Mark R. Hueschen

Organic electroluminescent devices

Electroluminescence from organic materials has the potential to enable low-cost, full-color flat-panel displays, as well as other emissive products. Some materials have now demonstrated adequate efficiencies (1 to 15 lumens/watt) and lifetimes (>5000 hours) for practical use; however, the factors that govern lifetime remain poorly understood. This article provides a brief review of device principles and applications requirements and focuses on the understanding of reliability issues.

High-power truncated-inverted-pyramid (AlxGa1−x)0.5In0.5P/GaP light-emitting diodes exhibiting >50% external quantum efficiency

A truncated-inverted-pyramid (TIP) chip geometry provides substantial improvement in light extraction efficiency over conventional AlGaInP/GaP chips of the same active junction area (∼0.25 mm2). The TIP geometry decreases the mean photon path-length within the crystal, and thus reduces the effects of internal loss mechanisms. By combining this improved device geometry with high-efficiency multiwell active layers, record-level performance for visible-spectrum light-emitting diodes is achieved. Peak efficiencies exceeding 100 lm/W are demonstrated (100 mA dc, 300 K) for orange-emitting (λp∼610 nm) devices, with a peak luminous flux of 60 lumens (350 mA dc, 300 K). In the red wavelength regime (λp∼650 nm), peak external quantum efficiencies of 55% and 60.9% are measured under direct current and pulsed operation, respectively (100 mA, 300 K).

FORMATION AND GROWTH OF BLACK SPOTS IN ORGANIC LIGHT-EMITTING DIODES

We report electroluminescence (EL) degradation studies of thin‐film organic light‐emitting diodes under ambient conditions. Bilayer organic ITO/TPD/Alq3/Mg/Ag devices were studied via EL and photoluminescence (PL) microscopy. In situ imaging of device luminescing areas and measurement of sample luminance were performed, allowing for a detailed study of black spot formation and luminance reduction under constant voltage stress conditions. Post‐stress devices were further characterized using PL microscopy, and it was found that black spots result from delamination of the metal at the Alq3/Mg interface initiated by pinholes on the cathode, caused by substrate defects.

Pulse-doped AlGaAs/InGaAs pseudomorphic MODFETs

MODFETs have been fabricated using heterojunctions consisting of AlGaAs and pseudomorphic InGaAs, grown on GaAs substrates. The large conduction band discontinuity (about 0.46 eV for 25% In and Al concentration) leads to a 2-D electron density as high as 2.3*10/sup 12/ cm/sup -2/, with electron mobilities of 7000 and 16000 cm/sup 2//V-s at 300 and 77 K, respectively. Such a high electron density in combination with reasonable transport properties leads to MODFETs with exceptional characteristics. Devices with 0.15-0.25- mu m gate length have room-temperature drain currents as high as 600 mA/mm and room-temperature transconductance as high as 500 mS/mm. The f/sub T/ is as high as 98 GHz, as determined by 20-dB/decade extrapolation of microwave data taken to 25 GHz. A comparison of the effect of bias on the total delay through standard and pseudomorphic MODFETs suggests that the excellent microwave performance exhibited by the pseudomorphic device arises from a reduction in parasitic and drain delays and not from a higher electron velocity under the gate. >

Improved microwave performance in transistors based on real space electron transfer

Experimental results on an improved type of transistor based on real space electron transfer are presented. Microwave measurements through 25 GHz show an extrapolated fT of 60 GHz and a measured fMAX of 18 GHz. These gain‐bandwidth products are approximately twice as high as any previously reported for this relatively new class of device. This improvement in performance results from a novel device design which incorporates a doped, pseudomorphic InGaAs channel, a GaAs collector drift region, and a collector‐up structure.

Pulse doped MODFET's

We have designed and fabricated a novel variation of the conventional modulation-doped heterostructure FET, based on growth by molecular beam epitaxy. The doping of the AlGaAs is confined to a thin layer, about 100Å thick, close to the heterointerface. This increases the forward voltage of the Schottky gate, reduces the dependence of threshold voltage on MBE growth parameters, allows the use of high doping levels without excessive gate leakage, and reduces trapping effects associated with silicon incorporation into AlGaAs. Extrinsic transconductances greater than 300 mS/mm and Fts greater than 33 GHz (for a 0.8 µm gate length) have been measured at room temperature.

Polymer thermosetting organic light-emitting devices

We report a systematic study of novel single- and double-layer thermosetting light-emitting devices (LEDs) based on triarytamines for hole transport layer and fluorenes for the emitting and electron transport layer. These devices possess high-thermal stability, high-quantum efficiency, and high-bandgap emission (blue and green). We have fabricated dot matrix displays based on analogs of these materials.

Effect of indirect minima carrier population on the output characteristics of AlGaInP light-emitting diodes

We show that carrier transfer to the indirect X level in the confining layer is responsible for most of the substantial decrease in the efficiency of AlGaInP light-emitting diodes (LEDs) operating at short wavelengths. Carrier transfer to the confining X level was obtained by reducing the separation between the AlGaInP direct Γ minimum and the X levels by varying the Al composition in the active region and by the application of hydrostatic pressure. Carrier transfer to the confining X level appeared as an additional peak in the electroluminescence (EL) and resulted in a significant decrease of the LED efficiency. A simple model of the EL emission that takes into account carrier population in the X minima was found to be in excellent agreement with the measured EL behavior.

IIA-5 Pulse-doped AlGaAs/InGaAs pseudomorphic MODFET's

bottom electron supplying layer. By introducing buried buffer layers with a high potential barrier, we are able to dope the bottom electron supply layer much heavier and hence generate more twodimensional electrons before the onset of a parasitic conducting channel. The structures were grown by MBE in the following sequences on semi-insulating substrates: GaAs buffer/high barrier buffer/n+GaAs/Ino,,5Gao~ssAs/n+-Alo~sGao,7As/n+-GaAs. The high barrier buffer layers were made of either a AlGaAs/GaAs superlattice or a buried atomically planar beryllium-doped P+-GaAs layer. A reference layer with an undoped GaAs buffer layer was also grown for comparison. A high extrinsic DC g, of 408 mS/mm and a full channel current of 610 mA/mm at room temperature are obtained for 1.2-pm gate length FET’s with buried buffer layers while FET’s without the buried high barrier buffer layers could not be pinched off. An fr of 21.5 GHz and an fmax of 80 GHz are extrapolated from measured S-parameter data from 0.5 to 26.5 GHz with a 6dB/octave slope. We have obtained a 2DEG sheet charge density as high as 1.7 X 10”/cm2 before the onset of parasitic conduction in the AlGaAs layers, while Hall data indicates larger densities. Preliminary CW power measurements show 0.45 W/mm with 11db linear gain and 36-percent power-added efficiency at 10 GHz and 0.36 W/mm with 7-dB linear gain and 22-percent efficiency at 18 GHz. More detailed measurements are presently being performed at higher frequencies, and the results will be reported. We have demonstrated the enhancement of 2DEG sheet charge densities while reducing parasitic conduction through electron supplying layers in double heterojunction MODFET’s by introducing buried high electron barrier buffer layers. These structures are capable of extending the operating frequency of multiple heterojunction power MODFET’s while increasing the associated power densities through the improvements in both current density and fmax.

High-current GaAs/AlGaAs MODFETs with f T over 80 GHz

We have designed and fabricated single-well, double-heterojunction, MODFETs which show up to 80% larger drain current, and up to 30% higher fT, than a good single-heterostructure MODFET. These improved electrical properties result from a substantial increase in the two-dimensional electron density, achieved by augmenting a pulse-doped (1) MODFET with a second doping pulse buried in an AlGaAs/GaAs superlattice below the 2D channel. In this study, we varied the sheet dose of Si donors in the buried doping pulse from 0 to 2 × 1012cm-2. The resulting increase in 2DEG sheet electron density allowed us to achieve maximum drain currents greater than 600 mA/mm for the buried pulses with highest doping, and values of fTas high as 82 GHz for a buried pulse with lower doping.