Abstract: Flexible organic light emitting diode (OLED) will be the ultimate display technology to customers and industries in the near future but the challenges are still being unveiled one by one. Thin-film encapsulation (TFE) technology is the most demanding requirement to prevent water and oxygen permeation into flexible OLED devices. As a polymer substrate does not offer the same barrier performance as glass, the TFE should be developed on both the bottom and top side of the device layers for sufficient lifetimes. This work provides a review of promising thin-film barrier technologies as well as the basic gas diffusion background. Topics include the significance of the device structure, permeation rate measurement, proposed permeation mechanism, and thin-film deposition technologies (Vitex system and atomic layer deposition (ALD)/molecular layer deposition (MLD)) for effective barrier films.

Abstract: The purpose of this paper is to give an overview of the state-of-the-art of metal oxide thin-film transistors (TFTs). First, the question of how to achieve high-performance oxide TFTs is addressed, including the exploration of new channel materials, the realization of low-resistance ohmic contacts and the implementation of high-k dielectric materials as the gate insulator. The electrical instability of the oxide TFTs is also discussed, which is critical for their application in flexible backplane electronics: special attention is given to the understanding of the degradation mechanism of oxide TFTs against bias thermal stress (BTS) and light illuminated BTS. Finally, the recent application of oxide TFTs in active matrix displays, such as electronic paper, liquid crystal displays and organic light-emitting diodes, is addressed.

Abstract: In this paper, we review the key issues related to the fabrication of large-area organic light-emitting devices (OLEDs) for lighting applications. We discuss the origin of a short-circuit problem, luminance non-uniformity, hot spot, efficiency reduction (power loss), and heat generation and present the way of suppressing them. We also introduce three different application areas of large-area OLED lighting panels. They can be integrated with a solar cell for power recycling or inorganic LEDs for emotional lightings. The feasibility of using OLEDs for the application of visible-light communications is also reviewed.

Abstract: This work presents a physically based analytical model for the threshold voltage in junctionless nanowire transistors (JNTs). The model is based on the solution of the two-dimensional Poisson equation and includes the dependence on JNT width, height and doping concentration. The quantum confinement has also been taken into consideration in the model formulation. The model is validated using experimental results of nMOS and pMOS JNTs, and three-dimensional TCAD simulations where the nanowire width and height, doping concentration, gate oxide thickness and temperature have been varied. The gate oxide capacitance is also addressed aiming to adequately calculate the capacitance in non-planar devices. The temperature influence on the threshold voltage of JNTs is also analyzed. The presented model shows excellent agreement with both experimental and simulated data, adequately describing the JNT threshold voltage.

Abstract: Ta-based ohmic contacts to gallium nitride high electron mobility transistor (GaN HEMT) epitaxial structures were investigated. Two metallization schemes were considered: Ta/Al/Ni(Ta)/Au and Ta/Al/Ta. The latter was superior in terms of lower contact resistance (R-c) and wider process window. The metallizations were applied to two different heterostructures (GaN/Al0.14Ga0.86N/GaN and Al0.25Ga0.75N/GaN). The lowest measured R-c was 0.06 and 0.28 Omega mm, respectively. The main advantage of the Ta-based ohmic contacts over conventional Ti-based contacts was the low anneal temperature. The optimum temperature of annealing was found to be 550-575 degrees C. From optical and scanning electron microscopy, it was clear that excellent surface morphology and edge acuity were obtained at these low temperatures. This facilitates lateral scaling of the GaN HEMT. TEM images were taken of the contact cross sections onto which EDX measurements were performed. The aim was to investigate the microstructure and the contact mechanism. Storage tests at 300 degrees C for more than 400 h in air ambient showed no deterioration of R-c.

Abstract: Discovery of self-organized epitaxial quantum dots (QDs) resulted in multiple breakthroughs in the field of physics of zero-dimensional heterostructures and allowed the advancement of optoelectronic devices, most remarkably, lasers. The most advanced and well-understood results are obtained for lasers based on Stranski–Krastanow InGaAs–GaAs three-dimensional QDs; even significant progress in the understanding of basic lasing properties is also achieved for QDs made of II–VI materials and 'native' QDs formed by nanoscale alloy phase separation in the InGaN–AlGaN material system.

Abstract: Metal oxide (MO) semiconductors have great potential for photovoltaic (PV) application owing to some optimal bandgaps and a variety of possible combinations of the materials. The progress is limited due to lack of high-quality materials, reliable process and theoretical study and models which can guide the development. This paper reports on the numerical modelling of MO semiconductor PV cells. The effects of the bandgap structure, material, doping concentration and layer thickness on the proposed oxide solar cells have been investigated. It was found that, in an ideal case of no defects and no interface states, wide-gap MO, CuO and Cu2O can form a heterostructure n+/p/p+ cell with efficiency up to 28.6%, demonstrating great potential for development.

Abstract: Photosensitive heterojunctions n-TiO2/p-CdTe were fabricated by dc reactive magnetron deposition of TiO2 thin films with n-type conductivity onto freshly cleaved p-CdTe single-crystal substrates (1 1 0). Their electrical properties were investigated and the dominating current mechanisms were analyzed at forward and reverse biases in the scope of the tunnel-recombination and emission-recombination models. The obtained surface-barrier structures n-TiO2/p-CdTe possessed the following photoelectrical parameters under 100 mW cm?2 illumination: the open-circuit voltage Voc = 0.69 V, the short-circuit current Isc = 6 mA cm?2 and the fill factor FF = 0.42.

Abstract: We operate an on-demand source of single electrons in high perpendicular magnetic fields up to 30 T, corresponding to a filling factor ν below 1/3. The device extracts and emits single charges at a tunable energy from and to a two-dimensional electron gas, brought into well defined integer and fractional quantum Hall (QH) states. We discuss ways to tune the emission energy as well as the sharpness of its distribution. We conclude that it can be used for sensitive electrical transport studies, e.g.of excitations and relaxation processes in QH edge states.

Abstract: In this study, we systematically investigated the bending effect on the electrical properties of flexible organic thin-film transistors (OTFTs) fabricated on stainless steel substrates. We found that the compressive strain resulted in an increased mobility, while the tensile strain degraded the electrical performance. We further used a transfer line method to extract the channel and parasitic resistances under either compressive or tensile strain. The results indicated that the parasitic resistance increased apparently under the tensile bending condition, which probably could be attributed to the damage of the source/drain contacts. Additionally, we deduced that mechanical strains influence the energy barrier height between the grains of pentacene thin films, thereby resulting in the variation of channel resistances. Overall, the flexible OTFTs fabricated on the metal foils exhibited high mechanical flexibility and stability.

Abstract: We report the fabrication and characteristics of a ZnO thin-film transistor (TFT) using a 50 nm thick ZnO film as an active layer on an Al2O3 gate dielectric film deposited by atomic layer deposition. Lowering the deposition temperature allowed the control of the carrier concentration of the active channel layer (ZnO film) in the TFT device. The ZnO TFT fabricated at 110 °C exhibited high-performance TFT characteristics including a saturation field-effect mobility of 11.86 cm2 V−1 s−1, an on-to-off current ratio of 3.09 × 107 and a sub-threshold gate-voltage swing of 0.72 V decade−1.

Abstract: Currently, wafer-fusion technology represents an effective technique for enhancing the performance of long-wavelength vertical cavity surface-emitting lasers (VCSELs) based on classical double heterostructures with multi-quantum well active regions. Using the example of 1310 nm wavelength VCSELs, we demonstrate the status of this technology for wafer-level scale, wavelength-controlled devices with high performance, capable of operation in a wide temperature range up to 90 °C with single-mode output power levels in excess of 1 mW and a side mode suppression ratio (SMSR) in excess of 40 dB. No degradation was observed in a qualification lot that operated at 10 mA and 90 °C for 2000 h.

Abstract: In this paper, for the first time, we propose a partial silicon-on-insulator (P-SOI) lateral double-diffused metal-oxide-semiconductor-field-effect-transistor (LDMOSFET) with a modified buried layer in order to improve breakdown voltage (BV) and self-heating effects (SHEs). The main idea of this work is to control the electric field by shaping the buried layer. With two steps introduced in the buried layer, the electric field distribution is modified. Also a P-type window introduced makes the substrate share the vertical voltage drop, leading to a high vertical BV. Moreover, four interface electric field peaks are introduced by the buried P-layer, the Si window and two steps, which modulate the electric field in the SOI layer and the substrate. Hence, a more uniform electric field is obtained; consequently, a high BV is achieved. Furthermore, the Si window creates a conduction path between the active layer and substrate and alleviates the SHE. Two-dimensional simulations show that the BV of double step partial silicon on insulator is nearly 69% higher and alleviates SHEs 17% in comparison with its single step partial SOI counterpart and nearly 265% higher and alleviate SHEs 18% in comparison with its conventional SOI counterpart.

Abstract: We have investigated the jitter, repetition frequency and spectra of a quantum-dot mode-locked laser operating at 40 GHz under continuous wave external light injection using an external cavity laser or under optical feedback. For the case of the continuous wave light injection we realized a shift of the optical emission of up to 15 nm. With optical feedback we have found a ninefold jitter reduction, down to a record low value of 1 ps over an integration range from 10 kHz to 1 GHz. Optical feedback fixes the repetition frequency while changing current and voltage applied to the diode. This is due to the fact that stable mode locking occurs at frequencies which are integers of the feedback loop frequency.

Abstract: Undoped silicon nanowire (Si NW) field-effect transistors (FETs) with a back-gate configuration have been fabricated and characterized. A thick (200 nm) Si3N4 layer was used as a gate insulator and a p++ silicon substrate as a back gate. Si NWs have been grown by the chemical vapour deposition method using the vapour–liquid–solid mechanism and gold as a catalyst. Metallic contacts have been deposited using Ni/Al (80 nm/120 nm) and characterized before and after an optimized annealing step at 400 °C, which resulted in a great decrease in the contact resistance due to the newly formed nickel silicide/Si interface at source and drain. These optimized devices show a good hole mobility of around 200 cm2 V−1 s−1, in the same range as the bulk material, with a good ON current density of about 28 kA cm−2. Finally, hysteretic behaviour of NW channel conductance is discussed to explain the importance of NW surface passivation.

Abstract: The SrTiO3 (STO) thin fi lms on aP t/Ti/SiO2/Si substrate were synthesized using a sol–gel method to form a metal–insulator–metal structure. This device shows the bipolar resistance switching (BRS) behavior for a compliance current Icc of less than 0.1 mA but exhibits soft breakdown at a higher level of compliance current. A transition from the BRS behavior to the stable unipolar resistive switching behavior (URS) was also observed. We found that the BRS behavior may be controlled by the structure interface while the URS behavior is likely bulk controlled. Our study indicates that the external compliance current is a key factor in resistance switching phenomenon of STO thin films. (Some figures in this article are in colour only in the electronic version)

Abstract: A low-cost Al-doped ZnO (AZO) thin film was deposited by radio-frequency magnetron sputtering with different Ar/O2 flow ratios. The optical and electrical properties of an AZO film were investigated. A highly conductive AZO film was inserted between the amorphous InGaZnO (a-IGZO) channel and the metal Al electrode to form a heterojunction source/drain contact, and bottom-gate amorphous a-IGZO thin-film transistors (TFTs) with a high κ HfON gate dielectric were fabricated. The AZO film reduced the source/drain contact resistivity down to 79 Ω cm. Enhanced device performance of a-IGZO TFT with Al/AZO bi-layer S/D electrodes (W/L = 500/40 µm) was achieved with a saturation mobility of 13.7 cm2 V−1 s−1, a threshold voltage of 0.6 V, an on-off current ratio of 4.7 × 106, and a subthreshold gate voltage swing of 0.25 V dec−1. It demonstrated the potential application of the AZO film as a promising S/D contact material for the fabrication of the high performance TFTs.

Abstract: We present an analysis of the catastrophic optical damage effect that is artificially provoked in 808 nm emitting broad area diode lasers by single current pulses. The kinetics of the sudden degradation process, monitored with a nanosecond temporal resolution, is linked to the damage pattern observed. This involves in situ tracing of emission power and hot-spot motion within the cavity as well as the verification of the resulting defects by defect spectroscopy and cathodoluminescence mapping. A complementary model is presented which explains the shape of the observed defect pattern. The combination of unidirectional energy transfer to defects by laser light within the laser cavity, spatially isotropic defect growth, and the presence of shadowing effects explain the complex damage pattern observed in the gain material, including effects of defect branching. The study is made with standard industrial devices making the findings directly applicable for device testing and performance improvements.

Abstract: Vertical cavity surface emitting lasers (VCSELs) emitting at 850 nm have experienced explosive growth in the past decade because of their many attractive optical features and incredibly low-cost manufacturability. This review reviews the foundations for GaAs-based VCSEL technology as well as the materials and device challenges to extend the operating wavelength to both shorter and longer wavelengths. We discuss some of the applications that are enabled by the integration of VCSELs with both active and passive semiconductor elements for telecommunications, both in vivo and in vitro biosensing, high-density optical storage and imaging at wavelengths much less than the diffraction limit of light.

Abstract: Organic–inorganic hybrid-type nonvolatile memory thin-film transistors using an organic ferroelectric gate insulator and an oxide semiconducting active channel are a very promising solution to the memory devices having both features of low-cost and high-performance, which are embeddable into the next-generation flexible and transparent electronics. In this paper, we discuss some important issues for this proposed device, such as device structure design, process optimization and memory array integration. Promising feasible applications and remaining technology issues to solve were also discussed.

Abstract: We review various GaAs-based axial and radial nanowire heterostructures grown on (1 1 1)B GaAs substrates by metal organic chemical vapor deposition via vapor–liquid–solid mechanism. Transmission electron microscopy, time-resolved photoluminescence and micro-Raman spectroscopy have been used to understand the crystal structure, non-radiative surface and bulk defects, carrier lifetime and strain effects on the bandgap energy.

Abstract: Zinc oxide thin films were deposited by atomic layer deposition (ALD). The structural and mechanical properties of the thin films were investigated by x-ray diffraction, transmission electron microscopy, atomic force microscopy, and nanoindentation. Diethyl zinc was used as the chemical precursor for zinc and water vapor was used as the oxidation agent. The samples were deposited at 150 °C and at a pressure of 2.1 × 10−1 Torr in the ALD reactor. A growth rate of 2 A per cycle was calculated in the ALD process window. The Nano Indenter XP was used in conjunction with the continuous stiffness method in depth control mode in order to measure and to analyze the mechanical properties of hardness and modulus of ALD ZnO thin film samples. For comparison, we benchmarked the mechanical properties of single crystal bulk ZnO samples against those of our ALD ZnO thin films.

Abstract: This study examined the effects of a cell area on the cell performances in ITO-free organic solar cells (OSCs) based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM). Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films with two different sheet resistances (Rsh) were used as polymeric transparent anodes for cost-effective ITO-free OSCs. Changes in the power conversion efficiency (PCE), the fill factor (FF), the short-circuit current (Jsc), and the open-circuit voltage (Voc) that resulted from changing the cell area or sheet resistance of transparent electrodes were systematically investigated. With increasing cell area from 4.5 to 49.5 mm2, the device performance of ITO-free OSCs was continuously decreased mainly due to the decrease in the FF and the series resistance (Rs). In addition, the performance of OSCs was critically dependent on Rsh of the PEDOT:PSS electrode. Upon reducing Rsh of the polymer anode from ~200 to ~90 Ω/, the FF and PCE showed better values at an identical large cell area and exhibited a relieved cell performance degradation with increasing cell area, suggesting that the sheet resistance of transparent electrodes is a dominant factor to limit cell efficiencies in practical large-area solar cells.

Abstract: The structural, electronic and optical properties of GaAs1-xBix alloy are investigated using the density functional theory based on the full potential linearized augmented plane wave method as implemented in the Wien2k package. The exchange correlation potential is treated by generalized gradient approximation. We have studied the effect of Bi composition on equilibrium volume, energy band gap, electron and hole effective masses and dielectric function. For the electron and hole effective masses and the dielectric function variations versus bismuth composition, our results represent an attempt to investigate their qualitative trends.

Abstract: The effect of structure parameters on the electroluminescence and photoconductivity of multilayer structures with self-assembled Ge(Si)/Si(0 0 1) islands has been studied. The highest intensity of the room-temperature electroluminescence in the wavelength range of 1.3–1.55 µm has been observed for the islands grown at 600 °C. The same diode structures with Ge(Si)/Si(0 0 1) islands have demonstrated room-temperature photoconductivity signals in the wavelength range of 1.3–1.55 µm. The observed overlap of the electroluminescence and photoconductivity spectra obtained for the same structures with Ge(Si) islands makes these structures a promising material for the fabrication of a Si-based optocoupler. Less degradation after neutron irradiation has been observed for the electroluminescence and photoconductivity signals from multilayer structures with Ge(Si) self-assembled islands in comparison with bulk silicon structures. This result is associated with more effective confinement of charge carriers in the multilayer structures with Ge(Si) islands.

Abstract: The diode ideality factor has been subjected to theoretical discussions from the viewpoint of light-emitting diodes (LEDs). As radiative recombination in LEDs is incongruent with the Sah–Noyce–Shockley (SNS) analysis, the SNS recombination plane was replaced with carrier confinement structures of modern LEDs, e.g., quantum wells (QWs). We calculated the voltage partitioning factors to indicate how carrier concentration changes as a function of terminal voltage. As local minority carriers determine the net recombination rate at QWs, we have deduced that the partitioning factor of the local minority carriers becomes the ideality factor of the LED. By applying incomplete ionization of acceptors in InGaN/GaN LEDs, a conclusion has been reached such that the partitioning factor of local minority carriers as well as the diode ideality factor can be nearly as large as 3.

Abstract: Electroluminescence characteristics of an n-ZnO nanotubes/p-GaN heterostructure light-emitting diode (LED) have been investigated at forward and reverse bias. Distinctly different emission spectra have been observed and the location of the recombination of electron–hole is analyzed under both configurations. The forward-biased emission spectrum shows two peaks centered at around 450 and 560 nm, while the reverse-biased spectrum exhibits a single emission peak at 650 nm. By comparing the current transport mechanisms, it is suggested that the violet-blue emission peak (450 nm) observed only under forward bias is originating from the heterojunction of the ZnO nanotubes/p-GaN LED. The influence on the emission intensity of the device with the increase in temperature at constant current is studied in the range from 25 to 65 °C, to check its compatibility for practical applications and under harsh conditions.

Abstract: Following the great discovery of the electrically conducting polymer, doped polyacetylene, which was honorably recognized in 2000 with the Nobel Prize in chemistry, conjugated molecules, i.e. organic semiconductors, have become an attractive class of active elements for various electronic or opto-electronic applications. Significant effort has been made in both academia and industry to investigate π-conjugated molecules for their unique electrical or opto-electrical properties over the last three decades. The discovery of electroluminescence in conjugated small molecules in 1982 and in polymers in 1989 was a major breakthrough, bringing those molecules to commercial applications within reach for the first time in (opto-)electronic devices, such as organic light-emitting diodes (OLEDs), photovoltaic cells (OPVs), and field-effect transistors (OFETs). Nowadays, we use OLED displays in everyday life in mobile devices. The potential of these devices, which have been fabricated with conjugated molecules, lies in the possibility to combine the advantages of solution processability, chemical tunability and material strength of polymers with the typical properties of plastics, to realize low-cost, large-area electronic devices on flexible substrates by solution deposition and direct-write graphic art printing techniques. The articles in the flexible OLEDs and organic electronics special issue in Semiconductor Science and Technology deal with a diversity of topics and effectively reflect the current status of research from all over the world on various organic electronic devices, including OLEDs, OPVs, and OFETs. Firstly, S Park et al describe the recent progress in thin-film encapsulation techniques for flexible AM-OLED and large-area OLED lightings, and their applications are discussed by J-W Park et al. Flexible active-matrix OLEDs on plastics require stable and flexible thin-film transistors processed at low temperature. Metal oxide thin-film transistors are proposed as one of the best candidates for the purpose, and J K Jeong discusses their status and perspectives. Next, several excellent research articles on OFETs follow. In particular, Y-Y Noh et al introduce an interesting method to control charge injection in top-gated OFETs by insertion of various self-assembled monolayers in their paper entitled 'Controlling contact resistance in top-gate polythiophene-based field-effect transistors by molecular engineering'. We would like to thank all the authors for their contributions, which combine new results and profound overviews of the state of the art in flexible OLEDs and organic electronics areas; it is this combination that most often adds to the value of topical issues. Special thanks also go to the staff of IOP Publishing, particularly Ms Alice Malhador, for contributing to the success of this effort. In this special issue, many wonderful reviews and research articles provide a detailed overview of recent progress in OLEDs, OPVs and OFETs as well as a scientific understanding of the device physics with these materials. We sincerely believe this special issue is a timely publication and will give productive information to a broad range of readers. Flexible OLEDs and organic electronics Contents Thin film encapsulation for flexible AM-OLED: a review Jin-Seong Park, Heeyeop Chae, Ho Kyoon Chung and Sang In Lee Large-area OLED lightings and their applications J W Park, D C Shin and S H Park Controlling contact resistance in top-gate polythiophene-based field-effect transistors by molecular engineering Yong-Young Noh, Xiaoyang Cheng, Marta Tello, Mi-Jung Lee and Henning Sirringhaus Branched polythiophene as a new amorphous semiconducting polymer for an organic field-effect transistor Makoto Karakawa, Yutaka Ie and Yoshio Aso Influence of mechanical strain on the electrical properties of flexible organic thin-film transistors Fang-Chung Chen, Tzung-Da Chen, Bing-Ruei Zeng and Ya-Wei Chung Frequency operation of low-voltage, solution-processed organic field-effect transistors M Caironi, Y-Y Noh and H Sirringhaus Nonvolatile memory thin-film transistors using an organic ferroelectric gate insulator and an oxide semiconducting channel Sung-Min Yoon, Shinhyuk Yang, Chun-Won Byun, Soon-Won Jung, Min-Ki Ryu, Sang-Hee Ko Park, ByeongHoon Kim, Himchan Oh, Chi-Sun Hwang and Byoung-Gon Yu The status and perspectives of metal oxide thin-film transistors for active matrix flexible displays Jae Kyeong Jeong Vertical phase segregation of hybrid poly(3-hexylthiophene) and fullerene derivative composites controlled via velocity of solvent drying Tao Song, Zhongwei Wu, Yingfen Tu, Yizheng Jin and Baoquan Sun Variations of cell performance in ITO-free organic solar cells with increasing cell areas Jun-Seok Yeo, Jin-Mun Yun, Seok-Soon Kim, Dong-Yu Kim, Junkyung Kim and Seok-In Na