Temperature-dependent electrical conductivity in thermally carbonized porous silicon
Introduction
During the first years after the report of a strong visible photoluminescence [1], the main attraction of PSi based on hopes to use PSi as a material for optoelectronic applications. Soon the research spread also to new interesting fields of applications. Now, a great deal of current interest in the PSi research is focused on other applications than optoelectronic. A growing number of new potential applications of PSi, in which the photoluminescence is not required, has been reported. Such applications include the use of PSi in gas sensors [2], [3], biomaterial and drug-delivering applications [4], [5], and sensing in liquid solutions [6], for example. However, the major part of the applications suffer from a lack of stability. For that reason, novel stabilization treatments are increasingly needed for different kinds of applications .
Thermal oxidation and annealing at high temperatures were the only useful stabilizing treatments for PSi for years. During the last years, several other stabilizing treatments have been introduced. The stabilizing studies have been focused mainly on the substitution of an initial hydrogen termination with a more stable SiC bonds, for example, with hydrosilylation of alkenes and alkynes. This can be done on the PSi surface in several ways [7], [8], [9]. On the hydrosilylation treatments, one of the most interesting is a thermal hydrosilylation [9], since it is not depending on the sample thickness or on the type or resistivity of the initial material, and it is simple to do.
A thermal carbonization of PSi by acetylene also produces a stable and chemically inert SiC surface [10], [11]. Recently it has been reported that instead of the SiC surface, a hydrocarbon termination with a good treatment efficiency can be achieved with a lower treatment temperature [12]. Since the specific surface area almost remains the same after the carbonization, and coalescence of pores or any coarsening were not observed and, in addition, because the thermally carbonized PSi only slightly oxidized even in the accelerated test conditions [13], it was found to be a potential candidate for humidity and gas sensor applications. In our previous paper, we constructed a first humidity sensor based on the thermally carbonized PSi [14]. The sensor showed a good sensitivity and repeatability with a moderate response time.
Although, the temperature dependency of the humidity or gas sensor could be a disadvantage in some cases, we believe that the possibility to integrate a humidity and temperature sensor on the same silicon-based chip could be seen as an advantage. In addition, studies to avoid the use of metallic oxides mixtures in negative temperature coefficient thermistors have led to the development of fabrication processes for wide bandgap semiconductors, such as SiC and diamond, to employ these materials for temperature sensor applications. Encouraging results have been reported, for example, highly sensitive thermistors based on polycrystalline cubic SiC [15].
In this work, we will describe the first results on the temperature dependence of a DC conductivity in thermally carbonized PSi. The samples were treated at different temperatures and the thickness of the samples were varied. The influence of these parameters on the conductivity has been discussed.
Section snippets
Experimental
The PS samples were prepared by anodizing the Si(1 0 0) wafers in an HF (40%)–ethanol mixture (HF:EtOH 1:1); the process was performed in dark to minimize the room temperature oxidation [16]. The current density was 50 mA/cm2 and a porosity of about 65% (gravimetrically determined). The etching times were varied from 30 to 210 s corresponding to the film thickness from 2.8 to 9.1 μm. The resistivity of the p+-type was 0.015–0.025 Ω cm. Before the carbonization treatment, the samples were dried in
Results and discussion
Due to the carbonization, the resistance of the porous layer decreases remarkable. In the sample treated at 850 °C the resistance is 10−5 times lower than in the as-anodized sample with the same thickness. The resistance is also dependent on the treatment temperature (Fig. 2); as the treatment temperature increases, the resistance decreases. The decrease of the resistance was an expected observation because of previous reports.
The previous reports concerning the interaction of carbon with the
Conclusions
We have studied the temperature dependence of the electrical resistance in the thermally carbonized PSi. The results show that the temperature dependence is dependent on the treatment temperature and on the thickness of the porous layer. The resistance–temperature characteristic of the thermally carbonized PSi was found to be described by a thermistor equation. The temperature sensitivity 5%/K at 25 °C is good compared with the previously reported values. That would open new prospects for future
Acknowledgements
This work is financially supported by Academy of Finland.
Jarno Salonen was born in 1967. He received his MSc degree in 1995 and PhD degree in 1999 from the University of Turku. His research interests include chemical treatments and stabilization of porous silicon and its sensor applications. He currently works as a Academy Research Fellow at the University of Turku.
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Jarno Salonen was born in 1967. He received his MSc degree in 1995 and PhD degree in 1999 from the University of Turku. His research interests include chemical treatments and stabilization of porous silicon and its sensor applications. He currently works as a Academy Research Fellow at the University of Turku.
Mikko Björkqvist was born in Turku in 1972 and received his MSc degree in physics 1999 and PhD degree in 2003 from the University of Turku. He is presently working on characterization and stabilization of porous silicon sensor structures. He is now working as a post-doctoral researcher at the University of Turku.
Jaani Paski was born in Kokkola in 1973 and received his MSc degree in physics 2003 from the University of Turku. He is presently working with porous silicon sensors and with porous silicon drug delivering vehicle applications. He is now a graduate student at the Graduate School of Materials Research.