High detectivity and responsivity in black phosphorus/SnS₂ heterostructure with broken-gap energy band alignment

van der Waals heterostructures (vdWHs) based on atomically two-dimensional materials have great potential in the next generation of electronics and optoelectronic devices due to its wide variety of materials and the ultrathin morphology features. ^1–3) The library of 2D materials makes it possible to design heterostructures by integrating proper material properties. Among them, black phosphorus (BP) has been studied as a promising candidates for future photodetection applications due to BP possess layer-dependent gap direct bandgaps (from 0.3 eV for bulk to 2.0 eV for monolayer), high carrier mobility (up to 1000 cm² V⁻¹ s⁻¹), and large coverage detection range. ^4–6) In addition, tin disulfide (SnS₂) is a promising candidate for field effect transistors (FETs), ⁷⁾ logic circuits, ^8,9) and photodetectors ^10,11) because of its appropriate indirect bandgap (2.08–2.44 eV), high optical absorption coefficient exceeding 10⁴ cm⁻¹, and large on/off ratio (>10⁶).

In general, three types of band alignments can be distinctly classified when different 2D materials are stacked together, including: straddling gap (type-I), ¹²⁾ staggered gap (type-II) ^13,14) and broken-gap (type-III). ^15–17) Thus far, most reported photodetectors have been realized with type-II band alignment to utilize the excellent electron–hole pair separation ability in heterostructures. ¹⁸⁾ However, the photodetectivity of photodetectors is typically limited by either low photoresponsivity or a high dark current. ^19,20) For example, photovoltaic device based on BP/MoS₂ vdWHs can reach a high ${I}_{{\rm{p}}{\rm{h}}}/{I}_{{\rm{d}}{\rm{a}}{\rm{r}}{\rm{k}}}$ ratio of ≈10³, while the photoresponsiviy is only ≈1 m W⁻¹. ²¹⁾ Recently, tunneling field effect transistors (TFETs) based on type-Ⅲ vdWHs have shown great potential in the field of low-power optoelectronic devices due to their low OFF-state current, which may exhibit excellent performance in photodiodes. ^22–24)

In this work, we report a high photodetectivity and photoresponsivity photodetector based on a BP/SnS₂ vdWHs. The BP/SnS₂ vdWHs shows strong interlayer coupling verified by Raman, which promotes highly efficient charge transfer in the channel. Thus, the BP/SnS₂ vdWHs exhibits ultrahigh photodetectivity of 6.72 × 10¹² Jones (${I}_{{\rm{p}}{\rm{h}}}/{I}_{{\rm{d}}{\rm{a}}{\rm{r}}{\rm{k}}}$ ratio of ≈2 × 10³) and high photoresponsivity of 295.3 A W⁻¹. The dark-carrier tunneling was highly suppressed by the depletion region of the junction whereas the photocarrier tunneling was efficiently improved by the high direct tunneling (DT) current when illuminated, which has been confirmed by the energy band structures and electrical characteristics fitted with DT. The experiments demonstrate that BP/SnS₂ vdWHs possess type-III (broken-gap) band alignment, indicating their application in TFETs devices and low-power photodetector.

2.1. Device fabrication

2.2. Device characterization

Figure 1(a) shows a schematic illustration of the BP/SnS₂ heterostructure device. Few layers p-type BP and n-type SnS₂ flakes, mechanically exfoliated from their bulk materials, were transferred onto a 300 nm SiO₂/Si substrate. Figure 1(b) presents a typical scanning electron microscopy image of the heterostructure device. Multiple metal electrodes were fabricated on the top of BP and SnS₂ regions by EBL and subsequent deposition of 10 nm/50 nm Cr/Au. As shown in Fig. 1(c), the thicknesses of the BP and SnS₂ flakes were determined to be 8 nm and 16 nm by atomic force microscopy, respectively and the inset shows the surface morphology. The Raman spectra of BP/SnS₂ heterostructure and isolated BP and SnS₂ was shown in Fig. 1(d). For the isolated BP region, the BP shows three dominant peaks at ${{\rm{A}}}_{{\rm{g}}}^{1}$ (436.5 cm⁻¹), ${{\rm{B}}}_{2{\rm{g}}}$ (358.8 cm⁻¹) and ${{\rm{A}}}_{{\rm{g}}}^{2}$ (463.7 cm⁻¹), which is consistent with previous studies. ^11,25) While for the isolated SnS₂ region, the SnS₂ shows one main peak at ${{\rm{A}}}_{{\rm{g}}}^{1}$ (313.4 cm⁻¹). All these peaks are contained in the spectra of BP/SnS₂ overlapped region, indicating the successful formation of BP/SnS₂ heterostructure. Figures 1(e), 1(f) is the transmission electron microscopy (TEM) imaging and the energy dispersive X-ray specteometry (EDS) elemental mapping of the heterostructure of the device. According to the TEM imaging, the interface of all layers is clear and flat. Furthermore, the EDS analysis of O elements demonstrates that the BP/SnS₂ heterostructure is not oxidized. In order to prevent the oxidation of BP, Liu' group synthesized the Te-doped and S-doped BP, which change the conduction band minimum (CBM) shifting close to the redox potential of O₂/O₂ ⁻, and suppresses the oxidation process of BP. ^6,26)

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In order to quantitatively probe the charge transfer at the heterostructure, we measured a series of electrical characteristics in the same device. Figures 2(a), 2(b) shows the transfer characteristics of individual BP and SnS₂ channel FET, indicating typical p-type (threshold gate voltage ${V}_{{\rm{t}}{\rm{h}}}$ of ≈−20 V) and n-type (threshold gate voltage ${V}_{{\rm{t}}{\rm{h}}}$ of ≈1 V) characteristics, ^8,27,28) respectively. All electrical measurements were performed under ambient conditions (room temperature of 300 K). The nearly linear output curves observed for BP FET under different gate voltages shows that the contact resistances between the Cr/Au metal electrode and the two materials are very low, ^27,29) but SnS₂ FET has a small rectification ratio due to work function mismatch in the insets of Figs. 2(a), 2(b). It is worth noting that we achieved sufficiently high on/off ratios (10⁶) in SnS₂ FET with thin flakes and they operated in the depletion-mode. However, the thicker BP FET has a high dark current. In general, the on/off ratio of a depletion transistor is limited by the thickness of the channel material, as the distance between the gate oxide surface and the channel increases, the gate field gradually weakens or even disappears completely. In other words, a large leakage current flows through the expanding undepleted portions as the channel thickness increases, resulting in a poor on/off ratio. ³⁰⁾ Majority carrier concentrations per unit area for the respective channels (hole for BP, electron for SnS₂) at V_g = 0 V are estimated by ³¹⁾ ${p}_{{\rm{B}}{\rm{P}}}\left({n}_{{\rm{S}}{\rm{n}}{{\rm{S}}}_{2}}\right)$ = ${q}^{-1}{C}_{g}| {{V}}_{{\rm{th}}}|$ = 1.54 × 10¹² cm⁻² (7.7 × 10¹¹ cm⁻²), where q = 1.6 × 10^–19 C, ${C}_{g}$ = ${C}_{{\rm{S}}{\rm{i}}{{\rm{O}}}_{2}}$ = 1.23 × 10^–8 ${\rm{F}}\,\cdot \,{{\rm{cm}}}^{-2}$ for 300 nm SiO₂ layer. Figure 2(c) shows the band alignment of the BP/SnS₂ heterostructure before and after contact. The CBM (${E}_{C}$) and valence band maximum (${E}_{V}$) values of BP (SnS₂) were previously reported to be −4.2 eV (−5.2 eV) and −4.6 eV (−7.4 eV), respectively. ^17,32,33) Among these, BP and SnS₂ can be calculated to be broken-gap type-III with the conduction band of SnS₂ a little lower than the valence band of BP.

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To examine the carrier transport through the BP/SnS₂ vdWHs under different bottom gate biases, a ${V}_{{\rm{d}}{\rm{s}}}$ of up to 0.5 V was applied to the electrode on the BP side, and the electrode on SnS₂ was grounded. As shown in Fig. 3(a), the forward current rapidly increases to several tens of nA, while the reverse current reach to order of pA, showing clear rectification characteristics. We further studied the forward current transmission to investigate the rectification behavior and found two unique transmission mechanisms that can explain the transmission characteristics of our device, i.e. DT or Fowler–Nordheim tunneling (FNT), ⁹⁾ as shown in the I–V curve replotted in Fig. 3(c). The I–V curve can be modeled based on a tunneling barrier with the Simmons approximation, ^20,34) where DT occurs at low voltage and FNT dominates at higher voltage. The DT and FNT can be expressed by the following relation:

$$\begin{eqnarray}&&{I}_{{\rm{D}}{\rm{T}}}\propto V\exp \left(-\displaystyle \frac{4\pi d\sqrt{2m* \varphi }}{h}\right),\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{I}_{{\rm{F}}{\rm{N}}{\rm{T}}}\propto {V}^{2}\exp \left(-\displaystyle \frac{8\pi d\sqrt{2m* {\varphi }^{3}}}{3heV}\right)\,,\end{eqnarray} \tag{ 2 }$$

where $\varphi$ is the barrier height, d is the tunneling barrier width, h is the Plank constant and $m*$ is the effective mass of an electron. From the above Eqs. (1) and (2), we draw the Fowler–Nordheim plot, i.e. $\mathrm{ln}\,\left(I/{V}^{2}\right)$ and $1/V$ plot, as shown in Fig. 2(c). This plot clearly shows the transition from a logarithmic regime at small ${V}_{{\rm{d}}{\rm{s}}}$ for DT to a linear regime with a negative slope at high ${V}_{{\rm{d}}{\rm{s}}}$ for FNT, representing region I (blue) and region II (orange), respectively. The threshold voltage of FNT ${V}_{{\rm{t}}{\rm{h}}}$=1/8 = 0.125 V corresponds to $\varphi /e,$ indicating that the tunneling barrier is about 0.125 eV. According to the electrical characteristics, the band diagrams of reverse bias, small forward bias and forward bias when the gate voltage is zero are respectively shown as Fig. 4(d). Under reverse bias, due to the depletion of holes in BP and the accumulation of electrons in SnS₂, the extremely low measured current results in almost no charge flowing through the heterostructure. At small forward bias (${V}_{{\rm{d}}{\rm{s}}}$ < 0.125 V), the high trapezoidal tunneling barrier at the interface dominates the charge transfer of DT. While, under a larger forward bias voltage (${V}_{{\rm{d}}{\rm{s}}}$ > 0.125 V), a lower triangular tunneling barrier at the interface appears with FNT. Moreover, we investigated the impacts of SnS₂ channel length on the electrical characteristics of BP/SnS₂ heterostructures. As shown in the Fig. 3(b), the device composed of 12 μm SnS₂ channel shows a normal forward diode characteristic, that is, current in forward ${V}_{{\rm{d}}{\rm{s}}}\,$is much larger than that in reverse ${V}_{{\rm{d}}{\rm{s}}}.$ When the SnS₂ channel decreases to 4 μm, the forward current has a significant increase compared to the case of 12 μm SnS₂ channel. Simultaneously, the reverse current keeps constant at a small ${V}_{{\rm{d}}{\rm{s}}}$ and then increases dramatically at a critical reverse voltage of about −0.15 V, which matches the performance of Zener diode. ³⁵⁾ Here, the case with 8 μm SnS₂ channel is considered to be a middle state transformed from forward diode to Zener diode. In our case, the negative differential resistance phenomenon was obscured for two reasons: one is that the diffusion current and drift current are enhanced by the thermal energy at the room temperature; ³⁶⁾ the other is that the junction region needs to be composed of heavily doped p-type regions and heavily doped n-type regions. ¹⁵⁾

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The widths of the depletion layers at the BP and SnS₂ sides can be estimated by using the depletion model implemented on the conventional p–n heterostructures ³⁴⁾

$$\begin{eqnarray*}&&{x}_{p}=\sqrt{\displaystyle \frac{2{N}_{d}{\varepsilon }_{1}{\varepsilon }_{2}{V}_{{\rm{b}}{\rm{i}}}}{q{N}_{a}\left({\varepsilon }_{1}{N}_{a}+{\varepsilon }_{2}{N}_{d}\right)}}\end{eqnarray*}$$

and

$$\begin{eqnarray*}&&{x}_{n}=\sqrt{\displaystyle \frac{2{N}_{a}{\varepsilon }_{1}{\varepsilon }_{2}{V}_{{\rm{b}}{\rm{i}}}}{q{N}_{d}\left({\varepsilon }_{1}{N}_{a}+{\varepsilon }_{2}{N}_{d}\right)},}\end{eqnarray*}$$

where ${N}_{d}$ = 7.7 × 10¹⁷ cm⁻³, ${N}_{a}$ = 5.9 × 10¹⁷ cm⁻³, calculated by ${N}_{d(a)}$ = n(p)/${t}_{{\rm{S}}{\rm{n}}{{\rm{S}}}_{2}\left({\rm{B}}{\rm{P}}\right)},$ from hole and electron doping concentrations and thicknesses of BP and SnS₂ with $n\left(p\right)\,\approx \,$7.7 × 10¹¹ (1.54 × 10¹²⁾ cm⁻², ${t}_{{\rm{S}}{\rm{n}}{{\rm{S}}}_{2}\left({\rm{B}}{\rm{P}}\right)}\,\approx \,$13(20) nm. ${\varepsilon }_{1}$ = 17.7 and ${\varepsilon }_{2}$ = 7 are the dielectric constants of SnS₂ and BP, ^17,37) respectively, and the ${V}_{{\rm{b}}{\rm{i}}}\,\approx$ 0.125 V is the built-in potential at the junction extracted from the FNT curve above. Consequently, ${x}_{p}$ $\left({x}_{n}\right)$ was calculated to be ≈ 4 (3.5) nm，as shown in Fig. 2(c).

Considering that the energy band structure is different from the familiar type-II p–n diodes, we have carried out photoresponsivity measurements to further explore the optoelectrical performance. Figure 4(a) illustrates the BP/SnS₂ based vdWHs photodetector. The monochromatic light emitted by the laser sheds on the photodetector, generating a large number of electron–hole pairs in both the SnS₂ and BP layers. These light-induced charge carriers tunnel is ultralow due to the depletion region between the BP/SnS₂ heterostructure. When the device is under illumination, photon-excited carriers are generated in the 2D semiconductors, and these charge carriers can drift across the junction to generate photocurrent, resulting in a large ${I}_{{\rm{p}}{\rm{h}}}/{I}_{{\rm{d}}{\rm{a}}{\rm{r}}{\rm{k}}}$ ratio of up to 2 × 10³ at a reverse bias of −64 V as shown in Fig. 4(b).

Figure 4(c) demonstrates the ${I}_{\mathrm{ph}}-{V}_{\mathrm{ds}}$ at ${V}_{g}$ = 0 V under optical irradiation (=365 nm) with various incident light powers. The photocurrent ${I}_{\mathrm{ph}}$ is defined as ${I}_{\mathrm{illumination}}-{I}_{\mathrm{dark}},$ where ${I}_{\mathrm{illumination}}$ and ${I}_{\mathrm{dark}}$ are the ${I}_{\mathrm{ds}}$ with and without light illumination, respectively. In addition, the differences in the I−V curves were also visualized by plotting the current versus voltage (${I}_{{\rm{d}}{\rm{s}}}$ versus ${V}_{{\rm{ds}}}$) on a logarithmic scale [Fig. 4(d)]. When ${{V}}_{{\rm{ds}}}$ is negative value, the current shows a significant increase under light, indicating that the junction has a strong light response. Figure 4(e) plots the relationship between the light responsivity ($R={I}_{{\rm{p}}{\rm{h}}}/PS$) and ${V}_{g}\,$of the heterostructure under different illumination intensities, where $P$ is the illumination power, $S\,\,$is the effect response area and ${I}_{{\rm{p}}{\rm{h}}}$ is the photocurrent. Moreover, the data of ${I}_{{\rm{p}}{\rm{h}}}$ is obtained from Fig. 4(c), and the high photoresponsive performance can be obtained at ${V}_{g}$ = −16 V (295.3 A W⁻¹). Another important parameter of photodetector is photodetectivity, which is described by the following equation $D* =\,R\,{S}^{1/2}/{\left(2e{I}_{{\rm{d}}{\rm{a}}{\rm{r}}{\rm{k}}}\right)}^{1/2},$ showing the ability to detect weak light signals. Figure 4(f) shows the photoresponsivity $R$ and photodetectivity $D*$ as a function of ${P}_{{\rm{i}}{\rm{n}}},$ at ${V}_{{\rm{d}}{\rm{s}}}$ = 0.5 V. It is worth noting that type-III band-aligned photodiode structure has a larger bandgap, which will produce a high potential barrier, thereby suppressing the generation of dark current and the photodetectivity can be estimated to be $\approx$6.72 × 10¹² Jones under a light intensity of 0.53 mW cm⁻², which is 10–1000 times higher than that of other SnS₂-based photodetector. ^10,38,39)

In conclusion, we have demonstrated a highly sensitive photodetector using vdWHs with broken-gap energy band alignment composed of BP and SnS₂. The BP/SnS₂ vdWHs shows strong interlayer charge coupling verified by Raman. Multifunctional diode is achieved by changing the channel length of SnS₂, which results from the increase of effective drain bias voltage. Furthermore，the BP/SnS₂ vdWHs exhibits ultrahigh photodetectivity of 6.72 × 10¹² Jones (${I}_{{\rm{p}}{\rm{h}}}/{I}_{{\rm{d}}{\rm{a}}{\rm{r}}{\rm{k}}}$ ratio of $\approx \,$2 × 10³) and high photoresponsivity of 295.3 A W⁻¹. The dark-carrier tunneling was highly suppressed by the depletion region of the junction whereas the photocarrier tunneling was efficiently improved by the high DT current when illuminated，which has been confirmed by the energy band structures and electrical characteristics fitted with DT.

This work was supported by the National Key R&D program of China (Grant No. 2019YFB2005600), the National Natural Science Foundation of China (Grant No. 51732010, 51802341), the China Postdoctoral Science Foundation (Grant No. 2020M671592).