Picosecond Time Resolution with Avalanche Amorphous Selenium

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Picosecond Time Resolution with Avalanche Amorphous Selenium Andy LaBella, Jann Stavro, Sebastien Léveillé, Wei Zhao, and Amir H. Goldan ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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Picosecond Time Resolution with Avalanche Amorphous Selenium Andy LaBella,† Jann Stavro,† Sebastien Léveillé,‡ Wei Zhao,¶ and Amir H. Goldan∗,¶ †Department of Biomedical Engineering, College of Engineering and Applied Sciences, Stony Brook University, Stony Brook, NY, US ‡Analogic Canada, Montreal, QC H4R 2P1, Canada ¶Department of Radiology, School of Medicine, Stony Brook University, Stony Brook, NY, US E-mail: [email protected] Phone: 1 631 638 8537 Abstract Ultrafast photodetection has traditionally been performed with crystalline photodetectors, which tend to suffer from low production yield, suboptimal detection efficiency and operational limitations that restrict their potential applications. Amorphous selenium is a unique, disordered photosensing material in which carrier transport can be shifted entirely from localized to extended states where holes get hot, resulting in deterministic, non-Markovian impact ionization avalanche, causing selenium to exhibit characteristics similar to crystalline photoconductors. For the first time, we’ve fabricated a multi-well selenium detector using nano-pillars that achieves both avalanche gain and unipolar time-differential charge sensing. We experimentally show how these features together improve selenium’s temporal performance by nearly four orders of magnitude, allowing us to achieve picosecond timing jitter suitable for a variety of

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ultrafast applications. Such a detector would be a viable low-cost, high production yield alternative for picosecond photodetection and imaging.

Keywords Amorphous Selenium, Picosecond, Photodetector, Nano-Frisch Grid, Multi-Well, Avalanche Picosecond photodetectors have a wide range of applications in both physics and medicine. Recent research in this field has focused on two-dimensional crystals, including graphene 1–5 due to its photodetection speeds on the order of single-digit or even subpicoseconds, as well as black phosphorus 6–9 and oxyselenides 10,11 due to their picosecond timing and ultrawide, tunable band gaps based on their structural designs. These detectors can also stack via van der Waals interactions with other materials to form so-called heterostructures that take advantage of the unique properties of each material, such as the fast timing of graphene and the detection efficiency of a high-Z material like WSe2 . 3–6 While two-dimensional crystals have been used for fluorescence imaging, microscopy and nano-imaging in both the optical and infrared regimes, 11–16 the commercial applications of such thin lateral structures are very limited due to their low scalability, costly production process, insufficient photon detection efficiency arising from weak light absorption, and low sensitivity due to the absence of a gain mechanism (with the exception of black phosphorus demonstrating photoconductive gain, which comes at the cost of poor timing). 7,17–20 In contrast to crystalline photodetectors, disordered materials are produced more efficiently over a larger area and are much cheaper to make. Previously, picosecond timing was achieved in both amorphous silicon 21–23 and arsenic sulfide, 23–26 but neither material took off in ultrafast applications due to the trade-offs required to achieve picosecond timing, including the detectors’ lateral structures similar to, albeit not as thin as, those of twodimensional crystals, low sensitivity due to the absence of avalanche gain, and poor charge

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collection efficiency. Amorphous selenium (a-Se) has demonstrated similar detection capabilities to crystalline detectors despite being composed of a disordered network of predominantly ring molecules. 27 Carriers in a-Se can get hot at high electric fields and produce impact ionization avalanche for enhanced effective quantum efficiency. 28–30 The two key features of the avalanche phenomenon in selenium are that first, only holes get hot and impact ionize as seen from the large difference between electron and hole impact ionization rates (see Supplementary Fig. 1), and second, the avalanche process is noise-free and non-Markovian (see Supplementary Note 1 and Supplementary Fig. 2). 31 In addition, a-Se is a room-temperature semiconductor with wide band gap and ultra-low leakage current even at high fields, and thus does not require cooling. As a result, a-Se has been commercialized as the photosensor in the first ever avalanche optical camera, 32,33 and also has recently been used for large-area x-ray imaging. 29 Despite its promising photodetection capabilities and high fabrication yield over large areas, a-Se’s utilization in picosecond sensing has been hindered due to its extremely low carrier mobilities and bipolar charge sensing, resulting in poor timing resolution. However, by combining the material’s unique property of impact ionization avalanche gain when operated at high electric fields with our novel multi-well detector geometry by implementing nano-Frisch grids (∼ 300 nm) that results in unipolar time differential charge sensing, picosecond timing sufficient for many ultrafast applications can be achieved. Conventionally, a planar photoconductive material is fitted between two parallel contacts, namely the common and collecting/pixel electrodes, to form a sandwich cell (Fig. 1a). In a planar detector geometry, photocurrent is bipolar, meaning it is a function of motion of both holes and electrons throughout the bulk. For planar selenium detectors, because holes have much higher mobility than electrons (i.e., µh /µe ≥ 30), the common electrode is kept at a positive potential while the collecting electrode (or the collector) is at virtual ground and connects to the readout electronics for signal capture. Such planar geometry, assuming negligible small pixel effect, 34 means that the collector is sensitive to

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Figure 1: The nano-scale multi-well avalanche selenium detector. a) Schematic diagram of our multi-well selenium detector and experimental setup. A blue picosecond laser is directed at a 3×1 mm2 detector. A shielded RF probe is connected to the 50 Ω transmission line (i.e., collector) to detect the induced photocurrents. b) Cross-sectional diagram of our proposed multi-well selenium detector. Insulated pillars are spaced uniformly along the collecting electrode. Two pillars form one well in which carrier motion is detected, while the rest of the bulk is electrostatically shielded from carrier motion by the Frisch grid electrodes within the pillars. c) Photocurrent response signals for a planar detector. Response time is proportional to electric field strength as it also influences carrier mobility. d) Photocurrent response signals for our multi-well detector. Inset shows a scanning electron microscope cross-section of the multi-well detector, showing the pillar/grid width of 300 nm and pitch of 2.5 µm.

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real-time bulk carrier transport, and thus the impulse response is limited by the carrier transit-time (i.e., carrier’s effective lifetime in the detector before being immobilized) across the detector thickness, L. Let us consider a selenium detector within which a sheet of n electron-hole pairs (EHPs) are optically induced in the immediate vicinity of the common electrode, which is actually the case for time-of-flight (TOF) measurements using, for example, ultra-fast blue lasers with wavelengths λ ∼ 400 nm, where the absorption mean free path `α inside a-Se is ∼ 20 nm. According to the Shockley-Ramo theorem, the induced current on the collector due to carrier displacement inside the detector is proportional to the time-derivative of the weighting potential, VW . 35 Note that the conceptual VW is dimensionless and is the potential that would exist in the detector with the corresponding collecting electrode raised to unity and all other electrodes grounded. Given that the photo-induced electrons are neutralized by the common electrode almost instantly due to their ultra-short drift length (`α ∼ 20 nm), the impulse response function for this planar R detector is given as iP (t) = (nq/Th ) (1 − g(t| Th , σh2 ))dt, where q is the electronic charge, Th = L/µh E is the hole transit time across L, µh is the hole mobility, E is the applied electric field, and g(t| Th , σh2 ) is the Gaussian distribution function of hole carriers with its standard deviation σh represented by the detector’s non-dispersive broadening during Th . 36 Now consider a new device with its weighting potential at zero everywhere in the bulk except for a very small region near the collector where it rises sharply to one. Given that electrons and holes drift in opposite directions along the axis of the electric field, charge sensing becomes unipolar. Also, the collector is completely insensitive to bulk event times of the drifting photo-induced carriers (both electrons and holes), and the photoresponse is independent of both the photoconductor material (whether single crystalline or disordered) and its carrier transport mechanism (whether coherent or incoherent drift via band transport, multiple-trapping, or hopping). For the case above where a sheet of EHPs are generated close to the common electrode inside a-Se, the new impulse response is

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expressed as iUTD (t) = − Th (diP (t)/dt) = nq g(t| Th , σh2 ). Note that iUTD is proportional to the time-derivative of iP and that response time at the collector is only limited by the Gaussian spreading of the hole charge packet (and of course the RC time constant of the readout electronics), hence the name ‘unipolar time-differential (UTD)’ charge sensing. Such a detector is fabricated by utilizing the multi-well geometry and embedding Frisch grids (i.e., evenly spaced metal electrodes) inside insulating pillars, as depicted in Fig. 1b. 35,37 Using a 45 ps pulse laser tuned to 405 nm and a 12 GHz electrical RF-probe, we measured the impulse response functions of our fabricated planar and multi-well selenium detectors using the TOF transient photoconductivity technique. We performed the TOF measurement at various electric fields from E = 5 − 25 V/µm. As expected, the planar detector’s impulse response shows a semi-rectangular pulse with a soft plateau, due to inhomogeneous field distribution, followed by an exponential decay displaying sigmoidal decay characteristics (i.e., integral of the total Gaussian drift spread as shown in Fig. 1c). However, the multi-well detector’s response shows a Gaussian pulse centered at Th that verifies the time-differential property (Fig. 1d). The scanning electron microscope (SEM) cross-section of the fabricated device is shown in the inset of Fig. 1d. This time-differential Gaussian TOF, which is similar to a typical time distribution of Charpak’s multi-wire proportional chamber (MWPC), 38 signifies the ability to reach the intrinsic physical limit for pulse speed in non-dispersive solids. Note the shorter time-to-peak and the larger amplitude as E increases, which is due to the conservation of charge. In addition, we experimentally confirmed the shift in carrier transport in a-Se from activated hopping transport between strongly localized states all the way to non-activated and uninterrupted transport in extended states under high electric fields, with their corresponding wavefunctions ψ depicted in Fig. 2a. Although extended-state transport is band-like transport, carriers still undergo appreciable energy and momentum relaxation scattering events, which are actually desirable to yield deterministic and non-Markov

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avalanche gain. Due to the high scattering events of hot holes in extended states, the impact ionization process cannot be ballistic and holes must be accelerated over a finite time period before acquiring enough kinetic energy for the next impact ionization event. This “delay time” is expected to reduce (and potentially eliminate) the excess noise in a-Se because the multiple scattering events along with the associated acceleration and deceleration causes averaging of the distance traveled over the finite delay time before an impact ionization occurs. Thus, the history of phonon scattering and energy/momentum relaxation events yields non-Markov branching of hot holes and conversion from an inherently stochastic process to a noise-free deterministic gain (see Supplementary Note 1 and Supplementary Fig. 2). For a negatively biased multi-well detector with 18 µm pillars and a thick bulk region (L ∼ 200 µm), we observe an asymmetric, semi-Gaussian UTD response of ultra-low mobility electrons with µe = 0.004 cm2 V−1 s−1 due to phonon-assisted electron hopping transport across strongly localized states (Fig. 2b). The carrier wavefunction is strongly localized due to strong electron scattering where the scattering mean-free-path `s is substantially less than the interatomic spacing, a (`s a). The large skewness at the tail of Fig. 2b shows that electron drift spreading is non-gaussian and dispersive due to the nature of hopping transport at relatively low E, as expected from the electron ψ. Also, the soft plateau preceding the UTD response shows a pronounced and non-ideal bipolar component due to the low spatial frequency f g of the Frisch grids (Fig. 2b inset), which function to shield the collector from sensing both electrons and holes outside the wells. In this case, the large bipolar component of the photocurrent is due to electron drift outside the wells. The length of the photosensing region, which is equal to the height of the pillars (10 µm), also contributes to the relatively wide photoresponse since charge motion is being sensed over a large length (Fig. 2b). By reducing the pillar width to 3.5 µm and reversing the applied bias such that we now collect holes, which are the faster carriers in a-Se with their trap-limited mobility µh = 0.14 cm2 V−1 s−1 , we improved the photoresponse timing

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Figure 2: Unipolar time-differential photocurrent measurements from hopping to extended-state transport. Results are for various thickness, grid electrode pitch and applied electric field showing the shift from strongly localized states to extended state transport as the electrode pillars go from micro- to nano-scale. a) Conjectured form of the carrier wavefunctions showing the fluctuating probability amplitude on atoms for transport occurring in strongly localized, localized, quasi-extended, and extended states. Below each plot is the corresponding measured UTD impulse response function. Insets show the scanning electron microscope cross-sections of the detectors used for each measurement. b) Photoresponse of a negatively-biased, thick multi-well detector with low Frisch grid frequency ( f g = 27 mm-1 ) which results in imperfect shielding and a strong bipolar photocurrent. c) Reversing the electric field bias to collect holes and increasing the grid frequency ( f g = 105 mm-1 ) significantly improves the timing and reduces the bipolar component. d) Decreasing the detector thickness and further improving the grid frequency ( f g = 200 mm-1 ) completely eliminates the bipolar impulse component and makes deep trapping negligible, enabling ideal UTD charge sensing. e) Operating the thin multi-well detector (with f g = 240 mm-1 ) under high electric field shifts the transport from activated, trap-limited mobility, to non-activated, band-like extended state transport where hot holes undergo impact ionization avalanche.

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by two orders of magnitude (Fig. 2c). The photo-induced hole carrier packets still undergo many scattering events with `s < a but their diffusive Brownian motion is similar to short free flights in quasi-extended states followed by frequent capture and thermal-release events due to shallow traps (i.e., localized states with their ψ shown in Fig. 2a). The hole transport is non-dispersive, as shown from the measured Gaussian UTD TOF in Fig. 2c. Note that the reduced bipolar component is attributed to the higher f g (Fig. 2c inset). Decreasing the detector thickness (L ∼ 15 µm) and pillar height (3.5 µm) improved the photoresponse timing (Fig. 2d), and further decreasing the pillar width to nano-scale (300 nm) and increasing f g (Fig. 2d inset) completely eliminated the bipolar component of the UTD TOF photocurrent (also refer to Fig. 1d). Finally, decreasing the pillar height to 1 µm, operating in an electric field sufficient to achieve avalanche gain (E > 70 V/µm), and encapsulating the grid electrodes inside insulators to inhibit charge injection from the grid into a-Se (Fig. 2e inset) enabled us to achieve non-activated band-like extended state transport with `s > a similar to that of crystalline semiconductors. This is due to the elimination of shallow trapping via increased hot hole mobility of µh = 1 cm2 V−1 s−1 (Fig. 2e). The small ringing following the pulse in Fig. 2e is likely due to reflections at the probe-to-pixel connectors. A certain amount of ringing (±10%) is also expected in the sampling scope. In summary, UTD photoresponse timing for the multi-well selenium detector improved from phonon-assisted hopping transport with 20 µs FWHM (Fig. 2c) to band-like extended-state hot-hole transport with 2 ns FWHM (Fig. 2e), representing an improvement of four orders of magnitude. In addition, we achieved perfectly unipolar charge sensing with high sensitivity and impact ionization avalanche by going from microto nano-scale pillars (Fig. 2b-e). The effective quantum efficiency η ∗ was measured over a wide range of electric fields from E = 5 − 100 V/µm for the encapsulated multi-well detector with its SEM shown in the inset of Fig. 2e, where the collector and pillars are conformally insulated with parylene (Fig. 3a). For sub-avalanche fields (E < 70 V/µm), η ∗ increases with increasing E as

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geminate recombination is reduced, in accordance to the Onsager dissociation model, 39 and saturates very close to unity for blue wavelengths. Above the avalanche threshold, η ∗ rises sharply via impact ionization of hot holes due to their uninterrupted band-like transport in extended states. Maximum electric field before breakdown was 100 V/µm, at which we achieved avalanche multiplication gain of M = 40 (Fig. 3a). Recall that the impulse response TOF measurements of the thin multi-well detector operated at avalanche fields showed a Gaussian photoresponse with a width of 2 ns (Fig. 2e).

Figure 3: Time-resolution measurements. a) Experimental results showing avalanche gain in a 15 µm multi-well detector as a function of the applied electric field. For E < 70 V/µm, which is the region where impact ionization avalanche isn’t achieved in selenium, photogenerated charge recombination occurs and strengthens while decreasing the field. For E > 70 V/µm, where we achieve impact ionization of hot holes, multiplication gain follows an exponential trend while increasing the field. b) Time-resolved multi-well detector photocurrent measurements following many laser impulse excitations, showing the Gaussian UTD photoresponses and the timing jitter. c) Histogram of photoresponse timing jitter is measured based on the width of the spread shown in (b) which also has a Gaussian distribution. Detector performance is characterized by the FWHM of the jitter plot which is 145 ps at ∼ 90 V/µm. d) Photoresponse jitter as a function of electric field, all of which were sufficient for impact ionization avalanche as shown in (a). Timing jitter improvements are largely due to increases in fast carrier mobility, making the photoresponse more reliable.

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The timing jitter, ∆t, of a detector is the variation in delay between the absorption of the impulse light (start) and the generation of an output electrical pulse (stop). We measured a series of impulse responses under the avalanche electric field (Fig. 3b) and acquired a histogram of start-stop time intervals over multiple laser trigger cycles. Although the measured responses in Fig. 3b are a convolution of the true detector impulse response with the optical pulse width (as well as the impulse response of the sampling head, connectors, and transmission lines), timing jitter in our experimental setup is dominated by the detector impulse response. The histogram of this photocurrent timing jitter is shown in Fig. 3c which has a perfectly gaussian distribution whose width depends on the operating electric field and shows a two-fold improvement from 260 ps at the onset of avalanche at 75 V/µm to 120 ps at 100 V/µm (Fig. 3d). A plausible model for the photoresponse consists of an initial current pulse via hot photoexcited carriers, which are in relatively mobile extended states and then rapidly impact ionize, producing exponential avalanche multiplication gain. Note that the carrier recombination process, albeit slower in extended states and possibly irrelevant to the impulse response photocurrent, may still affect the saturation behavior of the detector when used at high repetition rates. Thus, timing improvement as a function of electric field can be attributed to the extended state transport occurring further away from the mobility edge where carriers experience less phonon scattering events. In addition, carrier drift mobility loses its thermally activated behavior entirely and saturates at 100 V/µm, which is to say the transport mechanism becomes more band-like, 40 making the overall impulse response narrower. We experimentally proved that a-Se can simultaneously achieve unipolar time-differential charge sensing and avalanche gain, thus enabling it to reach picosecond time resolution. The novel avalanche devices were fabricated using nano-scale pillars to form a highdensity multi-well geometry. This is the first demonstration of both picosecond timing and avalanche gain in a detector based on a disordered material that is manufacturable over a

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large area. Future work will focus on improving the timing by optimizing our fabrication process and further reducing the nano-pillar height (< 1µm) and width (< 100 nm). Our proposed multi-well avalanche selenium detector is a lower cost and higher production yield alternative to crystalline photodetectors. Our experimental results set the stage for future improvements and developments of ultrafast photosensor technologies based on amorphous selenium. Methods. Detector fabrication was performed at the Center for Functional Nanomaterials (CFN) at the Brookhaven National Laboratory (BNL), which is a DOE funded user facility for the fabrication and study of nanoscale materials (www.bnl.gov/cfn/). For the pillars in the multi-well detector, we used HD8820 polyimide with high breakdown voltage as the dielectric material and tungsten as the conductor for the Frisch grids. We fabricated the nano-pattern well structure with submicron alignment accuracy using the CFN’s JEOL JBX6300FS electron beam lithography system. We etched the pattern using the Oxford Instruments Plasmalab 100 Deep Reactive Ion Etching (RIE) tool, which uses an inductively coupled plasma (ICP), to achieve high aspect ratio nano-scale pillars. We sent the samples to Specialty Coating Systems Inc. to conformally encapsulate the pillars with fluorinated parylene-HT. In order to characterize our fabrication, we used the Hitachi 4800 scanning electron microscope (SEM) to image the photolithographically processed substrate and to evaluate the fabrication process. The SEM cross-section samples were prepared with focused-ion beam (FIB) milling using the FEI Helios NanoLab FIB microscope. We also incorporated conventional sample polishing in case FIB was destructive to the atomic structure of our sample. The canonical method for charge carrier transport studies of highly resistive, low mobility amorphous solids (such as a-Se) is the time-of-flight (TOF) transient photoconductivity measurements. Our detectors were characterized using optical TOF measurements to determine charge transport and avalanche properties. We used a PiLas 45 ps laser source at 405 nm for optical impulse excitation of a 3×1 mm2 multi-well selenium pixel along

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with a D-COAX Model W4.0 × L6.5 12 GHz probe station for measurement readout. The transient current generated from the detector at the onset of impulse-like excitation was amplified and converted into voltage with a Femto HSA-Y-60 4 GHz current amplifier, which was also used to minimize space-charge perturbation and effects due to polarization, lag, and ghosting, and to maintain the small-signal measurement accuracy from the lowest laser excitation intensity. The transient voltage was time-resolved with a LeCroy 640ZI oscilloscope with 4 GHz bandwidth and 40 GS/s. Supporting Information. Plots comparing local impact ionization coefficients of different crystalline semiconductors with amorphous selenium. Experimental plot comparing excess noise and avalanche gain of crystalline indium arsenide with amorphous selenium. Description of excess noise in APDs and why amorphous selenium has noiseless impact ionization avalanche.

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Acknowledgments We gratefully acknowledge financial support from the National Institutes of Health (R21 EB024849 and R21 EB025300). For device fabrication and characterization, we used the nanofabrication facility at the Center for Functional Nanomaterials (CFN), which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DESC0012704. We’d like to thank Aaron Stein and Ming Lu at CFN for assistance with device fabrication and Kim Kisslinger at CFN for FIB and SEM.

Author Contributions A.L. and A.H.G. analyzed the results and drafted the manuscript. A.H.G. and J.S. fabricated the devices and carried out the experiments. S.L. deposited the selenium. A.H.G. and W.Z. conceived and designed the detector structure and experiments. All co-authors contributed to and proofread the manuscript. 18


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Additional Information Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI Competing Interests: The authors declare no competing financial interests. ORCID Amir H. Goldan: 0000-0001-6513-5801 Andy LaBella: 0000-0002-3510-0322

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Multi-Well Selenium Detector Structure and Timing

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Picosecond Time Resolution with Avalanche Amorphous Selenium

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