of the close-to-industry approaches is devices made of 2D semiconductor/Bulk semiconductor (such as Silicon, Germanium) heterostructure. Several studies have been conducted on this kind of electronic and optoelectronic devices, which has promising results to solve the serious challenges of the semiconductor industry [1,12-17]. As one of promising material, 2D SnS2 is new environmental friendly and earth abundant semiconductor material that shows high electron mobility (230 cm2 V−1 s−1), high responsivity (722 A W−1) and the fast photoresponse time (5 µs) comparing with the other 2D materials which makes it desirable for novel and safe electronic and optoelectronic devices [18-20]. In this work, we used p-type bulk Silicon and n-type SnS2 heterostructures to obtain high performance tunneling diode and photodiodes which is close to semiconductor industry process. EXPERIMENTAL DETAILS 3 ACS Paragon Plus Environment

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Crystal growth of SnS2 was performed using chemical vapor transport (CVT) method as following: tin and sulfur precursors are placed in a vacuumed and sealed ampoule with stoichiometric ratio of 1:2. The ampoule was placed on the furnace at 700-degree for two weeks. The SnS2 flakes were mechanically exfoliated on PMGI/PMMA double layer polymer films. Using micro- transferring machine, the desired flake was transferred on a prepared Si/SiO2 substrate with 20 µm SiO2 gap. The gap was made by a shadow mask during plasma enhanced chemical vapor deposition of 300 nm SiO2 film on low doped (1- 10 Ω. Cm) P-type Si wafer. After flake transferring, electrodes were deposited by Cr/Au metallization process. The electrical characterization of the devices is measured using the IVIUM CompactStat instruments. Also for optical measurements, IVIUM MODULIGHT was used. For STS measurements Nanosystem Pars SS2 was used. RESULTS AND DISCUSSION X-ray Diffraction (XRD) spectrum of grown material showed the proper growth of the crystalline SnS2 (Figure 1a). Micro Raman spectra from the transferred layer using 532 nm laser (Witech Alpha300 R) exhibits a main peak centered at 315 cm−1 (Figure 1b) correspponding to A1g (out-of-plane) peak of SnS2 [18]. Thickness of SnS2 flake was measured about 9nm (~ 12 layer) by atomic force microscopy (Park Scientific CP-Research, VEECO). A schematic of the SnS2/Silicon diode is illustrated in Figure 1d, As can be seen the 2D n-type SnS2 few layer is located on 3D p-type silicon and a p-n junction is created. Optical microscopy and scanning electron microscopy (SEM) images of this device shown in figure 1e and 1f.


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Figure 1. a) XRD spectrum of grown crystal shows high quality crystallite of SnS2. In inset depict image of ampoule used as crystal growth reactor. b) Raman spectra of SnS2 layer shows the characteristic peak of this material. c) AFM image and height profile from edge of the flake. d) Schematic of SnS2/Silicon Heterojunction d) optical microscopy image of this device e) Scanning electron microscopy image of device. In order to probe the energy band structures in SnS2/Silicon heterostructure, scanning tunneling spectroscopy (STS) is conducted and represented in Figure 2a. According to STS results, Silicon is a p-type semiconductor with 1.11 eV energy gap and SnS2 is a n-type semiconductor with 2.39 eV energy gap. Considering Silicon and SnS2 electron affinities (4.05 eV and 5 eV, respectively) [17,21], and the STS data, the band structure of this heterostructure has been illustrated in Figure 2b. In this structure, the gap between valence band of Silicon and conduction band of SnS2 is low and only about 0.16 eV which can facilitate the tunneling process.


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Figure 2. a) STS spectra of Silicon and SnS2/Silicon heterostructure vallence band and conduction band of SnS2 and heterostructure are shown with dash lines. b) Band diagram of Silicon and heterostructure. I-V data of this p–n diode in linear and logarithm sales are illustrated in Figure 3 a, and b . Reagrd to the curves, the device behavior is similar to backward diode and presenting precursor to negative differential resistance (NDR) in forwarding bias [1]. Schematics in figure 3c demonstrate carrier transport in the device using their band alignments of the heterojunction in various bias voltages.


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Figure 3. I – V characteristics of the device, on linear (a) and logarithmic(b) scales. c) energy band diagram at various bias voltages. In (A) Silicon vallence band is above of SnS2 conduction band which causes BTBT. In (B) no current will occur. In (C) electric current is derived from the drift and BTBT mechanism. In (D) like a normal diode, current is due to the drift mechanism. In reverse bias, electrons tend to move from the conduction band of SnS2 to the valence band of Silicon by band to band tunneling (BTBT). In small forward bias, many of electrons move from the conduction band of SnS2 to the conduction band of Silicon by drift mechanism and a few electrons tend to move from the conduction band of SnS2 to the valence band of Silicon by BTBT. By increasing forward bias, the conduction mechanism changes to drift as dominate process. The competition between drift and BTBT in forward bias make a candidate to use as NDR based devices.


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Two important parameters of backward diodes are reverse rectification ratio (Re) and curvature ∂2𝐼

coefficient. The curvature coefficient is 𝛾 =

∂𝑉2 ∂𝐼 ∂𝑉

𝐼𝑅𝑒𝑣𝑒𝑟𝑠𝑒

and Reverse rectification is defines as 𝑅𝑒 = 𝐼𝐹𝑜𝑟𝑤𝑎𝑟𝑑.

For the current device, the calculated curvature coefficient was

𝛾 = 20.03 𝑉 ―1

which is

comparable to similar devices made by Murali et al [22]. To identify the Reverse rectification values, we measured electro-optical properties of the device under different wavelengths illumination such as Red (635 nm, 260 mW/cm2), Amber (595 nm,82 mW/cm2), Cyan (510 nm, 78 mW/cm2), White (29 Klux), Green (535 nm,91 mW/cm2), Blue (475 nm, 278 mW/cm2) and Royal Blue (460 nm, 255 mW/cm2) from an identical light to sample distance in dark and light conditions. The measured reverse rectification values are listed in table 1: Table 1. Reverse rectification values of device in different wavelenghts illumination Dark

46.5

White

166

Royal Blue

Blue

Green

Cyan

Amber

Red

(460 nm)

(475 nm)

(535 nm)

(510 nm)

(595 nm)

(635 nm)

149

176

168

177.5

172

151

The increase in Re, in the presence of light, reflects the good response of this device to light. For further studies, the effect of light on electrical properties of a device has been investigated. Results in Figure 4 a, b and c show the photoinduced I – V curves of the device, on linear and logarithmic scales and for forward bias.


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Figure 4. photo induced I – V curves of device, on linear (a) and logarithmic (b) scales. c) I-V curves in forward bias. d) I-V curves in near to zero-zero point in various wavelength (Inset: The effect of wavelength on the short-cut current and open-circuit voltage). In this p-n diode photovoltaic effect has been observed. In Figure 4d, I-V curve is shown near the zero-zero point. Because of the different optical power density of the used LEDs, different open circuit voltage (VOC) and short circuit current (ISC) values are observed in Figure 4d. The ISC values are between 160 nA to 330 nA and the VOC value ranges from 210-260 mV (Figure 4d inset). It is well known that external quantum efficiency (EQE) and photoresponsivity are crucial parameters of photodetectors. Photoresponsivity (R) is defined as electrical output per optical input 9 ACS Paragon Plus Environment

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and calculated by R = (J / PLight ), where J is current density, and PLight is the optical power density of the incident light. EQE is the conversion efficiency of photons to electrons and can be calculated by EQE = (JSC / PLight) (hc / eλ), where JSC is short circuit current density, h is the Planck constant, e is the electron charge, c is the velocity of light, and λ is the wavelength of the incident light. Another important parameter of photodetector is photodetectivity which is capability of the device to detect weak optical signals. Photodetectivity calculates by D* = R.(S)1/2/ (2 e Idark)1/2, where S is effective area (87.15 μm2). EQE, Photoresponsivity and photodetectivity of the SnS2/Silicon diode ware depicted on Figure 4a, b and c. As an example, according to plots, Photoresponsivity, EQE and photodetectivity in the case of Royal Blue photons are EQE =490% , R =3740 AW-1 and D* ≈4.1 × 1010 Jones, respectivity. has been previously (Table 2.) reported [19,21,23,24].

Figure 5. a) external quantum efficiency of device in varius wavelenght, b) photoresponsivity of device in varius wavelenght at 0V and -1V bias, c) photodetectivity of device in varius wavelenght at 0V and -1V bias. d) response of device to pulsed light source e) rising time and falling time of device. 10 ACS Paragon Plus Environment

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The dynamic response time of this device tested using pulsed green laser (365 nm wavelength and 5mW power). In Figure 5d, the response of the device to the pulsed light has been reported for 18 cycles. As it can be seen the photoresponse to the light is very fast, stable and repeatable. From response of the device to one cycle light exposure (Figure 5 e), the rising time (increase from 10% to 90% of the peak value) and decay time (decrease vice versa from 90% to 10% of the peak value) measured as 15 μs and ~ 5 μs, respectively. These ultrashort response time values reflect low capacitance of the p-n junction. In Table 2, photodetector parameters of this type of the device have been compared by several similar devices. As it can be seen the responses of our device made by facile method, are much better than commercial photodectors and can easily compete with similar new devices made by other two dimentional devices made by complicated processes. Table 2. Compare similar devices Device

Photoresponsivity Photodetectivity Photoresponse Reference (AW-1) (Jones) time (μs)

Commercial Silicon 0.6 photodetector

N/A

0.001

[25]

Black phosphorus/MoS2

0.17

4.9 × 108

N/A

[26]

Black phosphorus/WSe2

0.5

1010

80

[27]

TiS3/Si heterojunction

0.034

2.5 × 108

2 × 104

[28]

MLG/h-BN/ZnO

1350

106

[29]

Few-layer BP

90000

3 × 1013

103

[30]

MoS2-GaTe

21.83

8.4 × 1013

N/A

[23]

41014

53

[31]

MoS2/graphene/WSe2 104 11

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N/A

1.7 × 106

[32]

graphene sandwiched 6 WSe2/GaSe

N/A

30-35

[33]

MoS2/Si heterojunction

0.3

1013

3

[24]

WSe2/SnS2 Heterostructure

244

1.29 × 1013

13 × 103

[34]

WS2/Si

5.7

N/A

670

[21]

SnS2/Silicon

3740

4.1 × 1010

5-15

This Work

MoS2−IGZO

1.7

MODELING Next, for further investigation of observed tunneling current, numerical calculations using Wentzel–Kramers–Brillouin approximation have been used. The direct tunneling probability in the BTBT process is approximated using the following equation [12].

{

2

𝐸

}

{

𝑇𝐵𝑇𝐵𝑇 = 𝑒𝑥𝑝 ― 𝑒𝜉∫0 𝑔𝑘(𝐸)𝑑𝐸 = 𝑒𝑥𝑝 ―

𝜉1 𝜉

}.

2 𝐸

𝜉1 = 𝑒∫0 𝑔𝑘(𝐸)𝑑𝐸

Equation 1.

Where e, 𝜉, Eg and k are electron charge, electric field, energy gap and imaginary wave vector, respectively. According to theoretical studies in a p-n junction, the tunnel current is equal to the following equation [35]. 𝐼𝐵𝑇𝐵𝑇 =

𝑔𝑠𝑔𝑣𝑒 ℎ

{ (1 + cosh( ) )}𝑇

𝑘𝐵𝑇𝑙𝑛

1 2

∆𝐸 𝐾 𝐵𝑇

Equation 2.

𝐵𝑇𝐵𝑇

Where KB, T, gv, gs, h and ∆𝐸 are Boltzmann constant, temperature, valley degeneracy, spin degeneracy, plank constant and tunneling energy window, respectively. By defining fitting 𝑔𝑠𝑔𝑣𝑒

parameter as a=

ℎ

𝑘𝐵𝑇 , b=∆𝐸 + 𝐸𝑔 and c=

∈ 𝑊𝑑 𝑒𝑉𝑑𝑠 + 𝑏

( ∈ is a built-in voltage and is a depletion

width) direct tunneling current for the BTBT process can be approximated as:


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{

1

𝐼 = 𝑎𝑙𝑛 2(1 + cosh

(

𝑒𝑉𝑑𝑠 ― (𝐸𝑔 ― 𝑏) 𝐾 𝐵𝑇

) )}𝑒𝑥𝑝( ―

𝑐 𝑒𝑉𝑑𝑠 + 𝑏 𝑒𝑉𝑑𝑠 + 𝑏

)

Equation 3.

Where Vds is voltage between source and drain, respectively. Fitting on experimental data points using the above equation, gives us estimated parameters for a, b and c. In Figure 6a, the tunneling current in dark condition and the fitted equation are shown.

Figure 6. a) I-V curves of experimental and fited BTBT model for dark condition, b) calculated built-in voltage and depletion width in various wavelength. Having the parameters from fitting and using equations 4, the depletion width and built-in voltage can be defined in Vds= 0, built-in voltage ( ∈ ) is equal to b parameter and 𝑊𝐷 calculated by: 𝑊𝐷 =

𝑐 𝑒𝑉𝑑𝑠 + 𝑏 𝑒∈

Equation 4.

The values calculated for depletion width and built-in voltage for dark and different wavelengths exposure are presented in Figure 6b and Table 3. Table 3. Fitting parameters of device in various wavelengths.


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a (pA)

b (eV)

c (eV)1/2

Built-in Depletion width voltage at Vds= at Vds= 0 (nm) 0 (V)

Royal Blue

0.219

0.908

0.826

0.908

0.867

Blue

0.199

0.964

0.892

0.964

0.908

Green

0.191

0.927

0.861

0.927

0.894

White

0.207

0.946

0.834

0.946

0.858

Cyan

0.181

0.936

0.923

0.936

0.954

Amber

0.183

0.936

0.883

0.936

0.913

Red

0.215

0.946

0.756

0.946

0.777

DARK

0.064

0.906

2.147

0.906

2.254

Light incident on the depletion region of a photodiode generated electrons and holes flowing to the n- and p-type side of the junction, which produces a narrower width of depletion region. Despite the decrease in depletion width, the built-in voltage remains constant due to the photo voltage resulting from carriers separation. According to the data in presences of light, the depletion width is decreasing (from 2.2 nm in dark to 0.77-0.95 nm in light) which leads to increasing in photo-current. Due to narrow depletion width in SnS2/Silicon heterojunction, photoresponsivity is very high (~ 4000 AW-1). The strong electric field created in this area leads to very efficient separation of the photo-generated electron-hole pairs, at small reverse voltages (~ -0.1 V). In this voltage, Silicon valence band is above of SnS2 conduction ban, which causes BTBT with increased electrical current. The configuration of SnS2 and Silicon bands and narrow depletion width of SnS2/Silicon heterojunction have led to the backward diode behavior and BTBT carrier transport mechanism in 14 ACS Paragon Plus Environment

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this device. High performance (high photoresponsivity ~3740 AW-1 and fast responsivity ~5-15 μs) along with the easy and close to industry fabrication process imply potential application for commercial high-performance photodetectors.

CONCLUSIONS In conclusion, we introduced a versatile and highly responsive photodetector based on two dimensional heterostructure of transferred few layers of SnS2 on p-type Silicon, using a simple and standard method. These types of new backward diodes, present high photoresponsivity (3740 AW-1), fast photoresponse time (5-10 µs) and high quantum efficiency (490%). The mechanism of the device can be explained by band to band tunneling model and band bending characteristics of the van der waals heterostructure due to ultra-thin depletion layer. Clean hetero-junction and precise engineering of the band structure at junction, lead to very fast and sensitive transport of photo-carriers. This device structure is fully compatible to semiconductor industry process and can be easily adapted with commercial silicon-based technology for novel applications

AUTHOR INFORMATION Corresponding AuthorS *E-mail: Ali Esfandiar; [email protected] and Azam Iraji zad;

[email protected]

Author Contributions SAH and AE designed the experiment and wrote the manuscript. AE fabricated devices and performed the structural characterizations. SAH did electro-optical measurements and modeling. 15 ACS Paragon Plus Environment

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AI and SMM commented on the characterizations and the presentation of the results. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interest. Acknowledgement AE acknowledges support from Research and Technology Council of the Sharif University of Technology. AZM is thankful to financial supports from Iran National Science Foundation (Plan No. 93-36789).

REFERENCES

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High photoresponsive backward diode by two dimensional SnS2/Silicon heterostructure Seyed Ali Hosseini,a Ali Esfandiar*b , Azam Iraji zad* ab, Seyed Hossein Hosseini-Shokouh b, Seyed Mohammad Mahdaviab , a Institute

for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588-89694, Iran

b Department

of Physics, Sharif University of Technology, Tehran 11155-9161, Iran.

*Corresponding authors: [email protected] , [email protected]


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High photoresponsive backward diode by two dimensional SnS2

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