IR Devices Research Articles

Uncooled infrared photodetectors in Poland

AbstractThe history and present status of the middle and long wavelength Hg1-xCdxTe infrared detectors in Poland are reviewed. Research and development efforts in Poland were concentrated mostly on uncooled market niche.Technology of the infrared photodetectors has been developed by several research groups. The devices are based on mercury-based variable band gap semiconductor alloys. Modified isothermal vapour phase epitaxy (ISOVPE) has been used for many years for research and commercial fabrication of photoconductive, photoelectromagnetic and other devices. Bulk growth and liquid phase epitaxy was also used. At present, the fabrication of IR devices relies on low temperature epitaxial technique, namely metalorganic vapour phase deposition (MOCVD), frequently in combination with the ISOVPE.Photoconductive and photoelectromagnetic detectors are still in production. The devices are gradually replaced with photovoltaic devices which offer inherent advantages of no electric or magnetic bias, no heat load and no flicker noise. Potentially, the PV devices could offer high performance and very fast response. At present, the uncooled long wavelength devices of conventional design suffer from two issues; namely low quantum efficiency and very low junction resistance. It makes them useless for practical applications. The problems have been solved with advanced 3D band gap engineered architecture, multiple cell heterojunction devices connected in series, monolithic integration of the detectors with microoptics and other improvements. Present fabrication program includes devices which are optimized for operation at any wavelength within a wide spectral range 1–15 μm and 200–300 K temperature range. Special solutions have been applied to improve speed of response. Some devices show picoseconds range response time. The devices have found numerous civilian and military applications.

The sensitivity threshold of IR devices based on array photodetectors

A noise model of IR staring arrays is proposed that takes into account the combined action of the spatial and temporal components of the noise. It is used as a basis for obtaining simple relationships for calculating the sensitivity threshold of IR staring devices. How the residual spatial noise affects the relationship in the sensitivity between a staring array and a scanning linear array of photosensitive elements is considered. It is shown that the operation of IR staring devices under conditions of incomplete suppression of spatial noise makes it possible to narrow the spectral range.

Weathering MindStorms with drizzle and DIODE in CS0

We introduce Drizzle, our new simple object-oriented scripting language for programming LEGO MindStorms robots, and DIODE, the Drizzle Integrated Object-Oriented Development Environment. Drizzle and DIODE have evolved from a student project into useful resources for our CS0 course. In this work we describe Drizzle and DIODE and discuss our experiences with them in our CS0 course.We are dismayed by the low retention rates of undergraduate majors in CS1 and CS2. To attempt to alleviate this problem, we introduced a new required CS0 course in Fall 2004. Its goals are to give students a breadth-first overview of CS and to expose them to algorithmic problem solving before immersing them in Java. We decided to use the MindStorms robots to accommodate student diversity, to introduce a fun element to the course, and to force the students to focus on problem analysis instead of detailed syntax. Our approach complements [2].Drizzle is a simple object-oriented scripting language. It supports repetition, selection, simple assignment statements, logical operators, numeric variables, and predefined objects representing components of a robot, i.e., sensors, motors, and the LCD display. Drizzle is purposely minimal to be suitable for students without prior programming experience. To encourage documentation, Drizzle supports comments.DIODE is the IDE that allows a MindStorms robot to be programmed in Drizzle. To achieve this, we used the leJOS API [3] to implement method call abstractions supporting the predefined objects. The IDE includes an intuitive GUI. It allows for automatic compilation of Drizzle code, including a translator from Drizzle to Java utilizing the predefined MindStorms objects. DIODE also directly interfaces to the robot's IR device. We provided these features because our typical user was familiar with text editing but was not familiar with command-line tools.Our CS0 course is currently taught in one 120-student section and meets 3 hours per week for 14 weeks. At least 3 hours are spent on each of algorithmic thinking and concepts of programming languages before Drizzle and DIODE are formally presented. The students see an actual problem with pseudocode and the corresponding Drizzle solution. A problem is then posed as a "Drizzle Team Challenge". Students must complete individual solution plans. The instructor then forms the student groups. Next, the teams produce solution plans. Finally, they are allowed to program with the actual robot. The lab has 3 computers with DIODE installed and 6 robots with 6 tracks. During the Fall 2004 offering, the 14 teams consisted of between 5 and 8 students. The large class size and small number of robots made smaller groups infeasible. No team solved the problem totally correctly during the first lab session. Not surprisingly, the smaller groups were more effective. All teams submitted at least a partial solution to the problem; 12 completed the solution. Students were excited about working with the robots.We are using Drizzle (and DIODE) again during Spring 2005. We expect to have smaller groups. We hope to evaluate the effectiveness of Drizzle; some possible approaches are additional questions in the course evaluation, surveys at the beginning and end of CS1, and analyses of CS1/CS2 retention rates. We may also compare the usage of Drizzle with the standard MindStorms approach and Alice [1].

Organic carbons and TOC in waters: an overview of the international norm for its measurements

Many chemical measurements are subjected to international recommendations and rules. When applied to environment, they must also respect national rules. In this paper, rules to measure TOC issued in the period 1973–2002 are presented. Over 20 rules and guidelines, both national and international, are presented all aiming to increase the environmental monitoring capacity of ecologists and researchers through the use of the best instruments commercially available and manufactured by the market farms. The tracking of CO 2, obtained by oxidative system, is the heart of TOC modern analysers. Practically, all the methods foresee the determination of the CO 2 produced by the degradation of the organic matter present in solution, often IR devices are used. Also, other detectors can be used (conductivimeter, CO 2electrode) but they must be characterised by mechanical resistance and stability, not interfered by other gases, not easily corroded. The thermal oxidation methods are distinguished in low-temperature method (below 100 °C) and high-temperature method (between 600 and 1200 °C) with the presence also of mixed oxidation systems. In many countries, TOC determination is ruled in order to avoid that manufacturers of scientific equipment manage at their commercial convenience on this field. Anyway, the rules imposed by internationally recognised organisms must be actually followed. They establish the use of reference solutions, specific methods of measure and detection limits that must be respected by the analytical instruments. Due to the wide field of application involving TOC measurements, especially related to water (drinking, surface, waste), this kind of instruments is largely present everywhere. At present, definitive rules about TOC measurement are not laid down by law; so, we consider it interesting to present the chronological list of the rules with their content concerning the measurement method, the sensitivity and detection limit, and the matrices that must be subjected to the control. Consideration about the measurement quality and the correspondence between rules and analytical power are out of the interest of this work.

Growth and properties of 40 mm diameter Hg 1−xCd xTe using the two- stage Pressurised Bridgman Method

The growth of high quality large 40 mm diameter HgCdTe ( x = 0.210 ) crystals using a new two-stage Pressurized-Bridgman (P-B) method with a starting charge of x = 0.06 has been reported recently (J. Crystal Growth 263 (2003) 273). New information on the equipment which used N 2 -pressurized chambers as safety features for the synthesis and crystal growth stages in order to avoid the explosion of the quartz ampoules due to the high mercury pressure is reported. Detailed characterization studies on crystals grown using this technique are discussed. The investigation showed that the large HgCdTe crystals grown at a low synthesis temperature of 720 °C and a low growth temperature of 710 °C had good compositional uniformity, crystallinity and electrical properties. Double-crystal rocking curve investigations showed that the best full-width at half-maximum was 14.4 arc sec. Both n type and p type wafers could be obtained after a low temperature heat treatment in a Hg atmosphere for about three weeks. Typical n type HgCdTe wafers had a carrier concentrations n 77<2×10 14 cm −3, mobility μ 77>1.2×10 5 cm 2/Vs and minority carrier life-time τ>2.0 μs (the best was 7.4 μs). For p type HgCdTe wafers, the carrier concentration was p 77<1.2×10 16 cm −3, mobility μ 77>500 cm 2/Vs. High performance HgCdTe IR devices have been prepared with these materials demonstrating the state-of-the-art quality of the large HgCdTe crystals.

Micro-machined pyroelectric thin film IR sensors

In this paper, we report our recent success in preparation and fabrication of Si-based pyroelectric thin film micro-IR sensors using micro-machining technology and systematical characterization results. Ferroelectric PT, PZT, PLZT and multilayered PZT/PT, PZT/PLZT thin films have been prepared on the Pt coated silicon water using a modified sol-gel method. These ferroelectric thin films have been patterned and micro-machined to make thin film infrared sensors and prototype pyroelectric IR devices on Si membrane have been obtained. The interesting self-biased phenomenon has been observed in PZT/PLZT multilayer IR sensors, and it is found that the polarization in ferroelectric thin films is strongly correlated to the pyroelectric property. Experimental results also show that multilayer approach is an important technique in improving the pyroelectric responses.

Photoconductivity and Electrical Properties of Pb 1 - x Sn x Te thin films

PbSnTe can be used in the fabrication of IR photoresistors and IR photodiodes. PbSnTe films are economic, if compared with bulk materials. Due to the importance of this material, \( Pb_{1 - x} Sn_x Te \) thin films were prepared with x=0.0, 0.1, 0.2, 0.3 and 0.4 by a flash evaporation technique. The interplanar spacing d, and consequently the reflecting planes (h k l) were identified, from the powder X-ray diffraction analysis. The as-deposited thin films of \( Pb_{1 - x} Sn_x Te \) were polycrystalline. As tin substitutes for lead in the prepared films, the interplanar spacing d decreases and the unit cell dimension a also decreases. Energy dispersive X-ray analysis (EDX) was carried out. From the composition of these films determined by EDX excess Te was detected for x=0.0 and 0.1, while a slight Te deficiency was observed for x=0.2, 0.3 and 0.4. Besides a slight Sn deficiency was found for films of x=0.1, 0.3 and 0.4. On the other hand a slight excess of Sn was present for x=0.2. The electrical properties, example, the electrical resistivity, the Hall coefficient and Hall mobility, were studied in the temperature range 100–450 K. The electrical transport properties of the investigated films are governed by scattering mechanisms, associated with the intergrain barrier height. A.c. photoconductivity measurements were carried out by using the frequency-dependence method. The carrier lifetime was determined for all the investigated films at temperatures of 100, 200, 300 and 400 K. This ranged from 12–20 ms at 100 K. The steady-state photoconductivity, photoconductivity, photosensitivity and the carrier lifetime were studied in the temperature range 100–450 K. The photosensitivity increases sharply at a temperature ∼100 K. Combining the results of the photoconductivity and the electrical measurements, it appears that x=0.1 is a suitable composition for further study to prepare IR device, since it has the best photosensitivity and good electrical properties.

Status of HgCdTe-MBE technology for producing dual-band infrared detectors

Progress on achieving reproducible growth of high performance, dual-band IR detector structures in HgCdTe grown by molecular beam epitaxy (MBE) is described. The reproducibility achieved in the MBE growth of n–p–n device structures comprising HgCdTe epitaxial layers with different composition and doping characteristics was evaluated from the run-to-run precision in the alloy composition, dopant concentration and dislocation density. For a series of 25 growth runs, the standard deviation of the alloy composition in the n-type absorbing layer was 0.002; the yield for the in situ n- and p-type doping process was >95%; and the average dislocation density was <5×10 5 cm −2. In situ optical diagnostics, including spectroscopic ellipsometry and an optical absorption flux monitor were used for the real-time determination of the alloy composition and Cd flux during MBE growth of the two-color device structures. Focal plane arrays with 128×128 elements were fabricated for the simultaneous detection of two sub-bands in the MWIR spectrum. Average R o A values exceeding 1×10 6 and 2×10 5 Ω cm 2 were measured at 77 K for diodes operating at 4.0 and 4.5 μm, respectively, and the quantum efficiency was greater than 70% in each band. These results on MBE growth and device performance demonstrate that HgCdTe MBE technology is poised for the modest-scale production of advanced IR devices.

Ex situ variable angle spectroscopic ellipsometry studies of electron cyclotron resonance etching of Hg 1−xCd xTe

The development of low damage anisotropic etching is important for the fabrication of mercury cadmium telluride (Hg 1− x Cd x Te) based IR devices. Typical methane-hydrogen chemistries have been shown to etch Hg 1− x Cd x Te causing changes in the surface composition and roughness. To investigate these changes, Hg 1− x Cd x Te samples with (111)A and (111)B orientations were systematically etched in hydrogen and methane mixtures, with 0%, 5%, 10%, and 20% methane in hydrogen using an electron cyclotron resonance plasma etching tool. Results were compared with reactive ion etched samples. Ellipsometry, atomic force microscopy (AFM) and scatterometry show that as etching proceeds there is considerable surface roughening. Quantitative measurements of roughness as measured by the three techniques reveal that etching causes vertical roughening between 2 nm and 20 nm. AFM measurements correlated well with scatterometry measurements. Ellipsometry measurements only weakly correlated with the other techniques owing to the sensitivity of ellipsometry to roughness features less than the wavelength of light as analyzed using effective medium theory. Ellipsometry results show that etching changes the surface alloy composition as well as roughness, and in several cases resulted in apparent vertical grading of composition. The addition of a few per cent methane in hydrogen reduced surface roughness.

MOVPE of narrow band gap II–VI materials

The high interdiffusion of mercury in Hg-based heterostructures is at the root of the development of numerous original MOVPE techniques which are reviewed bearing in mind the present requirements of IR devices. Doping, device performance and heteroepitaxy on GaAs substrates are then addressed as the most significant breakthroughs.

Monte Carlo simulations of internal photoemission in silicide infrared detectors

A Monte Carlo simulation of the internal photoemission process in silicides is presented. The results are compared with those of other authors and found to be in excellent agreement. The simulation has been extended to a 3-D geometry. A number of possible new configurations for IR devices are suggested which could vastly improve the quantum efficiency of these devices.

Position tunable volume reflection gratings for narrowband filtering applications (FWHM &lt; 5 nm) in optical fibre systems

We report a significant improvement in the performance of position-tuned volume reflection gratings in dichromated gelatin for fibre-based applications in the 1200–1600 nm range. Full widths at half-maximum of less than 5 nm have been obtained over the whole tuning range, with high peak efficiencies for DCG films of 410 μm thickness. These films are considerably thicker than have been reported elsewhere for IR or visible wavelength devices.

IV–VI compounds on fluoride/silicon heterostructures and IR devices

We present recent results on the growth of IV–VI lead-chalcogenide narrow gap semiconductors on silicon and subsequent IR device fabrication. Heteroepitaxy is achieved using intermediate stacked approximately 2000 Å thick epitaxial CaF2BaF2 buffer layers which allow us to overcome the large lattice mismatch (up to 20%) and, even more important, thermal expansion mismatch between lead chalcogenides and silicon. By growing different lead chalcogenides such as PbS, Pb1−xEuxSe, PbTe and Pb1−xSnxSe we have fabricated sensor arrays with cut-off wavelengths covering the whole thermal IR range from 3 microm up to above 12 microm.

MBE of Pb 1−xEu xSe for the use in IR devices

Pb 1−xEu xSe is investigated as an IR device material. This ternary material has a strong increase of the band gap energy with the europium content, while the lattice constant is expected to hardly change; this is favourable for the growth of heterostructures. Electrical characteristics of samples deposited on BaF 2 are described as well as the characteristics of lasers grown with this material.

Effects influencing the structural integrity of semiconductors and their alloys

The bond length and energy changes of the constituents of alloys relative to their pure crystal values are calculated from an extension of Harrison’s method. It is demonstrated that the already weak HgTe bonds are destabilized by adjacent CdTe, HgS, or HgSe, but are stabilized by ZnTe. It is also argued that dislocation energies and the hardness of semiconductors vary as a high inverse power of the bond length of the constituents. Hence, the shorter ZnTe bond as an additive should improve the structural properties of HgTe and CdTe. Experiments that support these predictions are noted. The electronic transport properties of 0.1 eV band gap HgZnTe are about the same as those of HgCdTe, and the structural properties of the Zn compound are superior; thus, we conclude that HgZnTe is likely to be the better material for IR devices.

Turbulent transport of carbon dioxide over a paddy field

Turbulent fluctuations in CO2 concentrations over a paddy field are measured by a fastresponse device with an open sensing path. This IR device coupled with a sonic anemometer constitutes an eddy correlation instrument to measure CO2 fluxes. Three experiments were conducted in the surface layer over paddy 90 cm high. The stability (z − d)/L ranged from -0.14 to 0.20, where L denotes the Monin-Obukhov length.

Pyroelectric response to infrared radiation signals at the F R(LT)-F R(HT) transition in lead titanate-zirconate

Pyroelectric voltage transients obtained from modified Pb(Ti0.07Zr0.93)O3 ceramics in response to step ir signals were studied between 20 and 100 °C, in which range the FR(LT)-FR(HT) transition occurs. The temperature dependences of peak voltage and total released charge exhibit a pronounced peak at about 85 °C with a small temperature hysteresis indicating that this transition is not purely of second order. The pyroelectric coefficient between room temperature and the transition temperature increases by about a factor 10. The dynamical behavior and the dielectric properties are discussed and their potential for the development of ir devices operating at or near this phase transition is outlined.