Wafer-scale production of vertical SnS multilayers for high-performing photoelectric devices

Malkeshkumar Patel , Hong-Sik Kim and Joondong Kim *
Photoelectric and Energy Device Application Lab (PEDAL), Department of Electrical Engineering, Incheon National University, 119 Academy Rd. Yeonsu, Incheon, 406772, Korea. E-mail: joonkim@incheon.ac.kr

Received 12th May 2017 , Accepted 20th July 2017

First published on 21st July 2017


This study achieved wafer-scale, high quality tin monosulfide (SnS) layers. By using a solid-state reaction, the vertically aligned SnS layers spontaneously grew with sulphur reduction from the sputtered SnS2 particles without any post processes. The quality of the SnS vertical layers was observed by high resolution transmission electron microscopy, which confirmed an interlayer space of 0.56 nm for a perfect match to the theoretical value. The phase purity of SnS was confirmed by Raman spectroscopy. The intrinsic energy band gap value (1.6 eV) of SnS is attractive for photoelectric devices. To form a heterojunction, the vertical SnS layers were grown on a n-type Si substrate. Due to the nanoscale size and vertical standing features of the SnS layers, a significantly low reflection (<5%) was realized for the SnS/n-Si heterojunction device. As a photovoltaic cell, the device provides a higher open circuit voltage (>300 mV). For photodetection, the response speed is faster than 15 μs for near infrared wavelength photons, which is a 1000 times improvement over the horizontally shaped device. The vertically standing SnS layers show high photoreactive performance, which confirms that the functional design of 2D materials is an effective route to achieve enhanced photoelectric devices, such as photodetectors and solar cells.


1. Introduction

Two-dimensional (2D) materials have emerged as promising entities with the realization of the fascinating features of the graphene monolayer.1–3 Recently, the van der Waals layered materials of graphenes and black phosphorenes strongly demonstrated the high feasibility of low-dimensional surface science with tailored surface atoms that exhibit exceptional physical properties, and these materials have been used in optoelectronic devices,4 chemical sensors,5 catalysts6 and solar energy devices.7,8 Few layered 2D materials offer an enhanced surface in which the properties of surface atoms can be controlled.9–13

Tin monosulphide (SnS) is an interesting 2D material that has direct and controllable energy band gap values (1.3 eV–1.7 eV). The intrinsic p-type conductive SnS has a strong absorption coefficient (α, >5 × 104 cm−1) and high carrier mobility (10[thin space (1/6-em)]000–38[thin space (1/6-em)]000 cm2 V−1 s−1).7,14–16 The predicted mobility of SnS is 1.46× and 8.92× higher than the black phosphorus in single layer and double layer, respectively.16 In addition, the earth abundant SnS is non-toxic and air-stable and can be processed in cost-effective ways.

Due to its excellent optical and electrical properties, SnS was applied in devices with different features, such as SnS film-based photodetectors,17 core–shell shaped photodetectors,11 gas sensors,18 gate tunable field effect transistors,13 and photoelectrochemical cells.7,14

Recently, layered SnS exhibited anisotropic properties. The optical anisotropy of orthorhombic SnS layers contributes an antireflection feature that results in high photoactivity.7 A strong anisotropic photoresponse was obtained by the core–shell SnS–SnSxSe(1−x) heterostructure.11 Structural anisotropy in monolayer SnS can produce strain-tunable electronic and optical properties.12 2D SnS layers showed remarkable in-plane anisotropy and high carrier mobility, even superior to that of black phosphorus.16 The anisotropic optical properties of the single crystal SnS functionalize the dielectric dispersion.19 The anisotropic thermoelectric transport properties of SnS led to 40–60% more lattice conductivity when in in-plane than in the cross-plane.20 Considering such plane orientation dependent properties of SnS layers, it seems desirable to control SnS layers in the vertical direction for enhanced device performance. Various 2D materials, such as HfS2,21 MoS2,22 MoSe2 and WSe2,23 ReS2,24 and SnS2,25 have shown the control of vertical growth.

To review the growth methods of SnS, Sucharitakul et al. prepared few layered SnS on a Si substrate using the vertical Bridgman method at 900 °C.13 Wang et al. reported SnS layers on graphene using molecular beam epitaxy.26 SnS flakes were synthesised using the thermal evaporation of SnS powder at 600–800 °C.27 Ultrathin nanobelts of SnS were synthesised using the SnS powder at 600 °C.11 By using the solid-state reaction, a copious amount of SnS2/SnS nanotubes was obtained for ordered superstructures.28 Meanwhile, the vertical growth of SnS has not been investigated to date. Moreover, the size of the SnS growth area is significantly limited.

In this work, we report a wafer-scale growth of vertical standing layers of SnS (Fig. 1a). The vertically grown SnS layers were spontaneously obtained by the sulphur reduction of sputtered SnS2 particles without any post processes. The physical, elemental, optical and electrical properties of the vertical SnS layers were systematically investigated. The vertically standing SnS layers were grown onto a Si substrate. Due to the intrinsic p-type semiconducting property of SnS, the heterojunction was simultaneously formed by the spontaneous growth of SnS on the n-type Si. The type-II heterojunction device exhibited photovoltaic performances with a high open-circuit voltage of 300 mV.


image file: c7nr03370b-f1.tif
Fig. 1 (a) Wafer-scale (∅ 3 inch) growth of vertical standing SnS layers on a Si substrate. Cross-sectional and topographic FESEM images for SnS grown at (b, d) 25 °C and (c, e) 300 °C, respectively. Energy dispersive spectrum analyses are presented by elemental percentage values as shown in (d) and (e). (f) HRTEM image of the SnS sample prepared at 300 °C to indicate the clear SnS layers, with an interlayer distance of 0.56 nm, electron diffraction pattern including d-spacing and hkl indices shown in the inset. (g) The crystal structures of orthorhombic SnS are shown using the crystallographic information file (CIF, COD-9008785.cif). Gray and yellow spheres represent the Sn and S atoms, respectively. The interlayer space was theoretically calculated by 5.59 Å.

As a photodetector, the p-SnS/n-Si device was extremely responsive to a tiny signal power light. The vertically standing SnS layers demonstrated high photoreactive performances, which confirmed that the functional design of the 2D material is a route for enhanced photoelectric devices, such as photodetectors and solar cells.

2. Experimental

2.1 Growth of SnS layers

Large area SnS2 and SnS films (∅ 3 inch) were deposited using the amorphous SnS2 target material (Target: SnS2, 99.999%, iTASCO, TSNALT0027, ∅ 2 inch). To form the SnS layer, the substrate temperature was kept at 300 °C with a SiC heater to reduce the sulphur from the sputtered SnS2 particles. During the sputtering, an RF power of 50 W was applied to the SnS2 target in the presence of flowing Ar gas (30 sccm), while a deposition pressure of 6 mTorr was maintained for all the samples. A base pressure of 3 × 10−6 Torr was achieved prior to flowing the Ar gas. The prepared samples of SnS2 and SnS were free from post-treatments. The deposition time of 10 minutes was kept constant for all the samples. Crystalline (001) Si wafers and glass were used as substrates after they were cleaned by a sequence of isopropyl alcohol, acetone, and distilled water under ultra-sonication and then dried under flowing nitrogen gas.

2.2 Fabrication of the devices

Intrinsic p-type SnS layers were deposited on an n-Si wafer for the p–n heterojunction fabrication. The 100 nm thick SnS film served as a photoactive layer (light absorbing). The top ITO layer (∼300 nm) was deposited at 300 °C using ∅ 4-in DC magnetron sputtering. A DC power of 3.7 W cm−2 was applied to the ITO target under an Ar flow condition of 50 sccm. The back Al (200 nm) electrode was deposited at room temperature using DC sputtering. The photoelectric device had a structure of ITO/SnS/n-Si/Al. These devices were isolated using a diamond stylus from the front (ITO) contact. The isolated inner geometry acted as an active area of the device.

2.3 Sample characterization

The cross section and morphology of the SnS2 and SnS layers were studied using a field emission electron microscope (FESEM, JEOL, JSM_7800F). A secondary electron imaging (SEI) detector was used for featuring the secondary electrons emitted from very close to the specimen (SnS) surfaces. The elemental composition of the films was determined by energy dispersive spectroscopy (EDS) attached to the SEM with an accelerating voltage of up to 20 kV. Transmission electron microscopy (TEM, TALOS F200X) with TEM point and lattice resolutions of 0.25 nm and 0.1 nm, respectively, were utilized. Grown layers of SnS were dispersed into an ethanol solution using ultrasonication for 30 minutes. This solution was dropped onto Cu grids for TEM observation. The phases of tin sulphide were examined by Raman spectroscopy (confocal Raman imaging, WITec alpha300) with an excitation wavelength of 532 nm (Laser: helium–neon) with a spatial resolution of <0.3 μm and spectral resolution of <0.01 nm. The transmittance, absorbance and reflectance characteristics of the samples were characterized by a UV-visible diffused reflectance spectrophotometer (Shimadzu, UV-2600) in the range of 300 nm to 1100 nm. The electrical properties of SnS samples were characterized by a Hall measurement system (Hall measurement system, AHT5573R, Ecopia) using the van der Pauw contact method.

2.4 Device characterization

A source measurement unit (SMU, Keithley, 2440) coupled with a solar simulator (AM1.5, McScience, K3000) and a photovoltaic power meter (McScience, K101) was used to record the JV characteristics under varying sunlight and in the dark. The top ITO layer and back Al contact were connected to the positive and the negative terminals of the SMU, respectively. The carrier collection efficiency of the prepared photoelectric device was studied using a quantum efficiency measurement system (McScience, K3100) coupled with a monochromator (Oriel Cornerstone 130 1/8 m) and a lock-in amplifier (McScience, K102). The MS characteristics of the fabricated devices were studied using a potentiostat/galvanostat (ZIVE SPI, WonA Tech) at an AC amplitude of 10 mV and a frequency of 1 kHz with a sample interval of 50 mV from the reverse bias to the forward bias scan direction in the DC bias from −1.5 V to 1.5 V. The transient photoresponse of the device was studied by the chronoamperometry method under pulsed monochromatic light (wavelength 850 nm) by varying the bias and light intensity. A function generator (MFG-3013A, MCH Instruments) was applied to the light source. Light intensity was calibrated using a power meter (KUSAM-MECO, KM-SPM-11).

3. Results and discussion

Cross-sectional FESEM images of the samples were prepared at 25 °C and 300 °C as shown in Fig. 1b and c, respectively, and reveal the morphological transformation from the compact film to vertically oriented layers. These samples were prepared on a (001) plane oriented crystalline Si wafer. The state-of-the-art large area sputtering process provided uniform and compact amorphous SnS2 films (Fig. 1b and d). Meanwhile, the SnS layers were formed at an increased temperature of 300 °C (Fig. 1c and e). The EDS investigation is summarized in Fig. 1d and e to show Sn%, S% (atomic), and S/Sn. The Sn to S ratio was decreased from 1.912 to 1.092, which indicates the sulphur reduction from the sputtered SnS2 to the SnS. Surface morphology and cross-sectional images show 2D SnS layers with an average thickness of 22 nm. SnS layers were continuously grown vertically to have a height of 400 nm and a width of 500 nm. The vertically aligned SnS nanoscale sheets have an enhanced surface which is an advantage for light interaction.

The vertical layers of SnS were analysed by high-resolution TEM (HRTEM) multiple SnS layers with an interlayer distance of 0.56 nm, as shown in Fig. 1f. The estimated interlayer spacing closely matches the ideal interlayer spacing (5.59 Å) in the lattice b direction of an orthorhombic SnS unit cell (mineral, herzenbergite; space group, Pbnm (62)) as shown in Fig. 1g. The electron diffraction pattern (the inset of Fig. 1f) clearly confirms the crystalline nature of the SnS layers to form nanoscale sheets. The calculated d-spacing values of 1.93 Å, 2.73 Å, 2.85 Å, 4.04 Å, and 5.58 Å correspond to the (002), (400), (111), (110), and (200) hkl planes, respectively. These d-spacing values match well with the ideal X-ray diffraction patterns of COD-9008785.cif (Table S1 of ref. 7).

To elucidate the phases, these samples were examined by Raman spectroscopy at room temperature using an excitation wavelength of 532 nm, as shown in Fig. 2. Raman spectroscopy of the SnS sample emerged as a strong phase characterization technique because of strong vibrational properties.7,17,18,27,29–34 A photograph of the samples prepared on Si is shown in the inset of Fig. 2. The Raman spectra of these samples clearly indicate the phase transformation of the deposited materials. The room temperature deposited film is composed of SnS2 and Sn2S3 phases, corresponding to the observed Raman peaks around 305–315 cm−1. The sputtered SnS2 particles are converted into the vertical layers of SnS with an orthorhombic crystal symmetry.7 Raman spectroscopy of the vertical SnS layers exhibited Raman peaks at 92, 150, 176, 191, 222, and 288 cm−1, which were well matched with those of the high purity SnS samples. The observed peaks were assigned to the one Ag, two B3g (LO), two Ag (LO) and one B2g modes, respectively.17,27–29,31,32,34 Similar to the high purity SnS samples, the vertically aligned SnS layers exhibited an intense peak at 92 cm−1, which is in good agreement with various scattering geometrical dependents.27,29 In the planar shape of SnS, the peak intensity at 92 cm−1 was weaker than the peak at 190.7 cm−1. Meanwhile, the vertically aligned SnS has a stronger peak at 92 cm−1 than at 191 cm−1.27 This may be due to the Raman wave-packets appearing at deviated Raman peak positions (Ag and B3g) for the vertically aligned SnS layers.7,27,29 The minor humps around 305–315 cm−1 indicate the presence of SnS2/SnS with ordered superstructures.28 Thus, small shifts of SnS and SnS2 modes with respect to the signals of bulk materials may have originated from the charge transfer from the SnS2 layer to the SnS layer with the misfit strain.28


image file: c7nr03370b-f2.tif
Fig. 2 Raman spectrum measured at the room temperature. The inset shows photographs of the prepared samples of SnS2 and SnS on a Si wafer.

The sulphur reduction process is the Sx depletion to cause the phase transformation from SnS2 to SnS. At an enhanced temperature of 300 °C, the Sx depletion is caused from the SnS2 dissociation due to the sublimation of SnS2.35 The thermodynamic reaction of SnS2 in a Knudsen cell suggests a melting in SnS2 above 250 °C, which induces the change in enthalpy (ΔH°) of 1.68 kcal mol−1.36

Fig. 3a shows the transmittance (T) characteristics for investigating the optical properties of these tin sulphide samples deposited on glass substrates. The SnS2 sample exhibited a broad T profile from the visible to the NIR range. Meanwhile, the SnS sample presents low T values. This significant change in T values is due to the structural and morphological transformation of the SnS2 film into SnS layers. The inset image in Fig. 3a clearly shows a different transparency. The SnS2 sample is evidently semi-transparent and the SnS sample is too dark to absorb the visible light. Fig. 3b confirms the strong absorbance characteristics of the vertical SnS layers for visible as well as NIR photons. The vertically aligned SnS layers showed saturated absorbance tendencies signifying a strong absorption coefficient (α), which can be estimated by the relation, image file: c7nr03370b-t1.tif, where d, R and λ are the thickness of the film, the reflectance, and the photon wavelength, respectively. In Fig. 3b, the saturated and transitional absorption tendencies specify the photon absorption features taken by the surface and the bulk part of the vertical SnS layers, respectively, on a base-film.37 Therefore, vertical SnS layers on a base-film with an anisotropic optical property exhibit stronger light absorption tendencies than horizontally oriented layers.


image file: c7nr03370b-f3.tif
Fig. 3 Optical properties of tin sulphide samples prepared on a glass substrate. (a) Transmittance (the inset shows a photograph of the SnS2 and SnS samples), (b) absorbance, and (c) Tauc plot of SnS2 and SnS films. (d) Reflectance spectra.

The Tauc plots of SnS and SnS2 samples were obtained to measure the direct optical band gap (Eg) (Fig. 3c). The Eg values of the amorphous SnS2 film and SnS layers were 2.12 eV and 1.6 eV, respectively. The Eg value (1.6 eV) of SnS was higher than conventionally reported values (1.3 to 1.45 eV). This difference is due to the nanoscale dimensions of the vertical SnS layers.7,32,38,39 To investigate the reflection profiles, the vertical SnS layers were grown on a Si substrate and a glass substrate, as shown in Fig. 3d. As clearly observed, the reflection effect of Si is inevitable. The ITO layer effectively reduced the high reflection of bare Si to introduce more photons into the Si. Moreover, the vertically grown SnS coating effect obviously surpassed the serious Si reflection. The overall R value was less than 5% in the UV-visible to NIR region. This clearly indicates that the vertical SnS layers efficiently drive the incoming photons and simultaneously serve as an antireflection entity.

The vertically grown SnS layers were investigated by Hall measurement and revealed p-type characteristics with an acceptor carrier concentration (NA) of 1017 cm−3 and a Hall mobility of 15 cm2 V−1 s−1 at room temperature. The hole–carrier mobility of 15 cm2 V−1 s−1 was obtained by a Hall measurement system. More accurate data can be achieved by a field effect transistor to reach the high mobility values of 4200 cm2 V−1 s−1.16

Considering the p-type SnS layers, an n-type Si wafer was adopted for the type-II heterojunction. The SnS-based photoelectric device had a structure of ITO/p-SnS/n-Si/Al, as shown in Fig. 4a. To provide a transparent conductor, the ITO film was applied as a top contact. The bottom contact was achieved by an Al metal layer. The top ITO contact and bottom Al contact were connected to the positive and negative terminal of the SMU, respectively. Initially, the current–voltage (JV) characteristics showed an Ohmic tendency because of the short circuit between the top ITO with the bottom Al contact during the ITO deposition process, as shown in Fig. S1. To resolve this, a diamond stylus was used to isolate the ITO surface, which also defined the active device area (Fig. 4b). After this electrical isolation, the device exhibited excellent dark JV characteristics. The topography of the ITO layer is presented in Fig. 4c. A cross-sectional SEM image (Fig. 4d) clearly presents the 100 nm-thick SnS layer between the Si substrate and the top ITO contact. Because of the layered topography of the SnS film, a three-dimensional TCO layer was formed, and it was different from the conventional planar ITO shape. The cross-sectional image also shows clear interfaces and a vertically standing SnS structure below the ITO layer.


image file: c7nr03370b-f4.tif
Fig. 4 (a) Schematic of the device with a structure of ITO/SnS layers/Si/Al. (b) The device under test showing the active area after the isolation using a diamond stylus. SEM images of the device (c) planar (ITO topography), and (d) cross-section highlighting the SnS layers sandwiched between ITO and a Si wafer.

In order to examine the junction properties of the ITO/p-SnS/n-Si/Al device, the JV characteristics were obtained under dark conditions (Fig. S2) and under light illumination (Fig. 5a). The light intensity was modulated from 0.1 mW cm−2 to 100 mW cm−2, and the corresponding photocurrent was increased from 20 μA cm−2 to a saturated 23 mA cm−2 at −1 V of bias condition. The logarithmic JV characteristics revealed a low dark saturation current density of 2.36 nA cm−2 (Fig. S2), and the diode operation regions are noted as the depletion space charge region (SCR), ideal diode, high injection, and series resistance regions. By plotting the light JV characteristics in logarithm, distinctive changes under light illumination conditions are clearly presented (Fig. 5b).


image file: c7nr03370b-f5.tif
Fig. 5 (a) Linear JV characteristics of the SnS/Si heterojunction device. (b) Logarithmic JV characteristics under varying light intensities under atmospheric conditions. (c) External quantum efficiency as a function of incidental photon wavelengths. (d) Spectral responsivity of the photoconductive SnS device at various constant bias potentials. (e) Energy band edges of c-Si and SnS layers. (f) Energy band diagram of the prepared device (type-II heterojunction) of the Si/SnS interface.

The photovoltaic operation was clearly observed to provide the photocurrent under an extremely low light intensity (0.1 mW cm−2), which resulted in a high photo-current to dark current density ratio (Jhv/Jd) of 200. The estimated Jhv/Jd value is certainly higher than 6000 at a light intensity of 100 mW cm−2 (1 sun).

This ITO/p-SnS/n-Si/Al device offered an open circuit voltage (VOC) of 300 mV for a 1 sun condition. This obvious photo-reactive performance is certainly of high interest for the vertical SnS layers of a photoelectric device. The JV curves plotted in the III–IV quadrants (Fig. S3) clarify the photovoltaic operation of this device.

The device possesses strong bias-dependent Jhv properties. The obtained Jhv is attributed to the strong optical absorption of the vertically grown SnS layers. However, the device offered negligible photoactivity with a larger forward bias than the VOC. In order to investigate the dependency of the light intensity, the device was characterized for the carrier collection performances. Fig. 5c shows the external quantum efficiency (EQE) of the ITO/p-SnS/n-Si/Al device, measured for λ from 300 nm to 1100 nm. By varying the bias potential value, the EQE profiles were changed, as shown in Fig. 5c. For the negative bias (−1 V), a strong bias effect was detected due to the extended SCR, which enhanced the QE values. However, the reduced SCR with increased forward biases readily caused the serious recombination effect, resulting in decreased QE values.

Overall, the SnS/n-Si junction possessed efficient EQE values for a broad λs from 550 nm to 1000 nm. Moreover, the higher EQE value (>72.5%) of around λ = 800 nm strongly suggests that the vertical SnS layers make an excellent NIR photoelectric entity. The photon wavelengths are very close to the energy value of 1.55 eV, which corresponds to the estimated Eg of the SnS layers. The lower EQE performance for the UV region can be explained by the strong surface absorptions of SnS. Even in the extremely low light intensity (10−10 W cm−2) of the EQE system (Fig. S4), the remarkably high carrier collection efficiency appeared to demonstrate the potential of the vertical SnS layers as light–matter entities. Further, the low light sensitivity of the photoconductive SnS device was evaluated by the spectral responsivity (R). Measured photocurrent as a function of photon wavelength was applied to compute the spectral R, given by the relation R = Ihv/Pin, where Ihv and Pin are the photocurrent and light intensity, respectively. Fig. 5d shows the spectral R of the SnS device at various constant bias potentials (−1 V to 0.3 V). The SnS photoconductive device revealed R values as high as 50 A W−1 in the NIR region at −1 V bias. This suggests that SnS layers have high sensitivity to the low intensity of photons, which is useful in the development of a low light sensitive photodetector.40 In order to understand the photoactive performance, the energy band edges of SnS (p-type) and Si (n-type) materials are shown in Fig. 5e, where Ec, Ev, Ef, and Δϕ are the conduction band, valence band, Fermi level, and work function difference, respectively. We can see that both materials have very similar electron affinity values resulting in a Δϕ of 1.2 eV. Hence, the intimate contact of the SnS layers to the n-Si established the band bending, as shown in Fig. 5f. As clearly presented, the conduction band is continuous at the interface, while the valence band has a barrier off-set (ΔEv) of 0.48 eV. Under light illumination, the incoming photons generate photo-generated carriers. Due to the higher potential of the conduction band, electrons flow from the vertical SnS layers to the n-Si side. Meanwhile, the holes move in the opposite direction. The electrons can be collected by the Al-back contact through the Si, and holes can be transported to the ITO-front contact through the SnS layers. Considering the value of Δϕ = 1.2 eV, ideally, this device can provide a VOC value as high as 1.2 V. This clearly indicates that the SnS/n-Si heterojunction device would provide a much enhanced output voltage from 0.3 V by improving the fill factor values and profiles.

In addition, the dark JV characteristics of the SnS/Si heterojunction device (Fig. 6a) had a rectifying ratio as high as 600. The energy band diagram for SnS/n-Si, as shown in Fig. 5f, indicates a nearly continuous conduction band and a band off-set of 0.48 eV of the valence band at the interface. Fig. 6b shows the MS characteristics measured at 1 kHz under dark conditions. The positive slope of A/C2vs. V attributed to the n-Si and linear intersection shows a flat band potential (∅bi) value of 0.4 V. This estimated value is much lower than the ideal 1.2 V (∼Δϕ). The surface states of the Si wafer and possibly of the SnS layer may have lowered the flat band potential. The donor concentration of the base n-Si wafer was estimated using the relation image file: c7nr03370b-t2.tif. Here εo, εr, q and A are the permittivity of free space, relative dielectric constant, free electron charge, and area of the device, respectively. The estimated ND value of 5.6 × 1014 cm−3 is in compliance with the Si wafer specifications. It is certain that the Si surface may have caused serious surface states. This can reduce the ∅bi and charge collection efficiency. However, in a reverse bias condition, photogenerated carriers eliminated the traps and exhibited an excellent photocurrent.


image file: c7nr03370b-f6.tif
Fig. 6 (a) Dark JV characteristics of the SnS/Si heterojunction device. The inset shows the linear JV curves. (b) Mott–Schottky characteristics at the room temperature.

To investigate the photoresponse, the ITO/p-SnS/n-Si/Al device was illuminated by varying the light intensity and bias voltages. Fig. 7a shows the excellent photoresponses to a light wavelength of 850 nm (biased at −1 V), which is very close to the photon energy of the SnS band gap. Meanwhile, significantly different photoresponse profiles were obtained at zero bias to show spikes at the rise and fall edges (Fig. 7b). This observation indicates the presence of the surface states at the interface of the SnS layers and the Si substrate. Under a bias condition, these surface states have less effect on the charge carrier transport because of the high electric field and extended space charge region. Hence, a square shaped photoresponse was attained. In contrast, serious transitional spikes appeared under the zero bias operation, which reflected the surface states for charging and discharging steps. To analyse the ITO/p-SnS/n-Si/Al photodetector, the light intensity was modulated, as shown in Fig. 7c and d (Fig. S5).


image file: c7nr03370b-f7.tif
Fig. 7 Photoresponses of the SnS/Si heterojunction device for (a) photoconductance mode (−1 V) and (b) self-bias mode (0 V). Under light illumination (λ = 850 nm, 20 mW cm−2). Effect of light intensity on the transient photoresponses under (c) low intensity (10 μW cm−2 to 150 μW cm−2) and (d) high intensity (5 mW cm−2 to 20 mW cm−2).

The key parameters of a photodetector, the photoresponse ratio (Jhv/Jd), responsivity (R), detectivity, rise time (τr) and fall time (τf), were estimated and are summarized in Table 1. The details for estimating these parameters can be found elsewhere.41,42 The R value of 8.3 mA W−1 at a zero bias for λ = 850 nm signifies an important feature of the SnS layers, the detectivity value of 5.3 × 109 Jones with the remarkable τr and τf of 34 μs and 15.1 μs, respectively (Fig. S6). This remarkable speed is 1000 times faster than that of a SnS photodetector using horizontal layers (nanobelt based photodetector: τr of 36 ms & fall time 7 ms; nanoparticle based photodetector: τr of 0.27 s & fall time 0.4 s).17,18 This clearly indicates the significance of the anisotropic optical absorption and carrier transport in SnS layers. Under a biased operation, R and D increased with a value of 0.25 A W−1 and 1.3 × 1011 Jones, respectively. Remarkably, the device exhibited a high photoresponse ratio of 462.8 for 20 mW cm−2 intensity of NIR light (850 nm) due to the significantly high photocurrent with a very small dark current. The light intensity-dependent transient photoresponse revealed an ultra-sensitivity for a tiny light intensity (10 μW cm−2), with excellent R and D of 1.19 A W−1 and 7.1 × 1011 Jones, respectively.

Table 1 The figure of merit parameters of the SnS/Si heterojunction device. The wavelength of the light source was 850 nm
Condition   J hv/Jd R (A W−1) D (Jones) τ r (μs) τ f (μs)
Bias 20 mW cm−2 −1 V 462.8 0.25 1.3 × 1011 41 40.7
0 V 21.4 0.0083 5.3 × 109 34 15.1
Light intensity (mW cm−2), −1 V 0.01 1.3 1.19 7.1 × 1011 222 80
0.05 4.1 0.78 4.5 × 1011 80 82.7
5 161.9 0.29 1.7 × 1011 40.5 40.7
20 522.1 0.25 1.4 × 1011 40.4 40.5


The linear dynamic range (LDR) measurement was carried out to investigate the linearity of the SnS photodetector by LDR (dB) = 20 log10(Pmax/Pmin), where Pmax and Pmin are the highest and the lowest power, respectively, according to the incident power.43,44 The LDR profiles were obtained under NIR light illumination with −1 V bias or self-power mode (zero bias). The LDR values are quite excellent to have 60 dB for the self-power mode and 72 dB at −1 V bias, which are equivalent for a 10 to 12 bit NIR photodetector.45 The self-power operation has the feature of photocurrent saturation for high light intensity (>20 mW cm−2), which is attributed to surface recombination. However, a small bias operation (−1 V) clearly resolves that. This signifies the potential of SnS 2D layers beyond graphene.40,43,46

In order to investigate the stability, the SnS photodetector has been used to measure the photocurrent under atmospheric conditions. The pulse-shape NIR light (λ = 850 nm, pulse duration 0.423 s) was illuminated for 200[thin space (1/6-em)]000 cycles, as shown in Fig. 8b. We can find the consistent photocurrent density values to confirm the excellent stability of the SnS photoreaction.


image file: c7nr03370b-f8.tif
Fig. 8 (a) Light intensity dependent photocurrent profiles of the SnS photodetector. (b) Photocurrent stability of the SnS photodetector by NIR light for 200[thin space (1/6-em)]000 cycles measured under atmospheric conditions at zero bias.

This study showed excellent photoactive performances of vertically grown SnS layers, which confirms their potential for photoelectric devices, including photodetectors and photovoltaic cells.

4. Conclusions

In summary, we reported wafer-scale grown SnS layers for improved photoelectric applications. High quality and vertically standing SnS layers were directly achieved by the sputtering method without any post processes. We confirmed that the solid-state reaction of sulphur reduction is a clear and stable approach for large-scale SnS production. The phase purity, morphology, and optical and electrical properties of vertical SnS layers were comprehensively studied. Intrinsic p-type SnS layers were directly grown on n-type Si wafers for high-performing photoelectric devices. By applying the SnS layers on the surface, a significantly low light reflection (<5%) was achieved for the broad wavelengths due to the tiny size and vertical standing feature. Moreover, the quality and vertical contact of the SnS layers provided a remarkably fast photoresponse time. Interface engineering of the SnS/Si will provide further improvements for photodetectors and photovoltaic cells.

Acknowledgements

The authors acknowledge the financial support of the Basic Science Research Program through the National Research Foundation (NRF) of Korea by the Ministry of Education (NRF-2015R1D1A1A01059165), the Korea Research Fellowship Program through the NRF by the Ministry of Science, ICT and Future Planning (NRF-2015H1D3A1066311) and the National Research Foundation of Korea funded by the Korean government (MSIP) (2010-0027963).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr03370b

This journal is © The Royal Society of Chemistry 2017