It has long been known that a plurality of nanocrystallites in silicon carbide (SiC) would give rise to an enlargement of the energy gap of the SiC shifting any emitted light towards the UV region because of quantum confinement, which allows the relaxation of momentum selection rules by confining the charge carriers spatially, thus allowing direct band gap transitions. See U.S. Pat. No. 5,376,241 entitled, “Fabricating Porous Silicon Carbide” by A. D. Kurtz et al., which issued on Dec. 27, 1994 and is assigned to Kulite Semiconductor Products, Inc, the assignee herein. In that particular patent, it teaches the formation of porous SiC, which is formed under electrochemical anodization. The patent also describes the production of the semiconductor through the use of ultraviolet light to illuminate the surface of the semiconductor. In this manner, by controlling the light intensity, the potential and THE doping level, a porous layer is formed in the semiconductor and thus one produces porous SiC. The porous SiC can be employed for UV light sources, such as LEDs and diode lasers. Porous SiC can be utilized as a filtering chemical process to be used to provide heterojunction devices.
See U.S. Pat. No. 5,376,818 entitled, “Large Area P-N Junction Devices Formed from Porous Silicon” issued on Dec. 27, 1994 to A. D. Kurtz et al. and assigned to the assignee herein. That patent shows the formation of porous SiC, which is produced under electrochemical anodization. The patent teaches that when a potential is applied to the semiconductor and ultra-violet light illuminates the surface of semiconductor, one can control the light intensity, the potential in doping level, to form a microporous structure in the semiconductor to thus produce porous SiC. The structure enhances the quantum confinement of energetic carriers and which device is highly sensitive to stress.
Reference is also made to U.S. Pat. No. 5,834,378 entitled, “Passivation of Porous Semiconductors for Improved Optoelectronic Device Performance and Fabrication of Light-Emitting Diode Bases on Same”. The patent issued on Nov. 10, 1998 to A. D. Kurtz et al. and is assigned to the assignee herein. That patent describes a method which improves the photoluminescent performance of a porous semiconductor. According to the method, a monolayer of passivating material is generated on a pore wall of the porous semiconductor to passivate the porous semiconductor. This layer substantially eliminates dangling bonds and surface states which are associated with the porous semiconductor layer. The resulting passive porous semiconductor layer exhibits a quantum efficiency of approximately five percent. It is indicated that one monolayer of passivating material can be an oxide which is generated by placing the bulk semiconductor substrate into a furnace. Also described is a heterojunction light emitting device which employs a passivated porous semiconductor layer.
U.S. Pat. No. 5,939,732 issued on Aug. 17, 1999 and is entitled, “Vertical Cavity Emitting Porous Silicon Carbide Light Emitting Diode Device and Preparation Thereof” and is assigned to the assignee herein and is invented by A. D. Kurtz et al. That patent teaches a multi-layered light emitting device, which has an active light emitting layer of porous silicon carbide and a sequence of layers of porous SiC underneath which serve as a quarter wavelength multi layer mirror. In this manner, one obtains electroluminescent emission of narrow visible light in the deep blue to UV range in a highly directed pattern. Thus, as indicated above, the nanocrystallites in SiC give rise to an enlargement of the energy gap and shifts emitted light towards the UV region. The same effect has also been demonstrated in silicon. Moreover, when LEDs are made from such materials, the emitted light is shifted towards the UV, the shifting inversely proportional to the size of the nanostructure. It is also well-known that the width of the energy gap may also be effected by the application of stresses (see for instance, deformation potentials). The use of deformation potentials as effecting the energy gap is well-known and is text book material. Thus, it is indicated and known that the effect of stress can cause a change in the frequency of emitted light of an LED or the light resonance of the structure.
In graphite, a similar effect can occur. Normal graphite is a semi-metal, but in a nanostructure it can be a conductor or a semiconductor. For example, see an article entitled, “Nanotubes for Electronics”. This article appeared in Scientific American in the December 2000 issue. The article describes nanotubes and is written by Phillip G. Collins and Phaedon Avouris. In that article it is clear that nanotubes are utilized because of their unique electronic properties. Carbon nanotubes essentially can be used to perform the same function as silicon does in electronic circuits, but at a molecular scale, where silicon and other semiconductors do not work. In particular, when the dimensions of the nanotube are of the same order of magnitude as the electron wavelength, then these quantum effects can occur at those dimensions.
See also an article entitled, “Cavity Quantum Electrodynamics” which appeared in Scientific American in April, 1993 by Serge Haroche and Jean-Michel Raimond. This article explains the operation of atoms and photons and their behavior in small cavities. The article shows that new sensors can be developed utilizing such techniques.
In any event, because of the function and operation of nanotubes, it has been determined that application of stress can change a conductor to a semiconductor by changing the energy gap where the quantum confinement leads to a large change in the electrical properties. Essentially, the electrical properties of nanostructures, such as nanotubes, which exhibit quantum confinement can be changed by the application of various stresses, thus leading to a means of measuring such stresses.
Therefore, the present invention contemplates the formation of a high frequency, high temperature sensor which employs nanotubes and where the nanotubes are subjected to pressure or other external conditions to vary the electrical properties of the nanotubes according to a desired application.
The nanotubes, which are conductive, will respond to an applied pressure to produce an output voltage or current proportional to the amount of pressure applied. These devices, because of their high temperatures properties, as well as because of the electrical properties provide improved transducer structures or sensor structures useful in generating sensors in the optical and microwave ranges.