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Quantum engineered field-effect transistors for warm, wide-bandwidth,
near quantum-limitedTerahertz heterodyne receivers
Mark Sherwin
UCSB
Physics Department
and
Institute for Terahertz Science and Technology
Herschel Space Observatory
Bilpratt et. al., A&A 518 L1 (2010)
Heterodyne Instrument for Far Infrared (HIFI)
Th. De Grauw, et. al., A&A 518 L6 (2010)
HIFI spectrum of Orion hot core
Th. De Grauw, et. al., A&A 518 L6 (2010)
35Cl/37Cl ratio in dense molecular clouds
J. Cernicharo et. al., A&A L115 (2010)
H2O in cometary atmosphere
3x105 km
P. Hartogh et. al., A&A 518, L150 (2010)
Coherent detection (e. g., heterodyne mixing)
Frequency
Sig
nal
fLO= 1 THz to 5 THz
fsigfIF = 1 - 10 GHz
• Receiver Noise:
Top = TA+ TM + TAmp / M
• Integration Time:
(TR)2
Mixer
TM
M
PLO
fLO
TAmpfIF
IF AmpBackend
SpectrometerfIF = | fLO - fsig|
HETERODYNE RECEIVERf / f = 107 - 108
Quantum Limit
(double sideband)TM≥hf/2kB
TQ= 25K @ 1 THz
TA
DSB system noise temperature on HIFI
Th. De Grauw, et. al., A&A 518 L6 (2010) Operating temperature 1.7K
Heterodyne receivers for future missions
• Higher frequency (HD @ 2.7 THz, OI@4.7 THz)
• Lower noise
• High temperature operation (40-100K)
• Wider bandwidth (>15 GHz vs. 4 GHz for phonon-cooled HEB)
• Low LO power
• Arrays
Superconducting hot-electron bolometers
Superconducting hot-electron bolometers
Theory, proposed mixers
Proposed mixers
• Mixer noise temperature (classical theory): 200K DSB
• Operating temperature: Top=30-100K
• LO frequency: 1.5-5 THz
• IF bandwidth: >15 GHz
• LO power requirement: – ~1 µW@ Top=30K
– ~100µW@ Top =100K
• Planar, suitable for arrays
Quantum-engineered FET as hot-electron bolometer
SourceDrain
IF
THz
Front gate (to antenna)
Back gate (to antenna)
absorberdetector
“Tunable antenna-coupled intersubband terahertz (TACIT) sensor”
Relevant physics in quantum wells
• Absorption: intersubband transition in quantum well
• Detection: temperature-dependent mobility of 2-D electron gas
IF
THz
absorber
IF
THzdetector
Example: intersubband absorption in square quantum well
Intersubband absorption in square well
0 20 40 60 80Position (nm)
Ene
rgy
(meV
)250
150
50
0
100
200
Al0.3Ga0.7AsGaAs
in-plane wavevector k
Ene
rgy
Absorption vs. dc electric field, constant Ns
2.4 THz 4.8 THz
Absorption peak vs. dc electric field, charge density
Experiment:Williams et. al., PRL, 2001)
Theory:Ullrich and Vignale Ibid.
ns=1010 cm-2
ns=13x1010 cm-2
Time-dependent Local density approximationAll parameters from experiment
Frequency (T
Hz)2.4
4.8
Intersubband absorption below 1 THz
Detection: temperature-dependent mobility
IF
THzdetection
• Mobility of 2-D electron gases vs. temperature
• Mobility determined by electron temperature Te.
Detection
• Hot-electron bolometric
IF
THzdetection
1R||
dR||
dT
⎛
⎝ ⎜
⎞
⎠ ⎟ ≡γ =0.01−0.03K−1@50K
ρ=dVdP
=IR||γT1
CV
=IR||γT1
1NSAkB
Speed
• Phonon cooling
• Diffusion cooling– 20 GHz IF bandwidth demonstrated in 4µm long millimeter-wave mixer. (M.
Lee et. al., APL 2001)
J. N. Heyman et. al., PRL 74, 2682 (1995)
109
1010
1011
0 0.04 0.08 0.12
1/T (K-1)
50 25 10
T (K)IF
bandwidth (G
Hz)
0.16
1.6
16
Coupling efficiency: impedance of active region
LV(t)
Area ACharge Q+ + + + + + + + + + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - - - - - - - - - - - - -2-DEG=
Dipole sheeton springs
On resonance
€
C =ε 0εA
L
R =q2
ε 2ε 02m *
Ns f12n(T)
2πf( )2A2Γ
R can be 20 in future designs
C can be tuned out byrf embedding circuit design
Source-Load coupling can be>90%!
The math
€
Z ω( ) =Vω
Iω
€
Z(ω*) =Vω*
Iω*
=L
iω*εε0A+
NSe2 f12n(T )ε 2ε0
2Am *ω*2 2Γ=
1iω*C
+ Rzz
€
= 1iωεε0A
L −χ 2D ω( )
εε0
⎛
⎝ ⎜
⎞
⎠ ⎟€
Vω = EωL − P2D /εε0€
χ2D ω( ) =NSe2 f12n(T )
m *1
ω*2 −ω2 + i2ωΓ
€
P2D(ω) = χ 2D(ω)Eω
M. Sherwin et. al., Proceedings of 2002 Monterey Submm workshop
LV(t)
Area ACharge Q+ + + + + + + + + + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Can couple efficiently to ~10,000 electrons!
LV(t)
Area ACharge Q+ + + + + + + + + + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - - - - - - - - - - - - -2-DEG=
Dipole sheeton springs
On resonance
49 Ohms*
18 Ohms*
*on resonance for high-quality 40 nm square well, Te=50 K, f=2.3 THz, A=5µm2,L=0.15 µm
Theory for NEP, TM
• NEP (direct detector)
– =insertion loss, =1/R(dR/dT), NS= sheet density, A=active region area
• Double sideband noise temperature (mixer)€
NEP = α −12kB
NS A
τ
4
γ 2+ Te
2
€
TM = α −1 4
γ 2Te
+ Te
⎛
⎝ ⎜
⎞
⎠ ⎟
M. Sherwin et. al., Proceedings of 2002 Monterey Submm workshop
Johnson noise Thermal conductionnoise
Performance limits for TACIT mixer
10,000 electrons106 cm2V-s @ low temp.Bulk LO phonon scattering
First-generation devices
• Twin slot coupled to coplanar waveguide
• Demonstrated direct detection, electric field tuning
• Difficult fabrication with very low yield
• Quantum well far from optimal
• Microwave embedding circuit far from optimal
• Collaboration with W. R. McGrath, Paolo Focardi
Completed TACIT detector
Source
Drain
Front gate bias line
Back gate bias line
10nm
130
nm
10.8 nm
197
nm
500
nm
1000
nm
200
nm
70 n
m9 nm
70 n
m
3 nm
0.5
mm
subs
trat
e
buff
er
etch
sto
p
cap Si d
elta
Dop
ing
(101
2 c
m-2)
Coupled quantum wellsba
rrie
rs
cap
GaAs Al0.3Ga0.7As
Sample structure design and growth
Intersubband absorption characterization
THz in (FTIR) THz out
bolometer
1.6 THzDesign frequency
theory
Electrons in quantum well
Fabrication of 1st generationTACIT detector
Epoxy Bond and Stop-Etch (EBASE), Sandia
1. Process front side
2. Epoxy bond to GaAs wafer
3. Etch away substrate
4. Process back side, etch vias
Experimental Set Up
IF
THz
GaAs Host Substrate
Silicon LensUCSB Free Electron Laser at 40-80 cm-1
Sample in a Helium Flow Cryostat, T=20-100K
Current amplifierSources and measures channel
oscilloscope
0 V
Gate Voltage
0
5
10
15
-5 0 5 10 15
Signal /mV
Time/ microseconds
5s, 1mW
0
10
20
30
40
50
-5 0 5 10 15
Signal /mV
Time/ microseconds
Response to fast THz pulses
1.5 ns 3.5 ns
Tunability with gate voltage
300
400
500
600
700
800
900
1000
0.25
0.26
0.27
0.28
0.29
0.3
0.31
0.32
0.33-1 -0.5 0 0.5 1 1.5
signal/ nanoAmps
signal/milliVolts
front gate - back gate Voltage
Sample ATb=100KTe~120Kf=1.53 THzFEL
Sample BTb=77KTe~100Kf=1.6 THzMoleculargas laser
Pho
tocu
rren
t (n
A)
Photovoltage (m
V)
Charge density held constant.
Origin of “double peak”
2000 2100 2200 2300 2400 2500-50
0
50
100
150
200
250
300field = -0.2 mV/Angstrom
Energy / meV
2000 2100 2200 2300 2400 2500
field = 0.2 mV/Angstrom
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
45
50
55
60
65
70
Intersubband Resonance
Frequency /THz
Intersubband Resonance
Frequency /cm
-1
-0.2 -0.1 0 0.1 0.2
Electric field (mV/Angstrom)
Inte
rsub
band
ab
sorp
tion
fre
quen
cy (
TH
z)
New design*
• Easier fabrication• Eliminate some parasitics• Can rapidly iterate to optimize
Inspired by Chris McKenney’s thesis, Cleland group.
TACIT mixer development strategy
• Microwave engineering• MBE growth of wafers with high mobility,
narrow intersubband linewidth.• Develop fabrication process• Test and iterate
40
TACIT specs to enable new missions
• High-temperature operation• Wide bandwidth• High frequency• Low noise
41
Herschel cryogenics
• Herschel– Large dewar drives up mission costs– Duration limited to 3.5 years
42
Long-duration mission with cryocooler
• Atmospheric Infrared Sounder (AIRS)– Detectors @ 58K– Long-lifetime cryocooler– Launched 2002
43
Potential platforms for TACIT mixers
• Explorer-class missions– Astrophysics– Planetary science
• Long-duration ballooning• SOFIA
44
Summary and conclusions
• THz heterodyne spectroscopy: important science• TACIT mixers offer improvements over state of art
• Timely to develop TACIT mixers in concert with new mission concepts
45
Superconducting HEB actual
TACIT mixer theory
Operating temp. 2K 30-100K
Bandwidth 4 GHz >15 GHz
Noise temperature @ 2.5 THz
1000K 200K
Acknowledgments
• W. R. McGrath (JPL): Antenna design• P. Focardi (JPL): Antenna impedance, mode matching
theory• G. B. Serapiglia (UCSB->law school): processing,
characterization, experiments• Sangwoo Kim (UCSB-> Tanner Research Labs): room-
temperature devices.• M. Hanson (UCSB): sample growth• A. C. Gossard (UCSB): sample growthFunding: NASA, NSF
46
Insertion losses for this device and experiment
• Matching antenna pattern– Only 4% coupled into antenna mode*– Device fried before could be improved– Mode matching can be increased to >90%+
Antennapattern
Illumination pattern
*Computed following Goldsmith, “Quasioptical systems”
+Focardi, McGrath and Neto, IEEE MTT 2004.
Zload=(1.7-i39)
Pin
€
PLoad
PIn
=1−ZS − ZL
*
ZS + ZL
2Zsource=(20-i40)
• Matching antenna and load impedances- Only 2.5% coupled from antenna into load
Responsivity at Tbath=80K
• Roptical=V/Pin=0.1 V/W
• Rinternal=Roptical/(total insertion loss)=107±75 V/W
• Relectrical
• Theory: Rinternal =(1100±400) V/W€
Relectrical =
V
I−
dV
dI
⎛
⎝ ⎜
⎞
⎠ ⎟
2V= (400 ±150)V /W
Electrical transport data
This sample
106 mobility2-DEG
Electrical transport data
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