1. A light-emitting device, comprising:
a substrate comprising gallium arsenide;
an active region comprising an n-type spacing layer and a p-type spacing layer; and
a tunnel junction structure comprising a p-type tunnel junction layer adjacent the p-type spacing layer, an n-type tunnel junction layer and a tunnel junction between the p-type tunnel junction layer and the n-type tunnel junction layer, in which:
the p-type tunnel junction layer comprises a layer of a p-type first semiconductor material including gallium and arsenic; and
the n-type tunnel junction layer comprises a layer of an n-type second semiconductor material including indium, gallium and phosphorus;
wherein the second semiconductor material has an indium fraction that provides the second semiconductor material with a lattice constant that matches that of the first semiconductor material, where the first semiconductor material is gallium arsenide, and the second semiconductor material is indium gallium phosphide of which the indium gallium phosphide has an indium fraction of 0.4 to 0.6.
2. A light-emitting device, comprising:
a substrate comprising gallium arsenide;
an active region comprising an n-type spacing layer and a p-type spacing layer; and
a tunnel junction structure comprising a p-type tunnel junction layer adjacent the p-type spacing layer, an n-type tunnel function layer and a tunnel junction between the p-type tunnel junction layer and the n-type tunnel function layer, in which:
the p-type tunnel junction layer comprises a layer of a p-type first semiconductor material including gallium and arsenic; and
the n-type tunnel junction layer comprises a layer of an n-type second semiconductor material including indium, gallium and phosphorus;
wherein the second semiconductor material has an indium fraction that provides the second semiconductor material with a lattice constant that matches that of the first semiconductor material, where the first semiconductor material is gallium arsenide, and the second semiconductor material is indium gallium phosphide and the n-type tunnel junction layer additionally comprises a layer of an n-type third semiconductor material sandwiched between the layer of the second semiconductor material and the p-type tunnel junction layer, and the third semiconductor material comprises indium, gallium and arsenic.
3. The light-emitting device of claim 2, in which the second semiconductor material and the third semiconductor material have respective compositions that provide strain compensation between the layers.
4. The light-emitting device of claim 2, in which:
the first semiconductor material is gallium arsenide;
the second semiconductor material is indium gallium phosphide; and
the third semiconductor material is indium gallium arsenide.
5. The light-emitting device of claim 2, in which the p-type tunnel junction layer additionally comprises a layer of a p-type fourth semiconductor material sandwiched between the layer of the first semiconductor material and the layer of the third semiconductor material, the fourth semiconductor material comprising gallium, arsenic and antimony.
6. The light-emitting device of claim 5, in which the fourth semiconductor material and the third semiconductor material have respective compositions that provide strain compensation between the layers.
7. The light-emitting device of claim 5, in which:
the first semiconductor material is gallium arsenide;
the second semiconductor material is indium gallium phosphide
the third semiconductor material is indium gallium arsenide; and
the fourth semiconductor material is gallium arsenide antimonide.
8. A surface-emitting laser, comprising:
opposed semiconductor reflectors the same conductivity type, the reflectors defining an optical cavity; and
an active region and a tunnel junction structure located in the optical cavity, in which:
the active region comprises an n-type spacing layer and a p-type spacing layer;
the tunnel junction structure comprises a p-type tunnel junction layer adjacent the p-type spacing layer, an n-type tunnel junction layer and a tunnel junction between the p-type tunnel junction layer and the n-type tunnel junction layer;
the p-type tunnel junction layer comprises a layer of a p-type first semiconductor material including gallium and arsenic; and
the n-type tunnel junction layer comprises a layer of an n-type second semiconductor material including indium, gallium and phosphorus;
wherein the second semiconductor material has an indium fraction that provides the second semiconductor material with a lattice constant that matches that of the first semiconductor material, where the first semiconductor material is gallium arsenide, and the second semiconductor material is indium gallium phosphide of which the indium gallium phosphide has an indium fraction of 0.4 to 0.6.
9. A surface-emitting laser, comprising:
opposed semiconductor reflectors if the same conductivity type, the reflectors defining an optical cavity; and
an active region and a tunnel junction structure located in the optical cavity, in which:
the active region comprises an n-type spacing layer and a p-type spacing layer;
the tunnel junction structure comprises a p-type tunnel junction layer adjacent the p-type spacing layer, an n-type tunnel junction layer and a tunnel junction between the p-type tunnel junction layer and the n-type tunnel junction layer;
the p-type tunnel junction layer comprises a layer of a p-type first semiconductor material including gallium and arsenic; and
the n-type tunnel junction layer comprises a layer of an n-type second semiconductor material including indium, gallium and phosohorus;
wherein the second semiconductor material has an indium fraction that provides the second semiconductor material with a lattice constant that matches that of the first semiconductor material, where the first semiconductor material is gallium arsenide, and the second semiconduct material is indium gallium phosphide and the n-type tunnel junction layer additionally comprises a layer of an n-type third semiconductor material sandwiched between the layer of the second semiconduct material and the p-type tunnel junction layer, and the third semiconductor material comprises indium, gallium and arsenic.
10. The surface-emitting laser of claim 9, in which the second semiconductor material and the third semiconductor material have respective compositions that provide strain compensation between the layers.
11. The surface-emitting laser of claim 9, in which:
the first semiconductor material is gallium arsenide;
the second semiconductor material is indium gallium phosphide; and
the third semiconductor material is indium gallium arsenide.
12. The surface-emitting laser of claim 9, in which the p-type tunnel junction layer additionally comprises a layer of a p-type fourth semiconductor material sandwiched between the layer of the first semiconductor material and the layer of the third semiconductor material, the fourth semiconductor material comprising gallium, arsenic and antimony.
13. The surface-emitting laser of claim 12, in which the fourth semiconductor material and the third semiconductor material have respective compositions that provide strain compensation between the layers.
14. The light-emitting device of claim 12, in which:
the first semiconductor material is gallium arsenide;
the second semiconductor material is indium gallium phosphide
the third semiconductor material is indium gallium arsenide; and
the fourth semiconductor material is gallium arsenide antimonide.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. An apparatus for reconstructing a 3D image of at least a first region of an object from truncated cone beam projection data acquired by means of an x-ray device having a source-detector assembly moved along an acquisition trajectory such that a cone beam between the source and the detector encompasses said first region, the apparatus comprising:
extrapolation means for processing the truncated projection data to derive pseudo-projection data associated with projection directions outside the detector;
first 3D reconstruction means for reconstructing an intermediate CT image covering at least the first region of the object based on the truncated projection data completed with the pseudo-projection data;
registration means for computing registration data representing a spatial transformation selected to register a prior CT image, representing a second region of the object including the first region and larger than the first region, with the intermediate CT image;
forward projection means for processing the truncated projection data and the registered prior CT image to derive forward projection data associated with projection directions outside the detector; and
second 3D reconstruction means for synthesizing a reconstructed CT image covering at least the first region of the object based on the truncated projection data completed with the forward projection data.
2. The apparatus as claimed in claim 1, wherein the registration means comprise means for determining a template of voxels of the intermediate CT image which are representative of contours present in the intermediate CT image, and means for determining a spatial transformation of the prior CT image which maximizes a local correlation measurement, at the voxels of the template, between the intermediate CT image and the transformed prior CT image.
3. The apparatus as claimed in claim 2, wherein the means for determining the template comprise means for determining a set of voxels of the intermediate CT image having respective x-ray attenuation values above a threshold, and means for selecting the voxels of the template whereby each voxel of the template has a respective local neighborhood including at least a predefined number of voxels of said set.
4. The apparatus as claimed in claim 3, wherein the means for selecting the voxels of the template are arranged to select voxels of the intermediate CT image having maximum edge strength values among the voxels having local neighborhoods including at least the predefined number of voxels of said set.
5. The apparatus as claimed in any one of claims 2, wherein the maximized local correlation measurement is of the form
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where N is the number of voxels of the template, j is a voxel index, n(j) denotes a neighborhood of voxel j, b1 and ti are x-ray attenuation values of voxel i, within n(j) in the intermediate CT image and in the transformed prior CT image, respectively, and bi and to are the mean values of the x-ray attenuation values within no) in the intermediate CT image and in the transformed prior CT image, respectively.
6. The apparatus as claimed in claim 1, wherein the forward projection means comprise first computing means for obtaining first forward projection data with a coarser resolution than the truncated cone beam projection data by means of the registered prior CT image, and second computing means for interpolating the first forward projection data to provide the forward projection data with the same spatial resolution as the truncated projection data.
7. The apparatus as claimed in claim 1, wherein the forward projection means %4ycomprise linear transformation means to fit the forward projection data with the truncated cone beam projection data in a region located along the border of the detector.
8. A computer program product loadable in a processing unit of a computer for reconstructing a 3D image of at least a first region of an object from truncated cone beam projection data acquired by means of an x-ray device having a source-detector assembly moved along an acquisition trajectory such that a cone beam between the source and the detector encompasses said first region, the computer program product including instructions to execute the following steps when run by the processing unit of the computer:
extrapolating the truncated projection data to derive pseudo-projection data associated with projection directions outside the detector;
reconstructing an intermediate 3D CT image covering at least the first region of the object based on the truncated projection data completed with the pseudo-projection data;
registering a prior CT image, representing a second region of the object including the first region and larger than the first region, with the intermediate CT image;
processing the truncated projection data and the registered prior CT image to derive forward projection data associated with projection directions outside the detector; and
synthesizing a reconstructed CT image covering at least the first region of the object based on the truncated projection data completed with the forward projection data.
9. The computer program product as claimed in claim 8, wherein the instructions for registering the prior CT image with the intermediate CT image comprise instructions for determining a template of voxels of the intermediate CT image which are representative of contours present in the intermediate CT image, and instructions for determining a spatial transformation of the prior CT image which maximizes a local correlation measurement, at the voxels of the template, between the intermediate CT image and the transformed prior CT image.
10. The computer program product as claimed in claim 9, wherein the instructions for determining the template comprise instructions for determining a set of voxels of the intermediate CT image having respective x-ray attenuation values above a threshold, and instructions for selecting the voxels of the template whereby each voxel of the template has a respective local neighborhood including at least a predefined number of voxels of said set.
11. The computer program product as claimed in claim 10, wherein the instructions for selecting the voxels of the template select voxels of the intermediate CT image having maximum edge strength values among the voxels having local neighborhoods including at least the predefined number of voxels of said set.
12. The computer program product as claimed in claim 8, wherein the instructions to derive forward projection data comprise instructions for obtaining first forward projection data with a coarser resolution than the truncated cone beam projection data by means of the registered prior CT image, and instructions for interpolating the first forward projection data to provide the forward projection data with the same spatial resolution as the truncated projection data.
13. The computer program product as claimed in claim 8, wherein the instructions to derive forward projection data comprise instructions to fit the forward projection data with the truncated cone beam projection data in a region located along the border of the detector.