Hopping conduction and its photoquenching in molecular beam epitaxial GaAs grown at low temperatures

As the growth temperature of molecular beam epitaxial GaAs is increased from 250 to 400°C, the dominant conduction changes from hopping conduction to band conduction with a donor activation energy of 0.65 eV. A 300°C grown layer is especially interesting because each conduction mechanism is dominant in a particular temperature range, hopping below 300K and band conduction above. Below 140K, the hopping conduction is greatly diminished (quenched) by irradiation with either infrared (hv≤1.12 eV) or 1.46 eV light, but then recovers above 140K with exactly the same thermal kinetics as are found for the famous EL2. Thus, the 0.65 eV donor, which is responsible for both the hopping and band conduction, is very similar to EL2, but not identical because of the different activation energy (0.65 eV vs 0.75 eV for EL2).     

Hot-electron velocity characteristics at AlGaAs/GaAs heterostructures

Two dimensional electron gas velocity characteristics under stationary and nonstationary conditions at a modulation-doped AlGaAs/GaAs heterostructures are investigated based on Monte Carlo simulation. In contrast to three-dimensional electron gas, a steeper increase in velocity is expected for two-dimensional electron gas closely dependent on the low-field high mobility. These calculations suggest that the high mobility can be effectively utilized in a high-speed logic application of such heterostructure devices.     

GaAs/AlGaAs quantum well infrared photodetector with low noise

A novel kind of multi-quantum well infrared photodetector（QWlP） is presented. In the new structure device,a p-type contact layer has been grown on the top of the conventional structure of QWlP,then a small tunneling current is instead of the large compensatory current,which made the device low dark current and low noise characteristics. The measured result of dark current is consistent with the calculated result, and the noise of the new structure QWIP is decreased to one third of the conventional QWlP.     

Very low threshold current ridge-waveguide AlGaAs/GaAs single-quantum-well lasers

Ridged-waveguide AlGaAs/GaAs single-quantum-well lasers were fabricated from a molecular-beam-grown GRIN-SCH wafer in which a superlattice buffer layer was introduced. Fabricated diodes exhibited excellent lasing characteristics including a very low threshold current of 5 mA with a T0 value as high as 160 K.     

Degradation and recovery of AlGaAs/GaAs p-HEMT irradiated by high-energy particle

Results of an extensive study on the irradiation damage and its recovery behavior resulting from thermal annealing in AlGaAs/GaAs pseudomorphic high electron mobility transistors (HEMTs) subjected to a 220-MeV carbon, 1-MeV electrons and 1-MeV fast neutrons are presented. The drain current and effective mobility decrease after irradiation, while the threshold voltage increases in positive direction. The decrease of the drain current and mobility is thought to be due to the scattering of channel electrons with the induced lattice defects and also to the decrease of the electron density in the two dimensional electron gas region. Isochronal thermal annealing shows that the device performance degraded by the irradiation recovers. The decreased drain current for output characteristics recovers by 75% of pre-rad value after 300°C thermal annealing for AlGaAs HEMTs irradiated by carbon particles with a fluence of 1×10¹² cm⁻². The influence of the materials and radiation source on the degradation is also discussed with respect to the nonionizing energy loss. Those are mainly attributed to the difference of particle mass and the probability of nuclear collision for the formation of lattice defect in Si-doped AlGaAs donor layer. A comparison is also made with results obtained on irradiated InGaP/InGaAs p-HEMTs in order to investigate the effect of the constituent atom. The damage coefficient of AlGaAs HEMTs is also about one order greater than that of InGaP HEMTs for the same radiation source. The materials and radiation source dependence of performance degradation is mainly thought to be attributed to the difference of mass and the possibility of nuclear collision for the formation of lattice defects in Si-doped donor layer.     

GaAs/AlGaAs and InGaAs/AlGaAs MODFET inverter simulations

Although MODFET's have exhibited the fastest switching speed for any digital circuit technology, there is as yet no clear consensus on optimal inverter design rules. We therefore have developed a comprehensive MODFET device model that accurately accounts for such high gate bias effects as transconductance degradation and increased gate capacitance. The device model, which agrees with experimental devices fabricated in this laboratory, is used in the simulation of direct-coupled FET logic (DCFL) inverters with saturated resistor loads. Based on simulation results, the importance of large driver threshold voltage not only for small propagation delay times but for wide logic swings and noise margins is demonstrated. Furthermore, minimum delay times are found to occur at small supply voltages as seen experimentally. Both of these results are attributed to the reduction of detrimental high gate bias effects. The major effect of reducing the gate length on delay time is to decrease the load capacitance of the gate. Using 0.25-µm gates, delay times of 5 and 3.6 ps at 300 and 77 K, respectively, are predicted. Finally, the recently introduced In-GaAs/AlGaAs MODFET's are shown to have switching speeds superior to those of conventional GaAs/AlGaAs MODFET's.     

AlGaAs/InGaAs/GaAs quantum-well power MISFET at millimeter-wavefrequencies

An AlGaAs/InGaAs/GaAs quantum-well MISFET developed for power operation at millimeter-wave frequencies is described. The InGaAs channel is heavily doped to increase the sheet carrier density, resulting in a maximum current density of 700 mA/mm with a transconductance of 480 mS/mm. The 0.25-μm×50-μm device delivers a power density of 0.76 W/mm with 3.6-dB gain and 19% power-added efficiency at 60 GHz. At 5.2 dB gain, the power density is 0.55 W/mm. A similar device built on an undoped InGaAs channel had much poorer power performance and no speed advantage     

Defects in molecular beam epitaxial GaAs grown at low temperatures

We have utilized a variable energy positron beam and infrared transmission spectroscopy to study defects in GaAs epilayers grown at low temperatures (LT-GaAs) by molecular beam epitaxy. We have measured the Doppler broadening of the positron-electron annihilation gamma ray spectra as a function of positron implantation energy. From these measurements, we have obtained results for the depth profiles of Ga monovacancies in unannealed LT-GaAs and Ga monovacancies and arsenic cluster related defects in annealed LT-GaAs. We have also studied the effects of the Si impurities in annealed LT-GaAs. The infrared transmission measurements on unannealed LT-GaAs furnish a broad defect band, related to As antisites, centered at 0.370 eV below the conduction band.     

High-power InAlGaAs/GaAs and AlGaAs/GaAs semiconductor laser arrays emitting at 808 nm

Molecular beam epitaxy (MBE) growth, device fabrication, and reliable operation of high-power InAlGaAs/GaAs and GaAlAs/GaAs laser arrays are described. Both InAlGaAs/GaAs and AlGaAs/GaAs laser arrays reached maximum continuous wave output powers of 40 W at room temperature. The external quantum efficiency was 50% and 45% for the InAlGaAs/GaAs and AlGaAs/GaAs laser arrays, respectively. Threshold current density for InAlGaAs/GaAs and AlGaAs/GaAs lasers was 303 A/cm/sup 2/ and 379 A/cm/sup 2/, respectively. While the current of AlGaAs laser arrays went up significantly after 1000 h of operation at a constant power of 40 W, InAlGaAs laser arrays had an increase in the injection current of less than 4% after 3000 h at 40 W.     

Transport and noise in GaAs/AlGaAs heterojunction bipolartransistors. II. Noise and gain at low frequencies

For pt.I see ibid., vol.36, no.6, p.1015-19 (June 1989). Low-frequency noise measured in high-current-gain GaAs/AlGaAs double-heterojunction transistors is shown to originate from noise processes in the base. High base resistance associated with high current gain causes Johnson noise to be dominant at high frequencies and low bias, while at low frequencies interface 1/f and generation-recombination noise exceed Johnson noise over a bandwidth that increases with base current. At high forward bias, this 1/f noise saturates, but by then can extend over megahertz bandwidths. A low-frequency decrease in devices gain and an excess base voltage noise in this saturation region is explained by punchthrough and the mechanism for high gain in these devices     

Effects of low temperatures and intensities on GaAs and GaAs/Gesolar cells

GaAs and GaAs/Ge solar cells grown by metalorganic chemical vapor deposition (MOCVD) were characterized at very low temperature (-185°C) and solar intensity (0.25 suns) to simulate the cell behavior in a severe interplanetary environment. A comparison is also made with GaAs cells grown with the liquid-phase-epitaxy (LPE) technique. The analysis carried out from -185 to +50°C shows, in particular, different behaviors for GaAs/Ge cells with active and passive Ge substrates; the GaAs/Ge passive cell behaves as a GaAs on GaAs cell, indicating that from the thermal and optical point of view, Ge acts only as a mechanical support. The GaAs cell with an active Ga substrate is affected by a notch in the I-V curves, which is more evident at low temperatures but reduces at low intensities. The GaAs/GaAs MOCVD cell shows the best performance at low temperature and intensity with a conversion efficiency of 27.2%     

Electron mobility in a AlGaAs/GaAs/AlGaAs quantum well

An oscillatory dependence of the electron mobility on the quantum well (QW) thickness in a AlGaAs/GaAs/AlGaAs heterostructure with double-sided modulation doping has been observed experimentally. A steep decrease in mobility with increasing electron concentration in the QW is established. The conditions for an increase in mobility on introducing a thin barrier into the QW are determined. The first experimental observation of an increase in mobility by a factor of 1.3 in a QW of thickness L=26 nm upon introducing a thin (1–1.5 nm) AlAs barrier is reported.     

Self-aligned AlGaAs/GaAs HBT with low emitter resistance utilizingInGaAs cap layer

A self-aligned AlGaAs/GaAs heterojunction bipolar transistor (HBT) with an InGaAs emitter cap layer that has very low emitter resistance is described. In this structure, a nonalloyed emitter contact allows the emitter and base electrodes to be formed simultaneously and in a self-aligned manner. The reduction of emitter resistance greatly improves the HBT's transconductance and cutoff frequency. In fabricated devices with emitter dimensions of 2 μm×5 μm, a transconductance-per-unit-area of 16 mS/μm² and a cutoff frequency of 80 GHz were achieved. To investigate high-speed performance, a 21-stage ECL ring oscillator was fabricated using these devices. Propagation delay times as low as 5.5 ps/gate were obtained, demonstrating the effectiveness of this structure     

Properties of 2D electron gas in AlGaAs/GaAs heterojunctions with thin AlGaAs layers

Transport properties of a 2D electron gas located at a short distance from the surface (15–32.5 nm) in an AlGaAs/GaAs heterojunction were studied. A pronounced effect of the surface on the behavior of the conductivity of the 2D electron gas, as well as metallic gate screening of the scattering of two dimensional electrons by charged centers, were observed.     

Extremely low loss 4*4 GaAs/AlGaAs optical matrix switch

A 4*4 GaAs/AlGaAs optical matrix switch with an extremely low loss of 1.6 dB has been developed. This low loss switch was achieved by layer structure modification, to reduce free-carrier absorption, reduction of plasma damage while dry etching, and an improvement in the crystal quality.<>     

Ion beam milling-induced damage in AlGaAs/GaAs/AlGaAs single quantum well

Ion beam milling-induced damage in a 500 AA AlGaAs/40 AA GaAs/500 AA AlGaAs single quantum well structure was investigated using low temperature cathodoluminescence spectroscopy. The ion beam energy (500-1500 eV) dependence of luminescence intensity indicated that minimum damage is introduced at a beam energy of 500 eV. Most (80-85%) of the original luminescence intensity was recovered on annealing at 400 degrees C for 5 min.<>     

Effect of dopants on arsenic precipitation in GaAs deposited at low temperatures

High resolution x-ray diffraction using synchrotron radiation was used to characterize GaAs grown by MBE at low temperatures (LT-GaAs) LT-GaAs grown at 225°C is nonstoichiometric and exhibits a 0.15% lattice expansion along the growth direction. Annealing LT-GaAs results in arsenic clusters with a well-defined orientation relationship with the GaAs matrix and a relaxation of the LT-GaAs lattice. The arsenic precipitation corresponds to a classical case of diffsion controlled nucleation and growth followed by coarsening. While the rates of growth and coarsening in the n-doped and the p-doped samples are observed to be identical, the effects of the superlattice seem to accelerate the precipitation kinetics in the p-n superlattice sample. The enhanced coarsening in the p-n superlattice sample is consistent with a previously propsed model involving interaction between charged precipitate and arsenic defects.     

Fabrication of 2-D photonic bandgap structures in GaAs/AlGaAs

The authors report the first realisation of a two-dimensional photonic bandgap structure with a period of λ₀/2n in the GaAs/AlGaAs material system. The structure consists of a honeycomb lattice with a wall thickness of ~30 nm and a period of 160 nm. It was found that, to realise patterns of such small size and periodicity, it is crucial to control the shape of the exposed features