Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2001-04-13
2003-12-02
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S370130
Reexamination Certificate
active
06657194
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to multi-color infrared sensing devices. More specifically, the invention relates to monolithic multi-color infrared imaging arrays based on the direct growth of infrared sensitive mercury cadmium telluride material structure on custom-fabricated read-out electronics on specially oriented silicon substrates by Molecular Beam Epitaxy (MBE).
BACKGROUND OF THE INVENTION
Semiconductors are either naturally occurring or artificially synthesized materials in which the atomic arrangement gives rise to a specific atomic potential that forbids electrical carries (electrons or positive charges, known as holes) to freely move and therefore carry electrical currents. They act as insulators for as long as there is no additional energy provided to excite these carriers across the forbidden gap (called a band gap) that is generated by the atomic potential. An electrical current can be obtained by the excitation of electrons across the forbidden band. Necessary energy can be generated in different ways and of interest for radiation detection is the energy carried by the electromagnetic radiation waves. The incoming radiation has to be tuned (i.e. the radiation has to carry enough energy to be able to excite the electrons) with the band gap of the semiconductor in order to produce this excitation.
In a crystal, both short- and long-range order are important in defining single crystal structure. The atoms hold positions that can be associated with a well-defined grid (or lattice) having very small or nonexistent deviations from the grid positions through out the entire crystal. This periodicity in the atomic arrangement is of utmost importance for the electrical behavior of the crystal. A polycrystalline material has a short-range order, a specific geometrical positioning of the atoms in a lattice, but lacks long-range order. Only by performing a combination of translations and rotations can one recover the same geometrical arrangement of an initial test region. The polycrystalline material is formed by a multitude of grains consisting of individual single crystals. A long-range order means that by translating the crystal in any direction one recovers exactly the same structural arrangement of the atoms. A unit cell can be therefore defined, and the entire crystal can be regained by translations of this unit cell. An amorphous material lacks both short and long-range order, and consequently lacks any periodicity in its atomic arrangement.
FIG. 1
shows the unit cell for a cubic crystalline lattice and several crystal directions. The crystal planes are planes that contain atoms and are perpendicular to the respective direction. Shown as shaded, is the plane (100). Obviously, in the case of a cubic unit cell the chosen orientation of the reference system is arbitrary, thus the (100),
(010
) and (001) planes are equivalent. All the equivalent planes form a family of planes and are called by a generic name, which is one of the family member names. The atoms can occupy positions on the nodes of the grid or at intersections of principal lines within the unit cell (such as the center of lateral cubic faces or the intersection of body diagonals of the cube). The atoms can, as well, occupy positions at certain coordinates around the nodes or intersections of principal lines in what is called a basis. A cubic unit cell with atoms sitting at the nodal position as well as in the center of each cubic face is called a face centered cubic (fcc). Mercury cadmium telluride (HgCdTe or MCT) is an fcc lattice with a basis in which a secondary set of atoms is situated at ¼ of the cubic length away from the fcc atoms in the (111) direction.
Mercury cadmium telluride is a semiconductor widely used as an infrared detector material. It consists of elements positioned in group II (Hg, Cd) in the periodic table of elements and in group VI (Te). The crystalline MCT is formed as a ternary material from a HgTe (mercury telluride) crystal lattice in which a certain percentage of Hg atoms is being replaced by Cd atoms. By varying the amounts of Cd atoms in replacement of Hg atoms, the electrical properties of the entire crystal can be tailored to suit the absorption and subsequent conversion of the incident infrared radiation into electrical current. Thus, short wavelength infrared (SWIR) MCT has a Cd percentage that allows radiation absorption of short wavelengths. Similarly, mid-wavelength (MWIR) MCT has a Cd percentage that allows radiation absorption of medium wavelengths, and long wavelength (LWIR) MCT has a Cd percentage that allows radiation absorption of long wavelengths. The flexibility of matching the electrical behavior of the crystal to certain application requirements by adjusting the composition of the crystal is known as band gap engineering, and is one of the great advantages of MCT. Several techniques are available for producing MCT and by far, MBE is the most reliable.
Molecular beam epitaxy (MBE) is a chemical vapor deposition (CVD) method in which the crystal is grown on a template (substrate) from atomic and/or molecular fluxes obtained by thermal evaporation of the charge material. The growth process occurs in an ultra-high vacuum (UHV) environment to minimize the presence of foreign atoms. Polycrystalline and/or amorphous material are loaded into crucibles and constitute the charge. During the growth the substrate is kept at a predefined temperature to ensure that sufficient energy is transferred to the surface to achieve specific reactions. The fluxes are adjusted by the temperatures at which the charge materials are kept. In this way the incoming atoms/molecules from the charges have to spend a certain residence time on the surface while traveling/diffusing around in order to find a geometrical position that minimizes the surface energy.
In order to control and to enhance the electrical properties of the materials grown by MBE one can use this method to add certain impurities (dopants) to the primary material. This added control offers a large advantage since it reduces the post growth processing along with the costs and increases the yield factor.
The substrate is of paramount importance for the MBE growth of crystalline materials. Its choice is primarily dictated by the lattice parameters that have to closely match the ones of the intended new material. Exceptions are rare and mismatches create an unwanted density of defects/dislocations.
In order to act as a template, the substrate itself should be a single crystal and one has to expose the periodic arrangement of the bulk material. Typically, the bonds between atoms are saturated (i.e. an atom/ion uses all of its available electrons for bond formation with its neighboring atoms). At the surface, the lack of periodicity in the direction perpendicular to the plane forces the atoms lying on the surface to react (use their available electrons) and bond with other elements, different than those present in the bulk of the material. These elements that are present at the surface are called contaminants. Such a surface is useless for the MBE growth of single crystalline materials.
For the growth of MCT one can use as substrate bulk cadmium zinc telluride (CdZnTe) for which lattice matching occurs at a Zn percentage close to 4%. A constant demand of larger area detectors prevents the use of CdZnTe as substrates since they are available in limited sizes only. Bulk CdZnTe is also expensive and brittle, reducing further its use in production environments. When using CdZnTe as a substrate one is limited by the current device fabrication technology.
The crystals used as substrates (Silicon, CdZnTe and others) are fabricated by cooling a melt of material (pure elements or compounds) in a way that allows crystal formation. Once crystallized, the previously formed ingot is cut into wafers with various orientations. Since the wafer is a single crystal (hence it contains a large number of unit cells, to be viewed as “bricks”) its surface can have various morphologies. The surface orientation of the substr
Ashokan Renganathan
Boieriu Paul
Chen Yuan-ping
Faurie Jean-Pierre
Sivananthan Sivalingam
EPIR Technologies, Inc.
Hannaher Constantine
Perkins Jefferson
Piper Rudnick LLP
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