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Abstract
β-Ga2O3 semiconductor crystal is of wide band gap and high radiation resistance, which shows great potential for applications such as medical imaging, radiation detections, and nuclear physical experiments. However, developing β-Ga2O3-based X-ray radiation detectors with high sensitivity, fast response speed, and excellent stability remains a challenge. Here we demonstrate a high-performance X-ray detector based on a Fe doped β-Ga2O3 (β-Ga2O3:Fe) crystal grown by the float-zone growth method, which consists of two vertical Ti/Au electrodes and a β-Ga2O3:Fe crystal with high resistivity. The resistivity of the β-Ga2O3:Fe crystal exceeds 1012 Ω cm owed to the compensation of the Fe ions and the free electrons. The detector shows short response time (0.2 s), high sensitivity (75.3 µC Gyair−1 cm−2), and high signal-to-noise ratio (100), indicating great potential for X-ray radiation detection.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Semiconductor materials that can convert X-ray radiation to electric signals have received much attention in many fields such as scientific research, medical imaging, and industrial research [1–7]. Semiconductor materials that can be used in efficient X-ray detectors should possess several properties: high density, high resistivity, low crystal defect density, high stability [8–10]. High density can greatly absorb the X-ray radiation. High resistivity can suppress the dark current of the detector, enabling the detector to detect the X-ray radiation with low dose rates. Low crystal defect density can effectively improve the response speed of the detector. High stability can enhance the duration of the detector. So far, CdZnTe and α-Se have been successfully developed as the commercial X-ray detectors [11,12]. In the last few years, perovskite materials have attracted much attention for X-ray radiation detection [13,14]. However, the X-ray radiation detection performance of those semiconductor materials is still limited by several factors. CdZnTe possesses a relatively low resistivity of 1010−1011 Ω cm [15–17]. Furthermore, growing high-quality CdZnTe crystal with low carrier trap concentration remains a challenge. Consequently, above shortages limit its application in low dose rate and pulsed X-ray radiation detection fields. α-Se possesses a low absorption coefficient of X-ray radiation, resulting in a low sensitivity of 20 µC Gyair−1 cm−2. Perovskite materials are of disappointing stability, making it difficult for practice application. Therefore, it is necessary to pursue new semiconductor materials to overcome those limits and develop high performance X-ray radiation detectors.
In the last ten years, a phenomenon development of large-size β-Ga2O3 crystal growth has been witnessed, driven by the demand for solar blind UV detection [18–22]. Due to wide band gap, high density (6.44 g/cm3), and high tolerance to ionizing radiation, researchers began to explore whether β-Ga2O3 could be developed in X-ray detection [23–28]. For instance, the detector based on unintentional-doped (UID) β-Ga2O3 with a Schottky barrier diode structure and the detector based on amorphous Ga2O3 with vertical Ohmic contacts structure have been successfully employed to detect X-ray [24–26]. However, the detection performance is limited by the dark current of those detectors, which is largely attributed to the shallow donor impurities in the raw materials. Doping deep acceptor to compensate free electrons and put the Fermi level away from the conduction band to obtain high-resistivity β-Ga2O3 is considered to be a feasible method [29]. Several divalent metal cations are favorable candidates such as Mg2+, Fe2+. In addition, the high-resistivity β-Ga2O3:Fe crystal has been successfully grown reported in previous work [29]. At high temperature, the β-Ga2O3:Fe crystal still exhibits relatively high resistivity, which is owed to the fact that only a few electrons captured by the Fe ions would be released to conductive band. Therefore, the β-Ga2O3:Fe crystal could be a favorable candidate to develop high-performance X-ray detectors.
In this work, a high-resistivity β-Ga2O3:Fe-based X-ray radiation detector with a metal/semiconductor/metal (MSM) has been successfully fabricated and measured under X-ray radiation exposure. Owing to the high resistivity of the β-Ga2O3:Fe crystal, the detector exhibit outstanding performance, including high sensitivity, high signal-to-noise ratio (SNR), and short response time.
2. Experimental section
2.1 Preparation of β-Ga2O3 single crystals
The 0.02 mol% Fe doped β-Ga2O3 single crystals were grown by the float-zone method. High purity powder of Ga2O3 (6N) and Fe2O3 (4N) were used as the raw materials. The crystals were grown under flowing air atmosphere. The crystal growth rate was 4∼5 mm/h with the rotation speed of 8∼10 rpm.
2.2 Device fabrication
We began by cutting the as-grown crystals into pieces along the (100) plane and then polished them by a chemical mechanized polishing method to obtain flat surfaces. The β-Ga2O3:Fe crystal samples with the dimension of 7 × 7 × 1 mm3 were obtained and used to fabricate the X-ray detector. Before the detector fabrication, we cleaned the crystal samples with acetone, isopropanol, and deionized water. Then, the Ti/Au (20/50 nm) metal electrodes with a square area of 9 mm2 were deposited on both sides of the β-Ga2O3:Fe crystal samples. To obtain good Ohmic contacts between crystal and metal electrodes, a rapid thermal annealing at 850 °C for 30 seconds in a nitrogen atmosphere was carried out.
2.3 Characterizations
A UV-VIS-NIR spectrophotometer (Varian Cary 5000) and a X-ray diffraction (XRD) (Rigaku Ultima IV) were used to measure the characteristics of the β-Ga2O3:Fe crystal. A miniature DC X-ray source (50 kV, 10 W, Mini-X2, Amptek, USA) with a silver target and a semiconductor analyzer (Keithley 4200-SCS) were used to measure the current-voltage and current-time curves of the X-ray detector. The accelerating voltage of the X-ray source is set at 50 kV during all X-ray measurements. The average energy of X-ray photons is about 16 keV under the accelerating voltage of 50 kV, including Bremsstrahlung continuum and Ag characteristic peaks.
3. Results and discussion
To assess the potential of β-Ga2O3 as a promising material for X-ray radiation detection, the calculated linear attenuation coefficients of β-Ga2O3 and several typical materials by using the photon cross-section database from the National Institute of Standards and Technology are shown in Fig. 1(a). The attenuation coefficient of β-Ga2O3 is much greater than those of Si, SiC, and diamond but is slightly lower those of CdZnTe and MAPbBr3. Figure 1(b) shows the attenuation efficiency of Ti and Au metals to 16 keV X-ray photons. In this investigation, the Ti layer and the Au layer are 20 nm and 50 nm, respectively. Obviously, both the Ti layer and Au layer have an extremely low attenuation efficiency for 16 keV X-ray photons. Therefore, the metal electrodes have little impact on the X-ray absorption. Figure 2(a) shows the transmittance spectrum of the β-Ga2O3:Fe crystal at room temperature. A sharp band-edge absorption at 270 nm can be clearly observed. However, in the visible wavelength range longer than 400 nm, the β-Ga2O3:Fe crystal shows a relatively low transmittance of 60%. The band gap of β-Ga2O3:Fe crystal is about 4.35 eV, which is obtained from the inset of Fig. 2(a). Figure 2(b) shows the XRD patterns of the β-Ga2O3:Fe crystal. All the diffraction peaks can be greatly matched with the monoclinic phase β-Ga2O3 (PDF #41-1103), suggesting that doping Fe atoms does not change the crystal structure of β-Ga2O3.
Fig. 1. (a) Absorption coefficients of β-Ga2O3, CdZnTe, GaN, SiC, Diamond, α-Se, Si, and MAPbBr3 as a function of photon energy. (b) Attenuation efficiency of Ti and Au metals to 16 keV X-ray photons as a function of thickness.
Fig. 2. (a) Transmittance spectra of the β-Ga2O3:Fe single crystal. The inset is the plot of (αhν)2 as a function of photon energy. (b) XRD patterns of the β-Ga2O3:Fe single crystal and the reference.
A photoelectric process occurs during the interaction between X-ray and β-Ga2O3 crystal, which produces high-energy electrons. The induced high-energy electrons subsequently lose their energy in generating more detectable electron–hole pairs. The electron–hole pair ionization energy is deduced to be 10.13 eV, calculated by the empirical model as Δ = 1.43 + 2Eg [30]. Figure 3(a) presents the cross-sectional schematic of the fabricated detector. For convenience, the Ti/Au electrodes are connected to a printed circuit board (PCB), as shown in Fig. 3(b). The dark current density of the detector in Fig. 3(c) shows a linear increase with the increase in applied voltage, indicating excellent Ohmic contacts between crystal and metal electrodes. A dark current density of 1.8 nA/cm2 is obtained at the electric field of 0.2 V/ µm, which is much lower than that of the CdZnTe-based detector, equal to that of the AgI-based detector, and slightly greater than that of the α-Se-based detector [31]. According to the slope of the current density-voltage curve with the applied voltage from −200 V to 200 V, a high resistivity of 1.1×1012 Ω cm for the β-Ga2O3:Fe crystal is obtained. The value is much greater than the threshold requirement of 108−109 Ω cm for the semiconductor-based X-ray radiation detectors [32]. The Fe atoms in β-Ga2O3:Fe crystal act as acceptors and compensate the free electrons, resulting in the high resistivity [29]. The current density of the detector under X-ray irradiation as a function of applied voltage with a dose rate of 0.15 Gyair/s is shown in Fig. 3(d). When biased at 200 V the detector shows a high photo-to-dark current density ratio of 369, which is owed to the high resistivity of the β-Ga2O3:Fe crystal. To explore the transient response of the detector to the X-ray radiation, the X-ray source is alternately turned on and off with a switching period of 20 s. The sampling frequency of the measurement is 50 Hz. Figure 4(a) shows the current-time curves of the detector under X-ray irradiation with a dose rate of 831 µGyair/s at various applied voltages in the range from −200 V to −1000 V, it is obvious that the detector can be readily switched between “on” and “off” states. Owing to the high radiation resistance of the β-Ga2O3:Fe crystal, the photoresponse shows excellent reproducibility. Under X-ray exposure, the current instantly increases to a table value. The photocurrent of the previous reported β-Ga2O3:Fe-based X-ray radiation detector decreases with the increase in exposure time, indicating that our detector is of greater stability [28]. When the X-ray source is switched to the off state, the current decreases rapidly down to a stable value, which is greatly matched with the initial dark current value. As shown in Fig. 4(b), both the response time (10–90%) and the recovery time (90–10%) of the detector are measured to be less than 0.2 s, much shorter than that of the X-ray and UV detectors reported in the literatures [19,25].
Fig. 3. (a) Cross-sectional schematic of the fabricated detector. (b) Photograph of the fabricated detector. (c) J-V characteristic of the fabricated detector in dark. (d) J-V characteristics of the fabricated detector in dark and under X-ray irradiation with a dose rate of 0.15 Gyair/s on a logarithmic scale.
Fig. 4. (a) Current-time curves of the fabricated detector at different voltages. (b) Zoom-in time traces of the fabricated detector at different voltages. (c) Both the net induced current and sensitivity of the fabricated. (d) Signal-to-noise ratio of the fabricated detector as function of applied voltage, where the green dashed lines represent the SNR of 3.
In addition to the response speed and the recovery speed, sensitivity is an important parameter to evaluate the detection performance of an X-ray detector. It can be described by the following equation. [6]
where IX-ray is the current when the detector under X-ray exposure, Idark is the current when the detector under dark, D is the dose rate of the X-ray radiation, A is the effective illuminated area. Figure 4(c) plots both the net induced current (IX-ray-Idark) and the sensitivity at different applied voltages. When the applied voltage from −200 V to −1000 V, both the induced current and the sensitivity monotonically increase with the increase in applied voltage. This phenomenon is attributed to the increase of drift velocity of X-ray generated carriers and the decrease of recombination possibility [5]. When biased at −1000 V, a sensitivity of 75.3 µC Gyair−1 cm−2 is obtained. The comparison of the performance of the present detector with the reported X-ray detectors is shown in Table 1. The sensitivity of the β-Ga2O3:Fe-based detector is approximately 4-fold larger than those of the commercial amorphous selenium X-ray detectors, which is owed to the suppressed dark current and the enhanced carrier collection efficiency. The SNRs at various applied voltages are shown in Fig. 4(d), which can be calculated by the following equation [33]
Table 1. Summary of X-ray detection performances of the present detector and the reported detectors
To explore the applicable X-ray dose rate range and the response to X-ray radiation, we recorded the responses of the detector under higher X-ray dose rates. Obviously, all the currents density of the detector under the X-ray exposure with the dose rates in the range from 0.03Gyair/s to 0.15 Gyair/s linearly increase with the increase in applied voltage, as shown in Fig. 5(a). Figure 5(b) presents the current density-time curves of the detector under different X-ray dose rates when the X-ray source is switched to on and off states periodically. The experimental results exhibit excellent reproducibility and stability even under a larger X-ray dose rate. Notably, the detector shows rapid responses to the X-ray radiation with different dose rates. Figure 5(c) shows the net induced current density of the detector under X-ray exposure as a function of dose rate. The net induced current density almost linearly increases and shows no obvious saturation effects with the increase in X-ray dose rate. It should be noted that the X-ray source used has a maximum power of 10 W and a maximum acceleration voltage of 50 kV, so that it is difficult to obtain the response for X-ray with a very large dose rate. Based on the measurement results, it is estimated that the present detector can be well operated with the dose rate from 831 µGyair/s to 0.15 Gyair/s.
Fig. 5. (a) J-V characteristics of the fabricated detector under irradiation with different dose rates. (b) Current density-time curves of the fabricated detector under irradiation with different dose rates. (c) Net induced current density at different X-ray dose rates.
4. Conclusions
In conclusion, we successfully fabricated the β-Ga2O3:Fe-based X-ray detector with a vertical MSM structure. The detector shows excellent Ohmic contacts between metal electrodes β-Ga2O3:Fe crystal. Attributed to the high resistivity of 1.1×1012 Ω cm of the β-Ga2O3:Fe crystal, the detector exhibits excellent performances such as a low dark current density of 1.8 nA/cm2 (200 V), a high photo-to-dark current density ratio of 369 (200 V, 0.15 Gyair/s), a short response time of less than 0.2s (all X-ray tests), a high sensitivity of 75.3 µC Gyair−1 cm−2 (−1000 V, 831 µGyair/s), a high SNR of 145.6 (−1000 V, 831 µGyair/s), and a relatively low detection limit less than 831 µGyair/s. These findings reveal the great potential of the β-Ga2O3:Fe crystals for X-ray detection applications.
Funding
National Natural Science Foundation of China (11975168); Equipment Pre-Research Fund Key Project (6140922010601).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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