Characterization of a‐Se p–i–n junction fabricated using bidirectional electrolysis in NaCl(aq)

Onishi, M.; Saito, Ichitaro; Komiyama, K.; Takeno, Kentaro; Miyazaki, Wataru; Masuzawa, Tomoaki; Koh, Angel T. T.; Chua, Daniel H. C.; Yamada, Takatoshi; Mori, Yusuke; Okano, Ken

doi:10.1002/pssa.201532372

Cited by 4 publications

(4 citation statements)

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“…The films were dipped into saturated aqueous solution of sodium chloride (NaCl), and a DC voltage was applied between the substrate of a‐Se and a counter electrode made of copper. Through this process, Cl atoms in the solution are incorporated into the surface of a‐Se films, which changes natively weak p‐type a‐Se into n‐type . The current during electrolysis was kept at 1 μA to obtain high Cl concentration at the surface of the film.…”

Section: Methodsmentioning

confidence: 99%

“…We call this process “drive‐in”. Detailed procedure and characterization of Cl incorporation into a‐Se is presented elsewhere …”

Section: Methodsmentioning

confidence: 99%

“…In this study, a novel approach has been explored to increase the sensitivity of photodetectors made of a‐Se and diamond cold cathode. An electro‐chemical method was utilized to incorporate chlorine (Cl) atoms into a‐Se, which changes its conduction from weak p‐ to n‐type . The a‐Se film with p‐n junction was then mounted with a diamond cold cathode to form a prototype photodetector, which showed an improved signal‐to‐noise characteristic compared to the photodetector using as‐deposited a‐Se.…”

Section: Introductionmentioning

confidence: 99%

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Formation of p‐n Junction in a‐Se Thin Film and Its Application to High Sensitivity Photodetector Driven by Diamond Cold Cathode

Masuzawa

Ohata

John

et al. 2017

Physica Status Solidi (a)

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In this study, photoconductor made of amorphous selenium (a-Se) is studied to improve a sensitivity of vacuum-tube type photodetectors. An electrochemical process was applied to a-Se films to form a p-n structure within the film. The formation of p-n structure was confirmed by time-of-flight secondary ion mass spectroscopy as well as its electronic property. The a-Se film was then combined into a prototype photodetector, and enhancement of sensitivity was evaluated in terms of nominal quantum efficiency and signal-to-noise ratio. The results showed that the formation of a p-n junction within a-Se film increased the sensitivity of the photodetector, especially at relatively low operation voltages.

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Section: Methodsmentioning

confidence: 99%

“…We call this process “drive‐in”. Detailed procedure and characterization of Cl incorporation into a‐Se is presented elsewhere …”

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Formation of p‐n Junction in a‐Se Thin Film and Its Application to High Sensitivity Photodetector Driven by Diamond Cold Cathode

Masuzawa

Ohata

John

et al. 2017

Physica Status Solidi (a)

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“…Another way of introducing Cl impurities is through electrolysis [97], [98]. During the electrolysis process, a-Se is used as the anode to electrolyze a saturated NaCl solution.…”

Section: Impuritiesmentioning

confidence: 99%

Recent Developments of Amorphous Selenium-Based X-Ray Detectors: A Review

Huang

Abbaszadeh

2020

IEEE Sensors J.

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Amorphous selenium (a-Se) is a photoconductive material that has been intensively investigated from its early application in xerography to its present application in flat panel X-ray imagers. It can be deposited up to a few millimeters thick over a large area. Its high vapor pressure yields uniform coverage in novel device structures for lowcost and large-area applications. The evidence of avalanche multiplication in a-Se and application of a-Se in high-gain avalanche rushing photoconductor video-tubes goes back to the early 1980s. Over the past decade there has been increasing research interest in novel detector structures and integration of a-Se with new materials to leverage the avalanche properties. We summarize some of the shortcomings of a-Se such as low charge carrier mobility, low charge conversion efficiency, depth dependence, and high dark current at high electric fields. We then highlight recent developments in a-Se-based devices to address these shortcomings and enable picosecond timing performance and high detection efficiency.

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Effects of x-ray irradiation on charge transport and charge collection efficiency in stabilized a-Se photoconductors

Kasap

Yang

Simonson

et al. 2020

Journal of Applied Physics

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Stabilized amorphous selenium (a-Se) photoconductive layers are currently used in the majority of modern digital x-ray flat panel imaging detectors in mammography. We examine the effects of pre-exposure of a-Se to high-dose x-ray irradiation on both hole and electron lifetimes, τe and τh, respectively, without any field applied to the device. The x-ray irradiation was from an Al-filtered tungsten target x-ray tube. We show that reduction in τh and τe depends only on the total or accumulated dose, D, absorbed in a-Se, and not on the rate of dose delivery, dD/dt, over the range of 0.15 Gy/s–2.5 Gy/s or on the x-ray energy over 50–90 kVp, corresponding to a mean photon energy over 31.9 keV–44.7 keV. The x-ray induced effects on charge transport are independent of the x-ray intensity and x-ray photon energy but dependent on the total energy absorbed in a-Se. The latter finding allows x-ray induced drop in the carrier lifetimes to be simply and conveniently modeled by τo/τ = 1 + AD, where τo is the lifetime before x-ray exposure (equilibrium lifetime), τ is the lifetime after exposure, D is the absorbed total dose, and A is a constant, which is 0.203 (±0.021) Gy−1 for the hole lifetime and 0.0620 (±0.0090) Gy−1 for the electron lifetime, a factor of three smaller than that for holes. X-ray irradiation had no effect on hole and electron drift mobilities. Reduction in carrier lifetimes with the total dose was examined at 10 °C, 23.5 °C, and 35.5 °C, close to the glass transition temperature, where x-ray induced effects are stronger. A is independent of hole and electron lifetimes but has a strong temperature dependence, increasing sharply with temperature. After the cessation of x-ray irradiation, carrier lifetimes relax (increase) to their pre-exposed equilibrium values over time scales that depend on temperature. Recovery has been interpreted and analyzed in terms of considering the kinetics of the rate at which x-ray induced capture centers are removed, as the structure restores the equilibrium concentration of deep traps. The annealing process of excess hole traps has a fast and a slow decay component, with time constants τr1 and τr2, respectively. The recovery processes associated with τr1 and τr2 exhibit activation energies larger than those typically involved in electronic transitions and are comparable to bond breaking energies in Se–Se and Se–As. Near 35.5 °C, close to the glass transition temperature, τr2-recovery has a large activation energy, pointing to structural relaxation phenomena. Fast annealing (∼6 min) of excess hole traps at 35.5 °C is, in a general sense, in agreement with the disappearance of irreversible photoinduced effects and suppression of crystallization (strain relief) at the a-Se/substrate interface, as observed previously. In the case of recovery of the electron lifetime, single exponential decay in excess electron traps and clear activation energy of 1.91 eV/atom point to a probable Se–Se bond breaking involved in returning excess electron traps to equilibrium concentration. Interpretations based on x-ray induced excess valence alternation pair (VAP) and intimate VAP type defects are also considered, including conversion from neutral defects to charged VAP defects. The implications of the present findings on x-ray sensitivity of a-Se detectors through the charge collection efficiency (CCE) are also examined and discussed. An effective carrier lifetime concept is used to describe the effect of x-ray irradiation on carrier lifetimes, which is then used to find CCE in a pre-exposed a-Se detector. The results indicate that x-ray induced effects are negligible for nearly all practical applications of a-Se mammographic detectors in use provided that the detector is operated at a sufficiently high field and a-Se has sufficiently long initial lifetimes, i.e., it is a high quality electronic grade material.

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Characterization of a‐Se p–i–n junction fabricated using bidirectional electrolysis in NaCl(aq)

Cited by 4 publications

References 5 publications

Formation of p‐n Junction in a‐Se Thin Film and Its Application to High Sensitivity Photodetector Driven by Diamond Cold Cathode

Formation of p‐n Junction in a‐Se Thin Film and Its Application to High Sensitivity Photodetector Driven by Diamond Cold Cathode

Recent Developments of Amorphous Selenium-Based X-Ray Detectors: A Review

Effects of x-ray irradiation on charge transport and charge collection efficiency in stabilized a-Se photoconductors

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