Near-infrared Electroluminescence Research Articles

Highly efficient near-infrared hybrid organic-inorganic nanocrystal electroluminescence device

We report the use of PbS nanocrystals within a hybrid device that emits 1.2μm electroluminescence with an external quantum efficiency of 1.15% corresponding to an internal quantum efficiency of ∼5%–12% thus demonstrating a viable, low-cost, highly efficient near infrared organic electroluminescent device. Direct generation of the excited state on the nanocrystal result in eliminating competing processes that have previously led to the low reported efficiencies in near-infrared light emitting devices. Furthermore, the emission wavelength can be tuned to cover a wide range of wavelengths including the 1.3–1.5μm region without significant change of the efficiency.

Spectroscopic Analysis on Metal-Oxide-Semiconductor Light-Emitting Diodes with Buried Si Nanocrystals and Nano-Pyramids in SiOx Film

The ordered arrays of magnetic metal (including Fe, Co and Ni) nanotubes and nanowires encapsulated with carbon tubes are controllably synthesized by employing the array of C tubes as second-order template and combining with electrodeposition technique. The wall thickness and diameter of carbon nanotubes are uniform along the whole tubes; also the wall thickness of inner metal nanotubes is adjustable from 25 nm to solid nanowires. These composite structures are characterized by X-ray diffractometer (XRD), energy-dispersive spectrometry (EDS), Raman scattering spectrum, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The magnetic properties show that coaxial nanotubes and nanocables composite arrays all exhibit magnetic anisotropy with the easy direction perpendicular to axis of the metal nanotubes or nanowires except the Ni at C coaxial nanotubes array that has no preferable magnetization axis.

Spectroscopic Analysis on Metal-Oxide-Semiconductor Light-Emitting Diodes with Buried Si Nanocrystals and Nano-Pyramids in SiOx Film

The difference between the white and near-infrared electroluminescence of metal-oxide-semiconductor light-emitting diodes fabricated on 1,100 degrees C-annealed Si-rich SiO(x)/p-Si substrate with interfacial pyramidal Si dots (Si nano-pyramids) was characterized. By changing the substrate temperature and induced coupled plasma power during the plasma enhanced chemical vapor deposition of Si-rich SiO(x) films, the effects of the growth conditions on the defect- and Si nano-pyramid-related carrier transport and Si nanocrystal-related electroluminescence spectroscopy were also investigated. The annealed Si-rich SiO(x)/p-Si films grown at higher synthesized substrate temperate (350 degrees C) show the larger Si nano-pyramids precipitating near the Si/SiO2 interface. The indium tin oxide/Si-rich SiO(x)/p-Si/Al metal-oxide-semiconductor light-emitting diodes with Si-rich SiO(x) films exhibit different white-light electroluminescence spectra at wavelengths from 400 to 650 nm. The Si nanocrystal-related electroluminescence spectra at 650-850 nm are confirmed, whereas the electroluminescence spectra are shorter wavelengths is attributed to oxygen related defects. These defects become an electron-preferred transporting path within the Si-rich SiO(x) film, whose densities are decreased by increasing the substrate temperature or reducing the induced coupled plasma power. Defect-related white-light electroluminescence emits power for a relatively short lifetime. The lifetime can be lengthened and the electroluminescence power can be raised simultaneously by increasing deposition temperature to 350 degrees C and adjusting the induced coupled plasma power to a threshold of 30 W, which effectively increases the densities of Si nanocrystals and nano-pyramids in the Si-rich SiO(x) film with Si concentration of up to 40 at%. A nearly defect-free Si-rich SiO(x) sample can be grown under such conditions, which contributes to the most stable and largest near-infrared electroluminescence with the longest lifetime, although the power-current slope of purely Si nanopyramid related electroluminescence at near-infrared wavelengths is slightly lower.

C O 2 laser rapid-thermal-annealing SiOx based metal-oxide-semiconductor light emitting diode

Structural damage enhanced near-infrared electroluminescence (EL) of a metal-oxide-semiconductor light emitting diode (MOSLED) made on SiOx film with buried nanocrystallite Si after CO2 laser rapid thermal annealing (RTA) at an optimized intensity of 6kW∕cm2 for 1ms is demonstrated. CO2 laser RTA induced oxygen-related defects are capable of improving Fowler-Nordheim tunneling mechanism of carriers at metal/SiOx interface. The CO2 laser RTA SiOx film reduces Fowler-Nordheim tunneling threshold to 1.8MV∕cm, facilitating an enhanced EL power of an indium tin oxide/SiOx∕p-Si∕Al MOSLED up to 50nW at a current density of 2.3mA∕cm2.

Effects of processing parameters on electroluminescence of rf magnetron sputter deposited ZnS:ErF3

Zn S : Er F 3 alternating current thin film electroluminescent (ACTFEL) devices are fabricated by rf plasma magnetron sputtering. In a full factorial design-of-experiment study, increasing deposition temperature, duty cycle on the doped target, and sputter gas pressure all result in increases in the 1.55μm near infrared (NIR) electroluminescence (EL) irradiance at 20V above threshold (B20). An increase in the EL threshold voltage (Vth) upon increasing the duty cycle of the undoped target is also reported. Postdeposition annealing of ACTFEL devices at 425°C for 1h improves the NIR EL irradiance by decreasing the F concentrations in the ZnS:Er films. The origins of these effects are discussed in terms of negative ion resputtering, surface mobility of sputtered species, crystallinity, and the effects of atomic concentration on the EL and radiative relaxation processes. A maximum irradiance, B20, of 147μW∕cm2 is measured for the 1.55μm NIR EL peak from ZnS:Er ACTFEL devices produced using a deposition temperature of 150°C, a duty cycle of 75%, an argon sputtering gas pressure of 24mTorr, and postdeposition annealing at 425°C for 1h in nitrogen.

Enhanced Fowler-Nordheim tunneling effect in nanocrystallite Si based LED with interfacial Si nano-pyramids

The premier observation on the enhanced light emission from such a metal-SiO(x)-Si light emitting diode (MOSLED) with Si nano-pyramids at SiO(x)/Si interface is demonstrated at low biases. The Si nano-pyramids exhibits capability in providing the roughness of the SiO(x)/Si interface, and improving the Fowler-Nordheim (F-N) tunneling mechanism based carrier injection through the novel SiO(x)/nano-Si-pyramid/Si structure. HRTEM analysis reveals a precisely controllable size and concentration of the crystallized interfacial Si nano-pyramids at 10nm(height)x10nm(width) and within the range of 10(8)-10(11) cm(-2), respectively. With these Si nano-pyramids at a surface density of up to 1012/cm(2), the F-N tunneling threshold can be reduce from 7 MV/cm to 1.4 MV/cm. The correlation between surface density of the interfacial Si nano-pyramids and the threshold F-N tunneling field has been elucidated. Such a turn-on reduction essentially provides a less damaged SiO(x)/Si interface as the required bias for the electroluminescence of the MOSLED is greatly decreased, which thus suppresses the generation of structural damage related radiant defects under a lower biased condition and leads to a more stable near-infrared electroluminescence with a narrowing linewidth and an operating lifetime lengthened to >3 hours. An output EL power of nearly 150 nW under a biased voltage of 75 V and current density of 32 mA/cm(2) is reported for the first time.

Synthesis and 0.88 μm near-infrared electroluminescence properties of a soluble chloroindium phthalocyanine

A chloroindium phthalocyanine bearing phenoxyl substituents was synthesized and characterized by MS, 1H NMR and elemental analysis, which were consistent with the proposed structure; UV–vis absorption and photoluminescence (PL) spectra were also investigated. Near-infrared (NIR) organic light-emitting devices (OLEDs) were fabricated employing tris-(8-hydroxyquinoline) aluminum (Alq 3) and poly(vinylcarbazole) (PVK) which had been doped with chloroindium phthalocyanine. Room-temperature electroluminescence (EL) was observed near 0.88 μm due to transitions from the first excited singlet state to the ground state (S 1–S 0). As phthalocyanine possesses excellent solubility in common organic solvents, the light-emitting layer can be prepared by spin coating to simplify the fabrication process and so lower the manufacturing costs. The emission processes of this chloroindium phthalocyanine in the doped devices are discussed.

Exciton Formation and Annihilation during 1D Impact Excitation of Carbon Nanotubes

Near-infrared electroluminescence was recorded from unipolar single-wall carbon nanotube field-effect transistors at high drain-source voltages. High resolution spectra reveal resonant light emission originating from the radiative relaxation of excitons rather than heat dissipation. The electroluminescence is induced by only one carrier type and ascribed to 1D impact excitation. An emission quenching is also observed at high field and attributed to an exciton-exciton annihilation process and free carrier generation. The excitons' binding energy in the order of 270 meV for 1.4 nm SWNTs is inferred from the spectral features.

Room-Temperature Electroluminescence from Single-Period (CdF2/CaF2) Inter-Subband Quantum Cascade Structure on Si substrate

Near-infrared electroluminescence from a single-period (CdF2/CaF2) inter-subband quantum cascade structure on a Si substrate is reported. The CdF2/CaF2 heterostructure is a good candidate for realizing a Si-based short-wavelength quantum cascade laser because of its large conduction band discontinuity of 2.9 eV and small lattice mismatch with the Si substrate. In the experiment, an active region consisting of (CdF2/CaF2) heterostructures was grown epitaxially on the Si substrate by molecular beam epitaxy, and an Au/Al electrode was evaporated on the active region. The wafer was polished mechanically and cleaved. Electroluminescence from the device was observed in the near-infrared region at room temperature for the first time.

Near-infrared electroluminescence based on perylenediimide-doped tris(8-quinolinolato) aluminum

For the purpose of exploring near-infrared emission, the photoluminescence (PL) and electro- luminescence (EL) of N,N′-bis(neopentyl)-3,4:9,10-perylenebis(dicarboximide) (BNPTCD): tris(8-quinolinolato)aluminum (Alq3) mixed films were investigated. It was found that BNPTCD:Alq3 mixed films exhibit PL in the near-infrared wavelength region and that organic light-emitting diodes using Alq3 highly doped with BNPTCD as an emitting layer exhibit near-infrared EL with a peak at 805 nm originating from BNPTCD aggregates.

Synthesis and properties of novel poly( p-phenylenevinylene) copolymers for near-infrared emitting diodes

A series of novel near-infrared (NIR) electroluminescence (EL) copolymers based on a host poly( p-phenylenevinylene) (PPV) derivative of poly(2-methoxy-5-octyloxy- p-phenylenevinylene) (PMOPV) with different content of narrow band-gap (NBG) unit 4,7-bis(2-thienyl)-2,1,3-benzothiadiazolevinylene (DBTV) or 4,7-bis(2-thienyl)-2,1,3-benzoselenadiazolevinylene (TBSV) was prepared by Stille coupling reaction. All the copolymers are soluble in common organic solvents, most of which emit NIR light accompanied by gradually red-shifting with increasing the content of the NBG units. The peak EL emission of the copolymers with around 30% TBSV content is at about 800 nm.

Near-infrared electroluminescent devices based on colloidal HgTe quantum dot arrays

Crystalline 4.6 nm HgTe quantum dots, stabilized by 1-thioglycerol ligands, were synthesized by wet chemical methods. Room-temperature photoluminescence spectra of the dots, both in solution and as solid arrays, exhibited near-infrared emission. Light-emitting devices were fabricated by deposition of quantum dot layers onto glass∕indium tin oxide (ITO)∕3,4-polyethylene-dioxythiophene-polystyrene sulfonate (PEDOT) substrates followed by top contacting with evaporated aluminum. Room-temperature near-infrared electroluminescence from 1mm2 ITO∕PEDOT∕HgTe∕Al devices, centered at ∼1600nm, with an external quantum efficiency of 0.02% and brightness of 150nW∕mm2 at 50 mA and 2.5 V was achieved.

Oxygen defect and Si nanocrystal dependent white-light and near-infrared electroluminescence of Si-implanted and plasma-enhanced chemical-vapor deposition-grown Si-rich SiO2

The mechanisms for silicon (Si) defect and nanocrystal related white and near-infrared electroluminescences (ELs) of Si-rich SiO2 films synthesized by Si-ion implantation and plasma-enhanced chemical-vapor deposition (PECVD) are investigated. The strong photoluminescence (PL) of Si-ion-implanted SiO2 (SiO2:Si+) at 415–455 nm contributed by weak-oxygen bond and neutral oxygen vacancy defects is observed after 1100 °C annealing for 180 min. The white-light EL of a reverse-biased SiO2:Si+ metal-oxide-semiconductor (MOS) diode with a turn-on voltage of 3.3 V originates from the minority-carrier tunneling and recombination in the defect states of SiO2:Si+, which exhibits maximum EL power of 120 nW at bias of 15 V with a power–current slope of 2.2μW∕A. The precipitation of nanocrystallite silicon (nc-Si) in SiO2:Si+ is less pronounced due to relatively small excess Si density. In contrast, the 4-nm nc-Si contributed to PL and EL at about 760 nm is precipitated in the PECVD-grown Si-rich SiOx film after annealing at 1100 °C for 30 min. The indium-tin-oxide/Si-rich SiOx∕p-Si∕Al metal oxide semiconductor (MOS) diode is highly resistive with turn-on voltage and power-current (P–I) slope of 86 V and 0.7mW∕A, respectively. The decomposed EL peaks at 625 and 768 nm are contributed by the bias-dependent cold-carrier tunneling between the excited states in adjacent nc-Si quantum dots.

Clarifying the origin of near-infrared electroluminescence peaksfor nanocrystalline germanium in metal-insulator-silicon structures

The origin of room-temperature near-infrared electroluminescence (EL) of nanocrystalline germanium (Ge) embedded in oxide was investigated. The nanocrystals were synthesized by partial oxidation of silicon-germanium, Si0.54Ge0.46, films. Under constant current density bias in accumulation and inversion, Ge nanocrystals with diameters of 5 and 10nm exhibit strong luminescence at 1350nm. The 1350nm EL peak is only observed in the presence of elemental Ge and is enhanced with the formation of Ge nanocrystals. The introduction of hydrogen during annealing passivates the dangling bonds at the interfaces of nanocrystals, thus minimizing the energy spread in the 1350nm peak. The 1350nm peak intensity is a function of the injected carrier density, while the peak location remains constant in energy and independent of the applied bias. The results are a clear indication that the luminescence peak originates from radiative recombination of excitons confined in the nanocrystals.

Near-infrared electroluminescence at room temperature from neodymium-doped gallium nitride thin films

Strong near-infrared (NIR) electroluminescence (EL) at room temperature from neodymium (Nd)-doped gallium nitride (GaN) thin films is reported. The Nd-doped GaN films were grown by radio-frequency planar magnetron cosputtering of separate GaN and metallic Nd targets in a pure nitrogen ambient. X-ray diffraction data did not identify the presence of any secondary phases and revealed that the Nd-doped GaN films had a highly textured wurtzite crystal structure with the c-axis normal to the surface of the film. The EL devices were fabricated with a thin-film multilayered structure of Al∕Nd-doped GaN∕Al2O3–TiO2∕indium-tin oxide and tested at room temperate. Three distinct NIR EL emission peaks were observed from the devices at 905, 1082, and 1364nm, arising from the radiative relaxation of the F3∕24 excited-state energy level to the I9∕24, I11∕24, and I13∕24 levels of the Nd3+ ion, respectively. The threshold voltage for all the three emission peaks was ∼150V. The external power efficiency of the fabricated EL devices was ∼1×10−5 measured at 40V above the threshold voltage.

1.4 μ m 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 F34→H34 and F34→H36 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.

Electroluminescence from Tm-doped GaN deposited by radio-frequency planar magnetron sputtering

Room-temperature blue and near-infrared (NIR) electroluminescence ac (EL) has been demonstrated from Tm-doped GaN thin films prepared by rf planar magnetron sputter deposition. Blue and NIR EL emission peaks at ∼475 and ∼800 nm, respectively, were observed from the Tm3+ 4f intrashell transitions from the G41 and F43 excited states to the H63 ground state, respectively. The threshold voltage for the 475 nm blue emission was consistently 11 to 19 V higher than that for the 800 nm NIR emission. The EL intensity ratio of the 475 to 800 nm emission (I475/I800) was increased ∼900% with a 67% increase in the applied ac voltage from 120 to 200 V. These results indicate that the excitation process of EL from GaN:Tm ACTFEL devices is dominated by impact excitation of the Tm3+ luminescent centers by direct interaction between hot electrons and the ground state 4f electrons.

Visible and near-infrared alternating-current electroluminescence from sputter-grown GaN thin films doped with Er

A demonstration of visible and near-infrared (NIR) alternating-current electroluminescence from sputter-grown GaN thin films doped with Er is reported. The alternating-current thin-film electroluminescent (ACTFEL) devices were constructed using a standard single-insulating structure, Al/GaN:Er/aluminum–titanium–oxide/indium–tin–oxide/Corning 7059 glass. Visible emissions peaked at 550 and 665 nm as well as NIR emissions centered at 1000 and 1550 nm were observed from the fabricated ACTFEL devices operating at room temperature. The visible and NIR emissions at 550, 665, 1000, and 1550 nm were attributed to the Er3+ 4f–4f intrashell transitions from the 4S3/2, 4F9/2, 4I11/2, and 4I13/2 excited-state levels to the 4I15/2 ground state, respectively. The green 550 nm emission had a larger dI/dV and a higher threshold voltage, Vth than the NIR 1550 nm emission, which could result from the need for higher electron impact energy to impact-excite the Er ion into the higher-energy excited-states for emission of green light.

Near-infrared electroluminescence in polymer composites based on organic nanocrystals

IR electroluminescence was revealed in single-layer light-emitting diodes based on a type of electroactive polymer nanocomposites-electron-hole conducting aromatic polyimide and organic nanocrystalline particles of cyanine molecules, known as J-aggregates. These materials exhibit a very narrow emission band with a maximum at 815 nm. Dramatic increase of charge-carrier mobility was observed for these layers containing the J-aggregate nanocrystalline phase.

Near-infrared electroluminescence from conjugated polymer/lanthanide porphyrin blends

Near-infrared-emitting polymer light-emitting diodes (PLEDs) have been fabricated using blends of conjugated polymers and lanthanide tetraphenylporphyrin complexes. Host polymers include MEH–PPV and a bis-alkoxy-substituted poly(p-phenylene) (PPP–OR11), and the lanthanide complexes include Yb(TPP)acac and Er(TPP)acac (where TPP=5,10,15,20-tetraphenylporphyrin and acac=acetylacetonate). Electroluminescence (EL) is observed at 977 nm from devices fabricated using MEH–PPV or PPP–OR11 blended with Yb(TPP)acac, and EL is observed at 1560 nm from a device fabricated using a blend of MEH–PPV and Er(TPP)acac. Visible EL from the host polymers is strongly suppressed in all of the devices, however, in the device fabricated using the PPP–OR11 polymer blue emission from the host is completely quenched. Very efficient quenching of the EL from the host in the PPP–OR11 device is believed to occur due to efficient Förster energy transfer, which is facilitated by the excellent spectral overlap between the PPP–OR11 fluorescence and the Soret absorption band of the TPP ligand.