Compounds having compositions of Mo(3)Sb(7-x)Te(x) (where x = 1.5 or 1.6) have been investigated as candidate thermoelectric materials. These compounds are members of a class of semiconductors that includes previously known thermoelectric materials. All of these compounds have complex crystalline and electronic structures. Through selection of chemical compositions and processing conditions, it may be possible to alter the structures to enhance or optimize thermoelectric properties

Gascoin, Frank S.

Rasmussen, Julia

Snyder, G. Jeffrey

NASA Technical Reports Server

NASA Tech Briefs, May 2009 31
“Sun” to ≈106 is required; this is not pos-
sible without special devices — modal
filters — that equalize the wavefronts ar-
riving from the two telescopes. 
Currently, modal filters in the ≈10-µm
spectral range are implemented as sin-
gle-mode fibers. Using semiconductor
technology, single-mode waveguides for
use as modal filters were fabricated. Two
designs were implemented: one using an
InGaAs waveguide layer matched to an
InP substrate, and one using InAlAs
matched to an InP substrate. Photon De-
sign software was used to design the
waveguides, with the main feature all de-
signs being single-mode operation in the
10.5- to 17-µm spectral range. Prelimi-
nary results show that the filter’s rejec-
tion ratio is 26 dB
This work was done by Alexander Ksend-
zov, Daniel R MacDonald, and Alexander
Soibel of Caltech for NASA’s Jet Propulsion
Laboratory. For more information, contact
iaoffice@jpl.nasa.gov. NPO-44457
Mo3Sb7–xTex for Thermoelectric Power Generation
These materials could be segmented with lower-temperature thermoelectric materials.
NASA’s Jet Propulsion Laboratory, Pasadena, California
Compounds having compositions of
Mo3Sb7–xTex (where x = 1.5 or 1.6) have
been investigated as candidate thermo-
electric materials. These compounds
are members of a class of semiconduc-
tors that includes previously known
thermoelectric materials. All of these
compounds have complex crystalline
and electronic structures. Through se-
lection of chemical compositions and
processing conditions, it may be possi-
ble to alter the structures to enhance or
optimize thermoelectric properties.
For the investigation, each specimen of
Mo3Sb7–xTex was synthesized as follows:
1. A mixture of specified proportions of
Mo, Sb, and Te powders was heated
for 7 days at a temperature of 750 °C
in a covered boron nitride crucible in
an evacuated, sealed fused silica am-
pule.
2. The chemical reactions were
quenched in cold water.
3. In an argon atmosphere, the crucible
was opened and the reacted powder
mixture was ground in an agate mor-
tar.
4. The powder was annealed by again
heating it as in step 1.
5, The powder was formed into a dense
cylindrical specimen by uniaxial press-
ing in a high-density graphite die for 1
hour at a pressure of about 20 kpsi
(≈138 MPa) and temperature of 873
°C in an argon atmosphere.
The traditional thermoelectric figure
of merit, Z, is defined by the equation Z =
α2/ρκ, where α is the Seebeck coeffi-
cient, ρ is the electrical resistivity, and κ is
the thermal conductivity. Often, in cur-
rent usage, the term “thermoelectric fig-
ure of merit” signifies the dimensionless
product ZT, where T is the absolute tem-
perature. The thermoelectric compatibil-
ity factor, s, is defined by the equation s =
[(1 + ZT)1/2 – 1]/αT. For maximum effi-
ciency, s should not change with temper-
ature, both within a single material, and
throughout a segmented thermoelectric-
generator leg, the segments of which are
made of different materials.
Values of ZT and s were calculated
from measurements of the pertinent
physical properties of Mo3Sb7–xTex
specimens as functions at various tem-
CeFe4Sb12
200
200 400 600 800 1,000 1,200 1,400
0
0.1
7
6
5
4
3
2
1
0
0.2
0.3
0.4
0.5ZT
0.6
0.7
0.8
0.9
1.0
300 400 500 600
Temperature, K
Temperature, K
C
o
m
p
at
ib
ili
ty
 F
ac
to
r, 
V
–1
700 800 900
.x = 1.6
x = 1.5
1,000 1,100
Bi2Te3
Zn2Sb3 Te/Ag/Ge/Sb Alloy
Mo3Sb5.4Te1.6
SiGe
Ce0.9Fe3.5Co0.5Sb12
PbTe
Thermoelectric Figures of Merit and compatibility factors of Mo3Sb7–xTex and other compounds were
determined as functions of temperature.
https://ntrs.nasa.gov/search.jsp?R=20090020540 2019-08-30T06:57:27+00:00Z
32 NASA Tech Briefs, May 2009
Two-Dimensional Quantum Model of a Nanotransistor
Quantum effects that become important at the nanoscale are taken into account.
Ames Research Center, Moffett Field, California
A mathematical model, and software
to implement the model, have been de-
vised to enable numerical simulation of
the transport of electric charge in, and
the resulting electrical performance
characteristics of, a nanotransistor [in
particular, a metal oxide/semiconductor
field-effect transistor (MOSFET) having
a channel length of the order of tens of
nanometers] in which the overall device
geometry, including the doping profiles
and the injection of charge from the
source, gate, and drain contacts, are ap-
proximated as being two-dimensional.
The model and software constitute a
com p utational framework for quantita-
tively exploring such device-physics is-
sues as those of source-drain and gate
leakage currents, drain-induced barrier
lowering, and threshold voltage shift
due to quantization. The model and
software can also be used as means of
studying the accuracy of quantum cor-
rections to other semiclassical models.
The present model accounts for two
quantum effects that become increas-
ingly important as channel length de-
creases toward the nanometer range:
quantization of the inversion layer and
ballistic transport of electrons across the
channel. Heretofore, some quantum ef-
fects in nanotransistors have been ana-
lyzed qualitatively by use of simple one-
dimensional ballistic models, but
two-dimensional models are necessary
for obtaining quantitative results. Cen-
tral to any quantum-mechanical ap-
proach to modeling of charge transport
is the self-consistent solution of a wave
equation to describe the quantum-me-
chanical aspect of the transport, Pois-
son’s equation, and equations for statis-
tics of the particle ensemble.
Non-equilibrium Green’s function
(NEGF) formalisms have been success-
ful in modeling steady-state transport in
a variety of one-dimensional semicon-
ductor structures. The present model
for the two-dimensional case includes
the NEGF equations, which are solved
self-consistently with Poisson’s equa-
tion. At the time of this work, this was
the most accurate full quantum model
yet applied to simulation of two-dimen-
sional semiconductor devices. Open
boundary conditions (in which the nar-
row channel region opens into broad
source, gate, and drain regions) and
tunneling through oxide are treated on
an equal footing. Interactions between
electrons and phonons are taken into
account, causing the modeled transport
to deviate from ballistic in a realistic
manner. Electrons in the wave-vector-
space ellipsoids of the conduction band
are treated within the anisotropic-effec-
tive-mass approximation.
Self-consistent solution of the Pois-
son and NEGF equations is computa-
Drain Current Versus Gate Potential in a conceptual 25-nm-gate-length MOSFET commonly used as an
example for testing MOSFET-simulating software was calculated by means of a three-band and a one-
band version of the present quantum-based model. For comparison, the plot also shows results from
a classical diffusion-drift model and a quantum-corrected model embodied in a commercial MOSFET-
simulation computer program. A drain bias of 1 V and a gate oxide thickness of 1.5 nm were used in
the simulations.
0 0.2 0.4 0.6 0.8 1.0
10–10
10–9
10–8
10–7
10–6
10–5
10–4
10–3
10–2
D
ra
in
 C
u
rr
en
t,
 A
m
p
er
es
/M
ic
ro
n
Gate Potential, Volts
Vd = 1V
Classical-Diffusion-Drift
Quantum
(One-Band)
Quantum-Corrected
Quantum
(Three-Band)
peratures, and the s values of
Mo3Sb5.4Te1.6 were compared with
those of other state-of-the-art thermo-
electric materials (see figure). The ZT
values of both Mo3Sb5.5Te1.5 and
Mo3Sb5.4Te1.6 were found to increase
with temperature up to 1,050 K, which
is just below the decomposition temper-
ature. The fact that ZT for x = 1.6 ex-
ceeds that for x = 1.5 might be taken as
a hint that one could increase ZT by in-
creasing x, except for the observation
that attempts to synthesize Mo3Sb7–xTex
having x > 1.6 resulted in specimens
that appeared to be multiphase. Hence,
other approaches to doping may be
more promising.
The s value of Mo3Sb5.4Te1.6 was
found to increase from about 1 V–1 at
300 K (room temperature) to about 2
V–1 at 1,000 K. This doubling of s indi-
cates poor self-compatibility of
Mo3Sb5.4Te1.6 over the affected tempera-
ture range. However, Mo3Sb5.4Te1.6
could be suitable for segmentation: for
example, a Mo3Sb5.4Te1.6 segment
(which can withstand a temperature
>800 K) could be joined to a lower-tem-
perature PbTe segment at an interface
temperature at or slightly below 800 K,
where their s values are equal.
This work was done by G. Jeffrey Snyder, Frank
S. Gascoin, and Julia Rasmussen of Caltech for
NASA’s Jet Propulsion Laboratory. For more in-
formation, contact iaoffice@jpl.nasa.gov.
NPO-43862


Mo(3)Sb(7-x)Te(x) for Thermoelectric Power Generation

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