Towards fast and highly responsive SnSe2 based photodiode by

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Functional Inorganic Materials and Devices 2

Towards fast and highly responsive SnSe based photodiode by exploiting the mobility of the counter semiconductor Emma Panzi Mukhokosi, Basanta Roul, Saluru Baba Krupanidhi, and Karuna Kar Nanda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16635 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Applied Materials & Interfaces

Towards fast and highly responsive SnSe2 based photodiode by exploiting the mobility of the counter semiconductor Emma P. Mukhokosi,† Basanta Roul,†,‡ Saluru B. Krupanidhi† and Karuna K. Nanda*,† †

Materials Research Center, Indian Institute of Science, Bangalore-560012, India ‡

Central Research Laboratory, Bharat Electronics, Bangalore 560013, India

ABSTRACT: In photodetection, the response time is mainly controlled by the device architecture and electron/hole mobility, while the absorption coefficient and the effective separation of the electrons/holes are the key parameters for high responsivity. Here, we report an approach towards the fast and highly responsive infra-red photodetection using n-type SnSe2 thin film on p-Si (100) substrate keeping the overall performance of the device. The I-V characteristics of the device show a rectification ratio of

147 at ± 5 V and enhanced

optoelectronic properties under 1064 nm radiation. The responsivity is 0.12 A/W at 5 V and the response/recovery time constants were estimated as ~ 5725/3415 s respectively. Overall, the response times are shown to be controlled by the mobility of the constituent semiconductors of a photodiode. Further, our findings suggest that n-SnSe2 can be intergrated with well established Si

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technology with enhanced opto-electronic properties and also pave the way in the design of fast response photodetectors for other wavelengths as well.

KEYWORDS: DC sputtering, Selenisation, Mobility, SnSe2, IR-Photodiode 1. INTRODUCTION In photodetection, band gap, absorption coefficient and response speed are the key parameters for materials to be selected for an application. Band gap and absorption coefficient are intrinsic properties of the material whereas response speed may depend on device architecture. In telecommunication applications, high response speed, large band width and low noise have been aggressively pursued.1–4 Several photodetectors with different operational principles exist. These include photoconductors,5 photodiodes,6,7 photoelectrochemical8 and phototransistors. The response speed in a photoconductor is limited by the mobility of the material and the electrode spacing or channel width5 whereas the response speed in a photodiode is limited by the width of the depletion region and mobility at the interface.9 The transit time for a linear (Ohmic) photoconductor based on metal-semiconductor-metal structure is defined as,5,9

 transit 

l

(1)

 drift .E

where l is the electrode spacing or transistor channel (in the order of few mm to m), drift is electron/hole drift mobility and E is the electric field between drain and source. On the other hand, the transit time for a p-n junction is9

 transit 

W W  , vdrift [ drift Eo ]

(2)

where


2

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W

 (Vo  Va )[ N a  N d ] 2eN a N d

(3)

is the width of the depletion region, Vo,Va, Na Nd, drift, and Eo are built-in potential, applied potential, concentration of acceptor atoms, concentration of donor atoms, electron/hole drift mobility and built-in electric field, respectively. Photodiodes made from p-n junctions have narrow depletion width in the order of few nm.9 This indicates that, photodiodes can have a fast transit time in the order of few nano-seconds and femto-seconds. Monolayer and few-layers of transitional metal dichalogenides (TMDCS) such as MoS 210–14, WS2,15 MoSe216, WSe215,17–20, InSe21,22, GaSe23,24 and In2Se325 have been widely investigated as novel materials in optoelectronic and sensing device applications because of their unique properties such as thickness and layer dependent optical properties.26,27 These materials posses strong covalent and weak in-plane van der Waals bonding between the layers that enables single layers to be mechanically exfoliated.28,29 Among the layered, earth abundant and environmentally friendly materials is SnSe2, an n-type semiconductor having hexagonal lattice of CdI2-type crystal structure with interesting thickness and layer dependent optical properties such as tunable band gap.30 Recently, we have demonstrated band gap engineering of SnSe2 thin films and shown that films with band gap ~ 1.2 eV (direct) can be exploited for infra-red (IR) photodetection.30 However, the response time is ~7.76 s with a bias of 5 V and 2.5 s with a bias of 10 V.30 The sensitivity and responsivity were ~ 2.5% and ~ 2 mA/W respectively.30 We further exploited SnSe2/PEDOT:PSS heterostructure and found that the response time improves to 1.33 s at zero-bias as expected from equation 2.31 However, PEDOT:PSS suffers from low mobility and the response/recovery times can be improved further by considering a high mobility p-type semiconductor as one of the constituents. In this regard, p-Si has been chosen as the


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model case and grown n-SnSe2 for the purpose. We analytically estimate the width of the depletion region and transit time for the proposed device using equation 2. At zero-bias, Va=0 and by taking Vo ~ 0.03 V (difference in work functions of SnSe2 and p-Si), Na=7×1015cm-3, Nd=4.0×1017cm-3,30 n ~ 4 cm2/Vs,30 εr=9.871,32 q=1.603×10-19 C, we estimate W ~ 36 nm. The expected transit time for the hetero-junction device is estimated using equation 2 as ~ 1 ps (at built-in voltage of ~ 0.02V, E= 0.55 MV/m), whereas the transit time for SnSe2 and p-Si linear devices as estimated using equation 1 at bias voltage of 5 V (E=5.0 kV/m) is ~ 500 and 4 µs, respectively. Overall, it may be noted that the time constant can be improved for homo/heterojunction of semiconductors with high mobility. In addition to the time constant, the responsivity of a photodetector is also equally significant. p-Si taken as the substrate for the study has an indirect band gap of 1.12 eV, a mobility of ~ 450 cm2/Vs and absorption coefficient of 102 cm-1.9 On the other hand, bulk SnSe2 has a direct band gap between ~ 1.0 - 1.2 eV with higher absorption coefficient of ~ 104 cm-1 but low mobility of ~ 4 cm2/Vs,30,33,34 as compared to silicon. Overall, SnSe2 and p-Si have properties that complement each other and are considered for the constituents of the photodiode. We grow SnSe2 thin film of ~ 1000 nm on p-Si substrate and demonstrate that the two materials form a p-n junction with rectification ratio of  147 at  5 V. Though the sensitivity toward 1064 nm radiation is highest at reverse bias with an optimal peak at - 2 V, other detector parameters like responsivity, external quantum efficiency and detectivity are highest at forward bias due to large photo-current generated. The response/recovery time constants were estimated as ~ 5725/3415 s respectively.


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2. EXPERIMENTAL SECTION Thin film deposition: p-Si (100) substrate was first etched in hydrofluoric acid (15% HF) solution for 2 minutes to remove native oxide layer followed by sonicating in deionized water for 10 minutes. The substrate was then removed and purged with nitrogen gas to remove any residuals and then loaded in the sputtering chamber. The resistivity of p-Si substrate is ~ 2 cm (Sigma Aldrich) that corresponds to a carrier concentration of ~ 71015 cm3. After p-Si cleaning, a two-step process was involved in thin film deposition and growth.30 Similar procedures for SnSe2 film growth has been reported by Fernandes et al.35 These procedures include: i. DC magnetron sputtering of Sn metal target at 43 W for 20 minutes on p-Si substrate, and ii. Selenization at 450 oC for 1 h in a chamber flowing with Ar. Thin film Characterization: The crystal structure of the film was accessed by X-ray diffraction (XRD) using X’pert-PRO PANAlytical instruments with CuKα (1.5418 Å). The surface morphology of the film and roughness were determined using scanning electron microscope (SEM, Ultra55 FE-SEM Karl Zeiss EDS) and non-contact mode atomic force microscope (AFM, A.P.E. Research A100-AFM). Veeco Dektak 6M surface profilometer was used to measure the thickness of the film. Diffuse Reflection Spectrum (DRS) of the film was obtained using UVVis-NIR spectrophotometer (Perkin Elmer-lambda 750 Instruments). Raman study was carried out using Visible LabRAM HR instruments with a 532 nm laser. X-ray photoelectron spectroscopy (XPS) measurements were performed using Axis Ultra DLD (from Kratos) high resolution instrument with automatic charge neutralization equipped with MgKα X-ray source. The system was maintained in ultra-high vacuum at a base pressure of 6.8 × 10-9 Torr. The C 1s peak at 284.60 eV was taken as reference to correct the binding energy values of the sample.


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Device fabrication and current-voltage measurements: The Cr/Au (6/80 nm) metal electrodes were deposited by the standard thermal evaporation method. Photodetection was perfomed by illuminating the film with 1064 nm laser radiation and the current (I) – voltage (V) characteristics were measured using a Keithley SMU2400 source meter.

3.

RESULTS AND DISCUSSION

The XRD pattern of SnSe2 thin film is shown in Figure 1a. The hexagonal phase of SnSe2 with space group P-3m1 was confirmed with no evidence of secondary phases and the pattern was indexed to (001), (100), (101), (102), (003), (110), (111), (103) and (201) planes of SnSe2 at 2θ values of 14.47, 27.08, 30.81, 40.11, 44.26, 47.71, 50.16, 52.61 and 57.85o, respectively in accordance with ICSD 43594. The lattice parameters are found to be a=b=3.79 Å and c=6.12 Å. The Si peak at 56.37o originate from the substrate. As shown in Figure 1b, two Raman active modes at 115.3 and 183.5 cm-1 confirm the SnSe2 phase.30,36 The peak at 115.3 cm-1 is for Eg mode is due to in-plane strecthing and the peak at 183.5 cm-1 is for A1g mode due to out of plane stretching of selenium atoms and the peak positions are consistent with other reports.30,36


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p-Si(100)/SnSe2

P42/mnm (ICSD-SnO2)

P-3m1 (ICSD-SnSe2)

Fd-3m (ICSD-Si)

(b) 960

(c) 10

(201)

Si

Intensity (c/s)

(102)

(003) (110) (111) (103)

(101)

(100)

Intensity (a.u)

(001)

P4/nmm (ICSD-SnO)

Intensity (103 c/s)

(a)

A1g

920

880 Eg

8

Experimental data Cummulative fit Sn3d5/2

Sn 3d

Sn3d3/2 SnO2

6 4

840

1000

Experimental data Cummulative fit Se 3d5/2 Se 3d3/2

800 600 400

30 40 2 ()

SeOx

50

80

60 Se 3d

120 160 200 240 Raman shift (cm-1)

62 60 58 56 54 52 50 48 46 Binding Energy (eV)

20

4

15

3

10 5 0 400

2 500

496 492 488 484 Binding Energy (eV)

480

5 (f) 100

(e) 25

200

280

2 Reflectance 1 Absorbance 0 800 1200 1600 2000 Wavelength (nm)

Absorbance (a.u) (h)

(d) 1200

20

Reflectance (%)

10

Intensity (c/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 0 0.0

Egdir ~ 1.0 0.5

1.0

1.5 2.0 h(eV)

0.1 eV 2.5

3.0

Figure 1. (a) Powder XRD patterns of SnSe2, bare Si substrate and ICSD data of SnO (ICSD-15516), SnO2 (ICSD-9163), Si (ICSD-51688) and SnSe2 (ICSD-43594), (b) Raman spectrum of SnSe2, (c-d) High resolution XPS spectra of Sn 3d and Se 3d, (e) UV-visible DRS and absorbance spectra versus wavelength obtained from Kubelka-Munk function and (f) Tauc plot of SnSe2 thin film on Si substrate.

High resolution XPS spectra of the SnSe2 film are shown in Figure 1c and d. After deconvultion of Sn 3d core level spectrum shown in Figure 1c, four peaks at binding energies at 486.1, 487.4, 494.5 and 495.7 eV are identified that correspond to 3d5/2 and 3d3/2 of Sn. The peaks with highest intensity at binding energies of 486.1 and 494.5 eV are an indication of Sn4+ oxidation state and are consistent with other reports.37,38 A peak shift of ~ 1.5 eV in the positions of Sn 3d is observed as compared to that on SLG substrate30 which is believed to be due to


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substrate effect. Similar peak shifts in the positions of binding energy for Ti4+ of upto 1.5 eV have been observed in TiO2 supported on different substrates (SiO, MgO, Ag, SnO),39 as well as the positions of binding energies for Sn 3d of n-SnO2 on semiconducting p-type-GaN and insulating c-Al2O3 substrates.40 The weak relative intensity peaks with binding energies at 487.4 and 495.7 eV are due to the oxidised amorphous phase of SnO2 that is expected as a result of the sample exposure to the atmosphere.41–43 We have deconvulted Se 3d core level spectrum as shown in Figure 1d into three peaks identified at binding energies of 55.6, 54.6, and 53.7 eV. The intense peaks at 54.6 and 53.7 eV are due to Se 3d3/2 and Se 3d5/2 states respectively,30,37 and the smaller peak at 55.6 eV can be assigned to SeOx.44 Figure 1e shows DRS and absorbance versus wavelength of SnSe2 on p-Si substrate. The absorbance was obtained by converting DRS using Kubelka Munk function.30 A direct band gap of ~ 1.0  0.1 eV was estimated by making Tauc plot as shown in Figure 1f and is in accordance to bulk band gap of SnSe2 reported by other groups.33,34 The band gap of similar sample developed on soda lime glass (SLG) substrate is ~ 1.2 eV.30 AFM image shown in Figure 2a reveals uniform and well grown grains with a surface roughness of ~ 83 nm. Figure 2b shows the height profile across the surface of the film with an average peak to valley height of 67.4 nm. The film thickness of ~ 1000 nm (deviation is  10 nm) is shown in Figure 2c. The surface morphology was further characterized by SEM as shown in Figure 2d and e and revealed plate-like crystallites oriented randomly. The optical image of the device configuration is shown in Figure 2f. Figure 2g shows the I-V characteristics of the device under dark and IR-illumination with different power density (48 - 250 mW/cm2). The device shows a rectifying behaviour characterised with a leakage current in dark of 0.15 A at an applied voltage of -5 V, resulting into a rectification ratio (defined as the ratio of forward to reverse current at a given voltage) of ~ 147 at ± 5 V (Idark at +5 V ~ 22 A, Idark at -5 V ~ 0.15


8

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A). The large leakage current across the junction can be attributed to a thin depletion layer (to be discussed later) and comparable to other two-dimensional SnSe2 based photodetector.45,46 Similar order of leakage current has also been observed in MoSe2/Si and MoS2/Si photodiodes.47,48


9


(a)

(b)

(e)

(d)

(c)

(f) 1E-3

SnSe2

(g)

(d)

1E-4

Current (A)

SnSe2 p-Si substrate

p-Si wafer

SnSe2

1E-4 1E-5 1E-6 1E-7 1E-8 0.01

1E-7

Dark 250 mW/cm2 221 202 176 160 144 125 103 48

1E-10 -5 -4 -3 -2 -1 0 1 2 3 4 5 Bias (V)

(h) Dark 250 mW/cm2 221 202 176 160 144 125 aV 103 e I ( 48 I

1E-6

1E-9

200 nm

1E-3

1E-5

1E-8

1 mm

Current(A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

nI gio e R

I

V (I 

)

)

n gio

Re

0.1

Bias (V)

1

Figure 2. (a) AFM image of SnSe2 thin film with root mean surface roughness ~ 83 nm, (b) height profile extracted across the AFM image, (c) step height of ~ 1000 nm taken from Dektak profilometer (vertical scale is in nm), (d) SEM image at the interface of SnSe2/Si, (e) SEM image taken on top surface of SnSe2, (f) Optical image of the device, (g) I-V characteristics of the device at different illumination power densities and (h) I-V on a log-log scale with two distinct regions I & II.

Various transport mechanisms such as thermionic emission(TE), space charge limited current (SCLC), quantum mechanical tunneling through barriers, trap filled limited current (TFLC) and


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minority carrier injection are known to occur in hetero-junction devices.49 Within a given voltage range and temperature, one of the mechanisms is dominant. We have investigated the dominant transport mechanism under forward bias using two models, each with a mathematical relationship between current and voltage given by; i.

Exponential (Therminionic emission model i.e I eAV or I=Is[exp(qV/kBT)-1)

ii.

Power law (TFLC): IV n1 for n ≥ 1

We have divided the I-V curves into two regions labelled I and II as shown in Figure 2h and determined the correlation coefficients (r) between I and V for each of the models. The value of r close to unity suggests the best model within a given voltage range. Region I correlates to exponential model and region II correlates to power law model. From the exponential region, we estimate the series resistance Rs, ideality factor and barrier height using Cheung’s method.50 By incorporating the series resistance into the circuit, the diode equation51 results into

I  I s [exp(

For

q(V  Rs I ) )  1] k BT

(V  Rs I ) 

I  I s [exp(

(4)

k BT q , equation (4) results into

q(V  Rs I ) )] k BT

(5)

Rearranging equation (5), we have

V

k BT k T Ln( I )  B Ln( I s )  Rs I q q

(6)

and by differentiating equation (6) with respect to Ln(I ) , results into

k T d (V )  Rs I  B d (ln( I )) q

(7)


11


By making plot of

d (V ) vs I of equation (7) as shown in Figure 3a, we evaluate values of d ( Ln( I ))

Rs = 92287.4 Ω and =(0.1192 × q)/(kBT)= 4.61 from the slope and intercept of the graph. To obtain the Schottky barrier height B, from equation (6), we define a function

H (I )  V 

k BT I Ln( )  Rs I   B q AA * T 2

(8)

Having extracted values of  from equation (7), we obtain values of H(I) and make a plot of H(I) versus I which is linear as shown in Figure 3b whose slope provides Rs = 90569.4  and intercept yields value of c = B. We evaluate B as 0.756 eV. The value of series resistance is the sum total of resistance due to substrate, SnSe2 and Cr/Au.

(b) 4.1

(a) 0.8 y=94577.5 I + 0.1

y= 90569.4  I + 3.5

4.0

0.6 H(I) (V)

dV/dLn(I)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

3.9 3.8 3.7

0.2 3.6

0.0

0

1

2

3 4 I(A)

5

6

3.5

0

1

2

3 I(A)

4

5

6

Figure 3. (a-b) d(V)/d(Ln(I)) vs I and H(I) vs I plots used to extract diode parameters using Cheung’s method.

The figures of merit for a photodetector are sensitivity (S), responsivity (R), external quantum efficiency (EQE), detectivity (D*) and response/recovery time (tresponse/trecovery). They are defined as i.

Sensitivity is the ratio of photocurrent to dark current i.e,


12

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S

I ph (9)

I dark

where I ph is the photo-current and Idark is the dark current, ii.

Responsivity is the ratio of photocurrent in response to optical power density impinging on the detector.9 i.e

R 

I ph P A ,

(10)

A is the area, Pλ is the power density, iii.

External quantum efficiency is the number ratio of electrons flowing out of the device in response to impinging photons i.e

EQE  iv.

hcR q

(11)

Specific detectivity (D*) is the measure of the detectors sensitivity by assuming that shot noise from dark current is the major contributor to the total noise.52 i.e (12)

D* = Rλ /(2qJo )1/2 and v.

The response time of a photodetector is usually measured between 10% (90%) to 90% (10%) of the generated signal under modulated excitation intensity, either on the rising or falling edge.5 Photodetectors based on p-n junctions normally operate at reverse bias due to low dark

current that enables the device to have high sensitivity. We determine the optimal operating


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voltage of the device from dark-current and photocurrent of Figure 2g. We evaluate photocurrent and determine photo-detector parameters of S, R EQE, and D* as shown in Figure 4(ad). In general, S, R, EQE and D* increases with increasing power density (power density varies from 48 mW/cm2 to 250 mW/cm2) and increasing reverse bias voltage with optimum peak at ~ -2 V. The increase in detector parameters with illumination power density is quantified by, 31,53

N ( P,  ) 

P , hc

(13)

where N is the number of excited electron-hole pairs, P is the incident illumination laser power per second,  is the wavelength of incident photons, h is Planck constant and c is the speed of light. Equation 13 implies that the number of electron-hole pairs generated is proportional to P at constant λ and also proportional to incident λ at constant P.31 Under the reverse bias, the photocurrent generated is mainly controlled by drift of minority charge carriers (Figure 7d). The peculiar optimum voltage of ~ -2 V can be explained based on band diagram and band offsets in the section that follows. To get further insights into the generation and recombination rates inside the photo-diode, we carry out power dependent I-t measurements as shown in Figure 5a. The dependence of photocurrent on power density shown in Figure 5b was fitted using the power law54, Iph α Pm and the exponent m which determines the response characteristic of a photodetector with incident illumination power density is 0.94 and suggests efficient seperation of charge carriers across the interface.30 Sensitivity similarly increases with increasing power density at constant voltage of -2 V as shown in Figure 5c. Figure 5d shows the ON and OFF IR response characteristic of the device which is well retained even after six cycle. From Figure 5e and f, we estimate the response and recovery time constants as  100 ms. The device can be operated at forward bias in order to achieve significant values of responsivity, external quantum


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efficiency and detectivity. At a forward bias, the dark-current is large due to reduced barrier potential (based on band diagram) that leads to the drop in sensitivity of Figure 6a as compared to that of Figure 4a. In general, responsivity, detectivity and external quantum efficience (Figure 6(b-d)) increases with increasing forward bias and decrease with increasing illumunation power density (0 – 5 V), a common charactersitic of photodetectors based on layered materials such as WS2,54 graphene,55 black phosphorous,56 MoS2,57 and InSe.58 This effect has been considered to be due to the reduction in photogenerated charge carriers available for extraction under photon flux due to Auger processes or saturation of recombination/trap states that influnce the life time of the generated carriers.54–56,59 It is worth to note that, the responsivity at 5 V is about 120 mA/W at illumination power density of 250 mW/cm2 which is several orders of magnitude higher than that of SLG/SnSe2 photoconductor and SLG/SnSe2/PEDOT:PSS reported earlier in our previous studies.30,31 The carrier mobility of p-Si reported at ~ 450 cm2/Vs9 is two orders of magnitude more than that of SnSe2 of ~ 4 cm2/Vs30 and several orders of magnitude more than that of PEDOT:PSS reported between 0.0095 - 0.0128 cm2/Vs31,60 and this explains the reasonably higher responsivity in the present device structure. Figure 6e shows the dependence of photocurrent on power density for the device operated at forward bias of 5 V. We fit this dependance of photocurrent on power density using the power law as shown in Figure 6f and find the value of m as ~ 0.96, an indication of efficient seperation of electron-hole pairs.30,31 Photodetectors based on p-n junctions can as well work under photo-voltaic mode. The photogenerated charge carriers can be seperated by the built-in electrostatic potential. We have investigated our device under zero-bias. The transient response is shown in Figure 6g. We have calculated the sensitivity of the device at zero-bias as ~ 37.5 % (Dark current ~ 40 nA, Photocurrent ~ 15 nA). At zero bias, when the device is shone with light of energy greater or


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equal to the band gap, electron-hole pairs are generated at the interface and are separated by a built-in electrostatic potential (strong built-in electric field) at the interface. The built-in electrostatic potential primarily depends on the difference in the work function of the two semiconductors and is estimated to be ~ 0.03 eV in our case. The low built-in potential and thin depletion layer is expected to give rise to a very low photocurrent. The responsivity at zero bias is calculated as ~ 7.8 A/W. This responsivity at zero-bias is three times higher than that obtained in SnSe2/PEDOT:PSS hetero-structures operated at zero-bias.31 The higher responsivity at zero-bias can be attributed to photon absorption of the two semiconductors and higher mobility of p-Si compared to that between SnSe2 and PEDOT:PSS.31 The response and recovery rates at 5 V forward bias are estimated as ~ 90 ms as shown in Figure 6h and i and are limited by the instrument used. We improve on the instrument set-up as shown in the schematic of Figure 6j which involves a mechanical optical chopper used to create optical pulses from the continuous wave laser, a DC voltage source to drive the device and a digital storage oscilloscope terminated with a resistor R of 1 M.61 The time-dependent response is shown in Figure 6k. We fit one cycle using first order exponential equation31 as shown in Figure 6l and find the response/recovery time constants as ~ 5725/3415 s respectively. As shown in Table 1, the general response is very fast as compared to our previousy reported SnSe2 and SnSe2/PEDOT:PSS devices fabricated on soda lime glass substrate30,31 and of the same order to other photodiodes where the counter semiconductor is either p/n-Si substrate.47,48,62–64 These better photosensor properties are attributed to optimal bandgap of the two semiconductors, higher mobility of p-Si substrate and built-in potential that efficiently seperates the photogenerated charge carriers across the junction. Unlike commercial Si photodiodes which are developed from single crystalline Si with response time reported as 50 ps,5 SnSe2 grown on p-Si


16

Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


wafer presents a polyscrystalline nature with possible defect states which leads to charge recombinations and slows down the response/recovery rates of the photo-generated charge carriers. We further test the performance of the device at a higher voltage of 10 V. Figure S1a shows the power dependent photoresponse of the device under 1064 nm laser. We fit this power dependence of photocurrent as shown in Figure S1b using the power law,54 and the exponent m which determines the response characteristic of a photodetector with incident illumination power density is found to be  1.1. The value of m is very close to unity, an indication that the photogenerated current can be attributed to efficient separation of electron-hole pairs.30 Figure S1(c - e) shows the variation of responsivity, external quantum efficiency and specific detectivity (D*) with power density respectively. Rλ, EQE and D* increase with increasing power density due to increase in photocurrent at 10 V and is in accordance with Equation 13. The increase in photo-current is mainly caused by decreased barrier potential. Figure S1f shows the timedependent photocurrent at 10 V and shows that the SnSe2 device is quite stable even after 7 cycles of constant power illumination of 250 mW/cm2. To access the stability of the device, we have tested the performance of the device after six months stored under ambient conditions. The device shows reproducible response to IR of different illumination power densities at 5 and 10 V as shown in Figure S2a and the photocurrent is proportional to P1.1 at 10 V and P0.75at 5 V as shown in Figure S2b. Figure S2 (c–e) shows the variation of R, EQE and D* with power density at 5 and 10 V biases. Surprisingly, R, EQE and D decrease with increasing power density at 5 V, while and they all increase with increasing power density at 10 V. The trend is similar to that discussed earlier. Figure S2f shows the time-dependent photocurrent at 5 and 10 V and the device is stable after six cycles of constant power illumination of 250 mW/cm2. Figure S2g shows the power dependent-photoresponse at reverse bias of 2 V with similar trend as observed


17


for freshly prepared sample. Figure S2h shows the transient response at -2 V and Figure S2i shows the response of the device at zero-bias. All the tests indicate the stability of the device for long-term applications.

6 4

Responsivity (mA/W)

Sensitivity

8

(b) 0.30

250 mW/cm2 221 202 176 160 148 125 103 48

2 0 -5

250 mW/cm2 221 202 176

0.25 0.20

160 148 125 103 48

0.15 0.10 0.05 0.00

-4

(d)2.0

-2 -3 Bias (V)

-1

250 mW/cm2 221 202 176

1.5

0 160 148 125 103 48

1.0

-5

-4

-2 -3 Bias (V)

(c) 0.03

EQE(%)

(a)10

Detectivity (109Jones)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 37

0

-1 250 mW/cm2 221 202 176

160 148 125 103 48

0.02 0.01

0.5

0.00 0.0 -5

-4

-3 -2 Bias (V)

-1

0

-5

-4

-3 -2 Bias (V)

-1

0

Figure 4. (a-d) Sensitivity, responsivity, external quantum efficiency and detectivity as a function reverse bias voltage.


18

Log(Iph)

-6.4

-1.0 -0.8 Log(P)

50 100 150 200 Power density (mW/cm2)

-0.6 (f) 0.0

-0.4

-1.2

-0.4

s

Iph(A)

-0.8

-0.8 -1.2

s

0

ON-STATE 50 100 Time (s)

150

200

-1.6 25.0

250

0m

-1.2

ry = 10

-6.6 100 150 200 250 300 -1.4 Time (s) (e) 0.0 OFF-STATE

25.1 Time (s)

25.2

-1.6 34.3

ov e

TA

 rec

50

Iph P0.94

-6.2

(c)10 9 8 7 6 5 4 3 2

0m

-0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6

0

TE

-S ON

-6.0

Iph(A)

Iph(A)

(d) 0.0

(b) -5.8

OFF-STATE

0.05 W/cm2

0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6

0.25 W/cm2

Iph(A)

(a)

0 =1 e ns po  res

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sensitivity

Page 19 of 37

34.4 Time (s)

34.5

Figure 5. (a-f) Dependence of photocurrent on power density, ln Iph vs ln P plot, Sensitivity vs power density, temporal response, response and recovery rates at reverse bias of 2 V.


19


(a) 5.0

(b)140

250 mW/cm 221 201 176 160 148 125 103

40 30 20 10 0

1

(g) -20

2 3 Bias (V)

4

Photocurrent (A)

0

50

150 100 Time (s)

0

(k)

100 80 60

EQE (%)

15.1 Time (s)

0.3

4

5

80 60

15.2

160 140 120 100 80 60 40 20

19.7

0.1 0.15 0.2 0.25 Power density (W/cm2)

19.8 19.9 Time (s)

20.0

Photovoltage (mV)

Photovoltage (mV)

R

Pulsed light

2 3 Bias (V)

(l) 0.4

0.2

(j)

1

(f) 140 120 100

50 100 150 200 250 300 0.05 Time (s) (i)180

120

20 15.0

200

0

40

40

ON-STATE -70

5

s 0m

-60

4

=9

-50

250 mW/cm2 221 201 176 160 148 125 103

ery

-40

2 3 Bias (V)

5V

160 0 V (h) 140

OFF-STATE

-30

5

1

Iph(A)

(e) 160 140 120 100 80 60 40 20 0

0

(c) 16 14 12 10 8 6 4 2 0

Photocurrent (A)

0

5

2

50

0

Photocurrent (nA)

4

0.048 W/cm2

2 3 Bias (V)

ms

Detectivity (109Jones)

(d) 60

1

20

90

0

40

Photocurrent (A)

0.0

60

on se =

1.0

80

esp

2.0

100

tr

3.0

0.25 W/cm2

Sensitivity

4.0

250 mW/cm2 221 201 176 160 148 125 103

120

Responsivity (mA/W)

250 mW/cm2 221 202 176 160 144 125 103

t recov

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

0.3

0.1 0.0

0.2

-0.1

0.1

-0.2 -0.3 -2000-1500-1000 -500

0

500 1000 1500 2000

Time (s)

0.0

0

200 400 600 800 1000 1200 Time (s)

Figure 6. (a-d) Sensitivity, responsivity, external quantum efficiency and detectivity as a function forward bias voltage, (e-i) Dependence of photocurrent on power density, Log Iph vs Log P plot, detector response at zero-bias, response and recovery rates at forward bias of 5 V. (j-l) Schematic of set-up for high speed measurements, time dependent response obtained with an optical chopper and fitted exponential response/recovery rates.


20

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b) p-Si

n-SnSe 2

p-Si

n-SnSe2

Evac

Evac ~ 5 eV

~ 4.85 eV

~ 4.05 eV ~ 5.02 eV

EC EF

1.12 eV ~ 1.0 eV

EV

V=0 (c)

(d)

- + n-SnSe2

+ n-SnSe 2

p-Si

p-Si

V0

Figure 7. (a) Energy band diagram of SnSe2 and p-Si before equilibrium, (b) Energy band diagram of SnSe2 and p-Si at equilibrium and IR illumination, (c) Energy band diagram of SnSe2 and p-Si under forward bias and IR illumination (d) Energy band diagram of SnSe2 and p-Si under reverse bias and IR illumination. The work function of SnSe2 and p-Si is taken as 5.0 and 5.02 eV, respectively. The closed and open circles represent electrons and holes respectively.

The mechanism of charge collection and transportation across the junction is illustrated in Figure 7. It may be noted that the work function of SnSe2 as reported in the literature is between 5.0 and 5.3 eV.65,66 The resistivity of p-Si(100) wafer is ~2 Ωcm that corresponds to a carrier concentration of ~ 71015/cm3. Considering the bandgap and electron affinity of Si as 1.12 and 4.05

eV

respectively

and

using

the

equation

p=NVexp[-(EF–EV)/kBT,

where

NV=2(2mh*kBT/h2)3/2= 2.51018 cm-3 is the density of states in the valence band at room temperature, mh*=0.217mo is the hole effective mass for Si(100),67 the work-function of p-Si is


21


Page 22 of 37

estimated to be ~ 5.02 eV. Similarly, the electron affinity for SnSe2 is estimated to be 4.85 eV by considering the carrier concentration (ne) as ~ 41017cm-3,30 me*=2.9mo,68 and work function as 5.0 eV.65 As we observed rectifying characteristics for our SnSe2/p-Si, we construct the band diagram taking the work function as 5.0 and 5.02 eV for SnSe2 and p-Si, respectively that is in accordance with the recently proposed band diagram for SnSe2/p-Si.65 Figure 7a shows the energy band diagram of isolated SnSe2 and p-Si semiconductors. When the two materials are in contact, a space charge region with an electrostatic potential qVo ~ 0.02 eV (difference in the work function of p-Si and SnSe2) is created and a built-in electric field that is directed from nSnSe2 toward the p-Si is established at the junction. Under IR illumination at zero bias as shown in Figure 7b, the photogenerated charge carriers are separated by the built-in electrostatic potential and collected at the electrodes. Under forward bias (VF), an external electric field oriented opposite the built-in field as shown in Figure 7c reduces electrostatic potential barrier to q(Vo - VF) and majority photogenerated charge carriers are able to overcome the potential barrier and are collected at the electrodes. Under reverse bias (VR) as shown in Figure 7d, an external electric field oriented towards the built-in electric field results into enhancement of electrostatic potential barrier to q(Vo +VR). The photogenerated minority carriers can pass through the interface easily due to a strong external electric field as compared with that without bias. It is well established that the barrier height seen by electrons and holes in diodes takes account of the band offsets.9,69 The conduction band offset is estimated using Anderson model as, ∆EC=SnSe2_

p-Si = 0.8 eV and that of the valence band offset for type-II heterojunctions as ∆EV =∆Eg + ∆EC = 0.92 eV. The physical implication of these barrier heights is the energetic barrier for electrons is lower than for holes. By adding the built-in potential barrier at the interface, the turn-on voltages for electrons and holes can be estimated as ∆EC + q(n - p) = 0.8+ 0.02 = 0.82 eV and


22

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


∆Ev + q(n - p) = 0.92 +0.02 = 0.94 eV respectively. The maximum sensitivity of the device observed at ~ - 2 V (Figure 4a) can be related to the turn-on voltage of ~ 0.94 V and suggests that the current is dominated by holes for this device operated in reverse bias. As Si is a well established materials for IR photodetector, we examined the IR (1064nm) response of bare p-Si substrate. Similar electrical contacts of Cr/Au (6/80 nm) were deposited on the substrate as shown in the schematic of Figure 8a. The metal-semiconductor-metal configuration forms a nearly ohmic I-V characteristics at room temperature as shown in Figure 8b. Under 2 V, Si substrate generates a photocurrent of ~ 7.1 A with sensisitity of 16.4 % (Idark = 6.1 A) and responsivity of ~ 0.5 mA/W as shown in Figure 8c and d. In this context, it may be noted that photodetectors made from silicon based material have a detection limit between wavelength of 170 – 1100 nm.5 Previous studies have shown that commercial silicon photodiodes have responsivity in the range of 500 mA/W at 880 nm wavelength and is negiligable for wavelengths less than 405 nm and larger than 1100 nm.5 n-SnSe2 and p-Si have properties that complement each other and the two materials form a diode when combined with enhanced photo-detector properties. The sensitivity of Si/SnSe2 hetero-structure under 2 V reverse bias is 984.6 %, an enhancement of  60 times with respect to bare p-Si substrate and the responsivity under 2 V bias is ~ 20 mA/W, an enhancement of ~ 40 times as compared to bare Si substrate at the same voltage.


23


(a)

(b) 25

Dark 2 IR(250 mW/cm )

Current (A)

20 15 10 5 0

(c) 0.6

0.4

0.2

0.0

0

1

2 3 Bias (V)

4

5

0

1

2 3 Bias (V)

4

5

(d) 3

Responsivity (mA/W)

Sensitivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

0

1

2 3 Bias (V)

4

5

2

1

0

Figure 8. (a - d) Schematic of bare p-Si wafer as a device, I-V characteristic under dark and IR (1064 nm) illumination, sensitivity and responsivity versus bias of p-Si wafer device.


24

Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Comparison of photoresponse parameters. Rλ

5 & 10

λcutt-off (nm) 1064

0

Device

Bias (V)

Ref

0.6 - 2 mA/W

Response time 2.5 - 7.52 s

SLG/n-SnSe2/Cr/Au SLG/n-SnSe2/pPEDOT:PSS

1064

1.4 - 2.6 A/W

1.33 s

31

Commercial Si photodiode

880

500 mA/W

50 ps

5

n-WS2/p-Si

-

340-1100

0.5-5.7 A/W

670 S

62

n-MoS2/p-Si

0

808

300 mA/W

3 S

48

p-GaSe/n-Si

-

532

-

~ 60 S

63

47

n-MoS2 /n-Si

-2

365, 500, 182, 93, 270, -, & 0.27 S 650, 808 - mA/W & 1310 650 11.9 A/W 30.5 S

Si/Bi2Te3

-

1064

3.64 mA/W

-

70

Bare p-Si substrate

2&5

1064

0.5&5 mA/W

-

This work

SnSe2/p-Si

-2, 0, 2 5 1064 & 10

Gr/n-MoSe2/p-Si

0.3, 0.0078, 20, ~ (5725) s 120 & 160 m A/W

30

64

This work

4. CONCLUSIONS In summary, we have fabricated a photodiode based on n-SnSe2 and p-Si (100) substrate. The two materials form a p-n junction with rectification ratio of ~ 147 at ± 5 V which we have explored as an efficient IR-photodiode. The responsivity is 0.12 A/W at 5 V and the response/recovery time constants were estimated as ~ 5725/3415 s. The better detector properties have been attributed to optimal band gap, efficient seperation of photo-generated


25


Page 26 of 37

charge carriers and higher mobility. Further, our findings suggests that n-SnSe2 can be intergrated with well established Si technology with enahanced opto-electronic properties and also pave the way in the design of fast response photodetectors for other wavelengths as well. ASSOCIATED CONTENT Supporting information. Photoresponse of the device under forward bias of 10 V and varying illumination power densities. Test for the photoresponse stability of the device after six months at

varying illumination power densities and bias of -2, 0, 5 and 10 V. AUTHOR INFORMATION Corresponding Author *E-mail: [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. 5. ACKNOLEDGEMENT E. P.Mukhokosi is grateful to Indian Institute of Science through the Office of International Relations for the Ph.D scholarship. Prof. S.B.K ackowledges support from INSA Fellowship. Dr. Kausik Majumdar of Electrical Communication department of IISc is acknowledged for the assistance of high speed measurements. Mr. Rohit Pant is acknowledged for the useful comments on the paper. Ajay Gautam is acknowledged for the SEM experiment.


26

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6. REFERENCES

(1)

Zhao, C.; Liang, Z.; Su, M.; Liu, P.; Mai, W.; Xie, W. Self-Powered, High-Speed and Visible-Near Infrared Response of MoO3-x/n-Si Heterojunction Photodetector with Enhanced Performance by Interfacial Engineering. ACS Appl. Mater. Interfaces 2015, 7 (46), 25981–25990.

(2)

Li, B.; Yan, X.; Zhang, X.; Luo, Y.; Lu, Q.; Ren, X. Controllable Photoresponse Behavior in a Single InAs Nanowire Phototransistor. Appl. Phys. Lett. 2017, 111, 113102.

(3)

Gao, Y.; Tao, L.; Tsang, H. K.; Shu, C. Graphene-on-Silicon Nitride Waveguide Photodetector with Interdigital Contacts. Appl. Phys. Lett. 2018, 112, 211107.

(4)

Eng, P. C.; Song, S.; Ping, B. State-of-the-Art Photodetectors for Optoelectronic Integration at Telecommunication Wavelength. Nanophotonics 2015, 4 (1), 277–302.

(5)

Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Photocurrent Generation with Two-Dimensional van Der Waals Semiconductors. Chem. Soc. Rev. 2015, 44 (11), 3691–3718.

(6)

Deng, W.; Jie, J.; Shang, Q.; Wang, J.; Zhang, X.; Yao, S.; Zhang, Q.; Zhang, X. Organic Nanowire/Crystalline Silicon p - n Heterojunctions for High-Sensitivity, Broadband Photodetectors. ACS Appl. Mater. Interfaces 2015, 7 (3), 2039–2045.

(7)

Zhang, X.; Liao, Q.; Liu, S.; Kang, Z.; Zhang, Z.; Du, J.; Li, F.; Zhang, S.; Xiao, J.; Liu, B.; Ou, Y.; Liu, X.; Gu, L.; Zhang, Y. Poly(4-Styrenesulfonate)-Induced Sulfur Vacancy Self-Healing Strategy for Monolayer MoS2 Homojunction Photodiode. Nat. Commun.


27


Page 28 of 37

2017, 8, 15881. (8)

Yu, Y.; Zhang, Z.; Yin, X.; Kvit, A.; Liao, Q.; Kang, Z.; Yan, X.; Zhang, Y.; Wang, X. Enhanced Photoelectrochemical Efficiency and Stability Using a Conformal TiO2 Film on a Black Silicon Photoanode. Nat. Energy 2017, 2, 17045.

(9)

Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, John Wiley & Sons, 2006.

(10)

Late, D. J.; Huang, Y. K.; Liu, B.; Acharya, J.; Shirodkar, S. N.; Luo, J.; Yan, A.; Charles, D.; Waghmare, U. V.; Dravid, V. P.; Rao, C. N. R. Sensing Behavior of Atomically ThinLayered MoS2 Transistors. ACS Nano 2013, 7 (6), 4879–4891.

(11)

Choi, W.; Cho, M. Y.; Konar, A.; Lee, J. H.; Cha, G. B.; Hong, S. C.; Kim, S.; Kim, J.; Jena, D.; Joo, J.; Kim, S. High-Detectivity Multilayer MoS2 Phototransistors with Spectral Response from Ultraviolet to Infrared. Adv. Mater. 2012, 24 (43), 5832–5836.

(12)

Dai, J.; Zeng, X. C. Bilayer Phosphorene: Effect of Stacking Order on Bandgap and Its Potential Applications in Thin-Film Solar Cells. J. Phys. Chem. Lett. 2014, 5 (7), 1289– 1293.

(13)

Chang, Y.; Zhang, O. W.; Zhu, O. Y.; Han, Y.; Pu, J.; Chang, J.; Hsu, W. Monolayer MoSe2 Grown by Chemical Vapor Deposition for Fast Photodetection. ACS Nano 2014, 8 (8), 8582–8590.

(14)

Kallatt,

S.;

Nair,

S.;

Majumdar,

K.

Asymmetrically

Encapsulated

Vertical

ITO/MoS2/Cu2O Photodetector with Ultrahigh Sensitivity. Small 2018, 14, 1702066. (15)

Perea-Lõpez, N.; Elías, A. L.; Berkdemir, A.; Castro-Beltran, A.; Gutiérrez, H. R.; Feng, S.; Lv, R.; Hayashi, T.; Lõpez-Urías, F.; Ghosh, S.; Muchharla, B.; Talapatra, S.;


28

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Terrones, H.; Terrones, M. Photosensor Device Based on Few-Layered WS2 Films. Adv. Funct. Mater. 2013, 23 (44), 5511–5517. (16)

Xia, J.; Huang, X.; Liu, L. Z.; Wang, M.; Wang, L.; Huang, B.; Zhu, D. D.; Li, J. J.; Gu, C. Z.; Meng, X. M. CVD Synthesis of Large-Area, Highly Crystalline MoSe2 Atomic Layers on Diverse Substrates and Application to Photodetectors. Nanoscale 2014, 6 (15), 8949–8955.

(17)

Gutiérrez, H. R.; Perea-López, N.; Elías, A. L.; Berkdemir, A.; Wang, B.; Lv, R.; LópezUrías, F.; Crespi, V. H.; Terrones, H.; Terrones, M. Extraordinary Room-Temperature Photoluminescence in Triangular WS2 Monolayers. Nano Lett. 2013, 13, 3447–3454.

(18)

Ramakrishna Matte, H. S. S.; Gomathi, A.; Manna, A. K.; Late, D. J.; Datta, R.; Pati, S. K.; Rao, C. N. R. MoS2 and WS2 Analogues of Graphene. Angew. Chemie - Int. Ed. 2010, 49 (24), 4059–4062.

(19)

Huo, N.; Yang, S.; Wei, Z.; Li, S.-S.; Xia, J.-B.; Li, J. Photoresponsive and Gas Sensing Field-Effect Transistors Based on Multilayer WS2 Nanoflakes. Sci. Rep. 2015, 4 (1), 5209.

(20)

Enyashin, a N.; Yadgarov, L.; Houben, L.; Popov, I.; Weidenbach, M.; Tenne, R.; BarSadan, M.; Seifert, G. New Route for Stabilization of 1T-WS2 and MoS2 Phases. J. Phys. Chem. C 2011, 115 (50), 24586–24591.

(21)

Feng, W.; Zhou, X.; Tian, W. Q.; Zheng, W.; Hu, P. Performance Improvement of Multilayer InSe Transistors with Optimized Metal Contacts. Phys. Chem. Chem. Phys. 2015, 17 (5), 3653–3658.

(22)

Tamalampudi, S. R.; Lu, Y. Y.; Kumar U., R.; Sankar, R.; Liao, C. Da; Moorthy B., K.;


29


Page 30 of 37

Cheng, C. H.; Chou, F. C.; Chen, Y. T. High Performance and Bendable Few-Layered InSe Photodetectors with Broad Spectral Response. Nano Lett. 2014, 14 (5), 2800–2806. (23)

Late, D. J.; Liu, B.; Luo, J.; Yan, A.; Matte, H. S. S. R.; Grayson, M.; Rao, C. N. R.; Dravid, V. P. GaS and GaSe Ultrathin Layer Transistors. Adv. Mater. 2012, 24 (26), 3549–3554.

(24)

Zhou, Y.; Zhou, Y.; Nie, Y.; Liu, Y.; Yan, K.; Hong, J.; Jin, C.; Yin, J.; Liu, Z.; Peng, H. Epitaxy and Photoresponse of Two-Dimensional GaSe Crystals on Flexible Transparent Mica Sheets. ACS Nano 2014, 8 (2), 1485–1490.

(25)

Jacobs-Gedrim, R. B.; Shanmugam, M.; Jain, N.; Durcan, C. A.; Murphy, M. T.; Murray, T. M.; Matyi, R. J.; Moore, R. L.; Yu, B. Extraordinary Photoresponse in TwoDimensional In2Se3 Nanosheets. ACS Nano 2014, 8 (1), 514–521.

(26)

Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS 2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805.

(27)

Xie, C.; Mak, C.; Tao, X.; Yan, F. Photodetectors Based on Two-Dimensional Layered Materials Beyond Graphene. Adv. Funct. Mater. 2017, 27, 1603886.

(28)

Yin, Z.; Li, H. H.; Li, H. H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H. Single-Layer MoS2 Phototransistors. ACS Nano 2012, 6 (1), 74–80.

(29)

Choi, W.; Cho, M. Y.; Konar, A.; Lee, J. H.; Cha, G. B.; Hong, S. C.; Kim, S.; Kim, J.; Jena, D.; Joo, J.; Kim, S. High-Detectivity Multilayer MoS2 Phototransistors with Spectral Response from Ultraviolet to Infrared. Adv. Mater. 2012, 24 (43), 5832–5836.

(30)

Mukhokosi, E. P.; Krupanidhi, S. B.; Nanda, K. K. Band Gap Engineering of Hexagonal


30

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SnSe2 Nanostructured Thin Films for Infra-Red Photodetection. Sci. Rep. 2017, 7 (1), 15215. (31)

Mukhokosi, E. P.; Krupanidhi, S. B.; Nanda, K. K. An Extrinsic Approach Toward Achieving Fast Response and Self-Powered Photodetector. Phys. Status Solidi A 2018, 215 (21), 1800470.

(32)

El-Nahass, M. M. Optical Properties of Tin Diselenide Films. J. Mater. Sci. 1992, 27 (24), 6597–6604.

(33)

Guillén, C.; Montero, J.; Herrero, J. Characteristics of SnSe and SnSe2 Thin Films Grown onto Polycrystalline SnO2-Coated Glass Substrates. Phys. Status Solidi Appl. Mater. Sci. 2011, 208 (3), 679–683.

(34)

Narro-Rios, J. S.; Ramachandran, M.; Martínez-Escobar, D.; Sánchez-Juárez, A. Ultrasonic Spray Pyrolysis Deposition of SnSe and SnSe2 Using a Single Spray Solution. J. Semicond. 2013, 34 (1), 013001.

(35)

Fernandes, P. a.; Sousa, M. G.; Salomé, P. M. P.; Leitão, J. P.; da Cunha,

a. F.

Thermodynamic Pathway for the Formation of SnSe and SnSe2 Polycrystalline Thin Films by Selenization of Metal Precursors. CrystEngComm 2013, 15 (47), 10278–10286. (36)

Wu, J.; Hu, Z.; Jin, Z.; Lei, S.; Guo, H.; Chatterjee, K.; Zhang, J.; Yang, Y.; Li, B.; Liu, Y.; Lai, J.; Vajtai, R.; Yakobson, B.; Tang, M.; Lou, J.; Ajayan, P. M. Spiral Growth of SnSe2 Crystals by Chemical Vapor Deposition. Adv. Mater. Interfaces 2016, 3 (16), 1600383.

(37)

Zhang, Y.; Liu, Y.; Lim, K. H.; Xing, C.; Li, M.; Zhang, T.; Tang, P.; Arbiol, J.; Llorca,


31


Page 32 of 37

J.; Ng, K. M.; Ib, M.; Guardia, P.; Prato, M.; Cadavid, D.; Cabot, A. Zuschriften Thermoelectricity Hot Paper Tin Diselenide Molecular Precursor for Solution-Processable Thermoelectric Materials Zuschriften. Angew. Chemie - Int. Ed. 2018, 130, 17309–17314. (38)

Choi, J.; Jin, J.; Jung, G.; Kim, M.; Kim, J.; Uk, S. ChemComm for Lithium Ion Batteries W. Chem. Commun. 2011, 47, 5241–5243.

(39)

V. M. Jimenez, G. Lassaletta, A. Fernandez, J. P. E. and A. R. G.-E. Substrate Effects and Chemical State Plots for the XPS Analysis of Supported TiO2 Catalysts. Surf. INTERFACE Anal. 1997, 25, 292–294.

(40)

Arca, E.; Mcinerney, M. A.; Shvets, I. V; Dong, Z.; Li, Y.; Devi, V.; Kumar, M.; Environment, M. Band Alignment of n-SnO2/p-GaN Hetero-Junction Studied by x-Ray Photoelectron Spectroscopy. J. Phys. D. Appl. Phys. 2014, 47, 215102.

(41)

Sun, M.; Zhao, Q.; Du, C.; Liu, Z. Enhanced Visible Light Photocatalytic Activity in BiOCl/SnO2: Heterojunction of Two Wide Band-Gap Semiconductors. RSC Adv. 2015, 5, 22740–22752.

(42)

Bian, H.; Tian, Y.; Lee, C.; Yuen, M.; Zhang, W.; Li, Y. Y. Mesoporous SnO2 Nanostructures of Ultrahigh Surface Areas by Novel Anodization. ACS Appl. Mater. Interfaces 2016, 8, 28862–28871.

(43)

Wang, Z.; Han, T.; Fei, T.; Liu, S.; Zhang, T. Investigation of Microstructure E Ff Ect on NO2 Sensors Based on SnO2 Nanoparticles / Reduced Graphene Oxide Hybrids. ACS Appl. Mater. Interfaces 2018, 10, 41773–41783.

(44)

Luo, H.; Xie, W.; Tao, J.; Pletikosic, I.; Valla, T.; Girija, S.; Krizan, W.; Zhu, Y.; Cava, R.


32

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


J.

Supplementary

Information:

Differences

in

Chemical

Doping

Matter

-

Superconductivity in Ti. Chem. Mater. 2016, 28, 1927–1935. (45)

Zhou, X.; Gan, L.; Tian, W.; Zhang, Q.; Jin, S.; Li, H.; Bando, Y.; Golberg, D.; Zhai, T. Ultrathin SnSe2 Flakes Grown by Chemical Vapor Deposition for High-Performance Photodetectors. Adv. Mater. 2015, 27 (48), 8035–8041.

(46)

Huang, Y.; Xu, K.; Wang, Z. X.; Shifa, T. A.; Wang, Q. S.; Wang, F.; Jiang, C.; He, J. Designing the Shape Evolution of SnSe2 Nanosheets and Their Optoelectronic Properties. Nanoscale 2015, 7 (41), 17375–17380.

(47)

Mao, J.; Yu, Y.; Wang, L.; Zhang, X.; Wang, Y.; Shao, Z.; Jie, J. Ultrafast, Broadband Photodetector Based on MoSe2/Silicon Heterojunction with Vertically Standing Layered Structure Using Graphene as Transparent Electrode. Adv. Sci. 2016, 3, 1600018.

(48)

Wang, L.; Jie, J.; Shao, Z.; Zhang, Q.; Zhang, X.; Wang, Y.; Sun, Z.; Lee, S. T. MoS 2/Si Heterojunction with Vertically Standing Layered Structure for Ultrafast, High-Detectivity, Self-Driven Visible-near Infrared Photodetectors. Adv. Funct. Mater. 2015, 25 (19), 2910– 2919.

(49)

Zhang, J.; Harrell, W. R. Analysis of the I–V Characteristics of Al/4H–SiC Schottky Diodes. J. Vac. Sci. Technol. B Microelectron. Nanom. Struct. 2003, 21 (2003), 872–878.

(50)

Cheung, S. K.; Cheung, N. W. Extraction of Schottky Diode Parameters from Forward Current-Voltage Characteristics. Appl. Phys. Lett. 1986, 49 (2), 85–87.

(51)

Lin, P.; Yan, X.; Zhang, Z.; Shen, Y.; Zhao, Y.; Bai, Z.; Zhang, Y. Self-Powered UV Photosensor Based on PEDOT:PSS/ZnO Micro/Nanowire with Strain-Modulated


33


Page 34 of 37

Photoresponse. ACS Appl. Mater. Interfaces 2013, 5 (9), 3671–3676. (52)

Xie, C.; Mak, C.; Tao, X.; Yan, F. Photodetectors Based on Two-Dimensional Layered Materials Beyond Graphene. Adv. Funct. Mater. 2017, 27, 1603886.

(53)

Cong, R.; Qiao, S.; Liu, J.; Mi, J.; Yu, W.; Liang, B.; Fu, G.; Pan, C.; Wang, S. Ultrahigh, Ultrafast, and Self-Powered Visible-Near-Infrared Optical Position-Sensitive Detector Based on a CVD-Prepared Vertically Standing Few-Layer MoS2/Si Heterojunction. Adv. Sci. 2017, 1700502.

(54)

Jia, Z.; Xiang, J.; Wen, F.; Yang, R.; Hao, C.; Liu, Z. Enhanced Photoresponse of SnSeNanocrystals-Decorated WS2 Monolayer Phototransistor. ACS Appl. Mater. Interfaces 2016, 8 (7), 4781–4788.

(55)

Konstantatos, G.; Badioli, M.; Gaudreau, L.; Osmond, J.; Bernechea, M.; de Arquer, F. P. G.; Gatti, F.; Koppens, F. H. L. Hybrid Graphene–quantum Dot Phototransistors with Ultrahigh Gain. Nat. Nanotechnol. 2012, 7 (6), 363–368.

(56)

Buscema, M.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; Van Der Zant, H. S. J.; Castellanos-Gomez, A. Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors. Nano Lett. 2014, 14 (6), 3347–3352.

(57)

Wang, X.; Wang, P.; Wang, J.; Hu, W.; Zhou, X.; Guo, N.; Huang, H.; Sun, S.; Shen, H.; Lin, T.; Tang, M.; Liao, L.; Jiang, A.; Sun, J.; Meng, X.; Chen, X.; Lu, W.; Chu, J. Ultrasensitive and Broadband MoS2 Photodetector Driven by Ferroelectrics. Adv. Mater. 2015, 27 (42), 6575–6581.

(58)

Feng, W.; Wu, J. Bin; Li, X.; Zheng, W.; Zhou, X.; Xiao, K.; Cao, W.; Yang, B.; Idrobo,


34

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


J. C.; Basile, L.; Tian, W.; Tan, P. H.; Hu, P. A. Ultrahigh Photo-Responsivity and Detectivity in Multilayer InSe Nanosheets Phototransistors with Broadband Response. J. Mater. Chem. C 2015, 3 (27), 7022–7028. (59)

Sun, Z.; Liu, Z.; Li, J.; Tai, G. A.; Lau, S. P.; Yan, F. Infrared Photodetectors Based on CVD-Grown Graphene and PbS Quantum Dots with Ultrahigh Responsivity. Adv. Mater. 2012, 24 (43), 5878–5883.

(60)

Rutledge, S. A.; Helmy, A. S. Carrier Mobility Enhancement in Poly(3,4Ethylenedioxythiophene)-Poly(Styrenesulfonate) Having Undergone Rapid Thermal Annealing. J. Appl. Phys. 2013, 114 (13), 133708.

(61)

Murali, K.; Majumdar, K. Self-Powered, Highly Sensitive, High-Speed Photodetection Using ITO/WSe₂ /SnSe₂ Vertical Heterojunction. IEEE Trans. Electron Devices 2018, 65, 4141–4148.

(62)

Lan, C.; Li, C.; Wang, S.; He, T.; Jiao, T.; Wei, D.; Jing, W.; Li, L.; Liu, Y. Zener Tunneling and Photoresponse of a WS2/Si van Der Waals Heterojunction. ACS Appl. Mater. Interfaces 2016, 8 (28), 18375–18382.

(63)

Yuan, X.; Tang, L.; Liu, S.; Wang, P.; Chen, Z.; Zhang, C.; Liu, Y.; Wang, W.; Zou, Y.; Liu, C.; Guo, N.; Zou, J.; Zhou, P.; Hu, W.; Xiu, F. Arrayed van Der Waals Vertical Heterostructures Based on 2D GaSe Grown by Molecular Beam Epitaxy. Nano Lett. 2015, 15 (5), 3571–3577.

(64)

Zhang, Y.; Yu, Y.; Mi, L.; Wang, H.; Zhu, Z.; Wu, Q.; Zhang, Y.; Jiang, Y. In Situ Fabrication of Vertical Multilayered MoS2/Si Homotype Heterojunction for High-Speed


35


Page 36 of 37

Visible-Near-Infrared Photodetectors. Small 2016, 12 (8), 1062–1071. (65)

Serna, M. I.; Hasan, S. M. N.; Nam, S.; El Bouanani, L.; Moreno, S.; Choi, H.; Alshareef, H. N.; Minary-Jolandan, M.; Quevedo-Lopez, M. A. Low-Temperature Deposition of Layered SnSe2 for Heterojunction Diodes. Adv. Mater. Interfaces 2018, 5, 1800128.

(66)

Shimada, T.; Ohuchi, F. S.; Parkinson, B. A. Work Function and Photothreshold of Layered Metal Dichalcogenides. Japanese J. Appl. Phys. Part 1-Regular Pap. Short Notes Rev. Pap. 1994, 33 (5A), 2696–2698.

(67)

Wang, C. S. First-Principles Electronic Structure of Si, Ge, GaP, GaAs, ZnS, and ZnSe. Phys. Rev. B 1981, 24 (6), 3393–3416.

(68)

Guo, C.; Tian, Z.; Xiao, Y.; Mi, Q.; Xue, J. Field-Effect Transistors of High-Mobility Few-Layer SnSe2. Appl. Phys. Lett. 2016, 109, 203104.

(69)

Yang, H.; Luo, H. M.; Wang, H.; Usov, I. O.; Suvorova, N. A.; Jain, M.; Feldmann, D. M.; Dowden, P. C.; DePaula, R. F.; Jia, Q. X. Rectifying Current-Voltage Characteristics of BiFeO3∕Nb-Doped SrTiO3 Heterojunction. Appl. Phys. Lett. 2008, 92 (10), 102113.

(70)

Liu, J.; Li, Y.; Song, Y.; Ma, Y.; Chen, Q.; Zhu, Z.; Lu, P.; Wang, S. Bi 2Te3 Photoconductive Detectors on Si. Appl. Phys. Lett. 2017, 110, 141109.


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TOC

Semiconductor1 Semiconductor2

1

2

Semiconductor1 Semiconductor3

1

3

 3 >  1 >  2 , 13 < 12 =Carrier mobility, =Response time


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Towards fast and highly responsive SnSe2 based photodiode by

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