Photoelectric Characteristics of n-ZnO/p-Si Heterojunction Photodetector

To solve the problems of the high experimental cost and single experimental result, the n-ZnO/p-Si heterojunction photodetector was simulated by TCAD software. Based on the established model, the photoelectric characteristics of n-ZnO/p-Si heterojunction photodetector were simulated and analyzed in terms of ZnO carrier concentration and thickness. The results show that the dark current is about 10−14 A, the photocurrent is about 10−8 A, and the photocurrent is at least three orders of magnitude larger than the dark current. The fast response time is 10−10 s. The maximum response of the device is 0.25 A/W in the ultraviolet band and the maximum response is 0.35 A/W in the visible light region. This simulated result establishes a theoretical foundation for further experimental research.


Introduction
The photodetector is a device that enables the conversion of light to electrical signals [1].Ideal photodetectors are used in semiconductor device physics, missile plume detection, ozone monitoring, and other fields, requiring high detector responsiveness.There are many studies on heterojunction photodetectors [2][3][4].ZnO is a wide bandgap semiconductor, which is suitable for photon detection.In recent years, ZnO has been widely studied because of its good photoelectric properties [5,6].Because ZnO is essentially an n-type material and it is difficult to obtain high-quality p-type ZnO, another p-type material in contact with ZnO is often used to construct ZnO-based p-n heterojunction photodetectors.In the past several years, some p-type semiconductor materials such as Si, GaN, SiC, etc. have been used to achieve p-n heterojunction [7,8].Due to the maturity of silicon-based microelectronics technology, silicon substrates are widely used for ZnO heterojunction photodiodes.This device can usually detect both ultraviolet and visible light simultaneously, which is highly advantageous in many industrial applications.In addition to a wide detectable spectral range, n-ZnO/p-Si heterojunction photodetectors have excellent optoelectronic performance, simple fabrication, relatively low deposition temperatures, and low production cost.This is also an important reason why researchers have applied them in the field of photodetectors.Researchers have reported some experimental and simulation analyses of the optoelectronic properties of n-ZnO/p-Si heterojunction photodetectors [9,10], but most of them are based on experimental studies, few are based on simulations, and there is no estimation of the response time of n-ZnO/p-Si heterojunction photodetectors as a function of ZnO material parameters.Therefore, this article simulates the effect of n-ZnO/p-Si photodiode on the photoelectric performance of devices with changes in ZnO parameters.Software simulation study can not only solve the problem of the high cost of experimental research, the most important thing is to be able to study the impact of multiple factors on the performance of the device.Of course, there is a certain degree of error in the simulation study, but this error is within the negligible range.

Device Structure and Simulation Model
Figure 1 shows the two-dimensional structural model of the device constructed by the device simulation module of the application software.The n-type ZnO with a thickness of 0.5 μm and a doping concentration of 1×10 19 cm -3 was grown on a highly doped p-type silicon material with a thickness of 12 μm.With the cathode electrode in contact with ZnO, the anode as the back electrode is distributed on the back of Si.During simulation, the effect of electrode thickness on device characteristics was not considered, so the electrode thickness was set to 0 during modeling.The incident light is set to be a vertical incident light source, and the incident light wavelength is 375 nm.The conductivity characteristics of p-n diodes are mainly controlled by minority carriers, and even if there is a slight change in minority carrier density, the current will undergo a significant change.From this perspective, p-n junction photodiodes are very sensitive to incident light.The p-n heterojunction photodetector is subjected to reverse bias, resulting in a wider space charge region.Under the influence of applied voltage and light, the device generates photogenerated carriers and passes through the place charge region at a saturation velocity, and the carriers also undergo collisional ionization during this movement process.Based on the above working principles, simulation models such as the carrier generation recombination model, mobility model, and collision ionization are selected in the simulation.The Newton iterative numerical calculation method is used to solve the equation.Because each model is supported by corresponding parameters in the calculation, more material parameters are used in the simulation, and the values of some of the parameters used in the simulation are listed in Table 1.

I-V Characteristics
An ideal photodetector should not generate current under no light conditions, but due to the presence of hot electron emission inside the semiconductor device, the device still generates a certain amount of current under a certain electric field.This leakage current output under light-free conditions by applying a reverse bias voltage to the detector is called dark current, which is one of the significant parameters affecting the performance of the device.The dark current is a noise signal and its presence affects the performance of the device, such as responsiveness.Therefore, when designing optoelectronic devices, the dark current should be reduced as much as possible to improve the performance of the device.The width of the depletion zone is an important factor determining the dark current of the device, as shown in the following equation.0 ( ) In the equation,  is the vacuum dielectric constant,  is the dielectric constant of ZnO material,  is the built-in potential of the p-n diode, V is the applied bias voltage, q is the electron charge, and N D is the donor concentration.
Figure 2 shows the I-V curves of the device under no light and light illumination conditions with an applied 5 V bias voltage.

Figure 2. I-V curve
It can be observed from Figure 2 that the simulated dark current of the n-ZnO/p-Si heterojunction photodetector is about 10 -14 A. Low dark current ensures low noise in the device.The light current is in the order of 10 -8 and the ratio of photocurrent to dark current is much larger than three orders of magnitude, which ensures the proper operation of the device, and the device has good rectification characteristics within the ±5 V voltage range.

Responsivity
Responsivity is an important parameter that characterizes the ability of a photodetector to convert incident light signals into electrical signals [11].The responsivity is calculated from the quantum efficiency, which is calculated from the spectral response curve.The equation is expressed as follows: In the equation, q is the electronic charge and η is the quantum efficiency, λ is the wavelength of the incident light, h is the Planck constant, and c is the speed of light.
Spectral response is simulated by using the Atlas emulator, the incident light is set as a vertical incident light source, and wavelength varies in equal intervals from 200 nm to 1000 nm in 100 nm steps.The  3 that the device responsivity is higher when the ZnO carrier concentration is low.This is because when the ZnO carrier concentration is low, the wider space charge region is distributed in the ZnO material layer.The wider space charge region of the ZnO absorption layer can separate more photo-generated carriers and improve responsivity.From Figure 3 (a), in the ultraviolet range, the device responsivity decreases with the increase of ZnO carrier concentration and the highest value of the device responsivity can be up to 0.25 A/W.From Figure 3 (b), it can be seen that there is no change in the responsivity of the device with the variation of ZnO carrier concentration in the visible light range.This is because the long wavelength limit of ZnO is around 365 nm, beyond the ultraviolet light range, changes in ZnO parameters will not have any impact on the device.The highest responsivity of the device in the visible light range is 0.35 A /W, which is highly consistent with the experimental results [12].4, it can be seen that when the thickness of ZnO is low, the device responsivity is higher.This is because when the ZnO film is thick, it will make it difficult for photo-generated carriers to accumulate on the surface of the ZnO absorption layer and will be separated by the electric field, resulting in a decrease in photocurrent and thus a decrease in responsivity.From Figure 4 (a), it can be seen that within the ultraviolet range, the maximum responsivity is up to 0.25 A/W.From Figure 4 (b), it can be seen that there is no change in device responsivity with the variation of ZnO thickness in the visible light range.

Response time
The important parameters of the photodetector, in addition to the simulation calculations mentioned above, also include a response time that reflects the performance of the detector.When light illuminates a photodetector, photogenerated carriers are produced, which move towards opposite electrodes in response to an electric field.Only when the electrons and holes reach the electrodes and form a photocurrent can they be detected in the external circuit.The time to convert incident light into photocurrent is the response time, which mainly includes the photocarrier transit time in the depletion region  , the diffusion time of photo carriers generated outside the depletion region of  , and the RC time constant of diff and photodiode.Their related equation is as follows.Observing the plot, it is found that the response time of the detector is about 10 -10 s and it is fast.In Figure 5, the response time increases with the increase of ZnO carrier concentration and thickness, but the photocurrent significantly decreases.This is because as the concentration of ZnO carriers increases, the width of the heterojunction space charge region narrows.A narrow space charge region increases the junction capacitance, the capacitor charging and discharging time, and the RC time constant.Therefore, the response time becomes longer under the influence of the time constant.As the thickness of ZnO increases, the barrier region expands to a certain extent, and the carrier transit time increases, resulting in a longer response time.In summary, the carrier concentration and thickness of ZnO heterojunction photodetectors should not be too large.

Conclusion
In a word, n-ZnO/p-Si Heterojunction photodetectors not only react to ultraviolet light but also to visible light, which breaks the limitation that the detector only responds to a certain type of light wave and is very useful in many industrial applications.This photodetector has a maximum response in the ultraviolet band of 0.25 A/W, and a maximum response in the visible band of 0.35 A/W, with a response time of about 10 -10 s and a fast response.This simulation calculation result provides a reliable theoretical basis for further experimental optimization of the heterojunction photodetector structure.

Figure 3 .
Figure 3.Effect of ZnO carrier concentration on responsivity Figure 3 (a) and Figure 3 (b) show the effects of ZnO carrier concentration changes on device responsivity in the ultraviolet and visible light bands, respectively.It can be seen from Figure3that the device responsivity is higher when the ZnO carrier concentration is low.This is because when the ZnO carrier concentration is low, the wider space charge region is distributed in the ZnO material layer.The wider space charge region of the ZnO absorption layer can separate more photo-generated carriers and improve responsivity.From Figure3(a), in the ultraviolet range, the device responsivity decreases with the increase of ZnO carrier concentration and the highest value of the device responsivity can be up to 0.25 A/W.From Figure3(b), it can be seen that there is no change in the responsivity of the device with the variation of ZnO carrier concentration in the visible light range.This is because the long wavelength limit of ZnO is around 365 nm, beyond the ultraviolet light range, changes in ZnO parameters will not have any impact on the device.The highest responsivity of the device in the visible light range is 0.35 A /W, which is highly consistent with the experimental results[12].

Figure 4 .
Figure 4. Effect of ZnO thickness on responsivity Figure 4 (a) and Figure 4 (b) show the effects of ZnO thickness changes in the ultraviolet and visible light bands on device responsivity, respectively.Observing Figure4, it can be seen that when the thickness of ZnO is low, the device responsivity is higher.This is because when the ZnO film is thick, it will make it difficult for photo-generated carriers to accumulate on the surface of the ZnO absorption layer and will be separated by the electric field, resulting in a decrease in photocurrent and thus a decrease in responsivity.From Figure4(a), it can be seen that within the ultraviolet range, the maximum

Figure 5 .
Figure 5. Response time curve Figure 5 (a) and Figure 5 (b) show the variation of the device response time under the influence of ZnO carrier concentration and thickness.Observing the plot, it is found that the response time of the detector is about 10 -10 s and it is fast.In Figure5, the response time increases with the increase of ZnO carrier concentration and thickness, but the photocurrent significantly decreases.This is because as the concentration of ZnO carriers increases, the width of the heterojunction space charge region narrows.A narrow space charge region increases the junction capacitance, the capacitor charging and discharging time, and the RC time constant.Therefore, the response time becomes longer under the influence of the time constant.As the thickness of ZnO increases, the barrier region expands to a certain extent, and the carrier transit time increases, resulting in a longer response time.In summary, the carrier concentration and thickness of ZnO heterojunction photodetectors should not be too large.

Table 1 .
Simulation Section Material Parameter Values