Bipolar switching in chalcogenide phase change memory.

Phase change materials based on chalcogenides are key enabling technologies for optical storage, such as rewritable CD and DVD, and recently also electrical nonvolatile memory, named phase change memory (PCM). In a PCM, the amorphous or crystalline phase affects the material band structure, hence the device resistance. Although phase transformation is extremely fast and repeatable, the amorphous phase suffers structural relaxation and crystallization at relatively low temperatures, which may affect the temperature stability of PCM state. To improve the time/temperature stability of the PCM, novel operation modes of the device should be identified. Here, we present bipolar switching operation of PCM, which is interpreted by ion migration in the solid state induced by elevated temperature and electric field similar to the bipolar switching in metal oxides. The temperature stability of the high resistance state is demonstrated and explained based on the local depletion of chemical species from the electrode region.

A multi-channel low-power system-on-chip for single-unit recording and narrowband wireless transmission of neural signal.

This paper reports a multi-channel neural recording system-on-chip (SoC) with digital data compression and wireless telemetry. The circuit consists of a 16 amplifiers, an analog time division multiplexer, an 8-bit SAR AD converter, a digital signal processor (DSP) and a wireless narrowband 400-MHz binary FSK transmitter. Even though only 16 amplifiers are present in our current die version, the whole system is designed to work with 64 channels demonstrating the feasibility of a digital processing and narrowband wireless transmission of 64 neural recording channels. A digital data compression, based on the detection of action potentials and storage of correspondent waveforms, allows the use of a 1.25-Mbit/s binary FSK wireless transmission. This moderate bit-rate and a low frequency deviation, Manchester-coded modulation are crucial for exploiting a narrowband wireless link and an efficient embeddable antenna. The chip is realized in a 0.35- εm CMOS process with a power consumption of 105 εW per channel (269 εW per channel with an extended transmission range of 4 m) and an area of 3.1 × 2.7 mm(2). The transmitted signal is captured by a digital TV tuner and demodulated by a wideband phase-locked loop (PLL), and then sent to a PC via an FPGA module. The system has been tested for electrical specifications and its functionality verified in in-vivo neural recording experiments.

Counting, timing, and tracking with a single-photon germanium detector.

We report the performance of a germanium quad-cell, composed of four avalanche photodiodes operated in the photon-counting regime, biased above the breakdown voltage. Each pixel detects a single photon in the wavelength range 1-1.5 microm with ~10% quantum efficiency and measures the photon arrival time with a time resolution of 100 ps. By adopting a multiplexed gating of the avalanche photodiodes we avoided the optical cross talk among pixels, thus achieving a noise-equivalent power of 3 x 10(-15) W/Hz((1/2)) for each pixel and demonstrating the tracking capabilities of such a detector.

Time-resolved photoluminescence measurements of InGaAs/ InP multiple-quantum-well structures at 1.3-µm wavelengths by use of germanium single-photon avalanche photodiodes.

A commercially available germanium avalanche photodiode operating in the single-photon-counting mode has been used to perform time-resolved photoluminescence measurements on InGaAs/lnP multiple-quantum-well structures. Photoluminescence in the spectral region of 1.3-1.48 µm was detected with picosecond timing accuracy by use of the time-correlated single-photon counting technique. The carrier dynamics were monitored for excess photogenerated carrier densities in the range 10(18)-10(15) cm(-3). The recombination time is compared for similar InGaAs-based quantum-well structures grown by use of different epitaxial processes.

Avalanche photodiodes and quenching circuits for single-photon detection.

Avalanche photodiodes, which operate above the breakdown voltage in Geiger mode connected with avalanche-quenching circuits, can be used to detect single photons and are therefore called singlephoton avalanche diodes SPAD's. Circuit configurations suitable for this operation mode are critically analyzed and their relative merits in photon counting and timing applications are assessed. Simple passive-quenching circuits (PQC's), which are useful for SPAD device testing and selection, have fairly limited application. Suitably designed active-quenching circuits (AQC's) make it possible to exploit the best performance of SPAD's. Thick silicon SPAD's that operate at high voltages (250-450 V) have photon detection efficiency higher than 50% from 540- to 850-nm wavelength and still ~3% at 1064 nm. Thin silicon SPAD's that operate at low voltages (10-50 V) have 45% efficiency at 500 nm, declining to 10% at 830 nm and to as little as 0.1% at 1064 nm. The time resolution achieved in photon timing is 20 ps FWHM with thin SPAD's; it ranges from 350 to 150 ps FWHM with thick SPAD's. The achieved minimum counting dead time and maximum counting rate are 40 ns and 10 Mcps with thick silicon SPAD's, 10 ns and 40 Mcps with thin SPAD's. Germanium and III-V compound semiconductor SPAD's extend the range of photon-counting techniques in the near-infrared region to at least 1600-nm wavelength.

Single-photon detection beyond 1 µm: performance of commercially available InGaAs/lnP detectors.

Commercially available InGaAs/lnP avalanche photodiodes, designed for optical receivers and range finders, can be operated biased above the breakdown voltage, achieving single-photon sensitivity. We describe in detail how to select the device for photon-counting applications among commercial samples. Because of the high dark-counting rate the detector must be cooled to below 100 K and operated in a gated mode. We achieved a noise equivalent power of 3 × 10(-16) W/Hz(1/2) to a 1.55-µm wavelength and a time resolution well below 1 ns with a best value of 200-ps FWHM. Finally we compare these figures with the performance of state-of-the-art detectors in the near IR, and we highlight the potentials of properly designed InGaAs/lnP avalanche photodiodes in single-photon detection.

Nanosecond single-photon timing with InGaAs/InP photodiodes.

We demonstrate that separate absorption and multiplication InGaAs/InP avalanche photodiodes can work biased above the breakdown voltage and detect the arrival time of single photons with 1-ns resolution and a noise-equivalent power of 1 x 10(-14) W/Hz((1/2)) at 150 K. We investigated the performance of various samples, cooling the detectors from different temperatures down to 50 K. These devices are suitable for the detection of short optical pulses in the near-infrared range up to a 1.55-microm wavelength, for the characterization of optical communication components, and for luminescence and radiative decay measurements.

Single-photon detection beyond 1 µm: performance of commercially available germanium photodiodes.

Germanium avalanche photodiodes (APD's) working biased above the breakdown voltage detect single optical photons in the near-infrared wavelength range. We give guidelines for the selection of devices suitable for photon-counting applications among the commercial samples, and we discuss in detail how the devices should be operated to achieve the best performance, both in terms of noise-equivalent power (NEP) and the timing-equivalent bandwidth. We introduce the driving electronics and we show that, in the measurements of fast optical signals, the adoption of single-photon techniques is very favorable, notwithstanding that presently available photodiodes are not designed for this purpose. On the contrary, in the detection of cw signals, the lower NEP values achieved in photon counting may not be sufficient to justify the replacement of conventional analog p-i-n germanium detectors, which offer comparable performance with a definitely larger sensitive area. Finally, we show that, by properly choosing theoperating conditions, some selected APD's achieve an 85-ps time resolution in the detection of optical photons at a 1.3-µm wavelength, which corresponds to a timing-equivalent bandwidth of 1.8 GHz. To the best of our knowledge, this time resolution is the lowest reported to date for single-photon detectors in the near infrared.

Subnanosecond single-photon timing with commercially available germanium photodiodes.

We demonstrate that commercially available germanium avalanche photodiodes can achieve single-photon sensitivity and subnanosecond time resolution at 77 K. Experiments show that carrier trapping phenomena give the main contribution to the detector noise. Therefore technological efforts will be welcome to overcome the present limitations that are due to the material quality. These devices are suitable for the detection of short optical pulses in the near-infrared range and in optical time-domain reflectometry characterization of short optical fibers with centimeter resolution at 1.3-1.5 microm. Futher improvements in the timing performance can be obtained by suitable device design.

High-accuracy picosecond characterization of gain-switched laser diodes.

A unique combination of the time-correlated photon-counting technique and single-photon avalanche diode detectors gives an accurate characterization of gain-switched semiconductor lasers with picosecond resolution. The high sensitivity and the clean shape of the time response reveal even small features (reflections and relaxation oscillations), making a true optimization of the laser-diode operation possible. The technique outperforms the standard characterization with ultrafast p-i-n photodiodes and a sampling oscilloscope. In addition, compared with other methods, it has favorable features that greatly simplify the measurement.