Researchers Yon Visell and Katja Seltmann co-anchor “Unknown Territories” on the UCSB radio station

photo of seltmann and visell
CCBER’s Seltmann and ECE’s Visell tap current scientific literature for their weekly cultural arts program on KCSB radio

UC Santa Barbara entomologist Katja Seltmann has an alter ego. She goes by the name Irene Moon, and she came into being decades ago when Seltmann first started creating science-inspired performance art and DJing on the radio as an undergraduate student at the University of Georgia.

Seltmann — or, rather, her Moon persona — is still riding the airwaves, joined by her partner, Yon Visell. Together they co-anchor “Unknown Territories,” an hour long program with the topic of all things science.

With diverse science backgrounds (Seltmann is the Katherine Esau Director of UCSB’s Cheadle Center for Biodiversity and Ecological Restoration (CCBER) and Visell is an assistant professor in the campus’s Department of Electrical and Computer Engineering, Department of Mechanical Engineering and Media Arts and Technology Graduate Program), the pair brings an unusual bent to their weekly radio show, which airs Tuesdays at 9 a.m.

“We both love radio as a medium for expressing ideas and for education, and it’s really fun to do,” Seltmann said. “This show has been great for us because one of the things about being a researcher is that you have to keep up with the literature. Each week, we read a variety of journals, including Science, Nature and the Proceedings of the National Academy of Sciences, to choose our topics. We also check social media to see what’s hot and what people are talking about.”

The couple’s broad range of expertise — biology, physics, engineering and cognitive sciences — informs the program’s content, which focuses on topical discussions aimed at communicating ideas about science through entertaining yet critical discourse.

According to Seltmann, “Unknown Territories” fills a critical niche. “We strive to promote science communications as well as to develop a larger dialogue with the community about scientific issues,” she noted. “And because KCSB is a public radio station that streams on the Internet, that community is not just Santa Barbara. There is a connection to the larger world. We feel strongly that everyone in society should be empowered to learn, evaluate and participate in science.”

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Novel mid-infrared materials and devices on InP: from metamorphic lasers to self-assembled nanocomposites

Laser diodes (LDs) emitting in the mid-infrared (mid-IR) spectral region (λ= 2 – 3 μm) are important for applications including molecular spectroscopy and gas detection. Quantum cascade lasers on InP have reached λ=3.0 μm continuous wave (CW) lasing at room temperature (RT), while type-I InAs quantum well (QW) LDs have reached λ= 2.4 μm. However, due to extremely high strain in the active regions for both technologies, demonstration of CW RT lasing at 2.4 – 3.0 μm remains difficult for InP-based lasers. A metamorphic InAsxP1-x graded buffer on InP can perform multiple functions in addressing this challenge, as it not only increases the critical thickness of InAs QWs to enable longer wavelength emission, but also functions as graded-index bottom cladding for optical confinement. We demonstrate room-temperature metamorphic type-I QW LDs on InP that take advantage of such multi-functional metamorphic buffers to achieve lasing at λ= 2.76 μm. The metamorphic LDs were grown on n-InP (001) substrates by solid source molecular beam epitaxy (MBE). We fabricated and tested 10 µm ridge-waveguide LDs, observing pulsed mode lasing up to 300 K at λ = 2.76 μm. The threshold current density at 77 K was 200 A/cm2 and increased to 14 kA/cm2 at 300 K.

In the second part of my talk, we present self-assembled growth of highly tensile-strained Ge nanostructures, coherently embedded in an InAlAs matrix (i.e. Ge/InAlAs nanocomposite) by using spontaneous phase separation. Ge is a very intriguing material for strain engineered nanocomposites, because strain can dramatically enhance its electrical and optical properties. Here, I employ spontaneous
phase separation during MBE growth as a fundamentally
new approach to forming highly tensile-strained Ge/InAlAs nanocomposite. Transmission electron microscopy reveals a high density of single-crystalline Ge nanostructures coherently embedded in InAlAs without extended defects, and Raman spectroscopy reveals a 3.8% biaxial tensile strain in the Ge nanostructures. I demonstrate that the strain in the Ge nanostructures can be tuned to 5.3% by altering the lattice constant of the matrix material, illustrating the versatility of epitaxial nanocomposites for strain engineering and the largest biaxial tension realized in Ge to date; the cross-over from indirect to direct is predicted at ~2% biaxial tension. Photoluminescence and electroluminescence results are then discussed to illustrate the potential for realizing devices based on this novel nanocomposite material.

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Transparent Conducting Oxide Clad Limited Area Epitaxy Semipolar III-nitride Laser Diodes

photo of Anisa MyzaferiBasal plane III-nitride laser diodes have been commercialized for wide ranging technologies, including pico projectors for solid state RGB displays, optical data storage and automotive headlights. Active research is focused in solid state lighting and visible light communications. Despite widespread commercialization, c-plane devices are affected by the inherent spontaneous and piezoelectric polarizations of the basal plane. These polarization effects are significantly reduced in semipolar planes of GaN, creating a vast design space for III-nitride optoelectronic devices.

III-nitride semipolar laser design and performance are considerably affected by the material composition and growth conditions of the cladding layers. The bottom cladding design is limited by stress relaxation of ternary alloys while the top cladding is limited by the growth time and temperature of the p-type (Al,In,Ga)N layers. These design limitations have been independently addressed by using limited area epitaxy (LAE) to enable n-AlGaN bottom cladding layers and by using thin p-GaN and transparent conducting oxide (TCO) top cladding layers.

In this work, we investigate a new laser design that simultaneously incorporates LAE-enabled n-AlGaN bottom cladding and thin p-GaN and TCO top cladding layers in (202 ̅1) III-nitride laser structures. We evaluate the performance of two different TCOs as the top cladding layer: indium-tin-oxide (ITO) and zinc oxide (ZnO). Thorough optical modeling of the LAE-TCO laser design will be discussed for various Al compositions in the LAE-enabled n-AlGaN bottom cladding and varying p-GaN thicknesses in the top cladding. The LAE-TCO laser design was first demonstrated using ITO as the top cladding layer, with pulsed lasing achieved at 446 nm with a threshold current density of 8.5 kA/cm2 and a threshold voltage of 8.4 V. Insights from the optical modeling in conjunction with improvements in the LAE-TCO laser fabrication process led to the demonstration of the LAE-TCO design using ZnO as the top cladding layer, with pulsed lasing achieved at 445 nm with a threshold current density of 5.6 kA/cm2 and a threshold voltage of 6.7 V. This notable improvement in threshold current density and voltage led to the first continuous wave (CW) operation of blue (202 ̅1) LAE-ZnO laser structures.

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The Origin of 1/f Noise, Maybe

The origin of 1/f noise (sometimes referred to as flicker or pink noise) in semiconductor devices has not been convincingly explained since it was first reported by Schokly in 1918. This paper presents a possible explanation namely that the noise arises due to turbulence in the flow of the electron (or hole) fluid/gas in the sample. Turbulence in fluid or gas flow arises naturally from the convective forces imposed by a fluid¹s viscosity. Historically, transport in semiconductor materials has been modelled using linear equations of drift and diffusion that do not exhibit turbulence. Transport of gases and fluids in the discipline of hydrodynamics also uses the equations of drift and diffusion, but in addition includes the Navier-Stokes equation to describe flows that are turbulent. The Navier-Stokes equation naturally produces turbulence as the Reynold¹s number exceeds a certain range. For a uniform flow the critical Reynolds number is large several thousand. However, for narrow jets impinging upon a reservoir, the Reynolds number may be more than two orders of magnitude smaller. Scattering of electron flow by phonons, impurities, and other carriers may precipitate turbulent flow to arise at low values of the Reynolds number in semiconductors. In this paper we report on Navier-Stokes simulations of the spectral properties of turbulent flows showing that they are consistent with a 1/f spectrum. We also report on a series of measurements of photodiodes as a function of illumination intensity to observe how the 1/f properties vary with current so that in the future we can compare that variation to our simulations.

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Dry Etched Features for Advanced Waveguide Design in GaN Laser Diodes

Blue and violet laser diodes (LDs) made from the (Al,In)GaN material system were first demonstrated in 1995 [1] and have since been commercialized for applications in data storage and display technology. As the material and device technology continues to mature, these laser diodes are being investigated for use in solid state lighting and wireless communications [2]. Competitiveness in these markets will require new device designs of GaN-based LDs to improve the efficiency and optical output power.

A key structural element of a laser diode is the set of cladding layers around a waveguiding core which, together, confine the optical mode. This confinement is dependent on the refractive index contrast between the cladding and core. However, it is difficult to grow lattice-mismatched AlGaN and InGaN layers with high enough composition and thickness to provide the required index contrast. Therefore, research efforts have begun to explore new low index cladding options such as transparent conductive oxides [3,4] and lattice-matched quaternary alloys [5].

In this talk, I explore an alternative cladding design using etched nano-structures to lower the effective refractive index and create a high index-contrast top cladding layer. I present detailed simulations, design, and fabrication of a blue (450 nm) laser diode. I also consider the effect of sub-surface dry etch damage which can destroy the light emitting active region. While prior work on light emitting GaN nano-structures required GaN regrowths or recovery anneals, I have developed a low-damage dry etch that avoids the etch damage issue [6]. The resulting process is a new fabrication method for surface etched nano-structures in GaN light emitting devices.

[1] Nakamura, S., et al., Japanese Journal of Applied Physics, Part 2: Letters, 35 (1996)
[2] Lee, C., et al., Optics Express, 23 (2015)
[3] Pourhashemi, A., et al., Applied Physics Letters 106 (2015)
[4] Mughal, A.J., et al., Electronic Letters. 52, 304 (2016)
[5] Katayama, K., et al., Electronics and Communications in Japan 98, 9 (2015)
[6] Nedy, J., et al., Semiconductor Science and Technology 30, 8 (2015).

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The UCSB student chapter of the Institute of Electrical and Electronics Engineer’s Photonics Society named 2016 Chapter of the Year

group photo of the 2016 ucsb ieee photonics society
Established four years ago, the student organization of young engineers at UCSB is recognized for its effort in promoting professional growth and career development in the field of photonics. “This international award exemplifies the strength of our local photonics community and the hard work of our members to promote relationships between students and industry professionals,” said Eric Stanton, chapter president.

Guided by faculty advisor John Bowers, director of UCSB’s Institute for Energy Efficiency, Stanton and 15 other student engineers devoted much time and energy over the past year to initiatives aimed at informing the community of the technology behind and use of photonics.

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Origins of Random Telegraph Noise, Bias Temperature Instability and Total Ionizing Dose -Defect-Oriented Degradations in Recent VLSIs

This paper explains Random Telegraph Noise (RTN) and Bias Temperature Instability (BTI) and Total Ionizing Dose (TID). They are correlated and caused from the same type of defects in gate and field oxides.

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Assistant Professor Yon Visell’s research on touch and vibrations featured in The UCSB Current article “A Sensitive Subject”

UCSB researchers catalog for the first time patterns of vibration on the skin of the hand that are part of how we sense the world through touch

photo of yon visell in the lab

“Most people don’t have a very clear picture of how touch sensation actually arises,” says Yon Visell, an assistant professor in the ECE Departent and in the campus’s Media Arts and Technology graduate program. While people are familiar with touch as consisting of the interaction between two surfaces — the skin and whatever it is in contact with — they are less aware of the subtle ways that touch sensing helps us to identify and navigate our surroundings, he said.

For instance, if your fingers are numb, you may still be able to move them, but be hesitant to pick up an object or send a text message, because of the lack of sensation — think of what happens when your foot or arm falls asleep. According to a study co-authored by Visell that appears in the Proceedings in the National Academy of Sciences, our hands in particular have access to rich tactile information that travels far beyond the tips of our fingers. This may help to explain some remarkable capabilities of the sense of touch — why, for example, people whose fingers have been anesthetized are still able to feel fine surface detail, as has been demonstrated in prior research.

“The way they seem to be able to do this is by using mechanical signals, or vibrations, that travel beyond the fingers, farther up the arm,” said Visell. “The hand has specialized sensory end organs distributed widely in it that can capture such mechanical vibrations at a distance.”

Their study used a specialized array of tiny accelerometers, or vibration sensors, worn on the sides and backs of the fingers and hands. With this device, the researchers were able, for the first time, to capture, catalog and analyze patterns of vibration in the skin of the whole hand that were produced during active touch. Actions such as tapping and sliding one or several fingers over different types of material, as well grasping, gripping and indirect tapping (using an object to tap on a surface) all gave rise to distinctive vibration signatures. “We can liken this to the different ways that a bell will sound if it is struck by a metal hammer or a rubber mallet,” said Visell.

“How do those signals reflect what it is that we’re doing and what it is we’re touching? Do parts of the hand nearer to the wrist receive significant information about the shape of the object that we’re touching, what it’s composed of, or how we’re touching it? How are different parts of the hand involved in touch sensing?” Visell said of the fundamental questions that motivated his group to pursue this research. “It is possible that the hand, like the ear, is able to use vibrations produced through contact in order to infer what is being touched, and how the hand is touching it.”

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Professors Steven Denbaars and Shuji Nakamura among 2016 Central Coast Innovation Award recipients

image of the CCI awards logo
Materials & ECE Professors Steven DenBaars and Shuji Nakamura among UCSB’s notable entrepreneurs and tech pioneers to be recognized at the 2016 Central Coast Innovation Awards.

The March 24th event, produced in partnership with UCSB’s Office of Technology & Industry Alliances (TIA), UCSB’s CNSI Incubator and the Pacific Coast Business Times, includes Startup Village (3:30-5:30) – a mix and mingle with the most promising emerging & mid-growth startup companies from the Central Coast and the Innovation Awards Reception (5:30-7:30).

UCSB 2016 Central Coast Innovation Award winners include:

  • Spirit of Innovation: Professor Galen Stucky (Materials and Chemistry)
  • Energy & Environmental Science: Soraa – the LED-based lighting company started by Professors Steven DenBaars (Materials & ECE), James Speck (Materials) and Shuji Nakamura (Materials & ECE), recipient of the 2014 Nobel Prize in Physics for his work on the bright blue LED
  • Most Disruptive Technology: Ecorithm – Professor Igor Mezić (Mechanical Engineering)
  • Agribusiness: Apeel Sciences (Technology Management Program – 2012 New Venture Competition winner)
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Test Data Analytics: Exploration of Hidden Patterns for Test Cost Reduction and Silicon Characterization

The manufacturing process for a modern integrated circuit encounters excessively long test time and produces huge amount of test data. There is valuable information hidden in the test data about the device under test (DUT), far more than the binary go/no go classification. Exploring the hidden correlations and patterns in the test data allows better understanding of the DUT and therefore leads to broad applications, such as test cost reduction and silicon characterization for discovering parametric variations and weak links in manufacturing process. A methodology with supporting statistical learning algorithms for test time and cost reduction through exploiting both spatial and inter-test-item correlations in the test data is proposed. A case study of a high-volume industrial device demonstrates that some test items can be identified for removal from the test program without compromising test quality and shows the significant reduction of test time. Next, a framework for characterizing systematic variations and failures through exploring the hidden patterns of test data from multiple test stages is developed. Experimental results demonstrate that the proposed framework reveals comprehensible and significant correlations in an industrial test dataset. Moreover, a software toolbox dedicated to test data analytics is also developed. The toolbox provides flexible and scalable functions for parsing, processing, learning and display test data for both designers and users in the area of test data analytics.

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