Coating processes – Direct application of electrical – magnetic – wave – or... – Ion plating or implantation
Reexamination Certificate
2001-03-29
2003-01-21
Meeks, Timothy (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Ion plating or implantation
C427S530000, C427S160000, C427S255110, C427S255310, C427S255350
Reexamination Certificate
active
06509066
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a family of lead chalcogenide photo-semiconductors (photoconductors) and, more particularly, to photoconductors having essentially homogeneous oxygen-containing shallow electron traps impregnated throughout the photodiode matrix.
BACKGROUND OF THE INVENTION
It is well understood that mankind's perception of the “real world” is limited to the extent of our five senses. For example, although electromagnetic radiation is in the form of gamma rays or microwaves, these radiations are not detectable to us unless some means is available to convert them into a perceptible form of radiation or other sensory stimuli (e.g., visible light, sound, heat, or physical contact). Fortunately, technology has developed materials, apparatus, and devices that can transform one form of electromagnetic radiation to another form of electromagnetic radiation. Typically, this is achieved by converting the radiation into electrical energy, which is then transformed back into electromagnetic radiation in another region of the spectrum. With the advent of semiconductor technology, these transformations can be readily performed.
One such device for converting electromagnetic radiation into electrical energy is a photo-semiconductor (photocoductor). Essentially, a photoconductor absorbs photons of electromagnetic radiation, thereby altering the conductivity of the radiation absorbing media in the photoconductor to generate a change in electrical current.
Similarly, a change in voltage can be created by using a photovoltaic cell which establishes a quantitative relationship between the impinging electromagnetic radiation and electrical current or voltage. The electrical current can then be converted back into a form of energy that can be perceived (i.e., sound, visible light, heat, movement, hard copy printout, etc.). Examples of components for converting electrical energy into other forms of energy include audio amplifiers, cathode ray tubes, light emitting diodes, semiconductor lasers, resistors, and transponders.
One very useful form of electromagnetic radiation is infrared radiation or infrared light. Infrared radiation is not visibly perceptible but yet is ever present in our natural environment. All matter that generates heat from such diverse processes as radioactive decay, chemical decomposition, or metabolic processes emit infrared radiation. Within the spectrum of electromagnetic radiation, infrared radiation has wavelengths ranging from 0.75-3 micron (shortwave infrared)(SWIR), 3-5 micron (midwave infrared)(MWIR), 8-14 micron (longwave infrared)(LWIR), and 14-1000 micron (far infrared).
Transmission of infrared signals can be either through the natural atmosphere, gases, vapors, mists, or through condensed media such as liquids or solids (e.g., optical fibers or waveguides). Detection of these signals is then achieved with photoconductors and finally converted into perceptible information using light emitting diodes, liquid crystals, cathode ray tubes, or other visible light generators.
During the last few years, small bandgap semiconductors have become important in the fabrication of infrared detectors. Since it is difficult or impossible to produce p−n junctions by other means, ion implantation offers a promising alternative. For example, n-type doping occurs by proton irradiation of Hg
1-x
Cd
x
Te (x=0.5,0.31;0.25) for the spectral range between 1.6 and 6 micron and PbTe and Pb
1-x
SnTe (x=0.12) for ranges up to 5 to 8 micron, respectively. In recent years ion cluster beam deposition and ion assisted deposition has been performed during epitaxy (i.e., layers having the desired doping are grown on a wafer) but no reports of using oxygen ion for doping lead chalcogenides are known to the present inventor.
Many semiconductors have band gaps less than 1 eV and, therefore, have thresholds in the infrared. For example, three lead salts, PbS, PbSe, and PbTe have band gaps on the order of between 0.2 eV and 0.4 eV. Some alloys such as Hg
1-x
Cd
x
Te, with x varying from 0 to 1, have band gaps ranging from 0.04 to 1.5 eV, which corresponds to wavelengths between 0.8 and 30 micron, covering essentially the entire infrared region. The preferred infrared detecting materials in the current art are lead chalcogenides such as lead sulfide, lead selenide, lead telluride, and lead/tin mixtures of these chalcogenides. Lead salts, especially lead selenide and lead sulfide materials, provide some of the most sensitive materials for detecting infrared energy at certain wavelengths. An infrared detector utilizing these materials usually comprises a thin film of lead selenide or lead sulfide on a substrate with electrical leads connected to opposite sides of the thin film or layer.
As with all transmission of data or information, it is advantageous to have the highest possible signal to noise ratio (S/N). However, in the case of low-cost infrared detectors currently available, the S/N values are inadequate, especially for weak signals being transmitted over great lengths.
When a semiconductor is subjected to photoexcitation of radiation flux with photon energies exceeding the band gap, an internal photochemical reaction occurs wherein the creation of electron-hole pairs form through the excitation of an electron from the valence band into the conduction band. The lifetime of the excited states typically ranges from 10
−3
to 10
−9
seconds. Upon illumination, the properties of the semiconductor are modified by the creation of these electron-hole pairs, increasing conductivity, known as photoconductivity. A direct correlation exists between the amount of photoconductivity and the light intensity impinging on the semiconductor photoelectric cell.
A mechanism for deleteriously limiting photoconductivity is known as “recombination”, where the holes and free electrons collide and recombine with each other essentially annihilating the charge pair. Recombination occurs at all levels of detection and always reduces the efficiency of the irradiation process, the driving force for this process being electrical charge attraction. Methods of “trapping” one or both of the charge carriers have been devised that serve to reduce the rate of recombination and thereby make the photoconductive process more efficient. Free conduction electrons can theoretically be trapped by using “shallow traps” located in the forbidden zone between the conduction and valance bands of the semiconductor. These shallow traps comprise doubly charged anions such as oxide ions. Therefore, it is beneficial to incorporate such traps into the semiconductor matrix in order to increase the sensitivity of the photoconductor.
In order to manufacture photoconductors of the type discussed supra, the microelectronics industry makes great use of epitaxy, the crystalline growth of a thin film of semiconductors or semiconductor devices on a substrate of similar type. The substrate acts as a mechanical and thermal support, but must not short circuit the components. Thus substrates usually have very high electrical resistance.
The thin film of lead salt can be formed either by vacuum evaporation or deposition onto the substrate or by chemically depositing onto the substrate from solution. The vapor deposition technique has been found to be extremely difficult to control. On the other hand, a chemical deposition from solution technique also has been far from satisfactory in that frequently the precipitation of the lead salt has had poor adherence to the substrate.
Attempts have been made to increase sensitivity of these materials by incorporating oxygen into a doubly-ionized lattice site in the matrix by use of chemical vapor deposition. Once the lead salt is deposited on the substrate, it is sensitized by oxidizing. This is accomplished by maintaining the substrate and the lead salt at an elevated temperature in an air or oxygen atmosphere for a predetermined time. It is known in the industry that the sensitization of the lead salt material, such as a lead sulfide or lead selenide
BAE Systems Information and Electronic Systems Integration Inc.
Meeks Timothy
Salzman & Levy
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