In the far-infrared (THz) field, single-walled carbon nanotubes (SWCNTs) are promising candidates for room-temperature and self-powered thermal detectors due to their low specific heat capacity, high stability, relatively large Seebeck coefficients, and the ability to be doped diversely. Especially when the nanotubes are aligned, the photoresponse becomes polarization-sensitive. In this work, we integrated a bowtie antenna stereostructure with a horizontally aligned SWCNTs film to generate new resonance modes and gain mechanisms to improve the polarization extinction ratio (PER) and the response of carbon nanotube far-infrared detectors. While the antenna size changes, we can get PERs higher than 6481 from 0.5THZ to 1.5 THz.
We numerically studied a simple and novel graphene-based dielectric microcavity narrowband perfect absorber. By electrically modulating the graphene Fermi level and critical coupling theory, our absorber achieves 100% perfect absorption at 8.0 μm in the mid-infrared, and the FWHM (full width at half-maximum) of the absorption peak is only 21 nm. We use the optical constants obtained from experimental data, including the mobility and surface conductivity of graphene, so the results are more applicable. In addition, the relatively simple structure of the dielectric microcavity graphene structure has a high tolerance for manufacturing defects.
Long-wavelength (8-14μm) infrared detection ability using third-generation infrared focal plane array (FPAs) is a desideratum for aerography, military and communication. These optical bands contain tremendous information about CO2 levels, atmospheric quality and biological activity. HgCdTe infrared photodetectors are able to reach high degree of performance even to be background limited. However, the material growth process, doping techniques and capability of defect control become increasingly difficult for the shrinking bandgap. Besides, the dark current characteristic and associated noise behavior are very sensitive to the detector fabrication processes. Thereby, the growth of p-type epitaxial layer is a fundamental and significant subject for long-wavelength HgCdTe infrared photodetector.
The requirement of the infrared technology applied on meteorological satellites is the key driving force for the development of infrared technology in Shanghai institute of technical physics (SITP), Chinese Academy of Sciences. The meteorological satellites have become a main detection method for the weather and ocean observation, there are totally 15 meteorological satellites that were launched into both sun synchronous and geostationary orbit and more satellites are under construction to be the second generation ones. The infrared remote sensors are the main payloads on-board on all these satellites. By these infrared remote sensors one can obtain the remote sensing data for ocean colour, water vapour, weather forecasting, and get the atmospheric temperature profile and humidity profile, etc. As the key technology in the infrared remote sensor, the infrared detector technology is developed mainly using the HgCdTe material, meanwhile the quantum well infrared photodetector and type II super-lattice infrared detector are also developed.
Low dimensional semiconductors have attracted enormous attention in recent years. Owing to the special dimension confinement, photodetectors based on low dimensional materials and their hybrid systems exhibit considerable performance at room temperature. This differs from traditional thin-film infrared photodetectors which require liquid nitrogen cooling. In this paper, we introduce uncooled photodetectors based on one dimensional (1D) nanowires, two-dimensional (2D) materials, 2D hybrid structures and 1D/2D heterostructures. We illustrate their working mechanisms and reveal the potential for practical infrared detection.
The configurations of nuclei in Au catalyzed Si nanowire growth were investigated through an ab-initio thermodynamic-combined approach. We discussed the relation between the configurations and formation energies of the lateral walls of the nucleus in nanowire growth numerically by the classical nucleation theory. The nucleation model was parameterized by the formation energies of surfaces, interfaces and steps calculated in first-principles methods. The configurations of the nuclei were determined by the Wulff theorem. Moreover, we found configurations of the nuclei are different in two different Si-Au contact structures. This study provides an important basis to understand the step-flow process in nanowire growth.
Detection in the very long wave infrared range (LWIR, 12-15µm) using third-generation infrared focal plane array (FPAs) is essential for remote atmosphere sounding. Indeed, these wavelengths are particularly rich in information about humidity and CO2 levels and provide additional information about cloud structure and temperature profile across the atmosphere. However, the dark current characteristic and associated noise behavior of the HgCdTe photodiode in the wavelength range of 12-15µm, operating at ~77K, are very sensitive to surface passivation techniques as well as to surface material treatments. For current HgCdTe material and device technology, detection of LWIR and VLWIR energy is the subject of current research. Within this range of shrinking band-gaps in detector material, precise control of the quality of the surface passivation and treatment is of great importance. The underlying physics of dark current mechanism is theoretically investigated by using a previously developed simultaneous current extraction approach and numerical simulations.
In addition, HgCdTe electron avalanche photodiodes (e-APD) have been widely used for low-flux and high-speed application. To better understand the dark current transport and electron-avalanche mechanism of the devices and optimize the structures, we perform accurate numerical simulations of the current-voltage characteristics and multiplication factor in planar and mesa homojunction (p-i-n) HgCdTe electron-avalanche photodiodes.
We investigative the field distribution in nanostructured metal waveguide arrays. Firstly, we analyze a simple discrete system containing two adjacent metallic waveguides (N=2). The propagation constants β1 and β2 can be calculated by a rigorous field analysis approach. According to the supermode theory of conventional dielectric waveguide arrays, we can also obtain the expressions of propagation constants. So we can obtain the coupling constant and the perturbation constant of the expressions in the supermode theory. Next, we consider a system that contains five adjacent metal waveguides (N=5). The propagation constants and the wavefunctions of the supermodes can be obtained according to the coupling constant, the perturbation constant, and the supermode theory. The incident light is located at the input of the 4st waveguide. The initial excited field can be expressed as a sum of supermodes. The total field is formed by the superposition of supermodes. The variation of field amplitude with propagation distance is obtained and can predict the precise positions of the field distribution. To demonstrate the analytical results, we numerically simulate the field distribution in the waveguides (N=5) constructed with silver by the finite-difference time-domain method. The numerical simulation results show a good agreement with theoretical expectations.
This paper reports on the disappearance of photosensitive area extension effect and the novel
temperature dependence of junction performance for mid-wavelength HgCdTe detectors. The
performances of junction under different temperatures are characterized by laser beam induced current
(LBIC) microscope. The physical mechanism of temperature dependence on junction transformation is
elaborated and demonstrated using numerical simulations. It is found that Hg-interstitial diffusion and
temperature activated defects jointly lead to the p-n junction transformation depended on temperature,
and wider band gap compared with the long-wavelength HgCdTe photodiode may correlate with the
disappearance of photosensitive area extension effect.
Zinc blende ZnSe longitudinal twinning nanowires (Type I) and a sandwich structure with the wurtzite ZnSe inserting into the zinc blende ZnSe longitudinal twinning nanowires (Type II) are fabricated via a simple thermal evaporation method. The growth of them might be caused by the crystal plane slip during the phase transformation process from wurtzite ZnSe to zinc blende ZnSe nanowire. The wurtzite ZnSe might have two origins: 1) The phase transformed wurtzite from zinc blende. At first, during the temperature rising stage in the experiment, before the temperature approached to the transformation temperature (Ttr), ZnSe in zinc blende phase might begin to nucleate and grow. Once the temperature is higher than Ttr, the zinc blende products would transform to wurtzite phase. 2) The new-born nuclei grown wurtzite phase at high temperature for it is reported that the wurtzite phase is more stable at higher temperature. During the cooling period, the source material is exhausted and no more nucleation would occur. Some of the wurtzite products would transform to zinc blende phase when the temperature is lower than Ttr. During the process, it is reasonable that the ZB phase begins to form from the outer sides of an individual nanowire. Once the process completes, the longitudinal twinning ZB nanowire would be obtained; otherwise, the sandwich-structured nanowire forms.
A novel mask technique, combining high selectivity silicon dioxide patterns over high aspect-ratio
photoresist (PR) patterns has been exploited to perform mesa etching for device delineation and electrical
isolation of HgCdTe third-generation infrared focal plane arrays (IRFPAs). High-density silicon dioxide film
covering high aspect-ratio PR patterns was deposited at the temperature of 80°C and silicon dioxide film
patterns over high aspect-ratio PR patterns of HgCdTe etching samples was developed by standard
photolithography and wet chemical etch. Scanning electron microscopy (SEM) shows that the surfaces of
inductively coupled plasma (ICP) etched samples are quite clean and smooth. The etching selectivity between
the novel mask and HgCdTe of the samples is increased to above 32: 1 while the side-wall impact of etching
plasma is suppressed by the high aspect ratio patterns. These results show that the combined patterning of
silicon dioxide film and thick PR film is a readily available and promising masking technique for HgCdTe
mesa etching.
In this paper, the physical mechanism of unipolar barrier structures is elaborated for dark current
suppression. To better understand the performance characteristics of the devices and optimize the
structures, we have performed numerical drift-diffusion simulations of both n-side and p-side InAs
based unipolar barrier photodiodes with AlAs0.18Sb0.82 barriers, as well as conventional pn junction
detectors. Numerical simulation was used to calculate the current-voltage (I-V) characteristic and R0A
values for InAs unipolar barrier photodiodes and traditional pn junction photodiodes. The performances
of different device structures have been investigated for temperatures from 150 K to 350 K. Comparing
to conventional devices, the unipolar barrier device has shown significant performance improvement.
The polarity inversion of laser beam induced current (LBIC) signal at low temperature and high
laser power density in As-doped p-type HgCdTe is investigated in this paper. It is found that the
polarity of LBIC signal reverses at 87 K compared to that at 300 K and the high laser power density is
also an important factor in inducing the LBIC signal reverse. The results demonstrate that the shape of
the LBIC signal profile is strongly dependent on the temperature of the device and the laser irradiation.
To provide a reasonable analysis for this interesting fact, a photocarrier spreading mode is presented in
this paper.
We report on the temperature-dependent extension of n-type inversion regions in mercury cadmium telluride (HgCdTe) photodiodes at low temperatures (87 K) compared to inversion regions at room temperature (300 K). Laser-beam-induced-current (LBIC) measurement techniques are used to obtain the photosensitive area extensions of n-type inversion in HgCdTe photodiodes for typical n+-on-p HgCdTe photovoltaic IR detectors. The effect of temperature on the extension of n-type conversion region is investigated by considering the sign of the LBIC signal. Theoretical results show that the hole concentration decreases in multidoped HgCdTe due to the freeze-out effect as the temperature decreases. Consequently, hole concentration is much lower than electron concentration at 87 K. The n-type inversion region extension is shown to be caused with the p-to-n type conversion.
The optical bandgap and photoresponse characteristics of middle-wavelength infrared (MWIR) mercury-cadmium-telluride (HgCdTe) photodiodes have been performed based on a self-consistent solution of the Poisson's equation, the electron/hole continuity equations, and three-generation-recombination processes as Auger, Shockley-Read-Hall and optical generation recombination. Three different carrier-density approximations: (i) parabolic conduction-band approximation, (ii) Bebb's nonparabolic expression, and (iii) Harman's nonparabolic approximation, are proposed to calculate the optical bandgap and photoresponse of MWIR HgCdTe photovoltaic devices by considering the carrier degeneracy and the nonparabolic conduction band. It is found that omitting nonparabolic effect can lead to an enormous deviation in the simulation result, especially for heavily doped HgCdTe devices. On the basis of the calculated results of photoresponse, the parabolic conduction-band and Harman's nonparabolic approximations can lead to the response peak shift to short and long wavelengths, respectively.
This paper reports on the temperature-dependent extension of n-type inversion regions in
HgCdTe photodiodes at low temperatures (87 K) compared to inversion regions at room
temperature (300 K). Laser-beam-induced-current (LBIC) measurement techniques are used
to obtain the photosensitive area extensions of n-type inversion in HgCdTe photodiodes for
typical n+-on-p HgCdTe photovoltaic IR detectors. The effect of temperature on the extension
of n-type conversion region is investigated by considering the sign of the LBIC signal.
Theoretical results show that the hole concentration decreases in multi-doped HgCdTe as the
temperature decreases. Consequently hole concentration is much lower than electron
concentration at 87 K. It is demonstrated that the n-type inversion region extension is caused
with the p-to-n type conversion.
The current-voltage and photo-response characteristics of middle wavelength infrared
(MWIR) HgCdTe photodiodes have been performed based on a self-consistent solution of the
Poisson's equation, the electron/hole continuity equations, and three
generation-recombination processes as Auger, Shockley-Read-Hall and optical generation
recombination. Three different carrier density approximations, (1) parabolic conduction band
approximation, (2) Bebb's non-parabolic expression, and (3) Harman's non-parabolic
approximation, are proposed to simulate the I-V curve and photo-response of MWIR HgCdTe
photovoltaic devices by considering the carrier degeneracy and the non-parabolic conduction
band. It is found that omitting non-parabolic effect can lead to an enormous deviation in the
simulation result, especially for heavily doped HgCdTe devices. Based on the calculated
results of photo-response, the parabolic conduction band and Harman's non-parabolic
approximation can lead to the response peak shift to short and long wavelength, respectively.
The history and milestones of HgCdTe infrared detector technology in China has been reviewed,
including the material growth, device processing and design. It is also presented that the HgCdTe
infrared detector has been used well in space remote sensing technology. The current status of
HgCdTe technology is focused on focal-plane arrays (FPAs) fabricated with HgCdTe grown on
different substrate, including GaAs and Si substrate, by epitaxy method. The FPA imaging,
material growth process and interface engineering have been discussed.
Rapid thermal annealing effects on GaNAs/GaAs epilayer with about 1.0% N are experimentally analyzed. Both the as-grown
and annealed samples are studied by photoluminescence at different temperature (11K~290K). Exciton
localization and delocalization are investigated in detail. It exhibits quite different optical properties for the localized and
delocalized excitons before and after annealing, this is attributed to the nitrogen reorganization inside the GaNAs layer,
which homogenizes initial nitrogen composition fluctuations present in the as-grown alloy.
For most commonly used GaAs/AlGaAs n-type quantum well infrared photodetectors (QWIPs), the normal incident
absorption is not possible because of the transition rule. The optical grating is required to achieve high absorption
quantum efficiencies. When some gratings are patterned on the metal plate, the polarization direction can be changed
greatly because of the diffraction effect. Finite difference time domain (FDTD) method has been used to investigate the
effect of a reflection metal grating on the couple efficiency previously. However, the authors only take one metal grating
and apply periodic boundary condition along the grating direction due to the computation limit. For a real QWIP system,
such simulation is crude. In this work we consider a real GaAs/AlGaAs QWIP with a wavelength response around 15um
and use FDTD method to investigate the effect of a reflection metal grating on the electric field pattern and the couple
efficiency. The simulating results show that the electric field pattern is not periodic for every metal grating in a real
QWIP system. We have also studied the influence of the substrate thickness and the grating period on the electric field
pattern and the couple efficiency. These results offer a guideline for the design of QWIP.
Electronic properties of both Pb and S vacancy defect in PbS(-100) have been studied using the first principles density
functional theory (DFT) calculations with the plane-wave pseudopotentials. The densities of states are computed to
investigate the effect of the Pb and S vacancy on the electronic structure, respectively. In the case of S vacancy defect,
the Fermi energy shifted to the conduction band making it an n-type PbS (donor). While in the case of Pb vacancy the
DOS do change appreciably.
By optimizing the growth technique of molecular beam epitaxy (MBE), we've prepared high quality InAs quantum dot samples, with high density (up to 1.2 x 1011 cm-2) and highly homogeneous size. Strong room temperature photoluminescence (PL) have been observed. The PL study reveals four confined-states QD transitions, which shows filling effect of the excited states. Dependence of these recombinations on excitation intensity and temperature has also been investigated.
Conductive atomic force microscopy (C-AFM) has been used to probe the local current of InAs surface quantum dots (SQDs) on doped GaAs layer. It is found that current about picoampere can be drawn from individual QD while bias of a few hundred mV is applied between tip and the sample. Further I-V studies discover that surface quantum dots usually has a threshold bias near 200 mV for current transmission, whereas bias of 400 mV is needed when the conductive tip is located on wetting layer.
ZnO nanoparticles had been successfully prepared by annealing the precursors at different temperature, which were produced by the chemical precipitation method. The annealing temperature is a key parameter to prepare ZnO nanoparticles. The microstructure of the resultant nanoparticles was studied by means of XRD, TEM and PL spectra. The ultra-violet emission as observed at room temperature.
Ion implantation enhanced intermixing of quantum well has become an important technology in device fabrication and material modification. We report the intermixing effect in a single asymmetric coupled quantum well (GaAs/AlGaAs) at different ion implantation dose by photoluminescence. More than 80meV of blue shift of the interband transition was observed before rapid thermal annealing process. It indicates that the intermixing has almost finished during the implantation process. A diffusion length of 1nm is obtained by the theoretical analysis.
In this paper we present the observation of the interband transition in the GaAs (100) surface Si-delta-doping potential. Samples with different surface doping concentrations (Ns equals undoped, 3.0 X 1012, 6.3 X 1013, 2.4 X 1014 and 3.6 X 104 cm-2) have been studied at room temperature in the MBE high vacuum chamber using modulated photo-reflection (PR) spectroscopy technique. The MBE chamber guarantees that all the sample surfaces are free of oxidation or uncontrollable contamination. The optical transition of at GaAs bandedge around 1.41 eV is strong and is almost independent of Ns. A relatively weak feature above 1.42 eV has been observed which is clearly enhanced and blue-shifted following the increase of Ns. The experimental results have been analyzed and well explained based on the self-consistent Schrodinger-Poisson equations. The theoretical analysis indicates that it is not proper to attribute the PR spectral peak of 1.42 eV simply to be certain subband-related optical transition. The observed spectral peak of 1.42 eV is more likely to be related to the high-index confined-levels in the half-V-shape conduction band at the sample surface.
In this letter, self-aligned dual implantation technique was successfully used to speed up the carrier transportation from sidewall quantum well (SQWL) to quantum wire (QWR) region in V-groove AlGaAs/GaAs QWR structure. Photoluminescence (PL) and time resolved photoluminescence (TRPL) show that the lateral confinement was enhanced after intermixing by intermixing the necking region. Lifetime was obviously enlonged after selective intermixing, which comes from the enhanced lateral carrier confinement. Strong hot exciton relaxation process in QWRs region is observed after selective intermixing.
Photoreflectance (PR) spectroscopy system is combined with molecular beam epitaxy (MBE) to accomplish in-situ PR measuring of the Si surface (delta) doping on GaAs (001) with different concentrations at different temperatures. The features observed on the high-energy side of the fundamental gap are attributed to transitions involving electronic subbands in the half V-shaped potential well. We find that the Si (delta) -doping-related spectral structure first shifts to high energy side with the doping concentration increasing, then almost stop shifting with the doping concentration higher than 2.4 X 1014 cm-2 when temperature increases, at certain doping concentration the Si (delta) - doping-related transition shifts toward low energy side. The dependence of the transition on doping concentration is well explained by using a simple theoretical model.
Manufacture of AlGaAs/GaAs QDs by visible light lithography and etching is accomplished in this paper, and the size distribution is studied by smaill-spotted PL. The broadening of PL peaks which is caused by the fluctuation of quantum well is studied.
A comprehensive and spectroscopic investigation, including absorption (AB), photoluminescence (PL) and photoreflectance (PR) experiments on the electronic states and their optical transitions in some near-surface and surface quantum well structures of semiconductors are performed and reported here in this paper. The strain relaxation as a function of capping layer, the electronic states on the surface quantum well and the dependence of transition related surface Si (delta) doping on doping concentration.
A long-wavelength 128 by 1 GaAs/AlGaAs multiple quantum wells (MQWs) infrared detector (IRD) array is presented. A good photo-current responsibility Rp equals 2.02 X 106V/W is obtained with the cut-off wavelength being (lambda) c equals 8.6 micrometer and as a result, we obtain a good remanent heat image of a room-temperature subject.
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