Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1999-03-01
2001-03-27
Bowers, Charles (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S078000, C257S190000, C257S201000, C257S614000, C438S095000, C438S518000
Reexamination Certificate
active
06208005
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention pertains in general to infrared radiation absorbing material Hg
1-x
Cd
x
Te and more specifically to such material structures which are fabricated by use of the interdiffused multilayer growth process.
BACKGROUND OF THE INVENTION
One process for the production of the infrared absorbing semiconductor material, Hg
1-x
Cd
x
Te, is termed MOCVD-IMP (Metalorganic Chemical Vapor Deposition-Interdiffused Multilayer Process). With this manufacturing process, alternating layers of CdTe and HgTe are grown with a total period thickness in the range of 20-120 nm (nanometers). After these layers have been grown by use of the MOCVD process, the group of layers are annealed which causes them to interdiffuse and form a homogeneous HgCdTe alloy. The mole fraction of the cadmium in the alloy is termed the “x” value, and this determines the wavelength of response for the infrared detector. This process is disclosed in U.S. Pat. No. 4,566,918 entitled “Utilizing Interdiffusion Of Sequentially Deposited Links Of HGTE And CDTE”. This patent issued on Jan. 28, 1986.
With the conventional approach for manufacturing an interdiffused multilayer HgCdTe material, there is a maximum 0.33 percent mismatch in the lattice constants between the CdTe and the HgTe. This results in the production of strain at the interface. Although the individual layer thicknesses for the CdTe and the HgTe are thinner than the critical thickness for the onset of dislocation formation, there is still left a residual strain to accommodate for the weak, elastic constants of the HgTe and the underlying interdiffused HgCdTe. This mismatch in the lattice constants contributes to dislocation formation, which can be as high as 1-5×10
6
cm
−2
.
A significant problem encountered in the design and use of semiconductor infrared detectors is that of dislocation defects in the HgCdTe alloy. These dislocations compromise the transport properties of the semiconductor, which in turn also reduce the performance of the HgCdTe infrared detectors. Furthermore, infrared detector devices requiring heterostructures with two or more dissimilar Hg
1-x
Cd
x
Te alloy compositions have additional misfit dislocations at the interfaces due to the different lattice constants. These dislocations also reduce the performance of HgCdTe detector structures with heterostructures and heterojunctions. Therefore, there is a need for a method of manufacture, and a resulting infrared sensitive material structure, which has a reduced defect density.
SUMMARY OF THE INVENTION
A selected embodiment of the present invention is a method for fabricating HgCdTe based material structure which has a reduced defect density. The method includes the step of forming a cadmium zinc telluride (Cd
1-y
Zn
y
Te) buffer layer on a cadmium telluride based substrate. The substrate may include zinc or selenium. Next, on the buffer layer, alternating layers of mercury telluride and cadmium zinc telluride are epitaxially grown. The mercury telluride has a given lattice constant. The buffer layer and the cadmium zinc telluride layers have a mole fraction of zinc which produces within these layers a lattice constant which is substantially similar or identical to the lattice constant of the mercury telluride layers. Finally, the structure is annealed to interdiffuse the mercury telluride and the cadmium zinc telluride layers to produce a homogeneous mercury cadmium zinc telluride alloy on a cadmium zinc telluride substrate with a lattice matched CdZnTe buffer layer.
A still further embodiment of the present invention is an infrared radiation material structure having reduced dislocation defects. This structure includes a cadmium telluride based supporting substrate which may include zinc or selenium with a mole fraction of 4%±1%. It further includes a buffer layer which is epitaxially grown on the substrate wherein the buffer layer comprises cadmium zinc telluride. A homogeneous alloy structure of mercury cadmium zinc telluride is formed by vapor phase epitaxy on the buffer layer of alternating layers of mercury telluride and cadmium zinc telluride. The Cd
1-y
Zn
y
Te layers and the buffer layer have a zinc mole fraction y to produce therein a lattice constant which is substantially similar to the lattice constant of the mercury telluride layers. The multiple pairs of mercury telluride and cadmium zinc telluride layers are annealed to form the homogeneous alloy structure.
The composition of the alloy is controlled by varying the relative thicknesses of mercury telluride (HgTe) and CdZnTe.
In still further embodiments of the present invention, multiple alloy structures can be formed for detecting single or multiple wavelength bands of infrared radiation but all with the same lattice constants. This is achieved by epitaxial growth of alternating thin layers of HgTe and lattice matched CdZnTe. Only the relative thicknesses of the two lattice matched pairs are varied to adjust the alloy compositions. The number of pairs are selected depending on the overall thickness of the homogeneous alloy. Further, CdTeSe can be substituted for the CdZnTe. Instead of cadmium zinc telluride layers, cadmium telluride selenide layers can be used with a composition that is substantially lattice matched to mercury telluride.
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“Effects
Bowers Charles
Lee Hsien-Ming
Lockheed Martin Corporation
Sadacca Stephen S.
Sidley & Austin
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