Time response of Cd_0.9Zn_0.1Te crystals under transient and pulsed irradiation

There has recently been tremendous interest in using Cd_0.9Zn_0.1Te to develop room-temperature X-ray or γ-ray counting detectors, because of its favorable photoelectrical properties of high atomic number, sufficiently large band-gap for high resistivity and low leakage current, and high intrinsic mobility–lifetime product.^1–5 During the past two decades, CdZnTe (CZT) crystals with bulk resistivity in excess of 10¹⁰ Ω·cm have become available. To date, most researchers have focused on improving the energy resolution of CZT detectors, and various electrode configurations and metal contacts have been explored. Although the remarkable progress in developing CZT detectors has mainly been in the field of spectral measurement, a less common but rewarding application of CZT detectors is the monitoring of intense and rapidly changing pulsed gamma-ray flux. Therefore, the properties of a fast time response and good drift properties of charge carriers of the detector are desirable.⁶ 

In this paper, we propose a CZT-based detector for the measurement of transient and pulsed radiation. The time response and drift properties of charge carriers in the CZT detector have been studied under the pulsed-radiation condition. This study investigates the pulsed time response of CZT detectors operating in current mode. In an experiment, a pulsed electron beam and pulsed X-rays are used as transient and pulsed sources, and it is found that thorough detector penetration is necessary to ensure that the measured time response is independent of the category of incident radiation (X-rays or electron pulses). Furthermore, we observe a rapid decrease in the time response with an increase in the electric field. These results are of interest to the application of CZT as a gamma-ray detector since the charge collection efficiency can be increased by strengthening the electric field beyond a threshold, and the results also provide an understanding of the charge carrier trapping effects and space-charge accumulation mechanisms. In addition, the response of electronic devices such as an amplifier is not taken into account in the measurement. Therefore, the time response of CZT detectors measured employing the above method is a more direct measurement than employing a conventional method such as measuring the α-particle spectral response.⁷ 

The CZT crystal was supplied by Northwestern Polytechnical University. The crystal was grown employing the modified vertical Bridgman method. The resistivity of the crystal sample was on the order of 10¹⁰ Ω·cm, and the electron mobility–lifetime product exceeded 1 × 10³ cm²·V^–1, which indicates high carrier transport properties.⁸ The CZT detector was fabricated with planar electrodes by evaporating gold contacts on the top and bottom surfaces of the crystal. In addition, a high-quality optical polish was applied to the side surfaces of the detector. The typical dimensions of the detector were Φ8 mm × 0.7 mm.

To investigate the properties of contacts in the detector, the current–voltage (I–V) curve of the Au/CZT/Au detector was measured at room temperature (Figure 1). Bias voltages of both polarities were applied and, as expected, both polarity curves showed good linearity between –100 and 100 V, which indicates good ohmic contact. Additionally, bias voltage up to 1000 V (14,286 V/cm) was applied to the detector at room temperature in the dark, and the leakage current only reached about 30 nA.

To further study the response of the detector irradiated by pulsed rays, it is necessary to perform a pulsed-ray radiation experiment for the detector. In particular, accurate measurement of the time response of the detector requires that pulsed sources possess sufficiently fast timing characteristics compared with those of the detector. Figure 2(a) is an assembly diagram of the CZT detector for pulsed experiments. The pulsed electron-beam and X-ray experiments were carried out at the Tsinghua Thomson Scattering X-ray Source (TTX) in Beijing, and at the Sub-nanosecond Rep-rate Pulse Hard X-ray Generator Radiation Facility (SRPX) (Figure 2(b)) in Xi'an, respectively. TTX, as shown in Fig. 3, has a photocathode radio-frequency (RF) electron injector and a femtosecond terawatt laser system and serves as a tunable monochromatic X-ray source. A 3-m S-band SLAC-type traveling-wave accelerating section is placed after a BNL/KEK/SHI-type 1.6-cell S-band photocathode RF gun, to boost electron pulses to 40–50 MeV. A magnetic bunch compressor is used to compress electron bunches to below 1 ps. Therefore, the TTX is capable of producing tunable, very-high-brightness X-ray or electron pulses at short wavelengths (<1 Å) and short durations (<1 ps).^9,10 SRPX is capable of producing tunable, hard X-ray pulses at about 100 keV with short durations of ∼200 ps.¹¹ 

In the pulsed radiation response experiments, a high voltage across the detector was applied by a PS350 direct-current power supply. The induced signals from the detector were transmitted to recording oscilloscopes (specifically, digital oscilloscopes with bandwidths of 4 and 12.5 GHz respectively) through low-loss cables with good noise shielding. The CZT detector was located behind a collimator, 2 m from the implosion center, and biased at at least 1000 V/cm.

In the electron-pulse response experiment, the initial spatial distribution of electron pulses was assumed to be identical to that of an ultraviolet laser, which has duration of 8 ps (full-width at half-maximum) and 2-ps (10–90%) rising edges, and is transversely uniform with a hard-edge radius of 1.0 mm. Figure 4 shows the waveforms measured by the detector irradiated with an electron pulse at TTX. The response waveforms of the detector were recorded by a digital oscilloscope with bandwidth of 12.5 GHz. The pulsed response was measured under applied bias of 100–600 V. All response curves have been normalized by the peak amplitude.

Figure 4 shows that the pulsed profiles sharply rise within ∼2 ns and decrease immediately after. The rise time of the response waveforms depends on the time from implosion of the electron beam and the intrinsic response time of the detector. Additionally, the figure clearly shows that the profiles have the same rise time for different applied voltages. The rise time of the CZT detector can be determined as 2.1 ns (10–90%). Since the rise time of the electron beam (2 ps) is much shorter than that of the CZT detector (2.1 ns), the time response waveform of the detector is considered here to be the output current waveform for a δ-function excitation pulse. Therefore, a shorter rise time of the response waveform is achieved when using detectors with a shorter response time.

In the pulsed X-ray response experiments, a single X-ray pulse with a rising edge (and full-width at half-maximum) of 200 ps and energy of about 100 keV at SRPX was used to irradiate the CZT detector. Figure 5 shows the typical current signal versus time for the detector, with bias voltage ranging from 100 to 600 V applied to the front electrode. The response waveforms from the CZT detector were recorded by a digital oscilloscope with bandwidth of 4 GHz. The amplitudes of the waveforms in Fig. 5 are normalized. The figure clearly shows that pulsed curves have exactly the same rise time for different applied bias voltages. The rise time can be read as 2.1 ns from the first peak.

Figures 4 and 5 show that the rise time of the CZT detector irradiated by pulsed X-rays (Fig. 5) agrees well with that obtained in the electron-beam experiment (Fig. 4), indicating that the time response does not depend on the category of incident radiation (X-rays or electron pulses) within experimental error. Additionally, given the effect of thorough detector penetration, it is concluded that the time response is independent of the characteristics of the excited pulse. Furthermore, thorough detector penetration is necessary in the experiment since short-range particles induce a different time response depending on the crystal face. In that case, the slowly moving holes have to travel the whole length of the detector in a much longer time than that for quickly moving electrons.^12,13

Figure 4 also shows the fall of the curves becoming faster with strengthening of the electric field. This is likely due to a relaxation process, which can be considered a complicated combination of electron–hole recombination, carrier trapping, and electron–acoustic phonon interaction. Nevertheless, it is notable that the 10–90% rise times of all curves shown in Fig. 4 are exactly the same (2.1 ns). Furthermore, the results in our experiments directly correspond to electron-beam excitation, and therefore, CZT detectors have potential in future timing measurements.

The output waveform is a complicated combined result of the waveform of the input pulse, drift velocities and collection time of the carriers in the detector volume, the diffusion velocities of the carriers, and the inductance and capacitance of the detector and its associated packaging, which should not be ascribed to one or two factors. Under ideal conditions (i.e., without trapping), the current transient should be constant after the excitation pulse terminates, and end abruptly as the generated charge carriers reach the counter electrode. Therefore, the current transients could provide a clear measurement of the transit time of the charge carriers across the crystal.¹⁴ However, a decay, characterizing carrier trapping, is often observed after including the trapping effects, which complicates the current shape. Under the trapping and detrapping effects, electrons and holes that generate signals are more easily able to break loose from shallow traps (dispersive trap-controlled mobility) as the electric field strengthens. The mobility mechanism might be ascribed to a combination of phonon scattering, impurity scattering, and other phenomena,¹⁵ which have not yet been proved.

The results obtained in the experiments indicate that the CZT detector developed and tested can be used for fast pulsed nuclear radiation detection, and its time resolution of 2.1 ns indicates its excellent timing characteristics. The result also demonstrates the feasibility of the method in studying the timing characteristics of CZT materials. In addition, accurate measurements of the time response indicate that the transient pulse response experiment is useful for characterizing semi-insulating materials.

More refined measurements and analysis of transport phenomena are needed to provide detailed information on material inhomogeneities that affect and even degrade the performance of the detector. Further investigations on local crystal properties (e.g., mobility and drift time analysis) will be reported in the near future.

The authors thank Northwestern Polytechnical University for providing CZT samples, Su Chunlei for the use of X-ray facilities, and Du Yingchao for the use of electron-beam facilities. This work was funded by the National Hi-Tech Project of China (Grant No. 2009AA050705) and the State Key Laboratory of Multiphase Flow (Xi'an Jiaotong University).