S. M. Lord

Large, low‐voltage absorption changes and absorption bistability in GaAs/AlGaAs/InGaAs asymmetric quantum wells

Three‐step asymmetric coupled quantum wells have unique excitonic properties, particularly under bias. We demonstrate these properties through the absorption changes in quantum well optical modulators. The samples consisted of p‐i‐n diodes with an active region of 20 coupled wells, each coupled well containing a 50 A GaAs well and a 20 A In0.2Ga0.8As well separated by a 10 A Al0.33Ga0.67As barrier. Analysis of the structure shows that field‐induced enhancement and suppression of electron and hole envelope wave function overlap can be observed through a corresponding increase or decrease in exciton absorption peaks. Our devices showed suppressed absorption with bias for the electron‐heavy hole 1 exciton and enhanced absorption with bias for the electron‐heavy hole 2 exciton. Stress‐related effects on the electron‐light hole 1 exciton are also observed. Absorption change per applied bias is five times lower than at the zero‐field exciton wavelength in quantum well devices utilizing the conventional quantum‐...

Intersubband transitions in high indium content InGaAs/AlGaAs quantum wells

We report the first observation of intersubband transitions in InyGa1−yAs(y=0.3,0.5)/ AlGaAs quantum wells. These quantum wells were grown on a GaAs substrate with a linearly graded InGaAs buffer to achieve strain relaxation before growth of the quantum wells. Measured intersubband transition energies of 316 and 350 meV are among the largest ever reported. Asymmetric step In0.5Ga0.5As/AlGaAs quantum wells designed for second harmonic generation measurements also demonstrate strong intersubband absorption at 224 meV corresponding to the 1‐2 transition. With the large conduction band offsets (larger than 800 meV) available in this material system, extension to larger intersubband transitions energies for quantum well photodetector and nonlinear optics applications should be possible.

1.3 μm Exciton resonances in InGaAs quantum wells grown by molecular beam epitaxy using a slowly graded buffer layer

We achieve sharp excitonic resonances near 1.3 μm in InGaAs quantum wells grown on a GaAs substrate by molecular beam epitaxy using a slowly graded InGaAs buffer layer. Our results indicate that linear grading is preferable to step grading for high In composition InGaAs on GaAs. SIMS results confirm the linearity of the grading. Cross-section TEM analysis of the graded sample reveals dislocations within the buffer but none which are threading up to reach the quantum wells. In addition to a well-defined exciton at 1.25 μm under zero bias, we observe pronounced quantum confined Stark effect in the absorption spectrum of a sample including a graded buffer layer while the spectra from samples with step-graded buffers exhibit no excitonic features

Hydrogen passivation of nonradiative defects in InGaAs/AlxGa1−xAs quantum wells

The effects of diffusion of monatomic hydrogen and deuterium in InGaAs/AlGaAs quantum wells were studied using photoluminescence (PL) and secondary‐ion‐mass spectroscopy. The multiquantum‐well structures were grown by molecular‐beam epitaxy and hydrogenated with a remote plasma. A significant increase in 77 K PL integrated intensity of bound excitons was observed after hydrogenation. This is attributed to the passivation of nonradiative recombination centers within InGaAs/AlGaAs quantum wells. A series of studies demonstrating the increase in passivation efficiency with increasing Al concentration in the barriers, the stability of the hydrogenation upon annealing to temperatures of up to and above 450 °C, the ratio of the deuterium concentration for samples with different barrier thicknesses, and the comparison of strained versus relaxed quantum wells, all strongly suggest that the passivated nonradiative recombination centers are interface defects. The stability of this hydrogen passivation at temperatur...

High contrast asymmetric Fabry–Perot electro‐absorption modulator with zero phase change

By analyzing the exciton line shapes of quantum wells, we can determine the wavelengths and biases at which they exhibit large absorption changes and zero refractive index changes relative to zero bias. To use this effect, we placed GaAs/AlGaAs quantum wells in the intrinsic region of a reverse‐biased p‐i‐n diode asymmetric Fabry–Perot modulator. We then developed a sample that could be post‐growth processed to optimize all relevant parameters. We used computer simulation to determine the exact post‐growth correction required and produced reflection modulators with 90% reflection change and zero relative phase change.

Low‐voltage, low‐chirp, absorptively bistable transmission modulators using type‐IIA and type‐IIB In0.3Ga0.7As/Al0.33Ga0.67As/ In0.15Ga0.85As asymmetric coupled quantum wells

Coupled InGaAs quantum‐well systems which use field‐induced spatial separation of electron and hole states to modulate the magnitude of exciton optical absorption, and hence transmission have been theoretically analyzed and experimentally demonstrated. The samples consisted of p‐i‐n diodes with an active region of 20 coupled wells, each coupled well containing a 50 A In0.3Ga0.7As well and a 30 A In0.15Ga0.85As well separated by a 10 A Al0.33Ga0.67As barrier. One structure was grown with the thinner well on the n‐type side of each coupled quantum well while in the other sample the thinner well was oriented toward the p‐type side. By applying bias to the structures, either the lowest electron or hole states effectively switch wells, thereby enhancing certain exciton resonances and quenching others. The two devices grown, despite their similar structure, operate through the field‐induced switching of opposite carrier types. Because this method of modulation does not require excitons to Stark shift, the devic...

1.3 μm electroabsorption reflection modulators on GaAs

We demonstrate a reflection electroabsorption modulator grown on a GaAs substrate operating near 1.3 μm, the dispersion minimum for silica fibers. The device was grown by molecular beam epitaxy and uses a novel technique of integrating the bottom quarter‐wave mirror into a buffer with linearly graded In composition. The active area consisted of thirty InGaAs quantum wells with GaAs barriers. The mirror was formed by layers of InGaAs and InAlAs where the In concentration was graded from 0% to 35%. A maximum relative change in reflectivity, ΔR/R, of 73% at 1.33 μm was achieved. Experimental results agree with simulations performed using the transfer matrix technique.

Strain relaxation in compositionally graded epitaxial layers

A model for strain relaxation in linear compositionally graded epitaxial layers is derived. Equilibrium dynamics for misfit dislocation generation is used in each compositional segment of the graded layers through the film thickness. A local relaxation thicknesses in the compositionally graded layers is calculated as a function of film thickness. The calculated critical thicknesses show good correspondence with the depth of dislocation‐free regions which are observed at the top of graded layers with different grading rates. From analysis of the model, we explain the origin of dislocation structures and minimized threading dislocation density in the compositionally graded layers. The compositional grading produces a large equilibrium dislocation spacing compared to that of highly mismatched conventional stepwise growths, and thus enables misfit dislocations to glide freely without dislocation–dislocation interactions. At successive local relaxation thicknesses, lateral dislocations pile up horizontally by ...

GaAs/AIAs Quantum Wells for Electro-Absorption Modulators

Summary form only given. The authors demonstrate both theoretically and experimentally that the quantum-confined Stark effect leads to larger absorption changes in GaAs/AlAs quantum wells compared to the conventional GaAs/AlGaAs material due to the higher Gamma confinement. The lower indirect valleys in the AlAs do not degrade the performance, and the exciton resonance is maintained at higher energy shifts. The improvement in exciton oscillator strength is 14% at zero bias and nearly 50% at 70-meV shift. The larger exciton strength at high fields has important applications for optical modulators and switches that operate at the long-wavelength side of the zero bias exciton. >

Electroabsorption modulators operating at 1.3 μm on GaAs substrates

This paper describes the growth and device performance of electroabsorption modulators on GaAs substrates operating near 1.3 μm, the dispersion minimum for silica fibres. The key to the successful molecular beam epitaxial (MBE) growth of these devices was the incorporation of a linearly-graded buffer layer beneath the InGaAs/AlGaAs multi-quantum-well active layer. Both transmission and reflection modulators are produced. For transmission devices, larger modulation is achieved when the buffer is graded more slowly: The maximum modulation reported was 22% for ΔT/TOcorresponding to a 0.86 dB contrast ratio with an insertion loss of roughly 5 dB at 1.34 μm. Antireflection coating a transmission modulator yields a reasonable reflection modulator. However, improved performance is reported for a reflection modulator using a novel technique of integrating the bottom quarter-wave mirror into a buffer with linearly-graded In composition. At 1.33 μm, a normally-off reflection modulator with an integrated mirror exhibited a ΔR/ROof 73%, a constrast ratio of 2.38 dB, and an insertion loss of 4 dB.