Frederique Pascal-Delannoy

MOCVD growth of Bi2Te3 layers using diethyltellurium as a precursor

Abstract The growth of Bi 2 Te 3 thin films by metal organic chemical vapour deposition (MOCVD) using diethytellurium and trimethylbismuth as tellurium and bismuth sources respectively is investigated on pyrex substrates. The results of growth rate, morphology, electrical and thermoelectrical properties as a function of growth parameters are given. The prepared films were always n -type. Film properties, such as electrical resistivity, mobility, carrier concentration, thermoelectric power and X-ray diffraction were studied at 300 K. For VI/V ratio greater than 6, we found an electrical resistivity lower than 9 μ Ω.m and a thermoelectrical power equal to 210 μ V/K. Hall mobility varies from 28 and 150 cm 2 /V.s. These initial results suggest a significant potential of MOCVD growth for large-scale production of thermoelectric material.

Preparation and characterization of MOCVD bismuth telluride thin films

Abstract The thermoelectric, electric and structural properties of Bi 2 Te 3 thin films grown by MOCVD have been investigated. The Seebeck coefficient shows that all the samples were n-type conductors decreasing from 213 to 129xa0μV/K when the carrier concentration increases from 9×10 19 to 3×10 20 xa0cm −3 . For high substrate temperature, good orientation of crystallites has been observed which can be directly related to the best values of Seebeck coefficient found. Hall effect has been studied in the temperature range from 110 to 450xa0K. The temperature dependence of the Hall mobility is found to be T −1 indicating lattice scattering. The good quality of Bi 2 Te 3 thin films growth by MOCVD observed allow to confirm the high potential of these deposition method which can be turned to be suitable for growing thin films for thermoelectrical material production.

Electrical and Thermoelectrical Properties of Sb2Te3 Prepared by the Metal-Organic Chemical Vapor Deposition Technique

The V2VI3 binary compounds such as Bi2Te3, Sb2Te3 and their alloys are narrow band±gap semiconductors with a high thermoelectric ®gure of merit Z x88 So=k, where S is the Seebeck coef®cient, o the electrical conductivity and k the thermal conductivity. These semiconductors have been extensively studied in recent years because of their promising applications especially for thermoelectrical devices [1, 2] and thermal [3] and humidity [4] sensors using the Seebeck and Peltier effects, respectively. Giani et al. [5] have grown Bi2Te3 on pyrex substrates by using the metal-organic chemical vapor deposition (MOCVD) technique in a horizontal quartz reactor. They have also studied in some detail the electrical and thermoelectrical properties of Bi2Te3. Venkatasubramanian et al. [6] have studied the MOCVD growth of Bi2Te3 and Sb2Te3 on a GaAs substrate and used their superlattice structures for thermoelectrical applications. Dauscher et al. [7] have elaborated Bi2Te3 thin ®lms by using pulsed laser deposition (PLD) and have shown that a congruent transfer of stoichiometry occurs from the target to the substrate over several cm and that a good crystallinity is achieved. Magri et al. [8] have investigated the properties of electrodeposited bismuth telluride ®lms and have shown that the ®lm composition depends on the electrolyte composition and the current density. Our studies were carried out in an attempt to make a detailed analysis of the behavior of Sb2Te3 thin ®lms regarding the effect of R x88 VI=V ratio on electrical and thermoelectrical properties. Sb2Te3 thin ®lms were grown using the MOCVD technique in a horizontal quartz reactor. Triethylantimony (TESb) and diethyltellerium (DETe) were used as antimony and tellurium sources, respectively. To avoid the possibility of any premature decomposition, TESb and DETe sources were both maintained at 20 8C. Hydrogen was used as the carrier gas with a ow rate equal to 3 slm to obtain good results. This is due to a better cracking ef®ciency for a ow rate of 3 slm found by Giani et al. [5]. The substrate temperature was ®xed at 450 8C during the deposition process and controlled by a thermocouple in direct contact with the substrate holder. The VI=V ratio (RVI=V x88 DETe partial pressure=TESb partial pressure) varied between 1 and 13. In addition, during the deposition of Sb2Te3, the partial pressure of the group V element (Sb) was kept constant and equal to 1 3 10 atm. A Philips X-ray diffractometer, using monochromatic CuKa radiation (e x88 1:54051 E A), was employed to obtain diffraction patterns from ®lms deposited on a Pyrex substrate. A wide range of e (from 58 to 308) was scanned so that all possible diffraction peaks could be detected. Surface morphology was examined by scanning electron microscopy (SEM). The composition of the deposited layers was measured using an energy dispersive X-ray (EDX) microanalyzer. To measure Seebeck coef®cients, heat was applied to the sample, which was placed between two small perfectly parallel brass cylinders. The temperature difference between these two cylinders was measured using thermocouples and a sensitive Keithley digital thermometer. The potential difference was obtained at the position of the two thermocouples using a sensitive digital multimeter. The Van Der Pauw technique was used at 300 K to evaluate the sample resistivity, its carriers mobility and its carriers concentration. The X-ray diffraction (XRD) pattern was compared with ASTM charts and showed that the deposited layers grew in (0 0 0 l)H and exhibited a polycrystalline phase characterized by the (1 0 1 5)H peak (Fig. 1). The same peak was observed by Mandouh [9] on vacuum-deposited Sb2Te3 thin ®lms and disappeared upon annealing at 473 K. The surface morphology and crystallinity of the deposited thin ®lms on the amorphous substrate were found to depend strongly on the VI=V ratio, and its aspect seemed quite different. The SEM micrograph shown in Fig. 2 is of one of the Sb2Te3 layers deposited at 450 8C with the VI=V ratio x88 7. It was observed that the shape of the crystallites was hexagonal but not symmetrical, which is in good agreement with the X-ray data and the fact that the crystallites are randomly oriented with respect to the amorphous substrate. EDX analysis showed that the binary compound was always stoichiometric. Opposite results were obtained on Bridgman Sb2Te3 [10], where progressive loss of Te occurred and where the sample composition contained excess Sb atoms,

Effect of antimony concentration on the electrical and thermoelectrical properties of (Bi1−xSbx)2Te3 thin films grown by metal organic chemical vapour deposition (MOCVD) technique

Abstract The electrical and thermoelectrical performances of p-type (Bi1−xSbx)2Te3 elaborated by metal organic chemical vapour deposition (MOCVD) in horizontal quartz reactor on pyrex substrate are discussed. The quality of the deposition layers is controlled by X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Hall effect measurements are used to determine resistivity and holes mobility. It is found that the electrical properties of thin films depend strongly on growth parameters and exhibit a polycrystalline structure. The measurement of Seebeck coefficient (S=235xa0μV/K) and the resistivity (ρ=15xa0μΩxa0m) leads us to confirm the significant potential of the MOCVD method to produce a good material promising for thermoelectric application.

Investigation of Pt/Ti bilayer on SiNx/Si substrates for thermal sensor applications

Pt/Ti thin films on SiNx/Si substrates have been investigated for thermal sensor applications on SiNx membrane. Therefore, Pt/Ti adhesion during KOH etching of silicon and high temperature coefficient of resistance (TCR) are the principal aims of this study. ac sputtering and electron beam evaporation have been investigated for metal deposition. Vacuum annealing is used to improve the Pt/Ti characteristics. Stress characterizations and adhesion strength are evaluated by an x-ray diffraction pattern and adhesive tape test, respectively. TCR and resistivity were finally measured to confirm the compatibility with good thermal sensor sensitivity. Pt/Ti films elaborated by electron beam evaporation and vacuum annealed present the best characteristics for thermal sensor applications: good adhesion is obtained even after 5 h in KOH etching solution, electrical resistivity is about 15 μΩu200acm and TCR is 3.3×10−3/°C. Finally, by using these platinum thin films, a thermal accelerometer has been manufactured and tilt ...

Growth of (Bil−xSbx)2Te3 thin films by metal-organic chemical vapour deposition

Abstract The effects of VI/V ratio on electrical and thermoelectrical properties of p-type (Bi 1− x Sb x ) 2 Te 3 elaborated by metal-organic chemical vapour deposition (MOCVD) in horizontal quartz reactor are discussed. The deposited layers exhibit a polycrystalline structure and an improvement in the intensity is observed with increasing the VI/V ratio. The quality of the layers is measured by means of the Energy dispersive X-ray (EDX) microanalyser and scanning electron microscopy (SEM). It is observed that the layers are stoichiometric when the VI/V ratio exceeds 3 and the surface texture is improved with increasing this ratio. The electrical properties of the thin films dependent on the VI/V ratio. The measurements of the Seebeck coefficient suggest a significant potential of MOCVD growth for large-scale production of thermoelectric materials.

Fast humidity sensor using optoelectronic detection on pulsed Peltier device

Abstract In order to develop a new way to use a Peltier device in a fast humidity sensor, we have studied the optical signal of a photodetector when water condensation appears on the cold side of a commercial Peltier module. The thermoelectric device is used at pulsed rate. During the cooling the applied current is stopped as soon as a variation of the optical response, due to the appearance of water droplets, is observed. A reverse pulse is then applied to return quickly to the ambient temperature. We have measured the response time of a small commercial Peltier module. The experimental thermal time constant τth measured during the cooling using a thermocouple is typically τth = 3.6 s. A calculated estimation of the reverse pulse time is given in order to optimize the return to the equilibrium room temperature for low relative humidity. The last part of this work is devoted to measurements of the relative humidity HR. The delay time TC of the optical detection is plotted as a function of humidity level. TC is function of the Peltier current supply Ipeltier: with Ipeltier equal to 700 mA, we have observed a delay time TC in the range 0.25 s ≤ TC ≤ 12.2 s for 15% ≤ HR ≤ 70%.

Fabrication of Thermoelectric Sensor and Cooling Devices Based on Elaborated Bismuth-Telluride Alloy Thin Films

The principal motivation of this work is the development and realization of smart cooling and sensors devices based on the elaborated and characterized semiconducting thermoelectric thin film materials. For the first time, the details design of our sensor and the principal results are published. nFabrication and characterization of Bi/Sb/Te (BST) semiconducting thin films have been successfully investigated. The best values of Seebeck coefficient (α(T)) at room temperature for Bi2Te3, and (Bi1−xSbx)2Te3 with x = 0.77 are found to be −220u2009µV/K and