John A. Modolo

Radiation Effects in MOS Capacitors with Very Thin Oxides at 80°K

Radiation induced flatband voltage shifts are measured at 80°K in MOS capacitors with oxides 6.0-50 nm thick. Previous studies have found that for relatively thick oxides (greater than 20 nm) the flatband voltage changes with radiation dose as the square of the oxide thickness suggesting that the holes created by the ionizing radiation in the oxide are uniformly created and trapped. For the thinner oxides examined in the present work, significantly smaller shifts than predicted by the oxide thickness squared dependence were observed indicating that many of the generated holes are escaping the thin oxide. Physical mechanisms to explain this effect, of which recombination of trapped holes by carrier tunneling appears the most important, are discussed.

Generation of Interface States by Ionizing Radiation in Very Thin MOS Oxides

The creation of interface states Dit by ionizing radiation is investigated in MOS capacitors as a function of oxide thickness in the range 6-50 nm. A comparison of the thickness dependence in etchback and asgrown oxides supports the idea that the number of defects at the Si-SiO2 interface increases with oxidation time. For relatively thin oxides (tox<12 nm), the rate of increase in Dit is significantly smaller than would be extrapolated from the behavior of thicker oxides for both oxide types. This effect is probably caused by tunneling of trapped holes near the oxide interfaces.

Gallium arsenide metal-semiconductor-metal photodiodes as optoelectronic mixers for microwave single-sideband modulation.

Gallium arsenide (GaAs) metal-semiconductor-metal (MSM) photodetectors have unique properties including high-bandwidth, linearity, and biphase response that make them suitable as mixers and programmable weights for microwave and communications applications. An optical technique for microwave single-sideband modulation that uses GaAs MSM photodiodes as mixers is reported. It uses MSM Schottky photodiodes formed in a GaAs/Al(0.3)Ga(0.7)As materials system to detect microwave in-phase and quadrature signals on optical carriers. Modulation of the photodetector bias voltages results in a single-sideband modulation of the microwave signal. Radio frequency and undesired-sideband suppression of 36 and 27 dB, respectively, were achieved. The optical wavelength was 850 nm, and the bandwidth of the photodetectors was > or = 29 GHz.

Programmable frequency excision and adaptive filtering with a GaAs/AlGaAs/GaAs heterojunction photoconductor array

The first demonstration of acousto-optical excision using a GaAs photodetector array is reported. A maximum isolation of 40 dB was obtamed using a six element GaAs interdigitated finger double heterojunction photoconductor array. The center frequency was 45 MHz, and the swept bandwidth was 6 MHz. The response of discrete GaAs heterojunction photoconductors was linear with bias voltage, potentially making them useful for performing multiplication functions in optical analog filters. New concepts of using these photoconductors for optical adaptive filters are presented.

Planar, linear GaAs detector-amplifier array with an insulating AlGaAs spacing layer between the detector and transistor layers

Monolithic, high-speed planar, linear, parallel channel, ten-element GaAs detector-amplifier arrays with a 70- mu m detector center-to-center spacing have been fabricated using a GaAs-AlGaAs-GaAs epitaxial structure grown on semi-insulating GaAs. The AlGaAs layer provided excellent electrical isolation between the transistor and n-type photoconductor epitaxial layers. Rise and fall times of integrated detector-amplifier array channels of 650 ps and 1.1 ns, respectively, were measured at 0.84- mu m wavelength. The sensitivity of single, discrete, detector-amplifier channels was better than -34 dBm.<<ETX>>

High‐speed planar GaAs photoconductors with surface implant layers

Selective implantation of silicon into GaAs is demonstrated as a simple method for modifying the response characteristics of low‐doped planar GaAs photoconductors for optoelectronic circuits with varying requirements. Response times and sensitivities of the photoconductors were strongly dependent on the implantation dose and energy. Rise times and full width at half‐maximum (FWHM) values of devices receiving low‐dose implants were of the order of 40–150 ps. Rise times and FWHM values of devices which received higher dose implants were in the ranges 50–140 ps and 1–5 ns, respectively. The sensitivity of devices which received higher dose implants was about a factor of 100 (20 dB in optical power) greater than that of devices which received lower dose implants.

Photodetectors fabricated on heteroepitaxial GaAs/Si structures grown by molecular beam epitaxy

Abstract Photodetectors were fabricated on GaAs/Si epitaxial structures grown on high resistivity Si substrates by modulated molecular beam epitaxy. Our efforts were focused on enhancing the substrate surface quality and optimizing the deposition conditions during the early stages of growth of the GaAs on the Si substrates. The photodetector structures investigated consisted of a 300A thick GaAs/AlAs nucleation layer, a 5-period GaAs (100A)/AlAs(100A) accomodation layer grown at 300°C, two GaAs/InGaAs strained layer superlatices separated by a GaAs spacer layer, and a 2μm undoped GaAs active photodetector layer, a 0.2μm n-Gaas and a n + -GaAs contact layer. Various photodetector configurations fabricated in these GaAs/Si material structures were investigated. The photodetector response measurements were made using a 840nm wavelength pulsed laser with a pulse width of 5ns and rise (t r ) and fall (t f ) times of 200ps. Typical rise and fall times of the photodetectors were in the 1–2 ns and 3–6 ns ranges, respectively. The responsivity and quantum efficiency values of these photodetectors were in the ranges of 0.5–1.0 A/W and 0.3–1.5, respectively.

GaAs/AlGaAs/GaAs Heterojunction Photoconductor Array For Programmable Frequency Excision And Adaptive Filtering

The first demonstration of acousto-optical excision using a GaAs photodetector array is reported. A maximum isolation of 40 dB was obtained using a six element GaAs interdigitated finger heterojunction photoconductor array. The center frequency was 45 MHz, and the swept bandwidth was 3 MHz. The response of discrete GaAs heterojunction photoconductors was linear with bias voltage potentially making them useful for performing multiplication functions in optical analog filters. Concepts of using these photoconductors for optical adaptive filters are discussed.

GaAs metal-semiconductor-metal photodector mixers for microwave single-sideband modulation

A new optical technique for microwave single sideband modulation is reported. It uses metal-semiconductor-metal Schottky photodiodes formed in a GaAs/Al0.3Ga0.7As materials system to detect microwave in-phase and quadrature signals on optical carriers. Modulation of the photodetector bias voltages results in a single sideband modulation of the microwave signal, rf and undesired-sideband suppression of 36 dB and 27 dB, respectively, were achieved. The optical wavelength was 850 nm, and the bandwidth of the photodetectors is greater than or equal to 29 GHz.

Planar, Al0.3Ga0.7As-passivated-base, heterojunction bipolar phototransistors.

New planar GaAs heterojunction bipolar phototransistors have been designed and demonstrated. The devices use a GaAs/Al(0.3)Ga(0.7) As molecular-beam-epitaxy materials system with an Al(0.3)Ga(0.7) As passivated, 10-nm-thick base; a depleted, high-low emitter; and a low emitter-base capacitance. Electrical contact to the emitter is made by a set of parallel, ohmic fingers and to the collector by an ohmic contact formed in a large, approximately 1.48-microm deep via. Rise times in response to impulse optical excitation at 810 nm were 747-891 ps except at the two lowest optical excitation powers measured. Photocurrent gains measured at 810 and 850 nm were 0.67-19, depending on experimental conditions. These devices are promising for use in heterodyne photodetector arrays for coherent optical processing channelizers requiring a 100-MHz bandwidth.