Utpal Das

Electron and hole impact ionization coefficients in GaAs‐AlxGa1−xAs superlattices

Electron and hole multiplication and impact ionization coefficients have been measured with pure carrier injection in p+‐n−‐n+ and p+‐n diodes grown by molecular beam epitaxy. Values of the electron and hole ionization coefficient ratio α/β=2–5 are measured for superlattices with well width Lz≥100 A and α/β>10 is measured in a graded band‐gap superlattice with a total well and barrier width LB+LZ=120 A. The ratio decreases and becomes less than unity for smaller well sizes. This is caused by an increase in β(E) while α(E) remains fairly constant. The results have been interpreted by considering varying hole confinement and scattering in the coupled quantum wells.

Investigation of the Interface Region Produced by Molecular Beam Epitaxial Regrowth

The interface region generated by molecular beam epitaxial regrowth has been studied in detail. Regrowth was carried out on epitaxial GaAs after a variety of realistic device processing steps. Combinations of wet chemical etching and ion milling with and without annealing were used with the objective of establishing the best procedure for integrated technologies during regrowth. Capacitance voltage measurements showed perturbations in the carrier profile corresponding to depletion and accumulation regions at the interface which are directly related to interface states at and around the regrowth interface. The measured concentration of the interface states are in the range 1.2 × 1010 to 7.05 × 1011 cm−2. The former is one of the lowest reported till date. The concentration of deep traps in the regrown layer and interface, observed by deep level transient spectroscopy, is much lower than the interface state density. Their contribution to carrier perturbation is insignificant, except in one case where an electron trap has a rather high concentration. Results of secondary ion mass spectroscopy indicate that the presence of carbon at the regrown interface is not principally responsible for creating the high resistivity interface region. Our data favor the concept of a disordered region created at the interface during regrowth. Interface state density and trap densities are much larger in the wet chemically etched samples, which is further supported by the results of temporal photoresponse measurements on junction photodiodes. The overall characteristics of the dry etched regrowth interfaces seem to be much more promising than the wet chemical etched ones.

Nonlinear effects in coplanar GaAs/InGaAs strained-layer superlattice directional couplers

We report on the performance characteristics of InGaAs/GaAs strained‐layer superlattice coplanar ridge‐type directional couplers realized by molecular beam epitaxy. The measured power transfer characteristics with 1.15 μm incident photoexcitation demonstrate nonlinear coupling due to absorption associated with the tails of the excitonic resonances in the quantum wells. From a theoretical fit of the measured data, the nonlinear refractive index coefficient, n2, of the multiquantum well is found to be 2.25×10−7 cm2/W. This agrees very well with a value of n2=1.9×10−7 cm2/W obtained independently on the same material from interferometric measurements.

MATERIAL PROPERTIES AND OPTICAL GUIDING IN InGaAs-GaAs STRAINED LAYER SUPERLATTICES-A BRIEF REVIEW

Due to the absence of lattice-matching requirements, strained-layer superlattices offer a large tunability in bandgap and other material properties suitable for device applications. Encouraging progress has been made in the molecular-beam epitaxial and metalorganic-vapor-phase-epitaxial growth of strained-layer superlattices and in their characterization. These have been briefly reviewed here. Since a strained-layer superlattice allows the use of InzGa,_,vAs layers with x-values up to - 1.0, a large variation of the refractive index from that in GaAs occurs due to mismatch strain and alloying. This variation in refractive index has been calculated. The increase in refractive index can be used to form optical guides in the SLS and such guides with good vertical confinement is demonstrated. Preliminary measurements of the impact-ionization parameters and deep-level traps in these materials are also reported. a/@ values close to and slightly greater than unity are measured. A single electron trap with thermal activation energy equal to 0.16 eV is identified.

Use of triethylindium and bisphosphinoethane for the growth on InP by chemical beam epitaxy

We have demonstrated the first CBE growth of InP using bisphosphinoethane as a group V source. Mirrorlike surface morphology and excellent reflection high‐energy electron diffraction patterns were observed. Room temperature and 77 K Hall mobilities for a 2.0 μm thick InP epitaxial layer were 4200 and 22 000 cm2/V s, with carrier densities of 5.7×1015 and 4.0×1015 cm−3, respectively. Although a high n‐type impurity concentration is observed at the epitaxial layer‐substrate interface, the epitaxial layer background impurity concentration is low enough for device fabrication. The full width at half maximum linewidth of the dominant donor bound exciton is 0.84 meV.

Variation of refractive index in strained InxGa1−xAs‐GaAs heterostructures

InxGa1−xAs‐GaAs heterostructures and strained‐layer superlattices can be used as optical waveguides. For such applications it is important to know explicitly the refractive index variation with mismatch strain and with alloying in the ternary layer. Starting from the Kramers‐Kronig integral dispersion relations, we have developed a model from which the refractive index change in the ternary layer of InxGa1−xAs‐GaAs heterojunctions can be calculated. The results are presented and discussed. The expected changes in a superlattice have been qualitatively predicted.

Orientation‐dependent phase modulation in InGaAs/GaAs multiquantum well waveguides

The electro‐optic effect and phase modulation in In0.2 Ga0.8 As/GaAs multiple quantum wells have been experimentally studied for the first time. The experiments were done with 1.06 and 1.15 μm photoexcitation which are, respectively, 25 and 115 meV below the electron–heavy hole excitonic resonance. Strong quadratic electro‐optic effect was observed near the excitonic edge in addition to the linear effect. These are characterized by r63 =−1.85×10−19 m/V and (R33 −R13 )=2.9×10−19 m2 /V2 . In addition, we observe a dispersion in the value of r63 . The relative phase shifts are higher in the strained system at 1.06 μm than in lattice‐matched GaAs/AlGaAs.

Low‐loss optical waveguides made with molecular beam epitaxial In0.012Ga0.988As and In0.2Ga0.8As‐GaAs superlattices

We demonstrate for the first time low‐loss optical guiding in In‐doped GaAs. Ridge waveguides are made with single In0.012Ga0.988As ternary layers and In0.2Ga0.8As‐GaAs superlattices. Attenuation constants of ∼1.3 dB/cm are measured and the principal loss mechanism is identified to be scattering at the ridge walls. It is expected that improved fabrication techniques will lead to guides with attenuation ≤0.5 dB/cm.

InGaAs/GaAs multiquantum-well electroabsorption modulator with integrated waveguide

A monolithically integrated guided-wave modulator has been realized by using molecular-beam epitaxial regrowth and ion-milling techniques. The guiding and modulating regions consist, respectively, of In-doped GaAs and GaAs/In(0.34)Ga(0.66)As strained-layer multiquantum wells. Modulation is achieved by field-enhanced electroabsorption in the multiquantum wells. The insertion loss of the modulator is 0.9 dB, and the transmission loss in the guides is </=1 dB/cm. The temporal response of similar GaAs/InGaAs as-grown photodiodes to pulsed laser excitation is characterized by a rise time of 115 psec.

F INDUCED LAYER DISORDERING OF GAAS/INGAP QUANTUM WELLS

F implantation (80–175 keV) induced GaAs/InGaP quantum well disordering was performed in a conventional furnace at 600–750 °C and in lamp annealing at 850 °C. Group V intermixing is found to be substantially enhanced for certain implantation and anneal conditions. Either the group III intermixing leading to lower band gaps or group V intermixing leading to higher band gaps may be made to dominate by choosing the process conditions. Only 50% reduction in integrated luminescence intensities from the as-grown layer makes this quantum well disordering process suitable for device fabrication.