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Abstract
Two-dimensional materials have gained considerable attention owing to their exceptional optoelectronic properties. Among these, black phosphorus (BP) stands out for its tunable bandgap and high carrier mobility. However, its application is limited by its instability in the ambient condition. The emergence of black arsenic (b-As), which offers good environmental stability, is a promising 2D material candidate for black phosphorus, exhibiting tremendous potential in optoelectronic properties. Here, we demonstrate a high-performance b-As photodetector based on the dominance of the photoconductive effect, achieving a broadband spectral range from 520 nm to 1550 nm. This self-powered photodetector exhibits a rapid photoresponse speed, with impressive rise and fall times of 118 μs and 115 μs, respectively. Furthermore, characterized by a high responsivity of 1.826 A·W−1 and outstanding external quantum efficiency of 436%, the photodetector demonstrates its potential in IR optical communication and imaging capability. Our study introduces a novel photodetector material with broadband detection, fast photoresponse, high responsivity, and versatility, thereby providing a competitive alternative for the development of advanced optoelectronic devices.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The photodetector (PD) is a crucial optoelectronic device that can transform optical signals into electrical signals via the photoelectric effect [1,2]. It has a widespread application in many aspects of our lives, including optical communication, imaging, optoelectronics, sensing applications, security monitoring, etc [3–5]. Nowadays, different conventional semiconductor materials are applied in various scenarios due to differences in their detection bands and material properties, exhibiting excellent optoelectronic performance; such as Si in the visible spectrum (VIS, 0.38–0.75 $\mu$m), $\text {In}_{\text {1-x}}\text {Ga}_\text {x}\text {As}$ in the near-infrared (NIR, 0.75–1.4 $\mu$m) and $\text {Hg}_{\text {1-x}}\text {Cd}_\text {x}\text {Te}$ in the entire IR region (1-30 $\mu$m) [6–8].
Despite their maturity, these devices still suffer from numerous drawbacks, particularly when encountering the growing demand for enhanced standards in technology. These limitations include poor performance, costly fabrication processes, constraints imposed by low-temperature cooling requirements, and the complexities of coordinating multiple detectors operating at different wavelengths [9]. Therefore, there arises an urgent need for a high-performance photodetector that offers a wide spectral range, high sensitivity, and fast response speed.
Two-dimensional (2D) materials, which are known for their atomic-scale thickness and unique photoelectric properties, have gained significant attention in recent years for their promising potential in various technological applications. They are stacked through weak van der Waals (vdW) interactions, allowing for a broader detection range by adjusting the number of layers through mechanical exfoliation. Photodetectors based on these 2D materials have high optical transparency, rapid photoresponse, and low dark current, effectively addressing certain limitations of their counterparts [10,11]. Graphene (Gra), as the first discovered 2D material, has demonstrated remarkable photoelectric properties such as a high carrier mobility ($2\times 10^5 \ \text {cm}^2 \text {V}^{-1} \text {s}^{-1}$) and broad spectral response (ranging from ultraviolet to THz spectrum). However, its inherent zero bandgap limits its applicability in certain optoelectronic devices. A monolayer of Gra has demonstrated low responsivity ($\sim \text {1 mA}\cdot {W}^{-1}$) and extremely low light absorption (2.3${\% }$) [12–14]. Other 2D materials are often impeded by various drawbacks as well, notably a slow response speed owing to the significant trapping effects of photocarriers in transition metal dichalcogenides (TMDs) and a high dark current for topological insulators [15,16].
Black phosphorus (BP), with a sizable bandgap (0.3-1.5 eV) achieved by adjusting the number of layers, exhibits great potential for photodetection. Nevertheless, challenges, such as ambient instability, overshadow its applications [17]. As a rare chemical form of arsenic, black arsenic (b-As) holds great potential as a novel semiconductor, serving as an attractive candidate for next-generation photodetectors to BP [18,19]. Unlike BP, which undergoes surface oxidation and degradation when exposed to oxygen and moisture in the air, b-As displays greater stability in the ambient conditions even when exposed for one month [19,20]. b-As is of puckered honeycomb structure, and shares a structural configuration similar to that of BP, suggesting that they share similar physical and chemical characteristics. Furthermore, the layered nature of b-As offers a level of versatility. Theoretical studies have confirmed that bulk-layered b-As exhibit a bandgap of 0.3 eV, while their monolayer counterparts show a higher value between 1 to 1.5 eV [21–23]. This tunability is particularly advantageous for achieving wide-band detection capabilities, which are essential for sensing and imaging applications. Additionally, few-layered b-As is predicted to have high carrier mobilities (several thousand square centimeters per volt-second) [24]. However, despite these promising attributes, research on b-As PDs is insufficient and lacks comprehensiveness compared to other two-dimensional materials.
Herein, we successfully fabricated high-performance photodetectors utilizing micro/ nanofabrication techniques to precisely exfoliate and transfer b-As nanoflakes. The results demonstrate that our b-As PD exhibits a broadband wavelength range from visible (520 nm) to short-wavelength infrared (1550 nm). Notably, it shows excellent photoelectronic properties with the responsivity (R) of $\sim$1.83 A$\cdot$W$^{-1}$ and external quantum efficiency (EQE) of $\sim$436${\% }$. Scanning photocurrent mapping reveals that our b-As PD is predominantly by the photoconductive effect (PCE). Moreover, this work highlights its potential applications in optical communication and imaging, demonstrating its feasibility as a promising alternative to BP.
2. Results and discussion
The b-As PD was built as the schematic diagram shown in Fig. 1(a). Our PD adopts a metal-semiconductor-metal (MSM) configuration, with metal-semiconductor contacts established on both sides of the semiconductor, giving rise to the formation of two ohmic contacts. Two metal electrodes (Cr/Au=5 nm/50 nm) were deposited onto the low-resistance substrate with an insulating layer of Si$\text {O}_2$ (300 nm) using ultraviolet lithography technology. The semiconductor between these electrodes is b-As, whose layers are held together by relatively weak van der Waals forces, allowing for easy exfoliation in the plane direction. Utilizing the micro/nanofabrication technique, we mechanically peeled off the bulk b-As with tape and transferred onto the substrate with the aid of polydimethylsiloxane (PDMS) as an intermediate carrier. Figure 1(b) displays the side-view sketch of the stacked b-As crystal lattice. Notably, within each layer, the zigzag (ZZ) and armchair (AC) are two critical directions denoted for the x- and y-axes [25–23]. Each arsenic atom adopts $\text {sp}^3$ hybridization, forming covalent bonds with three adjacent arsenic atoms, thereby giving rise to a puckered honeycomb structure [18]. As shown in Fig. 1(c), the Raman spectrum exhibits three remarkable peaks corresponding to the $\text {A}_{\text {g}}^1$ (221.4 $\text {cm}^{-1}$), $\text {B}_{2\text {g}}$ (228.1 $\text {cm}^{-1}$), $\text {A}_{2\text {g}}^1$ (255.3 $\text {cm}^{-1}$) vibration modes at the ZZ, 45$^\circ$, AC directions, respectively. These observed peaks correspond to the findings of previous reports, providing evidence of the high quality of our fabricated b-As nanoflakes [27]. Additionally, Fig. 1(d) is the atomic force microscopy (AFM) image of the transferred b-As film onto the $\text {SiO}_2$/Si substrate using a scanning probe microscope. The region outlined by the yellow dashed line represents the source and drain electrodes, whereas the grey region indicates the location of the material. Along the red line is the height profile of our material, as depicted in Fig. 1(e). It can be clearly seen that the thickness of our exfoliated b-As nanoflake is $\sim$24 nm. This level of thickness control is critical for optimizing the performance of the photodetector since thin nanosheets can generate lower dark currents.
Fig. 1. (a) Schematic diagram of b-As PD. Electrode: Au (50 nm)/Cr (5 nm) Substrate: $\text {SiO}_2$/Si (b) Atomic structures of b-As. The upper panel is the side view of the b-As crystal lattice; the bottom panel is two types of edges in b-As nanoribbons: armchair edge (AC) and zigzag edge (ZZ). (c) Raman spectra in three directions (ZZ, 45$^\circ$, AC). (d) AFM topography of b-As PD. (e) Height profile along the red line in (d). The inset shows the schematic diagram of AFM.
We now turn our focus towards studying the photoelectric properties of the b-As PD using a laser wavelength of 1550 nm, a critical frequency for optical communication [28]. All photoelectric measurements were conducted at room temperature and under ambient conditions. Figure 2(a) illustrates the current-voltage (${ {I}}_{\text {ds}}$-${ {V}}_{\text {ds}}$) characteristics under both dark and light illumination. The perfectly straight lines in both cases indicate a good ohmic contact between Au/Cr electrodes and the channel material. This reduces the loss of photocurrent ($ { {I}}_\text {ph}$=$ { {I}}_\text {light}$-$ { {I}}_\text {dark}$) caused by high contact resistance. The dark current under a 1 V bias is low ($\sim 0.8$ mA), highlighting the advantages of exfoliated b-As nanosheets. Furthermore, the device resistance under irradiation ($\sim 1.37$ k$\Omega$) is notably smaller than in the dark ($\sim 1.92$ k$\Omega$), providing evidence for the existence of photoconductive effect in the b-As PD. The time-dependent photocurrent spectrum with varying incident laser powers is shown in Fig. 2(b). The device exhibits a rapid ON/OFF photoresponse with a steady photocurrent, indicating its stability and resolvability. This is evident from the noticeable $ { {I}}_{\text {on}}$/$ { {I}}_{\text {off}}$ ratio of $\sim$1.46 for 7 mW, $\sim$1.39 for 5 mW, $\sim$1.30 for 4 mW, $\sim$1.22 for 3 mW, $\sim$1.14 for 2 mW, and $\sim$1.02 for 0.18 mW. These distinct ratios highlight the ability of the device to detect and respond to different incident power levels. Since the photoresponse time in a single circle exceeds the limit of conventional photoelectric detection systems, we adopted the oscilloscope method to measure the response speed of photocurrent at 1 V bias voltage. The rise time ($\tau _r$) and fall time ($\tau _f$) are defined as the period for the photocurrent to rise from $10{\% }$ to $90{\% }$ and decrease from $90{\% }$ to $10{\% }$ of the maximum stable value [29]. As shown in Fig. 2(c), $\tau _r$ and $\tau _f$ are measured to be $\sim$118 $\mu$s and $\sim$115 $\mu$s, respectively. The ultrafast response speed can be attributed to the rapid separation and recombination of electron-hole pairs facilitated by the bias voltage (inset of Fig. 2(d)), highlighting its versatility in dynamic optical environments. For the same reason, as the bias voltage increases, the absolute value of the photocurrent also increases, as displayed in Fig. 2(d). The photocurrent reaches up to $\sim$2.7 mA under 7 mW illumination at a bias voltage of 1.0 V. To verify the power dependence of the b-As-based PD, we plotted and fitted the photocurrent-power ($ { {I}}_\text {ph}$-$ { {P}}_\text {in}$) curve based on the power law ($ { {I}}_\text {ph}$ $\propto$ $ { {P}}^\alpha$, where $\alpha$ is the ideal factor that measures the quality of PD) in Fig. 2(e) [30]. Five bias voltages from -1 V to 1 V are applied to the device, and the corresponding ideal factors $\alpha$ are calculated to be 0.88 (1 V), 0.87 (0.5 V), 1.12 (0 V), 0.91 (-0.5 V) and 0.90 (-1 V), respectively. These values are very close to 1, indicating the high performance of b-As PD. Considering the influence of both the power and bias voltage on the photocurrent, Fig. 2(f) presents a 2D color-mapping plot of the absolute value of $ { {I}}_\text {ph}$ with varying $ { {V}}_\text {ds}$ and $ { {P}}_\text {in}$. The photocurrent remains zero under the no-bias condition, gradually increasing as both the bias voltage and irradiation power increase. This reflects the sensitivity of the b-As PD to joint modulation of both factors.
Fig. 2. (a) $ { {I}}_{\text {ds}}$ -$ { {V}}_{\text {ds}}$ curves in the dark and under laser irradiation with 1 mW incident power. The inset is a front view of b-As PD. (b) Time-resolved photocurrent at different laser incident powers. (c) Rising time and falling time of photoresponse in one period. (d) The absolute value of photocurrent versus voltage under 7 mW laser power. The inset shows the separation of electron-hole pairs under light radiation. (e) The fitting curve of photocurrent versus incident power at different biases. (f) 2D color-mapped plot of photocurrent with voltage and incident power. The darkness of colors represents the absolute magnitude of the photocurrent. Double dash represents contour lines.
Furthermore, the photoelectric properties with respect to wavelength are investigated at a fixed 400 $\mu$W power. Figure 3(a) shows the time-resolved photocurrent over a wide spectral range. The b-As PD displays a broadband photoresponse, ranging from the VIS spectrum to short-wavelength infrared (SWIR). Even at a wavelength of 1550 nm, it continues to demonstrate a remarkable ON/OFF behavior, although the on/off ratio may decrease. This observation agrees well with the narrow bandgap property of b-As. In order to evaluate the performance of the constructed b-As PD from the VIS to SWIR spectrum, several key metrics are selected for detailed characterization. Responsivity (R) as a fundamental figure-of-merit, is used to quantify the electrical output per optical input. External quantum efficiency (EQE) measures the efficiency of converting incident photons into electrons that contribute to the external circuit. These metrics are calculated using the following formulas:
where $h$ is planck constant ($\sim 6.63\times 10^{-34} \ \text {m}^2 \cdot \text {kg} \cdot \text {s}^{-1}$), e is unit charge ($\sim 1.60\times 10^{-19}$ C), $ { {P}}_\lambda$ and $ { {R}}_\lambda$ are the incident power and responsivity corresponding to the wavelength $\lambda$, respectively. The wavelength-dependent R and EQE are calculated from 520 nm to 1550 nm wavelength, as shown in Fig. 3(b). R reaches its maximum value $\sim$1.826 A$\cdot$W$^{-1}$ at a wavelength of 520 nm and then decreases sharply to $\sim$1.514 A$\cdot$W$^{-1}$ at 1550 nm. Similarly, EQE falls from $\sim$436${\% }$ to $\sim$121${\% }$, a significantly higher value compared to the Si$\text {P}_2$-based device ($\sim$105%) proposed by Wang et al. and BP detector ($\sim 50{\% }$) by Chang et al. [31,32]. The reason for EQE exceeding 1 is attributed to the internal gain of the device. In addition, Noise equivalent power (NEP) and specific (or normalized) detectivity ($D^*$) are also considered crucial figures-of-merit for assessing device performance. NEP characterizes the sensitivity of the system and is defined as the signal power that gives a signal-to-noise ratio of one in a one-hertz output bandwidth [33]. $D^*$ provides a more direct evaluation of the performance of our photodetector [34]. They are calculated using the following formulas where $k_b$ is the Boltzmann constant ($\sim 1.38 \times 10^{23} \ \text {m}^2 \cdot \text {kg}\cdot \text {s}^{-2} \cdot \text {K}^{-1}$), $T$ is 300 K, $I$, $\Delta f$, $ { {R}}_\text {d}$, $A_\text {d}$ are the dark current, operating bandwidth (1 Hz), resistance under the irradiation, effective device area, respectively. As shown in Fig. 3(c), as the wavelength increases, there is an escalation in NEP from $7.35 \times 10^{-12} \ \text {W}\cdot \text {Hz}^{-1/2}$ to $8.86 \times 10^{-12} \ \text {W}\cdot \text {Hz}^{-1/2}$, while $D^*$ declines from $3.33 \times 10^7$ Jones to $2.76 \times 10^7$ Jones. Figure 3(d) shows a comparison of responsivity between the b-As PD and other photodetectors across wavelength from visible to mid-infrared (MIR) range. It is evident from the graph that our work demonstrates a high responsivity exceeding 1 A$\cdot$W$^{-1}$ among $\text {Bi}_2 \text {S}_3$ (0.01 A$\cdot$W$^{-1}$ at 1310 nm) [35], BP (0.02 A$\cdot$W$^{-1}$ at 1550 nm) [36], 1T'-MoT$\text {e}_2$ ($1\times 10^{-5}$ A$\cdot$W$^{-1}$ at 2200 nm) [37], N$\text {b}_2$GeT$\text {e}_4$/Mo$\text {S}_2$ ($1\times 10^{-4}$ A$\cdot$W$^{-1}$ at 1100 nm) [38], MoS$_2$/BP/MoS$_2$ (0.61 A$\cdot$W$^{-1}$ at 1650 nm) [39], diamonds ($\sim 0$ A$\cdot$W$^{-1}$ at 700 nm) [40], MoT$\text {e}_2$ (0.2 A$\cdot$W$^{-1}$ at 1340 nm) [41], Si ($7.2\times 10^{-5}$ A$\cdot$W$^{-1}$ at 1550 nm) [6] and Ge (1 A$\cdot$W$^{-1}$ at 1520 nm) [42]. Moreover, it maintains outstanding stability over a broad spectrum, exhibiting excellent optoelectronic characteristics.Fig. 3. (a) Time-resolved photocurrent at 400 $\mu$W incident power under different laser wavelengths. (b) Responsivity and external quantum efficiency of PD under different wavelengths. (c) Noise-equivalent power and specific detectivity of PD to different laser wavelengths. (d) Responsivity comparison of our work with other photodetectors.
The bias-driven scanning photocurrent microscope (SPM) characterization reveals the photoresponse mechanism under 1550 nm and 638 nm wavelength in the b-As PD, as depicted in Fig. 4(a), (b). The laser spot diameter for scanning photocurrent measurements was 1 $\mu$m and 3 $\mu$m for 638 nm and 1550 nm irradiation, respectively. In the absence of bias, we observe a small photocurrent primarily localizes at the junction of the metal-semiconductor interface. This phenomenon arises due to the built-in electric field generated by the contact, which spatially separates electron-hole pairs near the junction. As the applied bias voltage between S and D electrodes increases, it gradually offsets the voltage induced by the built-in electric field. Consequently, photocurrents are generated within the channel, all of which move in the same direction. The magnitude of photocurrent within the channel grows as the bias voltage further climbs, and a reverse current emerges under negative bias. This observation confirms the dominance of the photoconductive effect in governing the photocurrent response mechanism within the VIS to NIR spectral range. In order to explain the details of the photoconductive effect, the band schematic diagrams before and after contact are illustrated in Fig. 4(c), (d). As b-As is a p-type semiconductor, its Fermi level is close to the valence band (VB). A higher work function is observed in the metal compared to that of the semiconductor b-As, giving rise to a significant potential difference between the two materials. This leads to the energy bands bending upwards towards the metal at the interface, creating a built-in electric field points from b-As to the metal. The voltage generated by this built-in electric field induces the photocurrent near the junction under zero bias conditions. Figure 4(e), (f) show behaviors of the electron-hole pairs under dark and light conditions. Under illumination, the electrons of b-As absorb photons and become excited, crossing the bandgap into the conduction band (CB) when acquire sufficient energy. This process increases the total number of free electrons within b-As, consequently reducing the resistance. Under an external bias, these charge carriers are driven towards the two electrodes, giving rise to the formation of photocurrent within the channel.
Fig. 4. (a-b) Scanning photocurrent mapping graphs of b-As PD at different bias under illumination of 1550 nm and 638 nm laser wavelength. The regions between the yellow dashed line are drain (D) and source (S). The area of the material is delimited by the grey line. (c-d) Energy band diagram before and after gold-semiconductor contact. (e-f) Energy band diagram in dark and under light illumination. The red sphere represents electrons, while the blue sphere represents holes.
Given the excellent photoresponse of the device, we explore further the potential application of b-As PD by employing the device as a signal receiver in an IR communication system. Figure 5(a) is the schematic diagram of the optical system under the illumination of 1550 nm wavelength. In order to test our optical communication system, a letter containing “As” is converted into American Standard Code for Information Interchange (ASCII) by the program, as shown in Fig. 5(b). “1” in binary version represents a high voltage state, while “0” stands for a low voltage state. The amplitude of the input voltage controls the opening or closing of the shutter, thereby governing the flow of light. Notably, the electrical signal after the photodetector can be accurately distinguished by the computer, forming an output letter of “As”. This demonstrates the promising role of our photodetector in enhancing the performance and reliability of optical communication systems on the Internet of Things (IoT).
Fig. 5. (a) Schematic diagram of an optical communication system. (b) The transmitted letter “As” is the input and output signal in the ASCII code. (c) Schematic operating illustration for IR imaging. (d) The wavelength-dependent transmittance curve for TCE, AC, IPA, ALC, and $\text {H}_2$O. (e) The imaging results for five liquids and air under irradiation of 1550 nm.
Additionally, the IR imaging system was developed using the PD serving as a single sensing pixel, as shown in Fig. 5(c). The infrared light with a wavelength of 1550 nm passes through the colorimeter dish, which is positioned on a computerized 2D translation stage programmed to move both horizontally and vertically with a precision step size of 1 mm. It is received by the detector and generates position-dependent photocurrent. Figure 5(d) displays the transmittance curves of five kinds of reagents with varying wavenumbers, as detected by Fourier-transform infrared spectroscopy (FTIR). Notably, at the irradiation of 1550 nm these reagents exhibits transmittances of $\sim$89${\% }$, $\sim$75.7${\% }$, $\sim$34.9${\% }$, $\sim$7.3${\% }$, $\sim$1.4${\% }$ for Trichloroethylene (TCE), Acetone (AC), Isopropyl Alcohol (IPA), Alcohol (ALC), and $\text {H}_2$O, respectively. The image for six colorimeter dishes (air as the control group) produced by the computer is displayed in Fig. 5(e), based on the different transmittance of light by various liquids. The shape of the colorimeter dishes and the level of the liquid surface is depicted clearly in the image, demonstrating the potential of b-As PD for infrared imaging applications. The capability of accurate detection and exceptional reproducibility makes the device as a valuable component in next-generation optical systems.
3. Conclusions
In summary, we presented a novel b-As-based photodetector with remarkable photoelectronic performance. The constructed b-As detector offers broadband detection capabilities ranging from 520 nm to 1550 nm, positioning it as a versatile tool for various applications. It exhibits sensitivity to the light illumination, with the rise period and fall period to be $\sim$118 $\mu s$ and $\sim$115 $\mu s$, respectively. Furthermore, the photodetector demonstrates impressive performance metrics, including a decent responsivity of $\sim$1.826 A$\cdot$W$^{-1}$ and an exceptional EQE of $\sim$436${\% }$ at 520 nm. The scanning photocurrent mapping under the illumination of 520 nm and 1550 nm confirms the device is dominated by photoconductive effect. In addition to its technical performance, the b-As photodetector also shows promising prospects in optical communication and imaging. The robust performance and versatility suggest b-As as a competitive successor to BP for wide-spectrum detection, optical communication, optical sensing, and other applications.
Acknowledgments
The authors acknowledge the support of Xiamen University Malaysia and the State Key Laboratory of Infrared Physics for the use of their characterization equipment facilities.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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