Detection of the soft X-ray with photon energy between 0.5 and 10 keV with a wavelength even shorter than EUV (i.e., ∼0.1 to 2 nm) plays an important role in a wide range of key application areas, such as medical imaging, nuclear safety inspection, space exploration, and material analysis^[1–5]. Traditional detection systems are mainly based on gas ionization detectors and scintillation detectors^[6,7]. Given the growing demands of soft X-ray detection facilities with more compact volumes and photon energy capabilities, semiconductor-based soft X-ray detection technology is experiencing fast growth^[8,9]. Owing to the benefits of Si technology in mature manufacturing, high material quality, and low cost, Si-based soft X-ray detectors are dominating the market^[10,11]. However, as limited by the material properties, especially the relatively lower bandgap (1.12 eV) and threshold displacement energy (∼15eV), Si detectors are faced with problems of thermal-carrier-related leakage even at room temperature, as well as radiation-induced degradations^[12–16]. These factors thus pose critical requirements for additional cooling and radiation hardening modules, which can introduce considerable increase in the power consumption and total cost, inhibiting especially the applications in harsh environments with strong radiation and large temperature variations like deep space exploration and nuclear safety inspection. Wide bandgap semiconductors (e.g.,  SiC, GaN, and diamond), on the other hand, are becoming a new driving force for the development of radiation detection technology based on their wider bandgaps and advantageous radiation resistance as compared to traditional Si-based radiation detectors^[17,18]. Considering the advanced development of SiC technology, especially the attractive merits of low background carrier concentration (∼1014cm−3) and available epi-layers with a thickness up to 100 µm, the SiC-based detector promises lower parasitic capacitance and lower operation voltage, which will directly benefit single photon detection with photon energy resolution and lower detective energy limit in the soft X-ray region^[19,20].

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 ∼100μm thick epilayer, a high collection ratio of photo-generated carriers, and ultra-low dark current for single photon detection^[21]. The other is peripheral amplification electronics for extremely weak photoelectric signal processing, considering that the avalanche diode cannot be applied due to the thick depleted layer^[22]. Therefore, a high amplification factor with low noise and excellent linearity is necessary for electronics. In general, with electron–hole pair generation and collection in semiconductor detectors, front-end electronics composed of charge-sensitive amplifiers (CSAs) can be utilized for the precise analysis on charge coupling and amplification^[23,24]. However, considering the difference in charge collection dynamics, parasitic parameters, and impedance matching requirements, traditional solutions for Si-based detectors can hardly be used^[25]. Furthermore, wide bandgap semiconductor SiC has relatively higher electron-hole pair creation energy (7.8 eV/e-h) as compared to Si material (3.6 eV/e-h), which means nearly a half reduction in photo-generated carriers in the SiC detector^[26]. These have raised unique requirements on the supporting electronics investigation, so as to guarantee low noise and precise charge collection for the subsequent signal amplification and analysis.

In this work, a p-i-n-type 4H-SiC soft X-ray detector with an 80-µm-thick absorption layer and low intrinsic doping is fabricated. Based on the specially developed recovering strategy for mesa-etching-related damage, the detector achieves ultralow leakage current at ∼1.8pA level at room temperature. Specific investigations on electronic amplification strategies for the p-i-n 4H-SiC soft X-ray detectors have also been conducted, where an active reset switch is introduced in the CSA circuit. In the traditional CSA circuit, in addition to the feedback capacitor for the charge collection, a feedback resistor is usually used for the charge release from the feedback capacitor to maintain the direct current (DC) operating point at the input stage of the amplifier. This feedback resistance will then introduce extra thermal noise. Here, an active reset switch composed of a voltage comparator, a pulse generator, and a reset diode has been utilized to actively control the capacitor charge release process, achieving both a low noise figure and a fast reset of the voltage on the feedback capacitor. Ultimately, by precisely tuning the duration of the shaping amplification stage, the 4H-SiC detector has demonstrated a linear energy response capability for soft X-ray photons, achieving an energy standard deviation below 5.7% and a linearity error below 0.5% for the studied energy range.

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 n+-type 4H-SiC substrate using the chemical vapor deposition (CVD) technique. The epilayer consists of a 0.3-µm-thick Al-doped p+-layer with a doping concentration of 1×1019cm−3, an 80-µm-thick lightly doped i-layer, and a 10-µm-thick highly doped n+-layer from the top to the bottom. The device fabrication starts with mesa etching, defining the active area as 1.5mm×1.5mm through lithography and processing in an inductively coupled plasma (ICP) system with a SF6/O2 atmosphere. To achieve full trench isolation, the etch depth is set at ∼0.6μm to ensure that the mesa bottom can reach the lightly doped i-layer. Subsequently, a thermal oxidation process at 1050°C in dry O2 is performed to grow a 15-nm-thick SiO2 layer, which will then be wet-etched to obtain a fresh SiC surface. Subsequently, a 30 nm high-temperature SiO2 and a 1-µm-thick SiO2 based on 350°C plasma enhanced CVD is grown for the passivation layer. After opening the contact window at the top of the wafer by wet chemical etching, a Ni/Ti/Al/Au (35/50/100/100 nm) metal stack and a thin metal layer of Ni (100 nm) are evaporated, respectively, on the bottom of the wafer and the top contact window. A rapid thermal annealing at 850°C in N2 is then performed for the ohmic contact metallization. Ultimately, the detector is mounted on a TO-8 header with a Be window and wire-bonded with Au wires for subsequent characterizations, as shown in Fig. 1(b).

As presented in Fig. 1(c), an extremely low dark current of ∼1.8pA at the bias of −180V and a minor increase in dark current with the reverse bias can be observed. The C–V characteristics of the detector are shown in Fig. 1(d), where a doping concentration of ∼5×1013cm−3 in the lightly doped i-layer can be acquired based on the slope of the 1/C2 versus the voltage curve. The relatively low doping concentration of the i-layer in this device allows a low operation voltage of 120 V for the full depletion of the 80 µm absorption layer. The relatively low capacitance (below 5 pF) and the low leakage current of the device guarantee superior dark current performance for single photon detection with low electronic noise.

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.

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., ∼1.8pA, the leakage-related voltage change is very slow, as presented in Fig. 3. Then, with the threshold voltage for the voltage comparator set at −2.25V in this circuit, the ratio of operating time to reset time is approximately 70,000 : 1 (225 ms : 3.2 µs). That is, the dead time for this feedback strategy is negligible, contributing to a preferable duty circle for effective operation time.

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 ∼180mV and a pulse rise edge within 160 ns, revealing the fast detection and signal-generating process of the 4H-SiC detector and the CSA module. Correspondingly, the amplification factor of the electronics can be calculated to be 30.56 mV/keV. Meanwhile, the preferably fast response time of this system can further facilitate the time of arrival (TOA) measurement for soft X-ray photons, supporting wider application domains like pulsar navigation and solar activity observations.

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., Ti-Kα=4.51keV, Mn-Kα=5.89keV, Ni-Kα=7.48keV, Ge-Kα=9.89keV, and Y-Kα=14.96keV, with a linearity error below 0.5%. Then, together with the noise floor suppressed below 1 keV, soft X-ray detection with photon energy resolution capability and excellent linearity has been achieved.

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.