Performance Evaluation of an Amorphous Selenium Photodetector at High Fields for Application Integration

Polyimide (PI) has been utilized as a hole-blocking layer (HBL) in high-resistivity photoconductors such as amorphous selenium (a-Se) and amorphous lead oxide (a-PbO). It prevents hole injection from the positively biased electrode and suppresses the dark current of the detector. To determine the performance parameters of an a-Se/PI detector for readout design and specification for different radiation detection applications, knowledge of the accurate voltage drop within the photoconductor is crucial. Here, we precisely determine the voltage drop across a-Se when interfaced with a thin layer of PI (<inline-formula> <tex-math notation="LaTeX">$1 \mu \text{m}$ </tex-math></inline-formula>) and characterize its temporal resolution (rise time and jitter), dynamic range, and quantum efficiency (QE) as a function of the electric field. The photoresponse of the detector is characterized using light-emitting diodes in the ultraviolet (UV, 355–400-nm wavelength) and visible (400–533-nm wavelength) regions of the light spectrum. We find the voltage drop across a-Se as 91% of the total applied voltage. This results in a QE near unity at 405 nm for corrected fields beginning at 45 V/<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>. At fields of 50 V/<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>, we observe rise times of 3.7 ns and jitter of 130 ps.

For the practical operation of a-Se detectors at electric fields above 10 V/µm, dark current suppression is essential to enhance the signal-to-noise ratio (SNR) [17]. Hole injection has been recognized as the dominant contributor to dark current and has motivated research for a more effective hole-blocking layer (HBL) for a-Se-based detectors [18], [19], [20]. Relative to the bandgap of the material, an adequate holeblocking contact should have either a large potential barrier for holes between the positively biased metal contact and the a-Se layer or a large number of hole traps and a very low hole mobility. In addition, electrons should be able to flow freely through this HBL. Most importantly, the layer should be compatible with the large-area electronics semiconductor fabrication process.
Various a-Se-based photodetector structures with different HBLs have been proposed and investigated [21], [22], [23], [24], [25]. As an example, in HARP TV camera tubes, nanometer-thin films of cerium oxide (CeO 2 ) have been utilized as an HBL [26], [27], [28]. CeO 2 is a commonly used n-type widebandgap material that prevents injection of holes from the anode by forming a large potential barrier to holes [29]. Although CeO 2 has been successfully used for HARP devices, deviation from stoichiometry in CeO 2 affects its hole-blocking capability by lowering the potential barrier for holes, making the fabrication process complex [30], [31], [32], [33]. Techniques capable of producing bulk CeO 2 films with good stoichiometry are high-temperature processes, which are incompatible with deposition on a-Se [34]. Colloidal quantum dots (QDs) have been developed as a low-temperature alternative to bulk CeO 2 , however, have still suffered from poor stoichiometry [35], [36]. Recent works have shown more successful synthesis techniques and ultralow dark currents, however still require high temperatures and leave room for improvement in device performance [24], [37], [38].
Alternatively, polymers such as polyimide (PI) and perylene tetracarboxylic bisbenzimidazole (PTCBI) have been utilized as an HBL. PI demonstrated better performance compared to PTCBI for a-Se detectors and recently in an amorphous lead oxide (a-PbO) detector [39], [40]. The detectors using a PI layer make use of a simple spin-coating fabrication process based on widely available semiconductor materials that can be easily integrated into current large-area manufacturing processes. When a thin layer of PI is interfaced with a thick layer of the amorphous photoconductor, the large band offset at the PI/a-Se interface prevents the injection of holes from the metal electrode to a-Se at high electric fields. Under light exposure, the resistivity of the photoconductive layer (a-Se) is reduced, and the high electric field dropping across PI promotes charge transport through the PI layer and prevents charge accumulation at the PI/a-Se interface [10]. In addition, PI minimizes physical stress-related crystallization of a-Se and improves reliability [41]. Compared with other oxide semiconductor HBLs, the spatial uniformity, low dark current, and low processing temperatures make PI thin films appealing for a-Se-based indirect conversion X-ray detectors.
Previous works demonstrated the application of PI as an HBL [10], [40], [42], [43], [44]. The electric field across selenium was calculated numerically based on a parallel resistor-capacitor model of a-Se and PI, and limited characteristics of signal performance-especially at high fields-were provided. In this work, we instead experimentally determine the amount of voltage drop across each layer (15 µm of a-Se and 1 µm of PI) by first measuring the hole transit time in a-Se of a device with no PI. We then measure the transit time of a device with PI and adjust the voltage to yield the same transit time as a function of electric field, relating these results to previous studies. Then, we characterize the quantum conversion efficiency, dynamic range, and temporal resolution (pulse shape, rise time, and jitter) as a function of the electric field, with previously unreported results for high fields. These measurements on single-pixel a-Se interfaced with PI provide the knowledge to determine the performance parameters of the detector for readout design and specification for different radiation detection applications.

A. Device Fabrication
Samples with and without PI were fabricated during the same process, allowing for direct comparison. Indium tin oxide (ITO) on glass substrates was cleaned via ultrasonication in acetone and isopropyl alcohol for 10 min each, rinsed with deionized water, and dried with nitrogen. For the samples with a blocking layer, a diluted PI layer (2 g of HDMS PI-2610 to 1 mL N-methyl-2-pyrrolidone) was spun at 500 r/min for 5 s, then 1450 r/min for 60 s, dried at 90 • C for 120 s, then ramped to 350 • C at 4 • /min to hard bake for 30 min, and finally allowed to gradually cool to room temperature on the now-off hotplate. Stabilized selenium (0.2% As, 10 ppm Cl-Amalgamet) was deposited via thermal evaporation in a dedicated selenium evaporator at 100 Å/s while rotating at 40 r/min; further details of the system may be found in [45]. Gold electrodes with 3, 4, and 5 mm diameters were deposited by electron beam evaporation at 1 Å/s. The final devices consisted of ITO, 1.1 µm PI, 15.3-µm a-Se, and 100-nm Au. An image of the final device and a schematic of the architecture can be found in Fig. 1. The sample without PI has the same architecture and thicknesses, omitting the HBL.

B. Device Characterization
Devices were characterized by impedance, transient photocurrent time of flight (TOF), and photocurrent measurements. LCR measurements were performed with an Agilent Authorized licensed use limited to the terms of the applicable license agreement with IEEE. Restrictions apply.
where I ph is the measured photocurrent, e is the fundamental charge, P 0 is the intensity of incident light, and hν is the incident photon energy. For the purpose of indicating the signal length, transit time is taken as 0 ns to the time at which the signal fall edge is 10% of the signal plateau or peak. Rise time and jitter measurements were taken using the TOF setup, pulsing light every 30 s for 20 repetitions, and using a 10%/90% of the plateau or peak to define the signal rise.
III. RESULTS AND DISCUSSION To reach electric fields in which a-Se achieves stable performance, low leakage currents, and high conversion efficiency, an HBL must be applied. The dropped voltage across a-Se, when combined with an HBL, will be reduced from the total voltage applied. It is important to understand what the actual field across selenium is to gauge the performance of the device. Most commonly, the capacitance or resistance of the layers is used to estimate the dropped voltage across selenium. To find the capacitances of selenium and PI, devices with and without PI were prepared using the same deposition of selenium and gold, making them identical except for the PI layer. LCR measurements give the capacitance of a device with selenium and another with both selenium and PI, and we can extract the capacitance of PI using The resulting capacitances and dielectric constant can be found in Table I. Capacitances fall within reasonable values for stabilized a-Se; however, calculated values for PI are higher than expected, resulting in a relative permittivity higher than typically found in literature and reported in the supplier's datasheet [47], [48], [49], [50]. As information for this particular PI with the fabrication recipe used has not previously been reported, it is possible that the material properties are slightly different from those previously established. Taking the voltage across a-Se as the ratio of 1/C Se : 1/C T -the impedance of a-Se and the total impedance, respectively-we can estimate the dropped voltage across the selenium layer as 88%-90% of the total voltage applied.
On the other hand, if we use resistivity values of 3E15 cm −1 for selenium [51] and 1E16 cm −1 for PI [52] and assume resistors in series, we have a dropped voltage across selenium of 81% of the total applied voltage. Unfortunately, both these models ignore the properties of the photoconductor in a metal-insulator-semiconductor-metal device and do not capture the full picture of how the PI affects the field across the selenium.
To more accurately calculate the percentage of the applied voltage that is dropped across the selenium layer, we compared the hole transit time (τ T ) of the devices taken from transient photocurrent TOF. Fig. 2(a) shows τ T for each device as a function of applied voltage. A 1/x fit was made to the Se-only device, indicating the expected transit time of holes through the Se layer for voltages from 50 to 750 V.
It can be seen in Fig. 2(a) that the transit times for the PI device are slightly higher than the expected transit time (solid line) for all applied voltages. This is expected since the applied voltage (V A ) will be split between the PI (V PI ) and a-Se (V Se ), resulting in a lower effective voltage and, therefore, longer τ T across Se. The actual transit times across the a-Se layer will be the same for the same V Se in the devices. To determine the voltage drop across Se, we match the transit time of the PI/a-Se device to that of our Se-only curve, using V Se = c · V A , where c is a scaling factor. A value of c = 0.91, that is, a voltage drop across selenium of 91% of the total applied voltage/field, gives the best match of transit times for the PI/a-Se device to the expected transit times, shown in Fig. 2(b). The inset shows the corrected hole mobility for the PI/a-Se device alongside the Se-only device, showing good agreement between the two. The value of 91% is most similar to the impedance model given above, though gives a slightly higher voltage drop across a-Se.
With the adjusted field more accurately estimated, we can report the quantum conversion efficiency (QE) at 405 nm as a function of the corrected fields, shown in Fig. 3. The QE is reported for the Se layer, after reductions in intensity due to reflection and absorption in the glass, ITO, and PI layers are taken into account. a-Se starts with a poor conversion efficiency at low fields and increases exponentially until it reaches 100% conversion at about 45 V/µm. This is in line with the Onsager model for fields below impact ionization and is in line with previous studies on a-Se with a blocking layer [39], [53], [54], [55], [56]. This also indicates that the device is of high enough quality to evaluate and provide parameters for readout electronics. In addition to the QE, the dynamic range-understanding how the device will behave at various intensities-is important for selecting signal-processing parameters. An optical setup utilizing collimated LEDs was used to find the peak photocurrent of a-Se devices. Fig. 4 shows the current density generated by a-Se at various intensities for four wavelengths: 365, 405, 470, and 533 nm, all at 50 V/µm. The inset shows the QE as a function of wavelength, which follows the expected trend based on the mobility gap of a-Se, ∼2.2 eV. The current densities reported in Fig. 4 do not include any correction for attenuation of the incident light from the glass, ITO, and PI, which will vary by wavelength. For each, a linear fit can be seen in the plot along with the r 2 values. The current density is linear with intensity for all wavelengths, though fits for the 365-and 533-nm plots have less-than-ideal goodness of fit, possibly due to lower current densities and increased error. At 365 nm, and to a lesser extent 405 nm, the device will have greater attenuation of light due to PI's absorbance in the UV, so less light will reach the device to generate current. At 533 nm, the QE of the device is significantly reduced, which leads to a lower signal from exposure. The linearity of the device across all wavelengths is in accordance with previous studies of a-Se and allows us to extrapolate current densities at higher intensities than the LEDs used were able to achieve, though limits on linearity at high intensities are known for a-Se [10], [57], [58], [59], [60], [61].
To understand the limitations of the device at low intensities, we performed QE and signal-to-noise measurements at 470 nm and 50 V/µm, respectively. These results can be found in Fig. 5. At higher light intensities, the conversion efficiency is as expected and matches literature [39], [59], [60]. As the intensity drops below 60 nW/cm 2 , the conversion efficiency begins to decrease quickly. This can be explained by a simple trap-filling model. With the low dark currents from the inclusion of the PI layer, some trap states may not be filled by injected charge. At high light intensity, these remaining states may be satisfied with enough excess photogenerated charge pairs to make the reduction in carriers negligible. As the light intensity is lowered and the total number of charge pairs generated is reduced, the filling of trap states from the photogenerated charge pairs begins to have an influence and lower the conversion efficiency. Using a simplistic, illustrative model for charge trapping that assumes a number of traps that can be filled by photogenerated electron-hole pairs that give QE∼[(# generated ehps)−(# traps)]/(# generated ehps), a similar curve shape to the QE decreasing behavior with decreased input intensity can be observed. Future studies will probe into the origin of this QE drop at low light levels, further exploring effects at the interface and due to space charge. Regardless of its origin, the drop in QE must be taken into consideration for applications requiring low photon detection.
Also important to selecting parameters for device readout are the signal shape and time. Fig. 6 shows the signal from the device when pulsed with 25 ps of ∼115 nJ, 355 nm light at 11, 30, and 50 V/µm. The shape of the signal changes drastically with the reduced time of hole transit. At 11 V/µm, we can see something akin to a rectangular pulse, with a plateau height of 7 mV and transit time of 130 ns. The transit time at 30 V/µm reduces by more than a quarter to 34 ns, as expected, and though a slightly rectangular shape remains, we begin to see softening at the rise and fall edges. At 50 V/µm, this changes to a quick rise and fall, peaking at 115 mV and a transit time reducing to 17 ns.   Finally, the average rise time and jitter for the device biased at 11, 30, and 50 V/µm was found by pulsing the sample every 30 s for 20 repetitions. The results, along with the transit time above, are reported in Table II; a plot of all 20 signals for the 50-V/µm bias can be seen in Fig. 7. We see that the rise time increases with increasing bias. With an increase in voltage, there is a reduction in the signal jitter, going from almost 1 ns at 11 V/µm to 225 ps at 30 V/µm, and finally to 130 ps at 50 V/µm. The large standard error is primarily due to the low number of measurements and slight variations in light intensity; traditionally, many more repetitions would be performed. However, due to the limitations of the experimental setup, we were unable to achieve greater numbers. Future work will automate the process and allow for more accurate measurements to be made.
At 10 V/µm, the field is strong enough to enable the proper conduction in the detector, giving a stable rise time. However, we see a slight increase in the rise time at 30 and 50 V/µm. This is due to our system being slew rate limited with a time constant close to 3 ns. As the amplitude increases at higher fields, this results in a longer time for the signal to increase from 10% to 90%.
Jitter is affected by various factors such as thermal noise, shot noise, and fluctuation in the electric field. At higher electric fields, the jitter is reduced due to improved charge carrier mobility, as demonstrated above, and the stronger field overcoming the resistivity of the PI [39].

IV. CONCLUSION
We have directly compared devices with and without PI to extract the dropped voltage across the a-Se layer. With the appropriate fields in place, parameters for a 15-µm a-Se device with a 1-µm PI layer have been reported, including QE, linearity and dynamic range, pulse shape, transit time, rise time, and jitter. Overall, we can conclude that our device is on par with those presented in previous works, though opportunities for optimization remain, especially for increasing fields to impact ionization levels. This work provides a starting point for selecting readout electronics for a variety of applications, including particle detection, vacuum ultraviolet, and indirect X-ray detectors. His research interests include the characterization of electronic materials and devices, medical and molecular imaging technologies, and instrumentation for radiation detection applications.
Jennifer Ott received the B.Sc. degree in chemistry and the M.Sc. degree in radiochemistry from the University of Helsinki, Helsinki, Finland, in 2014 and 2015, respectively, and the D.Sc. (Tech.) degree from the School of Electrical Engineering, Aalto University, Espoo, Finland, in 2021.
She conducted her doctoral research with the Helsinki Institute of Physics, Helsinki, Finland. She is currently a Postdoctoral Researcher at the University of California Santa Cruz, Santa Cruz, CA, USA. Her multidisciplinary work focuses on the development of semiconductor sensors and high-performance readout electronics for particle physics, nuclear physics, and medical imaging. She is currently an Associate Professor at the Department of Electrical and Computer Engineering (ECE), University of California Santa Cruz, Santa Cruz, CA, USA. Her research interests include radiation detection and instrumentation for molecular imaging, computational problem solving, and quantitative characterization of biological processes.