In recent years, the awareness of the earth's environmental problems has increased. In order to reduce the amount of carbon dioxide exhaust, interest has been growing in thermoelectric conversion devices that exploit the Seebeck effect for providing an electrical generating system that uses unutilized waste energy. The principle of operation of a thermoelectric converting device is schematically shown in FIG. 1.
In the case of the thermoelectric converting device shown in FIG. 1, a p-type semiconductor and an n-type semiconductor are bonded by a metal, the bonded portions are heated, and the non-bonded portion is cooled. Thereby, the Seebeck effect (in which an electromotive force is generated that is equivalent to the energy difference in the Fermi levels at the ends of the sample when a temperature gradient has been applied) of a semiconductor is produced, and the system functions as a battery.
Such thermoelectric conversion device are in commercial use as auxiliary batteries for artificial satellites, and the effective use of this energy has an enormous potential because electrical power can be obtained, for example, from geothermal heat, the waste heat from factories, solar heat, the combustion heat of fossil fuels, and the like. However, although the development of materials that are stable even at high temperatures is being pursued, this development has yet to emerge from the laboratory level because intermetallic compound semiconductor materials that are formed from extremely toxic heavy metal elements are used.
When applying a temperature gradient to one or both of the ends of the semiconductor samples in the thermoelectric conversion material, according to the phenomenon in which a thermoelectromotive force that is proportional to the temperature gradient is generated (a phenomenon referred to as the Seebeck effect), the proportionality coefficient (the thermoelectromotive force per 1° C. temperature gradient) is called the Seebeck coefficient. The performance of a thermoelectric conversion material is generally evaluated by using the dimensionless figure of merit ZT. When the Seebeck coefficient, which indicates the electromotive force of the thermoelectric conversion material at an absolute temperature T, is denoted by S, the electrical conductivity is denoted by σ, and the thermal conductivity is denoted by κ, the performance of the thermoelectric conversion material is represented by the dimensionless figure of merit ZT=T(S2σ/κ). The characteristics as a thermoelectric conversion material become increasingly superior with an increasing value for ZT.
In a thermoelectric converting device, generally thermocouples are formed by p-type and n-type thermoelectric conversion materials that are bonded by a metal. These thermocouples are coupled and used in the form of modules in which thermocouples are connected serially in order to obtain the desired voltage. In consideration of the level of the conversion efficiency, the n-type and p-type thermoelectric conversion materials that are used in such thermoelectric converting devices frequently use Bi2Te3 intermetallic compound single crystals or polycrystals. Although it is known that Bi2Te3 exhibits the highest thermoelectric conversion performance (ZT=1) in a temperature range near room temperature, the conversion efficiency is low, and its use has only been commercialized in some wristwatches that utilize the difference between body temperature and the external air temperature.
Due to the fabrication of semiconductor superlattices in 1993 by a research group led by Dresselhaus et al. of the Massachusetts Institute of Technology in the US, a drastic improvement in the thermoelectric conversion performance was predicted theoretically (Non-patent Document 1), and partially demonstrated experimentally (Non-patent Document 2). According to the details of the theory, because the density of state is increased by confining the carriers in a quantum well (having a well width of approximately 1 nm), the square of the Seebeck coefficient increases in inverse proportion to the well width. Specifically, if the well width is equal to or greater than 10 nm, the Seebeck coefficient is substantially identical to that in bulk, and when the well width is 1 nm, the Seebeck coefficient is twice that in bulk, and at 0.1 nm, the Seebeck coefficient is approximately eight times that in bulk.    [Non-patent Document 1] L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B47, 12727 (1993).    [Non-patent Document 2] M. S. Dresselhaus et al., Proceedings of the 16th International Conference on Thermoelectrics, 12 (1997).
A quantum well is generally fabricated by molecular beam epitaxy (MBE) or chemical vapor deposition (CVD), which are semiconductor thin film growing processes, but because control of a quantum well width equal to or less than 1 nm is extremely difficult, the absolute value of the Seebeck coefficient that can be realized has only risen to approximately 2 times that in bulk.
Literature related to the present invention is disclosed in Patent Documents 1 to 4.
[Patent Document 1] Japanese Patent Application Publication No. JP-A-8-222775
[Patent Document 2] Japanese Patent Application Publication No. JP-A-2-231223
[Patent Document 3] Japanese Patent Application Publication No. JP-2003-257709
[Patent Document 4] Japanese Patent Application Publication No. JP-A-2004-363576