John Alfred Trezza

Substrate temperature and monolayer coverage effects on epitaxial ordering of InAs and InGaAs islands on GaAs

For InAs Stranski–Krastanov (SK) island growth on GaAs by molecular‐beam epitaxy, we show that the in‐plane island diameter varies exponentially with the growth temperature over the range of 390–540 °C. A transition region in SK growth between isolated island growth and island coalescing is investigated as functions of growth temperature and equivalent InAs layer‐by‐layer monolayer (ML) coverage in order to extend the isolated island regime for quantum confinement applications. InAs islands of 150 A in diameter have been grown. Growth of In0.5Ga0.5As islands indicates an increased 2D epitaxial region before island nucleation and a decreased island concentration compared to growth of InAs islands.

Effects of monolayer coverage, flux ratio, and growth rate on the island density of InAs islands on GaAs

We have studied the effects of monolayer coverage, V/III flux ratio and growth rate on the density of three‐dimensional growth induced isolated InAs islands grown on GaAs by molecular‐beam epitaxy. Within the isolated island growth regime, increasing the monolayer coverage increases the InAs island density with only a small increase in island size. Decreasing the V/III flux ratio or decreasing the growth rate increases the island density without changing the average in‐plane island diameter. We have observed island densities that are 80% of the ideal close‐packed island density. We propose a model explaining the island density increase with monolayer coverage; local variations in accumulated strain in the wetting layer vary the point at which local islanding is initiated. As more material is deposited more islands are nucleated and the island density increases. The island density increases with decreasing V/III flux ratio or growth rate by increasing the adatom surface diffusion in the underlying wetting ...

Structural and photoluminescence properties of growth‐induced InAs island columns in GaAs

InAs quantum dots layers produced by growth‐induced islanding are vertically stacked with a variable thickness GaAs spacer layer. Using transmission electron microscopy, dense planar arrays of randomly ordered InAs islands are found to be vertically aligned in columns. We have compared single, 2, 5, and 10 layers of InAs islands and the vertical alignment is maintained. Atomic force microscopy shows the island size is unchanged throughout the layering process. 8 K photoluminescence shows a spectral peak shift and linewidth narrowing that we associate with electronic coupling within the columns. Variations in the GaAs spacer region produce spectral changes that are consistent with this coupling.

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‐...

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.

Creation and optimization of vertical cavity phase flip modulators

The creation of vertical cavity phase flip modulators is demonstrated, both theoretically and experimentally. These modulators rely on both the ability to make Fabry–Perot cavities which switch the dominant mirror responsible for reflection and the use of excitons in a manner which allows them to provide large absorption changes with zero parasitic refractive index changes. The current device provides a 180° phase change while only changing reflectivity from 65% to 63% with an applied bias of 13.5 V while using an active region only 5000 A long. These devices can be placed into dense arrays and should have numerous applications for stackable optical switching and logic, high‐efficiency spatial light modulation, and, with appropriate optimization, low‐reflectivity‐change analog phase modulators.

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.

Two-state electrically controllable phase diffraction grating using arrays of vertical-cavity phase flip modulators

An electrically controlled, two state, phase modulated diffraction grating is created by fabricating a vertical-cavity phase flip modulator and etching the top of the structure into a series of stripes. The modulator switches phase between 0 and 180/spl deg/ with 7-V bias. Alternating stripes are electrically isolated from the switching voltage as a result of stripe etching. With zero bias, the etched device reflects all normally incident light normal to the device. With bias, the light is diffracted primarily into the first-order diffraction peaks. We demonstrate this device, depict the parasitic effects of etching, and show the device design limitations.

Optical image correlation using high-speed multiple quantum well spatial light modulators

We review GaAs Fabry-Perot vertical cavity multiple quantum well (MQW) spatial light modulators (SLMs) developed at Sanders, a Lockheed Martin Company, and demonstrate their use in optical image correlation. These MQW SLMs are reflective- mode modulators using electrically-tunable absorption to modulate the reflected intensity. The operation of the MQW SLMs with a newly-developed Labview graphical user interface is described. A compact Vander-Lugt image correlator is described which was configured using MQW SLMs: binary 128 X 128 pixel image input with a binary filter plane. In addition, the architecture of 8-bit gray-scale MQW-SLM module developed at Sanders is discussed. The performance of the image correlator was characterized using amplitude-encoded binary phase-only filters and various test targets including test imagery supplied by US Army AMCOM, and is compared with simulations for peak-to-secondary efficiencies on these data. Finally, high-speed (250,000 frames per second) target recognition of 128 X 128 pixels binary input imagery is demonstrated.