JSSSJournal of Sensors and Sensor SystemsJSSSJ. Sens. Sens. Syst.2194-878XCopernicus GmbHGöttingen, Germany10.5194/jsss-4-249-2015Investigation of InAsSbP quantum dot mid-infrared sensorsHarutyunyanV. G.harutyunyan@ysu.amGambaryanK. M.AroutiounianV. M.HarutyunyanI. G.Department of Physics of Semiconductors and Microelectronics, Yerevan State University, 1 A. Manoogian, Yerevan 0025, ArmeniaDepartment of Optics, Yerevan State University, 1 A. Manoogian, Yerevan 0025, ArmeniaV. G. Harutyunyan (harutyunyan@ysu.am)02July20154224925313May201523June2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://jsss.copernicus.org/articles/4/249/2015/jsss-4-249-2015.htmlThe full text article is available as a PDF file from https://jsss.copernicus.org/articles/4/249/2015/jsss-4-249-2015.pdf
This work presents the results of investigations of a low bias
mid-infrared(IR) photoconductive cell (PCC) with InAsSbP quantum dots (QDs).
The self-assembled nanostructures were grown on an InAs(100) substrate by
modified liquid phase epitaxy. The coarsening of the QDs due to Ostwald
ripening was discussed. Hysteresis with a remnant capacitance of 0.483 pF
and contra-directional oscillations on the PCC's capacitance–voltage
characteristic at 78 K were observed. Additionally, peaks at 3.48, 3.68 and
3.89 µm on the room temperature photoresponse spectrum of a
quantum dot photoconductive cell were detected. Room temperature
photo-sensing properties were investigated under an irradiation of
3.39 µm as well. At a power density of 0.07 W cm-2, the
surface resistance of quantum dot PCC was reduced by up to 7 %. A current
responsivity of 0.2 mA W-1 was measured at an applied voltage of
8 mV.
Introduction
The importance of the mid-infrared(IR) range is attributed to the
transmission window of the atmosphere, absorption spectra of some industrial
gases, etc. To satisfy the demands of state-of-the-art infrared
photodetectors, nowadays quantum well infrared photodetectors (QWIPs) and
quantum dot infrared photodetectors (QDIPs) are of great interest. The QWIP
approach has the advantages of availability of a mature III-V fabrication
technology and multi-spectral capability. This type of photodetector has a
narrow absorption spectrum that can be tuned by varying the quantum well
width and barrier layer compositions. There is a large range of material
combinations available allowing the tailoring of band gaps through stacking
of thin layers. As for QDIPs, this type of photodetector is predicted to have
superior performances compared to QWIPs (Phillips, 2002), such as sensitivity
to normal incidence infrared radiation, low dark current, and high
responsivity and detectivity. Many semiconductor material systems, such as
InGaAs/GaAs, InAsSb/GaAs and InAs/GaAs, were investigated for QDIPs
(Bhattacharya et al., 2005; He et al., 2010 and Lao et al., 2013). HgCdTe
(MCT) is a well-established solid solution, which has been the dominant
system for mid- and long-infrared photodetectors. However, MCT suffers from
instability and non-uniformity problems over a large area due to high Hg
vapour pressure. Theoretical studies predicted that only type-II superlattice
photodiodes and QDIPs are expected to compete with HgCdTe photodiodes
(Martyniuk and Rogalski, 2008). The InAs–InSb–InP system could be discussed
as an alternative material system for mid-IR applications. InAsSbP quaternary
alloy was successfully used for nucleation of quantum dots (QDs). In the
first experiment to grow nanostructures with this quaternary composition,
GaAs was used as a substrate (Krier, 1999). In that work the grown structure
was characterized by photoluminescence (PL) measurements at 4 K temperature
and the PL peak at 1.65 µm was observed. Recently, InAsSbP QDs
were successfully grown on InAs substrates as well (Gambaryan, 2010, and
Gambaryan et al., 2012). It was shown that using InAs substrates leads to the
sensitivity in the mid-IR range (Gambaryan et al., 2011, and Harutyunyan,
2013). Among quantum-size object fabrication techniques, the self-organized
Stranski–Krastanow (S–K) growth mode is an important one by which
dislocation-free nanostructures can be produced (Stranski and Krastanov,
1938). Depending on the growth conditions, the elastic strain can be relaxed
by the formation of QDs, quantum rings or even unique island–pit pairs
(Gambaryan, 2010 and Gambaryan et al., 2012). In the S–K method, the last
stage of nucleation in solid and liquid solutions is often characterized by
the growth of the largest (supercritical) clusters and solving of small
(subcritical) ones, which results in the decrease in the density of clusters
with the increase in their sizes. Such a process of coarsening of clusters is
usually termed “Ostwald ripening” in the literature, in connection with his
studies performed at the beginning of the twentieth century (Ostwald, 1901).
A major advance in the theory of Ostwald ripening was made by Lifshitz and
Slyozov (1961) and followed by Wagner (1961).
In this paper, we report our efforts to grow InAsSbP QDs on InAs substrates
by modified liquid phase epitaxy. Grown nanostructures were studied by atomic
force microscope (AFM) and scanning electron microscope (SEM). Quantum dot
mid-IR photodetectors were fabricated in the form of photoconductive cells
(PCCs) made of InAs substrates with InAsSbP QDs. Electrophysical and
optoelectronic properties of fabricated photodetectors are reported as well.
Experiment
To grow QDs, a modified liquid phase epitaxy (MLPE) was used. For the
formation of a liquid phase, In (7N) was used as a solvent and InAs, InP and
Sb (6N) were used as solutes. A lattice mismatch of 2 % between an
n-InAs(100) substrate with a thickness of 400 µm and a wetting
layer was chosen to provide the growth of InAsSbP nanostructures in S–K
growth mode. The surface morphology, size distribution and density of quantum
dots were studied using an atomic force microscope (AFM – Asylum Research
MFP-3D). Additionally, large-scale surfaces of grown structures were
investigated using a high-resolution scanning electron microscope (HR-SEM:
FEI Nova 600–Dual Beam). Au–Cr evaporation was performed to form contacts
on the sample's surface. Actually, a fabricated photoconductive cell (PCC)
consists of an InAs substrate, unencapsulated InAsSbP QDs and contacts
(Fig. 1). The active area of the photoconductive cell is 0.83 mm2.
Capacitance–voltage (C–V) characteristics were investigated by
high-precision capacitance spectrometry (QuadTech-1920 precision LCR meter).
Mid-IR photoresponse spectra were measured by an IRS-21 spectrometer.
Optoelectronic properties were investigated using a He–Ne laser at an
irradiation wavelength of 3.39 µm.
Schematic of the fabricated photoconductive cell with QDs.
AFM images of InAsSbP QDs grown on an InAs substrate. (a)
– plane view; (b) – oblique view.
(a) – SEM image of InAsSbP QDs grown by LPE on an InAs
substrate (S=17×17µm2); (b), (c)
– AFM images of QD coalescence.
PCC's dark current density versus applied voltage.
Capacitance–voltage characteristic of the PCC.
Results and discussion
Figure 2 presents the AFM images of unencapsulated InAsSbP QDs nucleated from
the quaternary In–As–Sb–P liquid phase during 10 min at a growth
temperature of 550 ∘C. From Fig. 2a one can notice that QDs are
sufficiently uniformly distributed over an S=4.5µm× 5 µm substrate surface. The average surface density
and heights of grown QDs were found to be 7×109 cm-2 and
21 nm, respectively. The average aspect ratio between QDs' widths and
heights equals 2.5±0.5. The SEM and AFM images of the nanostructures
nucleated at 20 min are shown in Fig. 3. The enlarged views of regions
denoted in Fig. 3a by white and black cycles are presented as insets in the
same image. One can notice the process of Ostwald ripening and bimodal growth
of QDs. First, an array of small-size QDs distributed over a relatively large
area (∼ 300 µm2) can be seen and, second, large-diameter
QDs with much lower surface density are quite visible as well. We believe
that coarsening of large QDs occurs via solving of small QDs with subsequent
surface diffusion of dissolved material towards large QDs. The detailed
analyses of measurements showed that small-size QDs have a mainly spherical
shape, while large ones with a diameter of ∼ 50 µm and
larger are ellipsoidal. Increasing the growth time leads to the
transformation of QDs' distribution functions versus their sizes. In
particular, distribution of QDs grown at 10 min was well fitted by the
Gram–Charlier function, while for 20 and 30 min by the Gaussian and
Lifshits–Slezov functions, respectively (Aroutiounian et al., 2013). A
similar change in the distribution function was observed at nucleation of
silicon nano-islands on a Si(001) surface (Bartelt et al., 1996). It was
shown, by means of detailed analysis of individual islands and close-lying
regions of substrate, that merging and coarsening of QDs occurs via
dissolving of small QDs with subsequent surface diffusion of dissolved
substances towards larger QDs, leading to their further coarsening. We assume
that a similar mechanism of QD coarsening occurs in our case as well. Our
studies showed also that at further increase of growth time ellipsoidal QDs
are transformed to elongated islands with dimensions up to a few micrometres.
For the fabrication of the photodetector, the sample prepared at 10 min
growth time was used. Figure 4 shows the dark current–voltage (I–V)
characteristic of the PCC. Nonlinear I–V behaviour is found at around
∼ 60 mV voltage. The result of the C–V characterization performed at
78 K and 106 Hz frequency is presented in Fig. 5. Measurements were
performed by increasing the signal voltage from 0 up to 0.9 V with further
decreasing back to 0 V as denoted by arrows in the figure. In addition, it
can be seen that oscillations on the C–V curve are observed. Oscillations
observed during increasing voltage are opposite-directed to the oscillations
observed during further decreasing of the voltage. From Fig. 5 it can also be
seen that the capacitance hysteresis with a remnant capacitance of ΔC = 0.483 pF is revealed. We assume that opposite-directed oscillations
revealed on the C–V curve occur due to the structure's total charge
oscillations, which are caused by the depletion and occupation of the QDs'
energy levels. Detected hysteresis can be explained by the remnant
polarization in the structure due to spatial separation of the charge
carriers in type-II InAsSbP QDs (Gambaryan et al., 2011) which are conserved
after shutting off the voltage. For photoresponse measurements, a testing
infrared photodetector (TIP) was fabricated and used. The TIP is a
traditional photoconductive cell made of InAs bulk crystal (without QDs). The
photoresponse spectra of the samples at room temperature were investigated by
applying a 2 mV low bias. For TIP only one peak at 3.48 µm
corresponding to the energy band gap of InAs (Eg = 0.355 eV) was
observed (Fig. 6). It can obviously be seen that the PCC, in comparison with
the TIP, has a photoresponse spectrum extended to longer wavelengths up to
4 µm with a narrow peak at 3.48 µm. As was shown in our
previous works, InAsSbP QDs grown on InAs form type-II band alignment
(Gambaryan, 2010). The schematic of the band structure of the grown structure
used for PCC in this work is presented in Fig. 7. Photoresponse peaks at
3.68 µm (B=0.337 eV) and 3.89 µm (C=0.319 eV)
wavelengths could be explained by the transitions into sub-band-gap levels
created by QDs. The surface resistance of the samples was also investigated
under irradiation at 3.39 µm. The relative surface resistance
change in the PCC versus power density is plotted in Fig. 8. At a power
density of 0.07 W cm-2, the resistance of the PCC was reduced by
7 %. Meanwhile, the surface resistance of the TIP at the same density was
reduced by 2 %. Thus, due to the growth of InAsSbP QDs, the resistance
change sensitivity of the PCC is increased by 3.5 times. The current
responsivity of the PCC was investigated at power density of
0.05 W cm-2 (Fig. 9). The current responsivity of 0.2 mA W-1
was achieved at an operating voltage of 8 mV, which exceeds the responsivity
of the TIP by 20 times at the same power density and bias. As a result,
fabricated photoconductive cells with InAsSbP quantum dots are important for
low bias operation at the mid-IR range. Reported results could be important
for developing the idea of photoconductive cells with quantum dots for
next-generation infrared devices.
Photoresponse spectra of the TIP and PCC at room temperature.
Schematic of the band structure of the type-II InAsSbP QDs grown
on InAs.
Relative resistance change dependence on power density. Rd is
the dark resistance and Rir is the resistance under irradiation.
PCC's current responsivity versus bias.
Conclusions
A low bias mid-IR photoconductive cell with InAsSbP QDs was fabricated.
Nonlinear current-voltage
behaviour was found at room temperature. The Ostwald ripening for InAsSbP
nanostructures is studied. Specific opposite-directed oscillations and
hysteresis with a remnant capacitance of ΔC = 0.483 pF were
observed at 78 K temperature. PCC's room temperature photoresponse spectrum
is extended to longer wavelengths up to 4 µm. Besides red shift,
two additional peaks were observed at 3.68 and 3.89 µm
wavelengths. It was shown that under an irradiation of 3.39 µm,
PCC's surface resistance changes 3.5 times more than that of TIP. Applying a
8 mV bias, a current responsivity of 0.2 mA W-1 for PCCs was
measured. This value exceeds the TIP's responsivity by 20 times. Thus,
fabricated low bias photodetectors with InAsSbP QDs can be successfully used
in the mid-IR range.
Acknowledgements
This work was performed in the framework of basic financial support by the
State Committee of Science of Armenia and the NATO SFP 984597
grant. Edited by: W. A. Minkina
Reviewed by: two anonymous referees
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