PACS: 07.57Kp 95.55 Rg 85.25Pb 85.60Gz Keywords: GaAs/AlGaAs TCR Bolometer Room temperature a b s t r a c t GaAs/AlGaAs multilayer heterojunction structures with different aluminum (Al) fractions and emitter doping densities were tested to identify optimum parameters for high temperature coefficient of resistance (TCR). Higher Al fractions and lower doping densities showed higher TCR. Additionally, p-doped heterojunction structures showed a higher TCR compared to an n-doped one with similar parameters. A p-doped multilayer superlattice heterojunction structure with 30 periods of GaAs/Al 0.57 Ga 0.43 As junctions, operating at room temperature showed a TCR of ∼4% and bolometer like infrared (IR) response up to 20 m with a D* of 1.7 × 10 6 Jones. This TCR is higher than that of VO x or ␣-Si bolometers. At low temperatures (50 K) some of these devices have shown TCR values of over 30%

A G U Perera

H C Liu

P K D D P Pitigala

P V V Jayaweera

S G Matsik

CiteSeerX

This article appeared in a journal published by Elsevier. The attached
copy is furnished to the author for internal non-commercial research
and education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling or
licensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of the
article (e.g. in Word or Tex form) to their personal website or
institutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies are
encouraged to visit:
http://www.elsevier.com/copyright
Author's personal copy
Sensors and Actuators A 167 (2011) 245–248
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical
journa l homepage: www.e lsev ier .com/ locate /sna
Highly sensitive GaAs/AlGaAs heterojunction bolometer
P.K.D.D.P. Pitigalaa, P.V.V. Jayaweeraa, S.G. Matsika, A.G.U. Pereraa,∗, H.C. Liub
a Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, USA
b Institute for Microstructural Sciences, National Research Council, Ottawa K1A 0R6, Canada
a r t i c l e i n f o
Article history:
Received 3 September 2010
Received in revised form 1 February 2011
Accepted 2 February 2011
Available online 25 February 2011
PACS:
07.57Kp
95.55 Rg
85.25Pb
85.60Gz
Keywords:
GaAs/AlGaAs
TCR
Bolometer
Room temperature
a b s t r a c t
GaAs/AlGaAs multilayer heterojunction structures with different aluminum (Al) fractions and emitter
doping densities were tested to identify optimum parameters for high temperature coefficient of resis-
tance (TCR). Higher Al fractions and lower doping densities showed higher TCR. Additionally, p-doped
heterojunction structures showed a higher TCR compared to an n-doped one with similar parameters.
A p-doped multilayer superlattice heterojunction structure with 30 periods of GaAs/Al0.57Ga0.43As junc-
tions, operating at room temperature showed a TCR of ∼4% and bolometer like infrared (IR) response
up to 20 m with a D* of 1.7 × 106 Jones. This TCR is higher than that of VOx or -Si bolometers. At low
temperatures (50 K) some of these devices have shown TCR values of over 30%.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
A wide variety of thermal and photon techniques have been
tested to detect IR radiation. Among them thermal detectors uti-
lize the temperature-dependent properties of the detector material
[1]. As an example, devices such as thermocouples and thermopiles
use the thermoelectric effect; bolometers and microbolometers [2]
respond to changes in resistance; Golay cells [3] use the thermal
expansion; and pyroelectric detectors [4] utilize the temperature
dependent spontaneous electric polarization that generates elec-
tric charges upon incidence IR irradiation (known as pyroelectric
effect). Bolometers utilize changes in the absorber tempera-
ture when irradiated with electromagnetic radiation. Change in
temperature is usually measured as a resistance change in the semi-
conducting material. The change in the relative resistance per one
degree of Kelvin (K) is defined as the temperature coefficient of
resistance (TCR, ˇ), and is the key parameter in determining the
performance of the material.
The TCR (ˇ) is defined as follows, where R denotes the resistance
and T denotes the temperature. Additionally, R is proportional to the
carrier concentration (N) hence, TCR is also proportional to carrier
∗ Corresponding author. Tel.: +1 404 413 6037; fax: +1 404 413 6025.
E-mail address: uperera@gsu.edu (A.G.U. Perera).
concentration.
ˇ = 1
R
(
∂R
∂T
)
≈ − 1
N
(
∂N
∂T
)
The maximum change in resistance for each degree of tem-
perature difference and low noise characteristics are desirable to
obtain the highest sensitivity in a thermal detector. Materials such
as vanadium oxide (VOx) or amorphous silicon (-Si) were utilized
in presently established bolometers. The main drawbacks of ther-
mal detectors (bolometers) compared to other photon detectors are
slow response time and high noise (such as 1/f noise, Johnson noise,
thermal and background fluctuation noise [5]) and low TCR values.
With the advancement in micro-machining techniques, miniature
micro-bolometer structures have been developed and often used
in uncooled thermal cameras. Although VOx has a good bolome-
ter performance with TCR ∼2% K−1, this exotic material is not well
suited for large-scale CMOS production, due to the high temper-
ature process used in fabricating VOx for post-CMOS processes.
On the other hand, amorphous silicon (-Si) exhibits better TCR
(∼2.8% K−1) compared to VOx and, germanium silicon alloy micro-
bolometers have the best TCR around 5% K−1. This paper presents
a GaAs/AlGaAs heterostructure with TCR values higher compared
to existing VOx or -Si bolometers and close to micro-bolometers
of Ge/Si alloy. A properly optimized GaAs/AlGaAs heterostructure
would be a competitive candidate, as a micro-bolometer with high
0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.sna.2011.02.017
Author's personal copy
246 P.K.D.D.P. Pitigala et al. / Sensors and Actuators A 167 (2011) 245–248
Table 1
List of detector structures used in TCR calculations and brief description of their parameters.
Sample Al % Doping density (p/n) No. of layers in super lattice TCR % (temperature (K))
HE0204 12 1 × 1018 (p) 12 30.6 ± 0.2 (50 K)
HE0205 12 3 × 1017 (p) 12 31.5 ± 0.2 (50 K)
HE0206 12 1 × 1017 (p) 12 33.4 ± 0.3 (50 K)
1329 12 4 × 1017 (n) 12 18.7 ± 0.2 (50 K)
SP001 28 3 × 1018 (p) 30 5.5 ± 0.4 (140 K)
SP002 37 3 × 1018 (p) 30 6.6 ± 0.3 (140 K)
SP003 57 3 × 1018 (p) 30 8.1 ± 0.4 (140 K)
4.0 ± 0.5 (300 K)
detectivity due to the high TCR, especially compared to existing
bolometers fabricated using VOx or Si. Results presented in this
paper show a competitive TCR value of 4% K−1 with detectivity (D*)
1.7 × 106 Jones for a 400 m × 400 m pixel.
2. Experimental
Samples considered in this study are multilayer heterojunc-
tion (HEJ) structures based on doped GaAs emitters and undoped
AlGaAs barriers. The dependency of the TCR on several different
detector parameters such as doping density and Al fraction is inves-
tigated. A summary of parameters of these device structures is
shown in Table 1. These detector structures were measured for
their current–voltage–temperature (IVT) characteristics with the
variation of temperature and voltage in small steps.
Although there are thin layers of emitter/barrier regions sand-
wiched in these HEJ superlattice structures they will not form
any discrete quantum states inside the barrier/emitter/barrier lay-
ers due to their thickness. These emitters are doped high enough
to have a scattering length similar to the emitter thickness. The
result is a bound 3D carrier distribution in the emitter region.
The structures were grown on semi-insulating GaAs substrates
and were processed by wet etching to form square pixels with
400 m × 400 m in area. Ti/Pt/Au was evaporated on to the top
and bottom contact layers. All the sample structures listed in Table 1
are prepared by the same procedure. A sample pixel from the struc-
ture SP003 that was suspended in midair at the center of a regular
sample holder using gold wire was measured for the bolometric
response. Two of such samples were measured, and both of them
have shown a similar response.
The IVT characteristics of each structure were measured using
a KEITHLEY 2400 source meter, after mounting them on a cold fin-
ger of a closed cycle refrigerator. The temperature was controlled
by a Lake Shore 300 temperature controller system. The noise cur-
rent density of the device was measured using a Stanford Research
SR785 Fourier Transform dynamic signal analyzer coupled with
a low noise current amplifier, Stanford Research SR570. The IR
response of the device was measured using a Perkin-Elmer 2000
FTIR system.
3. Results and discussion
Initially, three structures (HE0206, HE0205 and HE0204) were
used to study the dependence of TCR on the doping density. These
three HEJ samples have identical structures except for the emitter
doping densities, which are 1 × 1017, 3 × 1017, and 10 × 1017 cm−3
respectively. The results show a decreasing trend in the TCR with
increasing doping density. The TCR of these three detectors (sum-
marized in Table 1) were calculated at 50 K. At higher temperatures
the resistance of these devices becomes very small; hence, the safe
current limit is reached upon applying few millivolts. Therefore,
an accurate calculation of TCR is difficult at higher temperatures
for those three detectors. The dependency of TCR with the emitter
Fig. 1. Variation of TCR with doping density of a multilayer heterojunction structure.
The structure consists with 12 layers of GaAs/AlGaAs emitter barrier regions (squire
p-doped and circle n-doped). TCR values are calculated at the temperature 50 K with
1 V bias across the structure. The dotted lines (horizontal and vertical) show the
intercept point of the TCR interpolated curve for a 4 × 1017 cm−3 p-doped structure.
Inset: Variation of TCR with Al fraction of a multilayer heterojunction structure.
The structure consists of 30 layers of GaAs/AlGaAs emitter/barrier regions with
3 × 1018 cm−3 p-doped emitter regions. TCR values are calculated at the temperature
140 K with 1 V bias across the structure.
doping density, at 1 V bias voltage across the top and the bottom
contact, is illustrated in Fig. 1. A maximum TCR of 33.4 ± 0.3% K−1
was observed for 1 × 1017 cm−3 doped sample (HE0206) at 50 K.
Work by Wissmar et al. [6] shows a similar behavior with the doping
density for quantum well structures.
A further study using an n-doped structure (Sample # 1329)
with similar parameters as above structures except for the emitter
doping density of 4 × 1017 cm−3, gave a TCR value of 18.7 ± 0.2% K−1
at 50 K, with 1 V bias applied across the detector (shown by the
filled circle in Fig. 1). The p-doped structure having a doping density
closest to 1329 is the HE0205 (3 × 1017 cm−3 (p)), and it shows a
TCR around 31.5 ± 0.2% K−1 at 50 K. A TCR value of ∼31% K−1 was
estimated for a 4 × 1017 cm−3 p-doped device by interpolating the
data in Fig. 1 (TCR vs doping). This observation predicts that the TCR
of p-doped structures is higher compared to n-doped structures. A
study by Sun et al. [7] shows a low carrier density (N) and a steeper
gradient in carrier density with temperature (dN/dT) for p-doped
structures compared to the n-doped structure from temperatures
around 300 K down to 80 K. This implies a higher variation in the
resistance in p-doped structures compared to n-doped structures
with variation of temperature. As a result, a better TCR is possible
in p-doped structures compared to n-doped structures.
The inset to Fig. 1 shows a TCR variation with the Al fraction,
in the barrier layer, of p-doped GaAs/AlGaAs structures SP001,
SP002 and SP003. All three sample structures contain 30 periods
of undoped AlGaAs barriers and 3 × 1018 cm−3 p-doped GaAs emit-
ters. The Al fraction (x) in AlxGa1−xAs barrier layers are 28%, 37%
and 57% respectively in these three devices respectively. For com-
Author's personal copy
P.K.D.D.P. Pitigala et al. / Sensors and Actuators A 167 (2011) 245–248 247
Fig. 2. Noise current density (S(f)) and fitted data (for 1 V and 2 V bias) of the sample
illustrated in the inset. Inset: The noise spectra after baseline correction. Dotted lines
indicate the 1/f fitted curves for 1 V and 2 V biases.
parison of TCR values of these three samples, the measurements
were carried at 140 K (maximum operating temperature of SP001).
Above this temperature SP001 also shows very low resistance;
hence exceeding the safe current limit. The sample with 57% Al frac-
tion (SP003) shows the highest TCR value of 8.1 ± 0.4% K−1 at 140 K.
Additionally, the SP003 sample also shows a TCR of ∼4 ± 0.5% K−1
at around room temperature. This value is 2 times of the TCR value
of VOx bolometers (∼2% K−1) [8]. This behavior (TCR vs Al%) implies
a better TCR for samples with 100% Al (i.e. AlAs), but further studies
are needed to confirm this conclusion.
The specially mounted sample has the same structure as SP003
(the sample with 57% Al fraction). The noise current density of this
device structure at two different bias voltages at 300 K is shown
in Fig. 2. The device shows flat white noise with a noise cur-
rant density <10−22 A2/Hz (∼5 × 10−17 V2/Hz) at higher frequencies
(f > 1000 Hz). The theoretical limit in the Johnson noise for a device
with similar resistance is 1.4 × 10−23 A2/Hz (2 × 10−17 V2/Hz) cal-
culated by Nyquist equation, where kB is the Boltzmann constant,
T is temperature (300 K) and R is the device resistance (∼1.2 k).
VJ (f )√
f
=
√
4kBTR,
IJ (f )√
f
=
√
4kBT
R
The noise power density is around 5 × 10−22 A2/Hz
(∼8 × 10−17 V2/Hz) at the 10 Hz with 1 V bias (∼0.7 mA) for
the GaAs/AlGaAs (SP003) bolometer. This noise spectral density is
very low compared to noise values reported for different materials
including Si (∼4 × 10−12 V2/Hz at 1 Hz), VOx (∼10−12 V2/Hz at
0.1 Hz) and Ge/Si (∼7 × 10−13 V2/Hz at 250 Hz) alloy [6,9]. Inset to
Fig. 2 shows the noise spectra after baseline correction. It shows
1/f noise dominating over the Johnson noise at low frequencies in
the device.
The measured response spectrum of the specially mounted sam-
ple is given in Fig. 3. It shows a wavelength independent (flat)
response as expected for a bolometer. With a 2.25 V bias across
the top and bottom contacts, the device shows a detectivity (D*) of
∼1.7 × 106 Jones, with a responsivity of ∼3.7 mA/W at 10 m, oper-
ating at room temperature. The 2.25 V is the maximum safe bias
voltage that can be applied to the device. The IV characteristic curve
shows a drastic increase in the dark current in the device, due to
Joule heating, just above this voltage. The response peak appearing
in the range 2–6 m is believed to be cause by inter band transi-
tions (heavy hole, light hole and split-off hole bands) [10,11] due to
photonic excitations of carriers. The response of the second struc-
ture is also showing a similar response, and the values lies within
the uncertainty of measurements.
Fig. 3. Responsivity of the sample consisting of 30 layers of GaAs/AlGaAs junctions
with 3 × 1018 cm−3 p-doped GaAs regions and Al fraction of 57% (SP003), at different
bias voltages.
4. Conclusion
GaAs/AlGaAs multilayer heterojunction structures with dif-
ferent Al fractions and doping densities were tested for TCR
values. Structures containing the p-doped GaAs emitter lay-
ers showed a better TCR compared to a similar heterojunction
structure with n-doped emitters. Furthermore, a GaAs/AlGaAs
multilayer structure with 57% Al fraction in the barrier layers
showed a TCR value around 4% K−1, at room temperature. This
is a higher TCR value compared to that of VOx or -Si which
is 2% and 2.8% respectively. Therefore with further research p-
doped GaAs/AlGaAs heterostructures have the potential to be a
competitive candidate in the future production of bolometers
or microbolometers working at room temperature. Additionally,
the device demonstrates a low noise spectral density, which is
a requirement for good performance in a bolometer type detec-
tor.
Acknowledgements
This work was supported by the Georgia Research Alliance under
GRAUP, USNSF under the grant # ECS-0553051, and the US Army
Research Office under the grant # W911NF-08-1-0448 monitored
by Dr. William Clark.
References
[1] A. Rogalski, Comparison of photon and thermal detector performance, in: M.
Henini, M. Razeghi (Eds.), Handbook of Infra-red Detection Technologies, Else-
vier Science, Amsterdam, 2002, pp. 5–81.
[2] C.V. Hoof, P.D. Moor, PolySiGe uncooled microbolometers for ther-
mal IR detection, in: M. Henini, M. Razeghi (Eds.), Handbook of
Infra-red Detection Technologies, Elsevier Science, Amsterdam, 2002,
pp. 449–479.
[3] K. Yamashita, A. Murata, M. Okuyama, Miniaturized infrared sensor using sili-
con diaphragm based on Golay cell, Sensors and Actuators A: Physical 66 (1998)
29–32.
[4] P.W. Steinhage, R.R. Zeyfang, Performance of pyroelectric detectors and
vidicons based on modified Pb(Ti, Zr)O3 ceramics, Ferroelectrics 11 (1976)
301–304.
[5] P.W. Kruse, Can the 300-K radiating background noise limit be attained by
uncooled thermal imagers? in: Infrared Technology and Applications XXX,
Orlando, FL, USA, 2004, pp. 437–446.
[6] S. Wissmar, L. Hoglund, J. Andersson, C. Vieider, S. Savage, P. Ericsson,
High signal-to-noise ratio quantum well bolometer materials, in: Optical
Materials in Defence Systems Technology III, Stockholm, Sweden, 2006,
pp. 64010N–11.
[7] Y. Sun, N. Balkan, A. Erol, M.C. Arikan, Electronic transport in n- and p-type
modulation-doped GaInNAs/GaAs quantum wells Microelectronics Journal 40
(2009) 403–405.
Author's personal copy
248 P.K.D.D.P. Pitigala et al. / Sensors and Actuators A 167 (2011) 245–248
[8] M. Hu, M. Wu, Y. Lv, Y. Dou, M. Cui, Study on thermal sensitivity of a
VOx/SiO2/PS/Si multi-layer structure, Surface and Coatings Technology 201
(2007) 4858–4860.
[9] A. Ahmed, R.N. Tait, Noise behavior of amorphous GexSi1−xOy for
microbolometer applications, Infrared Physics & Technology 46 (2005)
468–472.
[10] W. Songprakob, R. Zallen, W.K. Liu, K.L. Bacher, Infrared studies of hole-plasmon
excitations in heavily doped p-type MBE-grown GaAs:C, Physical Review B 62
(2000) 4501.
[11] W. Songprakob, R. Zallen, D.V. Tsu, W.K. Liu, Intervalenceband and plasmon
optical absorption in heavily doped GaAs:C, Journal of Applied Physics 91
(2002) 171–177.


Highly sensitive GaAs/AlGaAs heterojunction bolometer

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=7164F0BF830A1F4753BB51DC04530642?doi=10.1.1.1070.4755&rep=rep1&type=pdf

Highly sensitive GaAs/AlGaAs heterojunction bolometer

Abstract

Similar works

Full text

Available Versions

CiteSeerX