Vol. 24, No. 15 | 25 Jul 2016 | OPTICS EXPRESS 17041
4.3 µm quantum cascade detector in pixel
configuration
A. H ARRER , 1,* B. S CHWARZ , 1 S. S CHULER , 2 P. R EININGER , 1 A.
W IRTHMÜLLER , 3 H. D ETZ , 4 D. M AC FARLAND , 1 T. Z EDERBAUER , 1 A.
M. A NDREWS , 1 M. R OTHERMUND , 5 H. O PPERMANN , 5 W.
S CHRENK , 2 AND G. S TRASSER 1,6
1 Institute
for Solid State Electronics, TU-Wien, Floragasse 7, 1040 Wien, Austria
Institute, TU-Wien, Guçhausstrasse 27-29, 1040 Wien, Austria
3 Laboratoire Temps-Fréquence, Université de Neuchâtel, Avenue de Bellevaux 51, 2000 Neuchâtel,
Switzerland
4 Austrian Academy of Sciences, Dr. Ignaz-Seipel-Platz 2, 1010 Wien, Austria
5 Fraunhofer Institute for Reliability and Microintegration (IZM), Gustav-Meyer-Allee 25, 13355 Berlin,
Germany
6 Center for Micro- and Nanostructures, TU-Wien, Floragasse 7, 1040 Wien, Austria
2 Photonics
* [email protected]
Abstract: We present the design simulation and characterization of a quantum cascade detector
operating at 4.3 µm wavelength. Array integration and packaging processes were investigated.
The device operates in the 4.3 µm CO2 absorption region and consists of 64 pixels. The detector
is designed fully compatible to standard processing and material growth methods for scalability
to large pixel counts. The detector design is optimized for a high device resistance at elevated
temperatures. A QCD simulation model was enhanced for resistance and responsivity optimization.
The substrate illuminated pixels utilize a two dimensional Au diffraction grating to couple the
light to the active region. A single pixel
√ responsivity of 16 mA/W at room temperature with a
specific detectivity D∗ of 5 · 107 cm Hz/W was measured.
© 2016 Optical Society of America
OCIS codes: (040.3060) Infrared; (110.3080) Infrared imaging; (230.5160) Photodetectors; (040.5570) Quantum
detectors.
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1.
Introduction
Mid-infrared detector arrays have been investigated intensively in the past years. Their applicability in a large variety of fields including thermal imaging, remote detection, astronomy and
military countermeasure systems promoted the research on so called focal plane arrays (FPA).
Microbolometer, HgCdTe [1, 2], InSb [3] and quantum well infrared photodetector (QWIP)
FPAs have already proven their performance and are well established technologies. Despite of
their performance, HgCdTe detectors are highly sensitive to their material composition and
material distribution. Moreover, batch production relies on the availability of growth and bandgap
compatible substrates with respect to absorption in bottom side illuminated designs. QWIP
FPAs have been scaled up to provide megapixel and dualcolor imaging [4, 5]. Photoconductive
QWIP FPAs require operation temperatures which are not accessible by thermo-electric coolers
(TEC) and suffer therefore from relatively heavy space consuming casings. Type II superlattice
Vol. 24, No. 15 | 25 Jul 2016 | OPTICS EXPRESS 17043
interband detectors proposed already in 1987 [6] offer focal plane array compatibility [8],
detection in the mid-infrared and long wave infrared [9, 10]. Operation at elevated temperatures
up to 420 K was √
demonstrated [7]. Thermal imaging up to 180 K with a specific detectivity D∗
12
of 1.05 · 10 cm Hz/W at 150 K was shown utilizing a diffusion barrier [11].
(a)
(b)
pixel mesa
(c)
10 µm
bottom contact
500 µm
Au grating
top contact
isolation trench
HH
j
100 µm
Fig. 1. Scanning electron microscope image of a section of the (a) 8×8 pixel array with
separate bottom contacts, (b) a detailed SEM image of a single pixel with the corresponding
bottom contact (square without grating structure) surrounded by the isolation trench. The
inset (c) shows the metal grating in detail.
Quantum cascade detectors (QCDs) offer the advantages of room temperature operation in
combination with the design freedom of quantum cascade based intersubband devices and
compatibility to standard InP based growth and fabrication technology. Evolved from photovoltaic
QWIPs [12] they utilize a LO-phonon extraction scheme which provides more design freedom. A
challenging aspect of QCD design is to maintain a high device resistance while the responsivity
remains reasonably high. An advantage of QCDs over well established detector types is their
designable operation wavelength, low noise photovoltaic operation mode and therefore room
temperature operation capability. With a well established material system detectors for different
operation wavelengths can be designed by adjusting the well and barrier dimensions, while the
material properties remain the same. For mobile, battery powered applications an uncooled
or thermo-electrically cooled mid-infrared imaging FPA is needed. Beside the single pixel
performance, scalability to FPA dimensions of several tens of millimeters is necessary for high
pixel counts. MBE and MOCVD growth on InP provides these requirements. QCDs have already
proven their room temperature performance at a wide wavelength range [13–15] and broadband
designs [16–18]. As shown by Hofstetter et al. QCDs around 4.3 µm wavelength provide CO2
sensing abilities [19] with high selectivity. Recently QCD performance and robustness at longer
wavelengths was increased by a diagonal transition design [20]. The InAs/AlAsSb material system
had been introduced for short wavelength high resistance QCDs [21]. Dependent on the desired
application the operation wavelength and several QCD parameters have to be chosen. QCD
design involves a trade-off between responsivity and device resistance which then influences all
other parameters such as the number of periods N, barrier thicknesses, the absorption α and the
specific detectivity D∗ . A description of the design parameters, their influence and optimization
considerations is presented in more detail in [22–24].
In this paper we demonstrate a quantum cascade detector in pixel configuration processed as
array [Fig. 1]. The detector is substrate bottom side illuminated with an Au diffraction grating
processed into the top contact layer. The active region is optimized for a high device resistance.
Thereby a resistance increase in the entire temperature range was achieved. Sufficient QCD
resistance for operation with the custom read out integrated circuit (ROIC) can be ensured from
80 K up to 240 K. The presented device is based on the InGaAs/InAlAs material system grown
Vol. 24, No. 15 | 25 Jul 2016 | OPTICS EXPRESS 17044
(b)
(a)
Wavelength (µm)
4.5
4
5
A
B C D E F G H I J A’
Responsivity (mA/W)
30
80K
25 120K
160K
20 200K
240K
15
280K
10 300K
5
0
2000 2150 2300 2450 2600
Wavenumbers (cm−1 )
Fig. 2. Band diagram (a) of one QCD period with the corresponding extraction efficiencies
for all levels of the extractor and LO-phonon staircase calculated for 300K. The active
region consists of 20 periods. The simulated QCD responsivity (b) is based on the simulated
absorption efficiency of a 45◦ mesa device.
lattice matched by molecular beam epitaxy (MBE) on semi-insulating InP substrate.
2.
2.1.
QCD design
Heterostructure
At shorter mid-infrared wavelengths a resonant tunneling extraction scheme is more favorable
than a diagonal optical transition. Due to the higher energy of the optical transition in this
wavelength range back scattering to the ground level of the active region is reduced. The band
structure of one period is illustrated in Fig. 2(a). The active transition takes place in well A,
where A’ denotes the active well of the next period. The simulated extraction efficiencies are
calculated for all subsequent levels including the LO-phonon staircase (B-J). An electron in the
ground level of well A can be excited to one of the three degenerated excited states in well A
(delocalized between A, B and C) which have extraction efficiencies of 81.84%, 80.58% and
86.04%, respectively. These efficiencies express the probability of an electron, excited to the
corresponding level, to reach and to be captured in the ground level of the next QCD period (well
A’). The extraction efficiency is calculated as a weighted average of these levels.
The total extraction probability calculated considering the absorption strength for each
level allows for extraction optimization from the active well (A/A’) to the first extractor level
(B/B’). In order to identify bottlenecks, the extraction efficiency is calculated also for all
following LO-phonon staircase levels. The transition between well J and A’ is chosen to
exhibit a higher energy difference to prevent thermally excited carriers from back-filling the
lower staircase levels of the previous period. This is important to achieve a temperature
insensitive responsivity as thermal back-filling would reduce the absorption efficiency at elevated
temperatures. The device consists of 20 periods with a total active region thickness of 1.36 µm
sandwiched between a 800 nm InGaAs top contact layer and a 500 nm InGaAs bottom contact
layer, both Si doped to 1.5e18 cm−3 . A period of the active region consists of the layer
structure 4.3/6.5/0.85/6.5/0.9/6.5/1.0/5.5/1.2/4.5/1.45/4.5/1.7/4.5/2.05/4.0/2.4/4.0/2.85/3.0 nm
starting with the active well A, InGaAs in bold, InAlAs normal and the 8e17 cm−3 doped well
underlined.
Vol. 24, No. 15 | 25 Jul 2016 | OPTICS EXPRESS 17045
The Vienna Schrödinger Poisson (VSP2) frameworks semi-classical Monte-Carlo transport
simulator originally developed for quantum cascade laser design was extended for QCD design
and simulations [20, 25]. The tools allow for simulation of the intersubband scattering rates for
short wavelength detectors, the absorption, differential device resistance, responsivity and the
specific detectivity. The model accounts for all possible scattering transitions and is based on three
consecutive periods. For extraction efficiency simulation, a scattering event contributes to the
current flow, if it scatters from the middle period to the right or to the left adjacent period. From
the resulting scattering matrix the outbound scattering rates of the ground levels are deleted and
all electrons are initially set to be in the excited state(s) of the active well (A/A’). The scattering
matrix is then applied n times to the initial population. After n events the electrons accumulate in
the ground states which have no outbound scattering. The ratio between the amount of electrons
in the ground state of A to A’ then gives the total extraction efficiency. The absorption efficiency
η eff is modeled for 45◦ mesa devices considering the facet transmission Tfacet ≈ 70% and all
P
possible optical excited intersubband transitions by the absorption coefficient αisb = i, f α i f in
the active region.
!!
1
η abs = Tfacet 1 − exp − √ αisb Np L p ,
(1)
2
where Np is the number of QCD periods and L p the length of a period. The simulated responsivity
R(λ) is calculated based on the absorption efficiency. In contrast to QWIPs the capture probability
pc for QCDs can be assumed to be close to unity. The peak responsivity is then given by
Rp =
λq
λq
Is
=
η abs gp =
η abs pe
Ps
hc
hcNp
(2)
with the total detector current Is , the total incident radiation power Ps , the elementary charge q,
Plancks constant h, speed of light c, absorption efficiency η abs , photodetector gain gp and the
extraction efficiency pe . In-plane non-parabolicity is included in the model to correctly predict the
red-shift with increasing temperatures [Fig. 2(b)]. The QCD differential resistance model is based
on the work of Delga et al. [26]. All scattering transitions between subbands are replaced by a
conductance. An equivalent conductance circuit is extracted, which is solved by node analysis to
obtain the device resistance. The model accounts for inter period transitions by periodic repetition
of the scattering matrix.
2.2.
Optical and array design
The QCD is designed as a substrate bottom side surface-normal illuminated pixel device. A QCD
array was processed and flip-chip bonded to a CMOS ROIC. Since intersubband detectors are
only sensitive to light with the electric field component polarized in growth direction a coupling
scheme to the active region is required. Light coupling is achieved by a two dimensional 2nd
order Au grating with a grating period of p = 1.4 µm, depth d = 450 nm and duty cycle 0.4. The
pixel to pixel pitch is 225 µm in both directions, with a pixel size of 109 µm × 109 µm. The pixel
pitch is determined by the size of the prototype RIOC amplifier circuit for each pixel. This size
can be reduced in future designs.
The optical simulations for light coupling were conducted with the active regions absorption
αisb (λ) retrieved from our QCD model. Absorption was taken into account only in growth
direction to account for the intersubband selection rule. For all materials the mid-infrared complex
refractive indices n(λ) were taken into account. The grating depth, duty cycle and grating period
were optimized for maximum coupling efficiency. The Au layer of the grating is used as the top
contact of the pixels and serves as the bonding pad for Au thermo-compression flip-chip bonding.
Vol. 24, No. 15 | 25 Jul 2016 | OPTICS EXPRESS 17046
(a)
Wavelength (µm)
4.5
4
(b)
240
18
80K
16
14 120K
12 160K
10 200K
8 240K
6 280K
4 300K
2
0
2000 2100 2200 2300 2400 2500 2600 2700
1011
1010
109
108
107
106
105
104
103
102
R0 (Ω)
Responsivity (mA/W)
5
Wavenumbers (cm−1 )
Temperature (K)
160
120 100
80
EA = 228 meV
0.005
0.0075
0.01
0.0125
1/T (K−1 )
Fig. 3. Photocurrent spectrum and temperature performance of the 4.3µm 20 period QCD
design measured with a 100µm × 100µm mesa device. The illumination is through a 45◦
polished facet.
3.
Results
The QCD material was first characterized as a 100 µm × 100 µm mesa device. As intersubband
devices are subjected to the intersubband selection rule, a 45◦ facet was polished onto the sample
for light coupling. The photocurrent spectra were recorded by FTIR (Fourier transform infrared)
spectroscopy illuminated by a broadband globar source. In order to determine the detectors
wavelength dependent responsivity R(λ) the power density spectrum and total power of the
broadband globar source were recorded. With respect to the measured parameters the responsivity
from Eq. (2) can be expressed as
R(λ) =
Is (λ)
.
Pi (λ)M
(3)
The photocurrent spectrum Is (λ) is obtained by the FTIR recorded photocurrent spectrum
normalized by the measured total photocurrent Itot . The total incident power Pi is determined
by the spatial power distribution of the focused globar source and its fraction impinging on
the 45◦ facet. The factor M accounts for the polarization selection and the cryostat window
transmission. The cryostat ZnSn window transmission was determined to be 75%. For mesa
devices the polarization was accounted with a factor of 0.5 since 45◦ facet devices are only
sensitive to light polarized in growth direction (M = 0.75 × 0.5). Due to the 2D metal array the
pixel detectors are sensitive to unpolarized light (M = 0.75).
Figure 3 shows the temperature dependent responsivity and differential resistance of a 45◦
mesa device. The two pronounced valleys are due to the strong CO2 absorption bands. In
contrast to the simulation a responsivity increase between 80 K and 160 K can be observed. This
may be accounted to carrier freeze-out or the increasing dominance of coherent tunneling and
electron-electron scattering which are not included in the simulation model. The characteristic
red-shift with increasing temperatures is due to strong in-plane non-parabolicity. Due to the
optimization for higher operation temperatures, the CO2 absorption is mostly aligned with the
photocurrent peak at temperatures between 240 K and 300 K. The measured responsivity is
slightly below the simulated performance illustrated in Fig. 2(b). This mismatch can be accounted
to the Ti /Au metal-semiconductor contact resistances and the contact layer to first QCD period
resistances, which are not included in the simulator model. From previous experiments we noticed
that the design of the contact layer to the first QCD period is relevant, especially if the contact
Vol. 24, No. 15 | 25 Jul 2016 | OPTICS EXPRESS 17047
layer would be low doped [27]. The pixels are processed as sparse pixels which do not cover
the maximum available area. This design decision is due to the resistance limitations of the
prototype read out circuit. With the pixel mesa size of 109 × 109 µm a resistance of 13.9 kΩ
at 240 K operation temperature could be realized which is well within the ROIC specifications
[Fig. 4(b)]. From the finished array single pixels were directly wire bonded. The single pixel
measurements were conducted prior to flip-chip bonding. Figure 4(a) shows the single pixel
responsivity of a typical processed array with a SiNx anti-reflection coating (ARC). As for the 45◦
mesa devices strong CO2 absorption is observed. The responsivity increases between 80 K and
160 K and decreases towards 18 mA/W at the design temperature. Due to the high coupling and
absorption efficiency of the pixel design the responsivity decrease between liquid N2 temperature
and room temperature is less pronounced in comparison to 45◦ mesa devices. The simulated
peak absorption efficiency of a single pixel based on the real sample geometry is illustrated in
Fig. 4(d).
Grating coupling is designed around the peak at 2350 cm−1 with the electric field component in
growth direction shown in the inset of Fig. 4(d). A slightly higher doping was used in the model
to match the simulations with the experiment. The optimization for increased QCD resistance
results in a high detectivity even at room temperature [Fig. 4(c)].
The arrays substrate side was polished and reduced in thickness to ≈ 250 µm. For the presented
8 × 8 array complete substrate removal is not necessary due to the small total size, large pixel
pitch and comparable high operation temperature which results in a low ∆T ≈ 70 K. The finished
8 × 8 pixel device is packaged into a 16 pin TO-8 housing [Fig. 5(b)] with a spot welded sealed
cap purged with N2. The assembly consists of the two stage TEC, the thermo-compression bonded
array and ROIC, a Ge window soldered to the cap and a Cu cold shield [Fig. 5(a)]. The cold
shield is thermally connected with the TEC. The Au thermo-compression bonded prototype array
with the ROIC is wire bonded to the TEC element and to the pins of the TO-8 housing [Fig. 5(c)].
4.
Conclusion and outlook
Based on our customization of the VSP2 frameworks single particle Monte-Carlo transport
simulator we have shown a high performance QCD designed at the CO2 absorption bands
around 4.3 µm. These detector specific models enabled for a resistance increase and an over all
performance increase in comparison to previously shown III/V material based mesa devices
at this wavelength [14, 19, 20, 22, 28]. The calculation of the extraction efficiencies for every
extractor level allows to identify bottlenecks and to increase the total extraction efficiency pe .
Processing improvements for the grating structure could further improve pixel performance.
With the performance of our QCD design we show the potential of this detector type and
envision its further improvement to be used for arrays in imaging applications. Remote sensing
would benefit from spectrally narrow detection as provided by QCDs and provide additional
scene information based on the design wavelength and the thereby detectable substances such as
CO2.
5.
5.1.
Materials and methods
Array processing
The MBE grown material is cleaned with acetone in an ultrasonic bath and isopropanol prior to
all processing steps. A 420 nm SiNx hardmask is deposited for the 2nd order diffraction grating
etch process and structured by electron beam lithography. After the e-beam lithography the
grating dimensions were evaluated by atomic force microscopy. The SiNx hardmask is etched
with an CHF / O2 dry etch process optimized for highly anisotropic etching. The grating is
structured into the top contact layer by reactive ion etching. In the next step the pixels are
structured by reactive ion etching with a SiNx nitride hardmask. The pixels are isolated from
Vol. 24, No. 15 | 25 Jul 2016 | OPTICS EXPRESS 17048
(a)
(b)
Wavelength (µm)
4.5
4
5
10
5
0
2000 2100 2200 2300 2400 2500 2600 2700
Wavenumbers (cm
−1
80
EA = 243 meV
measurement
simulation
0.005
)
0.0075
1/T (K
(c)
0.01
−1
0.0125
)
(d)
1013
√
D∗ (cm Hz/W)
Temperature (K)
160
120 100
R0 (Ω)
15
80K
120K
160K
200K
240K
280K
300K
1011
1010
109
108
107
106
105
104
103
102
20
1012
15
1011
10
∗
DBLIP
10
10
9
10
5
108
7
0
10
100
150
200
250
Temperature (K)
300
RPeak (mA/W)
Simulated absorption eff.
Responsivity (mA/W)
20
240
0.5
300K
0.4
0.3
0.2
0.1
0
-0.1
2000 2150 2300 2450 2600
Wavenumbers (cm−1 )
Fig. 4. Photocurrent spectrum and temperature performance (a) of the 4.3µm 20 period QCD
design measured with a 109µm × 109µm pixel device. The illumination is through the ARC
coated substrate side. The two visible valleys in the spectrum are due to the strong CO2
absorption in the beam path. The simulated and measured differential resistance R0 (b) show
good agreement in the entire temperature range. The measured peak responsivity RPeak
and the specific detectivity D ∗ (c) were measured by a calibrated globar source. The field of
∗
view Θ for DBLIP
is π. The grating coupling simulation of the absorption efficiency (d) of a
single pixel with α abs (λ) provided by the QCD simulator. The inset depicts the normalized
electric field component Ey due to the metal grating coupling.
each other by a wet chemically etched 6 µm wide isolation trench which cuts through the bottom
contact layer. The next step involves the deposition of an insulation layer of 350 nm SiNx and
the opening of the contact windows. The Ti/Au (10 nm/440 nm) contacts are sputter deposited
using a negative resist lift-off process. The relatively high Au layer thickness is a requirement
for the Au thermo-compression flip-chip bonding process. After contact deposition the sample
is cleaned with an ultrasonic acetone bath followed by isopropanol and cleaved to the final
array size. The arrays have to be cleaved with minimal remaining edge width surrounding the
array to make the ROIC’s digital and analog I/O bond-pads accessible by the bond wires. The
samples InP substrate bottom side is then polished by wet mechanical polishing. The 580 nm
SiNx anti-reflection coating is then deposited by plasma enhanced chemical vapor deposition
directly onto the polished substrate bottom side. Au thermo-compression flip-chip bonding is
Vol. 24, No. 15 | 25 Jul 2016 | OPTICS EXPRESS 17049
(a)
(b)
(c)
4 mm
Fig. 5. The housing of the prototype consists of (a) the cap, the base with the two stage
thermo-electric cooler, the bonded ROIC with the QCD array, a cold shield and the antireflection coated Ge window. The array wire bonded viewed in illumination direction is
depicted in (b). The packaged array (c) is purged with N2. Flip-chip bonding and packaging
as depicted was developed and conducted by Faunhofer IZM Berlin.
done directly onto the grating surface of the pixels and the corresponding bottom contacts. Due
to the relatively low mesa height no planarization step is necessary.
Acknowledgments
This work was supported by the FP7 EU-project ICARUS, the Austrian Science Funds (FWF)
within SFB project F49-09 NextLite and the framework of the Doctoral School “Building Solids
for Function" (project W1243).