A review of miniaturised humidity sensors is presented. Recent achievements in capacitive-, hygrometric-, gravimetric-, optical- and integrated sensors are discussed. Attention is paid to a general perspective of miniaturised humidity sensors, emphasising on integration issues and technological challenges. A table summarising most of the reviewed devices is included.

Recent achievements in miniaturised humidity sensors—a review of transduction techniques

Revealing travel patterns and city structure with taxi trip data

https://iopscience.iop.org/article/10.1088/2053-1583/aaf836/pdf

A roadmap for electronic grade 2D materials

Arrays of bismuth telluride (Bi2Te3) nanowires with diameters of ∼25, ∼50, and ∼75 nm have been produced by electrochemical deposition into porous anodic alumina templates. Scanning electron microscopy confirms that the nanowire arrays are dense with a narrow distribution of nanowire diameters. The structure of the nanowires was assessed immediately after deposition, after annealing to ∼80% of the melting point, and after melting/recrystallization. As determined by XRD analysis, there is strong fiber texture in the arrays that depends on both the nanowire diameter and the postdeposition processing conditions. Bright-field/dark-field imaging and diffraction in the transmission electron microscope reveal that the as-deposited nanowires are polycrystalline with a bamboo-type grain structure that does not change significantly upon annealing, and a similar grain structure is obtained after melting and resolidification.

Structure of Bismuth Telluride Nanowire Arrays Fabricated by Electrodeposition into Porous Anodic Alumina Templates

Bismuth–telluride-based alloy thin film thermoelectric generators are fabricated by a flash evaporation method. We prepare Bi 0.4 Te 3.0 Sb 1.6 (p-type) and Bi 2.0 Te 2.7 Se 0.3 (n-type) powders for the fabrication of the flash evaporated thin films. The overall size of the thin film thermoelectric generators, which consist of seven pairs of legs connected by aluminum electrodes, is 20 mm by 15 mm. Each leg is 15 mm long, 1 mm wide and 1 μm thick. We measure the output voltage and estimate the maximum output power near room temperature as a function of the temperature difference between hot and cold junctions of the thin film thermoelectric generators. In order to improve the performance of the generators, a hydrogen annealing process is carried out at several temperatures from 25 °C to 250 °C. The highest output voltage of 83.3 mV and estimated output power of 0.21 μW are obtained from a hydrogen annealing temperature of T a  = 250 °C and a temperature difference of Δ T  = 30 K. The hydrogen annealing temperature of T a  = 250 °C also results in the best electrical performance for both p-type thin film (Seebeck coefficient = 254.4 μV/K, resistivity = 4.1 mΩ cm, power factor = 15.9 μW/cm K 2 ) and n-type thin film (−179.3 μV/K, 1.5 mΩ cm, 21.5 μW/cm K 2 ).

Fabrication and characterization of bismuth–telluride-based alloy thin film thermoelectric generators by flash evaporation method

Preparation and characterization of MOCVD bismuth telluride thin films

MOCVD growth of Bi2Te3 layers using diethyltellurium as a precursor

The V2VI3 binary compounds such as Bi2Te3, Sb2Te3 and their alloys are narrow band±gap semiconductors with a high thermoelectric ®gure of merit Z  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  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  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  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 carrier's mobility and its carrier's 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  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,

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

Growth of Ga1−xAlxSb and Ga1−xInxSb by organometallic chemical vapor deposition

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

A. Foucaran

Papers

Preparation and characterization of MOCVD bismuth telluride thin films

MOCVD growth of Bi2Te3 layers using diethyltellurium as a precursor

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

Growth of Ga1−xAlxSb and Ga1−xInxSb by organometallic chemical vapor deposition

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