Abstract
The response mechanism of a Four-Quadrant Photodetector (QPD) in an experimental setting was studied by irradiating a single QPD cell with a millisecond-pulsed laser. The response signal of the irradiated QPD cell varied with energy flux, pulse width, and applied bias, and comprised four main stages: an initial stage, decreasing barrier stage, holding stage, and recovery stage. Not only was the response signal of the irradiated cell affected by laser irradiation, but also the responses of the other three cells. This response in non-irradiated cells is mainly related to the common region structure, electric field, carrier generation and recombination in the QPD. The performance of each cell in the QPD can be distinguished, due to the differences in response signals between the irradiated cell and the other three. The research results have good application prospects in the fields of laser guidance and atomic force microscopy.
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Introduction
The Four-Quadrant Photodetector (QPD) has attracted much attention as an accurate position detector that operates with a low voltage1,2. A QPD can obtain a spot position by comparing the response signals from each of its four cells. Therefore, the study of its response signal under irradiation is important. An output current model of a cell detector has already been reported3, as well as a relaxation phenomenon in which the temperature affects a QPD’s signal4,5.
QPDs can be easily damaged, because they absorb laser energy6,7; this weakness is considered by the international scientific community as a significant technological problem8. Consequently, it is important to study how laser irradiation affects the performance of these detectors, and to understand how laser irradiation interacts with the QPD. In the past few years, many studies have investigated thermal damage in silicon PN-junctions9,10,11. Wang et al.12,13 studied the effect of an external circuit on damage to an avalanche photodiode (APD) by adjusting the bias or capacitance; they also investigated the influence of internal doping by isolating the capacitance.
To increase QPD resilience, a position detection system was designed based on a silicon-based positive-intrinsic-negative QPD and Nd: YAG laser. The response signal of the QPD cells was modulated by the changing the laser energy density, pulse width, and applied bias in the experiment. The QPD response mechanism was simulated by the semiconductor physics. These results provide both theoretical and experimental guidance on the design of the detection system.
Experimental testing and theory
Figure 1 shows the experiment setup, which included the optical and current measuring system. The QPD used in this experiment is a four-quadrant detector model GT111, manufactured by the 44th Research Institute of China Electronics Technology Group Corporation. An Nd: YAG laser (1064 nm) was used with 10 J maximum output and a pulse width which varied from 0.5 to 3.0 ms. The pulse repetition frequency could be varied from 1 to 10 Hz. A beam splitter was used to split the laser beam into an energy meter and a focusing lens with focal length 15 cm. The laser pulse width and energy were recorded by a THORLABS DET10 A/M probe and a NOVA II energy meter. The current measuring system, which used a Tektronix PWS2721, applied a bias in the range 0 to 40 V. The measurement system also included sampling-resistors (R1, R2, R3, R4), current-limiting resistors (Rlimiting), and a silicon-based QPD. These resistors connected the current measurement system of the four cells of the QPD. Only Cell 1 in the QPD was irradiated by the laser. The response signal from each quadrant was observed using the oscilloscope.
Experiment setup for a silicon-based QPD irradiated by a millisecond pulse laser.
Figure 2 shows the mechanism of carrier transportation in the QPD detectors with a silicon three-layer P + -I-N + structure. The doping concentration in the photosensitive area (P + -region) was 5.0 × 1019/cm3, and the depth of the junction was 1.5 μm. The absorption area (I-region, substrate semiconductor) between the P + -region and N + -region had a phosphorus ion doping concentration of 1.0 × 1012/cm3 and a thickness of 280 μm. The substrate layer (N + -region) had a thickness of 2 μm and a doping concentration of 1.0 × 1020/cm3. The four cells in the QPD were isolated by a 210-micron-wide substrate semiconductor. The response mechanism of the four cells was studied when irradiating a single quadrant cell. Based on the structure of the QPD with four cells and a common substrate, the mechanism of laser-induced carrier movement (electrons and holes) in the QPD is described in Fig. 2.
A diagram of a QPD and the movement of particles inside a QPD when a laser irradiates Cell 1.
To consider the influence of the electrons and holes formed in Cell 1 on the neighboring Cell 2, it is assumed that all regions are continuous and finite. Taking Cell 1 and Cell 2 as examples, the influence of applied bias on each cell is analyzed, and an expression for the QPD response signal was derived.
The photo-generated current is equal to the sum of the drift and the diffusion current, expressed as,
Here \({\text{J}}_{\text{L}}(\text{T},\text{x},\text{y},\text{t})\)\(J_{L} (T,x,y,t)\) is the photo-generated current density, \({\text{J}}_{\text{diff}}(\text{T},\text{x},\text{y},\text{t})\) \(J_{diff} (T,x,y,t)\) is the diffusion current density, and \({\text{J}}_{\text{dr}}(\text{T},\text{x},\text{y},\text{t})\)\(J_{dr} (T,x,y,t)\) is the drift current density.
where \(\text{L}(\text{T})\)\(L(T)\) is the diffusion length. \(\text{D}(\text{T})\) \(D(T)\) is the diffusion coefficient, and \(\tau\) is the lifetime.
The response signal for Cell 1 includes the initial stage, the decreasing barrier stage, the holding stage, and the recovery stage:
where \({\text{R}}_{1}={\text{R}}_{\text{i}}={\text{R}}_{\text{c}}\) \(R_{1} = R_{i} = R_{c}\), \(A_{i}\) is the PN-junction of Cell i, \(\varepsilon_{r}\) is the dielectric constant, \(T\) is temperature, and \(\text{W}(\text{T},\text{t})\) \(W(T,t)\) is the total length of the band gap.
The response signal model for Cell i is expressed as follows:
where \({\text{J}}_{\text{i}}(\text{T},\text{x},\text{y},\text{t})\)\(J_{i} (T,x,y,t)\) is the diffusion current density for Cell i. In addition, \({\text{V}}_{\text{i}}\) \(V_{i}\) is the applied bias and \(I_{0,i}\) is the current when the barrier is pulled down.
The resulting response signal (including the four stages noted above) and the laser pulse are shown in Fig. 3. The initial stage and the decreasing barrier stage are when the response signal peaks and then falls at the start of the irradiation. According to the theoretical model and experiment results, the duration of the barrier stage is about 25 μs (i.e., microsecond time scale). The holding stage is when the response signal decreases slowly from the peak to a relatively stable level, when the laser is turned off. The recovery stage is when the response signal rapidly declines after the irradiation stops, until the relaxation stage. After reaching the relaxation stage, the response signal rises to the second peak. Then, the response signal decreases until it recovers to its initial value.
Response signal time-series and (left) laser pulse signal time-series (right).
Results and discussion
Subsection
Influence of laser energy density on response signal
The response signal as a function of time for Cell 1 is shown in Fig. 4. An increase in the laser’s energy causes the recovery time to gradually increase. This is because the higher energy flux causes the non-equilibrium carriers inside Cell 1 to take longer to recover the original thermal balance. However, there are some deviations, which are due to differences related to the manufacture of individual QPDs and the heterogeneous distribution of their doping4,5. It can be seen from the results that there is little correlation between relaxation and energy flux.
Response signal time series at different fluences. (τ = 1.5 ms, U = 40 V).
Influence of laser pulse width on response signal
Figure 5 shows the response signal of Cell 1 with different pulse widths. The measured values are shown in Table 1. The peak values of each relaxation are respectively, A (1.91 ms, 35 μA), B (2.30 ms, 36 μA), C (2.66 ms, 38 μA). Therefore, when the energy density is below the damage threshold, the relaxation amplitude of the response signal does not change obviously at different pulse widths, mainly because the relaxation amplitude is not affected by the laser irradiation time when below the damage threshold (i.e., there is a low correlation with temperature). The above results are needed to determine whether the external bias is the main factor causing variations in the response signal.
Cell 1 response signal as a function of time, for τ = 1.5 ms and τ = 2.0 ms. A (1.91 ms, 35 μA), B (2.30 ms, 36 μA), C (2.66 ms, 38 μA).
Influence of applied bias on response signal
Figure 6 shows the response signal of Cell 1 as a function of time at 1.5 ms and 27 J/cm2. As the external bias increases, the average current in the holding stage increases. This increase is due to an increasing potential difference between the two ends of the PN-junction. The relaxation amplitude also gradually increases. Because the electric potential difference across the PN-junction increases, carriers in the cell moves faster under the action of the electric field, and the carrier concentrations and temperature gradient change rapidly until thermal equilibrium is reached. This result demonstrates that the external bias has a strong influence on the laser irradiated QPD.
Evolution of the response signal under different applied biases (τ = 1.5 ms, E = 27 J/cm2).
The holding stage and recovery stage, which this paper has already described, have never been previously observed. It is speculated that these phenomena are related to the special structure in the silicon-based QPD common I-region. This analysis combines the response signal evolution stage of each cell and the data from these measuring points (such as the peak of the recover stage).
Response signal of cells in the same conditions
Figure 7 shows the response signals of each cell under different energy fluxes. The current recorded in Cell 1 is significantly different to that in the other three cells. The response signals in Cells 2 and 4 are similar, but the response signal in Cell 3 is relatively weaker. The beam center is at the same distance from Cells 2 and 4 so it should have a similar influence on the temperature gradient and carrier gradient inside these two cells. The shape of four-stage trend are different for each cell.
Evolution of current in each quadrant (τ = 1.5 ms, E = 27.65 J/cm2, U = 40 V).
At the initial stage of laser irradiation, Fig. 8 indicates that the temperature of the PN junction has not yet increased. The absorption mechanism in the QPD is mainly photon absorption and free carrier absorption. Under the action of a strong electric field, the photo-generated carriers will form a current. Due to the limited resistance in the external circuit, photo-generated carriers cannot completely enter the external circuit. As a result, an imbalance of carriers develops in the PN-junction, creating a charge accumulation at both ends of the PN junction which generates an electromotive force14,15,16,17. As the photoelectromotive force subsequently decreases, carriers in Cell 1 will continuously diffuse into other cells. This weakens the electromotive force in Cell 1. After the laser irradiation, the concentration of non-equilibrium carriers in Cell 1 gradually decreases, and the temperature cools. The thermoelectromotive force become the main drivers of changes in the output signal.
Evolution of the response signal in CELL 1. τ = 2.0 ms, E = 27.65 J/cm2, U = 40 V.
A photogenerated electromotive force is the main reason for the current response seen in Cell 1. The temperature and carrier concentration gradient, which diffuse from Cell 1, then affect the current responses in Cells 2, 3, and 4. Their responses include the thermally generated electromotive force on the carriers. Since the response signals for Cells 2, 3 and 4 are similar, the results for Cell 2 are analyzed as an example.
Figure 9 shows the evolution of the response signal for Cell 2. In the initial stage, the response signal is negative. This is not random noise, and is produced for the following reasons. The carriers irradiated by the laser accumulate in the P-region near the hole carriers, and rapidly move to the other three cells. The other cells, which are originally in an equilibrium state, then move with the high-voltage field that is produced by the electromotive force associated with the P-region and N-region charge accumulation. In Cell 2, carrier gradients also form in the P-region and the N-region. Since there is no temperature rise in the P–N junction area, the carrier movement is caused by the change in the concentration gradient of carriers between the quadrants. In addition, in a photovoltaic detector, a Schottky barrier is formed when the metal substrate is in contact with the semiconductor15. Under the condition of reverse applied bias, the potential barrier is reduced, and the number of electrons flowing from the semiconductor to the metal is also reduced. The electrons flowing from the metal to the semiconductor form a current from the P-region to the N-region.
Evolution of the response signal in CELL 2. τ = 2.0 ms, E = 27.65 J/cm2, U = 40 V.
The above two processes are superposed in the output signals and cause a negative output signal from Cells 2, 3, and 4 during irradiation. In CELL 1, this signal is much smaller than the photo-generated current and can therefore can be ignored.
After the laser irradiation stops, the relaxation amplitudes of the response signals in Cells 2, 3, and 4 are lower than that in Cell 1 because their temperature gradients, carrier concentration gradients, and carrier speeds are far lower than those in Cell 1.
Conclusions
The response signal of a QPD was obtained when a single cell was irradiated by a millisecond pulsed laser. The results can improve the detection ability of a QPD. Based on differences in the response signals between the direct cell and the adjacent cells under the modulation of energy flux, pulse width and applied bias, the interaction between cells caused by their common structure and carrier concentration was analyzed. Results show that irradiation can redistribute the carrier concentration under a strong electric field, thereby aiding the function of a QPD in precise positioning. The response signal stability was found to be optimal at 2.5 ms. This method is of great significance for the application and development of a QPD in laser guidance and atomic force microscopy.
Data availability
The authors declare that the data supporting the findings of this study are available within the paper.
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Acknowledgements
We thank the Key Laboratory of Jilin Province Solid-State Laser Technology and Application for the use of the equipment.
Funding
This research was funded by National Natural Science Foundation of China, grant number 62005023. National Natural Science Foundation of China under Grants, grant number U2141239.
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Conceptualization, H. L. and W. C.; methodology, H. L.; software, W. C.; validation, H. L.; formal analysis, H. L.; investigation, W. C.; resources, H. L.; data curation, H. L.; writing—original draft preparation, W. C.; writing-review and editing, W. C.; supervision, H. L.. All authors have read and agreed to the published version of the manuscript.
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Chen, W., Liu, H. & Song, D. Mechanisms driving different QPD cells response signals revealed by a single cell irradiated with a laser. Sci Rep 15, 656 (2025). https://doi.org/10.1038/s41598-024-84875-2
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DOI: https://doi.org/10.1038/s41598-024-84875-2
