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][3][4]. At present, graphene has shown extremely high application value in many fields such as photoelectric detection, solar cell and optical modulator [5][6][7][8][9][10][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][13][14]. Recently, "three-dimensional (3D) graphene"-3D Dirac semimetal (DSM), a quantum topology material, has attracted much attention. As 3D graphene, Cd 3 As 2 has electronic band structure with zero band gap and linear energy dispersion [15][16][17][18]. And Cd 3 As 2 has extremely high carrier mobility, which can reach 9 × 10 6 cm 2 /(Vs) (below 5 K) [19]. Moreover, compared with graphene, the electronic properties of Cd 3 As 2 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 Cd 3 As 2 technology, varieties of photodetectors based on Cd 3 As 2 thin film or nanocrystal have been prepared and reported. Wang et al. reported the realization of a broadband photodetector based on Cd 3 As 2 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 Cd 3 As 2 crystals. The maximum R i of the photodetector is 0.27 mA/W [22]. Meng et al. experimentally proved the saturated absorption effect of Cd 3 As 2 thin film across 1-2 µm [23]. However, due to semimetal properties of Cd 3 As 2 , 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 Cd 3 As 2 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 Cd 3 As 2 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 Cd 3 As 2 [27,28]. Yang et al. reported photodetectors based on (Cd 1−x Zn x ) 3 As 2 /MoO 3 heterojunction [29], showing ultrahigh R i and detectivity (D * ) of 3.1 A/W and 6.4 × 10 10 Jones, respectively.
Compared with the inorganic, organic are more promising for the combination with Cd 3 As 2 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 cm 2 /(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][32][33]. In this article, we prepared wide-band photodetector with a heterojunction structure, which were fabricated by Cd 3 As 2 and (Cd 1−x Zn x ) 3 As 2 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 Cd 3 As 2 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 × 10 11 Jones) and faster response time (160/170 µs) than those of previously reported photodetectors based on the Cd 3 As 2 thin film. Use of DSM thin film and organic molecules also opens up a new path for the practical application of DSM materials.

Devices Preparation
Cd 3 As 2 and (Cd 1−x Zn x ) 3 As 2 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−x Zn x ) 3 As 2 thin film is ∼10 times smaller than that of pristine Cd 3 As 2 . Meanwhile, the photocurrent of (Cd 1−x Zn x ) 3 As 2 thin film is slightly smaller than that of Cd 3 As 2 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 Cd 3 As 2 into the lower energy states in ptype 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 rootmean-square (RMS) roughness of PbPc thin film on Cd 3 As 2 and (Cd 1−x Zn x ) 3 As 2 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 2 shows the optoelectronic properties of DSM/PbPc thin film heterojunction devices. The I-V curves of Cd 3 As 2 /PbPc and (Cd 1−x Zn x ) 3 As 2 /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 10 3 , indicating that a heterojunction is formed between the Cd 3 As 2 thin film and the organic thin film. Compared with the Cd 3 As 2 /PbPc device, the dark current and the photocurrent of (Cd 1−x Zn x ) 3 As 2 /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/cm 2 with V bias = −1 V. The on-off ratio of (Cd 1−x Zn x ) 3 As 2 /PbPc photodetector is as high as 295 under the bias voltage of −1 V, almost 10 times that of Cd 3 As 2 /PbPc photodetector 29.7. The result show that heterojunction can significantly reduce the dark current of Cd 3 As 2 thin film devices, and the effect of doped Cd 3 As 2 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.
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: 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][35][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/cm 2 .The maximum R i of Cd 3 As 2 /PbPc device at 808 nm is 54.6 mA/W. The (Cd 1−x Zn x ) 3 As 2 /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 Cd 3 As 2 /PbPc is 8.62 × 10 10 Jones at 808 nm. The maximum D * of (Cd 1−x Zn x ) 3 As 2 /PbPc device at 650 nm is 1.93 × 10 11 Jones, which is the maximum value obtained in the devices based on Cd 3 As 2 . 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.
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 Cd 3 As 2 /PbPc device is 160/170 µs, and that of (Cd 1−x Zn x ) 3 As 2 /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 Cd 3 As 2 /PbPc reach up to 193 mA/W and 3.05 × 10 11 Jones at 0.03 mW/cm 2 , respectively. And, the R i and D * of (Cd 1−x Zn x ) 3 As 2 /PbPc reach up to 25 mA/W and 3.95 × 10 11 Jones at 0.03 mW/cm 2 , respectively.
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 Cd 3 As 2 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/cm 2 compared with the reported Cd 3 As 2 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−x Zn x ) 3 As 2 /MoO 3 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.
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.

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 Cd 3 As 2 /PbPc), and excellent D * of 3.95 × 10 11 Jones [at 808 nm of (Cd 1−x Zn x ) 3 As 2 /PbPc], which is the maximum value measured in all Cd 3 As 2 based thin film devices. Meantime, the device in this work has faster τ on /τ off (160/170 µs and 170/200 µs for Cd 3 As 2 /PbPc heterojunction and (Cd 1−x Zn x ) 3 As 2 /PbPc heterojunction) than the previous Cd 3 As 2 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.