Green Diodes Research Articles

Gain spectroscopy and vertical cavity devices in wide-gap semiconductors

The InGaN quantum-well diode laser is now reaching the threshold of commercial availability as an edge emitting blue/violet device, yet many questions remain concerning the details of its operation. For example, the threshold current density in the best cw InGaN QW lasers is still about one order of magnitude larger than that in the green ZnCdSe QW diode devices, in spite of the materials being quite similar in the effective mass band picture. On the one hand, the nitride semiconductors are remarkably tolerant of built-in crystalline defects at high current densities. On the other, the microscopics of optical gain are significantly influenced by crystalline `disorder’ that emanates from indium compositional anomalies in the active InGaN quantum-well material.

Therapeutic options for capillary papillary hemangiomas

PurposeTo describe the results and therapeutic complications of treatment of papillary capillary hemangiomas over the last 13 years. DesignRetrospective, noncomparative, interventional case series. ParticipantsFive cases of unilateral capillary papillary hemangiomas were reviewed. Three patients had von Hippel-Lindau disease, and two had no associated familial or systemic disease. MethodsTwo patients were treated with argon green laser, one patient received argon green and diode photocoagulation, and two patients underwent argon green photocoagulation and diode transpupillary thermotherapy (TTT); two eyes also underwent vitreoretinal surgery. Follow-up ranged from 1 to 13 years. Main outcome measuresThe presence of exudative retinal detachment associated with the activity of the hemangioma and the visual acuity were the two main parameters. ResultsPrompt argon green laser treatment was the most effective therapy; TTT was ineffective. Vitreoretinal surgery, transscleral drainage, and argon endolaser photocoagulation in one case resolved bullous retinal detachment secondary to tumor exudation. Pretreatment visual acuity (VA) levels ranged from 20/25 to counting fingers; posttreatment VA levels ranged from 20/25 to light perception. ConclusionsIf left untreated, papillary hemangiomas may evolve to exudative retinal detachment and marked VA decreases. Although we have not established an ideal therapy, we recommend appropriate treatment on diagnosis. Close follow-up and multiple treatments with argon laser are likely the best therapeutic course.

Green electroluminescent (Ga, In, Al)N LEDs grown on Si (111)

The authors report on GaInN/GaN green electroluminescent diodes (/spl lambda/=508 nm) grown on Si [111] substrate. The I-V characteristics of this LED at room temperature are contrasted with a similar diode grown on sapphire. The turn-on voltage is 6.8 V and the operating voltage is 10.7 V at 20 mA. The large density of dislocations present in these heteroepitaxial nitrides on Si materials manifests itself through high reverse currents and low shunt resistances.

Pressure Studies of Defects and Impurities in Nitrides

A review of the selected results concerning the role of hydrostatic pressure in studies of impurity and defect centers in III–V nitrides, and in InGaAsN with small amount of N is presented. Firstly, the shallow to deep impurity state transition for basic donors, i.e. Si and O in GaN and AlGaN and their DX-like center behavior is discussed. It corresponds to the localized state of the appropriate donor (resonant with the conduction band at low pressures) which enters the band gap of GaN or AlxGa1—xN at sufficiently high pressure and/or x value. Next, the concept of pressure application to prove the effective mass character of Mg acceptor in GaN is demonstrated. We also present results of an anomalous decrease of the pressure coefficient with increasing amount of In in InyGa1—yN for photoluminescence and electroluminescence. It is compared with the pressure induced changes of the photocurrent spectrum in green Nichia diodes which we consider as corresponding to the variation of the InyGa1—yN band gap with pressure. Finally, studies of the peculiarities of the conduction band structure and related transport properties as a function of pressure in InGaAsN are presented.

Luminescence and electrical properties of InGaN/AlGaN/GaN light emitting diodes with multiple quantum wells

Luminescence spectra of light-emitting diodes based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells are studied for currents in the range J=0.15 µA-150 mA. The comparatively high quantum efficiency for low J(J max=0.5–1 mA) is a consequence of a low probability for the nonradiative tunnel current. The current-voltage characteristics J(V) are studied for J=10−12–10−1 A; they are approximated by the function V=ϕk+mkT· [1n(J/J 0)+(J/J 1)0.5] + J · R s. The portion of V∞(J/J 1)0.5 and measurements of the dynamic capacitance indicate that i-layers adjacent to the active layer play an important role. The spectra are described by a model with a two-dimensional density of states with exponential tails in multiple quantum wells. The rise in T with increasing J is determined from the short-wavelength decay of the spectrum of the blue diodes: T=360–370 K for J=80–100 mA. An emission band is observed at 2.7–2.8 eV from green diodes at high J; this band may be explained by phase separation with different amounts of In in the InGaN.

Influence of pressure on photoluminescence and electroluminescence in GaN/InGaN/AlGaN quantum wells

We have measured photoluminescence and electroluminescence in two different types of high-brightness single-quantum-well light emitting diodes manufactured by Nichia Chemical Industries with InxGa1−xN active layers (x=0.45 and x=0.15), under hydrostatic pressures up to 8 GPa. We discovered that the pressure shift of the primary luminescence peak in each diode is very small: 12 and 16 meV/GPa for the green and blue diodes, respectively. The observed pressure coefficients are much lower than those characteristic of the energy gap in GaN (≈40 meV/GPa) or the energy gap in InN (≈33 meV/GPa). This kind of behavior is usually associated with recombination processes involving localized states. These localized states may be associated either with band tails (arising from In fluctuations in the active layer or from high density of defects), and/or with localized excitons of various types.

Efficient green electroluminescent diodes based on poly (2‐dimethyloctylsilyl‐1,4‐phenylenevinylene)

An exceptionally high photoluminescene efficiency (60%) is reported for the title compound, a nwe, solution‐processable derivative of PPV. It is shown that the compound can be used to make relatively efficient green electroluminescent diodes, and methods of further improving the efficiency of the device, e.g., incorporation of an electron‐conducting, hole‐blocking material, are described. Electroluminescence, photoluminescence, and UV‐vis absorptioni spectra are presented, together with the IV and light characteristics of the devices.

Side-chain electroluminescent polymers

In order to eliminate problems linked to the use of small emitting molecules or conjugated polymers, a novel type of electroluminescent material is investigated. Side-chain polymers have been conceived, synthesized and tested. Blue, green and oragen diodes have been made. It is demonstrated that a hole-transporting layer increaess the light emission of the green diode. Doping of side-chain polymers has been studied to produce orange light.

An experimental study of indocyanine green dye-enhanced diode laser photocoagulation of primate retina

Classical galactosemia is an autosomal recessive disorder caused by a deficiency of the enzyme galactose-1-phosphate uridyltransferase. Undoubtedly, some of the short term complications are linked to the toxic effects of the accumulated abnormal metabolites (galactose-1-phosphate and galactitol). However, the physiopathology of neonatal liver failure remains unclear. We report the case of a 7-week-old girl who was first diagnosed with liver failure, hypoprotidaemia, ascites and generalized edemas. High citrulline (293 μmol/L), on initial plasma amino acid, suggested the diagnosis of citrin deficiency. As the citric acid cycle intermediates were non-detectable (oxoglutarate, succinate and citrate), a cataplerotic state was suspected. As a result, citrate (as an anaplerotic treatment) induced a clear improvement in her liver function. Four weeks later, this patient was switched to a galactose-free formula (as recommended in citrin deficiency with galactosemia) and her pathological status returned to normal. Citrin deficiency was later ruled out by molecular biology studies; then we reintroduced a galactose-containing formula which re-evoked rapidly vomiting, galactose aversion and hepatic cytolysis and the diagnosis of classical galactosemia was established. Our case clearly shows that cataplerosis could play a role in the pathophysiology of the neonatal liver disease observed in classical galactosemia.

Quantum Confined Structures and Coherent Light Emitters in II–VI Heterostructures

AbstractRecent developments in wide‐gap II–VI quantum wells and p–n‐junction heterostructures with emphasis on two‐dimensonal exciton physics are reported. Current device issues related to blue and green diode lasers and light emitting diodes in the ZnSe‐based configurations are briefly summarized. This is followed by experimental review of evidence which strongly suggests that exciton effects, which dominate interband absorption, are also of importance in both spontaneous and stimulated emission processes.

Blue and green diode lasers in ZnSe-based quantum wells

Laser diode operation has been obtained from (Zn,Cd)Se/ZnSe and (Zn,Cd)Se/Zn(S,Se) quantum well structures in the blue and the green. The devices, prepared on p- and n-type (In,Ga)As or GaAs buffer layers for lattice matching purposes to control the defect density, have been operated at near-room-temperature conditions and briefly at room temperature with uncoated end facets. Quasi-continuous wave operation has been obtained at T=77 K.

Degradation of green light emitting diodes LPE-GaP:N I. electron beam characterization and deep-level spectroscopy

Green light emitting diodes (LED) produced by LPE-technology are testet for different stress stages using the scanning electron microscope and the deep level transient spectroscopy (DLTS). Beside the n- and p-region the light output on unstressed LED is determined by a high efficient intrinsic interlayer adjoining the transition. The short-time deterioration of the electroluminescent efficiency is determined by the rapid degradation of the interlayer correlated with the creation of a PGa-antisite. The degradation of the n- and p-region causes the long-time decreasing electroluminescence efficiency. Grunes Licht emittierende Dioden (LED), hergestellt nach LPE-Technologie, werden in unterschiedlichen Belastungsstadien mit Hilfe eines Rasterelektronenmikroskops und DLTS untersucht. Eine hochkompensierte, hocheffiziente, den pn-Ubergang umgebende Zwischenschicht bestimmt wesentlich die Lichtausbeute an unbelasteten LED. Die Kurzzeitabnahme der Elektrolumineszenzeffizienz wird durch die schnelle Degradation der Zwischenschicht bei Ausbildung eines PGa-antisite Defekts verursacht. Die Degradation des n- und p-Gebietes wird durch die Langzeitabnahme der Elektrolumineszenzeffizienz angezeigt.

Cathodoluminescence measurements on green light-emitting GaP diodes

Green light-emitting GaP:N diodes (LED's) are investigated at room temperature using a scanning electron probe microanalyzer. Integral cathodoluminescence (CL) line scans across the p-n junction are carried out. The CL line scan is discussed in connection with the depth of the p-n junction and in comparison with the external quantum efficiency of the diodes. A higher concentration of non-radiative recombination centres are detected in some cases close to the p-n junction in the n-region. The injection ratio (under the forward-bias condition) is determined comparing CL with the electroluminescence. Mit einem Elektronenstrahlmikroanalysator werden grun leuchtende GaP:N-Dioden (LED's) bei Raumtemperatur untersucht. Linienscans der integralen Katodolumineszenz werden uber dem pn-Ubergang durchgefuhrt. Die Katodolumineszenzprofile werden in Verbindung mit der Tiefe der pn-Ubergange und mit der auseren Quantenausbeute der Dioden diskutiert. In einigen Fallen wird eine Ansammlung von nichtstrahlenden Rekombinationszentren im n-Gebiet nahe dem pn-Ubergang nachgewiesen. Aus dem Vergleich der Katodolumineszenz mit der Elektrolumineszenz wird das Injektionsverhaltnis unter Flusspannung bestimmt.

Electro- and cathodoluminescence studies on impurity segregation in green ZnTe diodes

We establish a correlation between defect contrasts present in electro- and cathodoluminescence on ZnTe. We show the role of b-acceptor (61 meV) in comparison with acceptor a (149 meV). b is related to lithium, contained in Te inclusions, and released after Zn or Te anneals.

Properties of group II elements in GaP light-emitting diodes

The radiative centers in red GaP diodes are known to be Zn(Cd)--O complexes. However, the production of green diodes requires suppression of the red component. This can be done by reducing the oxygen content (for example, by doping GaP with nitrogen from a stream of ammonia) or by ensuring a minimum injection of electrons to suppress the appearance of red luminescence, even if oxygen is present in the p region.

Electric properties of GaN light-emitting diodes

The mechanisms of electrical conduction processes in MIS GaN : Zn, yellow and green light-emitting diodes (LED’s), are analyzed by considering the I-V characteristics at room temperature and nitrogen temperatures and the temperature dependences of currents through the diodes with a constant forward bias. The diode current transport mechanisms are different between the two temperature regions, lower or higher than 150 K. The forward conduction current I is the sum of two contributions: I=I1+I2, where I1∝V1/2 exp(λV) and I2∝V exp(λ′V). At temperatures lower than 150 K, I1 dominates the current conduction controlled by tunnel-induced impact ionization. The electrons tunnel through a triangular potential barrier at the i-n junction. At temperatures higher than 150 K, I2 dominates as a result of electron thermal excitation, tunneling, and impact ionization. The electron thermal excitation occurs from the conduction band of the n layer to shallow donors of the i layer at the i-n junction. The thermal activation energy for the excitation depends on the Zn concentration in the i layer. The activation energies observed are 100 meV for the yellow LED and 14 meV for the green LED. From these activation energies and the donor binding energy, the Fermi levels of the i layers for the yellow and green diodes are estimated to locate 0.13–0.14 and 0.04–0.05 eV below the conduction band, respectively.

Green light emitting diodes simulate nuclear events

A comparison between a red and a green light emitting diode shows the advantage of the green one in wavelength matching with a photomultiplier. Stability and resolution are tested and show the ability of green diodes to simulate nuclear events.

Properties of GaP light-emitting diodes grown on spinel substrates

Red and green GaP electroluminescent diodes have been successfully fabricated from GaP grown heteroepitaxially on spinel substrates by a vapor phase/liquid phase two-stage process. Current-voltage and light emission characteristics of the diodes are compared with those grown on bulk substrates. Quantum efficiencies up to 0·1 per cent in the red and 0·01 per cent in the green have been obtained.

Optical-coupling efficiency of GaP : N green-light-emitting diodes

Optical-coupling efficiencies of slurry-diced GaP : N green-light-emitting electroluminescent diodes have been measured. Light losses due to absorption in such structures are complicated by the irregular diode geometry, the large refractive index, and the significant energy-dependent optical absorption present in the green spectral region. We show that the optical-coupling efficiency can be represented as a product of the red-emission random-path optical-coupling efficiency and a ratio of the integrated external spectrum to the integrated internal spectrum. Optical-coupling efficiencies of encapsulated green diodes in the range 25 ± 5% have been obtained from the measurements, i.e., three times more light is internally absorbed than is usefully transmitted to the ambient. Unencapsulated diodes have about one-half of this optical-coupling efficiency.

Gamma Irradiation and Annealing Effects in Nitrogen-Doped GaAs1-xPx Green and Yellow Light-Emitting Diodes

Irradiation of GaP (green) and GaAs.1P.9(yellow) light-emitting diodes from a 60Co source results in a decrease in quantum efficiency, peak spectral intensity, brightness and carrier lifetime of the respective devices. Damage constants expressed in terms of absorbed dose are Kιo = 4×10-8 rad(Si)-1 for the green diodes and Kιo = 3×10-8 rads(Si)-1 for the yellow diodes. Following isochronal annealing to 500°C the efficiency, lifetime, and peak spectral intensity recover to 75 percent of their respective initial values. Recovery is noted to start at 250°C.