Voxtel Blog

Voxtel Is at SPIE DSS This Week

April 20, 2015

Visit us April 21 to April 23 in booth 1117 at the SPIE Defense, Security, and Sensing (DSS) Expo at the Baltimore Convention Center in Baltimore, Maryland.

Synthesis of Colloidal PbSe Nanoparticles Using a Microwave-Assisted Segmented Flow Reactor

November 7, 2014

An Oregon State University / Voxtel team recently published a study evaluating the effect of nucleation temperature on PbSe nanoparticle size distribution, cryustallographic structure, particle shape, and particle composition. It was found that nucleation of Pb-rich species occurs in the microwave reaction zone, while PbSe nanoparticles form in the growth zone. The author’s peer-reviewed final manuscript, as accepted by the publisher is available here. The published article is copyrighted by Elsevier and can be found here.

Spatial mapping of electronic states provides clues that allow fine tuning of nanocrystal properties

October 31, 2014

Voxtel Nano scientists Thomas Allen and Peter Palomaki were recently published in the Journal of Physical Chemistry Letters in a collaboration with University of Oregon Assistant Professor George Nazin.  The paper entitled “Spatial Mapping of Sub-Bandgap States Induced by Local Nonstoichiometry in Individual Lead Sulfide Nanocrystals” explores the electronic structure of individual lead sulfide nanocrystals, which have applications in solar cells, photodetectors, and light-emitting devices.  Professor Nazin’s specialized scanning tunneling microscopy system allows scientists, for the first time, to spatially map the electronic states present in a single nanocrystal, providing clues for further tuning of the nanocrystal properties.  The observations made studying VoxtelNano’s lead sulfide quantum dots have already led to an improved understanding of devices currently in development at VoxtelNano.

NASA Tech Brief Magazine Reports on Voxtel’s Work on Materials for Thermoelectric Applications

September 2, 2014

To make cooling and electrical generation costs competitive with more traditional technologies that are not solid-state, Voxtel conducted work for the Marshall Space Flight Center on high-efficiency, easy-to-manufacture engineered nanomaterials for thermoelectric applications.  Developed thermoelectric (TE) materials were achieved using solution-processed nanocrystals. The published brief is available here. This work was conducted under NASA contracts NNM06AA41C and NNM07AA27C.

Paper Published: Pixelated Detector with Photon Address Event Driven Time Stamping and Correlation

June 26, 2014

A Voxtel/University of Oregon team has published the paper “Pixelated Detector with Photon Address Event Driven Time Stamping and Correlation.”

In the paper, we present the design, manufacture, and test results of an asynchronous event-driven address time-stamped (EDATS) pixelated array detector, operational over correlation time spans ranging from less than 10−6 to greater than 104 seconds. The pixelated EDATS detector was designed to measure the equilibrium density fluctuations on a nanometer-length scale using small angle x-ray scattering in x-ray photon correlation spectroscopy (XPCS) experiments. The detector sensor chip assembly (SCA) includes a custom readout integrated circuit (ROIC), hybridized to a silicon photodiode array, optimized for 500 eV to 2000 eV x-ray photons. The detector is shown to be capable of handling x-ray photon event rates of 100 million x-ray events per second, with less than 85 nanoseconds timing jitter per event.

The paper, which is published in the IEEE Transactions on Nuclear Science, is available for download here and through the IEEE Xplore Digital Library here.

Nanosecond Speed in Dual Threshold X-Ray Photon Counter

November 13, 2013

Voxtel strives to enable the world’s best next-generation opto-electronics systems, and the best systems start with the best research. To address the X-ray science community’s need for advanced X-ray detectors that can rapidly process scattered X-ray signals, we are developing a dual threshold X-ray photon counting detector (DT-XPC). Under SBIR Phase II funding from the Department of Energy, a Voxtel-led team designed and fabricated a dual-threshold X-ray photon counting ROIC that was confirmed to support the 10-MHz and 100-MHz count rates of the latest generation of X-ray synchrotron sources, like the accelerator at Argonne National Laboratory’s Advanced Photo Source. Senior CMOS Imager and Readout Design Engineer Jehyuk Rhee presented the team’s findings on October 29th local time at the  2013 Institute of Electrical and Electronics Engineers Nuclear Science Symposium and Medical Imaging Conference & Workshop on Room-Temperature Semiconductor X-Ray and Gamma-Ray Devices (2013 IEEE NSS/MIC/RTSD) in Seoul, Korea. The presentation is available for download here.


Achieving New Scales of Space-Time Correlations: Poster Presentation on Time-Resolved X-Ray Photon Detector Development and Fabrication

November 5, 2013

Senior CMOS Imager and Readout Design Engineer Jehyuk Rhee presented the findings of a Voxtel-led team on the development of a time-resolved X-ray photon detector. Characterization of several devices developed under the Department of Energy SBIR Phase II research and development program showed that the sensor has the required noise, bandwidth and timing resolution for running next generation XPCS experiments. The poster presentation was displayed during October 27 – November 2 at the  2013 Institute of Electrical and Electronics Engineers Nuclear Science Symposium and Medical Imaging Conference & Workshop on Room-Temperature Semiconductor X-Ray and Gamma-Ray Devices (2013 IEEE NSS/MIC/RTSD) and is available here.

Voxtel Paper Published: Instantaneous Receiver Operating Characteristic (ROC) Performance of Multi-gain-stage APD Photoreceivers

October 22, 2013

Voxtel’s CEO George Williams and Group Lead of Detector Products Andrew Huntington recently published the paper Instantaneous Receiver Operating Characteristic (ROC) Performance of Multi-gain-stage APD Photoreceivers, which they co-authored with  M.M. Hayat and D.A. Ramirez of the Center for High Technology Materials at the University of New Mexico.

The paper, which is published in the IEEE Journal of the Electronic Devices Society,  is available for download here and through the IEEE Xplore Digital Library here.

The complete abstract follows:

We describe the use of analytical and numerical models of a multi-gain-stage single-carrier multiplication (SCM) avalanche photodiode (APD) to generate time-resolved receiver operating-characteristic (ROC) curves. First, pseudo-DC analytic models of discrete multi-stage APDs are used to generate the statistical properties of the SCM APD necessary for ROC analysis. Next, numerical models are used to develop the joint probability density function (PDF) of the SCM APD gain and avalanche buildup time as a function of the device structure, material properties and local electric fields. The instantaneous (time-resolved) carrier count distributions resulting from photon- and dark-initiated impact ionization chains are used to calculate the mean and variance of the currents induced in the circuits of the photoreceiver over the integration times of the detection event. Last, autocorrelation functions are generated to allow the parameters of the signal, the noise and the signal embedded in noise—which are necessary for ROC hypothesis testing—to be calculated for the instances of the impulse response. It is shown that time-resolved ROC analysis of an APD photoreceiver, which includes the instantaneous properties of photon- and dark-initiated avalanche events, allows for better optimization of photoreceiver performance than does non-time-resolved ROC analysis.



IEEE Journal of the Electron Devices Society Publishes Voxtel Paper “Multi-gain-stage InGaAs Avalanche Photodiodes with Enhanced Gain and Reduced Excess Noise”

July 1, 2013

The IEEE Transactions on Electron Devices has accepted for publication a paper authored by George Williams, Madison Compton, and Andrew Huntington of Voxtel titled Multi-gain-stage InGaAs Avalanche Photodiode with Enhanced Gain and Reduced Excess Noise. The paper describes the device architecture and DC performance characteristics of Voxtel’s single carrier multiplication (SCM) avalanche photodiode (APD) technology, and provides measured data and numerical models, which verify that the SCM APD can be described, analytically, by models developed in the 1980s to describe superlattice and staircase APDs.

The published paper is available here.

The complete abstract follows:

We report the design, fabrication, and test of an InGaAs avalanche photodiode (APD) for 950-1650 nm wavelength sensing applications. The APD is grown by molecular beam epitaxy on InP substrates from lattice-matched InGaAs and InAlAs alloys. Avalanche multiplication inside the APD occurs in a series of asymmetric gain stages whose layer ordering acts to enhance the rate of electron-initiated impact ionization and to suppress the rate of hole-initiated ionization when operated at low gain. The multiplication stages are cascaded in series, interposed with carrier relaxation layers in which the electric field is low, preventing avalanche feedback between stages. These measures result in much lower excess multiplication noise – and stable linear-mode operation at much higher avalanche gain – than is characteristic of APDs fabricated from the same semiconductor alloys in bulk. The noise suppression mechanism is analyzed by simulations of impact ionization spatial distribution and gain statistics, and measurements on APDs implementing the design are presented. The devices employing this design are demonstrated to operate at linear-mode gain in excess of 6,000 without avalanche breakdown. Excess noise characterized by an effective impact ionization rate ratio below 0.04 were measured at gains over 1,000.



Discrimination of Photon- and Dark-Initiated Signals in Multiple Gaine Stage APD Photoreceivers

May 13, 2013

The IEEE Transactions on Electron Devices has accepted another paper authored by Voxtel staff for publication. The newest paper, “Discrimination of Photon- and Dark-Initiated Signals in Multiple Gain Stage APD Photoreceivers,” describes the different count distributions and statistical properties of photon-initiated avalanche events and dark current-initiated avalanche events. The paper shows that in Voxtel’s single-carrier multiplication (SCM) avalanche photodiode (APD), there is an ability to discriminate one from the other. The paper describes the numeric and analytical models that can be used to model these events in a photoreceiver.

A pre-publication version of the paper is available here.

The complete abstract follows:

We demonstrate the ability of linear mode single carrier multiplication (SCM) avalanche photodiode (APD) -based optical receivers to discriminate single-photon-initiated avalanche events from dark-current-initiated events. Because of their random spatial origin in discrete regions of the depletion region, in the SCM APD the dark-generated carriers multiply differently than the photon-generated carriers. This causes different count distributions and necessitates different statistical descriptions of the signal contributions from photon- and dark-originating impulse responses. To include dark carriers in the performance models of the SCM APD, we considered the influence of the spatial origin of the ionization chains on a receiver’s noise performance over the times the optical pulse is integrated by the receiver’s decision circuits. We compare instantaneous (time-resolved) numeric and pseudo-DC analytical models to measured SCM APD data. It is shown that it is necessary to consider both the distribution of spatial origin and the instantaneous properties of the ionization chains to describe statistically an SCM APD receiver. The ability of SCM APD receivers to discriminate single photon events from single dark events is demonstrated, and the effective gain and excess noise contributions of the light- and dark-initiated avalanche events and their influence on receiver sensitivity and signal-to-noise characteristics is shown.