8-9 and 14-15 Micron Two-Color 640x486 GaAs/AlGaAs Quantum Well Infrared Photodetector (QWIP) Focal Plane Array Camera

Abstract

An optimized long-wavelength two-color Quantum Well Infrared Photodetector (QWIP) device structure has been designed. This device structure was grown on a three inch semi-insulating GaAs substrate by molecular beam epitaxy (MBE). This wafer was processed into several 640x486 format monolithically integrated 8-9 and 14-15 micron two color (or dual wavelength) QWIP focal plane arrays (FPAs). These FPAs were then hybridized to 640x486 silicon CMOS readout multiplexers. A thinned (i.e., substrate removed) FPA hybrid was integrated into a liquid helium cooled dewar to perform electrical and optical characterization and to demonstrate simultaneous two-color imagery. The 8-9 micron detectors in the FPA have shown background limited performance (BLIP) at 70 K operating temperature, at 300 K background with f/2 cold stop. The 14-15 micron detectors of the FPA have reached BLIP at 40 K operating temperature at the same background conditions. In this presentation we discuss the performance of this long-wavelength dualband QWIP FPA in quantum efficiency, detectivity, noise equivalent temperature difference (NEAT), uniformity, and operability.

Full text

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8-9 and 14-15 pm Two-color 640x486 GaAs/AlGaAs Quantum Well
Infrared Photodetector (QWIP) Focal Plane Array Camera
S. D. Gunapala, S. V. Bandara, A. Singh**, J. K. Liu, S. B. Rafol, E. M. Luong, J. M. Mumolo,
N. Q. Tran, J. D. Vincent*, C. A. Shott*, J. Long*, and P. D. Levan**
Center for Space Microelectronics Technology, Jet Propulsion Laboratory
California Institute of Technology, Pasadena, CA 91 109
* Raytheon Infrared Center of Excellence, Goleta, CA 93 1 17
** Air Force Research Laboratory, Kirtland Air Force Base, NM 871 17
ABSTRACT
An optimized long-wavelength two-color Quantum Well Infrared Photodetector (QWIP) device structure has been designed.
This device structure was grown on a three-inch semi-insulating GaAs substrate by molecular beam epitaxy (MBE). This
wafer was processed into several 640x486 format monolithically integrated 8-9 and 14-15 pm two-color (or dual wavelength)
QWIP focal plane arrays (FPAs). These FPAs were then hybridized to 640x486 silicon CMOS readout multiplexers.
thinned (i.e., substrate removed) FPA hybrid was integrated into a liquid helium cooled dewar
optical characterization and to demonstrate simultaneous two-color imagery. The 8-9 pm detectors in the FPA have shown
background limited performance (BLIP) at 70 K operating temperature, at 300 K background with f/2 cold stop. The 14-15
pm detectors of the FPA have reached BLIP at 40 K operating temperature at the same background conditions. In this paper
we discuss the performance of this long-wavelength dualband QWIP FPA
equivalent temperature difference (NEAT), uniformity, and operability.
A
to perform electrical and
in quantum efficiency, detectivity, noise
1. INTRODUCTION
There are many applications that require long-wavelength dualband infrared FPAs. For example, a two-color FPA camera
would provide the absolute temperature of a target with unknown emissivity which is extremely important to the process of
identifying temperature difference between missile targets, war heads, and decoys. Dualband infrared FPAs can also play
many important roles in Earth and planetary remote sensing, astronomy, etc. Furthermore, monolithically integrated dualband
FPAs eliminate the beam splitters, filters, moving filter wheels, and rigorous optical alignment requirements imposed on
dualband systems based on separate FPAs or a broadband FPA. These dualband FPAs will also reduce the mass, volume, and
power requirements of the dualband systems. Due to the inherent properties such
tailorability, and stability (i.e., low l/f noise) associated with GaAs based QWIPs’”, it is an ideal candidate for large format
long-wavelength multi-color FPAs. GaAs based QWIPs will also provide higher uniformity, higher operability, and lower
cost as a result of mature GaAs growth and processing technology. In this paper, we discuss the first
wavelength and very long-wavelength infrared (VLWIR) imaging camera, based on a monolithically integrated 640x486
dualband QWIP FPA.
as narrow-band response, wavelength
demonstration of a long-
Until recently, the most developed and discussed two-color QWIP detector was the voltage tunable two stack QWIP.
device structure consists of two connected multi-quantum well (MQW) structures tuned to two different wavelength bands of
interest. This device structure utilizes the advantage of electric field domains formation to select the response of one or the
other detectors6,’ (MQW region). The major disadvantages of this type of two-color QWIP FPA are that, these detector pixels
need two different voltages to operate, and the long-wavelength sensitive segment of the device needs very high bias voltage
(> 8 V) to turn on the long-wavelength infrared (LWIR) detection. The other disadvantage is that the voltage tunable scheme
will not provide simultaneous data from both wavelength bands.
This

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2. DEVICE STRUCTURE
The LWIR and VLWIR dualband QWIP device structure described in this section can be processed into simultaneously
readable dualband FPAs with triple contacts to access the CMOS readout multiplexer' or interlace readable dualband FPA
(i.e., odd rows for one color and the even rows for the other color). The first approach require a special dualband readout
multiplexer which contains two readout cells per detector unit cell, whereas the second approach needs only an existing
single color CMOS readout multiplexer. The advantages of this scheme are that it provides simultaneous data readout and
allows the use of currently available single color CMOS readout multiplexers. However, the disadvantage is that it does not
provides a full fill factor for both wavelength bands. This problem can be eliminated by fabricating (n+l) terminals (e.g.,
three terminals for dualband) per pixel and hybridizing with a multicolor readout having n readout cells per detector pitch,
where n is the number of bands.
8-9 pm QWIP
- -
The device structure consists of
period stack, of VLWIR QWIP structure
and a second 18 period stack of LWIR
QWIP structure separated
doped 0.5 pm thick intermediate GaAs
contact layer (see Figure 1). The first
stack (VLWIR) consists of 30 periods of
a 500 8, A1,Gal-,As barrier and a 60 8,
GaAs well. Since the dark current of this
device structure is dominated by the
longer wavelength portion of the device
structure, the VLWIR QWIP structure
has been designed
quasibound intersubband absorption
peak at 14.5 pm. The second stack
(LWIR) consists of 18 periods of
p\ A1,Gal.,As barrier and a narrow
GaAs well. This LWIR
has been designed
a 30
14-15 pm QWIP
by a heavily
n+ GaAs CONTACT LAYERS
to have a bound-to-
Figure 1. Conduction band energy diagram of the long-wavelength and veiy long-
wavelength two-color infrared detector. The long-wavelength (8-9 pm)
sensitive MQW stack utilizes the bound-to-continuum
absorption. The vey long-wavelength (14-15 pm) sensitive MQW stack
utilizes the bound-to-quasibound intersubband absorption.
intersubband
a 500
40 8,
structure QWIP
to have a bound-to-
continuum intersubband absorption peak at 8.5 pm, since photo current and dark current of the LWIR device structure
relatively small compared to the VLWIR portion of the device structure. This whole dualband QWIP structure
sandwiched between 0.5 pm GaAs top and bottom contact layers doped with n
semi-insulating GaAs substrate by MBE. Then a 300 8, Alo,3Gao,7As stop-etch layer and a 1.0 pm thick GaAs cap layer were
grown in situ on top of the device structure. GaAs wells of the LWIR and VLWIR stacks were doped with n = 5 ~ 1 0 ' ~
5 ~ 1 0 ' ~ c m j respectively. All contact layers were doped to n = 1 ~ 1 0 ' ~ ~ m - ~ .
was intentionally increased by a factor of two to compensate for the reduced number of quantum wells
is worth noting that, the total (dark current
+ photo current) current of each stack can be independently controlled
carefully designing the position of the upper state, well doping densities, and the number of periods in each MQW stack. All
of these features were utilized to obtained approximately equal total currents from each MQW stack. Figure 1 shows the
conduction band energy diagram of this dualband QWIP device structure 9210.
is
is then
= 5 x 1017 ~ m - ~ , and has been grown on a
and
The GaAs well doping density of the LWIR stack
in the LWIR stack. It
by
Using both selective and non-selective dry etching, this MBE grown device structure was processed into
device structures. Au/Ge Ohmic contacts were evaporated onto the top, middle, and bottom contact layers. These test
detectors were back illuminated through a 45" polished facet
current-voltage curves and responsivity spectrums of these vertically integrated dualband QWIPs are shown in Figures 2, 3,
and 4. The responsivity of the LWIR detectors peak at 8.4 pm and the peak responsivity (Rp) of the detector is 509 mA/W at
bias VB = -2 V. The spectral width and the cutoff wavelength of the LWIR detectors are
respectively. The responsivity of the VLWIR detectors peak at 14.4 pm and the peak responsivity (Rp) of the detector is 382
mA/W at bias VB = -2.0 V. The spectral width and the cutoff wavelength of the LWIR detector are AhIh = 10% and h , = 15
pm respectively. The measured absolute peak responsivity of both LWIR and VLWIR detectors are small, up to about Vg =
-0.5 V. Beyond that, it increases almost linearly with bias in both LWIR and VLWIR detectors reaching Rp = 0.3 (at VB =
-2V) and 1 A/W (at Vg = -3V) respectively. This behavior of responsivity versus bias is typical for bound-to-continuum and
bound-to-quasibound QWIPs in LWIR and VLWIR bands respectively. The peak quantum efficiency of LWIR and VLWIR
detectors were 5.4% and 13.2% respectively at operating biases indicated
200x200 pm2 test
as described elsewhere5 and simultaneously measured dark
AXIL = 16% and h, = 9.1 pm
in Figure 4 for a 45" double pass.
t

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10-3
o-4
I I
I
I I I
I
I
DEVICE AREA = 4 X 10-4 cm2
1 0-5
-
a 10-6
5 10-7
10-8
2 10-9
v
10-10
1 0-1 1
10-12
- 5 - 4
-3
- 2 - 1
BIAS VOLTAGE (V)
0 2 1
3
4
5
Figure 2. Dark current vs. voltage curves of a L WIR detector at various temperatures. The device area is 4x1 0-4 cm2
I
I I I
I I I I
I
DEVICE AREA = 4 X 1 0 4 cm2
I
-
10-4 1 LC = 15.0 pm
in-12
I "
1
-5
-4
-3
- 2 - 1
BIAS VOLTAGE (V)
0 2 1
3
4
5
I
Figure 3. Dark current vs. voltage curves of a VL WIR detector at various temperatures. The device area is 4x1 0-4 cm2
The photoconductive gain g was experimentally determined using g
and in is the current noise, which was measured using a spectrum analyzer". The photoconductive gains of the LWIR and
VLWIR MQW stacks reached 1.38 and 0.25 respectively at VB = -2V. The peak detectivity is defined as Di = R, &I
where Rp is the peak responsivity, and A is the area of the detector. The area of both LWIR and VLWIR detectors is ~ x I O - ~
cm2. The peak detectivities of both LWIR and VLWIR detectors were estimated at different operating temperatures and bias
voltages using experimentally measured noise currents, and results are shown
test detector data, the LWIR detectors show BLIP at bias VB = -2 V and temperature T = 72 K for a 300 K background with
f/2 cold stop. The VLWIR detectors show BLIP with the same operating conditions at 45 K operating temperature.
= iz / 4eI,B, where B is the measurement bandwidth,
in,
in Figures 5 and 6. Based on single element

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0.6
I I I I I I
I I I
I
I
Vs,As = -2v
7
WAVELENGTH (pm)
Figure 4. Simultaneously measured responsivity spectrum of vertically integrated
QWIP detector.
L WIR and VL WIR dualband
ZERO BACKGROUND
" " -
0.5
1
1.5 3.5
BIAS VOLTAGE (-V)
2
3 2.5
Figure 5. Experimentally measured peak detectivity D* of L WIR detectors as a function of bias voltage at three
different operating temperatures.
3. FOCAL PLANE ARRAYS
It is well known that GaAs/AlGaAs based n-type QWIPs do not absorb infrared radiation incident normal to the surface.
Thus, researchers have invented various light coupling schemes such
reflector^'^, corrugated reflectors14, lattice mis-match strain material systems, and p-type materials15. Many of our previous
FPAs utilized random reflectors for efficient light coupling. Although random reflectors have achieved relatively high
quantum efficiencies with large test device structures, it is not possible to achieve the similar high quantum efficiencies with
random reflectors on small FPA pixels due to the reduced width-to-height aspect ratios. In addition, it is difficult to fabricate
as two-dimensional (2-D) periodic gratings12, random

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random reflectors for shorter wavelength detectors relative to very long-wavelength detectors (i.e., 15 pm) due to the fact that
feature sizes of random reflectors are linearly proportional to the peak wavelength of the detectors. For example, the
minimum feature size of random reflectors of 15 and 9 pm cutoff FPAs were 1.25 and 0.6 pm respectively, and it is difficult
to fabricate sub-micron features by contact photolithography. As a result, the random reflectors of the 9 pm cutoff FPA were
less sharp and had fewer scattering centers compared to the random reflectors of the 15 pm cutoff QWIP FPA. Thus, 2-D
periodic grating structure has used for efficient light coupling into dualband QWIPs. As we have discussed before5, two
passes of infrared radiation can be coupled to the QWIP detector structure by incorporating a 2-D periodic grating surface12
on top of the detectors.
I
ZERO BACKGROUND
I
L
1010
W n
1 09
Figure 6.
0.5 1 1.5
BIAS VOLTAGE (-V)
2
2.5
3
3.5
Experimentally measured peak detectivity D* of a VL WIR detector as a function of bias
different operating temperatures.
n
GRATINGS
LIGHT COUPLING
COLUMNS
voltage at three
EVEN ODD EVEN
I
I
I
I
I
I
ODD
22pm r
- 1
AulGe METAL CONTACTS
LWlR GaAslAtGaAs
MULTI-QUANTUM WELL LAYERS
HEAVILY DOPED GaAs
Figure 7. (a) Schematic side view of the interlace dualband FPA. The VL WIR detector pixels in even rows were
short circuited at the row-ends. The L WIR detector pixels in odd rows were short circuited by fabricating
VL WIR light coupling grating structure through the L WIR portion of detector pixels.