**Lithographically Defined Microporous Carbon GaN Growth Templates**

[Bruce Burckel (1815)]{.underline}

Gallium nitride (GaN) and aluminum nitride (AlN) are wide bandgap
semiconductors used to make blue and UV LEDs and lasers. Due to a lack
of bulk substrates, GaN and AlN must be grown on lattice-mismatched
substrates such as sapphire (Al~2~O~3~) and silicon carbide, yielding
high defect density which lowers performance and reliability of devices
fabricated in the material. Research has shown that heteroepitaxial
growth on patterned substrates reduces defect density by orders of
magnitude. I will present our recent progress toward using
lithographically defined carbon structures as a growth mask for
heteroepitaxial crystal growth of GaN. In this work, interferometric
lithography is used to create micron and submicron scale 2-D and 3-D
patterns in photoresist. The patterns are then converted to carbon via
pyrolysis in a reducing atmosphere yielding a growth mask with micron
scale periodic features. This talk covers the brief history of the
project as well as current challenges and future directions for the
research.

**&**

**Investigation of Nanosecond and Picosecond Excitation Schemes for
Laser-Induced Fluorescence Imaging Of Atomic Hydrogen in Flames**

[Waruna D. Kulatilaka (8353)]{.underline}

Atomic hydrogen is an important radical species in reacting flows due to
its high reactivity and diffusivity. Laser-induced fluorescence (LIF) is
used widely to measure atomic hydrogen concentrations in flames and
plasmas. In practical combustion environments, single-photon excitation
from the ground electronic state of atomic hydrogen is not possible
because the excitation wavelengths lie in the vacuum ultraviolet (VUV)
where the medium is optically thick. Instead, excitation wavelengths can
be shifted into the UV using various multi-photon schemes, as
demonstrated in the past. The quantitative measurement of atomic
hydrogen concentration in flames using two-photon-excited LIF (TP-LIF)
is often complicated by laser-dependent photolytic interferences and
population loss from excited states by collisional quenching and
stimulated emission (SE). In an analogous detection scheme for atomic
oxygen, avoidance of laser-induced interference was demonstrated by
using picosecond laser pulses instead of the more commonly used
nanosecond pulse durations. In this study, we directly compare the
nanosecond and picosecond excitation schemes for detecting atomic
hydrogen in flames. TP-LIF line imaging of H atoms is employed in a
range of premixed methane/air, hydrogen/oxygen and hydrogen/air flames.
Sources of photolytic interferences and the effect of stimulated
emission (SE) are studied for both excitation schemes.

**Hosted By:**

![](media/image1.wmf){width="0.9152777777777777in" height="0.8in"}

The Post-doctoral/Recent Staff Seminar (PReSS) series will be
videoconferenced between NM and CA. PReSS is an informal seminar held
once a month where post-docs or recently-hired staff present on a
research topic they are working on or are interested in. Through
attendance and participation, researchers in various organizations and
fields across Sandia gain exposure, learn about other work being
performed at Sandia, share ideas, and network.

[[http://www.sandia.gov/pd2p/index.html]{.underline}](http://www.sandia.gov/pd2p/index.html)