X. Y. Sun

Microwave attenuation of multiwalled carbon nanotube-fused silica composites

Multiwalled carbon nanotubes (MWCNTs) were used to convert radome materials to microwave absorbing materials. Dense MWCNT-fused silica composites were prepared by hot-pressing technique. The composites exhibit high complex permittivities at X-band frequencies, depending on the content of MWCNTs. The value of the loss tangent increases three orders over pure fused silica only by incorporating 2.5vol% MWCNTs into the composites. The average magnitude of microwave transmission reaches −33dB at 11–12GHz in the 10vol% MWCNT-fused silica composites, which indicates the composites have excellent microwave attenuation properties. The attenuation properties mainly originate from the electric loss of MWCNTs by the motion of conducting electrons.

1.4 μm band electroluminescence from organic light-emitting diodes based on thulium complexes

Near-infrared (NIR) electroluminescence (EL) devices have been fabricated employing thulium complexes as emitting materials. The EL emissions at 1.4 and 0.8 μm were observed from the devices of tris-(dibenzoylmethanato)-mono-(bathophenanthroline or 1,10-phenonthroline) thulium [Tm(DBM)3bath or Tm(DBM)3phen] at room temperature and assigned to 3F4–3H4 and 3F4–3H6 transitions of Tm3+ ions, respectively. By comparison with the NIR emissions of four Tm complexes with different ligands, it was found that the first ligand played a more important role for the Tm3+ ion emissions rather than the second one. In order to meet the requirement of optical communication, both Tm(DBM)3bath and erbium [Er] (DBM)3bath were incorporated into EL devices so that a broadened EL emission band ranging from 1.4 to 1.6 μm was obtained, showing the potential application of Tm complexes for optical communication systems.

Observation of 1.5μm photoluminescence and electroluminescence from a holmium organic complex

Electroluminescence (EL) and photoluminescence in both the visible and near-infrared spectral range were observed from a holmium(dibenzoylmethanato)3(bathophenanthroline) [Ho(DBM)3bath]. Five peaks at 580nm, 660nm, 980nm, 1200nm, and 1500nm, respectively, were attributed to the internal 4f electronical transitions of the Ho3+ ions. Except for the emissions of the Ho3+ ions, a broadband exciplex emission from 480nmto670nm appeared in the EL cases. The emission intensity of the exciplex at organic interface showed a tendency to saturation beyond a certain driving voltage, while the emissions of the Ho3+ ions kept increasing. This evolution of visible EL spectra was discussed in terms of the extension of the charge recombination zone. The 1500nm emission corresponding to the F55→I65 transition suggests that the Ho(DBM)3bath is a potential candidate for optical communications.

Improved electroluminescent performances of europium-complex based devices by doping into electron-transporting/hole-blocking host

High-efficiency, pure red organic light-emitting devices were fabricated by doping a europium-complex Eu(DBM)3pyzphen (DBM=dibenzoylmethane, pyzphen=pyrazino[2,3-f][1,10]phenanthroline) into 4,7-diphenyl-1,10-phenanthroline as emission layer. A maximum current efficiency of 5.1cd∕A at the current density of 1.2mA∕cm2 and a maximum luminance of 1400cd∕m2 were obtained from a 25wt% Eu(DBM)3pyzphen doped device. High efficiencies were maintained at high current densities and high luminance. For example, at the current density of 10mA∕cm2, the efficiency reached 3.8cd∕A. It means that the efficiency roll-off, a major obstacle to the development of Eu-complex ogranic light-emitting devices, was greatly alleviated. The mechanisms behind the improvement are discussed.

Investigation of dye-doped red emitting organic electroluminescent devices with metal-mirror microcavity structure

Organic electroluminescent (EL) devices with planar microcavity structure, indium-tin-oxide/Ag∕N,N′-diphenyl-N , N ′-bis(3-methylphenyl)-1,1′-biphenyl-4 , 4′-diamine/tris(8-hydroxyquinoline)-aluminum (AlQ):4-(dicyanomethylene)-2-methyl-6-(p-dimethyl aminostyryl)-4H-pyran/AlQ∕LiF∕Al, were fabricated. The Ag and Al layers acted as not only hole-injection layer and cathode, respectively, but reflective mirrors, resulting in strong microcavity effects, such as spectral narrowing and directional emission. The effects of device parameters on the EL performance were studied in detail and were discussed in terms of conventional microcavity theory. On-axis light magnification with a coefficient (EL enhancement ratio between cavity and noncavity devices) of ∼5 was observed, which was consistent with the theoretical calculation. At the same time, optimized microcavity device with bright pure red emission showed maximum luminance of 5140cd∕m2, peak at 624nm, Commission International de I’Eclairage coordinates of x=0....

Stable multiplication gain in GaN p?i?n avalanche photodiodes with large device area

Visible-blind p-i-n avalanche photodiodes (APDs) were fabricated with high-quality GaN epilayers deposited on c-plane sapphire substrates by metal-organic chemical vapour deposition. Due to low dislocation density and a sophisticated device fabrication process, the dark current was as small as similar to 0.05 nA under reverse bias up to 20V for devices with a large diameter of 200 mu m, which was among the largest device area for GaN-based p-i-n APDs yet reported. When the reverse bias exceeded 38V the dark current increased sharply, exhibiting a bulk avalanche field-dominated stable breakdown without microplasma formation or sidewall breakdown. With ultraviolet illumination (360 nm) an avalanche multiplication gain of 57 was achieved.

Photoelectric characteristics of metal/InGaN/GaN heterojunction structure

A heterojunction structure photodetector was fabricated by evaporating a semitransparent Ni/Au metal film on the InGaN/GaN structure. The photocurrent (PC) spectra show that both the Schottky junction (NiAu/InGaN) and the InGaN/GaN isotype heterojunction contribute to the PC signal which suggests that two junctions are connected in series and result in a broader spectral response of the device. Secondary electron, cathodoluminescence and electron-beam-induced current images measured from the same area of the edge surface clearly reveal the profile of the layer structure and distribution of the built-in electric field around the two junctions. A band diagram of the device is drawn based on the consideration of the polarization effect at the InGaN/GaN interface. The analysis is consistent with the physical mechanism of a tandem structure of two junctions connected in series.

Suppression of indium droplet formation by adding CCl4 during metalorganic chemical vapor deposition growth of InN films

In this work, the influences of CCl4 on the metalorganic chemical vapor deposition (MOCVD) growth of InN were studied for the first time. It was found that the addition of CCl4 can effectively suppress the formation of metal indium (In) droplets during InN growth, which was ascribed to the etching effect of Cl to In. However, with increasing of CCl4 flow, the InN growth rate decreased but the lateral growth of InN islands was enhanced. This provides a possibility of promoting islands coalescence toward a smooth surface of the InN film by MOCVD. The influence of addition of CCl4 on the electrical properties was also investigated.

Improved performance of europium-complex electroluminescent devices with metal-mirror microcavity

Eu-complex organic electroluminescent (EL) devices with a double metal-mirror Fabry-Perot microcavity structure were constructed to increase luminance efficiency and to improve colour purity. By optimizing the device structure, bright pure red EL from the Eu3+ ion was obtained. The peak luminance of 1160 cd m−2 at 19 V and CIE coordinates of (x = 0.675, y = 0.320) at 12 V were achieved in the cavity device. The narrow-band emission peak was not changed with increasing detection angle, and the colour saturation of the top 45°-off normal EL emission was even better than that of top normal emission in noncavity devices, overcoming the emission colour blue-shift with increasing detection angle in traditional microcavity devices. It was more important that a significant improvement in EL quantum efficiency at high current density was observed in the cavity device and the increasing mechanism of the EL efficiency was also revealed, which should be mainly ascribed to the shortened lifetime of the excited state of the Eu3+ ion.