Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-10-15
2001-08-07
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
Reexamination Certificate
active
06271526
ABSTRACT:
BACKGROUND
This specification relates to devices and techniques of coupling radiation energy to a light sensing array, and more particularly, to radiation coupling to a quantum-well infrared sensing array via evanescent waves.
Quantum-well semiconductor devices can be designed to respond to radiation energy to produce charge indicative of the amount of received radiation. The radiation-induced charge can then be converted into an electrical signal (e.g., a voltage or current) to be processed by signal processing circuitry. Many quantum-well devices use an intraband transition between a ground state and an excited state in the same band (i.e., a conduction band or a valance band) of the quantum-well structure to detect infrared (“IR”) radiation. The compositions of lattice-matched semiconductor materials of the quantum well layers can be adjusted to cover a wide range of wavelengths for infrared detection and sensing. In comparison with other radiation detectors, quantum-well structures can achieve a high quantum efficiency, a low dark current, compact size and other advantages. Infrared quantum-well sensing arrays may be used for various applications, including night vision, navigation, flight control, environmental monitoring.
A quantum well infrared sensor only responds to incident radiation with a polarization that is perpendicular to the plane of the quantum well layers. This is because only this polarization can induce an intraband transition at a desired infrared wavelength. Hence, the direction of the electric field of the received radiation must be parallel to the growth direction of the quantum well layers. One direct approach for light coupling is to orient the quantum well infrared photodetector at an angle to the incident infrared radiation (e.g., forty-five degree). The incident electric field will have a component along the growth direction of the quantum well layers to produce absorption of photons. Any additional scattering can enhance this absorption.
For applications based on imaging at focal plane arrays, the photodetector array is often oriented perpendicular to the scene to be imaged. Since the electric vector is essentially parallel to the quantum well layers in this arrangement, the quantum well layers absorb little or no light. One way to provide proper coupling is to use a random surface to scatter the incident radiation into the correct polarization for absorption. Alternatively, grating couplers with one or two-dimensional periodic profiles can be used to convert normally-incident radiation to waves propagating parallel to the quantum well layers.
SUMMARY
The present disclosure includes techniques and devices that couple radiation to quantum-well sensors via evanescent waves so that the polarization of the coupled energy is substantially perpendicular to the quantum-well layers. Efficient IR coupling can be achieved in various applications.
One embodiment of a semiconductor device includes substrate, a quantum-well sensing region in the substrate, a non-sensing region in the substrate, and a prism engaged to the substrate. The sensing region has a quantum-well structure of alternating semiconductor layers parallel to the substrate to absorb radiation at or near a resonance wavelength. The non-sensing region at least partially encloses the sensing region to form a resonant optical cavity whose optic axis is parallel to said substrate.
The prism includes a flat surface and a slanted surface that forms an angle with the flat surface. The flat surface is substantially parallel to and spaced from the substrate by an air gap less than one wavelength of the radiation to permit evanescent coupling to the sensing region. The angle of the slanted surface is configured to couple radiation to the flat surface at an incident angle that is equal to or greater than a critical incident angle so that a total internal reflection occurs at the flat surface.
This configuration produces an evanescent wave that propagates along the flat surface. When the beam incident to the flat surface includes a polarization in the incident plane, the evanescent wave has a polarization perpendicular to the flat surface and hence the quantum well layers. Under this condition, the radiation energy in the evanescent wave can be absorbed by the quantum-well layers.
The resonance peak of the optical cavity may substantially overlap with said resonance wavelength of said sensing region in order to efficiently absorb the radiation. The air gap between the prism and the substrate may be filled with a dielectric material which has an index of refraction less than that of the prism.
These and other aspects and associated advantages will become more apparent in light of the detailed description, the accompanying drawings, and the appended claims.
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Cwik Thomas
Yeh Cavour
California Institute of Technology
Fish & Richardson P.C.
Gabor Otilia
Hannaher Constantine
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