Magnetic properties of submicron size particles made from Fe/Co multilayers

Arrays of submicron size (0.15 μm) particles of 23 and 35 nm thick Fe(2 ML)/Co(6 ML) multilayers were investigated by magnetization measurements and magnetic force microscopy. The behaviour of elliptical particles is mainly determined by their shape anisotropy. Varying the lateral size and thickness of the particles there is a transition from multi-domain to single-domain states.     

Molecular beam epitaxy growth and characterization of self-assembled InAs quantum dots on (1 0 0) InAlAs/InP substrates

Self-assembled InAs quantum dots (QDs) on In_0.52Al_0.48As layer lattice matched to (1 0 0) InP substrates have been grown by molecular beam epitaxy (MBE) and evaluated by transmission electron microscopy (TEM) and photoluminescence (PL). TEM observations indicate that defect-free InAs QDs can be grown to obtain emissions over the technologically important 1.3–1.55 μm region. The PL peak positions for the QDs shift to low energy as the InAs coverage increases, corresponding to increase in QD size. The room temperature PL peak at 1.58 μm was observed from defect-free InAs QDs with average dot height of 3.6 nm.     

Development of a 4–15 μm infrared GaAs hyperspectral QWIP imager

In the on-going evolution of GaAs quantum well infrared photodetectors (QWIPs) we have developed a four band, 640 × 512, 23 μm × 23 μm pixel array which we have subsequently integrated with a linear variable etalon (LVE) filter providing over 200 spectral bands across the 4–15.4 μm wavelength region. This effort was a collaboration between NASA’s Goddard Space Flight Center (GSFC), the Jet Propulsion Laboratory (JPL) and the Army Research Laboratory (ARL) sponsored by the Earth Science Technology Office of NASA. The QWIP array was fabricated by graded molecular beam epitaxial (MBE) growth that was specifically tailored to yield four distinct bands (FWHM): Band 1; 4.5–5.7 μm, Band 2; 8.5–10 μm, Band 3; 10–12 μm and Band 4; 13.3–14.8 μm. Each band occupies a swath that comprises 128 × 640 elements. The addition of the LVE (which is placed directly over the array) further divides the four “broad” bands into 209 separate spectral bands ranging in width from 0.02 μm at 5 μm to 0.05 μm at 15 μm. The detector is cooled by a mechanical cryocooler to 46 K. The camera system is a fully reflective, f/4.2, 3-mirror system with a 21° × 25° field of view. The project goals were: (1) develop the 4 band GaAs QWIP array; (2) develop the LVE and; (3) implement a mechanical cryocooler. This paper will describe the efforts and results of this undertaking with emphasis on the overall system characteristics.     

Micromagnetic investigation of sub-1 0 0-nm magnetic domains in atomically stacked Fe(0 0 1)/Au(0 0 1) multilayers

Continuous films and nanostructures of atomically stacked epitaxial Fe(0 0 1)/Au(0 0 1) multilayers have been studied by soft X-ray and Lorentz microscopy as well as micromagnetic simulations. Domain imaging shows about 65 nm wide magnetic stripe domains, in which the magnetization is oriented perpendicular to the film plane. These results are confirmed by micromagnetic simulations, which also yield additional information about the internal structure of domains and walls.     

Gas source MBE grown wavelength extended 2.2 and 2.5 μm InGaAs PIN photodetectors

Using homo-junction structure and relative thin linear graded In_xGa_1−xAs as the buffer layer, extended wavelength InGaAs PIN photodetectors with cut-off wavelength of 2.2 and 2.5 μm at room temperature have been grown by using GSMBE, and their performance over a wide temperature range have been extensively investigated. For those 2.2 or 2.5 μm detectors with 100 μm diameter, the typical dark current (V_R = 10 mV) and R₀A are 57 nA/10.3 Ω cm² or 67 nA/12.7 Ω cm² at 290 K, and 84 pA/4.70 kΩ cm² or 161 pA/3.12 kΩ cm² at 210 K respectively. The thermal activation energies of the dark current are 0.447 eV or 0.404 eV for 2.2 or 2.5 μm detectors respectively.     

Nanoparticle-induced multi-functionalization of silicon: A plug and play approach

Here, we demonstrate a “plug and play” approach to achieve multi-functionalization of Si. In this approach, externally synthesized functional nanoparticles are introduced onto device quality Si wafers and the surface chemical bonds are manipulated. Sonochemically synthesized Fe₂O₃ nanoparticles are introduced onto Si from an alcohol suspension. On annealing this sample in ultra-high vacuum, the oxygen atoms change the bonding partner from Fe to Si and desorb as SiO at 750 °C. This results in the formation of nanoparticles of Fe on the surface and exhibits ferromagnetic behavior. Deposition of a thin layer (2 nm) of Si onto the sample containing the metallic Fe nanoparticles followed by annealing at 560 °C leads to optically active Si. Photoluminescence measurements show that this sample emits light at three different wavelengths, namely 1.57, 1.61 and 1.63 μm, when excited by He–Ne or Ar lasers. Oxidation of this material results in the formation of a selective capping layer of SiO₂. Thus we obtain multi-functional Si in an “all in one” form and we believe that this approach is universal.     

NIR, MWIR and LWIR quantum well infrared photodetector using interband and intersubband transitions

This paper presents the design, fabrication and characterization of a QWIP photodetector capable of detecting simultaneously infrared radiation within near infrared (NIR), mid wavelength infrared (MWIR) and long wavelength infrared (LWIR). The NIR detection was achieved using interband transition while MWIR and LWIR were based on intersubband transition in the conduction band. The quantum well structure was designed using a computational tool developed to solve self-consistently the Schrödinger–Poisson equation with the help of the shooting method. Intersubband absorption in the sample was measured for the MWIR and LWIR using Fourier transform spectroscopy (FTIR) and the measured peak positions were found at 5.3 μm and 8.7 μm which agree well with the theoretical values obtained 5.0 μm and 9.0 μm for the two infrared bands which indicates the accuracy of the self-consistent model. The photodetectors were fabricated using a standard photolithography process with exposed middle contacts to allow separate bias and readout of signals from the three wavelength bands. The measured photoresponse gave three peaks at 0.84 μm, 5.0 μm and 8.5 μm wavelengths with approximately 0.5 A/W, 0.03 A/W and 0.13 A/W peak responsivities for NIR, MWIR and LWIR bands, respectively. This work demonstrates the possibility of detection of widely separated wavelength bands using interband and intersubband transitions in quantum wells.     

Effect of heat-pretreatment of the graphite rod on the quality of SWCNTs by arc discharge

Single-walled carbon nanotubes (SWCNTs) have been synthesized in high yield by the dc arc discharge technique under heat-pretreatment of the graphite rod conditions. Before executing arc discharge, the graphite rods containing the catalysts were heat treated at 600, 700, 800 and 900 °C for 1–3 h, respectively. Effects of heat-pretreatment of the graphite rod on the quality of SWCNTs by arc discharge were investigated. The heat-treatment temperature and time were found to be crucial for a high yield of high-purity SWCNTs. Optimum parameter was found to be at the heat-treatment temperature of 800 °C for 2 h. The SWCNTs synthesized under the optimum condition have better field-emission characteristics. The turn-on field needed to produce a current density of 10 μA/cm² is found to be 1.9 V/μm and the threshold field where current density reaches 10 mA/cm² is 3.9 V/μm.     

Uncooled microbolometer detector: Recent developments at Ulis

Uncooled infrared focal plane arrays are being developed for a wide range of thermal imaging applications. Therefore, to answer these markets, a 35 μm pixel-pitch uncooled IR detector technology has been developed enabling high performance 160 × 120 and 384 × 288 arrays production. Besides a wide-band version from uncooled 320 × 240/45 μm array has been also developed in order to address process control and more precisely industrial furnaces control. The ULIS amorphous silicon technology is well adapted to manufacture low cost detector in mass production. After some brief microbolometer technological background, we present the characterization of 35 μm pixel-pitch detector as well as the wide-band 320 × 240 infrared focal plane arrays with a pixel pitch of 45 μm. Information on the new 640 × 480 array with a pixel pitch of 25 μm is also presented.     

Depth dependence of photoluminescence and chemical bonding in porous silicon

Porous silicon (PS) is studied by stepwise peeling of the surface layer to clarify the non-uniformity in the photoluminescence (PL) and correlate it with the in-depth chemical bonding and structure of the 30 μm thick layer. The PL intensity grows by an order of magnitude after the peeling off of the first 10 μm and decreases five times in the next 5 μm while the peak maximum position shifts from 730 to 800 nm. X-ray photoelectron spectroscopy (XPS) measurements show that Si–Si and Si–O bonds are present both on the surface and below, and the preferential oxidation state of silicon changes from 3+ and 4+ on the surface to 1+ and 2+ below 10 μm. Using Raman spectroscopy silicon nanocrystals are shown to exist. Their mean size can be estimated at about 3 nm. These results show that the strongest PL comes from a region in the PS layer where silicon nanocrystallites are surrounded by oxides with a low level of oxidation and not from the strongly oxidized surface layer.     

Si doped GaAs/AlGaAs terahertz detector and phonon effect on the responsivity

Terahertz detection capability of an n-type heterojunction interfacial work function internal photoemission (HEIWIP) detector is demonstrated. Threshold frequency, f₀, of 3.2 THz (93 μm) was obtained by using n-type GaAs emitter doped to 1 × 10¹⁸ cm⁻³ and Al_0.04Ga_0.96As single barrier structure. The detector shows a broad spectral response from 30 to 3.2 THz (10–93 μm) with peak responsivity of 6.5 A/W at 7.1 THz under a forward bias field of 0.7 kV/cm at 6 K. The peak quantum efficiency and peak detectivity are 19% and 5.5 × 10⁸ Jones, respectively under a bias field of 0.7 kV/cm at 6 K. In addition, the detector can be operated up to 25 K.     

MBE grown type-II MWIR and LWIR superlattice photodiodes

We report on the status of GaSb/InAs type-II superlattice diodes grown and fabricated at the Jet Propulsion Laboratory designed for infrared absorption 2–5 μm and 8–12 μm bands. Recent LWIR devices have produced detectivities as high as 8 × 10¹⁰ Jones with a differential resistance–area product greater than 6 Ohm cm² at 80 K with a long wavelength cutoff of approximately 12 μm. The measured internal quantum efficiency of these front-side illuminated devices is close to 30% in the 10–11 μm range. MWIR devices have produced detectivities as high as 8 × 10¹³ Jones with a differential resistance–area product greater than 3 × 10⁷ Ohm cm² at 80 K with a long wavelength cutoff of approximately 3.7 μm. The measured internal quantum efficiency of these front-side illuminated MWIR devices is close to 40% in the 2–3 μm range at low temperature and increases to over 60% near room temperature.     

Development of IR-FEL facility for energy science in Kyoto University

A mid-infrared free electron laser (FEL) has been constructed for energy science in the Institute of Advanced Energy, Kyoto University. The FEL system consists of a compact S-band Linac and an undulator to generate 4–13 μm coherent mid-infrared radiations. The Linac consists of a 4.5 cell rf gun with a thermionic cathode and a 3-m traveling-wave-type accelerator tube fed by 10 MW and 20 MW rf power, respectively. We have succeeded to produce 40 MeV, 40 mA and 3 μs electron beams. Last December, the 9.2 μm spontaneous emission from the undulator generated by 29.5 MeV electron beams was observed for the first time. Further optimization parameters of both the electron beam and the optical cavity are being pursued for an FEL lasing in the near future.     

Absorption coefficients of O3 at CO2 laser wavelengths

The O₃ absorption coefficients for the rotational lines P(12)–P(28) of the 9.4 μm emission band of the CO₂ laser are presented. Measurements were made in O₃–air dilute mixtures (20–600 ppm) at 25°C and a total pressure of 1013.25 h Pa using a frequency stabilized cw CO₂ laser and values have been determined with greater precision than in previously reported studies.     

Characteristics of thermal distortions of the laser mirror substrates filled with phase-change materials

Under irradiating of the laser power of 2 kW, the thermal deformations of the silicon mirror substrates with phase change materials are experimentally measured and numerically analyzed by using finite element methods, respectively. The experimental results show that when the absorbed laser power is 120 W and the laser irradiating time gets to three seconds, the thermal distortion of the silicon mirror substrates with paraffin/carbon powder is 0.25 μm, that of the paraffin/aluminum powder 0.33 μm, and that of the paraffin/copper powder 0.37 μm. The numerical calculation coincides with the experimental results.     

On the origin of 1.5 μm luminescence in porous silicon coated with sol–gel derived erbium-doped Fe2O3 films

Sol–gel derived Fe₂O₃ films containing about 10 wt% of Er₂O₃ were deposited on porous silicon by dipping or by a spin-on technique followed by thermal processing at 1073 K for 15 min. The samples were characterized by means of PL, SEM and X-ray diffraction analyses. They exhibit strong room-temperature luminescence at 1.5 μm related to erbium in the sol–gel derived host. The luminescence intensity increases by a factor of 1000 when the samples are cooled from 300 to 4.2 K. After complete removal of the erbium-doped film by etching and partial etching the porous silicon, the erbium-related luminescence disappears. Following this, luminescence at 1.5 μm originating from optically active dislocations (“D-lines”) in porous silicon was detected. The influence of the conditions of synthesis on luminescence at 1.5 μm is discussed.     

Research topics at Thales Research and Technology: Small pixels and third generation applications

Recent results obtained on building blocks for future third generation infrared focal plane arrays (FPAs) are presented. Our approach concerning the FPA performance assessment and small pixels modelling is exposed. We also demonstrate the ability of the quantum well infrared photodetector technology to answer the needs for compact (20 μm pitch) polarimetric FPAs. Finally, we present our first results on mid-wave infrared detectors at wavelengths below 4.2 μm.     

LWIR/SWIR switchable two color device based on InP/InGaAs integrated HBT/QWIP

A novel two color infrared (IR) device that allows fast electrical switching between the short wavelength IR (SWIR) band (0.9–1.6 μm) and the long wavelength IR (LWIR) band (8–12 μm) is presented. The integrated sensor is based on MOCVD grown, lattice matched (to InP substrate) epilayers of InGaAs/InP and consists of two, monolithically integrated sections of heterojunction bipolar transistor (HBT) and quantum well infrared photodetector (QWIP).     

Principle of π-phase plate long trace profiler for synchrotron radiation optics

A π-phase plate long trace profiler (πLTP) for testing aspherical optical elements in large dimension, especially the optical elements used in synchrotron radiation, is introduced. Based on the angle-to-position converting feature of a lens, a collimated diode laser beam out of a single mode optical fiber scans all the way across the surface under test (SUT). A diffracted pattern is used to precisely determine the position of the light spot on the back focal plane of the lens. A prototype based on the principle with a measurement range of 370 mm and a slope resolution of better than 0.25 μrad has been established. An accuracy of 0.7 μrad has been achieved.     

Enhanced magnetic properties of Nd–Fe–B thin films crystallized by heat treatment

Nd₂Fe₁₄B Φ phase crystallites were formed in Nd_16.7Fe_65.5B_17.8 thin films prepared by RF sputtering with subsequent heat treatment. The 2 μm-thick films were deposited onto 0.1 mm Mo sheets at an average substrate temperature (T_s) of 365°C. The enhanced magnetic properties of the magnetically anisotropic thin films were investigated using different heating rates (h_r) of 10°C, 20°C, 50°C and 100°C/min in an annealing experiment. Transformation from the amorphous phase to the crystalline phase is clearly manifested by the formation of fine crystallites embedded as a columnar matrix of Nd₂Fe₁₄B phase. High-resolution scanning electron microscope data of the cross-section of the annealed films show columnar stacking of Nd₂Fe₁₄B crystallites with sizes <500 nm. Transmission electron microscope observations revealed that the microstructure of these films having out-of-plane magnetization consists of uniformly distributed Φ phase with grain size around 400 nm together with small Nd rich particles. This grain size of Φ phase is comparable to the single domain particle diameter of Nd₂Fe₁₄B. Significant change in _iH_c, 4πM_r and 4πM_s with h_r was confirmed. Annealing conditions with a heating rate of 50°C/min to an annealing temperature (T_a) of 650°C for 30 min was consequently found to give optimum properties for the NdFeB thin films. The resulting magnetic properties, considered to be the effect of varying h_r were _iH_c= 1307–1357 kA/m, 4πM_r=0.78–1.06 T and 4πM_s=0.81–1.07 T.