Solar-blind deep-ultraviolet photodetectors based on solution-synthesized quasi-2D Te nanosheets

1 Introduction

The discovery of graphene, the first 2D material, in 2004 [1], led to 2D materials attracting widespread attention because of their unique photoelectric properties, such as strong light absorption [2], [3], [4], high light sensitivity [5], [6], fast light response speed [7], [8], [9], and wide light response range [10], [11]. Recently, tellurium, a new 2D material, which is a member of the chalcogen element family and a well-known p-type semiconductor with a narrow band gap energy of 0.35 eV at room temperature exhibiting intriguing properties, has attracted the attention of scientists [12], [13], [14], [15], [16]. Ye et al. demonstrated that solution synthesized air-stable quasi-2D Te nanosheets (Te NSs) can be applied as the basic materials for field-effect transistors (FETs) [17]. The prepared FETs exhibited air-stable performance at room temperature for more than 2 months, on/off ratios in the order of 10⁶, and field-effect mobilities of approximately 700 cm²/V s. Then, Wang et al. reported a high photoresponsivity and flexible photodetector based on van der Waals epitaxy synthesized 2D hexagonal Te nanoplates on a flexible mica substrate for the first time; the fabricated photodetector exhibited excellent stability and photoresponsivity that reached 389.5 A/W [17]. Amani et al. exhibited short-wave infrared photodetectors based on solution-synthesized environmentally stable quasi-2D Te nanofilms, the peak photoresponsivity of which can be adjusted from 1.4 μm (13 A/W) to 2.4 μm (8 A/W), and up to 3.4 μm [18]. Recently, Xie et al. demonstrated a high photoresponse electrochemical photodetector based on liquid phase exfoliation synthesized 2D nonlayered Te NSs for ultraviolet (UV) (350 nm) and visible light (475 nm) [19]. This indicates that despite the band gap of Te NSs being 0.35 eV, it can obtain UV light response.

Further, UV radiation of wavelength shorter than ~280 nm from sunlight cannot penetrate the atmosphere and reach the surface of the earth; 200–280 nm is typically referred to as the solar-blind spectrum region. Benefiting from advances in the semiconductor industry in the past few decades [20], [21], UV light detection, specifically solar-blind deep-ultraviolet (DUV) photodetection, has become an important subject in recent years because of its significant applications in various fields, including remote control, chemical analysis, flame detection, missile warning system, and secure space-to-space communication [22], [23], [24], [25], [26]. Recently, the optical absorption coefficients of bulk β-Te and few-layer β-tellurenes were systematically calculated by Wu et al. [27]. Moreover, layer-dependent absorption of layered β-tellurenes has been obtained, and it has been found that absorption of few-layer β-tellurenes begins at 175 nm to the visible region. In particular, the peak of absorptivity occurs in blue-violet visible regions. All these results indicate that layered β-Te is suitable for DUV light detection.

In this study, the optical and optoelectronic properties of solution-synthesized quasi-2D Te NSs are experimentally studied. Both the absorption and the photoresponse results confirm that quasi-2D Te NSs are solar-blind DUV sensitive materials. The experimental results demonstrate that the Te NS-based FETs exhibit an ultrahigh responsivity (R) of 6.5×10⁴ A/W, a remarkable external quantum efficiency (EQE) of 2.26×10⁶%, and a high detectivity (D^*) of 3.73×10⁸ Jones in the solar-blind band (261 nm), which indicate the great potential of Te NSs for high-performance solar-blind DUV photodetectors.

2 Experimental section

3 Results and discussion

Quasi-2D Te NSs, with a thickness of approximately 20 nm, were synthesized (with a small modification) via the facile hydrothermal method based on a former report by Ye et al., the details of which can be found in the Experimental section [17]. The crystal structure of Te, as shown in Figure 1A and B, is highly anisotropic, and the chains of Te are oriented along the c-axis and band together by strong van der Waals forces. Each Te atom shares covalent bonds with its nearest atoms with a systematic arrangement in a helical chain [28]. Figure 1B shows the 3D structure of Te, viewed along the [001] direction; in addition, zigzag layers can be seen, which are banded via weak van der Waals forces. The scanning electron microscopy (SEM) image of Te NSs reveals irregular shapes of lengths in the order of tens of micrometers and widths of a few micrometers (Figure 1C). Figure 1D shows the transmission electron microscopy (TEM) image of a section of Te NSs, and the corresponding high-resolution TEM (Figure 1E) image shows the continuous crystal lattice of the synthesized Te NSs with a lattice constant of 0.196 nm, which can be assigned to the (003) plane of Te. The inset in Figure 1E shows the selected area electron diffraction (SAED) pattern of the Te NS, and the typical bright diffraction spots correspond to the (001), (110), and (013) planes of Te, which verify that the synthesized Te NS has a single-crystalline structure. Figure 1F shows the atomic force microscopy (AFM) image of a Te NS of thickness 18.9 nm. An obvious absorption peak located at 278 nm can be detected in the ultraviolet-visible (UV-Vis) spectrum (Figure 1G), which can be attributed to the direct transition from band to band. In addition, four vibrational modes at 92 cm⁻¹, 105 cm⁻¹, 121 cm⁻¹, and 141 cm⁻¹ can be found in the Raman spectrum (Figure 1H), which correspond to the E₁-TO, E₁-LO, A₁, and E₂ modes of Te NSs, respectively, and are consistent with previous reports [29], [30]. The X-ray diffraction (XRD) pattern in Figure 1I shows the characteristic peaks of the synthesized Te NSs, which are similar to those in the simulated reference (PDF No# 36-1452) [31]. The UV-Vis absorption and the Raman and XRD results confirm the successful synthesis of quasi-2D Te NSs.

Figure 1:

Crystal structure of Te viewed from the (A) x-axis and (B) z-axis. (C) SEM and (D) TEM images of Te NSs. (E) HR-TEM image and the corresponding SAED pattern of Te NSs. (F) AFM image of a typical Te NS of thickness 18.9 nm. (G–I) UV-Vis absorption, and Raman and XRD spectra of Te NSs.

Figure 2A shows a 3D schematic of the Te NS photodetector. In this image, the source/drain electrodes are in direct contact with the Te NSs, and the current is generated by the bias voltage (V_d) applied across the two electrodes, which flows through the Te NS channel and is modulated by the back-gate voltage (V_g) applied to the p+-doped Si substrate. The surface morphology of the Te NSs was measured by AFM. As shown in Figure 2B, the thickness of the few-layered quasi-2D Te NSs is approximately 20 nm. The I_ds-V_ds curves of the device in the dark condition were measured at various V_g values from –5 to 5 V (Figure 2C), indicating that the device current can be tuned using the back-gate voltage. The representative transfer curves (I_d-V_g) of the device at V_d=1 V are shown in Figure 2D, which indicate a carrier mobility of approximately 100 cm²/V s and an on/off ratio of 10 in the Te NS-based transistor.

The characteristics of the Te NS-based photodetectors were further studied by examining the photoconductivity effect under various illumination wavelengths of 261–405 nm. The performance of the UV photodetection of the Te NS-based photodetectors can be evaluated by the output current under illumination at different powers. Figure 3 shows the I-V curves of the Te NS photodetectors under different light excitation powers of 261 nm (0.08 nW, 0.12 nW, and 0.14 nW in the dark). Figure 3A shows that the photocurrent increases gradually with increasing illumination power density, which can be attributed to the increase in the number of photons from the incident light. Under intense illumination, more photogenerated electron-hole pairs are generated in Te, which are driven to the electrodes in different directions guided by the drain-source electric field, resulting in a larger channel current. The laser power-dependent photocurrent curves are shown in Figure 3B. The photocurrent displays a positively related dependence on the applied V_ds, implying that a larger photocurrent can be readily obtained by increasing V_ds. Figure 3C–E show the R, EQE, and D^* curves of Te NS photodetectors under different illumination powers, which can be calculated using the equations provided in the equation section in Supporting Information. Furthermore, R is an important parameter to evaluate the performance of photodetectors. Figure 3C shows that R decreases with the increase in illumination power. Remarkably, the responsivity of the Te NS photodetectors reached up to ~6.5×10⁴ A/W with an illumination power of 0.08 nW at 261 nm. The EQE reflects the ratio of the number of photon-excited charge carriers and incident photons per unit time. The Te NSs based on photodetectors of EQE for DUV illumination (261 nm) on the incident illumination power density are presented in Figure 3D, which reveals that the photodetector exhibits an EQE of 2.26×10⁶%, when P=0.08 nW and V_ds=3 V. The high EQE of the Te NS photodetectors, which exceeds 100%, can be attributed to the additional gain inside the quasi-2D Te NSs. The carrier transit time in Te was calculated to be 1.36 ns, using the formula τ=L²/(μV_ds), where L is the distance between source-drain separation (L=3.7 μm, μ=100 cm²/V s, V_ds=1 V), which is significantly less than the carrier lifetime of Te, according to the time-dependent photocurrent, as shown in Figure 3F (on the order of seconds). Therefore, a large internal gain exists in the Te NS photodetector, resulting in EQE>100%. In addition to R and EQE, D^* is another key factor for a photodetector that describes the detection limit for a weak illumination signal. Figure 3E presents the relationship of D^* with V_d of a Te NS photodetector under different illumination powers. When the power of a 261 nm light is 0.08 nW (V_ds=3 V), D^* is 3.73×10⁸ Jones. It can thus be concluded that the R and EQE of the Te photodetector in the solar-blind DUV band are highest among those of the previously reported similar detectors, as shown in Table S1; however, D^* is relatively low.

Figure 2:

The structure and electronic characterization of the Te NS FET.

(A) Schematic of the Te FET. (B) AFM image of the Te NSs photodetector (scale bar: 5 μm). (C) I_ds-V_ds curves of the device in the dark measured at various V_g from –5 to 5 V by applying a voltage on Si. (D) I_ds-V_gs curves of the device in the dark measured at V_d=1 V.

Figure 3:

Optoelectronic characterization of the Te NS photodetector by illumination at 261 nm.

(A) I_ds-V_ds curves of the Te NS photodetector obtained in dark and with illumination at various excitation intensities (0.08 nW, 0.12 nW, and 0.14 nW) at V_g=3 V. (B) I_p-V_ds curves of the Te NS photodetectors for different excitation powers with illuminations of 0.08 nW, 0.12 nW, and 0.14 nW, which exhibit a linear dependence on the bias (V_g=3 V). (C) Responsivity of the Te NS photodetector vs. light illumination power. (D) EQE of Te NS photodetector vs. light illumination power. (E) D^* of the Te NS photodetector vs. light illumination power. (F) I-t response curve under an illumination of 261 nm, P=0.14 nW, and V_ds=1 V. Inset: dynamic response of the Te NS photodetector during on/off of the incident light with a temporal resolution.

Based on the dynamic response of the device to the on/off condition of laser (Figure 3F and inset), the rising and falling times were measured as ~2 s and ~5 s, respectively. Moreover, the photodetection performance of the Te NS photodetector under the UV bands of 360 nm and 405 nm was further investigated, and the results are shown respectively in Figures S1 and S2. The results indicate that the device exhibits good power-dependent photoelectric response to lasers of 360 nm and 405 nm, as well as high R and EQE.

The results of power-dependent photocurrent are summarized in Figure 4 to further investigate the performance of the Te NS photodetector. As shown in Figure 4A, under an illumination of 261 nm, the photocurrent at V_d=3 V increases monotonically with the increase in illumination power from 0.08 nW to 0.14 nW. Moreover, R decreases from 6.5×10⁴ A/W to 3.9×10⁴ A/W, EQE decreases from 2.2×10⁶% to 1.3×10⁶%, and D^* decreases from 3.7×10⁸ Jones to 2.2×10⁸ Jones with the increase in illumination power from 0.08 nW to 0.14 nW. The decrease in R, EQE, and D^* with the increase in light power could be attributed to hole-trapping saturation. The corresponding parameters of the device under light illuminations of 360 nm and 405 nm are shown in Figures S3 and S4, respectively. The R, EQE, and D^* of the Te NS photodetector as functions of illumination power decrease linearly, and their trends are consistent with those at 261 nm.

Figure 4:

Photoresponse of the Te NS photodetector at 261 nm.

(A) Power dependence of photocurrent. (B) Responsivity vs. laser illumination power with an excitation wavelength of 261 nm. (C) EQE vs. laser illumination power with an excitation wavelength of 261 nm, which indicates the high sensitivity of the device. (D) D^* vs. laser illumination power with an excitation wavelength of 261 nm.

The long-term stability of photodetectors under ambient conditions is another important property for evaluating these devices [32], [33], [34], [35]. To evaluate the stability of the device under ambient conditions, the photoelectric performance of the Te NS photodetector at different intervals was investigated. Optoelectronic measurement results were obtained from devices that had been exposed to air with a relative humidity of 90% at 22°C. The R, EQE, and D^* values of the Te NS photodetector are shown in Figure 5A–C. Although a small decline in photoresponse performance is observed initially for a few days, the R, EQE, and D^* remain almost constant at 2.19×10⁴ A/W, 7.55×10⁵%, and 1.24×10⁸ Jones, respectively, for as long as 90 days. Furthermore, these values are higher than those of most other 2D material-based DUV photodetectors, as shown in Table S1. These results indicate that the Te photodetectors can maintain long-term and stable high-performance under ambient conditions.

Figure 5:

Aging characterization of the Te NS photodetector under ambient conditions.

Photodetection of the Te NS photodetector at 261 nm. (A) Responsivity of the Te NS photodetector at V_g=3 V. (B) EQE of the Te NS photodetector at V_g=3 V. (C) D^* with the measured bias at V_g=3 V.

4 Conclusions

In this study, a UV photoconductive detector was fabricated based on quasi-2D Te NSs on a SiO₂/Si substrate with high responsivity over a broad UV spectral range of 261–405 nm. A responsivity of 6.5×10⁴ A/W at a solar-blind DUV band, an EQE of more than 2.26×10⁶%, and a high detectivity of 3.73×10⁸ Jones were obtained, which are highest among those of other 2D material-based solar-blind DUV photodetectors. Moreover, the Te NS DUV photodetector was found to be stable for a long time (as long as 90 days), when exposed to air with a relative humidity of 90% at 22°C without any encapsulation. The findings of this study indicate that quasi-2D Te NSs can provide a viable approach for future ultrahigh-responsivity and ultrahigh-EQE solar-blind DUV photodetectors.