Conductive isostructural compounds

Details Conductive isostructural compounds Conductive isostructural compounds

1999-10-11

2001-11-06

Koslow, C. Melissa (Department: 1755)

Compositions

Barrier layer device compositions

Group iii element containing binary compound; e.g., ga, as

C252S06230V, C252S519400, C136S238000, C136S239000, C136S240000, C136S241000, C136S259000, C136S261000, C136S264000, C250S338400, C250S370140, C257S065000, C257S431000, C257S467000, C257S613000, C257S614000, C257S615000

Reexamination Certificate

active

06312617

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to new compounds exhibiting crystal lattice morphologies and, more particularly, to conductive compounds having NaCl-type cubic crystal lattice structures.
BACKGROUND OF THE INVENTION
Semiconductor materials are at the core of current technological infrastructure and continuing advancements. Various semiconductors enable many technologies. For example, Si and Ge enable high speed computing, GaAs, InSb and their derivatives enable optoelectronics and communication devices, Si and GaAs are the vital components of solar energy converters, GaN and GaAs alloys enable solid state lasers, Bi
2
Te
3
alloys enable thermoelectric cooling, and PbS, PbSe and HgCdTe are used in medium and long wavelength radiation detection.
Semiconductor materials used in solid state lasers, photovoltaic cells, optoelectronic devices and radiation detection, for example, must not only be excellent electronic conductors, but also have the appropriate energy band configuration or “band gap” for those applications. As more than one material is used in layers, the difference in the band gap between the various semiconductor materials used is also critical. For example, photovoltaic cells can use two semiconductor materials to produce a rectifying heterojunction. The advantages of utilizing this design include the ability to choose materials with properties appropriate for each component of the device and the reduced necessity for compromise with the property requirements of other components of the device. An example of this is the use of a wide band gap “window” semiconductor material as a barrier layer on a more narrow band gap “absorber” semiconductor material. The amount of radiation absorbed and therefore the electrical current generated in the device, increases with the decreasing band gap width, while the diffusion potential obtainable within the device, and therefore the electrical voltage generated in the device, increases with band gap width. Thus, the absorber material is chosen to maximize the solar radiation absorbed and affords a reasonable diffusion potential, while window material is chosen to absorb a minimum amount of solar radiation. Therefore, the closer the actual band gap to the desired, theoretical band gap of the semiconductor materials, the more efficient the photovoltaic cell. Many current semiconductor materials are limited in that they do not allow for fine adjustment of the band gap. Furthermore, some semiconductor materials contain volatile elements causing changes in the composition of the materials and consequently unwanted changes in the band gap.
In thermoelectric devices, it is also critical to have semiconductor materials that have specific properties. Such devices may be used for heating, cooling, temperature stabilization, power generation and temperature sensing. Modern thermoelectric coolers typically include an array of thermocouples.
Thermoelectric devices are essentially heat pumps and power generators which follow the laws of thermodynamics in the same manner as mechanical heat pumps, refrigerators, or any other apparatus used to transfer heat energy. The efficiency of a thermoelectric device is generally limited to its associated Carnot cycle efficiency reduced by a factor which is dependent upon the thermoelectric figure of merit (zT) of the materials used in fabrication of the thermoelectric device. The figure of merit represents the coupling between electrical and thermal effects in a material. The basic thermoelectric effects are the Seebeck and Peltier effects. The Seebeck effect is the phenomenon underlying the conversion of heat energy into electrical power and is used in thermoelectric power generation. The complementary effect, the Peltier effect, is the phenomenon used in thermoelectric refrigeration and is related to heat absorption accompanying the passage of current through the junction of two dissimilar materials.
While thermoelectric materials such as alloys of Bi
2
Te
3
, PbTe and BiSb were developed thirty to forty years ago, the efficiency of such thermoelectric devices remains relatively low at approximately five to eight percent energy conversion efficiency.
Therefore it would be desirable to have semiconductor materials that are not only good conductors but have a range of band gaps to fit a wide number of applications. It would be further desirable to have materials in which the band gaps could be adjusted to give the desired band gap for the appropriate application. These materials should also be thermal and chemically stable.
Furthermore, it would be desirable to have thermoelectric materials that have a high thermoelectric figure of merit. Use of such materials would produce thermoelectric devices with high efficiencies.
SUMMARY OF THE INVENTION
The present invention relates to new isostructural compounds having the general formula A
n
M
m
M′
n
Q
2n+m
where A is an alkali metal, such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), or cesium (Cs) or the transition metal silver (Ag) or thallium (TI) and mixtures thereof, M is lead (Pb), tin (Sn), germanium (Ge), calcium (Ca), strontium (Sr), barium (Ba), any divalent transition metal or mixtures thereof, M′ is bismuth (Bi), antimony (Sb) or mixtures thereof, and Q is sulfur (S), selenium (Se), or tellurium (Te) and mixtures thereof. These compounds possess an NaCl-type cubic lattice crystal structure where A, M and M′ occupy the Na sites and Q occupies the Cl (chlorine) sites. This family of compounds combine isotropic morphology, an advantageous property for device processing, with low thermal conductivity and widely ranged electrical conductivity. Further, certain properties such as the electrical properties of the compounds can be controlled by varying the values for n and m. The isostructural compounds of the present invention are therefore good candidates for semiconductor applications in thermoelectronic devices, detectors, and photovoltaic cells, by way of non-limiting example.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.


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