Current–voltage characteristics of AlCdO Schottky contact on the polished and unpolished p-type Si surfaces with and without light illumination

In this study, the current-voltage characteristics of the AlCdO/unpolished p-type Si and AlCdO/polished p-type Si Schottky diodes with and without light illumination were examined. It is found that the Schottky barrier height (the series resistance) of the AlCdO/unpolished p-type Si Schottky diode is higher (lower) than that of the AlCdO/polished p-type Si Schottky diode. The power conversion efficiency of the AlCdO/p-type Si devices in the light (AM 1.5 G, 100 mW/cm²) was improved by increasing built-in potential at the AlCdO/p-type Si interfaces and reducing the device series resistance and surface reflectivity. It is shown that the device surface roughness plays an essential role in improving the device performance.     

Improved performance of mixed single layer top-emission organic light emitting devices using capping layer

The performance of top-emission organic light emitting devices (TEOLEDs) can be improved by using a thin capping layer on top of the semitransparent metal electrode. We investigated the emission properties of inverted mixed single layer TEOLEDs with the same device structure but different capping materials. The thickness of capping layer was optimized by calculation. The power efficiency of device was 2.5 times enhanced when 45 nm TPD capping layer was added. The enhancement is not simply dependent on the transmittance and reflectance of the top contact, but also on other complex phenomena such as the interference effects in the device. The results of properties and dependence of EL spectra on viewing angle for all devices indicated that the large enhancement factor may be related to the complex interference phenomenon in our mixed single layer devices due to the emitter center and recombination region is different from conventional heterojunction devices.     

The effects of active layer thickness on Programmable Metallization Cell based on Ag–Ge–S

Programmable Metallization Cell (PMC) is a newly developed non-volatile memory device based on chalcogenide solid electrolytes. In this paper, we studied the PMC’s electrical properties as a function of its active layer thickness. The PMC devices are fabricated based on Ag-Ge-S materials using thermal evaporation method. The fabricated devices have their active layer thickness ranges from 8 nm to 30 nm. The I-V characteristics of the fabricated devices are studied as a function of their active layer thickness. It is observed that the ‘ON’ resistance of PMC displays a decreasing trend as we increase the active layer thickness. This is because less deposition nuclei can be formed on the cathode/electrolyte interface in thinner devices, which limits the number of conduction links that can be formed. The SET voltage also decreases slightly as increase of the active layer thickness. This is probably because thinner Ag-Ge-S layer localizes Ag ions’ movements due to the large size of Ag+ ion. On the other hand, the RESET voltage increases as we increase the active layer thickness, which is because there are more conduction links to be ionized in thicker devices.     

Effect of rapid thermal annealing on pentacene-based thin-film transistors

The bottom contact pentacene-based thin-film transistor is fabricated, and it is treated by rapid thermal annealing (RTA) with the annealed temperature up to 240 °C for 2 min in the vacuum of 1.3 × 10⁻² torr. The morphology and structure for the pentacene films of OTFTs were examined by scanning electron microscopy and X-ray diffraction technique. The thin-film phase and a very small fraction of single-crystal phase were found in the as-deposited pentacene films. While the annealing temperature increases to 60 °C, the pentacene molecular ordering was significantly improved though the grain size only slightly increased. The device annealed at temperature of 120 °C has optimal electrical properties, being consistent with the experimental results of XRD. The post-annealing treatment results in the enhancement of field-effect mobility in pentacene-based thin-film transistors. The field-effect mobility increases from 0.243 cm²/V s to 0.62 cm²/V s. Besides, the threshold voltage of device shifts from −7 V to −3.88 V and the on/off current ratio increases from 4.0 × 10³ to 8.7 × 10³.     

一种微腔有机发光二极管的设计与性能研究

设计了PPV为发光层、Ag和DBR为上下反射镜、结构为Glass/DBR/ITO/PPV/Ag的微腔有机发光二极管。应用特征矩阵法,系统地研究了DBR和银镜的性质对器件性能的影响。结果表明:1)当金属厚度较小时,随着厚度的增加,器件的反射峰值不断增加并且蓝移,但是当厚度达到100 nm后,再增加厚度反射谱基本上没变化;2)随着DBR周期数的增加,器件的反射谱峰值不断增大,峰值半宽度不断变窄。3)器件的EL谱的峰值随着金属反射率的增加不断增大,而随着DBR反射率的增加是先增加后减小的。并给出了不同条件下DBR的最优化选择依据。     

Novel cationic water-soluble polyfluorene derivatives with ion-transporting side groups for efficient electron injection in PLEDs

Synthesis of cationic water-soluble polyfluorene derivatives with various side groups, which are used as electron injecting layers in polymer light emitting diodes, is described. Neutral polyfluorene derivatives containing bromo-alkyl terminal groups were synthesized by a palladium catalyzed Suzuki coupling reaction. The bromo-alkyl terminal groups in the neutral polyfluorenes were quaternized by treatment with a trimethyl amine solution. When a high work-function metal such as Ag is used as a cathode in a light emitting diode with an ITO/PEDOT:PSS/MEH-PPV/water-soluble polyfluorene/Ag configuration, effects of these water-soluble polyfluorenes on the device performance were investigated. In the case of poly[(9,9-bis((6′-(N,N,N-trimethylammonium) hexyl)-2,7-fluorene))-alt-(9,9-bis(2-(2-methoxyethoxy)ethyl)-fluorene)] Dibromide (WPF-oxy-F) containing ethylene oxide groups as the electron injecting layer, the electroluminescence efficiency of light emitting devices was significantly enhanced by about two orders of magnitude compared to that of a device without an electron injecting layer because migration of bromide ions via the ethylene oxide side groups led to large space charge. As a result, the injection barrier could be reduced between the emitting layer and Ag cathode resulting high electroluminescence efficiency.     

Ultra-thin and graded sliver electrodes for use in transparent pentacene field-effect transistors

The authors report the fabrication of efficient and transparent pentacene field-effect transistors (FETs) using a graded structure of ultra-thin silver (Ag) source and drain (S–D) electrodes. The S–D electrodes were prepared by thermal evaporation with a controlled deposition rate to form Ag layer with a graded structure, leading to a reduced injection barrier and smoothing the contact surface between the electrode and the pentacene channel. The sheet resistance of such Ag electrode was found to be as low as 9 Ω/sq. In addition, a hole-only behavior of device with Ag electrode characterized by current–voltage measurement and conductive atomic-force microscopy shows the injection property of high current flowing as compared with device using Au electrode, resulting in an efficient injection condition existing at the interface of the graded Ag/pentacene. Device characterization indicates the transparent pentacene FET with a graded ultra-thin Ag electrode and organic capping layer of N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine exhibits a high transmission rate of ∼75% in the range of visible light from 400 to 550 nm, a threshold voltage of −6.0 V, an on–off drain current ratio of 8.4 × 10⁵, and a field-effect mobility of 1.71 cm²/V s, thus significantly outperforming pentacene FETs with multilayer oxide electrodes or other transparent thin metal layers.     

Fabrication of Ge2Sb2Te5 based PRAM device at 60 nm scale by using UV nanoimprint lithography

Phase change memory is one of the most promising non-volatile memory for the next generation memory media due to its simplicity, wide dynamic range, fast switching speed and possibly low power consumption. Low power consuming operation of phase change random access memory (PRAM) can be achieved by confining the switching volume of phase change media into nanometer scale. Nanoimprint lithography is an emerging lithographic technique in which surface protrusions of a mold such as sub-100 nm patterns are transferred into a resin layer easily. In this study, crossbar structures of phase change device array based on Ge₂Sb₂Te₅ were successfully fabricated at 60 nm scale by two consecutive UV nanoimprint lithography and metal lift-off process, which showed on/off resistance ratio up to 3000.     

Thermal stability of SiC Schottky diode anode and cathode metalisations after 1000 h at 350 °C

The thermal stability of two commercially available silicon carbide Schottky diode types has been evaluated following a 1000 h non-biased storage test under vacuum at 350 °C. The Ti-based Schottky (Anode) contact shows excellent stability over the duration of the test with less than 5% change in either extracted Schottky barrier height or ideality values. The Al die attach metalisation on the anode also shows no evidence of degradation after the test. However, a considerable change in series resistance was observed for both diode types, with up to a factor of 100 measured for one of the diodes. The primary early failure mode is related to degradation of the NiAg Ohmic (cathode) die attach metalisation. Demixing of the NiAg alloy, leading to Ag agglomeration is proposed to be the underlying degradation mechanism involved resulting in delamination of the die attach metalisation and the corresponding series resistance increase.     

Au and non-Au based rare earth metal-silicide ohmic contacts to p-InGaAs

The aim of this study is to improve the electrical properties of ohmic contacts that plays crucial role on the performance of optoelectronic devices such as laser diodes (LDs), light emitting diodes (LEDs) and photodetectors (PDs). The conventional (Pd/Ir/Au, Ti/Pt/Au and Pt/Ti/Pt/Au), Au and non-Au based rare earth metal-silicide ohmic contacts (Gd/Si/Ti/Au, Gd/Si/Pt/Au and Gd/Si/Pt) to p-InGaAs were investigated and compared each other. To calculate the specific contact resistivities the Transmission Line Model (TLM) was used. Minimum specific contact resistivity of the conventional contacts was found as 0.111 × 10⁻⁶ Ω cm² for Pt/Ti/Pt/Au contact at 400 °C annealing temperature. For the rare earth metal-silicide ohmic contacts, the non-Au based Gd/Si/Pt has the minimum value of 4.410 × 10⁻⁶ Ω cm² at 300 °C annealing temperature. As a result, non-Au based Gd/Si/Pt contact shows the best ohmic contact behavior at a relatively low annealing temperature among the rare earth metal-silicide ohmic contacts. Although the Au based conventional ohmic contacts are thermally stable and have lower noise in electronic circuits, by using the non-Au based rare earth metal-silicide ohmic contacts may overcome the problems of Au-based ohmic contacts such as higher cost, poorer reliability, weaker thermal stability, and the device degradation due to relatively higher alloying temperatures. To the best of our knowledge, the Au and non-Au based rare earth metal-silicide (GdSi_x) ohmic contacts to p-InGaAs have been proposed for the first time.     

High field-effect mobility normally-off AlGaN/GaN hybrid MOS-HFET on Si substrate by selective area growth technique

The paper reports a normally-off n-channel AlGaN/GaN hybrid metal-oxide-semiconductor heterojunction field-effect transistor (MOS-HFET) on Si substrate with high field-effect mobility. To decrease the channel resistance and to improve the field-effect mobility, of the MOS-HFET, a selective area growth (SAG) technique is applied at the channel region. The fabricated MOS-HFET exhibits a good normally-off operation with the threshold voltage of 3.7 V, the specific ON-state resistance of 7.6 mΩ cm², and the breakdown voltage of over 800 V.     

基于Alq3光耦合层的新型顶发射蓝光有机电致发光器件

研制了一种结构为Ag/Glass/ITO/TAPC/mCP/mCP∶Firpic/TPBi/LiF/Al/Ag/Alq₃的顶发射有机电致发光器件,通过在ITO玻璃衬底背面生长一层Ag反射膜,使器件发出的蓝光被反射膜反射到顶电极出射。利用顶电极表面的Alq₃光耦合层有效地提升了金属复合阴极的透射率,降低了器件的微腔效应。实验结果表明,当光耦合层厚度为30nm时,获得了最大电流效率和最大亮度分别为8.91cd/A和5758cd/m²的蓝光顶发射有机电致发光器件(TEOLED);同时,在10V电压下,其色坐标为(0.157,0.320),当亮度从1cd/m²变化到5000cd/m²时,其色坐标仅漂移(0.002,0.010),表现出良好的色稳定性。     

Flexible organic light-emitting diodes with ZnS/Ag/ZnO/Ag/WO3 multilayer electrode as a transparent anode

We investigated the highly flexible, transparent and very low resistance ZnS/1st Ag/ZnO/2nd Ag/WO₃ (ZAZAW) multilayer electrodes on PET substrate as an anode in flexible organic light-emitting diodes (OLEDs). A theoretical calculation was first conducted to obtain the optimal thickness of the ZAZAW multilayer for high transparency. Its measured luminous transmittance was over 80% in the visible range with a very low sheet resistance of 2.17 Ω/sq., and it had good mechanical flexibility due to the ductility of Ag. Ag’s effect on optical and electrical properties was also studied. Flexible OLEDs devices that were fabricated on ZAZAW multilayer anode showed good hole injection properties comparable to those of ITO-based OLEDs due to the use of WO₃ as a hole injection layer. However, the electroluminescent properties of the ZAZAW-based OLEDs varied depending on WO₃ thickness. Although the transmittance of the ZAZAW electrode was reduced by tuning the WO₃ thickness to adjust the microcavity effect, the device efficiency could be enhanced above that of ITO-based OLEDs.     

Spectral response of blue-sensitive Si photodetectors in SOI

In this paper, we present a novel blue-sensitive Si photodetector. The detector is realized as a Si diode with a vertical PN junction in the silicon-on-insulator (SOI) thin film for normal incident light. Due to the thin SOI device layer, the photodetector shows a blue-shift spectral response with the peak external quantum efficiency (QE) of 69.6% at wavelength of 480 nm. The photodetector adopts a thin layer of SiO₂ as an antireflection coating and as a blocking layer for shallow ion implantation doping. The isolation trench etched through the SOI thin film to the buried oxide (BOX) provides fully electrical isolation. The device structure is simple and its performance is very high, therefore, it is in favor of monolithically integration with other micro/nanodevices.     

Fully depleted silicon on insulator MOSFETs on (1 1 0) surface for hybrid orientation technologies

Effective memory performance of the nonvolatile memory/thin film transistor (NVM/TFT) devices needs good TFT characteristics. The reduction in leakage current of the TFT devices was accomplished with the gate offset (GOF) structure. A simplified fabrication process for the GOF NVM is introduced in this study using the insulator over-etching approach. Nonvolatile memory devices on glass using SiO₂/SiO_x/SiO_xN_y stack with an offset length of 0, 0.2, 0.4, and 0.6 μm were investigated. The highly selective etching process and the short offset length help to avoid the problem of the gate aluminum collapsing on the source/drain electrodes. The TFT characteristics of the GOF structures displayed the remarkable improvement in leakage from 1.1 × 10⁻¹¹ A, for the TFT without an offset region, to the low OFF current of 1.34 × 10⁻¹² A for the device with a 0.6 μm offset length. The longer offset length gave the lowest OFF current. The degradation in transconductance and the threshold voltage was negligible with the g_m values of about 3 × 10⁻⁶ S and ΔV_th of about 0.2 V, respectively. The switching characteristics remained similar for all the devices. Additionally, the GOF structures slightly enhanced the retention characteristics. The memory window of the NVM without the offset after a retention time of 10,000 s was 58%, lower than the over 69% of the GOF devices. Therefore, the application of the GOF structure to reduce the leakage of the NVM/TFT proved to be effective.     

UV‐Curable Silver Electrode for Screen‐Printed Thermoelectric Generator

Fabricating thermoelectric generators (TEGs) using the screen‐printing process has advantages, including mass production, device scalability, and system applicability. However, the thick film formed through the process typically has low film density, and reduced performance, because of the presence of pores in the film created by the vaporization of the resin during high‐temperature annealing. During the soldering process used for thermoelectric module fabrication, the printed solder infiltrates into the screen‐printed electrodes through the micropores in the electrodes, causing cracks of the electrode film and an increase in resistivity. In this paper, an ultraviolet radiation (UV)‐curable process for screen‐printed electrodes is reported. The paste for the electrodes is synthesized by mixing Ag flakes that can be cured at low temperature with a UV resin. Scanning electron microscope images show that the UV‐curing process significantly reduces pores and thereby results in a smooth‐surfaced electrode layer. The film density after crystallization is also enhanced. TEGs composed of 72 couples with UV‐curable Ag electrodes generate a high power density of ≈6.69 mW cm^?2 at a temperature difference of 25 °C; the device resistance is ≈0.75 Ω, and the figure of merit of the device is recorded to be 0.57, which is the highest among the printed TEGs.     

Ultraviolet photodetector based on Au doped TiO2 nanowires array with low dark current

Au nanoparticles doped TiO2 nanowires (NWs) arrays with an average diameter of 100 nm were synthesized through a facile solvothermal method. Thereafter, metal/semiconductor/metal (MSM) structured detectors with Ag electrodes were fabricated on these NWs. The ultraviolet (UV) sensing characteristics of pure TiO2 and Au-doped ones (Au-TiO2) were investigated. Compared with pure TiO2, the Au-TiO2 NWs based device shows a much lower dark current of 1.5 nA at 3 V bias. The low dark current mechanism might be due to the promoted directional transmission of carriers induced by Au doping. The photoresponse is nearly one order of magnitude under 360 nm monochromatic illumination. The Au-TiO2 NWs detector with simple fabrication process, low noise and good overall performance provides a broad way in fabricating UV imaging arrays.     

Electrical properties at p-ZnSe/metal interfaces

In order to prepare low resistance ohmic contacts to p-ZnSn by the “deposition and annealing (DA)” technique which has been extensively used for GaAs and Si-based devices, formation of a heavily doped layer by the p-ZnSe/metal reaction is required. For p-ZnSe/Ni contacts, Ni and Se reacted preferentially at the ZnSe/Ni interface upon annealing at temperatures higher than 250°C. However, capacitance-voltage measurements showed that the net acceptor concentration (N_A-N_D) close to the p-ZnSe/Ni interface was reduced upon the Ni/ZnSe reaction, resulting in high contact resistance. For p-ZnSe/Au contacts, neither Au/ZnSe reaction nor reduction of the acceptor concentration were observed after annealing at temperatures lower than 300°C. This indicates that although the metal/p-ZnSe reaction is mandatory to prepare a heavily doped layer, the reaction induced an increase in the compensation donors in the p-ZnSe substrate. In order to increase the acceptor concentration in the vicinity of the p-ZnSe/metal interface through diffusion from the contact materials, Li or O which was reported to play the role of an acceptor in ZnSe was deposited with a contact metal and annealed at elevated temperatures. Ni or Ag was selected as the contact metal, because these metals were expected to enhance Li or O doping by reacting with ZnSe. However, the current density-voltage characteristics of the Li(N)/Ni and Ag(O) contacts exhibited rectifying behavior, and the contact resistances increased with increasing annealing temperature. The present results indicated that, even though the acceptor concentration in the p-ZnSe substrate increased by diffusion of the dopants from the contact elements, an increment of the compensation donors was larger than that of the acceptors. The present experiments indicated that preparation of low resistance ohmic contacts by forming a heavily doped intermediate layer between p-ZnSe and metal is extremely difficult by the DA technique.     

Over 1000 V/30 mA operation GaN-on-Si MOSFETs fabricated on Si substrates

We demonstrated the operation of GaN-on-Si metal-oxide-semiconductor field effect transistors (MOSFETs) for power electronics components. The interface states at SiO₂/GaN were successfully improved by annealing at 800 °C for 30 min in N₂ ambient. The interface state density was less than 1 × 10¹¹ cm^-2 eV⁻¹ at E_c − 0.4 eV. The n⁺ contact layers as the source and drain regions as well as the reduced surface field (RESURF) zone were formed using a Si ion implantation technique with the activation annealing at 1200 °C for 10 s in rapid thermal annealing (RTA). As a result, we achieved an over 1000 V and 30 mA operation on GaN-on-Si MOSFETs. The threshold voltage was +2.6 V. It was found that the breakdown voltage depended upon the RESURF length and nitride based epi-layer thickness. In addition, we discussed the comparison of each performance of GaN-on-Si with -sapphire devices.     

380‐nm Ultraviolet Light‐Emitting Diodes with InGaN/AlGaN MQW Structure

In this paper, we demonstrate the capabilities of 380‐nm ultraviolet (UV) light‐emitting diodes (LEDs) using metal organic chemical vapor deposition. The epi‐structure of these LEDs consists of InGaN/AlGaN multiple quantum wells on a patterned sapphire substrate, and the devices are fabricated using a conventional LED process. The LEDs are packaged with a type of surface mount device with Al‐metal. A UV LED can emit light at 383.3 nm, and its maximum output power is 118.4 mW at 350 mA.