A. Giani

Anemometer with hot platinum thin film

The techniques of micromachining silicon are used for the manufacture of an anemometer with low electric consumption and great sensitivity. To reduce the energy consumption, a suspended membrane of silicon rich silicon nitride SiNx makes it possible to carry out the heat insulation between the heater and the substrate. Platinum (Pt) thin film (3000 A) with titanium (300 A) adhesion layer on SiNx/Si substrate is used for the hot resistor. Among the methods of Pt deposition tested, electron beam evaporation gives the best results for the temperature coefficient of resistance (TCR) of Pt. Its response time is about 6 ms. Sensitivity in laminar and turbulent flow range are respectively 4.80 mV/(m/s)0.45/mW and of 0.705 mV/(m/s)0.8/mW for about 20 mW power supplied. The experiments show that the temperature rise of the sensor is not sensitive to the ambient temperature. Moreover, sensor response shows no significant changes according to parallel or perpendicular orientation of the gas flow.

Growth of Bi2Te3 and Sb2Te3 thin films by MOCVD

Abstract Metal organic chemical vapor deposition (MOCVD) has been investigated for elaboration of Bi 2 Te 3 and Sb 2 Te 3 using TMBi (Trimethylbismuth), TESb (Triethylantimony) and DETe (Diethyltellurium) as metal–organic sources. Their thermoelectric and physical properties were studied versus growth conditions. The MOCVD elaboration of Bi 2 Te 3 and Sb 2 Te 3 was carried out in an horizontal reactor for a temperature varying from 400 to 500°C, a total hydrogen flow rate D T varying from 3 to 6 l mm −1 and ( R VI/V ) ratio ranging from 1.5 to 15. The thin films were deposited on pyrex and silicon substrates. The partial pressure of the V element varied between 0.5 10 −4 to 2 10 −4 atm to obtain high growth rate for micro-peltier applications. The cristallinity was investigated by X-ray diffraction and we observed a typical preferential c -orientation. The SEM micrographs show the layers quality and confirms the hexagonal structure. The microprobe data indicate that the stoichiometry of Bi 2 Te 3 and Sb 2 Te 3 is constant for all thickness of the epitaxial films (0.3–7 μm). The films are always n-type conduction for Bi 2 Te 3 and p-type for Sb 2 Te 3 . Seebeck coefficient and the minimum values of the resistivity were found close to −210 and +110 μV K −1 , 9 and 3.5 μΩ.m for Bi 2 Te 3 and Sb 2 Te 3 , respectively. Electrical measurements (mobility and carrier density) were performed by Van der Pauw method. For the two materials, the best values of thermoelectrical properties were obtained at a growth temperature closed to 450°C and a VI/V ratio varying from 2 to 8. The thermoelectric properties of the two materials stay constant when the growth rate is increasing to value higher than 1.5 μm h −1 . This result is very interesting for thick film applications. The previous objective of these experimental results has been to perform the thermoelectric properties of n- and p-type films by establishing first suitable deposition conditions and the elaboration of ternary alloys is now possible.

Porous silicon layers used for gas sensor applications

Abstract In this communication we report on the elaboration of porous silicon layers for gas sensor applications. We describe our test system for gas sensors, and we investigate the electric characteristics of porous silicon layers (p type) under different gases and levels of humidity.

Influence of substrate temperature and target composition on the properties of yttria-stabilized zirconia thin films grown by r.f. reactive magnetron sputtering

Abstract The effects of substrate temperature and composition target on the optical and structural properties of ZrO 2 –Y 2 O 3 films have been investigated. The refractive index, extinction coefficient, packing density, inhomogeneity and structure are reported for substrate temperature range 150–400°C. It was found that pure zirconia layers grew preferentially towards the monoclinic (111) direction. The intensity of this monoclinic peak increases with increasing substrate temperature. The admixture of amounts of yttria to the zirconia matrix results in films having a single crystalline cubic phase. In the stabilized zirconia, the presence of molecules of the Y 2 O 3 dopant reduces the effect of substrate temperature on the film properties with regard to the pure ZrO 2 material. The maximum refractive index ( n =2.19 at λ =750 nm), nearly unity packing density and better homogeneity have been obtained from the sample containing 8 wt.% Y 2 O 3 for substrate temperature of 400°C. The extinction coefficient in the order of 10 −3 is reported in the visible spectrum, which makes the films useful for some optical applications.

Porous silicon layer coupled with thermoelectric cooler: a humidity sensor

Abstract In this work, an original humidity sensor is described. It is based on the study of the capacitance variation of a porous silicon layer (PSL) during water condensation induced by a commercial small-size thermoelectric cooler (TEC). The measurement principle is to detect the weak increase of capacitance created when water condensation occurs in a PSL stuck on a TEC. This important variation of capacitance is related to the high difference between the dielectric constant of PS ( e r e r ≅80). The dielectric constant of PS ranges from these of silicon oxide ( e r =3.9) to these of silicon ( e r =12) [H. Mathieu, Physique des semiconducteurs et des composants electroniques, Masson, 1987, p. 36]. Experimental measurements are performed in a climatic chamber for several values of relative humidity from 10% to 95% and for a TEC current equal to 0.43 A for the cooling part of the process. The analysis of the PS capacitance leads to information over the condensation formation during the TEC cooling. A quick increase of the capacitance appears after a delay time, τ , of 0.5–2 s from the start of the TEC cooling. The higher the humidity level, the faster the capacitance increase. It is possible to draw the capacitance reached after 1 s, from the start of the TEC cooling as a function of the relative humidity level.

Elaboration of Bi2Te3 by metal organic chemical vapor deposition

Abstract Bi2Te3 layers were elaborated for the first time using metal organic chemical vapor deposition. The films composition is stoichiometric when the following conditions are verified: substrate temperature lower than 500 °C, VI/V ratio greater than 3, TMBi partial pressure lower than 2 × 10−4 atm. By X-ray diffraction and MEB observation, we noticed the polycrystalline structure of the layers. The high thermoelectric power (+ 190 V K−1 for the n-type layer and −94 V K−1 for the n-type layer) of this material is promising for device applications.

Elaboration and characterization of InAsSb grown on GaSb and GaAs substrates

We study the growth of InAs0.91Sb0.09 lattice matched on GaSb presenting a bandgap of 0.29 eV (4.2 μm) at 300 K. The layers are grown on (100) GaSb and GaAs substrates at atmospheric pressure in a vertical reactor MOVPE (metal organic vapor phase epitaxy) system using trimethylindium (TMIn), trimethylantimony (TMSb) and arsine (AsH3). We report the results on growth rate and InSb incorporation versus growth parameters as temperature and TMSb partial pressure for different VIII ratios. Carrier density and mobility of InAsSb undoped layers are given. Optical photoconductivity measurements are presented with a wavelength cut off up to 5.4 μm at 300 K, and the absorption by atmospheric CO2 at 4.25 μm is clearly detected. The I-V curves of (n)InAsSb/(n and p)GaSb heterojunctions are presented. At room temperature, the dark current for the n/n heterostructure is 70 μA at −1 V, while the n/p junction is ohmic.

Electrical characteristics of GaIn(As)Sb layers grown by metal organic vapor phase epitaxy

Abstract We report the electrical characteristics of GaInSb and GaInAsSb layers grown by metal organic vapor phase epitaxy (MOVPE) on GaSb and GaAs substrates versus the growth parameters as growth temperature and V/III ratio. The effect of initial growth process including GaSb or stepped buffers is also described. On bulk Ga 0.6 In 0.4 Sb layer grown with a stepped buffer layer, the best carrier density and mobility are 1.4 × 10 16 cm −3 and 454 cm 2 V −1 s −1 , respectively. But the lower value (6.4 × 10 15 cm −3 ) than the previous one was obtained on a p/n structure containing a stepped buffer layer and a growth interruption between p- and n-layers. For the quaternary layer, the best results were obtained for samples grown outside the miscibility gap and close to the lattice matching.

Photodetection at 3.65 in the atmospheric window using heteroepitaxy

The photoconductive properties of layers deposited by organo metallic chemical vapour deposition (OMCVD) on GaAs substrates have been investigated through the characterization of a test photoconductor device. The spectral responses at 300 K and 77 K show that this material allows detection in the best region of transparency of the atmospheric window, around . The large increase of the response with decreasing temperature is attributed to the variation of the recombination time. Noise measurements between Hz and Hz indicate the presence of two types of fluctuations: the low-frequency one is inversely proportional to the frequency f and is partly due to non-optimized contacts; the higher-frequency one is due to a generation - recombination process. At , an estimated limit to detection of is deduced at 80 K in the ideal case of pure thermal noise.

A NEW THERMAL ACCELEROMETER BASED ON PYROELECTRIC MATERIALS

ABSTRACT The characteristics of a new pyroelectric sensor that measures acceleration are presented. This accelerometer is based on the heat convective fluxes modified by an applied acceleration. In order to present the performances of this new device, firstly, numerical simulations will be shown using computational fluid dynamic software. Secondly, experimental results will be presented for different measurement mode.