Optical and electrical properties of alkaline-doped and As-alloyed amorphous selenium films

Güneş, Ozan; Koughia, Cyril; Curry, Richard J.; Gholizadeh, A. Baset; Belev, George; Ramaswami, Kieran; Kasap, S. O.

doi:10.1007/s10854-019-01386-x

Cited by 7 publications

(3 citation statements)

References 27 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…We note that commercial a-Se detectors are based on a three-layer p-i-n structure in which the p-layer and n-layer are thin (a few microns) and the i-layer is approximately 200 μm. The p-layer (next to the substrate) is As 2 Se 3 and the n-layer (between the i-layer and the positive electrode) is an alkaline-(Cs) doped Se 1−x As x (x ≈ 0.02-0.05) alloy which is n-type [21,22]. The thin layers act as "blocking layers" and reduce the dark current, but X-ray absorption and transport occurring in the i-layer dictate device performance; the work reported here is most relevant to that region.…”

Section: Introductionmentioning

confidence: 99%

Frequency- and time-resolved photocurrents in vacuum-deposited stabilised a-Se films: the role of valence alternation defects

Jacobs

Belev

Brookfield

et al. 2020

J Mater Sci: Mater Electron

View full text Add to dashboard Cite

Stabilised amorphous selenium (a-Se) is currently used in the majority of direct conversion mammographic X-ray imaging detectors due to its X-ray photoconductivity and its ability to be uniformly deposited over large area TFT substrates by conventional vacuum deposition. We report experimental results on photocurrent spectroscopy (frequency-resolved spectroscopy (FRS) and single-time transients), on vacuum-deposited a-Se films. We show that all measured photocurrents depend critically on the relative time spent by the material in the light and in the dark. We identify that the observed pronounced variation in optical response depends on the density of trapped (optically injected) charge within 200 nm of the surface and show that it is the ratio of dark and light exposure time that controls the density of such charge. Our data confirm that the localised charge radically influences the photocurrent transient shape due to the effective screening of the applied field within 200 nm of the surface. The field modification occurs over the optical extinction depth and changes both the photogeneration process and the drift of carriers. Many aspects of our data carry the signature of known properties of valence alternation pair (VAP) defects, which control many properties of a-Se. Modelling in the time domain shows that light generation of VAPs followed by optically triggered VAP defect conversion can lead to near-surface charge imbalance, demonstrating that VAP defects can account for the unusual optical response. The stabilised a-Se films were deposited above the glass transition temperature of the alloy with composition a-Se:0.3% As doped with ppm Cl. Electron paramagnetic resonance measurements at temperatures down to 5 K did not detect any spin active defects, even under photoexcitation above band gap.

show abstract

Section: Introductionmentioning

confidence: 99%

Frequency- and time-resolved photocurrents in vacuum-deposited stabilised a-Se films: the role of valence alternation defects

Jacobs

Belev

Brookfield

et al. 2020

J Mater Sci: Mater Electron

View full text Add to dashboard Cite

show abstract

“…Figure 2 and the data in Table 1 for the film H make the film-to-bulk As content ratio 0.257/0.34 or 0.756. The average As content in the film when the bulk is 6.0% is 4.0%, i.e., a ratio of 0.667 in [ 7 ]. An average of 0.71 was used to obtain the average film content for the analysis of the dependence of the carrier ranges on the As content in the film.…”

Section: Formulation Of the Modelmentioning

confidence: 99%

The Effect of Fractionation during the Vacuum Deposition of Stabilized Amorphous Selenium Alloy Photoconductors on the Overall Charge Collection Efficiency

Kasap

2022

Sensors

Self Cite

View full text Add to dashboard Cite

The general fabrication process for stabilized amorphous selenium (a-Se) detectors is vacuum deposition. The evaporant alloy is typically selenium alloyed with 0.3–0.5% As to stabilize it against crystallization. During the evaporation, fractionation leads to the formation of a deposited film that is rich in As near the surface and rich in Se near the substrate. The As content is invariably not uniform across the film thickness. This paper examines the effect of non-uniform As content on the charge collection efficiency (CE). The model for the actual CE calculation is based on the generalized CE equation under small signals; it involves the integration of the reciprocal range-field product (the schubweg) and the photogeneration profile. The data for the model input were extracted from the literature on the dependence of charge carrier drift mobilities and lifetimes on the As content in a-Se1−xAsx alloys to generate the spatial variation of hole and electron ranges across the photoconductor film. This range variation is then used to calculate the actual CE in the integral equation as a function of the applied field. The carrier ranges corresponding to the average composition in the film are also used in the standard CE equation under uniform ranges to examine whether one can simply use the average As content to calculate the CE. The standard equation is also used with ranges from the spatial average and average inverse. Errors are then compared and quantified from the use of various averages. The particular choice for averaging depends on the polarity of the radiation-receiving electrode and the spatial variation of the carrier ranges.

show abstract

“…A combination of the two materials was shown to exhibit even better photo-detection performance with high quantum efficiencies [4,5]. This material combination is also a promising candidate for ultrasensitive flat-panel x-ray imagers [6][7][8]. Recent work revealed that multi-layer films of Se and As 2 Se 3 fabricated using rotational evaporation exhibit electrical properties associated with superlattice structures, altering the band structures of the base materials and resulting in sequential tunneling based transport phenomena observed as oscillations in the current-voltage (I-V) characteristics [9,10].…”

Section: Introductionmentioning

confidence: 99%

Spectroscopic ellipsometry of amorphous Se superlattices

John¹,

Okano²,

Sharma³

et al. 2021

J. Phys. D: Appl. Phys.

View full text Add to dashboard Cite

Superlattice structures have a variety of electrical and optical properties that allow for interesting applications like quantum cascade lasers and ultrasensitive photo-detectors. However, such structures require high-tech fabrication methods like molecular beam epitaxy, and this technology barrier means that these promising devices are not in widespread use. Using the simple method of rotational evaporation, we fabricated films with alternating multi-nanolayers of amorphous selenium (Se) and arsenic selenide (As2Se3). We investigated the optical properties of the individual materials, and the resulting multi-layer structure using spectroscopic ellipsometry. The results were modeled using Cody–Lorentz oscillators to obtain the refractive index (n) and extinction coefficient (k). The models showed the optical band gaps of Se and As2Se3 to be 1.97 and 1.69 eV, respectively. The absorption coefficient (α) of the multi-layer structure showed a series of five ‘steps’ in energy at 1.72, 1.82, 1.89, 1.97, and 2.04 eV. These are confirmed to stem from the transitions between confined quantum well levels due to the superlattice structure. In this way, the optical measurement using spectroscopic ellipsometry confirms the possibility of fabricating good quality nanostructutres using amorphous materials and rotational evaporation.

show abstract

Optical and electrical properties of alkaline-doped and As-alloyed amorphous selenium films

Cited by 7 publications

References 27 publications

Frequency- and time-resolved photocurrents in vacuum-deposited stabilised a-Se films: the role of valence alternation defects

Frequency- and time-resolved photocurrents in vacuum-deposited stabilised a-Se films: the role of valence alternation defects

The Effect of Fractionation during the Vacuum Deposition of Stabilized Amorphous Selenium Alloy Photoconductors on the Overall Charge Collection Efficiency

Spectroscopic ellipsometry of amorphous Se superlattices

Contact Info

Product

Resources

About