Yuriy Vygranenko

Stable indium oxide thin-film transistors with fast threshold voltage recovery

Stable thin-film transistors (TFTs) with semiconducting indium oxide channel and silicon dioxide gate dielectric were fabricated by reactive ion beam assisted evaporation and plasma-enhanced chemical vapor deposition. The field-effect mobility is 3.3cm2∕Vs, along with an on/off current ratio of 106, and subthreshold slope of 0.5V/decade. When subject to long-term gate bias stress, the TFTs show fast recovery of the threshold voltage (VT) when relaxed without annealing, suggesting that charge trapping at the interface and/or in the bulk gate dielectric to be the dominant mechanism underlying VT instability. Device performance and stability make indium oxide TFTs promising for display applications.

Low leakage p-NiO∕i-ZnO∕n-ITO heterostructure ultraviolet sensor

This letter reports a low leakage p-NiO∕i-ZnO∕n-ITO ultraviolet photodiode fabricated at room temperature by ion beam assisted e-beam evaporation. Analysis of its J-V characteristics, and time-dependent behavior, reveals that the dominant source of leakage current stems from deep defect states in the ZnO i layer, with its dynamic response at low signal levels limited by charge trapping in the absorption layer. Under a 5V reverse bias, the dark current density is 10nA∕cm2 and quantum efficiency is 18% at a wavelength of 380nm, with a photoresponse behavior that is linear over 5decades.

ZnO-based p-i-n and n-i-p heterostructure ultraviolet sensors: a comparative study

A comparative study is reported on p-NiO∕i-ZnO∕n-ITO and n-ITO∕i-ZnO∕p-NiO heterostructure ultraviolet sensors. In comparing reverse-bias current-voltage characteristics, dark current transients, and dark-current field dependence of both diodes, we observed that thermal and Poole–Frenkel generation currents dominate in the leakage. The difference in the leakage level and field dependence essentially depends on the p-i interface. Analysis of forward-bias current-voltage characteristics identifies the difference in Schottky barrier height between two diodes, which causes the difference in the contact leakage level as well. Furthermore, the p-i-n diode has better performance in photosensitivity and responsivity due to less optical loss in the top contact, and both diodes are promising for low levels of ultraviolet detection.

A-Si Amoled Display Backplanes on Flexible Substrates

In view of its maturity and low-cost, the amorphous silicon (a-Si) technology is an attractive candidate for active matrix organic light emitting diode (AMOLED) display backplanes on flexible substrates. However, the a-Si material comes with significant intrinsic shortcomings related to speed (mobility) and stability of operation, requiring novel threshold-voltage-shift (δVT) compensated thin-film transistor (TFT) pixel circuits and architectures to enable stable OLED operation. But given the dramatic progress in efficiency of OLED materials over recent years, the drive current requirement has been significantly lowered, thus relaxing the constraints on a-Si TFTs. For compatibility to plastic substrates, the a-Si TFT process temperature must be reduced from the conventional 300°C to ∼150°C or below, which tends to compromise the integrity of thin-film materials and device performance. Hence, optimizing the TFT process for high device performance with limited thermal budget is a necessary step towards flexible AMOLEDs with a-Si backplanes. This paper reviews the design and process challenges, and specifically examines the performance of TFTs and δVT- compensated integrated pixel driver circuits on plastic substrates with respect to current driving ability and long term stability. More importantly, lifetime tests of circuit degradation behaviour over extended time periods demonstrate highly stable drive currents and its ability to meet commercial standards.

Design, fabrication and characterization of an a-Si:H-based UV detector for sunburn applications

A thin-film a-Si:H pin detector was developed for selective detection of UVA (320–400 nm) radiation. In order for the fabrication technology to be transferable onto flexible substrates, all of the processing steps were conducted at temperatures less than 125 °C. The measured saturation current as low as 2 pA cm−2 and the ideality factor of 1.47 show that the pin diodes have a good quality i-layer as well as p–i and n–i interfaces. The film thicknesses were optimized to suppress the detector sensitivity in the visible spectral range, and the peak of spectral response was observed at 410 nm. The selectivity estimated from the ratio of the photocurrent generated by UVA absorption to the total photocurrent is 21%.

Photo-Induced Instability of Nanocrystalline Silicon TFTs

We examine the instability behavior of nanocrystalline silicon (nc-Si) thin-film transistors (TFTs) in the presence of electrical and optical stress. The change in threshold voltage and sub-threshold slope is more significant under combined bias-and-light stress when compared to bias stress alone. The threshold voltage shift after 6 h of bias stress is about 7 times larger in the case with illumination than in the dark. Under bias stress alone, the primary instability mechanism is charge trapping at the semiconductor/insulator interface. In contrast, under combined bias-and-light stress, the prevailing mechanism appears to be the creation of defect states in the channel, and believed to take place in the amorphous phase, where the increase in the electron density induced by electrical bias enhances the non-radiative recombination of photo-excited electron-hole pairs. The results reported here are consistent with observations of photo-induced efficiency degradation in solar cells.

Room Temperature Growth of Indium Oxide Films by Reactive Ion Beam Assisted Deposition

We report the growth of indium oxide thin films at room temperature by oxygen ion beam assisted e-beam evaporation for device applications. We examined the influence of deposition conditions on film properties including the crystal structure, resistivity, optical transmittance, stoichiometry, morphology, and intrinsic stress. X-ray diffraction analysis shows that the film structure changes from amorphous to polycrystalline with preferred (222) orientation when the discharge current increases from 0.5 A to 2.0 A. It is also observed that film resistivities can be tailored over a wide range, from 3x10 -4ω-cm to 2xl09ω-cm by modifying both the evaporation rate and the discharge current. X-ray photoelectron spectroscopy data reveal that the highly-resistive films are more oxygen-enriched than the highly-conductive counterparts due to the electrical activity of oxygen vacancies. All films studied in this work show an optical transmittance up to 80%. Thus, high-performance indium oxide films can be engineered by reactive ion beam assisted deposition to meet different application requirements of devices such as solar cells, photodetectors, OLEDs, transparent TFTs, and optical coatings.

Transient current in a-Si:H-based MIS photosensors

Large-area amorphous silicon (a-Si:H) sensor arrays are widely used for medical x-ray imaging, nondestructive testing and security screening. Most of the commercially available detectors are of the indirect conversion type, in which an x-ray phosphor screen is optically coupled to an array of a-Si:H sensors. The a-Si:H PIN photodiode and the MIS photoelectric converter are two alternative sensing elements used in these detectors. The major advantage of the MIS structure over PIN is fact that this device has the same layer sequence as the a Si:H TFT switch and therefore, they can be fabricated simultaneously resulting in an effective reduction in the lithography mask count. The main disadvantage of the MIS structure is the higher noise level due to transient dark current. The transient dark current originates from traps at the semiconductor-insulator interface and i-layer bulk defects. In this work we analyze the transient current transport in segmented-gate/SiN/a Si:H/n+/ITO structures under different biasing conditions and temperatures. Using a home-made setup the dark current decay was measured within an interval of 1 second in the temperature range from 294 to 353K. It is found that the dark current component associated with charge trapping at the insulator-semiconductor interface can be largely eliminated by adjusting the bias voltage during the refresh period. Under optimized biasing conditions and elevated temperatures the bulk current component becomes dominant.

High Performance Hydrogenated Amorphous Silicon n-i-p Photo-diodes on Glass and Plastic Substrates by Low-temperature Fabrication Process

We report on the fabrication and characterization of hydrogenated amorphous silicon (a-Si:H) films and n-i-p photodiodes on glass and PEN plastic substrates using low-temperature (150°C) plasma-enhanced chemical vapor deposition. Process conditions were optimized for the i-a-Si:H material which had a band gap of ~1.73 eV and low density of states (of the order 10 15 cm -3 ). Diodes with 0.5 μm i-layer demonstrate quantum efficiency ~70%. The reverse dark current of the diodes on glass and PEN plastic substrate is ~10-11 and below 10 -10 A/cm 2 , respectively. We discuss the difference in electrical characteristics of n-i-p diodes on glass and PEN in terms of bulk- and interface-state generation currents.

Segmented Amorphous Silicon n-i-p Photodiodes on Stainless-Steel Foils for Flexible Imaging Arrays

This paper reports the first successful attempt to fabricate amorphous silicon (a-Si:H) n-i-p photodiodes on a thin stainless-steel foil substrate for medical X-ray imaging applications. Two architectures of the n-i-p-photosensor, where the top electrode is based on amorphous or polycrystalline ITO, have been developed and characterized. The impact of critical fabrication steps including the deposition of semiconductor layers, dry etch of the NIP stack, diode passivation and encapsulation, as well as a contact formation on the device performance is presented and discussed. The test structures comprising segmented photodiodes with an active area ranged from 0.126 × 0.126 to 1 × 1 mm 2 have been fabricated on stainless-steel foils and on glass substrates for the purposes of process characterization. The fabricated samples are evaluated in terms of current-voltage, capacitance-voltage, and spectral response characteristics.