Yuri Vygranenko

Photolithographically defined polythiophene organic thin-film transistors

A photolithography process for the fabrication of organic thin-film transistor (OTFT) and integrated circuits will be presented. Fully encapsulated polythiophene OTFTs in the top-gate, bottom-gate, and dual-gate configurations have been demonstrated using this approach. Photolithography steps are incorporated for the definition of gate electrodes and source/drain contacts, isolation of transistors, and formation of vias and interconnects. These steps are vital to achieve device integration and to realize organic integrated circuits for applications such as flat panel displays and radio-frequency identification tags. The fabrication process is compatible with various substrates (e.g., glass and plastic), and enables the realization of discrete transistors and OTFT-based circuits through consecutive photolithographic steps and a tailored etch recipe for patterning of the polymer film. This article will provide a detailed examination of the fabrication approach, analyze the electrical performance of various ...

Low dark current and blue enhanced a-Si:H∕a-SiC:H heterojunction n-i-δi-p photodiode for imaging applications

This paper presents an a-Si:H∕a-SiC:H heterojunction n-i-δi-p photodiode with low dark current and enhanced short wavelength responsivity suitable for low-level light detection applications. Junction properties and carrier transport are investigated in terms of current-voltage characteristics, photocurrent transient measurements, and spectral photoresponse. It is demonstrated that introduction of a thin (∼40A) undoped a-SiC:H buffer (δi) at the p-i interface significantly reduces the reverse dark current and recombination losses at this interface. A dark current density of ∼10pA∕cm2 at reverse bias of 1V is achieved for the n-i-δi-p structure, in which the p-type a-SiC:H window layer and the undoped δi buffer layer have a band gap of 2eV.This paper presents an a-Si:H∕a-SiC:H heterojunction n-i-δi-p photodiode with low dark current and enhanced short wavelength responsivity suitable for low-level light detection applications. Junction properties and carrier transport are investigated in terms of current-voltage characteristics, photocurrent transient measurements, and spectral photoresponse. It is demonstrated that introduction of a thin (∼40A) undoped a-SiC:H buffer (δi) at the p-i interface significantly reduces the reverse dark current and recombination losses at this interface. A dark current density of ∼10pA∕cm2 at reverse bias of 1V is achieved for the n-i-δi-p structure, in which the p-type a-SiC:H window layer and the undoped δi buffer layer have a band gap of 2eV.

Amorphous Silicon Display Backplanes on Plastic Substrates

Amorphous silicon (a-Si) thin-film transistor (TFT) backplanes are very promising for active-matrix organic light-emitting diode displays (AMOLEDs) on plastic. The technology benefits from a large manufacturing base, simple fabrication process, and low production cost. The concern lies in the instability of the TFTs threshold voltage (VT) and its low device mobility. Although VT-instability can be compensated by means of advanced multi-transistor pixel circuits, the lifetime of the display is still dependent on the TFT process quality and bias conditions. A-Si TFTs with field-effect mobility of 1.1 cm2/Vmiddots and pixel driver circuits have been fabricated on plastic substrates at 150 degC. The circuits are characterized in terms of current drive capability and long-term stability of operation. The results demonstrate sufficient and stable current delivery and the ability of the backplane on plastic to meet AMOLED requirements

Phototransistor with nanocrystalline Si/amorphous Si bilayer channel

We report a field-effect phototransistor with a channel comprising a thin nanocrystalline silicon transport layer and a thicker hydrogenated amorphous silicon absorption layer. The semiconductor and dielectric layers were deposited by radio-frequency plasma enhanced chemical vapor deposition. The phototransistor with channel length of 24 microns and photosensitive area of 1.4 mm2 shows an off-current of about 1 pA, and high photoconductive gain in the subthreshold region. Measurements of the quantum efficiency at different incident light intensities and biasing conditions, along with spectral-response characteristics, and threshold voltage stability characterization demonstrate the feasibility of the phototransistor for low light level detection.

Two-dimensional a-Si:H n-i-p photodiode array for low-level light detection

This paper presents the design, fabrication process, and performance evaluation of a two-dimensional hydrogenated amorphous silicon (a-Si:H) n-i-p photodiode array, developed specifically for low-level light sensor applications. The design of the device is simpler than conventional active-matrix-arrays based on thin-film transistor (TFT) addressing electronics, owing to the utilization of the a-Si:H switching diodes for signal readout. The discussed technological developments are aimed to minimize the leakage current and to enhance the external quantum efficiency. The current-voltage characteristics of the sensing and switching diodes are analyzed to identify the sources of the excess leakage current. The optical losses in the photodiodes with an ITO/a-SiN/sub x/:H antireflection coating have been minimized using numerical modeling. Description of the peripheral electronics and associated timing diagrams along with the results of the detector characterization, including the linearity and response time measurements, are presented and discussed.

Optimization of the a -SiC p -layer in a -Si:H-based n-i-p Photodiodes

Our work is aimed at enhancing the external quantum efficiency (EQE) of n-i-p photodiodes by reducing the absorption losses in the p-layer and the recombination losses in the p-i interface. We have applied boron-doped and undoped hydrogenated amorphous silicon carbon alloy (a-SiC:H) grown in hydrogen-diluted, silane-methane plasma to both the p-layer and undoped buffer layer, thus tailoring the p-i interface. The current-voltage, capacitance-voltage, and spectral-response characteristics of fabricated photodiodes are correlated with the doping level, optical band gap, and deposition conditions for a-SiC:H layers. The optimized device exhibits a leakage current of about 110 pA/cm2 at the reverse bias of 5 V, and a peak value of 89% EQE at a wavelength of 530 nm. At shorter wavelengths, the EQE decreases down to 56% at a 400 nm wavelength. Calculations of transmission/reflection losses at the front of the photodiode show that observed short-wavelength sensitivity enhancement can be attributed to improved separation of electron-hole pairs in the p-layer depletion region.

Blue-enhanced thin-film photodiode for dual-screen x-ray imaging

This article reports on a-Si:H-based low-leakage blue-enhanced photodiodes for dual-screen x-ray imaging detectors. Doped nanocrystalline silicon was incorporated in both the n- and p-type regions to reduce absorption losses for light incoming from the top and bottom screens. The photodiode exhibits a dark current density of 900 pA/cm2 and an external quantum efficiency up to 90% at a reverse bias of 5 V. In the case of illumination through the tailored p-layer, the quantum efficiency of 60% at a 400 nm wavelength is almost double that for the conventional a-Si:H n-i-p photodiode.

Noise performance of high fill factor pixel architectures for robust large-area image sensors using amorphous silicon technology

Large area digital imaging made possible by amorphous silicon thin-film transistor (a-Si TFT) technology, coupled with a-Si photo-sensors, provides an excellent readout platform to form an integrated medical image capture system. Major development challenges evolve around optimization of pixel architecture for detector fill factor, signal propagation performance, and manufacturability, while suppressing noise stemming from pixel array and external electronics. This work analyzes a novel vertically integrated pixel design based on signal readout and noise performance, and compares with conventional co-planar and continuous detector architectures. In addition, the analysis will consider various substrate options including glass and robust substrates such as polymer and metal foil. Our evaluation have demonstrated state-of-the-art radiographic detector system with electronic noise under 2000 electrons at 150 p.s frame time for an imaging arrays on robust substrate.

Driving Scheme Using MIS Photosensor for Luminance Control of AMOLED Pixel

This paper presents a new driving scheme utilizing an in-pixel metal-insulator-semiconductor (MIS) photosensor for luminance control of active-matrix organic light-emitting diode (AMOLED) pixel. The proposed 3-TFT circuit is controlled by an external driver performing the signal readout, processing, and programming operations according to a luminance adjusting algorithm. To maintain the fabrication simplicity, the embedded MIS photosensor shares the same layer stack with pixel TFTs. Performance characteristics of the MIS structure with a nc-Si:H/a-Si:H bilayer absorber were measured and analyzed to prove the concept. The observed transient dark current is associated with charge trapping at the insulator-semiconductor interface that can be largely eliminated by adjusting the bias voltage during the refresh cycle. Other factors limiting the dynamic range and external quantum efficiency are also determined and verified using a small-signal model of the device. Experimental results demonstrate the feasibility of the MIS photosensor for the discussed driving scheme.

Optimization of p-type nanocrystalline silicon thin films for solar cells and photodiodes

We report on structural, electronic, and optical properties of boron-doped, hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) at a substrate temperature of 150°C Film properties were studied as a function of trimethylboron-to-silane ratio and film thickness. The film thickness was varied in the range from 14 to 100 nm. The conductivity of 60 nm thick films reached a peak value of 0.07 S/cm at a doping ratio of 1%. As a result of amorphization of the film structure, which was indicated by Raman spectra measurements, any further increase in doping reduced conductivity. We also observed an abrupt increase in conductivity with increasing film thickness ascribed to a percolation cluster composed of silicon nanocrystallites. The absorption loss of 25% at a wavelength of 400 nm was measured for the films with optimized conductivity deposited on glass and glass/ZnO:Al substrates. A low-leakage, blue-enhanced p-i-n photodiode with an nc-Si p-layer was also fabricated and characterized.