S. O. Kasap

Springer Handbook of Electronic and Photonic Materials

The Springer Handbook of Electronic and Photonic Materials has been prepared to give a broad coverage of a wide range of electronic and photonic materials, starting from fundamentals and building up to advanced topics and applications. Its wide coverage with clear illustrations and applications, its chapter sequencing and logical flow, make it very different than other electronic materials handbooks. Each chapter has been prepared either by experts in the field or instructors who have been teaching the subject at a university or in corporate laboratories. The handbook provides an accessible treatment of the material by developing the subject matter in easy steps and in a logical flow. Wherever possible, the sections have been logically sequenced to allow a partial coverage at the beginning of the chapter for those who only need a quick overview of the subject. Additional valuable features include the practical applications used as examples, details on experimental techniques, useful tables that summarize equations, and, most importantly, properties of various materials. The handbook also has an extensive glossary at the end being helpful to those readers whose background may not be directly in the field. Key Topics Fundamental Electronic, Optical and Magnetic Properties Materials Growth and Characterization Materials for Electronics Materials for Optoelectronics and Photonics Novel Materials Selected Applications Features Contains over 600 two-color illustrations Includes over 100 comprehensive tables summarizing equations, experimental techniques and properties of various materials Emphasizes physical concepts over extensive mathematical derivations Parts and chapters with summaries, detailed index and fully searchable CD-ROM guarantee quick access to data and links to other sources Delivers a wealth of up-to-date references Incorporates a detailed Glossary of Terms

Amorphous and Polycrystalline Photoconductors for Direct Conversion Flat Panel X-Ray Image Sensors

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI2 and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples.

Amorphous Semiconductors Usher in Digital X‐Ray Imaging

Unlike other major medical imaging methods, such as computed tomography, ultrasound, nuclear medicine and magnetic resonance imaging—all of which are digital—conventional x‐ray imaging remains a largely analog technology. Making the transition from analog to digital could bring several advantages to x‐ray imaging: Contrast and other aspects of image quality could be improved by means of image processing; radiological images could be compared more easily with those obtained from other imaging modalities; the electronic distribution of images within hospitals would make remote access and archiving possible; highly qualified personnel could service remote or poorly populated regions from a central facility by means of “teleradiology”; and radiologists could use computers more effectively to help with diagnosis—work that has already been initiated at the University of Chicago by Kunio Doi and his coworkers.

Direct-conversion flat-panel X-ray image sensors for digital radiography

Advances in active-matrix array flat panels for displays over the last decade have led to the development of flat-panel X-ray image detectors. Recent flat-panel detectors have shown image quality exceeding that of X-ray film/screen cassettes. They can also permit the instantaneous capture, readout, and display of digital X-ray images and, hence, enable the clinical transition to digital radiography. There are two general approaches to flat panel detector technology: 1) direct and 2) indirect conversion. The present paper outlines the operating principles for direct-conversion detectors based on the use of photoconductors. It formulates and reviews the required X-ray photoconductor properties for such applications and examines to what extent potential materials fulfill these requirements. The quantum efficiency, X-ray sensitivity, noise, and detective quantum efficiency factors are discussed with reference to current and potential large area X-ray photoconductors.

X-ray sensitivity of photoconductors: application to stabilized a-Se

The x-ray sensitivity of a high-resistivity photoconductor sandwiched between two parallel plate electrodes and operating under a constant field is analysed by considering charge carrier generation that follows the x-ray photon absorption profile and taking into account both electron and hole trapping phenomena but neglecting recombination, bulk space charge and diffusion effects. The amount of collected charge in the external circuit due to distributed generation of electrons and holes through the detector is calculated by integrating the Hecht collection efficiency with Ramos theorem across the sample thickness. The results of the model allow the x-ray sensitivity to be calculated as a function of the applied field, detector thickness and electron and hole ranges (µτ), given the field and energy dependence of the electron and hole pair creation energy, W±, and the energy spectrum of incident x-ray radiation. The sensitivity model was applied to stabilized a-Se that is currently used as a successful x-ray photoconductor in the recently developed flat panel x-ray image detectors. Recent free electron-hole pair creation energy versus electric field data at room temperature and appropriate electron and hole drift mobilities were used to calculate the sensitivity for monoenergetic x-rays at 20 and at 60 keV. For the 20 keV radiation, it was shown that a typical detector thickness of 200 µm (4 × attenuation depth at 20 keV) with currently attainable electron and hole trapping parameters in a-Se was operating optimally, the sensitivity of which can only be increased by further increasing the applied field. With the receiving electrode positively biased, the sensitivity was much more dependent on the hole lifetime than electron lifetime. The absence of hole transport results in a reduction in sensitivity by a factor of about 4.4, whereas the absence of electron transport results in a sensitivity degradation of only 22%. The ratio of hole trapping limited sensitivity to electron trapping limited sensitivity is about 0.3. For a detector of thickness 200 µm operating at 10 V µm-1, the maximum sensitivity is about 220 pC cm-2 mR-1, and this sensitivity degrades by more than 10% when either the electron lifetime falls below ~20 µs or the hole lifetime falls below ~5 µs. When the hole lifetime is very short so that the sensitivity is substantially reduced, the sensitivity versus thickness dependence at a given field exhibits a maximum (an optimal thickness) that is less than that for full absorption. In the case of 60 keV x-ray photons, it is more useful to examine the sensitivity as a function of detector thickness given the practical bias voltage limit. The sensitivity versus thickness behaviour for a given bias voltage exhibits a maximum, that is an optimal thickness, that is less than that for nearly full absorption. Electron lifetimes longer than ~200 µs and hole lifetimes longer than ~10 µs do not significantly affect the sensitivity.

Review X-ray photoconductors and stabilized a-Se for direct conversion digital flat-panel X-ray image-detectors

Recently developed direct conversion flat-panel X-ray image detectors provide not only potentially superior images but also enable a simple and convenient means of achieving digital radiography. The present flat-panel sensors use stabilized a-Se as an X-ray photoconductor to convert absorbed X-ray photons to collectable charge carriers. The present paper outlines the basics of the direct conversion flat-panel X-ray detector, formulates and reviews the required X-ray photoconductor properties and examines to what extent stabilized amorphous selenium (a-Se) fulfills these requirements and how it compares with other potential X-ray photoconductors that have been proposed. Charge transport and the electron–hole pair creation energy in stabilized a-Se are reviewed within the context of our present knowledge. Current materials problems and future potential developments are also critically discussed.

High index contrast waveguides in chalcogenide glass and polymer

We review various properties of chalcogenide glasses that make them promising materials for passive and active microphotonics. We then describe two processes for channel waveguide fabrication, using the chalcogenide glass As/sub 2/Se/sub 3/ as a core material and compatible polymers as claddings. In the first approach, waveguides are patterned directly in the chalcogenide film by photoexposure through a mask followed by selective wet etching. This technique has produced shallow rib waveguides with losses as low as 0.26 dB/cm and small modal area photonic wire waveguides with losses on the order of 10 dB/cm. In the second approach, waveguide patterning is achieved by using an organic photoresist as a mask for selective photodoping of silver into the chalcogenide glass. Selective wet etching produced strip waveguides with smooth and highly vertical sidewalls. We report preliminary light guiding results for these latter structures.

Density of localized electronic states in a-Se from electron time-of-flight photocurrent measurements

Electron time-of-flight transient photocurrents have been investigated in stabilized a-Se as a function of electric field, annealing, aging (relaxation), and alloying with As and doping with Cl.The distribution of localized states (DOS) in stabilized a-Se has been investigated by comparing the measured and calculated transient photocurrents. The samples were prepared by conventional vacuum deposition techniques. The theoretical analysis of multiple-trapping transport has been done by the discretization of a continuous DOS and the use of Laplace transform formalism. The resulting DOS has distinct features: A first peak at ∼0.30eV below Ec with an amplitude ∼1017eV−1cm−3, a second small peak (or shoulder) at 0.45–0.50 eV below Ec with an amplitude 1014–1015eV−1cm−3, and deep states with an integral concentration 1011–1014cm−3 lying below 0.65 eV, whose exact distribution could not be resolved over the time scale of present experiments. The influence of doping, aging, annealing, and substrate temperature on ...

Photoinduced refractive index change in As2Se3 by 633nm illumination.

Photodarkening of amorphous As2Se3 thin films was generated by a 633-nm HeNe laser. The refractive index and absorption coefficient of the chalcogenide glass was determined, both before and after exposure, by analyzing the materials transmission spectrum. In order to accurately determine the optical constants, the thin films non-uniform thickness was accounted for. The increase in the refractive index and the coefficient of absorption was investigated and was found to demonstrate saturation with increased exposure time. Index changes as high as 0.05, or 2%, were obtained in As2Se3, a promising glass for all-optical switching.

Time-of-flight drift mobility measurements on chlorine-doped amorphous selenium films

Time-of-flight drift mobility experiments have been carried out on pure and chlorine-doped amorphous selenium films. The temperature and field dependence of the drift mobility of both types of carriers have been investigated and the charge transport mechanism has been critically examined in terms of various shallow-trap-controlled transport processes, including hopping for the microscopic mobility. Addition of 30 and 40 wt ppm Cl to a-Se has been found to increase the hole drift mobility activation energy by approximately 0.16 eV. Two interpretations have been shown to be plausible: (i) Cl doping diminishes the existing charged centres (possibly Se1-) acting as shallow hole traps and/or changes the mode of microscopic transport, (ii) Cl introduces a new set of charged hole traps (probably Cl-) at approximately 0.45 eV above Ev. Furthermore, it is shown that at low temperatures the hole drift mobility may be described by an empirical expression similar to that due to Gill (1972, 1976).