F. Pascal-Delannoy

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.

Quartz Crystal Microbalance (QCM) used as humidity sensor

This paper describes an application of Quartz Crystal Microbalance (QCM) used as humidity sensor. Moisture apparition is detected by using a QCM associated with a Peltier module. When water condensation produced by the Peltier cooling appears on the QCM, a change of mass on the crystal sensitive surface results in the decrease of the resonant frequency. If we measure the delay time between the beginning of Peltier supply and the apparition of water condensation on the quartz, we determine the relative humidity and the condensation velocity. A study of thermal transfer is first presented. Relative humidity measurements are then realised in a climatic chamber. A theoretical approach is finally compared with the experimental results.

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.

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.

Design of a micromachined thermal accelerometer: thermal simulation and experimental results

This paper describes numerical simulation of a micromachined thermal accelerometer and experimental measurements. The sensor principle consists of a heating resistor, which creates a symmetrical temperature profile, and two temperature detectors symmetrically placed on both sides of the heater. When an acceleration is applied, the free convection is modified, the temperature profile becomes asymmetric and the two detectors measure the differential temperature. This temperature profile and sensor sensitivity according to the distance heater-detector have been studied using numerical resolution of fluid dynamics equations with the commercial code CFD2000/STORM: it shows that the optimum distance between the temperature detectors and the heater is about 300 μm. A thermal accelerometer with 3 pairs of detectors placed at 100, 300 and 500 μm from the heater was manufactured using the techniques of micromachining silicon and experimental measurements have shown a good agreement with the numerical simulations: the experimental optimum distance between heater and detectors seems to be close to 400 μm and the differential temperature of detectors is about 3 °C/g for an operating heater power of 54 mW and an heater temperature rise AT of 238 °C. The electrical sensitivity is then 2.5 mV/g.

Growth and characterization of metal-organic vapour phase epitaxial Ga1−xInxAsySb1−y quaternary layers

Abstract The growth in the miscibility gap and the characterization of MOVPE Ga1−xInxAsySb1−y quaternary layers have been undertaken. The experimental conditions have been determined to obtain quaternary epilayers in the miscibility gap grown on GaSb with a good morphology even for lattice mismatched layers, in varying ΣPIII and DH2. The material quality has been assessed by single and double X-ray diffraction. The spectral responses of GaInAsSb(p)/GaSb(n) heterojunction are given. A spectral response obtained under illumination from the GaSb side, presents a good response at 2.75 μm.

InGaSb/GaSb photodiodes grown by MOVPE

Abstract We report a systematic investigation of the growth by MOVPE of a series of InGaSb photodiodes lattice-mismatched on GaSb substrates. The samples were grown in two atmospheric pressure MOVPE reactors and with different organometallic sources. We have investigated the use of various intermediate layers between the substrate and the homojunction photodiode such as compositional ramps, steps and superlattices in order to prevent dislocations due to the lattice mismatch from reaching the crucial p/n junction at the top of the device. The top layers have been evaluated by X ray diffraction, SIMS profiling and imaging, Hall and resistivity measurements. From these layers, photodiodes were fabricated with mesa wet etching without any antireflecting coating. Electrical and optical characteristics of these photodiodes — dark current, capacitance and external quantum efficiency — were investigated at room temperature. The dark current and diffusion length are compared with a p/n InGaSb homojunction grown without intermediate layer and with a p/n InGaAsSb homojunction lattice-matched on GaSb.

Optical and electrical characterization of thick GaSb buffer layers grown on 2 in GaAs wafers

Abstract We have investigated the growth of thick, highly uniform GaSb buffer layers on 2 in GaAs substrates. We have found that ramping the temperature under an arsine flux optimizes the switching sequence between GaAs and GaSb. In this case, the large (about 8%) lattice mismatch which separates the GaAs and GaSb bond lengths is abruptly relaxed, and good quality GaSb can be homogeneously deposited. On such samples, the photoluminescence signal ranks as well (or even better) as that obtained for comparative homoepitaxial material; also, the electrical properties are among the best ever reported for GaSb grown by metal-organic chemical vapor deposition on GaAs.