Detection of the soft X-ray with photon energy between 0.5 and 10 keV with a wavelength even shorter than EUV (i.e.,
Chinese Optics Letters, Volume. 23, Issue 3, 031202(2025)
4H-SiC-based soft X-ray single photon detector with linear photon energy response
In this work, a 4H-SiC-based soft X-ray single photon detector with photon energy resolution capability is demonstrated. The 4H-SiC p-i-n detector with an 80-μm-thick epi-layer and low intrinsic doping exhibits a low leakage current of ∼1.8 pA at -180 V, guaranteeing superior dark current performance for single photon detection with low electronic noise. An amplification strategy employing an active switch in the charge-sensitive amplifier has also been developed, where feedback-resistance-related thermal noise has been well eliminated, contributing to lower electronic noise in the amplification stage. By tuning the shaping time in the analog-to-digital circuit for precise signal processing, an optimal photon energy resolution has been achieved with a duration time within 6.4 µs, achieving an energy analysis standard deviation below 5.7%. Ultimately, superior linearity has been obtained between the output pulse amplitude and the characteristic photon energy by utilizing a series of different metal targets, opening a new opportunity for advanced soft X-ray detection technology based on wide bandgap semiconductors.
1. Introduction
Detection of the soft X-ray with photon energy between 0.5 and 10 keV with a wavelength even shorter than EUV (i.e.,
The development of SiC-based soft X-ray detectors, however, is at its very early stage, especially for the single photon detector with photon energy resolution capability. The key challenges can be divided into two aspects. One is the fabrication of a SiC detector that requires an
In this work, a p-i-n-type 4H-SiC soft X-ray detector with an 80-
Sign up for Chinese Optics Letters TOC Get the latest issue of Advanced Photonics delivered right to you!Sign up now
2. Device Fabrication
The schematic cross-sectional structure of the 4H-SiC soft X-ray detector fabricated in this work is illustrated in Fig. 1(a). The device is grown on an
Figure 1.(a) Schematic diagram of the 4H-SiC p-i-n detector. (b) Photograph of the packaged device. (c) Dark I–V characteristics and (d) C–V characteristics of the detector at room temperature.
3. Results and Discussion
As presented in Fig. 1(c), an extremely low dark current of
For single photon detection and energy resolution, the photo-generated carriers in numbers of 100 to 1000 corresponding to one single soft X-ray photon need to be collected and analyzed. Since the carrier number is linearly dependent on the photon energy, the voltage output from the amplifier can be utilized for photon energy analysis, given realized linear amplification. The CSA circuit is usually adopted for first-stage amplification, which is the key stage for interpreting photon energy information into the voltage pulse height. In the CSA circuit, a feedback capacitor is used for charge coupling from the detector, where then a voltage change linearly related to the number of collected charges is generated on the capacitor. The smaller the feedback capacitor is, the larger the voltage change will be, which will then be fed into the amplifier circuit. To recover the voltage on the capacitor to the DC operating point at the input stage of the amplifier, a feedback resistor is usually adopted and connected in parallel with the feedback capacitor for the dissipation of the accumulated charges, as shown in Fig. 2(a). This resistor, however, will either enhance the thermal noise with a low resistance value or elevate the time constant for the capacitor charge release process, limiting the operation speed and thus the photon counting rate. Correspondingly, this work has introduced an active switch module as the substitute for the feedback resistor, as shown in Fig. 2(b). This switch consists of a comparator, a pulse generator, and a reset diode. When the voltage on the capacitor is below the threshold voltage of the comparator, the switch maintains the off state for high resistance to alleviate the thermal noise during the charge accumulation on the capacitor. Once the capacitor voltage reaches the threshold of the comparator, the active switch will turn on, rapidly discharging the feedback capacitor by feeding a current pulse into the capacitor through the reset diode, thus returning the circuit to its initial state. Since the effective resistance of the switch can be maintained over 1 GΩ, the switching circuit is well isolated from the capacitor with negligible influence on the signal coupling and amplification.
Figure 2.(a) Schematic of the resistor-capacitor feedback CSA circuit. (b) Schematic of the reset CSA circuit. AMP, amplifier.
On the other hand, the leakage current of the detector will also be fed into the capacitor, introducing a continuous change in the capacitor voltage. Therefore, the suppression of the leakage current is critical for noise control. In this work, due to the good control on the device leakage, i.e.,
Figure 3.Time-domain voltage evolution on the capacitor as a result of the detector’s leakage current and active switch turning on.
With this specifically established CSA featuring the active switch, the transient signals as a result of a single soft X-ray photon incidence can be captured, as shown in Fig. 4. The photon as excited from the Mn target with characteristic energy of 5.89 keV has produced a pulse voltage with an amplitude of
Figure 4.A transient output signal as a result of a single soft X-ray photon incidence with a characteristic energy of 5.89 keV.
To further demonstrate the photon energy analysis capability for the single soft X-ray photon, the analog-to-digital converter (ADC) module has been utilized for statistical analysis of the pulse height of a large amount of incident photons. That is, with a chosen metal target, characteristic photon energy with specific photo-generated carriers and thus uniform pulse height should be obtained. However, the electronic noise from both the detector and the amplification circuit will introduce the widening in the distribution of the pulse heights, while the widening effect can be quantified with the standard deviation[27]. Considering that, the original single photon signal with a sharp rise edge as shown in Fig. 4 will create challenges for pulse peak sampling, and a pulse reshaping unit becomes necessary in the ADC module, where the pulse will be transferred into Gaussian waveform with the pulse height preserved. This is beneficial for the pulse height recording and statistical analysis of the signals. Correspondingly, enough shaping time is required for the adequate acquisition of the former pulses, while a shaping time that is too long will lead to an integration effect of leakage-related noise on the other hand.
Various shaping time have been applied to seek the optimal value with the smallest full width at half-maximum (FWHM) of the pulse height distribution. As presented in Fig. 5(a), for the Mn target with the characteristic energy of 5.89 keV, an energy resolution as low as 0.78 keV has been obtained with a shaping time of 6.4 µs, corresponding to a standard deviation of 5.7% for photon energy analysis. It should also be noted that the energy resolution shows minor degradation with increasing shaping time, agreeing well with the low dark current of the fabricated SiC detector. Then, by utilizing a series of the metal targets, i.e., Ti, Mn, Ni, Ge, and Y, pulse height distributions with different centerline positions have been observed and presented in Fig. 5(b). The centroid value of the Gaussian distribution exhibits near-ideal linear dependence on the characteristic energies, i.e.,
Figure 5.(a) FWHM at 5.89 keV (characteristic energy) versus shaping time of the pulse shaping unit. (b) Energy resolution of the 4H-SiC soft X-ray single photon detectors under different characteristic energies.
4. Conclusion
In summary, based on the fabrication of the 4H-SiC detector with ultra-low room temperature leakage and the introduction of the active switch for the charge release from the feedback capacitor in the CSA circuit, this work has demonstrated a room temperature 4H-SiC soft X-ray single photon detector with linear energy response performance. Correspondingly, this work has explored and presented in detail the device fabrication techniques, amplification electronics strategies, and shaping time requirements for the photon analysis in the 4H-SiC soft X-ray detector, paving the way to advanced soft X-ray detection technology based on wide bandgap semiconductors. Meanwhile, for the future advancement of 4H-SiC-based soft X-ray detection technology, further optimization of the noise performance in the electronics is needed, and lower dark current with specific suppression of the ballistic deficit effect in the 4H-SiC detector is also preferred, which will facilitate a wider application of 4H-SiC-based soft X-ray technology.
[2] J. P. Halpern, M. RudermanReport. Soft x ray properties of the Geminga pulsar. Imaging the Nearby Seyfert 2 Galaxy NGC 1068, and Spectrum and Variability of Geminga(1993).
[6] G. Steinhauser, K. Buchtela. Gas ionization detectors. Handbook of Radioactivity Analysis, 245(2020).
[10] V. V. Zabrodsky, P. N. Aruev, V. L. Sukhanov et al. Silicon precision detectors for near IR, visible, UV, XUV and soft X-ray spectral range. Proceedings of the 9th International Symposium on Measurement Technology and Intelligent Instruments(2009).
[11] R. Korde, L. Randall Canfield. Silicon photodiodes with stable, near-theoretical quantum efficiency in the soft x-ray region. X-Ray Instrumentation in Medicine and Biology, Plasma Physics, Astrophysics, and Synchrotron Radiation(1989).
[17] O. Karadavut, S. K. Chaudhuri, J. W. Kleppinger et al. Performance-improved vertical Ni/SiO2/4H-SiC metal–oxide–semiconductor capacitors for high-resolution radiation detection. IEEE Trans. Nucl. Sci., 69, 1965(2022).
[22] G. E. Stillman, C. M. Wolfe. Avalanche photodiodes. Semiconductors and Semimetals, 291(1977).
[25] S. Capra, A. Pullia. Study of the effects of parasitic capacitance on large integrated feedback resistors for charge-sensitive preamplifiers. 2014 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC)(2014).
Get Citation
Copy Citation Text
Hao Qu, Weizong Xu, Jiuzhou Zhao, Dong Zhou, Fangfang Ren, Feng Zhou, Dunjun Chen, Rong Zhang, Youliao Zheng, Hai Lu, "4H-SiC-based soft X-ray single photon detector with linear photon energy response," Chin. Opt. Lett. 23, 031202 (2025)
Category: Instrumentation, Measurement, and Optical Sensing
Received: Jul. 31, 2024
Accepted: Sep. 24, 2024
Published Online: Apr. 1, 2025
The Author Email: Weizong Xu (wz.xu@nju.edu.cn), Hai Lu (hailu@nju.edu.cn)