newsbreaks
is sensed after being focused through
the lens. (In an alternate configuration,
both near-IR and terahertz light are incident from the same direction in free
space on a device with an area of 1. 17
mm2.) Terahertz radiation from 0.24
to 4. 9 THz was detected with a detection time constant of shorter than 30
ps, while the time constant for near-IR
light (pulses emitted by a Ti:sapphire
laser) was 150 ps. The smallest dimensions of the FET devices were quite
large at 3 µm, allowing easy fabrication.
The detectors can be used in either
pulsed or continuous-wave tabletop experiments. Contact Sascha Preu at
sascha.preu@physik.uni-erlangen.de.
III-V space-based PV cells
could go terrestrial
Photovoltaic (PV) cells made
from III-V semiconductor ma-
terials have the highest dem-
onstrated optical-to-electrical
conversion efficiency (around
40%); however, the epitax-ial-growth process used to
fabricate these cells is expensive and slow, formerly limiting them to space-based and
concentrating PV applications. But a new vapor-liquid-solid (VLS) growth process
from University of California,
Berkeley scientists makes it
possible to grow polycrystalline indium phosphide (InP) thin films on molybdenum (Mo) substrates, clearing a path towards the fabrication of large-area, ul-tra-efficient PV cells for terrestrial applications.
The growth process begins with deposition of indium films (with tunable
thickness from 0.2 to 2 µm) on 25-µm-thick Mo foils via either electron-beam
evaporation or electroplating, followed by e-beam evaporation of a 50-nm-
thick silicon oxide (SiOx) cap. This stack is then heated in hydrogen to a temperature that melts the indium, allowing introduction of a phosphorous vapor that
diffuses into the indium and, upon cooling, results in the precipitation of solid
InP crystals. Besides planar films, textured films with improved carrier collection
can be fabricated by introducing 1-µm-diameter silica beads, for example. After etching away the SiOx cap, luminescence and photogenerated current levels
were found to be comparable to or better than copper indium gallium selenide
(CIGS) thin-film PV materials. Contact Ali Javey at ajavey@berkeley.edu.
Phosphorous
450-800°C
Phosphorous
450-800°C
Indium phosphide
SiOx
Indium phosphate Mo
Textured InP
Molybdenum foil
a)
b) c)
SiOx
Indium Mo
1 µm 500 nm
Ultrafast detector continued from page 9 Single-shot continued from page 9
the fundamental and the second harmonic of a 30 fs pulse from a Ti:sapphire
laser, while at the same time part of the
fundamental was split off and chirped.
The mid-IR pulse was passed through
the medium to be analyzed; the chirped
and mid-IR pulses were then combined
in xenon to create a visible spectrum via
F WDFG. The researchers say that the
chirped-pulse upconversion method is especially applicable to mid-IR spectroscopy with attenuated total reflectance
(ATR) because the temporal and spatial
overlap of the MIR pulse and the chirped
pulse would not be changed by exchanging the sample in the ATR region.
Contact Takao Fuji at fuji@ims.ac.jp.
