Cu2ZnSn(S, Se)4 (CZTSSe) is a promising alternative absorber material for thin-film photovoltaic applications because of its earth-abundant constituents, tunable band gap, and high optical absorption coefficient. Using binary and ternary chalcogenide nanoparticles as precursors we have developed a chemical route to produce high efficiency CZTSSe photovoltaic (PV) devices via solution based methods. The printed CZTSSe films show an interesting microstructure consisting of an upper micrometer-sized polycrystalline layer (large-grain layer) and a bottom fine-grain layer. In this paper, we present our results on characterization of the layers including composition, electronic and optical properties. Based on the observed properties we develop a numerical model for the CZTSSe PV device and present the simulation results. We anticipate that the combination of detailed characterization and device model will help us better understand the limitations of our current devices and indicate potential improvement paths.
The terahertz region in the electromagnetic spectrum has attracted much research interest recently, because of its
potential applications in many areas, such as biological and medical imaging, free-space communications, and homeland
security. Here, a tunable quantum dot photodetector for terahertz detection based on intersublevel transitions is proposed
and simulated. The intersublevels are formed by a lateral electric field confinement on quantum wells. The intersublevel
spacing can be tuned and in hence different wavelengths in the terahertz region can be detected. Our simulation results
show a tunability of peak detection wavelength from ~3.3 to ~12 THz by only changing the electrical confinement
voltages and the peak absorption coefficients of the detection are in the range of 103 cm-1. The peak calculated
detectivity of the tunable photodetector is as big as 1.7x109 Jones. Compared with quantum dot terahertz photodetectors
produced by self-assembled growth method, the detector presented here is easier to be tuned and the effective sizes have
a much higher uniformity, because of the uniform electrical confinement.
Quantum well infrared photodetectors (QWIPs) have demonstrated applications in many different areas, such as
medical and biological imaging, environmental and chemical monitoring, and infrared imaging for space and night
vision. However, QWIPs still suffer from low quantum efficiency and detectivity compared with mercury cadmium
telluride (MCT) based interband photodetectors, which dominate current infrared detector market. Besides, n-type
QWIPs cannot detect the normal incident infrared radiations because of the polarization selection rules of intersubband
transitions. Here, we used periodic holes array perforated in gold film to convert normal-incident infrared light to
surface plasmon waves, which can excite the intersubband transitions and be absorbed by quantum wells (QWs). Our
3D FDTD simulation results show that electric field component in the QWs growth direction can be enhanced by more
than 5 times compared with the total electric field intensity without any plasmonic arrays. The experimental results
show that the photodetector has a peak detection wavelength at ~8 μm with a high detectivity of ~7.4×1010 Jones, and the
photocurrent spectrum was very close to the simulation result of the electric field enhancement spectrum.
The terahertz region (1-10 THz) has potential applications in many areas, such as chemical sensing, medical
imaging and free-space optical communications. With the demonstration of terahertz sources, it is quite necessary to
develop the detection technology in terahertz. Here we propose an electrically tunable quantum dot infrared
photodetector to detect the terahertz region. The proposed detector applies a lateral electrical confinement on the
quantum wells and forms a quantum disk in the quantum well area. The two-dimensional quantum confinement of
quantum disk combining the vertical confinement from the quantum barrier forms a quantum dot structure. Using the
energy states and intersublevel energy spacing in the quantum dot, the detector can be used to detect the terahertz region.
Changing the lateral electrical confinement, the intersublevel energy spacing can also be tuned and in hence different
wavelengths can be detected. Our modeling and simulation results show the tunability of peak detection wavelength of
the photodetector from ~3.3 to ~6.0 THz with a gate voltage applied on the detector from -2 to -5 V. The peak
absorption coefficients of the detection are shown in the range of 103 cm-1. Compared with current quantum dot
photodetectors produced by self-assembled growth method the detector proposed here is easier to be tuned and the
effective sizes have a much higher uniformity, because of using electrical confinement.
We present here a novel design to form an artificial quantum dot with electrical confinement and apply it to a Quantum
Cascade Laser structure to realize a Quantum Dot Cascade Laser. A two-dimensional finite element method has been
used to numerically simulate the novel design of electrical formation of an artificial quantum dot. The size of the
quantum dot is electrically tunable and can be applied to quantum cascade laser structure to reduce the non-radiative LO-phonon
relaxation. Numerical modeling with cylindrical symmetry is custom developed using Comsol multiphysics to
evaluate the electrical performance of the device and optimize it by varying design parameters, namely, the doping
density of different layers and thickness of the cladding and active regions. The typical s-, p-, d- and f- wave functions
have been calculated. Numerical simulations show that the energy level separation could be as large as 50 meV by
electrical confinement. We also demonstrate the road map for the fabrication of such a device using a maskless super
lens photolithography technique. We have achieved a uniform array of nano-contacts of size ~ 200nm, required for the
device, using photolithographic technique with a UV source of λ ~ 400nm. The entire processing involves 7
photolithographic steps. This new device - "Quantum dot cascade laser", promises low threshold current density and
high wall-plug efficiency.
With nanotechnology becoming widely used, many applications such as plasmonics, sensors, storage devices, solar
cells, nano-filtration and artificial kidneys require the structures with large areas of uniform periodic nanopatterns. Most
of the current nano-manufacturing techniques, including photolithography, electron-beam lithography, and focal ion
beam milling, are either slow or expensive to be applied into the areas. Here, we demonstrate an alternative and novel
lithography technique - Nanosphere Photolithography (NSP) - that generates a large area of highly uniform periodic
nanoholes or nanoposts by utilizing the monolayer of hexagonally close packed (HCP) silica microspheres as super-lenses
on top of photoresist. The size of the nanopatterns generated is almost independent of the sphere sizes and hence
extremely uniform patterns can be obtained. We demonstrate that the method can produce hexagonally packed arrays of
hole of sub-250 nm size in positive photoresist using a conventional exposure system with a broadband UV source
centered at 400 nm. We also show a large area of highly uniform gold nanoholes (~180 nm) and nanoposts (~300nm)
array with the period of 1 μm fabricated by the combination of lift-off and NSP. The process is not limited to gold.
Similar structures have been shown with aluminum and silicon dioxide layer. The period and size of the structures can
also be tuned by changing proper parameters. The technique applying self-assembled and focusing properties of micro-/nano-spheres into photolithography establishes a new paradigm for mask-less photolithography technique, allowing
rapid and economical creation of large areas of periodic nanostructures with a high throughput.
Short wave infrared (SWIR) imaging systems have several advantages due to the
spectral content of the nightglow and better discrimination against camouflage.
Achieving single photon detection sensitivity can significantly improve the image quality
of these systems. However, the internal noise of the detector and readout circuits are
significant barriers to achieve this goal. One can prove that the noise limitations of the
readout can be alleviated, if the detector exhibits sufficiently high internal gain.
Unfortunately, the existing detectors with internal gain have a very high noise as well.
Here we present the recent results from our novel FOcalized Carrier aUgmented Sensor
(FOCUS). It utilizes very high charge compression into a nano-injector, and subsequent
carrier injection to achieve high quantum efficiency and high sensitivity at short infrared
at room temperature. We obtain internal gain values exceeding several thousand at bias
values of less than 1 volt. The current responsivity at 1.55 μm is more than 1500 A/W,
and the noise equivalent power (NEP) is less that 0.5 x10-15 W/Hz1/2 at room temperature.
These are significantly better than the performance of the existing room temperature
devices with internal gain. Also, unlike avalanche-based photodiodes, the measured
excess noise factor for our device is near unity, even at very high gain values. The stable
gain of the device combined with the low operating voltage are unique advantages of this
technology for high-performance SWIR imaging arrays.