Atomic fluorescence mapping of optical field intensity profiles issuing from nanostructured slits, milled into subwavelength metallic layers

In this work, we report on the fabrication and characteristics of light-emitting diodes based on p-GaN/i-ZnO/n-ZnO heterojunction. A 30 nm i-ZnO layer was grown on p-GaN by rf reactive magnetron sputtering, then a n-ZnO was deposited by the electron beam evaporation technique. The current-voltage characteristic of the obtained p-i-n heterojunction exhibited a diode-like rectifying behavior. Because the electrons from n-ZnO and the holes from p-GaN could be injected into a i-ZnO layer with a relatively low carrier concentration and mobility, the radiative recombination was mainly confined in i-ZnO region. As a result, an ultraviolet electro-emission at 3.21 eV, related to ZnO exciton recombination, was observed in a room-temperature electroluminescence spectrum of p-i-n heterojunction under forward bias.     

Investigations of ZnSe-ZnO structures,by using the electrooptical and Auger depth profile methods

Analysis of the ZnSe-oxide structures formed by anodic oxidation has been performed. Electrooptical properties were analysed as a function of parameters of anodic oxidation by means of the electroluminescence method. Surface properties of ZnSe with a ZnO layer were investigated by using an Auger depth profile analysis. A theoretical treatment of the results based upon the application of thermionic and tunneling theories to double Schottky diodes was assumed.     

Infrared electroluminescence from erbium-doped spark-processed silicon

The infrared (IR) electroluminescence (EL) of erbium-doped spark-processed silicon (sp-Si) was investigated. For this, a device was constructed which consisted of a silicon wafer on which an erbium layer was vapor deposited, followed by spark-processing and rapid thermal annealing for 15 min at 900 °C in air. The metallization consisted of a 200 nm Ag layer (above the spark-processed area) and a 50 nm thick Al film (on the “back side”), containing a window through which the light could escape. Maximal light emission occurred near 1.55 μm, that is, at a wavelength where commonly used fiber optical materials have their minimum in energy loss. The processing parameters for most efficient light emission were an Er thickness of 200-300 nm, a spark-processing time of about 30 s, an n-type Si wafer having a low (3-5 Ω cm) resistivity, an operating temperature near room temperature, and an operating voltage between 25 and 40 V under reverse bias. The results are interpreted by postulating an energy transfer from sp-Si to the Er³⁺ ions involving the first excited state ⁴I_13/2 to ground state ⁴I_15/2. Further, impact excitation and hot electrons that are accelerated into the erbium doped sp-Si by the applied field (100 kV/cm) are considered.     

Photographic diagnosis of crystalline silicon solar cells utilizing electroluminescence

The photographic surveying of electroluminescence (EL) under forward bias was proved to be a powerful diagnostic tool for investigating not only the material properties but also process induced deficiencies visually in silicon (Si) solar cells. Under forward bias condition, solar cells emit infrared light (wavelength around 1000 to 1200 nm) whose intensity reflects the number of minority carriers in base layers. Thus, all the causes that affect the carrier density can be detected, i.e., the minority carrier diffusion length (or in other words, lifetime), recombination velocity at surfaces and interfaces, etc. (intrinsic material properties), and wafer breakage and electrode breakdown, etc. (extrinsic defects). The EL intensity distribution can be captured by Si CCD camera in less than 1 s, and the detection area simply depends upon the optical lens system suitable to the wide range of 1 cm–1.5 m. This fast and precise technique is superior to the conventional scanning method such as the laser beam induced current (LBIC) method. The EL images are displayed as grayscale, which leads to the difficulty of distinguishing the sorts of those deficient areas. Since the intrinsic deficiency is more sensitive to temperature than the extrinsic deficiency, the change in solar cell temperature can offer the difference in EL intensity contrasts. These effects upon the measurement temperature can be applied to categorize the types of deficiency in the crystalline Si solar cell.     

Two modes of electroluminescence from single‐walled carbon nanotubes

The electroluminescence from single‐walled carbon nanotube field effect transistors is spectrally resolved, and shows two distinct modes of light emission. The vast majority of nanotubes have spectrally broad emission consistent with the spectrum of blackbody radiation. Much more rarely, superposed on the broad emission is a single narrow (<50 meV) peak which is consistent with expectation for electron–hole recombination. The narrow emission is strong even at lower biases and in general has greater peak intensity than the broadband emission. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fabrication and electrical characterization of ZnO rod arrays/CuSCN heterojunctions

ZnO rod arrays/CuSCN heterojunctions are fabricated by depositing ZnO rod arrays films using two-step chemical bath deposition (CBD) and CuSCN thin films using successive ionic layer adsorption and reaction (SILAR) on ITO substrate successively. The structures and morphologies of ZnO films and CuSCN films, analyzed by X-ray diffraction (XRD) spectroscopy and metallurgical microscope, show that ZnO films are hexagonal wurtzite structure and consisted of vertical polycrystalline rods with diameter of 1 μm, CuSCN films are β-phase structure and consisted of elongated grains with length of 3 μm. Current–voltage (I–V) measurements of ZnO/CuSCN heterojunctions show good diode characteristics with rectification ratio about 48.3 at 3 V. The forward conduction is, respectively, determined by carrier recombination in the space charge region, defect-assisted tunneling and exponential distribution trap-assisted space charge limited current mechanism with the increase of forward voltage. Also, a band diagram of ZnO/CuSCN heterojunctions has been proposed to explain the transport mechanism.     

Thermal quenching of electroluminescence assisted by an electric field in thin films of tris-(8-hydroxyquinolinato) aluminum (III)

Temperature dependence of the electroluminescence (EL)-current efficiency of tris-(8-hydroxyquinolinato) aluminum (III) (Alq₃)-based organic light-emitting diodes (OLEDs) operated at a constant current density was investigated. The effects of temperature and electric field on photoluminescence (PL) efficiency of Alq₃ thin layers were also investigated. On the basis of these results, it was found that the EL efficiency decreases more markedly with increasing temperature than does PL efficiency. The temperature dependence of the EL efficiency can be interpreted in terms of the thermal dissociation of excitons that is assisted by the electric field.     

Organic electroluminescence channel avoiding triplet excitons

Polarons and bipolarons are main carriers in conducting polymers. It is shown that a bipolaron can open a channel of electroluminescence, which does not involve a triplet exciton, and can enhance the efficiency of electroluminescence. The dynamics of this channel is simulated.     

Synthesis and electroluminescence properties of benzothiazole derivatives

Benzothiazole-based blue fluorescent materials N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylbenzenamine (BPPA) and N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylnaphthalen-1-amine (BPNA) were synthesized for use in organic light-emitting diodes (OLEDs). Electroluminescent device with a configuration of ITO/NPB/BPPA/BCP/Alq₃/LiF/Al showed a maximum brightness of 3760 cd/m² at 14.4 V with the CIE coordinates of (0.16, 0.16). A current efficiency of 3.01 cd/A and an external quantum efficiency of 2.37% at 20 mA/cm² were obtained from this device. Molecules derived from BPPA and BPNA with incorporated dicyanomethylidene, which is a functional group for most red fluorescent molecules, were designed, synthesized and characterized to study the red fluorescence properties of the benzothiazole derivatives.     

Temperature-Dependent Transport Properties in Oxide p-n Junction above Room Temperature

Oxide p-n junctions ofp-SrIn_0.1Ti_0.9O₃/n-SrNb_0.01Ti_0.99O₃ (SITO/SNTO) are fabricated by laser molecular beam epitaxy. The current-voltage characteristics of the SITO/SNTO p-n junction are investigated mainly in the temperature range of 300--400K. The SITO/SNTO junction exhibited good rectifying behaviour over the wholetemperature range. Our results indicate a possibility of application of oxide p-n junction in higher temperatures in future electronic devices.     

Enhanced electroluminescence of ZnO nanocrystalline annealing from mesoporous precursors

An improved sol-gel method was used to prepare ZnO nanoparticles. EL results showed that slowing the addition of LiOH solution and heating in vacuum to obtain gel precipitation made the final ZnO samples’ emission peak blue shift to 520 nm. Simultaneously, the peak value of the sample processed with no templates was enhanced 4.68 times and that of the sample processed with ODA was enhanced 0.71 times. Two copolymers Pluronic P123 (P123) and Pluronic F-127 (F-127) were adopted respectively as template reagents. The obtained mesoporous ZnO precursors exhibited a surface area of 69.21 m²/g and 103.57 m²/g and an average pore size of 6.61 nm and 5.70 nm, respectively. After calcining in a muffle furnace in air, the obtained ZnO nanocrystalline samples from these precursors revealed stronger green emission than the samples dealt with ODA. Compared to the magnification multiple of 0.89 times of the sample processed with ODA, the peak intensity of the sample processed with P123 was 2.03 times higher than that of the sample processed with no template reagents, and the intensity of the sample processed with F-127 was 3.3 times higher. This may be due to the larger surface area of samples from the longer molecule chains of the two template reagents.     

Improved electroluminescence performance of ZnS:Cu,Cl phosphors by ultrasonic treatment

ZnS:Cu,Cl electroluminescence (EL) phosphors were prepared by high-temperature (1150 °C) solid-state reaction, subsequent ultrasonic treatment (t=0-60 min) and final low-temperature annealing process at 750 °C. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), UV-vis absorbance spectra, electron probe microanalyzer (EPMA) and photoluminescence (PL) spectra. EL performance was investigated on an EL lamp fabricated by screen-printing at 100 V and 400 Hz. Ultrasound irradiation leads to intensity reductions and width increases of some XRD diffraction peaks, and results in a slight red-shift of UV-vis absorption edge. It also exhibits strong influences on PL and EL properties of the phosphors. Generally, PL performance monotonically declines with the increase of ultrasonic time, while EL performance benefits from the ultrasonic treatment and is superior to that of the commercial ones. The defects in the microstructure induced by the ultrasonic treatment are the fundamental reason for the change of PL and EL performances.     

Photon emission from adsorbed C60 molecules with sub-nanometer lateral resolution

We present the first experimental demonstration of spatially resolved photon emission of individual molecules on a surface. A scanning tunneling microscope (STM) was used as a local electron source to excite photon emission from hexagonal arrays of C₆₀ molecules on Au(110) surfaces. Specifically, we show that in maps of photon emission intensities, C₆₀ fullerenes appear as arrays of individual light emitters 4 Å in diameter and separated by 10 Å. Comparison with simultaneously recorded STM images reveals, that most intense emission is detected when the STM tip is centered above a molecule. The results demonstrate the highest spatial resolution of light emission to date using a scanning probe technique.     

The coalescence of silicon layers grown over SiO2 by liquid-phase epitaxy

The coalescence of epitaxial silicon layers which are grown laterally over oxidized and patterned Si substrates is studied using various techniques of transmission electron microscopy (TEM). The epitaxial layers, the seam of coalescence and lattice defects formed by the coalescence are characterized. The epitaxial layer as a whole is found to be bent with respect to the substrate. Such misorientations, together with facetting of the growth fronts of the coalescing layers, may lead to the formation of solvent inclusions, dislocations and stacking faults at the seam of coalescence. However, under favourable conditions, the seam is found to be entirely defect-free.     

Dislocation generation in silicon grown laterally over SiO2 by liquid phase epitaxy

Epitaxial lateral overgrowth (ELO) on thermally oxidized and patterned (111) Si is effected by liquid phase epitaxy (LPE). It produces Si layers spreading out on the amorphous SiO₂ which are either perfectly grown defect-free or, coexisting, defective layers containing dislocations. High voltage electron microscopy of the defective layers reveals regular arrangements of the dislocations which result from glide and multiplication processes governed by the elastic interactions between the dislocations. The nucleation of the first dislocations during the ELO process is attributed to a slight warping of the substrates. A corresponding bending of the epitaxial layer induces mechanical stress, which may exceed the critical value at the oxide edges of the seeding windows where the first dislocations nucleate. The characteristics of the dislocation arrangements and lattice imaging results support this model. Suggestions are made for ways to reduce stress and, thus, avoid dislocation formation.     

Room temperature electroluminescence from ZnO/Si heterojunction devices grown by metal-organic chemical vapor deposition

Heterojunction light-emitting diodes with ZnO/Si structure were fabricated on both high-resistivity (p) and low-resistivity (p⁺) Si substrates by metal-organic chemical vapor deposition technology. Fairly good rectifications were observed from the current-voltage curves of both heterojunctions. Ultraviolet (UV) and blue-white electroluminescence (EL) from ZnO layer were observed only from ZnO/p⁺-Si heterojunction under forward bias at room temperature (RT), while strong infrared (IR) EL emissions from Si substrates were detected from both ZnO/p-Si and ZnO/p⁺-Si heterojunctions. The UV and IR EL mechanisms have been explained by energy band structures. The realization of RT EL in UV-visible and IR region on Si substrate has great applicable potential for Si-based optoelectronic integrated circuits.     

Voltage-controlled electroluminescence from SiO2 films containing Ge nanocrystals and its mechanism

Luminescent SiO₂ films containing Ge nanocrystals are fabricated by using Ge ion implantation, and metal–oxide–semiconductor structures employing these films as the active layers show yellow electroluminescence (EL) under both forward and reverse biases. The EL spectra are strongly dependent on the applied voltage, but slightly on the mean size of Ge nanocrystals. When the forward bias increases towards 30 V, the EL spectral peak shifts from 590 nm to 485 nm. It is assumed that the EL originates from the recombination of injected electrons and holes in Ge nanocrystals near the Si/SiO₂ interface, or through luminescent centers in the SiO₂ matrix near the SiO₂/metal interface. The mismatch of the injection amounts between holes and electrons results in the low EL efficiency. Received: 28 February 2000 / Accepted: 28 March 2000 / Published online: 5 July 2000     

Asymmetric Joule heat production at a point contact

In the vicinity of a point contact the Joule's heat production is asymmetrical with respect to the plane of the constricting orifice, provided the contact radius is small compared to the mean-free path of the electrons. This effect is measured onn-type Silicon for a variety of contact radii and explained theoretically. Possible applications for thermoelectric coolers are proposed.     

Characterization of solution-synthesized CdTe and HgTe

We report the characterization of solution-synthesized CdTe and HgTe nanocrystals by X-ray diffraction, transmission electron microscopy, and photoluminescence. Methanol solutions of sodium telluride and cadmium iodide or mercury iodide, respectively, are reacted to precipitate the nanocrystalline metal tellurides, while the sodium iodide byproduct remains in solution. The existence of crystalline CdTe, HgTe, and ternary HgCdTe compounds has been demonstrated by powder X-ray diffraction after a post-synthesis sintering process. Precipitated crystallites from this synthesis were analyzed by transmission electron microscopy, which revealed that crystal diameters can vary from approximately 1 nm to 100 nm and that crystals are stoichiometric within the detection limit of the electron microprobe technique. Narrow size ranges can be selected and investigated due to an in-situ separation process in the electron microscope. Photoluminescence is found at energies above the bulk exciton energy for CdTe and is attributed to near-band-gap recombination which is blue-shifted due to quantum confinement. Both low defect luminescence and dark field imaging suggest a high crystalline quality. A comparative characterization by photoluminescence, transmission electron microscopy, and X-ray diffraction evaluates the effects of heat treatments during and after synthesis.     

Infrared electroluminescence from a Si MOS structure with Ge in the oxide

We report on room-temperature infrared electroluminescence (EL) from metal-oxide-semiconductor devices made from Si. We compare the luminescence from RF sputtered oxide films containing SiO₂ with and without Ge by using a composite target and luminescence from a SiO₂ layer made by rapid thermal oxidation. The sputtered films were annealed in the temperature range 600-900 °C. This densifies the films and is likely to reduce the concentration of defects. A luminescence peak located around 1150-1170 nm is observed at current densities as low as 0.1 A/cm². The corresponding photon energy is close to that of the Si band gap. In addition, we observe several broad luminescence bands in the range 1000-1750 nm. These bands get stronger with Ge in the SiO₂ film. Some of these bands have previously been suggested and are directly associated with Ge. Since we observe that the intensity is correlated with the presence of Ge while the mere presence of the bands is not, we discuss the EL bands being due to defects which concentration is influenced by Ge in the oxide.