Kenneth Vaccaro

InAlAs-InGaAs based avalanche photodiodes for next generation eye-safe optical receivers

This paper presents results of three research and development efforts on the subject of avalanche photodiodes with InGaAs absorbers and InAlAs multiplication layers. The first portion of the paper presents results on 256x256 arrays of InAlAs-InGaAs APDs. These spanned more than 1.5 cm x 1.5 cm, had breakdown voltage variation of less than 2.5 volts and a dark current range between 1.5 and 3.5 nA at a gain of 10. The second portion of the paper presents single photon detection results of a receiver with a 50 micron aperture avalanche photodiode biased into sub-Geiger mode and a Maxim MAX3658 transimpedance amplifier. At temperatures of 200K and average avalanche gains approaching 1000 single photon detection efficiencies greater than 5% were observed with dark count rates of less than 500 kHz. At 175 K detection rates were as high as 14%. Finally, in the third portion of this paper, performance results of a novel impact ionization engineered InGaAs-InAlAs based avalanche photodiode are presented showing excess noise values lower than any previously published InGaAs based avalanche photodiode.

Observation of negative differential resistance in GaAlAs single-barrier heterostructure at room temperature

Observation of experimental negative differential resistance at room temperature due to electron tunneling in a multiple-step single-barrier GaAlAs heterostructure is reported. Theoretical investigations of a three-step single-barrier heterostructure were conducted using the transfer matrix method and the Tsu–Esaki approach to obtain the transmission coefficients and current–voltage characteristic, respectively. The system was designed based on these calculations, and grown with molecular beam epitaxy. The diode exhibited negative differential resistance at 300 K and had a peak-to-valley current ratio above unity, which was in agreement with the predicted values.

Sidewall profile control of thick benzocyclobutene reactively ion etched in CF4∕O2 plasmas

The feasibility of controlling the sidewall angle of thick benzocyclobutene (BCB) etched in a CF4∕O2 plasma using thick photoresist as an etch mask has been investigated. Sidewall angle, BCB etch rate, and BCB to photoresist selectivity as functions of chamber pressure and CF4 to O2 ratios are reported. Through the use of postdeveloped reflown photoresist, an optimum sidewall angle of less than 60° was achieved at 33mT chamber pressure and a 19% CF4∕O2 ratio. The method presented here achieves deep, residue-free etching of thick BCB with a sidewall profile suitable for e-beam evaporated and lifted metal for use in vertical interconnects.

Comparator-based measurement scheme for dark-count rates in single photon avalanche diodes

III-V single photon avalanche diode (SPAD) sensitivity at wavelengths > 1 /spl mu/m typically comes at the expense of higher dark-count rates and afterpulsing compared to silicon SPADs. Regarding the measurement of dark-count rates, conventional counters are limited by the deadtime required to quell afterpulsing effects; this led to the adoption of time-correlated single photon counting (TCSPC). In this paper, a technique for measuring the dark-count rates encountered in III-V SPADs using only a comparator and an averaging oscilloscope is reported. A detailed explanation of the technique is presented along with a semianalytical proof, simulation results comparing the techniques validity in comparison to TCSPC, and example measurements.

Properties of low-temperature (80-300° C) pyrolytic SiO 2 on Si and InP

We report on a new low-temperature pyrolytic deposition technology for silicon dioxide. We present data characterizing the electrical and optical properties of this dielectric deposited on Si and InP substrates. The effects of thermal processing are also reported. Deposition of high-quality SiO2 is achieved by reacting SiH4 and O2 at pressures of 2–12 Torr. Reactions occur by pyrolysis only, promoting stoichiometric SiO2 deposition and good interfacial properties. No plasma- or photo-enhancement is required. Deposition is achieved at temperatures as low as 80° C, the lowest temperature ever reported for pyrolytic SiO2 deposition. Rates as high as 65 Å/min at 100° C and 100-150 Å/min at 150-300° C are attained. The leakage current densities measured for both Si and InP MIS capacitors (e.g. 10-9 Å/cm2 for 150° C SiO2) are two to six orders of magnitude lower than values reported for plasma- and photo-enhanced SiO2 deposited at equivalent temperatures. The high-temperature integrity of this dielectric also makes it a promising annealing cap for group III-V compound semiconductors. Our annealing studies show that SiO2-capped indium phosphide surfaces remain specular up to 850° C.

Performance characterization of an InGaAs/InP single photon avalanche diode

III-V based single photon avalanche diodes (SPADs), avalanche photodiodes (APDs) operated in Geiger-mode, are ideally suited for ultra-weak signal detection in the near infrared for photon counting and photon timing applications. Spaceborne SPADs would provide a rugged, compact alternative to photomultiplier tubes with lower operating voltage requirements, stronger near-IR response, and the possibility for array implementation. Results from a performance characterization of an in-house fabricated In0.53Ga0.47As/InP SPAD are presented. Sensitivity (NEP) and timing resolution (δt) were investigated as a function of bias from T = 135 K to 165 K; an NEP ≈ 5 x 1015W/Hz1/2 at T = 150 K and δt ≈ 230 ps at T = 165 K were measured for λ = 1.55 μm light.

All optically driven MEMS deformable device via a photodetector array

In this paper we demonstrate the first optical actuation of a single-pixel, deformable-mirror MEMS device through a direct cascade with a photodetector. Photovoltaic, p-i-n, and avalanche photodetectors were successfully utilized. Mirror deformations were monitored by interferometry. Deformation is quasilinear at low light intensities, and saturates at higher intensities. Actuation at picowatt light intensities has been accomplished by cascading with an avalanche photodetector. We also describe the fabrication of an integrated device consisting of an all optically addressed deformable-mirror MEMS suspended over a p-i-n photodetector. Initial demonstration of optical actuation of the deformable mirror using the newly integrated device is also presented.

All optically-driven MEMS deformable device via an array of photodetectors

We are in the process of developing an all optically driven, deformable mirror device through the integration of an array of photodetectors with an array of MEMS deformable mirror devices. In this paper we demonstrate the optical actuation of a single-pixel, deformable-mirror MEMS device through a direct cascade with a photodetector. Deformation is quasilinear at low light intensities, and saturates at higher intensities. We also describe the fabrication of an integrated device consisting of an all optically addressed deformable-mirror MEMS suspended over a p-i-n photodetector. Initial demonstration of optical actuation of the deformable mirror using the newly integrated device is also presented. We have fabricated several membrane materials, membrane structures, and photodetector arrays.

Novel focal plane array integration technology for use in eye-safe imaging ladar receivers

A novel integration technology for the fabrication of active, or passive, focal plane array imagers has been developed. The integration scheme is based on the transfer of epitaxial layers to a surrogate substrate without critical alignment. Once the epitaxial layer is successfully transferred to the surrogate substrate, photodetector isolation, passivation, and fabrication are completed. To demonstrate the potential of the process, 320 x 256 arrays of InGaAs mesas were successfully transferred onto commercially-available focal plane array readout integrated circuits. Pitch and pixel resolution are limited by the available standard photolithography. InGaAs mesas transferred to silicon wafers with a pitch as small as 10 microns have been demonstrated. The process was optimized for the fabrication of high-performance vertical Schottky photodiodes. Dark-currents below 5 nA were observed with 44 mm diameter photodiodes. Responsivities of 0.55 A/W were obtained with a 1 micron InGaAs absorber. The new integration process can be used to easily achieve photodiodes with bandwidths higher than 20 GHz, without the use of an air-bridge.

Optically Actuated MEMS Deformable Mirror Device

We propose a new design, fabrication procedures and the operating theory for a very low-power optically actuated deformable mirror device. The deformable mirror consists of an array of AlGaAs PIN photo detectors bonded onto a transparent substrate, a thin film resistor, and a suspended membrane