Joseph L. Maserjian

Molecular beam epitaxy engineered III-V semiconductor structures for low-power optically addressed spatial light modulators

Device approaches are investigated for optically addressed SLMs based on molecular beam epitaxy (MBE) engineered III-V materials and structures. Strong photo-optic effects can be achieved in periodically δ-doped multiple quantum well structures, but are still insufficientfor high-contrast modulation with only single- or double-pass absorption through active layers of practical thickness. We use the asymmetric Fabry-Perot cavity approach that permits extinction of light due to interference of light reflected from the front and back surfaces of the cavity. Optically controlled modulation of the absorption in the active cavity layers unbalances the cavity and turns on the reflected output signal, thereby allowing large contrast ratios. This approach is realized with an all-MBE-grown structure consisting of GaAs/AlAs quarter-wave stack reflector grown over the GaAs substrate as the high reflectance mirror (≈ 0.98) and the GaAs surface as the low reflectance mirror (≈ 0.3). We use for our active cavity InGaAs/GaAs multiple quantum wells separated by periodically δ-doped GaAs barriers to achieve a sensitive photo-optic effect due to exciton quenching. High-contrast modulation (> 60:1) is achieved using a low-power ( 2 ) InGaAs/GaAs quantum well laser for the control signal.

Optically addressed spatial light modulators by MBE-grown nipi MQW structures

Promising approaches for achieving optically addressed spatial light modulators (O-SLMs) are investigated based on combining nipi and multiple quantum well structures. Theoretical aspects of photooptic effects achievable in such structures are treated. Test structures are grown by molecular beam epitaxy using two material systems, (In,Ga)As/GaAs and (Al,Ga)As/GaAs. Experiments show large optically controlled modulation of the absorption coefficient in the quantum well layers ( approximately 10(4) cm(-1)), a log power dependence on the control signal, millisecond and shorter time response, and generally predictable behavior. The results are encouraging for several different O-SLM device structures proposed.

Carrier recombination in a periodically /spl delta/-doped multiple quantum well structure

We have theoretically and experimentally investigated the optical-excitation-dependent carrier recombination lifetime in a periodically /spl delta/-doped InGaAs/GaAs multiple-quantum-well structure. The spatial separation of photogenerated electrons and holes results in an increased sensitivity to the optical excitation intensity in proportion to the increase in carrier lifetime. Experimentally, we find more than six orders of magnitude reduction in the carrier recombination rate over that for spatially direct transitions under low-excitation conditions. On the other hand, theory predicts intrinsic recombination rates for ideal structures far below those found experimentally. Various mechanisms such as electric-field-enhanced redistribution of the dopants during epitaxial growth, statistical variations in the separation of the dopants, and extrinsic recombination channels caused by misfit dislocations are discussed as possible origins for this discrepancy. >

Optical links for cryogenic focal-plane array readout

An optical link can provide an interface channel for a focal plane array that is immune to electro-magnetic interference (EMI) and can lower the heat load on the dewar. Our approach involves the use of fiber optics and an on-focal-plane optical modulator to provide an interface to the focal plane array (FPA). The FPA drives the modulator with an electrical signal. We evaluated specially fabricated AlGaAs/GaAs multiple-quantum-well (MQW) optical modulators, operating near 840 nm, for analog modulation, and we have used the results to calculate the performance of an optical interface link using experimentally determined device parameters. Link noise and dynamic range for an analog link were estimated from a separate experiment using pigtailed fiber components. The performance of the MQW modulator system is compared to alternative strategies. Significant improvement in performance in comparison to conventional electronic interfaces appears to be possible.

Long-wave infrared detectors based on III-V materials

Future NASA missions for earth observation and planetary science require large photovoltaic detector arrays with high performance in the long wavelength region to 18 microns and at operating temperatures above 65 K where single-cycle long-life cryocoolers are being developed. Since these detector array requirements exceed the state of current HgCdTe technology, alternative detector materials are being investigated as a possible option for future missions. Advanced growth techniques (e.g., MBE and MOCVD) of column III-V semiconductors have opened opportunities for engineering new detector materials and device structures. The technical approaches under investigation at JPL (with university and industry participation) include: quantum well infrared photodetectors, heterojunction internal photoemission (HIP) photodetectors, type-II strained layer superlattices, and nipi doping superlattices. Each of these options are briefly described with some of their pros and cons. A more detailed description is given for the HIP approach being pioneered at JPL.

Optical links for cryogenic focal plane array readout

An optical link can provide an interface channel for the focal-plane array that is immune to electromagnetic interference (EMI) and can lower the heat load on the dewar. Our approach involves the use of fiber-optics and an on-focal-plane optical modulator to provide an interface to the focal-plane array (FPA). The FPA drives the modulator with an electrical signal. We evaluated specially fabricated AlGaAs/GaAs multiple quantum well (MQW) optical modulators, operating near 840 nm, for analog modulation, and we have used the results to calculate the performance of an optical interface link using experimentally determined device parameters. Link noise and dynamic range for an analog link were estimated from a separate experiment using pigtailed fiber components. The performance of the MQW modulator system is compared to alternative strategies. Significant improvement in performance in comparison to conventional electronic interfaces appears to be possible.

Carrier lifetimes in periodically δ-doped MQW structures

The excitation dependent carrier recombination lifetime in periodically (delta) -doped strained InGaAs/GaAs multiple quantum well structures has been investigated both experimentally and theoretically. Experimentally, we find more than six orders of magnitude increase in the lifetime over that for undoped material due to the spatial separation of photogenerated carriers. This results in strong photo-optic effects and optical nonlinearities. Calculations, on the other hand, predict intrinsic recombination lifetimes in the periodically (delta) -doped material far above those found experimentally. Using secondary ion mass spectroscopy, transmission electron microscopy, cathodoluminescence imaging, and electron beam induced absorption modulation imaging we find evidence for misfit dislocation related recombination mechanisms that limit the carrier lifetime in the strained quantum well material.

Low-power optically addressed spatial light modulators using MBE-grown III-V structures

Device approaches are investigated for O-SLMs based on MBE engineered III-V materials and structures. Strong photo-optic effects can be achieved in periodically (delta) -doped multiple quantum well (MQW) structures. The doping-defined barriers serve to separate and delay recombination of the photo-generated electron-hole pairs. One can use this photo-effect to change the internal field across the MQWs giving rise to quantum-confined Stark shift. Alternately, the photo-generated electrons can be used to occupy the quantum wells, which in turn causes exciton quenching and a shift of the absorption edge. Recent work has shown that both of these predicted photo-optic effects can indeed be achieved in such MBE engineered structures. However, these enhanced effects are still insufficient for high contrast modulation with only single or double pass absorption through active layers of practical thickness. We use the asymmetric Fabry-Perot cavity approach which permits extinction of light due to interference of light reflected from the front and back surfaces of the cavity. Modulation of the absorption in the active cavity layers unbalances the cavity and turns on the reflected output signal, thereby allowing large contrast ratios. This approach is realized with an all-MBE- grown structure consisting of a GaAs/AlAs quarter-wave stack reflector grown over the GaAs substrate as the high reflectance mirror (approximately equals 0.98) and the GaAs surface as the low reflectance mirror (approximately equals 0.3). We use for our active cavities InGaAs/GaAs MQWs separated by npn (delta) -doped GaAs barriers to achieve sensitive photo-optic effect due to exciton quenching. High contrast modulation (> 60:1) is achieved with the Fabry-Perot structures using low power (< 100 mW/cm2) InGaAs/GaAS quantum well lasers for a write signal.

Optoelectronic studies of an electrically tunable infrared detector

Studies are reported of an MBE-grown, two-quantum-well structure which uses photon-assisted resonant tunneling between the two quasi-confined well states to provide a detection current. Bias applied across the device allows for tuning of the wavelength of the detected light by changing the difference in energy of the two states. Various charactrization measurements of this structure will be described, and their ramifications will be discussed.