Z. Djuric

A consideration of the use of metamaterials for sensing applications: field fluctuations and ultimate performance

We analysed some peculiarities of electromagnetic metamaterials convenient for plasmon-based chemical sensing with enhanced sensitivity. Owing to the vastly extended range of possible values of refractive index, a higher degree of design freedom is enabled in comparison to conventional surface plasmon sensors. After considering some possible approaches to use metamaterials as either surface or bulk adsorbents, we analysed the ultimate performance of such sensing devices which are limited by refractive index fluctuations caused by adsorption and desorption processes. We utilized an approach based on a parallel between the adsorption–desorption mechanism and the carrier generation–recombination in semiconductors. As a special case, we considered structures for the detection of a single gas and used our approach to calculate the spectral density of refractive index fluctuations. We believe that our analysis may be of importance in different practical fields, including e.g. environmental sensing, homeland security and biosensing.

Adsorption–desorption noise in micromechanical resonant structures

Abstract In this paper we analyze adsorption–desorption (AD) noise in resonant microelectromechanical structures. This noise is generated by instantaneous differences in the rates of adsorption and desorption of contaminant molecules to and from the resonator surface, which cause mass fluctuations and consequentially frequency fluctuations. Contrary to the usual statistical approach, we used the analogy between AD processes in resonant structures and generation–recombination processes in semiconductors. Starting from that general analogy and using the theory of Langmuir’s isotherm, since it is the most convenient for this case, we derived an exact expression for the AD fluctuations-induced phase noise for the case of one adsorption layer. Our numerical results show that AD induced phase noise is comparable to other noise sources and becomes dominant for very small resonator beam dimensions. The presented theory is applicable to the determination of the limiting performance of various micromechanical resonators and microcantilever sensors and also for the theoretical studying of AD processes in general. In addition, phase noise measurements and their comparison with the theory are convenient for the investigation of catalytic AD processes.

New generation of near-room-temperature photodetectors

The major drawback of IR photodetectors is the need for cooling to suppress thermal generation of free carriers resulting in noise. New ways to improve the performance of infrared photodetectors operated without cryogenical cooling are discussed, including the optimum design of the devices, the use of optical immersion of photodetectors to high refraction index lenses, and the optical resonant cavity. Another and very promising way, however, is the suppression of thermal generation, which is governed by the Auger mechanism by depletion of the semiconductor in charge carriers. The stationary depletion can be achieved by the use of exclusion, extraction, and magnetoconcentration effects. The combination of various methods would eventually enable us to achieve near-background-limited photodetection (near-BLIP) performance of IR detectors without cooling.

Composition and thickness control of CdxHg1-xTe layers grown by open tube isothermal vapour phase epitaxy

Abstract Lowered Hg pressure due to the Hg loss out of the growth chamber strongly influences the properties of Cd x Hg 1- x Te layers grown by open tube isothermal vapour phase epitaxy (ISO VPE). Conventional LPE slider systems are usually too leaky to growth low x -value (0.3-0.2) with proper thickness ( ≈ 20 μm). We developed a new semiclosed ISO VPE system which practically eliminated Hg loss. Layers of any desired x -value with thickness from 1 up to 100 μm were grown. Methods of layer thickness and composition control are proposed.

Infrared photodetector with electromagnetic carrier depletion

A method is presented for thermal noise reduction in a near room-temperature intrinsic IR photodetector. The method is based on suppression of the Auger generation-recombination processes using the electro-magnetic carrier depletion (EMCD) of a narrow gap semiconductor. The device is a lightly doped narrow gap semiconductor flake with a high backside surface recombination velocity, supplied with electrical contact band and placed in a magnetic field. Due to the action of the Lorentz force most of the device depletes charge carriers, which results in suppression of the Auger generation and recombination processes. As a result, the I-V characteristic becomes nonlinear, exhibiting regions of high positive and negative resistance. Thermal noise can be dramatically reduced, leading to a substantial improvement in performance. The ultimate detection may be determined either by background radiation or by Shockley-Read generation, depending on the ratio of the background photon flux to the recombination center concentration. Near-BLIP performance is predicted for 10.6-μm (Hg, Cd) Te devices, prepared from high-quality materials and operated at 225-250 K.

Vacuum MEMS sensor based on thermopiles - simple model and experimental results

This paper presents a simple model of MEMS thermopile based vacuum detector and experimental results obtained for sensors designed and fabricated at IHTM-IMTM. Sensors contain two thermopiles, each with 30 multilayer p+Si/Al thermocouples and p+Si or Al heater. Thermal isolating membrane has sandwich structure consisting of sputtered SiO2 and residual n-Si layer. Dependence of output thermopile voltage on pressure was measured for sensors with different membrane thickness. Tests were performed in pressure range (10-3-105) Pa. Experimental results were compared with theoretical predictions.

Some theoretical and technological aspects of uncooled HgCdTe detectors: a review

Abstract This work analyses a relatively new type of Hg 1−x Cd x Te detector which is used for the detection of CO 2 laser radiation (10·6 μm) and which operates at room temperature. In the first part of the work basic parameters of this detector type are given (spectral responsivity, detectivity, time constant) as well as their dependence on starting material parameters (absorption coefficient, carrier concentration, lifetime). The second part describes the production technology of epitaxial Hg 1−x Cd x Te layers using the method of isothermal vapour phase epitaxy, as well as the methods of making the detector. Finally, in the third part of the work, experimental results are presented.

A complete quantitative model of the isothermal vapor phase epitaxy of (Hg,Cd) Te

A quantitative model of isothermal vapor phase epitaxy is proposed. It can be applied to both closed and open tube systems. This model enables the prediction of compositional profiles of the layers grown by isothermal vapor phase epitaxy with dependence on the growth parameters and thermodynamical data of the (Hg,Cd)Te system. The dependence of compositional profiles of the ISOVPE layers on temperature and time of deposition, source to substrate spacing, mercury and inert gas pressures are discussed for both solid and liquid sources. Modification of the compositional profiles by the postgrowth annealing has also been studied. The proper choice of growth and annealing parameters makes the optimization of the profiles possible. The calculated profiles are compared with the experimental data and a satisfactory quantitative fit is found in most cases. The possible reasons for remaining discrepancies are discussed.

Silicon UV flame detector utilizing photonic crystals

In this paper we propose a silicon UV flame detector for combustion systems. In gas burners the relative intensity of flame radiation is dominant in the UV region. In the visible and IR regions the relative intensity of radiation of the incandescent surfaces is several orders of magnitude greater than the gas flame radiation intensity. Therefore it is required that the flame detector has a much greater sensitivity in the UV region. The propose detector is formed on n-type silicon on isolator wafer. In order to suppress sensitivity in the visible and the IR regions, the absorption region of the detector is greatly reduced, and a UV filter utilizing photonic crystal is designed. The p-n junctions are formed by very shallow diffusion of impurities. The contacts are made after the deposition of a thin oxide layer. The UV filter is then sputtered on the detector surface. The filter consists of a thin silver film, and a 1D photonic crystal made of twelve pairs of NaF/Y2O3 layers. The photonic band gaps of the crystal should suppress the propagation of the light with wavelengths greater than 0.35 micrometers . For the detector active area of 5 mm2, the thickness of the silver layer of 0.13 micrometers and a dark current of 1 nA, the noise equivalent power at 0.32 micrometers is 4.23 10-13 W/Hz1/2. The calculated flame signal to total signal ratio is 0.52.

Experimental determination of silicon pressure sensor diaphragm deflection

Abstract A method is presented for the accurate determination of the deflection of diaphragms commonly used for miniature piezoresistive and capacitive pressure sensors. The method utilizes a well-known apparatus for thin-film thickness measurements (Talystep), an instrument for accurate pressure measurement and control (Mensor) and a sample holder for simultaneous pressure application and diaphragm deflection measurement. The deflection measurements for a stiffened and a square diaphragm are presented and compared to analytically calculated results from the literature. High-precision deflection measurements reveal the existence of build-in stresses.