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We demonstrate that ordered microlens arrays with 10 μm diam poly-dimethyl-siloxane lenses attached to glass substrates increase the light output of organic light emitting devices (OLED) by a factor of 1.5 over unlensed substrates. The lenses, which are considerably smaller than, and not aligned to the OLEDs, outcouple light that is emitted outside of the escape cone of the substrate. We show that an electrophosphorescent device based on a fac tris(2-phenylpyridine)Iridium (Ir(ppy)3) doped emitting layer has its external quantum efficiency increased from 9.5% using a flat glass substrate, to 14.5% at low current densities using a substrate with microlenses. No change in the emission spectrum is observed for different viewing angles using the lens arrays.

Vertical stacking of organic light emitting devices (OLEDs) that emit the three primary colors is shown to be a means for achieving efficient and bright full-color displays. In Paper I, we addressed stacked OLED (SOLED) design and fabrication principles to optimize emission colors, operating voltage, and efficiency. Here, we present results on two different (metal-containing and metal-free cathode) SOLED structures that exhibit performance suitable for many full-color display applications. The operating voltages at 10 mA/cm2 (corresponding to video display brightnesses) are 6.8, 8.5, and 12.1 V for the red (R), green (G), and blue (B) elements of the metal-containing SOLED, respectively. The respective subpixel luminous efficiencies are 0.53, 1.44, and 1.52 cd/A, and the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates are (0.72, 0.28), (0.42, 0.56), and (0.20, 0.22). In the high transparency metal-free SOLED, an insulating layer was inserted between the two upper subpixels to allow...

We present a study of optically pumped waveguide and microcavity lasers based on vacuum-deposited thin films of small molecular weight organic semiconductors. Lasing action in waveguide lasers is characterized by high output peak power (50 W), high differential quantum efficiency (70%), low lasing threshold (1 μJ/cm2), and long operational lifetime (>106 laser pulses at 100 times the threshold pump power). Microcavity laser characteristics include 3 W peak output power, 300 μJ/cm2 lasing threshold, and lifetimes of >106 pump laser pulses (operating at 6 times the threshold power). We demonstrate wavelength variability from 460 to 700 nm by changing the composition of the organic films. The confinement of excitations on the dopant molecules leads to quantum dot-like behavior such as high temperature stability of the lasing threshold, output power, and emission wavelength in the temperature range from 0 to 140 °C. The linewidth of laser emission from microcavity structures is found to be 0.2±0.1 A and is tr...

Ultraviolet (UV) optical pumping followed by fluorescence wavelength downconversion of thin film organic light emitting materials deposited directly on the surface of Si p‐n junction diodes is found to be an accurate and rapid means to determine the film internal fluorescence efficiency. By measuring the photoresponse of the Si detectors in the UV, we find that the organic light emitting films of aluminum tris‐(8‐hydroxyquinoline) (Alq3), N,N′‐diphenyl‐N,N′‐bis‐(3‐methylphenyl)‐ 1,1′‐biphenyl‐4,4′‐diamine, and bis‐(8‐hydroxyquinaldine)‐chlorogallium (Gaq2′Cl), have internal fluorescence efficiencies of 0.30±0.05, 0.35±0.03, and 0.36±0.03, respectively. It has also been found that the organic films can be grown to a thickness which optimizes UV light conversion and can, at the same time, serve as antireflection coatings in the visible spectral region, thereby resulting in enhanced Si photodiode sensitivity extending from the UV to the infrared.

In the standard architecture for optical communications receivers there are typically three basic elements, a photodetector, a pre-amplifier, and a filter. If the signal is frequency multiplexed, there must be an additional mixing stage to separate the different incident optical channels. Sensitivity and gain are most critical for these elements since the signal is weakest and most susceptible to noise at the receiver front end. In commercial receivers, the photodetector and mixer are separate from the pre-amplifier and must be integrated by hybrid techniques. Interconnections between the pre-amplifier and the other components frequently require complex matching networks or interconnection schemes which introduce unwanted noise while reducing reliability. We have designed and tested a high sensitivity p-i-n/JFET, monolithically integrated photoreceiver which incorporates a p-i-n photodiode and a voltage tunable transimpedance preamplifier for 1.3-1.55 pm optical fiber communication. This design allows us to monolithically mix an optical signal with an electrical signal for use in applications such as frequency demultiplexing or coherent detection. Comparison of our results with alternative mixer technology indicates good efficiency with low signal distortion.

Using electro-optical pump and probe measurements, we studied optical properties of a thin film of tris-(8-hydroxyquinoline) aluminum (Alq3) doped with DCM2 laser dye under pulsed injection currents as high as 300 A/cm2. Pump-induced transparency and absorption were observed in the spectral ranges of 570–610 nm and 620–705 nm, respectively. We attribute these optical effects to molecular polarons formed under electrical excitation. Polaron absorption dominates the optical properties of electrically pumped Alq3:DCM2 films in the spectral range where optical gain is anticipated (i.e., from 630 to 700 nm). Polaronic effects may present a significant obstacle for realization of organic diode lasers based on this material system.

We report a semitransparent, two-color, stacked organic light-emitting device (SOLED) with high efficiency, low drive voltage, and minimal color distortion. The SOLED emits light from both device surfaces. The external quantum efficiencies of the green and red stacked elements are 1% and 0.4%, respectively, where only the photons emitted from the substrate surface are collected. The drive voltage for the top stack element is decreased to ∼12 V by using a modified indium tin oxide thin film deposition process. Color distortion and angular dependence of the emission spectra are minimal.

We introduce a class of low-reflectivity, high-transparency, nonmetallic cathodes useful for a wide range of electrically active, transparent organic devices. The metal-free cathode employs a thin film of copper phthalocyanine (CuPc) capped with a film of low-power, radio-frequency sputtered indium tin oxide (ITO). The CuPc prevents damage to the underlying organic layers during the ITO sputtering process. We present a model suggesting that damage-induced states at the cathode/organic film interface are responsible for the electron injection properties of the contact. Due to the low contact reflectivity, a non-antireflection-coated, metal-free transparent organic light-emitting device (MF-TOLED) is demonstrated with 85% transmission in the visible, emitting nearly identical amounts of light in the forward and backscattered directions. The MF-TOLED performance is found to be comparable to that of conventional TOLEDs employing a more reflective and absorptive cathode consisting of a semitransparent thin fil...

Photoluminescence of 1.3 μm wavelength strained multiple quantum well InGaAsP/InP laser structures has been used to understand the excess carrier redistribution between the quantum well and waveguide regions at a high level of excitation. A model is developed to describe the experimental results. The model suggests that space charge barriers play a significant role in the electron confinement in quantum wells at the high excitation range typical of laser diode operation.

Results are presented from polymer/molecular organic heterostructure light emitting diodes composed of a layer of the conjugated conducting polymer poly(p‐phenylene vinylene) (PPV), and a layer of fluorescent molecular compound tris(8‐hydroxy) quinoline aluminum (Alq). The external quantum efficiency of these heterostructure LEDs is ∼0.1%, which is over one order of magnitude higher than that of simple PPV LEDs. The electroluminescence (EL) spectra indicate that both materials in the device emit comparable amounts of light. The dependence of the EL spectra on the layer thicknesses and its independence on bias suggest that neutral excitons are formed in the Alq, far from the PPV/Alq interface, and subsequently diffuse into the PPV layer.

We present a systematic analysis of the relationship between the photoluminescence (PL), light emitting device electroluminescence (EL), and conducting properties of a series of metalquinolates, Mq3, where M is a metal (Al, Ga, In, or Sc), and q3 is tris‐(8‐hydroxyquinoline). We compare the solution and thin film PL quantum yields and spectra of each quinolate with the EL quantum efficiencies of organic heterojunction light emitting diodes using the compound as the emitter layer. Our results indicate that, contrary to previous reports, the relative PL yield is not a good indicator of the EL quantum efficiency of a particular material. Specifically, we find that while the PL of Alq3 films is four times that of Gaq3, light emitting devices made from these two materials have comparable electroluminescence quantum efficiencies and long‐term stabilities. Furthermore, the Gaq3 devices have an approximately 50% higher power efficiency than Alq3 structures, suggesting that Gaq3 is a superior emitter material for ...

We investigate the current transport and electroluminescence mechanisms controlling the efficiency of organic heterojunction, green light emitting devices consisting of tris‐(8‐hydroxyquinoline) aluminum (Alq) and a hole transport layer (HTL). From the dependence of the current‐voltage characteristics and luminescence efficiency on layer thickness and HTL composition, we find that current is controlled by trap‐limited transport of electrons injected into Alq. Electroluminescence efficiency, on the other hand, is controlled by hole injection from the HTL into Alq. The luminescence is generated by recombination of electrons localized at a high density trap in the Alq, and holes injected from the HTL. These results provide insight into the fundamental processes which control the operating voltage and luminescence efficiency of vacuum deposited organic light emitters.

We quantitatively study the electric field distributions in Hg0.56Cd0.44Te avalanche photodiodes recently reported for use in the 1.6–2.5 μm wavelength region using a two‐dimensional solution of Poisson’s equation. It is found that, contrary to previous reports, there is no evidence for large uniform avalanche gain in these diodes. Furthermore, to first order, it is shown that the ionization rate ratio of electrons to holes in the diodes demonstrated to date is on the order of unity. Hence, it is not expected that the present diodes will lead to low noise operation superior to Ge or In0.53Ga0.47As/InP avalanche photodetectors.

In previous work, we have successfully developed techniques to fabricate low-loss rib waveguides in a crystalline organic compound, namely, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA). Here, we demonstrate apparently the first optical directional coupler using an index-matching layer in similar crystalline organic semiconductor waveguides. We also observed switching of the guided light between the waveguides in the PTCDA couplers by pumping the surface of one waveguide in the coupling region with 0.514 micrometer wavelength light. A directional coupler typically consists of a pair of parallel channel waveguides with a gap between them. The gap width and the refractive index discontinuity between the guides and the gap are small enough to allow the guided light to couple from one waveguide to the other by the overlapping evanescent tails of the guided modes. However, there is a large refractive index discontinuity of 1.017 for TE modes at the interface between a PTCDA waveguide and an air gap at lambda = 1.06 micrometer. The large index discontinuity causes a small coupling coefficient and a long coupling length. The best way to increase the coupling coefficient between two rib waveguides is to fill the gap with a layer whose index is only slightly lower than the adjacent guides. In our experiments, we employed AZ 1400 photoresist as the index-matching layer material for PTCDA couplers. AZ 1400 photoresist has a relatively high index of 1.61 and low absorption at lambda = 1.06 micrometers. In this case, for a coupler having 0.5 micrometer thick, 2 micrometer wide waveguides and 1 micrometer gap, Kappa approx. - 0.50 mm-1, which is about 40 times greater than that for the coupler without an index-matching layer. Correspondingly, the coupling length is 3.14 mm.

We report on low‐dark‐current IN0.53GA0.47As photodiodes in which generation‐recombination current dominates diode leakage up to as high as 100 V. At higher voltages, tunneling currents become dominant, resulting in the soft breakdown characteristic widely observed in these materials. The dark current versus voltage characteristics have been fit to variations in current of over six orders of magnitude and a temperature range greater than 150 K using a theory which includes generation‐recombination, tunneling, and shunt components.

We report on deep levels in In0.53Ga0.47 As/InP heterostructure avalanche photodiodes grown by liquid‐phase epitaxy. Two electron levels have been identified and studied using both admittance spectroscopy and a static capacitance‐voltage technique. One level, found only in the InP layer, is acceptor‐like with an activation energy of etA = 0.20±0.02 eV and has a density in the bulk of the InP layer of NtA≊ 1015 cm−3. The density of the trap increases sharply at the In0.53Ga0.47 As/InP heterointerface to a value nearly ten times the background carrier concentration in this region. Filling of this trap at low temperatures reduces the conduction band discontinuity at the heterointerface from Δec = 0.19 to 0.03 eV due to repulsion of the bands. The second trap, which is characteristic only of the In0.53Ga0.47 As, has an activation energy of etB = 0.16±0.01 eV, and a density of between 3×1013 and 8×1013 cm−3.

A four-terminal epitaxial p-n-p junction field-effect transistor grown by molecular-beam epitaxy is shown to be an effective one-component gain control element when operated as a four-terminal device. Control of amplifier gain is demonstrated by using the back-gate terminal to adjust the transconductance of a standard three-terminal transistor. The effect of parasitic capacitance on amplifier gain and cutoff frequency is described for a common-source configuration.

We consider, in detail, the potential improvement in receiver sensitivity that can be realized using an avalanche photodiode (APD) rather than the conventional p-i-n diode in long-wavelength optical communications systems. Numerical computations are used to determine optimum gains and receiver sensitivities for several values of ionization coefficient ratios and dark currents. Sensitivities are considered for transmission bit rates of 45 Mb/s, 90 Mb/s, and 274 Mb/s — values characteristic of present long-wavelength systems. We find that general relationships and scaling laws between receiver sensitivity and the other critical parameters can be formulated if the sensitivity is calculated in units relative to the quantum limit. An important result is that the improvement in APD sensitivity depends strongly on dark current, but only weakly on the ionization coefficient ratio. Our calculations are compared with recent results obtained for In0.53Ga0.47As/InP APDs sensitive in the λ = 0.95 μm to 1.6 μm wavelength region. We also include a brief discussion comparing APD sensitivities with those obtained using phototransistors and majority carrier devices.

Heterojunction In0.53Ga0.47As/InP avalanche photodiodes have recently been observed to have a response to long‐wavelength optical pulses which contains both fast and slow components. We present evidence that the slow response is limited by charge pile‐up at the semiconductor heterointerface. We find that the speed of response depends on the degree of compositional grading in the heterointerface region. The response time can be significantly reduced for compositional grading lengths of greater than 600 A depending on the doping and depletion region width in the In0.53Ga0.47As layer.

A self‐aligned all‐In0.53Ga0.47 As junction field‐effect transistor structure grown by molecular beam epitaxy is described, using p+‐InGaAs as the confinement layer for the n channel. Devices have been made to operate in either the depletion mode or the enhancement mode. Transconductances of 90 mS/mm have been observed, with low capacitance (0.7 pF/mm) and low gate leakage.

Depletion-mode junction field-effect transistors (JFET's) with InGaAs p-n junctions grown on compensated Fe:InP or highly resistive In 0.52 Al 0.48 As isolation layers grown on n+-InP substrates have been fabricated using a combination of molecular-beam epitaxy and metalorganic chemical vapor deposition growth techniques. Using a self-aligned gate technology with a 1-µm gate length, devices with high transconductance (80 mS/mm), low leakage current (

The drift of drain‐to‐source current in enhancement mode InP‐based metal‐insulator‐semiconductor field‐effect transistors is analyzed. We simultaneously measure the time dependence of both the drain‐to‐source current (IDS) and the transconductance of these devices when operated in the saturation regime. From the comparison of these long‐term data, we conclude that the observed drift is caused by a variation in threshold voltage (VT), whereas the carrier mobility in the channel appears to be independent of time. We interpret these results in terms of charge trapping in the gate insulator and infer a slow surface state density of ≳1×1011/cm2. In spite of this relatively low density of states, large scale drifts in VT and IDS are observed. Apparently, this is the first report of the long‐term time dependence of the saturation transconductance and its correlation with IDS drift.

InGaAs photodetectors have been configured into linear arrays of 30 x 30 micron photodetectors spaced 50 microns apart. The devices have typical responsivities of 0.9 A/W (86-percent QE) at 1.3 microns and exhibit room temperature dark currents below 100 pA. A 256-element array has been mounted in a Reticon multiplexer and configured into a PAR optical multichannel analyzer to extend spectral response out to 1.7 microns. Individual InGaAs detectors have been fabricated for response out to 2.2 microns with dark current below 1 microA (-1V) and 50-percent QE at room temperature.

We compare the basic detector structures used in receivers for optical communication system applications. The three types of detectors most commonly considered for the direct detection of digital signals are the photoconductor, the p-i-n detector and the avalanche photodiode (APD). A comparison of these structures from a noise (and hence, sensitivity) and bandwidth perspective indicates that reversed biased detectors such as p-i-n photodiodes and APDs afford the greatest advantages, although the latter structure can be extremely difficult to fabricate.

Thermal, electrical, and noise characteristics of a GaAs junction FET are described. Low voltage noise [1.5+/-0.2 nV/(Hz)(1/2) at T =4.2 K] and insensitivity to temperature change in the range 1.3or =Tor =300 K make it suitable for low-temperature applications.