Three-Dimensional Dirac Semimetal/Organic Thin Film Heterojunction Photodetector With Fast Response and High Detectivity

As a typical three-dimensional Dirac semimetal (3D DSM), Cd₃As₂ possess ultrahigh carrier mobility, high level of full spectral absorption, fast electron transmission speed, and high photocurrent response, which enable wide applications in infrared photodetector. However, the large dark current of the detector based on Cd₃As₂ thin film limits the application of the small current response. Hence, we demonstrated heterojunction photodetectors based on n-type 3D DSM Cd₃As₂ (pristine and Zn doped) and p-type organic (PbPc) by depositing PbPc thin film on Cd₃As₂ (pristine and Zn doped) thin film using thermal deposition method. These photodetectors can detect the radiation wavelength from 405 to 1,550 nm at room temperature. It is remarkable that this thin film heterojunction photodetector exhibits high detectivity (3.95 × 10¹¹ Jones) and fast response time (160 μs) under bias voltage, which is significantly improved vs. that of Cd₃As₂-based devices. The excellent performances are attributed to the strong built-in electric field at the interface of p-n junction, which is beneficial for efficient photocarriers collection and transportation. These results show that DSM/organic thin film heterojunction has excellent performance in the application of photodetectors. By combining 3D DSM with organic to form heterojunction, it provides a feasible solution for high-performance photodetectors.

Introduction

Since the successful preparation of graphene in 2004 [1], graphene has been widely studied due to its excellent carrier transport performance, unique two-dimensional (2D) energy dispersion, and stable electrical and optical properties [2–4]. At present, graphene has shown extremely high application value in many fields such as photoelectric detection, solar cell and optical modulator [5–11]. However, due to the limitations of the natural ultrathin structure of 2D materials, graphene inevitably has shortcomings such as vulnerable to external dielectric interference, weak absorbance and lack of controllable physical space [12–14]. Recently, “three-dimensional (3D) graphene”—3D Dirac semimetal (DSM), a quantum topology material, has attracted much attention. As 3D graphene, Cd₃As₂ has electronic band structure with zero band gap and linear energy dispersion [15–18]. And Cd₃As₂ has extremely high carrier mobility, which can reach 9 × 10⁶ cm²/(Vs) (below 5 K) [19]. Moreover, compared with graphene, the electronic properties of Cd₃As₂ are better protected to prevent surface and environment change [20]. Owing to all these characteristics above mentioned, the 3D DSM provides an excellent platform for the fabrication of high-performance photodetectors.

With the gradually mature development of growing Cd₃As₂ technology, varieties of photodetectors based on Cd₃As₂ thin film or nanocrystal have been prepared and reported. Wang et al. reported the realization of a broadband photodetector based on Cd₃As₂ nanoplate and nanowire, which exhibits broadband detection capability (from 532 to 10.6 μm) and had a maximum responsivity (R_i) of 5.9 mA/W [21]. Yavarishad et al. demonstrated an infrared photodetector based on Cd₃As₂ crystals. The maximum R_i of the photodetector is 0.27 mA/W [22]. Meng et al. experimentally proved the saturated absorption effect of Cd₃As₂ thin film across 1–2 μm [23]. However, due to semimetal properties of Cd₃As₂, results in a large dark current of devices. One possible solution to the above predicament is to design a p-n heterojunction photodetector [24]. For example, Yang et al. demonstrated a wideband photodetector based on heterojunction of Cd₃As₂ thin film and pentacene, which can detect the radiation wavelength from 450 to 10.6 μm with a maximum R_i of 36.15 mA/W at room temperature [25]. After that, Yang et al. continued to prepared heterojunction devices which were fabricated by Cd₃As₂ thin film with organics (small molecules and polymers), whose detection range was from 450 to 10.6 μm with a maximum R_i of 729 mA/W [26]. On the other hand, in order to reduce the electron density and open the band gap, Zn doping is used to compensate for the residual electron concentration of Cd₃As₂ [27, 28]. Yang et al. reported photodetectors based on (Cd_1−xZn_x)₃As₂/MoO₃ heterojunction [29], showing ultrahigh R_i and detectivity (D^*) of 3.1 A/W and 6.4 × 10¹⁰ Jones, respectively.

Compared with the inorganic, organic are more promising for the combination with Cd₃As₂ thin film because of easy large area and low-cost fabrication. Lead phthalocyanine (PbPc) is a kind of hole transport material, but its hole mobility is only about 10⁻⁶–10⁻⁴ cm²/(Vs) [30]. Moreover, PbPc is selected as the p-type material in heterojunction photodetectors because it is a good candidate for near-infrared absorbing material [31–33]. In this article, we prepared wide-band photodetector with a heterojunction structure, which were fabricated by Cd₃As₂ and (Cd_1−xZn_x)₃As₂ thin film with the PbPc. Therefore, we designed and utilized the chemical doping and p-n heterojunction structure, which greatly improve the photoelectric performance of Cd₃As₂ thin film devices. The DSM/organic thin film heterojunction devices exhibit excellent photocurrent response and very small dark current. Moreover, these devices have higher D^* (the maximum D^* can reach 3.95 × 10¹¹ Jones) and faster response time (160/170 μs) than those of previously reported photodetectors based on the Cd₃As₂ thin film. Use of DSM thin film and organic molecules also opens up a new path for the practical application of DSM materials.

Experimental Section

Devices Preparation

Cd₃As₂ and (Cd_1−xZn_x)₃As₂ thin films were produced by molecular-beam epitaxy (MBE) system when the vacuum lower than 2 × 10⁻⁸ Pa, PbPc thin film and Au electrodes were prepared by thermal deposition when the vacuum lower than 6 × 10⁻⁴ Pa. Before growing the DSM layer, the substrates were degassed at 400°C for 30 min to remove gas molecules absorbed on the surface. Then, the CdTe buffer layer began to grow in order to achieve a better crystalline match. The DSM thin films deposition was carried out on the top of the buffer layer. The substrate with 100 nm DSM thin films was placed into the evaporation equipment to deposit 100 nm PbPc thin film. The electrodes is also prepared by the thermal evaporation method, and the thickness of the electrodes is 60 nm. Finally, the photodetectors of DSM/PbPc thin film were fabricated with a channel of 200 × 200 μm.

Characterization

The surface morphology of thin film was studied by atomic force microscopy (AFM). The photoelectric performances were measured by the Keithley 2636B source meter system and FS-Pro all-in-one multifunctional semiconductor parameter testing system. The laser power was measured by Ophir RM9 system, and the measurement range is 100 to 100 mW.

Results and Discussion

We directly prepare electrodes on the DSM thin films by thermal evaporation, Supplementary Figure 1 shows the schematic of DSM thin film device. From the Supplementary Figure 2, we can find that the dark current of (Cd_1−xZn_x)₃As₂ thin film is ~10 times smaller than that of pristine Cd₃As₂. Meanwhile, the photocurrent of (Cd_1−xZn_x)₃As₂ thin film is slightly smaller than that of Cd₃As₂ thin film. This result is mainly due to the doping of Zn ion, which reduces the conductivity of the thin film, but the response to light changes little.

Next, we prepared heterojunction photodetectors based on DSM and p-type organic. The structure of the vertical heterojunction devices is shown in Figure 1A, from top to bottom, the layers are Au, PbPc, DSM, and mica. Once the DSM thin film is in contact with the PbPc thin film, due to the difference in Fermi level between them, electrons flow from the n-type Cd₃As₂ into the lower energy states in p-type PbPc, forming a negative space charge region on the semiconductor surface. At this time, there is a certain electric field in the space charge region, which causes the energy band bend upward (Figure 1B). When the device is illuminated by light, photogenerated electrons/holes are generated at the interface between DSM and organic. Then they separate in the junction region and transport to the two electrodes, respectively, because of the built-in electric field. Figures 1C,D demonstrate surface topography of PbPc thin film on DSM thin films. The root-mean-square (RMS) roughness of PbPc thin film on Cd₃As₂ and (Cd_1−xZn_x)₃As₂ thin films are 12.83 and 14.27 nm, respectively. It can be clearly seen that the films made of the two materials are uniform and dense. Therefore, the film quality of the two composite thin films is very good, which can meet the needs of fabricating devices.

FIGURE 1

Figure 1. Structure and morphology of the devices. (A,B) Device schematic diagram and energy-band diagram of DSM/PbPc thin film heterojunction photodetector, respectively. (C,D) AFM surface diagram of PbPc thin film on Cd₃As₂ and (Cd_1−xZn_x)₃As₂ thin films, respectively.

Figure 2 shows the optoelectronic properties of DSM/PbPc thin film heterojunction devices. The I-V curves of Cd₃As₂/PbPc and (Cd_1−xZn_x)₃As₂/PbPc devices are shown in Figures 2A,D. The dark current of the device is small under the reverse bias voltage, while the dark current increases exponentially with the voltage under the forward bias voltage, which is consistent with the I-V curve of the typical photodiode, and has significant rectification characteristics. The rectification ratio at ± 1 V is close to 10³, indicating that a heterojunction is formed between the Cd₃As₂ thin film and the organic thin film. Compared with the Cd₃As₂/PbPc device, the dark current and the photocurrent of (Cd_1−xZn_x)₃As₂/PbPc photodetector are decreased from 7.5 to 0.42 nA, and 223 to 124 nA, when illuminated under 808 nm laser source whose laser power intensity is 2 mW/cm² with V_bias= −1 V. The on-off ratio of (Cd_1−xZn_x)₃As₂/PbPc photodetector is as high as 295 under the bias voltage of −1 V, almost 10 times that of Cd₃As₂/PbPc photodetector 29.7. The result show that heterojunction can significantly reduce the dark current of Cd₃As₂ thin film devices, and the effect of doped Cd₃As₂ devices is more obvious. In order to find the best bias voltage for the best test effect, these photodetectors use V_bias= −1 V as the main test bias voltage.

FIGURE 2

Figure 2. Optoelectronic properties of DSM/PbPc thin film heterojunction photodetectors. (A,D) I-V curves of Cd₃As₂/PbPc and (Cd_1−xZn_x)₃As₂/PbPc thin film heterojunction photodetectors, respectively, without and with 808 nm irradiation. (B,E) Photocurrents of different wavelengths for Cd₃As₂/PbPc and (Cd_1−xZn_x)₃As₂/PbPc thin film heterojunction photodetectors. The measurements were conducted under bias voltage of −1 V with laser power intensity of 2 mW/cm². (C,F) The photocurrent curves of the Cd₃As₂/PbPc and (Cd_1−xZn_x)₃As₂/PbPc devices under different negative bias voltage with a 2 mW/cm² laser power intensity.

When the DSM/PbPc devices are biased at −1 V, the photocurrent response of different wavelength bands (from 405 to 1,550 nm) is shown in Figures 2B,E. We can clearly see that the device has good photocurrent response from visible light to near-infrared region. As the wavelength gradually rises, the photocurrent falls first and then rises, which is related to the absorption spectrum of PbPc (Supplementary Figure 3). In addition, as shown in Figures 2C,F, the photodetectors can produce a much stronger photocurrent when it works under the bias voltage mode.

R_i is used to characterize the excellent performance of optoelectronic device. In general, R_i is the ability of the photodetector to convert optical signal into electrical signal, which is defined as the ratio of the photocurrent detected by the device to the power of the incident light source. D^* is an important index to measure the limit detection ability of photodetectors for weak signals. The formulas are as follows:

$\begin{array}{ll}{R}_i=\frac{I_{Ph}}{P}=\frac{I_L-I_D}{P}& (1)\end{array}$

$\begin{array}{ll}{D}^{*}=\frac{R_i\sqrt{S}}{\sqrt{2q{I}_D}}& (2)\end{array}$

Among them, I_Ph is photogenerated current on the active channel area, I_L is current under the illumination, I_D is dark current (without illumination), and P is laser power. Moreover, S and q, respectively, represent the active area and the charge per electron [34–36].

Figure 3A is the R_i diagram of the DSM/PbPc devices. The measurements were conducted under bias voltage of −1 V with laser power intensity of 2 mW/cm².The maximum R_i of Cd₃As₂/PbPc device at 808 nm is 54.6 mA/W. The (Cd_1−xZn_x)₃As₂/PbPc device has a maximum R_i of 38.8 mA/W at 650 nm. Figure 3B is the D^* calculated based on the measured experimental data. The maximum D^* of Cd₃As₂/PbPc is 8.62 × 10¹⁰ Jones at 808 nm. The maximum D^* of (Cd_1−xZn_x)₃As₂/PbPc device at 650 nm is 1.93 × 10¹¹ Jones, which is the maximum value obtained in the devices based on Cd₃As₂. The high D^* can be attributed to be the lower dark current at reverse bias voltage. The EQE curve is shown in Supplementary Figure 4. Accordingly, the largest values are 8.4 and 8.6%, respectively.

FIGURE 3

Figure 3. (A,B) The R_i and D* curves of DSM/PbPc thin film heterojunction photodetectors under bias voltage mode from 405 to 1,550 nm. (C,D) Response time curves of DSM/PbPc thin film heterojunction photodetectors under light illumination (at 808 nm).

Moreover, the response time has traditionally been considered to be a significant feature of photodetector [37, 38]. Figures 3C,D is the response time curves of the device. The response time of Cd₃As₂/PbPc device is 160/170 μs, and that of (Cd_1−xZn_x)₃As₂/PbPc device is 170/200 μs. It can be observed in the figure that there is an obvious “peak” in the rising process, which is usually related to the charge trapping effect existing in the carrier transport process inside the device [35, 39, 40]. Specifically, at the moment when light begins to irradiate the device, the photogenerated exciton at the interface of the device are dissociated into holes and electrons under the action of the built-in electric field, and then rapidly transport to the two electrodes, respectively. If there is charge trapping traps in a certain interface or layer, some of the free carriers will be filled with these traps. With the filling of the traps, the original built-in electric field is gradually weakened, resulting in the decrease of the number of carriers collected by the two electrodes in unit time, so the photocurrent gradually decreases. Until the recombination of carrier transmission and trap induction achieves a dynamic balance, the photocurrent can achieve a steady state, so the “peak” phenomenon can be seen in the rising part of the transient photocurrent [41, 42]. At the same time, there is a reverse “peak” in the falling process, which reflects the phenomenon that the interface charge cannot be effectively extracted when the light is stopped suddenly [43]. Moreover, the peak may also be related to the charge and discharge of the heterojunction capacitor.

We further investigated the incident light power dependence of the photoresponse in both the DSM/PbPc thin film heterojunction photodetector, and the major results are plotted in Figure 4, respectively (measured at V = −1 V in ambient conditions). The relationship among R_i, photocurrent and incident laser power is shown in Figures 4A,B. With the increase of optical power, the photocurrent of the device also increases, and the photocurrent tends to be saturated at higher optical power. This non-linear relationship is the final result of a series of complex processes, including the generation of photogenerated carriers in near the DSM/PbPc interface, the photogenerated carriers transport through the interface, and the photogenerated carriers are trapped and compounded by defects [44]. Also, with the increase of incident power, the R_i is inversely proportional to the incident power, and the higher R_i often occurs when the power is relatively low. Due to most of the defect states are not occupied at low power, and the holes generated after illumination are captured, so the R_i is high. With the increase of the incident light power, the captured states are occupied and tend to be saturated, and the built-in electric field decreases, and the R_i gradually decreases due to this effect. As shown in Figures 4C,D, the calculated R_i and D^* under illumination of 808 nm with various light intensity are obtained, respectively, at room temperature. We can see the R_i and D^* of two photodetector decreases with the power increasing. The R_i and D^* of Cd₃As₂/PbPc reach up to 193 mA/W and 3.05 × 10¹¹ Jones at 0.03 mW/cm², respectively. And, the R_i and D^* of (Cd_1−xZn_x)₃As₂/PbPc reach up to 25 mA/W and 3.95 × 10¹¹ Jones at 0.03 mW/cm², respectively.

FIGURE 4

Figure 4. (A,B) Variation of the photocurrent (left axis) and R_i (right axis) of DSM/PbPc photodetector with the different intensities of illumination at the laser wavelength of 808 nm. (C,D) R_i (left axis) and D* (right axis) curves of DSM/PbPc photodetector under light illumination with different power. The measurements were performed in ambient conditions with V = −1 V and the laser wavelength of 808 nm.

To understand the above-mentioned excellent ultra-high D^* and high response speed performance of device, the mechanisms are proposed here to elucidate this in detail. The response speed of photodetector is determined by the separation and transport speed of the photogenerated charge carriers. For our photodetector based on p-n heterojunction, there are three main reasons for the fast response speed: (1) The vertical heterojunction structure has intrinsically short transport length for carriers [45]. (2) The strong build-in electric field from the DSM and PbPc thin film can greatly facilitate the separation and collection of the photocarriers, inducing the high response speed [46, 47]. (3) Moreover, when the heterojunction is applied with reverse bias voltage, the electric field caused by reverse bias is consistent with the build-in electric field, so the barrier height is enhanced. Meanwhile, the photogenerated minority carriers can pass through the interface easier owing to the strong external electric field [48]. All of the features work synergistically for fast transport of photogenerated charge carriers at the respective electrodes. In addition, the high D^* can be attributed to be the excellent diode behavior with a lower dark current at reverse bias voltage. Moreover, the low reverse dark current thus is conductive to enhance low light detection capability of the photodetector.

Comparison of the performance of DSM/PbPc heterojunction photodetector with Cd₃As₂ based devices in the literature, some key parameters of the photodetector are summarized in Table 1. It is noted that the heterojunction devices fabricated by us have a relatively high D^* and a fast response speed with light intensity of 2 mW/cm² compared with the reported Cd₃As₂ based photodetectors. However, the excellent performance of the devices on the fast response speed was achieved by sacrificing Ri, the DSM/PbPc heterojunction devices fabricated by us have a relatively low Ri compared with the others. Especially, compared with the (Cd_1−xZn_x)₃As₂/MoO₃ heterojunction device. These excellent performances of the p-n heterojunction photodetector indicate that this device may have great potential applications in the low light signals photodetection.

TABLE 1

Table 1. Performance of photodetectors based on Cd₃As₂ material.

The unit stability of array device is an important performance index of array devices. In this work, we stochastically tested three different units in the DSM/PbPc thin film heterojunction array devices, respectively, as shown in Figures 5A,B. We found that the current properties of different units in each array device were basically consistent, and the units has good stability and repeatability. This result shows the potential of DSM thin film in the fields of array applications.

FIGURE 5

Figure 5. (A,B) Three different unit photocurrent diagrams of DSM/PbPc thin film heterojunction array devices, respectively. Under bias voltage of −1 V with laser power intensity of 2 mW/cm². The illustration is the physical picture of the array device, the parts selected in the red frame are the unit devices.

Conclusion

In conclusion, we have successfully fabricated 3D DSM/organic thin film heterojunction photodetectors. These devices have good R_i, up to 193 mA/W (at 808 nm of Cd₃As₂/PbPc), and excellent D^* of 3.95 × 10¹¹ Jones [at 808 nm of (Cd_1−xZn_x)₃As₂/PbPc], which is the maximum value measured in all Cd₃As₂ based thin film devices. Meantime, the device in this work has faster τ_on/τ_off (160/170 μs and 170/200 μs for Cd₃As₂/PbPc heterojunction and (Cd_1−xZn_x)₃As₂/PbPc heterojunction) than the previous Cd₃As₂ based devices. In this paper, the DSM and organic heterojunction photodetectors have the advantages of simple structure, easy preparation and low cost, which are suitable for industrial promotion. Moreover, the development direction of array is also studied, which can adapt to the future market demand for high-performance photodetectors.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

Author Contributions

JW designed and conducted this work. QL and MY conducted experimental measurement and data analysis. JZ, MY, HZ, and JG provided suggestions and assistance to this experiment. All authors discussed the experimental results.

Funding

This work was supported by National Outstanding Youth Science Fund Project of National Natural Science Foundation of China (No. 61922022), Science Fund for Innovative Research Group Project of the National Natural Science Foundation of China (No. 61421002), the Sichuan Science and Technology Program (Grant No. 2020JDRC0061).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fphy.2021.672591/full#supplementary-material

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