High Tg hole transport polymers for the fabrication ofbright and efficient organic light-emitting devices with an air-stablecathode

An organic electroluminescent device with a luminous efficiency of 20 Im/W, at 14 cd/m², and an external quantum efficiency of 4.6% has been fabricated using a high T_g hole transport polymer, a small molecule emission layer, and a LiF/Al cathode. The device quantum efficiency can be increased by tuning the ionization potential of the hole-transport moieties. When tested under pulsed voltage mode, in air at room temperature, and without any encapsulation, the device showed a high peak brightness of 4.4×10⁶ cd/m² at 100 A/cm² and an efficiency of 4.4 cd/A     

High-performance, 0.1 μm InAlAs/InGaAs high electron mobilitytransistors on GaAs

This letter describes the material characterization and device test of InAlAs/InGaAs high electron mobility transistors (HEMTs) grown on GaAs substrates with indium compositions and performance comparable to InP-based devices. This technology demonstrates the potential for lowered production cost of very high performance devices. The transistors were fabricated from material with room temperature channel electron mobilities and carrier concentrations of μ=10000 cm² /Vs, n=3.2×10¹² cm^-2 (In=53%) and μ=11800 cm²/Vs, n=2.8×10¹² cm^-2 (In=60%). A series of In=53%, 0.1×100 μm² and 0.1×50 μm² devices demonstrated extrinsic transconductance values greater than 1 S/mm with the best device reaching 1.074 S/mm. High-frequency testing of 0.1×50 μm² discrete HEMT's up to 40 GHz and fitting of a small signal equivalent circuit yielded an intrinsic transconductance (g_m,i) of 1.67 S/mm, with unity current gain frequency (f_T) of 150 GHz and a maximum frequency of oscillation (f_max) of 330 GHz. Transistors with In=60% exhibited an extrinsic g_m of 1.7 S/mm, which is the highest reported value for a GaAs based device     

Dynamic characterisation of Si/SiGe power HBTs

Si/SiGe power heterojunction bipolar transistors (HBTs) grown by MBE were dynamically characterised in the common-base configuration. At an emitter current density of 1.1×10⁵ A/cm², a maximum frequency of oscillation of 49 GHz was observed. At 10 GHz a maximum unilateral gain of 14 dB is available, and a CW output power of 1.3 W/mm for a device with 10 parallel emitter-fingers of 1×10 μm² each was predicted, from CW measurements     

Enhancing the Near-Infrared Spectral Response of Silicon Optoelectronic Devices via Up-Conversion

A silicon-based optoelectronic device that exhibits an enhanced response to subbandgap light is described. The device structure consists of a bifacial silicon solar cell with an up- converting (UC) layer attached to the rear. Erbium-doped sodium yttrium fluoride (NaY_0.8F₄ : Er_0.2 ³⁺) phosphors are the optically active centers responsible for the UC luminescence. The unoptimized device is demonstrated to respond effectively to wavelengths (lambda) in the range of 1480-1580 nm with an external quantum efficiency (EQE) of 3.4% occurring at 1523 nm at an illumination intensity of 2.4 W/cm² (EQE = 1.4 times 10^-2 cm²/W). An analysis of the optical losses reveals that the luminescence quantum efficiency (LQE) of the device is 16.7% at 2.4 W/cm² of 1523-nm excitation (LQE = 7.0 times 10^-2 cm²/W), while further potential device improvements indicate that an EQE of 14.0% (5.8 times 10^-2 cm²/W) could be realistically achieved.     

27.5-percent silicon concentrator solar cells

Recent advances in silicon solar cells using the backside point-contact configuration have been extended resulting in 27.5-percent efficiencies at 10 W/cm²(100 suns, 24°C), making these the most efficient solar cells reported to date. The one-sun efficiencies under an AM1.5 spectrum normalized to 100 mW/cm²are 22 percent at 24°C based on the design area of the concentrator cell. The improvements reported here are largely due to the incorporation of optical light trapping to enhance the absorption of weakly absorbed near bandgap light. These results approach the projected efficiencies for a mature technology which are 23-24 percent at one sun and 29 percent in the 100-350-sun (10-35 W/ cm²) range.     

Microchannel heat sinks for two-dimensional high-power-densitydiode laser arrays

The operation of a two-dimensional GaInAsP/InP diode laser array with CW power dissipation up to 500 W/cm² into a Si microchannel heat sink is discussed. The approximately 1×4-mm² laser array was used to characterize the heat sink, and the value of 0.040°C cm²/W was obtained for the thermal resistance per unit area. The extrapolated value for a 1-cm² heated area is 0.070°C cm²/W     

Power performance of InP-based single and double heterojunctionbipolar transistors

The microwave and power performance of fabricated InP-based single and double heterojunction bipolar transistors (HBTs) is presented. The single heterojunction bipolar transistors (SHBTs), which had a 5000 Å InGaAs collector, had BV_CEO of 7.2 V and J_Cmax of 2×10⁵ A/cm². The resulting HBTs with 2×10 μm² emitters produced up to 1.1 mW/μm2 at 8 GHz with efficiencies over 30%. Double heterojunction bipolar transistors (DHBTs) with a 3000-Å InP collector had a BV_CEO of 9 V and J_{c max} of 1.1×10⁵ A/cm², resulting in power densities up to 1.9 mW/μm² at 8 GHz and a peak efficiency of 46%. Similar DHBTs with a 6000 Å InP collector had a higher BV_CEO of 18 V, but the J _{c max} decreased to 0.4×10⁵ A/cm² due to current blocking at the base-collector junction. Although the 6000 Å InP collector provided higher f_max and gain than the 3000 Å collector, the lower J_{c max} reduced its maximum power density below that of the SHBT wafer. The impact on power performance of various device characteristics, such as knee voltage, breakdown voltage, and maximum current density, are analyzed and discussed     

Multichip module photovoltaic miniarrays

In this work a novel concept of photovoltaic mini array assembly is described based on a flip-chip multichip module technology. A number of arrays composed of 15 series-connected 2 mm² c-Si photovoltaic cells have been fabricated, achieving a packaging density of 40 cells/cm². Measurements of the dark I-V characteristic confirmed that the resistive losses of the approach are small and do not degrade the electrical characteristics of the array. Nearly 0.5 mW/cm ² delivered density power, and 7 V in open circuit conditions were typically measured under normalized solar spectrum (AM1.5 100 mW/cm ²), and around 6.3 mW/cm² and 8.5 V using a 150 W commercial lamp placed at 5 cm distance from the panel     

A Practical Implementation of Silicon Microchannel Coolers for High Power Chips

This paper describes a practical implementation of a single-phase Si microchannel cooler designed for cooling very high power chips such as microprocessors. Through the use of multiple heat exchanger zones and optimized cooler fin designs, a unit thermal resistance 10.5 C-mm² /W from the cooler surface to the inlet water was demonstrated with a fluid pressure drop of <35kPa. Further, cooling of a thermal test chip with a microchannel cooler bonded to it packaged in a single chip module was also demonstrated for a chip power density greater than 300W/cm². Coolers of this design should be able to cool chips with average power densities of 400W/cm² or more     

Low-resistance bandgap-engineeredW/Si1-xGex/Si contacts

Bandgap-engineered W/Si_1-xGe_x/Si junctions (p⁺ and n⁺) with ultra-low contact resistivity and low leakage have been fabricated and characterized. The junctions are formed via outdiffusion from a selectively deposited Si_0.7Ge _0.3 layer which is implanted and annealed using RTA. The Si _1-xGe_x layer can then be selectively thinned using NH₄OH/H₂O₂/H₂O at 75°C with little change in characteristics or left as-deposited. Leakage currents were better than 1.6×10^-9 A/cm² (areal), 7.45×10^-12 A/cm (peripheral) for p⁺/n and 3.5×10^-10 A/cm² (peripheral) for n⁺/p. W contacts were formed using selective LPCVD on Si_1-xGe_x. A specific contact resistivity of better than 3.2×10^-8 Ω cm² for p ⁺/n and 2.2×10^-8 Ω cm² for n⁺/p is demonstrated-an order of magnitude n⁺ better than current TiSi₂ technology. W/Si_1-xGe _x/Si junctions show great potential for ULSI applications     

4H-SiC avalanche photodiode with multistep junction extensiontermination

4H-SiC avalanche photodiodes (APDs) are fabricated with a multistep junction termination extension. The leakage current density has been dramatically reduced to as low as 1 μA/cm² and photo-responsivity up to 10⁵ A/W has been achieved. The 4H-SiC APDs can run very stably at power densities up to 10⁴ W/cm²     

High average and high peak brightness slab laser

A high average and high peak brightness Nd:YAG MOPA laser system composed of a laser-diode-pumped Nd:YAG master oscillator, flash-lamp-pumped slab power amplifiers and a phase conjugated mirror was developed. The system demonstrates an average output power of 235 W at a repetition rate of 320 Hz and a peak power of 30 MW at a pulse duration of 24 ns with M²=1.5. Both an average brightness of 7×10⁹ W/cm²·sr and a peak brightness of 1×10¹⁵ W/cm2·sr are achieved simultaneously. The system design rules that we confirmed suggest that by replacing lamp pumping in the amplifier with laser-diode pumping, an average output power of ~1 kW can be obtained at ~1 kHz with a higher average brightness of ~3×10¹⁰ W/cm²·sr and a higher peak brightness of ~3×10¹⁵ W/cm²·sr     

Extremely low-leakage GaAs P-i-N junctions and memory capacitorsgrown by atomic layer epitaxy

Leakage-limited P-i-N-i-P charge storage capacitors demonstrate a 1/e storage time of over 30 min at room temperature, corresponding to a current density of less than 50 pA/cm² at 1-V reverse bias for a 160×140-μm² capacitor. These storage times are comparable to those of the best MBE-grown structures reported to date. For the diodes tested, which range in size from 4×10^-4 to 4.9×10^-5 cm², leakage is dominated by generation around the etched diode perimeter. The relatively small bulk generation current is evidence of the high quality of the atomic layer epitaxy (ALE)-grown junctions     

Performance enhancement of GaInP/GaAs heterojunction bipolarphototransistors using dc base bias

GaInP-GaAs heterojunction bipolar phototransistors grown by metal organic vapor phase epitaxy (MOVPE) and operated with frontside optical injection through the emitter are reported with high optical gain (<88) and record high frequency performance (28 GHz). Heteropassivation of the extrinsic base surface is employed using a depleted GaInP emitter layer between the nonself-aligned base contact and the emitter mesa. The phototransistor's performance is shown to improve with increasing dc base bias in agreement with predictions of a recently reported Gummel-Poon model. Experimental results are reported for devices with optical active areas of 10×10 μm², 20×20 μm², and 30×30 μm², with peak measured cutoff frequencies of 28.5, 23.1, and 18.5 GHz, respectively, obtained at collector current densities between 2×10 ³ and 6×10³ A/cm²     

Fully-planar AlGaAs/GaAs HBT's achieved by selective CBE regrowth

This paper describes a novel fully planar AlGaAs/GaAs heterojunction bipolar transistor (HBT) technology using selective chemical beam epitaxy (CBE). Planarization is achieved by a selective regrowth of the base and collector contact layers. This process allows the simultaneous metallization of the emitter, base and collector on top of the device. For the devices with an emitter-base junction area of 2×6 μm² and a base-collector junction area of 14×6 μm², a current gain cut off frequency of 50 GHz and a maximum oscillation frequency of 30 GHz are achieved. The common emitter current gain h_FE is 25 for a collector current density J_c of 2×10⁴ A/cm²     

Low threshold voltage vertical-cavity lasers fabricated byselective oxidation

Novel vertical-cavity surface emitting lasers fabricated using selective oxidation to form a current aperture under a top monolithic distributed Bragg reflector mirror are reported. Large cross-sectional area lasers (259 μm²) exhibit threshold current densities of 150 A/cm² per quantum well and record low threshold voltage of 1.33 V. Smaller lasers (36 μm²) possess threshold currents of 900 μA with maximum output powers greater than 1 mW. The record performance of these oxidised vertical-cavity lasers arises from the low mirror series resistance and very efficient current injection into the active region     

Saturable absorbers based on impurity and defect centers incrystals

Saturation of near-infrared absorption and transmission dynamics are investigated in tetravalent-chromium-doped Gd₃Sc₂ Ga₃O₁₂, Gd₃Sc₂Al₃ O₁₂, and Mg₂SiO₄ crystals, as well as in reduced SrTiO₃ using 20 ps 1.08 μm laser pulses. An absorption cross section of (5±0.5)×10^-18 cm² in garnets and (2.3±0.3)×10^-18 cm² in forsterite is estimated for the ³A ₂-³T₂ transition of tetrahedral Cr⁴⁺. Q-switched and ultra-short pulses are realized in neodymium lasers using chromium-doped crystals as the saturable absorbers. Saturation of free-carrier absorption with ultra-short relaxation time is observed in SrTiO₃ at 10⁸-10 ¹⁰ W/cm² pump intensities, while at 10¹⁰-10¹¹ W/cm² three-photon interband transitions predominate. The free-carrier absorption cross section is estimated to be (2.7±0.3)×10^-18 cm²     

Silicon point contact concentrator solar cells

Experimental results are presented for thin high resistivity concentrator silicon solar cells which use a back-side point-contact geometry. Cells of 130 and 233 µm thickness were fabricated and characterized. The thin cells were found to have efficiencies greater than 22 percent for incident solar intensities of 3 to 30 W/cm²(30-300 "suns"). Efficiency peaked at 23 percent at 11 W/cm²measured at 22-25°C. Strategies for obtaining higher efficiencies with this solar cell design are discussed.     

Picosecond photorefractive and free-carrier transient energytransfer in GaAs at 1 μm

The strength, formation, and decay of photorefractive and free-carrier gratings written in GaAs by 43-ps pulses at a wavelength of 1 μm are investigated using picosecond-time-resolved two-beam coupling, transient grating, and degenerate-four-wave-mixing techniques. Photorefractive weak-beam gains of a few percent are measured at fluences of a few pJ/μm² (0.1 mJ/cm²), and gain from transient energy transfer is observed at fluences larger than ~10 mJ/cm² in the beam-coupling experiments. The roles of saturation and two-photon absorption in determining the final electron, hole, and ionized-donor populations and the roles of drift and diffusion in determining the quasi-steady-state photorefractive and free-carrier index modulations are discussed     

Large purely refractive nonlinear index of single crystal P-toluenesulphonate (PTS) at 1600 nm

Z-scan measurements at 1600 nm on single-crystal PTS (p-toluene sulfonate) with single, 65 ps pulses gave a complex nonlinear refractive index coefficient of n₂=2.2(±0.3)×10^-12 cm²/W at 1 GW/cm² and α₂<0.5 cm/GW. This is the first highly nonlinear, organic material to satisfy the conditions imposed by the figures of merit