Capacitive humidity-sensitivity of carbonized silicon nanoporous pillar array

doi:10.1016/j.matlet.2010.03.005

Materials Letters

Volume 64, Issue 11, 15 June 2010, Pages 1268-1270

https://doi.org/10.1016/j.matlet.2010.03.005 Get rights and content

Abstract

The synthesis and application of silicon carbide (SiC) electronic devices have been studied broadly. In this letter, we report that by a thermal treatment process, the surface of silicon nanoporous pillar array (Si-NPA) could be carbonized in a graphite vacuum furnace. The composition and the morphology of the carbonized Si-NPA (SiC/Si-NPA) were characterized by X-ray diffractometer and scanning electron microscope. The humidity-sensing properties of SiC/Si-NPA were measured at room temperature. The result shows that with the relative humidity (RH) being changed from 11 to 95%, the capacitance of the sensor increased from 7.65 to 145.94 nF at the testing frequency of 20 Hz, and a capacitance increment over 1900% was achieved. The response and recovery times were determined to be ∼ 10 s by carrying out vapor adsorption and desorption dynamic cycle experiment. The sensing performances are attributed to the unique surface structure, morphology and chemical characteristics of SiC/Si-NPA.

Introduction

Silicon carbide (SiC) has been applied in the fields as diverse as wafer growth, material processing, electronic devices and increasingly, sensors [1], [2], [3], [4]. Crystal SiC possesses a wide band-gap of ∼ 3.23 eV, which leads to its carriers being difficult to be thermally activated and the quantity of the excited electron-hole pairs being greatly reduced. Such an electronic structure makes SiC being a suitable material for fabricating electronic devices which could be operated at a high temperature or in harsh environments [3], [4]. Traditionally, SiC sensors are usually applied to detect the exhaust gases, which mainly contain hydrocarbons, CO and NO_x etc., and the devices with high sensitivity, rapid response rate and high thermal stability have been reported [5], [6]. SiC sensors are often constructed as field effect transistors (FET) or Schottky diodes, in which a thin layer of metal film is deposited on SiC as catalyst [5], [6], [7], [8]. Although the adsorption process of hydrogenous gas on SiC would surely cause the variation of its electrical parameters, SiC sensors directly based on the change of its capacitance or resistance were seldom reported. In the previous study, we reported the preparation and characterization of silicon nanoporous pillar array (Si-NPA), a unique silicon hierarchical structure prepared by a hydrothermal method [9], [10]. Based on Si-NPA, the capacitive humidity sensors with high sensitivity, short response and recovery times were obtained, and the good performance was attributed to the formation of the regularly arrayed and highly nanoporous silicon pillars on its surface [11], [12], [13]. Nevertheless, the surface nanoporous structure of Si-NPA decides that it must be highly reactive and easily oxidized even in ambient air at room temperature, and this might bring problems to the long-term stability of Si-NPA sensors.

In this letter, we will try to integrate the advantages of SiC as a sensing material and Si-NPA as a functional substrate to probe the capacitive humidity-sensing properties of carbonized Si-NPA (SiC/Si-NPA), which would be obtained by forming a thin layer of SiC on Si-NPA through a high temperature carbonization process. The surface morphology and elemental composition of SiC/Si-NPA were characterized and the room-temperature humidity-sensing properties were measured. Based on the experimental results, the sensing mechanism of SiC/Si-NPA was analyzed.

Section snippets

Experimental details

The preparation and the structural characterization of Si-NPA have been described previously [9], [10]. Similar to other groups [14], [15], [16], the carbonization of Si-NPA was realized by treating the sample in a graphite vacuum furnace at high temperature. Si-NPA wafers specified by 2.0 cm × 2.0 cm were placed in the furnace and then the furnace was pumped until its inner pressure was below 5 Pa. Heat up the furnace at a rate of 20 °C/min until the inner temperature reaches 1000 °C and then

Results and discussion

The XRD diffractogram of SiC/Si-NPA is shown in Fig. 1, in which four reflections could be distinguished. The reflexes locating at 35.5, 60.3 and 71.8° are indexed to the diffractions from the (111), (220) and (311) crystal planes of cubic SiC, respectively. The reflex locating at 73.1° is indexed to the diffraction originating from the (203) crystal plane of SiO₂, which might be formed natively on the surface of Si-NPA during its preparation process [10], [17].

The surface morphology of

Conclusion

A SiC/Si-NPA humidity sensor was fabricated by growing a cubic SiC film on Si-NPA through thermal treatment of Si-NPA disposed in a graphite vacuum furnace. The capacitive sensing properties of the sensor were measured at room temperature. The capacitance–RH curves showed the high sensitivity and the response and recovery times showed the quick response rate of the sensor. The sensing performances given by SiC/Si-NPA should be attributed to the unique surface structure, morphology and chemical

Acknowledgement

This work was supported by the Science and Technology Project on Key Problems of Henan Province (082101510007) and the National Natural Science Foundation of China (10574112, 50602040).

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Capacitive humidity-sensitivity of carbonized silicon nanoporous pillar array

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusion

Acknowledgement

Aerosp Sci Technol

Sensor Actuat B

Sensor Actuat B

Sensor Actuat B

Solid State Electron

Sensor Actuat B

Sensor Actuat B

Sensor Actuat B

Vacuum