Recombination dynamics in GaAs/AlxGa1- xAs quantum well structures

Time-resolved and excitation-dependent photoluminescence of GaAs/AlxGa1-xAs quantum well structures reveal that recombination takes place between free carriers, not excitons, at room temperature for carrier densities at and above the mid -10¹⁶cm^-3level. Other samples show trapping and release of carriers from traps, evidence of dynamic Shockley, Hall, and Read recombination for optically active traps. The traps can be saturated to a large extent. Further studies show that they are associated with interfaces between different materials and that they become active at a temperature around 150 K. Results from all samples indicate that the bimolecular radiative recombination coefficientBfor quantum wells is no larger than the value ofBfor bulk GaAs, and may in fact be smaller. This is one of the first studies of time-resolved luminescence of impurities in quantum well structures. Impurity decays at low temperatures are found to be quite slow. A spectral line which appeared to be longitudinal optical phonon-shifted emission is shown to be due to an acceptor impurity.     

Dispersion of the refractive index of GaAs and AlxGa1-xAs

The real part of the complex refractive index near the fundamental absorption edge is calculated for the ternary compound AlxGa1-xAs,0 leq x leq 0.3, as a function of frequency. An analytical expression fornis given which is derived from a quantum mechanical calculation of the dielectric constant of a semiconductor assuming the band structure of the Kane theory. The expression obtained is a function of bandgap energy, effective electron and heavy hole masses, the spin orbit splitting energy, the lattice constant, and the carrier concentration for n-type or p-type materials. The refractive index at the absorption edge is found as a function of the material parameters above. This enables one to express theoretical results in terms of basic material parameters only, with no adjustable constants. Comparison of theory with available experimental data is given for various reported values of the bandgap energy and effective masses as functions of mole fractionx.     

Doping effects and compositional grading in AlxGa1-xAs/GaAs heterojunction bipolar transistors

First-order analytical calculations were made for the energy-band diagrams for n-AlxGa1-xAs/p-GaAs heterojunctions for x = 0.15, 0.3, and 0.5 employing different compositional gradings and doping densities specifically for heterojunction-bipolar-transistor (HBT)applications. In the calculations most recently determined, conduction-band discontinuity ΔECof 65 percent of the bandgap difference ΔEgbetween the AlxGa1-xAs and GaAs, and the donor activation energies in n-AlxGa1-xAs of 60 and 160 meV for x = 0.3 and 0.5, respectively, were used. The results show that the position of the heterojunction spike barrier, and the depth and width of the notch in the conduction-band edge for a compositionally abrupt heterointerface depend on the respective doping densities on the p and n sides of the heterojunction. Also, for an abrupt heterointerface the difference in barrier heights for electron and hole injections varies between ΔEgand ΔEV(the valence-band discontinuity), depending on the doping densities and the applied bias, and is not necessarily the generally accepted value of ΔEV. Analytical expressions and curves were obtained to estimate the minimum compositional grading L for eliminating the spike barrier and the notch as a function of the doping densities and the applied bias.     

Self-focusing effects in pulsating AlxGa1-xAs double-heterostructure lasers

Using proton bombarded stripe geometry lasers which emit intense optical pulses, we have measured the width of the optical beam in the plane of the junction as a function of time during the pulse. The width of the beam is qualitatively proportional to the change in the carrier density. The width increases during the quiescent period between pulses where the carrier density increases by current injection and decreases during the emission of the pulse. For one laser studied in considerable detail, the full width at half intensity decreases from 9 μm at the start of the pulse to 7.2 μm at the end of the pulse. The reduction in the width results from the self-focusing of the beam. It is due to an increase in the refractive index and the decrease in the gain distributions near the center of the stripe. The reduction in the beamwidth concentrates the mode to a region of sufficiently higher average gain to compensate for the reduction in spatial gain distribution. The self-focusing acts to reduce the damping of the relaxation oscillations, and thus enhances the effect of other nonlinearities such as saturable absorption in causing pulsations. The thermal induced refractive index distribution across the stripe is shown to play a crucial role in the gain instability caused by self-focusing.     

Spectral broadening of pulsating AlxGa1-xAs double heterostructure lasers

The longitudinal mode spectrum of stripe geometry double heterostructure (DH) lasers exhibits a spectral broadening of ∼ 0.8 Å when the laser emits pulsations. The broadening results from a chirp to longer wavelengths which occurs during the ∼ 0.4 ns long pulse. The chirp is due to an increase of the refractive index resulting from the reduction in the carrier concentration during the pulse. Both plasma and band to band interactions affect the index change with the latter mechanism giving the dominant contribution. A comparison of the wavelength shift obtained during the pulse with the current dependent shift of the longitudinal modes at currents well below threshold confirms that the change in carrier concentration is responsible for the chirp.     

Investigation of parallel conduction in GaAs/ AlxGa1-xAs modulation-doped structures in the quantum limit

We present a detailed study of the transport in GaAs/ AlxGa1-xAs modulation-doped structures in the low field and high magnetic field quantum limit for varying amounts of parallel conduction in the AlGaAs region. We observe the apparent breakdown of quantum Hall effect behavior due to low mobility carriers in the parallel channel. The onset of conduction through the parallel channel by quantum transport measurments has been observed, along with a non-linear dose dependence due to photoexcitation.     

Electrooptic polarization modulation in multielectrode AlxGa1-xAs rib waveguides

A single mode rib waveguide (RWG) polarization modulator is described with a measured extinction ratio ≳27 dB, a power conversion ≈0.99, and a switching voltage of 12.5 V atlambda = 1.064 mum. The modulator is based on a modified stepDeltabeta-reversal configuration. Contact with CdO and an Au overlay ensures low optical losses (lsim1cm^-1) for both polarizations.     

High-field transport in organometallic VPE AlxGa1-xAs transferred-electron devices

The velocity-field characteristics of hot electrons in planar organometallic vapor phase epitaxial (VPE) AlxGa1-xAs (0.1 ≤ x ≤ 0.6) transferred-electron devices have been measured by the probe technique. The characteristics in layers with x ≤ 0.30 show normal subthreshold behavior. For x > 0.30, the characteristics exhibit high values of drift velocity. The effects are more enhanced when an undoped 0.4-µm GaAs buffer layer is grown first on the GaAs: Cr substrate before the ternary layer is grown. Monotonically increasing drift velocities with electric field, with a value of 2 × 10⁷cm/s at 6 kV/cm, have been measured in these devices. These effects and overshoots in the pulsed current-time profiles indicate the onset of real-space electron transfer at the heterointerface at high fields.     

Visible-light laser oscillation in indirect-bandgap AlxGa1-xAs (77 K)

Laser oscillation was accomplished at 77 K for the first time in indirect-gap AlxGa1-xAs (x = 0.46) incorporated with nitrogen atoms by ion implantation. Pumping was made by a coumarin 102 dye laser (480 nm). Threshold pumping power was6 times 10^{4}W/cm²in nitrogen-doped AlxGa1-xAs (x = 0.46), and laser oscillation was not observed in undoped samples. Nitrogen atoms, introduced into the undoped AlxGa1-xAs active layer, were found to work as isoelectronic traps and to be responsible for stimulated emission and laser oscillation.     

Transverse mode stabilized AlxGa1-xAs injection lasers with channeled-substrate-planar structure

A built-in passive waveguide mechanism is introduced in AlxGa1-xAs injection lasers by growing planar double heterostructure (DH) layers on a grooved GaAs substrate. The lasing mode is confined to the channel region due to excess absorption loss outside the channel. Stable fundamental mode oscillation is achieved up to twice the threshold current for a channel width of5-8 mu. Undesirable lasing behavior usually induced by transverse mode instability, such as nonlinear kinks in the light output versus current characteristics, are significantly reduced in the present lasers. The dc threshold current is 40-90 mA at room temperature. Median lifetime of 780 hours has been obtained during preliminary aging tests at a heat sink temperature of 70°C.     

Quantum-well AlxGa1 - xAs-GaAs heterostructure lasers grown by metalorganic chemical vapor deposition

Data are presented on photopumped single- and multiple-quantum-well AlxGa1-xAs-GaAs heterostructures grown by metalorganic chemical vapor deposition (MO-CVD) showing that continuous room-temperature (CW 300 K) laser operation of such structures is possible in the range 0-150 meV above the GaAs active region band edge (Delta E equiv hbar omega - E_{g} = 0-150meV). Optically pumped multiple-quantum-well heterostructure lasers of short cavity length (l lsim 20 mum), and thus high edge-to-edge cavity end losses, are shown to operate at photo-excitation threshold levels as low as 900 W/cm²(J_{th} sim 375A/cm²). As the quantum-well dimension is reduced toL_{z} lsim 100Å, single-active-layer heterostructures shift their laser operation to higher confined-particle states, or fail to operate altogether, whereas multiple-active-layer heterostructures continue to operate as lasers on the lowest confined-particle statesn = 1 e rightarrow hhandn' = 1' e rightarrow lhtransitions). For a multiple-quantum-well heterostructure of small enough GaAs active region size,L_{z} < 80Å, recombination radiation at the energy gap is cut off, and as expected, laser operation on the lowest confined-particle states (1 - 1') is shifted to high energy (hbar omega - E_{g} > 50meV).     

Modulation-doped MBE GaAs/n-AlxGa1-xAs MESFETs

Modulation-doped GaAs/n-Al0.3Ga0.7As MESFETs have been fabricated. At 77 K, DC transconductance of 160 mS mm^-1was observed, which is the highest transconductance value ever reported in this type of structure. The intrinsic transconductance was calculated to be 350 mSmm^-1, and the corresponding average electron drift velocity is 1.8 × 10⁷cm s^-1, which demonstrates the real advantage of this type of device in high-speed applications.     

Fast switching and storage in GaAs—AlxGa1-xAs heterojunction layers

We experimentally show the possibility of fast switching and storage in GaAs-AlxGa1-xAs heterojunction layers in accordance with previous theoretical predictions. The switching of electrons between layers is shown to occur (at least) on a nanosecond scale. The storage times which can be accomplished are of the order of minutes at 77K and much longer at lower temperatures. These times make the effect an attractive candidate for low temperature high-speed logic applications. It is speculated that for high Al mole fractionx, storage effects can even be achieved at room temperature.     

Band offset effect on transport in AlxGa1-xAs/GaAs heterojunction bipolar transistors grown by metalorganic chemical vapor deposition

AlxGa1-xAs/GaAs heterojunction bipolar transistors were grown with x in the emitter from 0 to 0.57 and the band offset effect on electron transport has been studied using a new technique. The data, though preliminary, indicate that the transport is dominated by electrons near the conduction-band minima when x < 0.45, but not when x = 0.57.     

Properties of p-N and n-N heterointerfaces in GaAs-AlxGa1-xAs heterostructure LED's

The V-I characteristics and photoresponse spectra of p-N and n-N GaAs-AlxGa1-xAs heterojunctions have been measured. Experimental results confirmed the existance of energy levels at the heterointerface. From photoresponse spectra the position of the interface levels was determined to be 0.7-0.8 eV below the conduction band of AlGaAs. Some properties of these levels are discussed.     

Double heterojunction AlxGa1-xAs/GaAs bipolar transistors (DHBJT's) by MBE with a current gain of 1650

Double heterojunction AlGaAs/GaAs bipolar junction transistors (DHBJT's) grown by molecular beam epitaxy (MBE) were fabricated and tested. Devices with 0.2-µm and 0.1-µm base thicknesses exhibited common emitter current gains of up to 325 and 1650, respectively, in a wide range of collector currents. To obtain such high current gains, growth conditions had to be optimized and controlled. These high current gains, compared with the previous best value of 120 obtained in a MBE-grown transistor, make the HBJT's very promising for low-power high-speed logic application.     

Determination of carrier saturation velocity in high-performance InyGa1-yAs/AlxGa1-xAs modulation-doped field-effect transistors (0 ≤ y ≤ 0.2)

We describe a new method for determining the carrier saturation velocity υsatin modulation-doped field-effect transistors (MODFET's). High-performance pseudomorphic InyGA1-yAs/AlGaAs MODFET's (0 ≤ y ≤ 0.20) grown by MBE were tested, and the dependence of υsaton InAs mole fraction y was obtained. It was found that each device with y > 0 had a higher υsatthan a conventional GaAs/ AIGaAs MODFET (y = 0). Further, we believe that there is an optimum InAs mole fraction such that υsatis maximized. A pseudomorphic MODFET structure optimizing υsatmay also optimize overall device performance.