Influence of Frohlich Interaction on Intersubband Transitions of InGaAs/InAlAs-Based Quantum Cascade Detector Structures Investigated by Infrared Modulated Photoluminescence

We demonstrate the use of an infrared modulated photoluminescence(PL)method based on a step-scan Fourier-transform infrared spectrometer to analyze intersubband transition(ISBT)of InGaAs/InAlAs quantum cascade detector(QCD)structures.By configuring oblique and parallel excitation geometries,high signal-to-noise ratio PL spectra in near-to-far-infrared region are measured.With support from numerical calculations based on the k·p perturbation theory,the spectra is attributed to intraband and interband transitions of InGaAs/InAlAs QCD structures.Temperature evolution results show that the k-dependent transitions caused by longitudinal optical phonon-assisted scattering(Frohlich interaction)plays an important role in the ISBT.These results suggest that this infrared modulated-PL method has great potential in characterizing QCD devices and conducting performance diagnostics.

A New Resonant Tunnelling Structure of Integrated InGaAs/GaAs Very-Long-Wavelength Quantum-Well Infrared Photodetector

We perform a theoretical study on a low dark current InGaAs/GaAs very-long-wavelength （〉 12 μm） quantum well infrared photodetector （VLW-QWIP）, based on a double barrier resonant tunnelling structure （DBRTS）. The ground tunnelling state of the central quantum well （QW） of the DBRTS can resonate with the first excited bound state of the doped InGaAs QW by adjusting the structure parameters of the DBRTS. Investigation of the carrier transport performance of this device is carried out based on quantum wave transport theory. It has been shown that the dark current in this device can be significantly reduced by two orders compared to conventional InGaAs/GaAs VLW-QWIPs, while the photocurrent is almost the same as those in conventional VLW-QWIPs. This DBRTS integrated VLW-QWIP structure may stimulate the experimental investigation for VLW-QWIPs at high operation temperatures.

QUANTUM MECHANICAL MODEL AND SIMULATION OF GaAs/AlGaAs QUANTUM WELL INFRARED PHOTO-DETECTOR-ⅠOPTICAL ASPECTS

A complete quantum mechanical model for GaAs?AlGaAs quantum well infrared photodetectors(QWIPs) is presented here. The model consisted of four parts: (1) Starting with the description of the electromagnetic field of the infrared radiation in the QWIP, effective component of the vector potential <| A z |> along the QWIP growth direction ( z axis) due to the optical diffraction grating was calculated. (2) From the wave transmissions and the occupations of the electronic states, it was discussed that the dark current in the QWIP is determined by the drift diffusion current of carriers thermally excited from the ground sublevel in the quantum well to extended states above the barrier. (3) The photocurrent was investigated by the optical transition (absorption coefficient between the ground state to excited states due to the nonzero <| A z |> ). (4) By studying the inter diffusion of the Al atoms across the GaAs?AlGaAs heterointerfaces,the mobility of the drift diffusion carriers in the excited states was calculated, so the measurement results of the dark current and photocurrent spectra can be explained theoretically. With the complete quantum mechanical descriptions of (1 4), QWIP device design and optimization are possible.

Growth Optimization, Strain Compensation and Structure Design of InAs/GaSb Type-II Superlattices for Mid-Infrared Imaging

InAs/GaSb type-II superlattce (T2SL) photodetector structures at the MWIR regime were grown by molecular beam epitaxy. The growth temperature and group-V soaking times were optimized with respect to interface and transport quality. Novel strain compensation schemes with insertion of InSb layers were proposed and tested to be efficient to tune the overall strain between tensile and compressive without degradation of interface and optical quality. The effect of the proposed methods is modeled by analytic functions.? Band structure calculations were also carried out for the proposed T2SL structures to assist optimizing sample designs. Single pixel photodiodes with a low dark current were demonstrated.

Detection Wavelength of Strained InxGa1-xAs/GaAs Very-Long-Wavelength Quantum Well Infrared Photodetectors

Detection wavelength is one of the key performance indices of infrared photodetectors. We study the character of detection wavelength of the strained InxGa1-xAs/GaAs very-long-wavelength （〉 12 μm） quantum well infrared photodetectors （VLW-QWIPs） characterized by the photoluminescence （PL） and photocurrent （PC） measurements. Based on the theoretical calculation and experimental data, we have built a practical model for the Inx Ga1-xAs/GaAs strained VLW-QWIPs, from which the interband transitions, intersubband transition and peak detection wavelength can be determined. Afterwards, the dependences of detection wavelength and device operation mode on the In mole fraction and InxGa1-xAs well width are presented, which will be helpful for device design and optimization.

Abnormal physics of group-Ⅱ telluride system:valence contribution of d electrons

The physical trend of group-I/tellurides is unexpected and contrary to the conventional wisdom. The present firstprinciples calculations give fundamental insights into the extent to which group-Ⅱ telluride compounds present special properties upon mixing the d valence character. Our results provide explanations for the unexpected experimental observations based on the abnormal binding ordering of metal d electrons and their strong perturbation to the band edge states. The insights into the binary tellurides are useful for the study and control of the structural and chemical perturbation in their ternary alloys and heterostructures.

Optical characterization of defects in narrow-gap HgCdTe for infrared detector applications

Narrow-gap Hg_(1-x)Cd_x Te material with a composition x of about 0.3 plays an extremely important role in mid-infrared detection applications. In this work, the optical properties of doped HgCdTe with x ≈ 0.3 are reviewed, including the defects and defect levels of intrinsic V_(Hg) and the extrinsic amphoteric arsenic(As) dopants, which can act as shallow/deep donors and acceptors. The influence of the defects on the determination of band-edge electronic structure is discussed when absorption or photoluminescence spectra are considered. The inconsistency between these two optical techniques is demonstrated and analyzed by taking into account the Fermi level position as a function of composition, doping level,conductivity type, and temperature. The defect level and its evolution, especially in As-doped HgCdTe, are presented. Our results provide a systematic understanding of the mechanisms and help for optimizing annealing conditions towards p-type As-activation, and eventually for fabricating high performance mid-infrared detectors.

Pulsations in Optically Pumped Formic Acid 433μm Far Infrared Laser

Pulsation behaviours of optically pumped Formic Acid(HCOOH)433μm laser are observed.Pulsation frequency varies continuously with tilting the output cavity mirror or changing the HCOOH pressure.Unstable pulsations are observed at a relative high pressure or by changing a cavity mirror with higher output loss.

Electrical Property of Infrared-Sensitive InAs Solar Cells

InAs infrared-sensitive solar cells are fabricated by using the films grown by the liquid phase epitaxy technique. The film microstructures are characterized by x-ray diffraction and scanning electronic microscopy. The current-voltage characteristics of the solar cells in the dark and under AM1.5 illumination at 300 K and 77 K are discussed. The conversion efficiency of p-InAs/n-sub InAs cells decreases when the thickness of the p-type film changes from 1.7 μm to 3.5 μm, which is caused by the reduced effective photons near p？n junction. The p-InAs/n-InAs/n-sub InAs solar cell with the conversion efficiency of 7.43% in 1-2.5 μm under AM1.5 at 77 K is obtained. The short circuit current density increases dramatically with decreasing temperature due to the weakened effect of phonon scattering.

Demonstration of Far-Infrared GaAs/AlGaAs Quantum Well Photodetectors for Broadband Wavelength Detection

We discuss the realization of broadband wavelength detection by demonstrating the longest cutoff wavelength(λc=28.6μm)far-infrared GaAs/AIGaAs quantum well infrared photodetectors(QWIPs).The responsivity is comparable to that of mid-infrared GaAs/AIGaAs and InGaAs/GaAs QWIPs,with a responsivity of 0.265 A/W and detectivity of 3.4x109 cm.Hz1/2/W at the peak wavelength of 26.9μm at 4.2K.Based on the temperature-dependent dark current and response results,it is expected that similar performance can be obtained at least up to 20 K.Several ways to expand the wavelength coverage are also addressed.

Infrared Reflection Spectroscopy on the Linear Chain Heisenberg Antiferromagnetic Crystal NENP

The vibration modes of crystal NENP[Ni(C_(2)H_(8)N_(2))_(2)NO_(2)ClO_(4)]in the wavenumber range of 50-5000cm_(-1) are studied by the infrared rejection spectra,.The dielectric function property of the sample is obtained by the Kramers-Kronin calculation on the reilGction spectra.The dielectric function polarization property induced by the vibration mode polaiization in far infrared region is investignted.

Far Infrared Resonance Transition Study of the Chain-End S=1/2 Modes in an S=1 Antiferromagnetic Chain

The unpaired spin S=1/2 states which occur at the ends of an S=1 Heisenberg-like antiferromagnetic chain,have been observed recently in low-temperature electron spin-resonance measurements of Ni(C_(2)H_(8)N_(2))_(2)NO_(2)ClO_(4) containing selected impurities.We present here a further study on this topic by far infrared transmission spectra under the magnetic field.The splitting of S=1/2 modes was clearly observed and explained by the interaction of S=1/2 modes at two nearby chains,which can be separated by vacancy-like defect.

Bismuth Effects on Electronic Levels in GaSb(Bi)/AlGaSb Quantum Wells Probed by Infrared Photoreflectance

GaSb（Bi）/Alo.2Gao.sSb single quantum wells are characterized by a Fourier transform infrared spectrometer- based photoreflectance method at 77K. Spatially direct and indirect transitions between the electronic levels at and above the effective band gap are well resolved. The shifts of the electronic levels with Bi incorporation are identified quantitatively. The results show that the upshift of the valence band edge is clarified to be dominant, while the Bi-induced downshift of the conduction band edge does exist and contributes to the band gap reduction in the GaSbBi quantum-well layer by （29±6）%.

A novel spin-FET based on 2D antiferromagnet

Engineering the electronic band structure of material systems enables the unprecedented exploration of new physical properties that are absent in natural or as-synthetic materials.Electronic structures of bilayer two-dimensional(2D)systems can be flexibly engineered by the external electric field.For example.

Recent progress on integrating two-dimensional materials with ferroelectrics for memory devices and photodetectors

Two-dimensional （2D） materials, such as graphene and MoS2 related transition metal dichalcogenides （TMDC）, have attracted much attention for their potential applications. Ferroelectrics, one of the special and traditional dielectric materials, possess a spontaneous electric polarization that can be reversed by the application of an external electric field. In recent years, a new type of device, combining 2D materials with ferroelectrics, has been fabricated. Many novel devices have been fabricated, such as low power consumption memory devices, highly sensitive photo-transistors, etc. using this technique of hybrid systems incorporating ferroelectrics and 2D materials. This paper reviews two types of devices based on field effect transistor （FET） structures with ferroelectric gate dielectric construction （termed FeFET）. One type of device is for logic applications, such as a graphene and TMDC FeFET for fabricating memory units. Another device is for optoelectric applications, such as high performance phototransistors using a graphene p-n junction. Finally, we discuss the prospects for future applications of 2D material FeFET.

Synthesis and Characterization of Mesoporous Titania Particles and Thin Films

Well-organized mesoporous titania particles and thin films were successfully synthesized by using tetrabutyl titanate as the inorganic precursor and triblock copolymer （Pluronic P123） as the template via evaporationinduced self-assembly process. The resulting materials were characterized by high resolution transmission electron microscopy （HRTEM）, X-ray diffraction （XRD）. Fourier-transform infrared spectroscopy （FT-IR）,Brunauer-Emmett-Teller （BET） and atomic force microscopy （AFM）. Macro shape of mesoporous titania would greatly influence the mesostructure of materials, and the probable reasons were also discussed.

A 10.7 μm InGaAs/InAlAs Quantum Cascade Detector

A 10.7μm quantum cascade detector based on lattice matched InGaAs/InAlAs/InP is demonstrated and characterized in terms of responsivity, resistivity and detectivity. The device operates in the 8 14μm atmospheric window up to 140 K and shows a peak reponsivity of 14.4mA/W at 78K. With a resistance-area product value of 159Ωcm^2, the Johnson noise limited detectivity D^＊J is 2.8 × 10^9 Jones （cmHz^1/2W^-1） at 78K.

Structural and electrical properties of SrTiO3 thin films as insulator of metal-ferroelectric-insulator-semiconductor （MFIS） structures

SrTiOs （STO） thin films were deposited on p-Si（100） substrates at various substrate temperatures from 300℃ to 700℃ by radio frequency （RF） magnetron sputtering technique. Their structure and electrical properties were investigated. It was found that the transition from amorphous phase to polycrystalline phase occurred at the substrate temperatures 300-400℃. Their crystallinity became better when the substrate temperatures further increased. The dielectric and leakage current measurements were carried out by using the Si/STO/Pt metal-insulator-semiconductor （MIS） structures at room temperature. It was found that the fixed charge density decreased and both the interface trap density and the dielectric constant increased when the substrate temperatures were increased. The leakage current mechanisms for STO MIS structures with STO films prepared at 700℃ followed the space charge limited current （SCLC） under the low applied electric field and the Poole-Frenkel emission under the high one. In addition, the resistivity for films prepared at 700℃ was higher than 10^11Ω.cm under the voltage lower than 10V （corresponding to the electric field of 1.54×10^3kV.cm^-1）. It suggested that the STO films prepared at 700℃ were suitable for acting as the insulator of metal-ferroelectric-insulator-semiconductor （MFIS） structures.

Ab Initio Study of Structural and Electronic Properties of Sodium Bromide

The structural and electronic properties of sodium bromide （NaBr） are investigated by the density functional theory （DFT） within the generalized gradient approximation （GGA） for the exchange and correlation energy. The equilibrium lattice constant, bulk modulus and its pressure derivative are obtained by fitting the calculated total energy to the third-order Birch-Murnaghan equation of state. The band structure along the higher symmetry axes in the Brillouin zone, the density of states （DOS） and the partial density of states （PDOS） are presented. The results have been discussed and compared with the available experimental and theoretical data.

Controlled Synthesis of Nanoscale CdTe Urchins

We presented a simple route to prepare nanoscale CdTe urchins in a tri-n-octylphosphine oxide（TOPO） system. CdTe urchins consisted of a core and several attached arms. The arms were ca. 3 nm wide, and their lengths could be controlled with the reaction time. The authors investigated the optical absorption and structural properties of the prepared CdTe. The lengths of the arms could be tuned into CdTe nanourchins, which led to a change in the photophysical properties of the nanoscale CdTe urchins. The results, including transmission electron microscopy（TEM） and absorption spectra, indicated that mesoporous silica and aminopropyltriethoxysilane（APTES） contributed to the formation of nanoscale CdTe urchins.