# Si:HgTe Colloidal Quantum Dots Heterojunction-Based Infrared Photodiode

Authors: Jinmei Jia; Huan Liu; Jijie Zhao; Yuxuan Du; Shuai Wen
Journal: Journal of Nanomaterials
 (2023)

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## Abstract

Integrated circuits and optoelectronics are currently dominated by silicon technology. However, silicon’s response wavelength is typically less than 1,100 nm, limiting the application of silicon in machine vision, autonomous vehicles, and night vision. For infrared photodetectors, HgTe colloidal quantum dots (CQDs) are promising materials. Because of the adjustable bandgap, it responds over a wide spectral range. However, the construction of a high-quality junction between Si and HgTe CQDs continues to be difficult, thus restricting the scope of its application. In this article, we describe the synthesis, characterization, and correlation of HgTe CQDs with reaction temperature and nanocrystal size. We then fabricated HgTe-CQDs/silicon infrared photodiodes and discussed how the silicon resistivity affected their performance. We found that the devices prepared from 9.1 nm HgTe quantum dots synthesized at 80°C and a silicon substrate with a resistivity of 20–50Ω·cm has optimal performance parameters. This results in a responsivity of 0.2 mA/W for 1,550 nm incident light at room temperature. These results provide a direction for future silicon-compatible HgTe quantum dot infrared optoelectronics.


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## Body

## 1. Introduction

Infrared detectors have been explored for many optoelectronic applications, particularly for night vision detection, infrared guidance, thermal imaging, and optical fiber communication [1–4]. Currently, the most common materials used for infrared photodetectors are HgCdTe and InSb. Both are mature material technologies that can be used in integrating focal plane arrays with silicon-based readout circuits and provide good quantum efficiencies, but the complex integrated manufacturing process makes these photodetectors expensive [5–8]. New photodetectors must be created due to the drawbacks of existing detectors. The characteristics of the ideal photodetectors should include rapid response, high responsiveness, ease of production, and low cost [9–11].In recent years, nanoparticles and quantum dots have attracted a great deal of interest in academia due to their unique properties and outstanding applications [12–15]. Colloidal quantum dots (CQDs) and nanoparticles are widely used in humidity sensors, antibacterial activity, anticancer activity, and optoelectronic devices [16–18]. CQDs, which provide solution processability, spectral tunability, and silicon-compatible fabrication, are providing solutions to such challenges. Near-infrared PbS-CQDs/silicon hybrid photoconductive have been illustrated. CQDs with narrower gaps are required to obtain longer wavelength infrared to further increase the spectral detection regions. Because of their strong absorbance in the mid-infrared and THz ranges, HgTe CQDs are promising infrared detector materials [19–22]. HgTe CQDs can be easily deposited on silicon-based readout circuits using the spin-coating method due to their solution processability [23]. As a result, it may be easily integrated with readout circuits without the need for a complicated electrical link between the detector and readout circuits. This can greatly simplify device manufacture and lower the cost [24–26]. Liu et al. [27] recently presented a HgTe CQDs photoconductive detector based on different ligand exchange processes on SiO2 substrates. However, the devices suffer from low responsivity (3.5 × 10−3 A/W) at infrared wavelengths. Utilizing a graphene common contact between short-wave infrared HgTe CQDs and a silicon substrate, Tang et al. [28] established a method for visible and infrared dual-channel detectors. Nevertheless, the photodiode for the CQDs has a detective of 5 × 109 Jones. In order to improve device performance, HgTe-CQDs/graphene and graphene/silicon contacts need to be optimized.In this work, we describe the easy solution-derived synthesis and characterization of HgTe CQDs materials. In parallel, we report a HgTe-CQDs/silicon infrared photodiode and compared the effect of planar silicon with different resistivity on the performance of silicon-based HgTe CQDs photodiode. Under illumination with different wavelength lasers, obvious light response proves the feasibility of the silicon-based HgTe CQDs photodiode. Our devices have a 0.2 mA/W response at 1,550 nm. The device has a normalized detectivity of 4.4 × 1010 Jones, a high responsivity of 40 mA/W at 980 nm, and an external quantum efficiency (EQE) of up to 5.1%.


## 2. Experimental Section

### 2.1. Materials

Mercury chloride (HgCl2, powder, 99%), tellurium power (Te, ALaddin, 99.99%), trioctylphosphine (TOP, ALaddin, 90%), oleylamine (OLA, ALaddin, 90%), dodecanethiol (DDT, ALaddin, 98%), methanol (ALaddin, 99.5%), toluene (99%), 1,2-ethanedithiol (EDT, ALaddin, 97%), and isopropanol (IPA, ALaddin, 99.7%). Except for oleylamine, which is dried overnight at 80°C under vacuum, all of these compounds are utilized as received. Highly hazardous mercury compounds are present. Take extra care when handling them.


### 2.2. HgTe Colloidal Quantum Dots Synthesis

In a 100 mL three-neck flask under an argon environment, the HgTe CQDs were synthesized [29]. A typical synthesis involved heating a combination of 342 mg of HgCl2 and 40 mL of oleylamine under vacuum to 110°C for 1 hr. In a three-neck flask, 1.27 g Te powder is combined with 20 mL of TOP. The flask is placed under vacuum and allowed to stand at ambient temperature for 5 min before being heated to 110°C. In addition, as the temperature is raised to 275°C. The solution is agitated until it turns a clear orange hue. When the bottle reaches room temperature, the hue changes to yellow. The final step is to put this liquid into a glove box filled with Ar. The solution is clear, showing that HgCl2 is completely dissolved. After decreasing the temperature to 80°C, the 0.63 mL of TOP:Te and 6.3 mL oleylamine were instantly injected before changing to argon. The reaction time can influence the nanoparticle sizes. By having removed the heating source and injecting a solution of 2 mL DDT and 18 mL toluene to stabilize the nanocrystals, the process was soon quenched. The nanocrystals were redispersed in toluene after being precipitated and cleaned three times with a methanol solution. Before using, this solution needs to pass through a 0.1 μm polytetrafluoro ethylene filter. Sonication was occasionally necessary to achieve a tidy colloid. Aggregation must be avoided while creating high-quality films.


### 2.3. Device Fabrication

Silicon substrates are cut into 1 × 1 cm sizes and thoroughly cleaned by sonication with acetone for 15 min, cleaned with ethanol for 15 min, washed with deionized water for 5 min, submerged in HF acid for 60 s, rinsed with deionized water for 10 s, then dried completely with N2 flow. During ligand exchange, mix 20 mL IPA and 0.2 mL EDT and a drop of HgTe-CQDs solution (40 μL) were spread over the surface and spun at 2,500 rpm for 40 s. Then a drop of dissolved EDT/IPA was put onto the HgTe CQDs film, allowed to wait for 60 s, and then spin-coating at 2,500 rpm for 1 min. The HgTe film was next washed with IPA before being annealed for 1 min at 60°C on the hot plate. The preceding procedures were repeated nine times and finally, top and bottom Ag electrodes were deposited onto the HgTe CQDs film and Si substrate with mask, respectively.


### 2.4. Characterization

X-ray diffraction (XRD) was used to investigate the crystalline nature of the samples (BrukerAXS D8). By using Fourier transform infrared spectroscopy (FTIR) (Bruker, Vertex70v), it is possible to measure the absorption spectra of HgTe CQDs at different temperatures while checking whether the ligand exchange is successful. A transmission electron microscope was used to measure the size of the HgTe CQDs (TEM, Talos, L120C). The electronic properties of the fabricated devices were characterized by measuring the current–voltage (I–V) curves using B2901A SYSTEM source meter with a pair of prods test systems. The bands used for the light source were 405, 520, 638, 980, and 1,550 nm. The semiconductor parameter analyzer platform design automation and the program LabExpress were used to measure the devices’ reaction times. The testing procedure was conducted in a protected, dark box at ambient temperature.


## 2.1. Materials

Mercury chloride (HgCl2, powder, 99%), tellurium power (Te, ALaddin, 99.99%), trioctylphosphine (TOP, ALaddin, 90%), oleylamine (OLA, ALaddin, 90%), dodecanethiol (DDT, ALaddin, 98%), methanol (ALaddin, 99.5%), toluene (99%), 1,2-ethanedithiol (EDT, ALaddin, 97%), and isopropanol (IPA, ALaddin, 99.7%). Except for oleylamine, which is dried overnight at 80°C under vacuum, all of these compounds are utilized as received. Highly hazardous mercury compounds are present. Take extra care when handling them.


## 2.2. HgTe Colloidal Quantum Dots Synthesis

In a 100 mL three-neck flask under an argon environment, the HgTe CQDs were synthesized [29]. A typical synthesis involved heating a combination of 342 mg of HgCl2 and 40 mL of oleylamine under vacuum to 110°C for 1 hr. In a three-neck flask, 1.27 g Te powder is combined with 20 mL of TOP. The flask is placed under vacuum and allowed to stand at ambient temperature for 5 min before being heated to 110°C. In addition, as the temperature is raised to 275°C. The solution is agitated until it turns a clear orange hue. When the bottle reaches room temperature, the hue changes to yellow. The final step is to put this liquid into a glove box filled with Ar. The solution is clear, showing that HgCl2 is completely dissolved. After decreasing the temperature to 80°C, the 0.63 mL of TOP:Te and 6.3 mL oleylamine were instantly injected before changing to argon. The reaction time can influence the nanoparticle sizes. By having removed the heating source and injecting a solution of 2 mL DDT and 18 mL toluene to stabilize the nanocrystals, the process was soon quenched. The nanocrystals were redispersed in toluene after being precipitated and cleaned three times with a methanol solution. Before using, this solution needs to pass through a 0.1 μm polytetrafluoro ethylene filter. Sonication was occasionally necessary to achieve a tidy colloid. Aggregation must be avoided while creating high-quality films.


## 2.3. Device Fabrication

Silicon substrates are cut into 1 × 1 cm sizes and thoroughly cleaned by sonication with acetone for 15 min, cleaned with ethanol for 15 min, washed with deionized water for 5 min, submerged in HF acid for 60 s, rinsed with deionized water for 10 s, then dried completely with N2 flow. During ligand exchange, mix 20 mL IPA and 0.2 mL EDT and a drop of HgTe-CQDs solution (40 μL) were spread over the surface and spun at 2,500 rpm for 40 s. Then a drop of dissolved EDT/IPA was put onto the HgTe CQDs film, allowed to wait for 60 s, and then spin-coating at 2,500 rpm for 1 min. The HgTe film was next washed with IPA before being annealed for 1 min at 60°C on the hot plate. The preceding procedures were repeated nine times and finally, top and bottom Ag electrodes were deposited onto the HgTe CQDs film and Si substrate with mask, respectively.


## 2.4. Characterization

X-ray diffraction (XRD) was used to investigate the crystalline nature of the samples (BrukerAXS D8). By using Fourier transform infrared spectroscopy (FTIR) (Bruker, Vertex70v), it is possible to measure the absorption spectra of HgTe CQDs at different temperatures while checking whether the ligand exchange is successful. A transmission electron microscope was used to measure the size of the HgTe CQDs (TEM, Talos, L120C). The electronic properties of the fabricated devices were characterized by measuring the current–voltage (I–V) curves using B2901A SYSTEM source meter with a pair of prods test systems. The bands used for the light source were 405, 520, 638, 980, and 1,550 nm. The semiconductor parameter analyzer platform design automation and the program LabExpress were used to measure the devices’ reaction times. The testing procedure was conducted in a protected, dark box at ambient temperature.


## 3. Results and Discussion

TEM images of HgTe nanoparticles prepared at the temperature of 60, 70, 80, 90, 100, and 110°C are presented in Figure1(a)–1(f) and the corresponding histograms of the size distribution are shown in Figure 2. The reaction time was maintained for 3 min while the reaction temperature increased from 60 to 110°C; the shape of the synthesized HgTe CQDs frequently took the form of triangles or distorted circle shapes, as well as the size of HgTe CQDs increased. Typically, at temperatures of 60 and 70°C, and the reaction time is 3 min, the size of HgTe quantum dots does not change significantly. As the temperature continues to increase, the CQDs begin to aggregate into a tetrapodic shape. The micrographs suggest that the HgTe nanoparticles are homogeneous and continuously grown with good orientation. The TEM images revealed that the CQDs ranged in size from 5 to 13 nm (Figure 3). The HgTe CQDs size distribution of temperature at 80°C and time at 3 min was homogeneous, which is very suitable to prepare thin films. They are bipolar and can form heterojunction junctions with N-type silicon. If the size is too large, the quantum dots are N-type and are not able to form heterojunction junctions with N-type silicon [29].Figure 1
TEM images of the HgTe CQDs and the corresponding histograms of the size distribution: (a) 60°C; (b) 70°C; (c) 80°C; (d) 90°C; (e) 100°C; (f) 110°C.Figure 2
Plot of HgTe CQDs size depending on the reaction temperature. The reaction time was maintained for 3 min.Figure 3
At the same time, change the temperature and observe the change in the regularity of particle size.As shown in Figure4, the spectrum of XRD of HgTe CQDs is produced at varied reaction temperatures. The large peaks are attributable to the nanostructure’s low crystallinity [30]. It is evident that as temperature rises, the diffraction peak’s full width at half maximum (FWHM) diminishes. According to Scherrer’s formula, the FWHM of the diffraction peak is inversely proportional to the crystal size. The XRD patterns showed some peaks at 23.9°, 39.59°, 46.8°, 57.1°, 62.8°, 71.7°, and 76.8° corresponding to the (111), (220), (311), (400), (331), (422), and (511) plane of zinc blende structure HgTe.Figure 4
XRD analysis of a film of HgTe nanoparticles.Figure5(a) displays a series of absorption spectra for samples of various sizes that were deposited on indium tin oxide glass substrates and generated over a 3 min reaction period at temperatures of 70, 80, and 90°C. It can be seen that the smaller particles give an absorption onset at higher frequencies. The first exciton absorption peak exhibits a red shift as the quantum dot’s size increases. It displays a monotonic trend in particle size-dependent absorption. The 10 and 6.7 nm CQDs’ absorption spectra clearly demonstrate intraband at 2,300 and 1,900 cm−1, respectively. This indicates that the HgTe CQDs have a broad absorption range.Figure 5
(a) The optical absorption spectrum of HgTe CQDs synthesized at different temperatures; (b) FTIR spectra of a HgTe CQDs film containing DDT and EDT ligands.
(a)(b)When synthesizing HgTe CQDs, we have used ligands with lengthy alkyl chains. The long-chain ligands can make the quantum dots have better dispersibility in the solution, which makes the synthesized quantum dots not easy to be oxidized, but hinders the transport ability between the quantum dots. To improve the conductivity of the CQDs film, we use EDT short-chain ligands (0.5 nm sizes) instead of DDT long-chain ligands. The ligand exchange process is finished using the spin-coating technique. As shown in Figure5(b), after the ligand exchange, the ligand peaks are weakened, indicating that C–H bonds are reduced, and long-chain ligands are substituted.To evaluate photoresponse characteristics, several parameters need to be determined. An important factor that describes the light–current transformation efficiency of the photodetector is called responsivity [31, 32]. The R is determined using the following formula as the photocurrent-to-light power ratio:(1)R=Ilight−Idarkp=IillP,where the photocurrent of the device is Iill, and the detector’s currents under light and darkness, respectively, are Ilight and Idark. P is the light power.In order to compare photodetectors with various structures and regions, the normalized detectivity (D∗), is another crucial factor for assessing the performance of photodetectors [33]. This parameter emphasizes the detector’s capacity for detection. That is, the greater D∗, the more sensitive the device is to detecting optical signals. The following is the D∗ calculation formula:(2)D∗=RS2qIdark,where R stands for responsivity, q for element charge, and S for the effect of light absorption area when illuminated. Another crucial performance metric for the detector is its EQE, which measures photoelectric conversion efficiency and represents the capacity to transform photons into carriers [34]. The following formula is commonly used to define it:(3)EQE=R×hcqλ=R×hvq,where v stands for the light frequency, h stands for Planck’s constant, λ for the incident light’s wavelength, and c for the speed of light. The relationship between the external quantum efficiency and responsivity is inverse.Table1 shows the electrical parameters for the five different kinds of silicon substrates used in this work. We tested carrier lifetime, diffusion coefficient, and mobility. It can be seen that the #5 sample presents a higher carrier lifetime, diffusion coefficient, and mobility. The higher the resistivity of silicon, the lower the impurity concentration and the higher the mobility, which is consistent with the theory. Due to the high doping concentration of the #1 sample, parameters such as carrier lifetime were not measured. Then we fabricated HgTe-CQDs/silicon photodetectors and measured their key parameters. The structure of the photodiode based on HgTe-CQDs/silicon heterojunction is shown in Figure 6(a). The photovoltaic voltages produced by HgTe-CQDs/silicon channel surfaces are employed for photoelectric detection. A light-absorbing layer is provided by the HgTe CQDs film. Figure 6(a) depicts the corresponding energy band diagram. We used a silicon substrate that was n-doped. On top of this n-contact, a p-type HgTe CQD layer is deposited. The actual photosensitive region is 0.2 cm2 and the device size is 1 × 1 cm2. The responsivity, reflection ratio, and dark current for HgTe-CQDs/Si photodetectors with different Si substrates are shown in Figure 6(b)–6(d), respectively. Clearly, the device uses the Si substrate with a resistivity of 20–50 Ω·cm and exhibits the highest responsivity and reflection ratio, as well as the lowest dark current, demonstrating its suitability for visible and near-infrared light detection.Figure 6
(a) The structure of the HgTe-CQDs/silicon photodiode is depicted schematically and band diagram of HgTe : Si device; (b) the responsivity of the HgTe-CQDs/silicon photodiode with the silicon of different resistivities; (c) rectification ratio of the HgTe-CQDs/silicon photodiode with the silicon of different resistivities; (d) dark current of the HgTe-CQDs/silicon photodiode with the silicon of different resistivities.
(a)(b)(c)(d)Table 1
Summary of silicon performance of different resistivity.
NumberSample (Ω·cm)Carrier lifetime (s)Diffusion coefficient (cm2 s−1)Mobility (cm2 V−1 s−1)10.001–0.005–––20.01–0.022.561.1142.8630.1–0.519.244.79184.9441–1030.386.28242.47520–5056.018.99347.10Thus, we carried out a detailed characterization of the device. Figure7(a) depicts the detected light response of HgTe-CQDs/silicon photodiode, demonstrates a broad spectral response between 405 and 1,550 nm. Figure 7(b) shows photoresponsivity curves of HgTe-CQDs/silicon photodiodes at 1,550 nm. The device responds at 1,550 nm with a responsivity of 0.2 mA/W (30 mW/cm2). I–V curves of HgTe-CQDs/silicon heterojunction devices were obtained in the dark and under 980 nm light irradiation to further investigate the electrical properties of the heterojunction and to learn more about the electrical characteristics of the heterojunction, I–V curves of HgTe-CQDs/silicon devices were obtained in the dark and under 980 nm light irradiation. The findings are displayed in Figure 7(c). The response time is described as the times from 10% and 90% of the rising edge and 90% to 10% of the falling edge [35]. A quick reaction is produced when the photodetector’s surface is exposed by light due to the generation of photogenerated carriers. The photodetector’s response gradually diminishes as the number of carriers grows until it approaches stability. The photodetector’s rise and fall times are 380 and 360 μs, respectively, as shown in Figure 7(d). By studying the response characteristics of HgTe-CQDs/silicon photodetector under different voltages, as shown in Figure 7(e). The sensitivity to light increases with increasing the bias voltage from 0 to 1 V, and Ion/Ioff values of 98, 350, 580, 980, 1,500, and 2,300 are obtained at bias voltages of 0, 0.2, 0.4, 0.6, 0.8, and 1 V, respectively. This demonstrates that this device is capable of producing and separating electron–hole pairs [36]. Light intensity is also an important factor in determining a photodetector’s photocurrent. As shown in Figure 7(f), the light intensity-dependent response of the HgTe-CQDs/silicon device was investigated further. The optical response was significantly improved when the incident light power intensity was increased from 5.1 to 234.3 mW/cm2. Then, we contrast a variety of performance metrics for a HgTe-CQDs/silicon photodetector with a variety of conventional photodetector types in Table 2. It can be seen that our silicon-compatible HgTe quantum dot device is in the leading position.Figure 7
(a) Photoresponse of HgTe-CQDs/silicon photodetectors under incident light of 405, 520, 638, 980, and 1,550 nm (30 mW/cm2) at a bias of 1 V; (b) photoresponsivity curves of the HgTe-CQDs/silicon photodiodes at 1,550 nm; (c) I–V curves of HgTe-CQDs/silicon heterojunction devices in the dark and under light illumination at 980 nm; (d) at 980 nm, rising and falling edges are used to estimate the rise time (Tr) and fall time (Tf); (e) photoresponse of HgTe-CQDs/silicon heterojunction under different bias voltages at 980 nm; (f) photoresponse of the HgTe-CQDs/silicon heterojunction under different light intensities at 980 nm.
(a)(b)(c)(d)(e)(f)Table 2
Si:CQDs PD performance is compared to CQDs-based PD in terms of device performance.
DevicesOperating temperature (K)Responsivity (A/W)Detectivity (Jones)Time response (ms)ReferenceSi/HgTe-based photodiode3004 × 10−24.4 × 10100.36This workHgTe CQDs-based photoconductor3003.5 × 10−36.6 × 107–[27]Si/Graphene/HgTe-based photodiode3000.95 × 109–[28]HgTe CQDs-based phototransistor260–3000.562 × 10100.012[37]Si/PbSe-based phototransistor300648.77.48 × 10100.0732[38]


## 4. Conclusion

To summarize, we described a silicon-based infrared photodiode made at room temperature using a solution technique. We explore the influence of silicon resistivity on the performance of these photodiodes. The silicon substrate with a resistivity of 20–50Ω·cm has optimal performance parameters. Further, these HgTe-CQDs/Si photodiodes had a broad response spectrum, ranging from 405 to 1,550 nm. The device reaches a high external responsivity of 0.2 mA/W for 1,550 nm incident light at room temperature. In addition, the photodetector offers quick response times (Tr/Tf = 380/360 μs) and a decent response of 40 mA/W at 980 nm. The remarkable performance of the HgTe-CQDs/silicon photodetector implies that this material has a bright potential in the optoelectronic industry.



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*Source: 4595819-2023-02-09.xml*