Quantum Dot Infrared Photodetectors and Focal Plane Arrays
Xuejun Lu
Department of Electrical and Computer Engineering, University of Massachusetts Lowell,
One University Avenue, Lowell, MA 01854 USA
*E-mail: xuejun_lu@uml.edu
Abstract
Quantum dot (QD) nanostrcutures with dimensions on the orders of the De
Broglie wavelength provide three-dimensional (3-D) quantum confinement of carriers.
The 3-D quantum confinement results in complete localization of electrons and holes,
which not only leads to split of the continuous conduction or valance bands of
semiconductors into discrete subbands, but also significantly changes the properties of
the subbands, including density of states (DOS), and lifetimes of the subbands. This
chapter reviews the properties of 3-D confined QDs and the transitions between the
subbands (intersubband transitions) with emphasis on quantum dot infrared photodetector
and focal plane arrays (FPA) based on the intersubband absorption. Device physics,
fabrication and characterization of QDIP and FPA are also presented.
1. Introduction
Quantum dots (QDs) are semiconductor nano-crystallites that have the
dimensions smaller than de Broglie wavelength of electrons in semiconductors [1-2].
There are generally two size groups of quantum dots generally obtained from different
methods. The first is colloidal QDs, such as CdSe and PbS [3-5]. The colloidal QDs can
1
be synthesized in various sizes and forms and can also be combined with conductive
polymers. The colloidal QDs have the dimension of 3-5 nm in diameters. Their working
wavelengths (emission or absorption) are in visible or near infrared (IR) region. The
second type is epitaxial QDs such as InAs. Epitaxial QDs are self-assembled nanocrystallites grown by molecular beam epitaxy (MBE) or metal organic chemical vapor
deposition (MOCVD) through the Stranski–Krastanow (S-K) growth mode [6-7]. The
epitaxial QDs have dimensions usually ~20 -40 nm in base and 5-8 nm in height [2].
They work at near infrared (NIR) through far infrared (FIR) regions. In this chapter, we
will be focusing on the inter-subband transition in InAs QDs for LWIR optoelectronic
devices.
Epitaxially grown semiconductor QDs have shown great promises in various
optoelectronic devices such as QD lasers [8-11] and QD infrared photodetectors (QDIP)
[9-17]. Due to the nano-scale quantum confinement, QDs exhibit atomic-like properties,
including discrete energy levels within the conduction (c) and valance (v) bands, deltafunction-like density of states (DOS) [18], reduced electron-phonon scattering [19], and
enhanced overlap of wavefunctions [20-22]. These quantum confined properties open a
new area of possibilities for unipolar optoelectronic devices covering a broad wavelength
range from middle infrared (MIR) through terahertz (THz) [14, 23-24] with significantly
improved performance features, including low threshold current, high photoconductive
gain and large EO coefficients [25-26]. These advantages make semiconductor QD nanocrystallites one of the most promising artificial materials for a great variety of
optoelectronic devices in IR through THz wavelength regions. Devices and technologies
in these wavelength ranges are of great importance to many civilian and homeland
2
security applications such as target detection and tracking, remote sensing, chemical
analysis and medical diagnostics [27-29]. This chapter focuses on reviews on the
properties of 3-D confined QDs and the transitions between the subbands (intersubband
transitions) and their applications in quantum dot infrared photodetector and focal plane
arrays (FPA). Device physics, fabrication and characterization of QDIP in will be
presented.
2. Properties of QDs
Due to the three dimension confinement of carriers, QDs show complete discrete
energy levels within the conduction band and valance bands. For an initial understanding,
we start with a simple quantum box with dimensions Lx, Ly and Lz. Here, we focus on the
conduction band. The wave function and energy state of a box-shaped QD are given in
Eq. 2.1, and Eq. 2.2, respectively:
c ,nlm
2
 
L
3/ 2
E c ,nlm  EC 
 n
sin 
 Lx
  l
x  sin 
  L y
  m
y  sin 
  Lz


z U c ( r ) ,

 2 2 2
 2 2 2  2 2 2
n

l 
m ,
2mc* L2x
2mc* L2y
2mc* L2z
(2.1)
(2.2)

where, n, l, m =1, 2, 3 …., U v (r ) is the wave function of a unit cell. mc* is the effective
mass of the electrons in conduction band. The 3-D quantum confinement leads to split of
the conduction band into atomic-like discrete energy levels. The energy separation
between these energy levels depends on the dimensions of the quantum box.
The atomic-like discrete energy levels show a delta-function like density of state
(DOS), QD (E ) , which can be written as:
3
 QD ( E )  g ( E n ) ( E  E n ) ,
(2.3)
where, g(En) is the degeneracy of the energy level En. Fig. 2.1(a), (b), and (c) show the
schematic drawing of the DOS of bulk materials, quantum well (QW) and QDs,
respectively.
Fig. 2.1, Schematic drawing of the DOS, QD (E ) . (a) Bulk materials, (b) quantum wells
(QW) and (c) QDs.
The atomic-like discrete energy level not only shows a complete different DOS
than bulk semiconductors, but also has a longer excited-state lifetime. This is primarily
due to the nonradiative relaxation caused by the thermally-activated electron-LO phonon
scattering process [30], as shown Fig. 2.2(a). In a QW structure, the energy states are
quantized only along the growth direction (z direction) while the in-plane energy states
4
are quasi-continuous. This quasi-continuous energy states make it easy to achieve
resonant electron-LO phonon scattering. The LO-scattering nonradiatively depopulates
electrons from the upper states to the lower states at a much faster (2,300 times) rate than
the radiative emission processes [31-32]. Such as fast nonradiative relaxing rate leads to
short excited state lifetime. In quantum dots, since discrete energy levels are offresonance with the LO-phonons. Thus, the LO-phonon nonradiative relaxation can be
substantially reduced, which leads to long excited state lifetime. This is typically referred
to as “phonon bottleneck” effect [33-34]. The long excited-state-lifetime was predicted
and experimentally observed using time –resolved photoluminescence (PL) technique [33,
35].
Fig. 2.2, LO-phonon scattering process, (a) Resonant LO-phonon scattering in QWs; (b):
nonresonant LO-phonon scattering in QDs.
The transitions rate between the intersubbands can be described using Fermi’s
Golden rule:
w jm 
2
 j H '  m 2 ( E jm   ) ,

5
(2.4)
where, m and j, are the wave functions of the subbands, H’ are the interaction
Hamiltonian of the incident light with QDs. Under electric dipole approximation [36], H’
can be written as [36, 37]:
 
H '  er  E ,
(2.5)


where,  er is the electric dipole moment and E is the electric field. Eq. (2.4) and (2.5)
also include the quantum selection rule for the intersubband transitions, i.e. the non-zero
matrix element H jm '   j | H ' |  m  0 . For normal incidence, the electric field is along

the x or y direction. Assuming E is along the x direction, for the simple quantum box
example, the matrix element H jm ' can be written as:
 j
H jm '   j  exE  m  C sin 
 Lx

 m
x   exE sin 

 Lx

x ,

(2.6)

where, C is a constant containing the integration of unit cell wavefunction U c (r ) . Due to
the quantization in the x direction, the normal incidence along the x direction has a nonzero matrix element H jm ' , indicating normal incidence absorption and detection
capability. A real QD usually has a lens or pyramid shape rather than a cube or box shape.
This would cause the mixture of wave-functions. Nevertheless due to the quantization in
the x direction, it has the normal incidence light absorption and detection capability. Such
normal incidence absorption eliminates requirement for surface gratings in quantum well
(QW)-based photodetectors and thus greatly simplifies the fabrication complexity of a
large format (1Kx1K) photodetector array and focal plane array (FPA).
6
3. Structure of a QD infrared photodetector (QDIP) and its advantages
As discussed in the previous section, due to the 3-D quantum confinement, QDs
show complete discrete energy levels within the conduction band and valance bands. The
transitions between the intersubbands can be used for infrared detection. The schematic
structure and simplified band diagram of a QD infrared photodetector (QDIP) are shown
in Fig. 3.1. It consists of vertically-stacked InAs quantum dots layers with GaAs capping
layers. The electrons are excited by the normal incident light and subsequently collected
through the top electrode and generate photocurrent. This is a uni-polar photodetector.
Only conduction band is involved in the photodetection and photocurrent generation
process.
Fig. 3.1, schematic structure and simplified band diagram of a QD infrared
photodetector (QDIP)
Due to the unique properties of QDs, QDIP offers several advantages as listed in
the following:
7
(1)
Normal incidence light detection capability
As discussed in the previous section, due to the3-D quantization, the normal
incidence along the x direction has a non-zero matrix element H jm ' . This enables normal
incidence absorption and detection. Such normal incident light detection capability is
especially suitable for two-dimensional (2-D) focal plane array (FPA).
(2)
Low dark current
Delta-function like density of state (DOS), the total states of QDs for a given
energy interval E will be much less than those of bulk materials, or quantum wells. The
major source of photodetector noise currents comes from the thermal excitation process
(thermal-electrons). The thermal excitation processes for unbiased and biased scenarios
are shown in figure 1 (a) and 1(b), respectively. Under zero bias (Fig. , the thermal
excitation process is balanced by the relaxation, and the thermal excitation rate equals the
relation rate, i.e.:
Rth  Rrelax 
N

,
(3.1)
Where, Rth is the thermal excitation rate and Rrelax is the relaxation rate, which is related
to the excited state lifetime , and N is the number of electrons on the excited state.
Under a high external bias, the thermally excited electrons can be effectively
collected before they relax to the ground states. Thus the collection rate equals the
thermal excitation rate. The electrons collected by the external bias form the dark current.
The dark current can therefore be written as:
I d  qRcollection  qRth  q
N

,
(3.2)
8
Fig. 3.2, Thermal excitation, relaxation, and collection processes in a photodetector. (a)
unbiased; (b) biased.
For QWIP, the number of electrons on the excited states NQW can be written as:
N QW   
m

Em
N (E)
dE ,
 q( E  E F ) 
1  exp 

kT

(3.3)
where, m is the index of the different energy bands in QWs, EF is the Fermi-level of the
quantum well, which depends primarily on the doping concentration (N0), N(E) is the
density of state of the QW. The dark current density in quantum wells can thus be written
as:
I d ,QW 
qdN QW
 QW

qd
 QW

m

Em
N (E)
dE ,
 q( E  E F ) 
1  exp 

kT

(3. 4)
where, d is the thickness of the active layers. For QDs, the number of electrons on the
excited states NQW can be written as:
9
N QD  
m
gm
,
 q( E m  E F ) 
1  exp 

kT


(3.5)
where, EF is the Fermi-level of the quantum dots, which depends primarily on the doping
concentration (N0) of the dots, Em is the energy of the energy level m of the dots, NQD is
the volume density of the dots, q is the charge of an electron, gm is the degeneracy of the
QD energy level.
The thermally generated dark current in quantum dots Id,QD can thus be expressed as:
I d ,QD 
qdN QD
 QD

qd
 QD

m
gm
,
 q( E m  E F ) 
1  exp 

kT


(3.6)
From Eq. (3. 4) and Eq. (3.6), one can see that QDIP shows lower noise current than
QWIPs [38] due to primiliarily two reasons:
(1) The excited state lifetime of QDs is much longer than that of QWs, which makes
the thermal excitation rate much smaller in QDs.
(2) The density of states of QDs are much lower than that of QWs, which allows
QDs to hold less thermally-generated electrons for dark current
Fig 3.3 shows the dark current density of the QDIPs made from different MBE
growths. Also shown in Fig.3.3 is the dark current density of a QWIP reported in [38].
10
Fig. 3.3, Dark current density of the QDIPs made from different MBE growths
As indicated in Fig. 3.3, the QDIPs from different MBE growths show very similar
level of the dark current density. An over two orders of magnitudes of lower dark current
can be obtained in QDIPs.
(3)
High photoconductive gain
The electron capture and reemission processes are shown in Fig. 3.4. Electrons
are captured into QDs mainly through the wetting layer or QW due to the continuous
energy levels and the large density of states of the wetting layer and QW [34]. Because
the electron de Broglie wavelength is comparable to the length scale of QD
heterostructures, the electron capture into the QW depends strongly on the well thickness.
An oscillation capture time as a function of the well thickness has been observed [35, 39].
11
Fig. 3.4, Temperature-dependent electron trap and thermal re-emission process
Once captured into the wetting layer or quantum well, the electrons are relaxed
into the QDs through phonon scattering [20, 33]. The thermal reemission out of the QDs
follows the temperature-dependent Arrhenius equation e(-Eb/KT) with activation energy (or
barrier energy) Eb [30, 33]. Electron can also be thermally re-emitted out of the QD. The
net electron capture probability is proportional to exp(-Ea/kT), where Ea is the activation
energy (or barrier energy), T is the absolution temperature, and k is Boltzmann’s
constant.
The electron capture and reemission processes also temperature-dependent
properties [31]. The electron capture probability pc is related to the electron relaxing into
a QD and reemission out of the QD by [40]:
pc   em /  relax ,
(3.7)
where, 1/relax and 1/em represent electron relaxing and reemission rates, respectively.
The Arrhenius-type pc and temperature T relation has been experimentally verified [31].
12
Fig. 3.5 shows such pc and temperature T relations at different biases. The slightly
lower activation energy (Ea) at higher bias is attributable to the quantum-fined Stark
effect [41-42].
Fig. 3.5, Temperature-dependent electron capture probability
When pc is small, photoconductive gain Gph of a quantum dot infrared photodetector
is related to the carrier capture probability pc by the follow equation [33].
G ph 
I collect
1
,

I ph
FNpc
(3.8)
where, N is the number of QD layers, and F is the QD filling factor, which is usually
0.35. The bias voltage dependent photoconductive (PC) gain at different temperatures is
shown in Fig. 3.6. A high PC gain of over 100 can be obtained. The high PC gain leads to
large photoresponsivity [40]. However, one should note that the PC gain would also
increase the dark current level as well. Under background noise limited situation, the PC
gain would increase the signal to noise ratio (SNR) [38-43].
13
Fig. 3.6, Photoconductive gain at different temperatures
4. QD Growth and characterization
Stranski–Krastanow (S-K) mode epitaxial growth of QDs through the by using
MBE or MOCVD [6-7] are generally used to obtain high-quality, defect-free QDs with
very good size uniformity. This method utilizes highly lattice-mismatched growth of
In(Ga)As on GaAs. Due to the strain built up in the lattice-mismatched epitaxial growth,
self-assembled QDs will form after a few mono layers (i. e. the critical thickness) of the
layer-by-layer growth. The typical growth growth temperature for GaAs is around 620°C.
The temperature can be measured by an optical pyrometer. QDs are around 470°C by
depositing a few monolayers (ML) of InAs on GaAs or InGaAs. Different QD size and
density can be obtained by changing the QD growth temperature and the amount of InAs
deposited.
14
The QD size and density can be measured using atomic force microscopy and
cross-section transmission electron microscopy (XTEM). The AFM images of the QDs
are shown in Fig. 4.1. The lateral size and the density of typical QDs grown by MBE are
~ 25 nm and ~ 2.9×1010 cm-2, respectively. A cross-sectional transmission electron
microscopy (XTEM) image of typical QDs grown by MBE is shown in Fig. 4.2. The
layered QDs can be clearly seen. The height of the QDs is typically ~ 6 nm. Note that the
QDs were aligned across all these layers, which clearly indicates that the QD growth was
strain driven.
(a)
(b)
Fig. 4.1, AFM images of typical QDs grown by MBE, (a) top view, (b) side view
Fig. 4.2, XTEM image of QDs grown by MBE.
15
The energy states of QDs can be measured by measuring the photoluminescence (PL)
emission from the QDs. Fig. 4.3 shows the simplified band diagram and the principle of
the PL. The PL process starts with the excitation of electro-hole pairs (EHP) generated by
a pump laser in the conduction and valance bands. The electrons and holes subsequently
diffuse and are captured by QDs. The recombination of EHP on the ground and excited
states of QDs give re-emissions (photoluminescence) at longer wavelength than the pump
laser.
Fig. 4.3, Simplified band diagram in the PL process.
Fig. 4.4 shows an example PL of InAs/GaAs QDs. Since the PL comes from EHP
recombination at QD energy levels, the PL contains important information energy states
of QDs. The peak wavelength and the width (full width half magnitude (FWHM)) reveal
the energy levels and the uniformity of the QDs, respectively.
16
Fig. 4.4, Example PL from InAs QDs in GaAs.
5. Fabrication of QDIP
After the growth, the wafer was processed into 100-µm-diameter circular mesas using
standard photo-lithography and wet-etching procedures. Figure 5.1 shows the flow-chart
of the fabrication process. It starts spin-coating of a thin-layer (~0.3µm thick) of
photoresist on top of the MBE grown QD wafer (Fig. 5.1(b)). The photoresist is then
patterned in individual 100µm-diameter circular pixels using standard photolithography
technology (Fig. 5.1(c)). The patterned wafer is then wet-etched using the
H2SO4:H2O2:H2O (1:80:80) solution for ~ 120 seconds to reach the bottom contact layer
(Fig. 5.1(d)). The depth of the etching is measured by using a profilometer or an optical
profiler. The top and bottom electrodes can be formed simultaneously on top of and
surrounding the mesas by standard E-beam metal-evaporation deposition, lift-off, and
17
rapid thermal annealing of n-type (Ni/Ge/Au) alloys (Fig. 5.1(e )-(j)).
Fig. 5.1, Flow chart of the FPA fabrication process
The pictures of 12×12 QDIP array and the individual QDIP are shown in Fig. 5.2(a)
and (b), respectively. The QDIP is then wire-bonded and mounted in an infrared (IR)
dewar with a ZnSe IR window that has more than 60% transmission over the 3-14-µm
broad-band IR region. The packaged QDIP in a liquid nitrogen (LN2) dewar is shown in
Fig. 5.3.
18
(a)
(b)
Fig. 5.2, Fabricated QDIPs: (a) 12×12 QDIP array and (b) an individual QDIP with an
100µm×100µm bonding pad.
Fig. 5.3, Photography of a packaged QDIP in an LN2 dewar.
19
6. QDIP characterization
QDIP characterization including the following: (1) detection spectrum; (2) dark
current and noise current; (3) noise and photoconductive gain calculation; (4)
photocurrent measurement; (5) photoresponsitivity and photodetectivity calculation, (6)
temperature dependent performance.
(1) Detection spectrum
The spectral response of the QDIP is typically measured by using a Fourier transform
infrared (FTIR) spectrometer. Fig. 6.1 shows the diagram of an FTIR spectrometer. A
typical FTIR spectrometer consists of a blackbody infrared (IR) source, a 50% beam
splitter, a fixed mirror and a mirror and an IR detector.
Fig. 6.1, Principle of the Fourier transform infrared (FTIR) spectrometer.
20
The working principle of the FTIR spectrometer is briefly described as the following: the
50% beam splitter divides the incoming IR beam into two optical beams that reflect back
from the fixed mirror and the moveable mirror respectively. The two reflected beams are
combined by the beam splitter and pass through a sample. Because of the constantly
changing position of the moveable mirror, the interference of the two reflected beams
generates an interferogram. Every data point (a function of the moving mirror position) of
the interferogram contained the information about every infrared frequency. The
frequency information can thus be obtained by doing an inverse Fourier transform as
shown schematically in Fig. 6.2. A transmission FTIR spectrum of a GaAs wafer is
shown in Fig. 6.2 as an example.
Fig. 6.2, Principle of Fourier transform infrared (FTIR) spectrometer.
The interference of the two reflected beams can be written as:
I ( x )  E1  E2  E12  E22  2 E1 E2 cos( ) 
2
I0
2

 2
1  cos 



x  ,

(6.1)
where, I0 is the intensity of the incident IR beam,  is the wavelength and x is the
position. For non-monochromatic IR source:
I ( x)  

0
I0
2

 2 
1  cos  x G ( )d ,



21
(6.2)
where, G() is the transmission profile of the sample.
 I
I0
 2
G ( )d   0 cos
0 2
0 2
 
 I
I
 2 
 0   0 cos
x G ( )d
0 2
2
  
I ( x)  


x G ( )d

,
(6.3)
The AC part of I(x), is actually Fourier transform of the transmission profile of the
sample, G(), which be obtained by performing a reverse Fourier transform.

 2 
G ( )   I ( x ) exp(  j 
x dx ,

  
(6.4)
In practical situation, the position x can not move from - to  . The resolution of the
FTIR system is determined by the maximum movement xmax:
Resolution 
1
,
2x max
(6.5)
Fig. 6.3 shows the QDIP spectral response measurement setup using an FTIR
spectrometer. The spectral measurement uses the QDIP to replace the original IR detector
of the FTIR. Since the IR source is broad band. The resulting spectrum is the spectrum
response of the QDIP.
Fig. 6.4 shows an FTIR photocurrent spectrum of the QDIP with the In 0.20 Ga 0.80 As
B
B
B
B
capping layers at both positive and negative biases. The inset of Fig. 6.4 shows the QDIP
heterostructure. The spectrum is peaked at 9.7 µm. The spectral width (FWHM) is ~ 1.2
µm. The ∆/ was measured to be ~12%, which indicates the photocurrent was generated
by the electron transitions between bounded states of the QDs.
22
Fig. 6.3, QDIP Spectral response measurement setup
Fig. 6.4, Photocurrent spectrum of a QDIP with In0.20Ga0.80As capping layers at different
biases.
23
(2). Dark current and noise current measurement
Dark current (Id) of the QDIP can be measured using a semiconductor parameter
analyzer or a source meter. Fig. 6.5 shows the dark current as a function of bias voltages
at different temperatures.
Fig. 6.5, Dark current of the QDIP as a function of bias voltages at different
temperatures
The spectral density of dark current (inoise, in A/Hz1/2), i.e. noise current can be
measured by obtaining the spectrum of the dark current at certain bias voltages. The
spectrum of the dark current can be obtained by a fast Fourier transform (FFT) spectrum
analyzer. Fig. 6.6 shows the noise current for different bias voltages. Note that the 1/f
24
noise dominates at low frequency range from DC to around 700Hz. To avoid 1/f noise,
the noise currents (inoise) at high frequency (1kHz) were used for generationrecombination (GR) noise analysis.he dark current induced noise is a white noise. The
noise floor can be measured starting f = 1000Hz.
Fig. 6.6, Noise current spectrum at different bias voltages
(3).
Photoconductive gain and photoresponsivity measurement
The noise current inoise contains GR noise current and thermal noise (Johnson
noise) current Ith :
2
inoise
 4eGn I d  I th2 ,
(6.6)
where, Gn is the noise gain, e is the charge of an electron (1.6×10-19C), and Id is the dark
current of the QDIP. The thermal noise current can be calculated using:
25
4kT
,
R
I th 
(6.7)
where, k is Boltzmann’s constant, T is the absolute temperature, and R is the differential
resistance of the QDIP, which can be extracted from the slope of the dark current. The
noise gain Gn can thus be calculated from Eq. (6.6):
Gn 
2
inoise
 4kT / R
,
4eI d
(6.8)
As a good approximation, when the electron-capture probability into a QD is
small [43], the photoconductive gain and noise gain are equal in a conventional
photoconductor. The photoconductive gain can thus be calculated. The calculated
photoconductive (PC) gain G for different bias voltages is shown in Fig. 3.6. Note that
the PC gain is larger at higher temperature, indicating a lower capture probability at high
temperature.
The photoresponsivity of the QDIP array was measured using the calibrated
cavity blackbody source at 1000K. The total number of incident photons on the QDIPs
can be determined by calculating the blackbody emission at the detector’s peak
wavelength and the spectral width. The photoresonsivity can be written as:

4I ph
Pd v
,
(6.9)
where, Iph is the photocurrent of the detector, Pd is the power spectral density of the
cavity blackbody at 1000K, is the solid angle of the photodetector, and  is the spectral
width of the detector. The solid angle of the photodetector can be written as:
26
 
D 2
R02
,
(6.10)
where, D is the diameter of the photodetector and the R0 is the distance from the
blackbody to the photodettector. Fig. 6.7 shows an example of the photoresponsivity of
an LWIR QDIP at different bias voltages. For QDIP, a high photorespnsivity of >
7.9A/W photoresonsivity can be obtained due to the high PC gain of QDIP. Fig. 6.7 also
clearly shows a temperature dependent photoresponsitivity [40].
Photoresponsivity (A/W)
1.0E+01
1.0E+00
1.0E-01
100K
130K
1.0E-02
150K
190K
1.0E-03
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Bias Volatge (V)
Fig. 6.7, Photoresponsivity of the QDIP at different bias voltages
(4).
Photodetectivity (D*) calculation
The photodetectivity D* can be calculated using [38, 43]:
D* 
 A
2
inoise
 iB2 ,noise

( eG ph / hv ) A
2
4GnoiseI d  4G ph
B
27
,
(6.11)
where, A is the detector area, iB,noise is the noise current due to the background radiation, 
is the absorption quantum efficiency, and B is the background photon absorbed by the
QDIP. Fig. 6.8 shows the bias-dependent photodetectivity D* of a QDIP at different
temperatures. At 77K, A high photodetectivity D* of 4.8×109cmHz1/2/W can be obtained
at the bias voltage of -1.5V. At 190K, a photodetectivity D* of is ~1.3×108 cmHz1/2/W at
the bias voltage of -0.25V.
1.0E+10
1/2
Photodetectivity (cmHz /W)
78K
100K
190K
1.0E+09
1.0E+08
1.0E+07
-1.5
-1
-0.5
0
0.5
1
1.5
Bias Volatge (V)
Fig. 6.8, Photodetectivity(D*) of an QDIP as a function of bias voltages for different
temperatures
7. QD focal plane array development and characterization
In infrared image acquisition, focal plane arrays (FPA) offer great advantages
over point-to-point or line-by-line scanning based imaging systems. It allows a faster
frame rate and enables staring mode image acquisition. A complete FPA consists of IR
28
photodetector array and integrated silicon read out circuits (ROIC) via the flip-chip
bonding
hybridization
technique.
This
section
will
present
fabrication
and
characterization of a QD FPA. Note that due to its surface normal IR incidence detection
capability, no surface grating is required in QD FPA. This would simplify the FPA
fabrication.
1.
320x256 Focal plane array fabrication and hybridaization
320256 focal plane array (FPA) is fabricated using the same photolithography,
wet etch and lift-off metallization procedures as shown in Fig. 7.1. Each pixel of the
320256 FPA has the dimension of 28 µm  28 µm on a 30 µm pitch. A picture of the
fabricated 320x256 FPA and a zoom in view are shown in Fig. 7.1(a) and (b),
respectively.
30 µm
24 µm
(a)
(b)
Fig. 7.1, Fabricated 320x256 focal plane array. (a) the 2z0x256 FPA chip; (b) zoom-in
view of the pixels.
29
The FPA is subsequently hybridized with an Indigo 9705 ROIC using the
standard indium evaporation, flip-chip bonding and substrate removal techniques. Fig.
7.2 shows a picture of the ROIC chips.
Fig. 7.2, Picture of ROIC chips.
The flip-chip bonding hybridization process is schematically shown in Fig. 7.3.
Fig. 7.3, Flip-chip bonding hybridization process
30
Fig. 7.4 (a) and (b) show the fully-packaged FPA in a ceramic chip carrier and a
thermal image of a researcher taken using the FPA without using any filter. The FPA
temperature was 67 K and the bias of the FPA was -0.7V. The integration time was set to
16.7 ms. Two-point non-uniformity correction at extended area blackbody temperatures
of 20°C and 30 °C was used to obtain the image.
(a)
(b)
Fig. 7.4 (a) Fully-packaged FPA with ROIC and (b) a thermal image of a researcher
taken using the FPA at an estimated FPA temperature of 67 K.
31
Fig. 7.5 (a) and (b) show images at MIR (3-5 µm) and LWIR (8-12 µm) bands
obtained at different biases, indicating imaging with on-demand voltage-tunable multispectral detection band selections.
(a)
(b)
Fig. 7.5, FPA images at different biases: (a) LWIR (8-12 µm) band, and (b) MIR (3-5
µm) band, indicating imaging with on-demand voltage-tunable multi-spectral detection
band selections.
The FPA performance is measured at blackbody temperatures of 10 C 20 C, 30
C and 50 C., respectively. The sensitivity of each pixel of the FPA can be calculated by:
32
S ( mV / 0 C ) 
mV (T2 )  mV (T1 )
,
T2  T1
(7.1)
where, T2 and T1 are initial and final temperatures of the blackbody, respectively. mV(T2)
and mV(T1) are the voltages of the pixel at temperature of T2 and T1, respectively. Fig. 7.6
shows the measured sensitivity map of the QD FPA at 30 C. The average sensitivity is
8.3mV/C with a standard deviation  of 8.3mV/C. The sensitivity map shows quite
uniform areas. The histogram of the FPA sensitivity is illustrated in Fig. 7.7. The narrow
distribution indicates the uniformity of the FPA sensitivity.
Fig. 7.6, Measured sensitivity map of the 320x256 FPA at 30 C. The average
sensitivity is 8.3mV/C with a standard deviation  of 8.3mV/C.
33
Fig. 7.7, Histogram of the 320x256 FPA at 30 C. The narrow distribution indicates
the uniformity of the FPA sensitivity.
The noise equivalent temperature difference (NET) can be estimated as:
i
NET  n
Ip
 kTB2  

 ,
 hc 
(7.2)
where TB is the blackbody temperature, c is the speed of light,  is the detection
wavelength, k is the Boltzmann constant, in is the noise current and Ip is the photocurrent.
In the FPA measurement, the noise current in is the average of noise currents at
blackbody temperatures of 20 C, 30 C and 50 C. Fig. 7.8 shows the histogram of the
NET of the FPA. An average NET value of 172 mK can be obtained at the blackbody
temperature of 30 C and the FPA bias voltage of -0.7 V. The standard deviation  of the
NET is calculated to be 40 mK. Since NET scales with the square of F#, a lower
NET can be expected for a smaller F# lens system.
34
Fig. 7.8, Histogram of the NET of the FPA. An average NET value of 172 mK was
obtained at the blackbody temperature of 30 C with a standard deviation  of 40 mK.
The cross-talk of the FPA can be characterized by using a fixed area blackbody
source. The fixed area blackbody source will be imaged on part of the FPA. By changing
the temperature of the blackbody and observing the responses of the pixels near the part
of the FPA that correspond to the blackbody source, one can determine the cross-talk
between the pixels. Fig. 7.9 shows the principle of the cross-talk measurement. As the
blackbody temperature is increased from 20 C to 50 C, the responses of the pixels near
the edge show no obvious change. This indicates low cross-talk between pixels.
35
Fig. 7.9, Principle of the cross-talk measurement by changing the fixed area blackbody
temperature from 20 C to 50 C.
8. Summary and conclusion
Due to the three-dimensional (3D) quantum confinement of carriers, quantum dot
infrared photodetectors (QDIPs) offer great advantages for infrared detection and sensing,
including intrinsic sensitivity to normal incident radiation and long excited state lifetime,
which allows efficient collection of photo-excited carriers and ultimately leads to high
photoconductive (PC) gain, high photoresponsivity. The normal incidence detection
capability greatly simplifies the fabrication complexity for a large format (1K1K) FPA.
The high photoconductive (PC) gain and high photoresponsivity provide a promising way
36
for low-level IR detection. Compared with quantum well infrared photodetectors (QWIP),
QDIP also shows lower dark current due to the 3D quantum confinement. The long
excited state lifetime together with low dark current makes QDIPs promising for hightemperature operation. QDIPs with high operating temperatures of T = 190K and T
=300K have been reported working at both middle MWIR and LWIR. Voltage-tunable
multi-spectral QDIPs have also been demonstrated using stacked QD layers with
different capping layers, i.e. InGaAs, GaAs and AlGaAs for short-wave (SWIR), middlewave (MWIR) and long-wave (LWIR) infrared detections, respectively. These works
clearly show the potentials of QDIP technology for highly sensitive, high operation
temperature and multi-spectral IR detection and imaging.
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