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 ultraviolet (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 to Kurtz, entitled “Fabricating Porous Silicon Carbide,” which issued on 27 Dec. 1994 and is assigned to Kulite Semiconductor Products, Inc, the assignee herein. That patent 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 UV 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, thereby producing porous SiC. The porous SiC can be employed for UV light sources, such as LEDs and diode lasers. Porous SiC can also be utilized as a filtering chemical process to provide heterojunction devices.
See U.S. Pat. No. 5,376,818 to Kurtz, entitled “Large Area P-N Junction Devices Formed from Porous Silicon,” issued on 27 Dec. 1994 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 ultraviolet 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, thus producing porous SiC. The microporous structure enhances the quantum confinement of energetic carriers, and the semiconductor device is highly sensitive to stress.
Reference is also made to U.S. Pat. No. 5,834,378 to Kurtz entitled, “Passivation of Porous Semiconductors for Improved Optoelectronic Device Performance and Fabrication of Light-Emitting Diode Bases on Same.” The patent issued on 10 Nov. 1998 and is assigned to the assignee herein. That patent describes a method for improving the photoluminescent performance of a porous semiconductor. According to the patented method, a monolayer of passivating material is generated on a pore wall of the porous semiconductor to passivate the porous semiconductor. This monolayer 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 generated by placing the bulk semiconductor substrate into a furnace. Also described is a heterojunction light emitting device employing a passivated porous semiconductor layer.
U.S. Pat. No. 5,939,732, which issued on 17 Aug. 1999 and is entitled “Vertical Cavity Emitting Porous Silicon Carbide Light Emitting Diode Device and Preparation Thereof,” is assigned to the assignee herein and 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 minor. 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 well-known that the width of the energy gap may also be affected by the application of stresses (see for instance, deformation potentials). The use of deformation potentials as affecting the energy gap is well-known and is textbook 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” in the December 2000 issue of Scientific American, pages 62 to 69. This article describes nanotubes and is written by Phillip G. Collins and Phaedon Avouris. In the article, it is clear that nanotubes are utilized because of their unique electronic properties. Carbon nanotubes can be used to perform essentially 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” by Serge Haroche and Jean-Michel Raimond, which appeared in Scientific American in April 1993. 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.
Given the above, it is well-known that there are many methods for measuring an applied force or stress. Historically, these measurements have been made using semiconductor based piezoresistive pressure transducers or strain gages, wherein a micromachined structure deflects under the load and results in the semiconductor material experiencing strain. The strain in the semiconductor material causes the electrical characteristics of the semiconductor to change, which in turn results in a change in the output signal of the device that is proportional to the applied force.
It is also known that a monolayer of graphite, also known as graphene, is a zero band gap semiconductor, and multilayers of graphene can be produced to create a small, controlled band gap. Similar to carbon nanotubes as demonstrated in U.S. Pat. No. 7,312,096, titled “Nanotube Semiconductor Structures with Varying Electrical Properties,” which is assigned to the assignee herein, a monolayer or multilayers of graphene can exhibit a change in electrical properties when strained by an applied force.
Conventional sensors do not take full advantage of the various beneficial properties of nanotubes and graphene described above. Therefore, there is a need for sensors that effectively uses the above described beneficial properties of nanotubes and graphene.