Relaxation study of AlGaAs cladding layers in InGaAs/GaAs (111)B lasers designed for 1.0-1.1 μm operation

InGaAs/GaAs-based lasers require thick AlGaAs cladding layers to provide optical confinement. Although the lattice mismatch between GaAs and AlGaAs is very low, relaxation may occur due to the thickness requirement for an AlGaAs waveguide of the order of microns. We have studied the relaxation of InGaAs/GaAs lasers with AlGaAs waveguides grown on GaAs (111)B substrates. We have observed by transmission electron microscopy (TEM) that certain AlGaAs layers show a high density of threading dislocations (TDs), whilst other AlGaAs layers remain essentially dislocation free. To explain the experimental results a model based on dislocation multiplication has been developed. TDs in the AlGaAs cladding layers are observed when the critical layer thickness (CLT) for dislocation multiplication has been overcome. Consequently, a design rule based on a modified CLT model for AlGaAs/GaAs (111)B is proposed.     

Gain-bandwidth product optimization of heterostructure avalanche photodiodes

A generalized history-dependent recurrence theory for the time-response analysis is derived for avalanche photodiodes with multilayer, heterojunction multiplication regions. The heterojunction multiplication region considered consists of two layers: a high-bandgap Al/sub 0.6/Ga/sub 0.4/As energy-buildup layer, which serves to heat up the primary electrons, and a GaAs layer, which serves as the primary avalanching layer. The model is used to optimize the gain-bandwidth product (GBP) by appropriate selection of the width of the energy-buildup layer for a given width of the avalanching layer. The enhanced GBP is a direct consequence of the heating of primary electrons in the energy-buildup layer, which results in a reduced first dead space for the carriers that are injected into the avalanche-active GaAs layer. This effect is akin to the initial-energy effect previously shown to enhance the excess-noise factor characteristics in thin avalanche photodiodes (APDs). Calculations show that the GBP optimization is insensitive to the operational gain and the optimized APD also minimizes the excess-noise factor.     

Optical characterization of individual semiconductor nanostructures using a scanning tunneling microscope

By injecting low-energy minority carriers from the tip of a scanning tunneling microscope (STM) and analyzing the light emitted from the tip-sample gap of the STM, it is possible to study the optical and electronic properties of individual semiconductor nanostructures with an extremely high spatial resolution close to the atomic scale. This technique has been applied to investigate the transport properties of hot electrons injected into AlGaAs/GaAs quantum well structures and the optical properties of single self-assembled InAs/AlGaAs quantum dots. The physical principles, usefulness and future expectations of this novel technique are discussed.     

Impact ionization and light emission in AlGaAs/GaAs HEMT's

Impact ionization and light emission phenomena have been studied in AlGaAs/GaAs HEMTs biased at high drain voltages by measuring the gate excess current due to holes generated by impact ionization and by analyzing the energy distribution of the light emitted from devices in the 1.1-3.1 eV energy range. The emitted spectra in this energy range can be divided into three energy regions: (i) around 1.4 eV light emission is dominated by band-to-band recombination between cold electrons and holes in GaAs; (ii) in the energy range from 1.5 to 2.6 eV energy distribution of the emitted photons is approximately Maxwellian; and (iii) beyond 2.6 eV the spectra are markedly distorted due to light absorption in the n⁺ GaAs cap layer. The integrated intensity of photons with energies larger than 1.7 eV is proportional to the product of the drain and gate currents. This suggests recombination of channel electrons with holes generated by impact ionization as the dominant emission mechanism of visible light     

Improved current gain and fT through dopingprofile selection in linearly graded heterojunction bipolar transistors

Analytical and experimental results are used to show that extension of a thin p-doped layer of base doping into the graded-gap region, close to the base, of an n-p-n AlGaAs/GaAs heterojunction bipolar transistor and removing n-type dopant from the rest of the linearly graded AlGaAs region improves current gain β and unity gain cutoff frequency f_T. Current gain is significantly improved by reducing recombination near the metallurgical interface and using the effective electric field from the grading to accelerate electrons as they are injected into the p-base. The doping profile also inhibits the formation of a potential minimum in which electrons can be stored in close proximity to the base. This greatly improves f_T, and does not hamper the current injection or increase the turn-on voltage. Space-charge recombination current is also reduced, due to the carrier density reduction associated with the effective electric field due to the graded gap     

Semiconductor lasers with asymmetric barrier layers: An approach to high temperature stability

A method for enhancing the temperature stability of injection lasers that is based on introducing asymmetric barrier layers on each side of the quantum-confined active region is suggested. The asymmetric barrier layers prevent electrons from escaping from the active region into the part of the waveguide region where holes are injected and prevent holes from escaping into the part of the waveguide region where electrons are injected. Parameters of the layers that allow implementation of the asymmetric-barrier design using pseudomorphic structures grown on GaAs substrates are determined. The calculation of the threshold characteristics of these laser structures demonstrates that suppression of electron-hole recombination outside the active region attained due to the use of asymmetric barrier layers leads to a significant decrease in the threshold current and an increase in the characteristic temperature of this type of lasers.     

A large enhancement of the maximum drift velocity of electrons in the channel of a field-effect heterotransistor

It is shown that the optical-phonon momentum quantization in a GaAs quantum well resulting from the introduction of an InAs quantum-dot barrier layer provides for the elimination of inelastic scattering of electrons by optical phonons and, thus, makes the acceleration of electrons above the saturation drift velocity possible. It is shown experimentally that the maximum drift velocity of electrons in high electric fields in AlGaAs/GaAs heterostructure with InAs quantum-dot barriers introduced into the GaAs quantum well exceeds the saturation drift velocity in bulk GaAs by as much as a factor of 10. Such a rise in the maximum drift velocity of electrons ensures increased maximum current density, transconductance, and cutoff frequency of the heterostructure field-effect transistor with quantum dots.     

Extremely strong and sharp photoluminescence lines from nitrogen atomic-layer-doped GaAs,AlGaAs and AlGaAs/GaAs single quantum wells

We have performed nitrogen atomic-layer doping into GaAs, AlGaAs, and AlGaAs/GaAs single quantum wells using atomic nitrogen cracked by a hot tungsten filament. While the atomic-layer-doped GaAs layers show a series of sharp and strong photoluminescence lines relating to excitons bound to nitrogen atoms at 8K, atomic-layer-doped AlGaAs layers show several broad nitrogen-related lines. For the atomic-layer-doped single quantum well at the center of the GaAs layer, the quantum well luminescence itself disappears and a dominant and sharp luminescence is observed at a longer wavelength. It is found that the As pressure during the atomic-layer doping greatly affects the luminescence characteristics.     

Elimination of drain I/V collapse in MODFETs through the use of thin n-GaAs/AlGaAs superlattice

We report on the elimination of collapse of drain I/V characteristics in modulation-doped field-effect transistors at 77 K by replacing the doped AlGaAs with a thin GaAs/AlGaAs superlattice where only the GaAs is doped. Such thin barriers (10 ?/15 ?) are transparent to the electrons making the electron transfer into the bulk GaAs (undoped) possible. Room-temperature transconductances of 180 mS/mm which increased to 210 mS/mm at 77 K under both dark and light conditions were obtained. Furthermore, the threshold voltage of these devices did not shift appreciably on cooling (+0.12 V), and no noticeable light sensitivity at 77 K was observed for this device structure.     

Analysis of AlGaAs/GaAs solar cell structures by opticalreflectance spectroscopy

The control of the thickness and composition of the GaAs cap and AlGaAs window layers is essential to the fabrication of high-efficiency AlGaAs/GaAs heteroface solar cells. The use of optical reflectance spectroscopy for the analysis of these structures is presented. The calculation of the reflectance of a system of thin films is described along with the computer program, REFIT, for the analysis of GaAs cell structures. Results are presented for structures at different stages in the solar cell fabrication process, and the method is applied to the analysis of the oxidation of high AlAs mole fraction AlGaAs layers     

Fabrication and numerical analysis of AlGaAs/GaAs tandem solarcells with tunnel interconnections

High-efficiency (20.2% at 1 sun, AM 1.5) Al_0.4Ga_0.6As/GaAs tandem solar cells are successfully fabricated by molecular-beam epitaxy. The interconnection between the AlGaAs top cell and the GaAs bottom cell consists of a GaAs tunnel junction sandwiched between AlGaAs layers and provides a high-quality tunnel junction. Numerical analysis suggests that an efficiency of 30% can be realized by increasing the carrier lifetimes of AlGaAs layers to 10^-8 s. An efficiency of 35% is expected to be obtainable by replacing the GaAs tunnel junction with an Al_0.33 Ga_0.67As tunnel junction     

The effects of heavy impurity doping on AlGaAs/GaAs bipolartransistors

The effects of heavy impurity doping on the electrical performance of AlGaAs/GaAs heterojunction bipolar transistors are examined. Electrical measurements of GaAs diodes and transistors demonstrate that the electron current injected into p⁺-GaAs is unexpectedly large. These results provide evidence for a large effective bandgap shrinkage in p⁺-GaAs. The results are presented in a form suitable for device modeling. For the heavy p-type doping commonly used in the base of an n-p-n AlGaAs/GaAs heterojunction bipolar transistor, the effective bandgap shrinkage is comparable in magnitude to the bandgap variation designed into the device by its compositional variation. Two examples demonstrate that such effects must be considered when analyzing or designing such devices     

A field-effect transistor with a negative differential resistance

We report the effect of negative differential resistance (NDR) in the drain circuit of a new type of selectively doped AlGaAs/ GaAs heterojunction transistor. The key new element of our structure is the presence of a subsidiary GaAs conducting layer, separated from the FET channel by an AlGaAs graded barrier. In this work the subsidiary layer is realized by the conducting substrate. The NDR effect arises due to the heating of channel electrons by the source-to-drain field, and the subsequent charge injection over the barrier. This effect is strongly influenced by the gate and substrate voltages. In a floating-substrate arrangement the current-voltage characteristics exhibit memory effects associated with retention of injected charge in the substrate. In this mode, the NDR is seen only at low temperatures with the peak-to-valley ratios in current at 77 K reaching values as high as 30. On the other hand, when the substrate is biased positively, the NDR results from a peculiar effect of dynamical channel depletion by the injected space charge which drifts on the downhill slope of the graded barrier. In this case, the NDR is observed even at room temperature.     

An increase in the electron mobility in the two-barrier AlGaAs/GaAs/AlGaAs heterostructure as a result of introduction of thin InAs barriers for polar optical phonons into the GaAs quantum well

Confinement and localization of optical phonons in narrow phonon wells with thin phonon barriers decreases the rate of electron-phonon scattering by polar optical phonons by a factor of many times. An increase in mobility and drift velocity of electrons is experimentally observed in strong electric fields upon introduction of thin phonon barriers into the AlGaAs/GaAs/AlGaAs quantum well.     

MOCVD生长带有MQB的量子阱激光器材料

用ＭＯＣＶＤ生长发射波长为８０８ｎｍ的ＡＬＧａＡｓ／ＧａＡｓ量子阱激光器材料。通过在激光器材料的波导中加入多量子势垒（ＭＱＢ）层，有效地限制电子在阱内的复合以及高能电子溢出阱外，从而降低了激光器的阈值电流，提高了它的特征温度。增加了ＭＱＢ后，器件的阈值电流密度Ｉ＿（ｔｈ）从原来的４００～６００Ａ／ｃｍ￣２下降到３００～４００Ａ／ｃｍ￣２，特征温度从１６０Ｋ提高到２１０Ｋ。     

Investigation of indium doping and incorporation in AlGaAs/GaAs double-barrier resonant tunnelling structures

In doping and incorporation in the barrier layers of AlGaAs/GaAs double-barrier resonant tunnelling structures (DBRTSs) have been studied. It was found that the peak-to-valley current ratio (PVCR) can be improved by the proper amount of In doping. This is attributed to the improvement in the quality of the AlGaAs barrier layers due to the high surface migration rate of In atoms that reduces group III vacancies. Also pseudomorphic In/sub x/(Al/sub 0.5/Ga/sub 0.5/)/sub 1-x/As/GaAs (x=0.12) strained-layer DBRTSs have been fabricated by incorporating a sufficient amount of In into the AlGaAs barrier layers. PVCRs as high as 27.5 at 77 K have been obtained. This is the first realisation of such DBRTSs with lattice-mismatched quaternary barrier layers.<>     

Reduction of backgating in GaAs/AlGaAs MESFETs by optimisation of active-layer/buffer-layer interface

Backgating measurements made on GaAs MESFETs with abrupt, graded alloy and graded superlattice interface AlGaAs buffer layers were compared to measurements made on conventional GaAs buffer-layer MESFETs. Only the superlattice interface structure showed a reduction in the backgating transconductance (by a factor of 24 compared to the GaAs buffer-layer FET). The lack of reduction in the backgating transconductance for the abrupt and graded alloy interface devices is attributed to traps resulting from GaAs growth on an AlGaAs layer.     

Monte Carlo modeling of femtosecond relaxation processes inAlGaAs/GaAs quantum wells

Results are presented from Monte Carlo simulations of the femtosecond relaxation of photoexcited electrons in AlGaAs/GaAs quantum wells. Two experiments are simulated: in one electrons are initially excited at high energies far from equilibrium, and in other electrons are excited at low energies close to the bottom of the band. The effects of electron-electron, polar optical phonon, and intervalley deformation potential scattering are studied. For comparison, subpicosecond relaxation in bulk GaAs is also discussed     

Time-of-flight experiments on 2D electron gases

Transport of electrons in GaAs/AlGaAs modulation-doped heterostructures is studied by means of the Time-of-Flight technique. By applying a constant background illumination, we are able to distinguish between lateral transport processes and effects of perpendicular transport across the GaAs/AlGaAs interface. It is shown that the initial part of the photocurrent signal is due to lateral transport only. This part of the signal is further investigated as a function of sample length and applied electric field.     

Characterization of AlGaAs microstructure fabricated by AlGaAs/GaAsmicromachining

An AlGaAs/GaAs micromachining technique that is compatible with laser diode fabrication process is described. AlGaAs structural layers and GaAs sacrificial layers are prepared by metal organic vapor phase epitaxy. Reactive dry etching with chlorine is used to fabricate high-aspect structures. Peroxide/ammonium hydroxide solution is used for selective etching of the sacrificial layer. Since the epitaxial layer has low stress, precise undeformed microstructures are obtained. Good compatibility with the LD process makes it possible to integrate microcantilever beams with LD's without degradation of LD characteristics. Microcantilever beams of AlGaAs are characterized by directly measuring stiffness and natural frequencies. A fracture test is also performed on the AlGaAs microcantilever beams. The average fracture stress of AlGaAs is found to be 1 GPa at 1% strain, which shows that the material is strong enough to support the micrometer scale structures