Science - USA (2020-03-20)

INSIGHTS | PERSPECTIVES

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field effects ( 5 ); however, these systems
have typically focused on high-temperature
sources (>1000°C), whereas more than 95%
of wasted heat in the United States—and
85% of the associated work potential—is
below 400°C ( 2 ). The direct conversion
of longer-wavelength thermal radiation
poses numerous challenges, such as lower
incident photon flux and the limited avail-
ability of efficient low-bandgap semicon-
ductors. An alternative approach is to use a
rectifying antenna, which finds wide appli-
cation at lower-energy microwave frequen-
cies. In these devices, incident oscillating
electromagnetic waves are funneled by an
antenna-like structure and drive a direct
electrical current through a fast diode ( 6 ).
At higher frequencies, ultrafast direct tun-
nel diodes in a metal-insulator-metal con-

figuration have shown promise ( 7 ), but the
large asymmetry needed in tunnel diodes at
the small voltages associated with IR ther-
mal radiation has remained a roadblock.
Davids et al. tackle this challenge and
demonstrate a bipolar metal-oxide semicon-
ductor tunnel junction diode that converts
incident photons in the long-wave IR part
of the electromagnetic spectrum (7 to 14
μm) to electricity. Incident electromagnetic
waves are coupled by a grating to an electro-
magnetic mode that confines light strongly
in a 3- to 4-nm silicon dioxide barrier be-
tween the metal grating and a base layer of
doped silicon (see the figure). The strong
electromagnetic field concentration drives
photon-assisted tunneling of electrons from
the doped p-type silicon into the metal and
to the n-type silicon part. Although the over-

all process shares superficial similarities to

a photovoltaic system because it uses a pn

junction, the current instead is generated

from photon-assisted tunneling between two

metal-oxide semiconductor diodes rather

than a depletion region.

The authors demonstrate a peak power

density of 61 μW/cm^2 for a 350°C radia-

tive source. This appears relatively modest

at first glance, as the achieved power cor-

responds to a considerably lower efficiency

than the Carnot limit. However, these re-

sults are orders of magnitude better than

previous unipolar metal-oxide semiconduc-

tor tunnel junction diodes ( 8 , 9 ). The use of

multiple interdigitated bipolar junctions—

which yields a periodic well structure where

charge is stored that can be additionally

pumped from p+ to metal and from metal

to n+, enables the leap in performance. This

process yields an open-circuit voltage that

is substantially higher than the actual volt-

age induced by the IR radiation across the

tunnel junction and overcomes the chal-

lenge of achieving high asymmetries at

small voltages.

Converting thermal radiation to elec-

tricity from low-temperature sources (or

higher-temperature ones such as the Sun)

can be more effectively harnessed through

photonic approaches that concentrate

light or enhance light-matter interaction.

These approaches are essential to enable

improved performance for thermophoto-

voltaic devices ( 4 , 5 ). In a similar vein, the

authors cleverly exploit both photonic de-

sign and materials properties to enhance

the photon-assisted tunneling effect. The

authors leverage a phonon resonance of

silicon dioxide in the 8-μm range, which

overlaps well with the blackbody spectrum

of thermal emitters in the 200° to 400°C

range. This specific resonance yields wave-

length ranges where silicon dioxide has

a near-zero permittivity ( 10 ) and enables

funneling of incident thermal radiation to

improve tunneling efficiency. Although this

initial performance is compelling, it leaves

a large fraction of incident thermal radia-

tion uncoupled at wavelengths away from

silicon dioxide’s phonon resonance. Intrigu-

ing opportunities and challenges thus lie in

combining photonic microstructures and

exploiting the intrinsic dispersion of dif-

ferent materials to make this photonic con-

centration more broadband. In doing so, a

larger fraction of incident thermal radiation

could be harnessed.

The authors’ use of the globally standard

complementary metal-oxide semiconduc-

tor (CMOS) platform allows for the scal-

ability that will eventually be needed for

this technology. However, a substantial

amount of wasted heat still lies at tempera-

tures below 250°C ( 2 ). Conversion of low-

temperature radiated heat to electricity, all

the way down to ambient (0° to 50°C) heat

rejected as thermal radiation to the sky ( 11 ,

12 ), represents an intriguing frontier for

energy efficiency but must ultimately be

able to deliver meaningful performance at

an attractive cost point. Indeed, the laws

of thermodynamics that Carnot glimpsed

in his prescient experiments two centuries

ago place fundamental constraints on our

ability to convert energy. Nonetheless, con-

siderable headroom still remains to better

control and harness our collective thermal

energy footprint at the very largest scales. j

REFERENCES AND NOTES

1. S. Carnot, Réflexions sur la puissance motrice du feu
   (1824).


2. U.S. Department of Energy, Advanced Research
   Projects Agency–Energy, Request for Information (RFI)
   DE-FOA-0001607 on Lower Grade Waste Heat Recovery
   (2016); https://arpa-e-foa.energy.gov/FileContent.
   aspx?FileID=80197dc7-ad62-4d58-a8c4-4bdbc-
   c0a52c6.


3. P. S. Davids et al., Science 367 , 1341 (2020).


4. A. Lenert et al., Nat. Nanotechnol. 9 , 126 (2014).


5. A. Fiorino et al., Nat. Nanotechnol. 13 , 806 (2018).


6. J. O. McSpadden, T. Yoo, K. Chang, IEEE Trans. Microw.
   Theory Tech. 40 , 2359 (1992).


7. S. Grover, O. Dmitriyeva, M. J. Estes, G. Moddel, IEEE
   Trans. Nanotechnol. 9 , 716 (2010).


8. P. S. Davids et al., Nat. Nanotechnol. 10 , 1033 (2015).


9. J. Shank et al., Phys. Rev. Appl. 9 , 054040 (2018).


10. A. Shahsafi et al., Phys. Rev. Appl. 10 , 034019 (2018).


11. P. Santhanam, S. Fan, Phys. Rev. B 93 , 161410 (2016).


12. A. P. Raman, W. Li, S. Fan, Joule 3 , 2679 (2019).

ACKNOWLEDGMENTS

Supported by a Sloan Research Fellowship in Physics (Alfred

P. Sloan Foundation).

10.1126/science.aba8976

~1 cm

A device with many junctions
By using many tunnel diodes (see inset), a larger voltage (V) is generated
from the IR radiation of the heat source. The light is funneled into a very thin
silica barrier between doped silicon and an aluminum grating.

Bipolar semiconducting

tunnel diode

The concentrated light (red)

drives charge carriers to tunnel

from the p-type to the n-type

silicon and thus generates a

rectifying current.

Silicon

dioxide

Broadband

IR thermal

radiation

Bipolar metal-oxide
semiconductor tunnel diode

n+ Si p+ Si

Aluminum

Silicon dioxide

Resistor

Bipolar semicconducting

n+ Si p+ Si

Resistor

V

IR light

concentration

250° to 400°C

heat source

Silicon (Si)

Converting heat into power
Infrared (IR) radiation from lower-temperature heat sources is usually wasted, but certain specialized devices
can convert this radiation into electricity. A bipolar semiconductor tunnel diode accomplishes this through a
combination of light funneling and light-facilitated electron tunneling.

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