Patent Application
20050056829
Embodiments of the present invention are generally related to the field of photodetectors and more specifically related to semiconductor based radiation detectors.
Detectors may be fabricated in many ways, and may serve many purposes. For all detectors, sensitivity and signal-to-noise ratios are important to successful operation. When attempting to detect x-rays, photodetectors are preferably highly sensitive to x-rays and relatively insensitive to other electromagnetic radiation. Photodetectors are constructed with photoconductor sensors. The photoconductors can either be intrinsic semiconductor materials that have high resistivity unless illuminated by photons, or diode structures that have small currents due to the blocking effect of the diode junction unless illuminated.
Among semiconductor materials considered for x-ray detectors are selenium, mercuric iodide and lead iodide. The two iodide compounds have a higher mobility product, require a much lower polarizing voltage than selenium, and have additional advantages such as greater temperature stability. However, each of mercuric iodide and lead iodide has physical parameters that affect their performance and ease of use in single layer x-ray detectors.
In mercuric iodide, the carrier mobility is measured to be higher than lead iodide and the lag time is found to be lower. The lower carrier mobility means that it is difficult to use a thick layer of lead iodide, which is more efficient in absorbing a greater fraction of incident x-ray photons, especially at higher photon energies that increase detector sensitivity. However, mercuric iodide is more chemically reactive toward typical contact materials (e.g., aluminum) than is lead iodide and considerable problems have been experienced with contact corrosion in flat panel detectors coated with mercuric iodide.
As mentioned above, photoconductors may also have diode structures based on either a p-i-n or p-n configuration.
As mentioned above, the p-i-n structure may be used to detect x-rays that are incident on either of the p-doped semiconductor material layer 282 or the n-doped semiconductor material layer 285. In operation of p-i-n photodiode 150, a reverse-bias voltage is applied across the photodiode and x-rays are made incident upon the intrinsic region 283. The electron-hole pairs then separate under the applied electric field and quickly migrate toward their respective poles. The electrons move toward the positive pole and the holes move toward the negative pole. Conventional photodiodes have narrow intrinsic regions 283. Due to the narrowness of the intrinsic region 283 and also due to the high mobility of the intrinsic material, there is little chance that the carriers will recombine before they arrive at the interface with the doped material. The electrons and holes then collect near the respective interface with the doped material. The change in resistivity results in a change in one or both of a voltage or current between top conductor 281 and second conductor 286, which may be measured in a surrounding system (not shown).
One problem with prior diode structure photoconductors is that dark (leakage) current limits the usefulness of the high x-ray sensitivity of photoconductor sensors. One solution to substantially reducing such dark current is by using p-n heterostructures of photoconductors. Diodes structures (p-n and p-i-n) may be composed of two or more dissimilar semiconductor materials, thereby forming a heterojunction. For example, one prior photodetector consists of a layer of cadmium telluride and a layer of cadmium sulfide forming a heterojunction. The cadmium telluride is deposited so that it is a p-type material (excess holes) and the cadmium sulfide is deposited so that it is an n-type material (excess electrons). An external voltage applied across the heterojunction of the two materials produces a p-n junction that acts as a photodiode. As discussed above, radiation induced electron-hole pairs give rise to electrical currents that flow in proportion to the incident radiation. The p-n junction, when reversed biased, inhibits dark current from flowing across the junction.
The performance of a photoconductor may be judged by various criteria including sensitivity. Sensitivity refers to the current produced by a photoconductor with respect to the electromagnetic power. A photoconductor with high sensitivity will produce more current for a given intensity of incident radiation than one with a low sensitivity. Sensitivity is affected by the mobility of the electrons in the material. Semiconductor materials with a higher mobility have a higher sensitivity, if other parameters are similar, because the electrons can move at a greater speed. One problem with prior heterojunction photoconductors is that they exhibit low sensitivity.
A photodetector is described. In one embodiment, the photodetector comprises a first semiconductor material, a second semiconductor material coupled to the first semiconductor material, and a contact coupled to the second semiconductor material. The second semiconductor material being less corrosive than the first semiconductor material to the contact.
In another embodiment, the photodetector comprises a plurality of semiconductor materials forming a heterojunction. The plurality of semiconductor materials comprises a first semiconductor material and a second semiconductor material coupled to the first semiconductor material. The first and second semiconductor materials may be halides.
In one particular embodiment, the first semiconductor material comprises lead iodide and the second semiconductor material comprises mercuric iodide.
Other features and advantages of the present invention will be apparent from the accompanying drawings, and from the detailed description, which follows below.
The present invention is illustrated by way of example and not limitation in the accompanying figures in which:
In the following description, numerous specific details are set forth such as examples of specific components, processes, etc. in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that these specific details need not be employed to practice the present invention. In other instances, well known components or methods have not been described in detail in order to avoid unnecessarily obscuring the present invention.
The terms “top,” “bottom,” “front,” “back,” “above,” “below,” and “between” as used herein refer to a relative position of one layer or component with respect to another. As such, one layer deposited or disposed above or below another layer, or between layers, may be directly in contact with the other layer(s) or may have one or more intervening layers. The term “coupled” as used herein means connected directly to or connected indirectly through one or more intervening layers or operatively coupled through non-physical connection (e.g., optically).
A photodetector having a heterojunction structure is described that may operate to reduce contact corrosion and/or reduce dark current while maintaining high x-ray sensitivity. Instead of using a single, thick semiconductor photoconductor layer (e.g., of mercuric iodide), a multiple layer heterojunction structure is employed.
In one particular embodiment, for example, the photodetector includes a layer of lead iodide (PbI2) and a thicker layer of mercuric iodide (HgI2) disposed above to form a bi-layer PbI2—HgI2 coating film. A thin top layer of lead iodide may also be disposed above the mercuric iodide layer to form a three-layer PbI2—HgI2—PbI2 “sandwich” structure. Alternatively, other semiconductor materials may be used for any of the layers as discussed below.
An advantage of such a structure is that the outer contacts on the coating film may be protected from chemical attack by the mercuric iodide layer because of the presence of the intervening layer(s) of relatively unreactive lead iodide, while maintaining a high mobility in the photoconductor due to the thicker higher mobility layer. The mobility of electrons in mercuric iodide is higher than the mobility of holes. In lead iodide, the mobility of holes is higher than electrons. In general, the mobility of electrons in mercuric iodide is higher the mobility of holes in lead iodide, thus making mercuric iodide a better photoconductor material because it can more effectively collect charges with a lower bias. As such, by using a thicker layer of mercuric iodide, the overall carrier transport properties of the photoconductor may be dominated by the mercuric iodide layer that constitutes the bulk of the photodetector's thickness.
Further, such a structure may operate to reduce dark current in the photoconductor. Although the band gaps of mercuric iodide and lead iodide are approximately the same (e.g., differing by less than 10%), the slight difference in the band gap and carrier mobilities in mercuric iodide and lead iodide may lead to quasi p-n junction behavior at the layer interfaces that may operate to reduce dark current, in particular, when operated under reverse bias conditions.
In one particular embodiment, first semiconductor material 240 may be composed of PbI2 and second semiconductor material 230 may be composed of HgI2. As noted above, an HgI2 layer 230 is preferably thicker than a PbI2 layer 240. HgI2 may have superior properties for x-ray detection, making its inclusion in one embodiment of photodetector 200 advantageous. However, use of HgI2 as a semiconductor material, by itself, in a photodetector may be problematic, as it may be chemically reactive with one or both conductors 260 and 210. As such, a thin layer of PbI2 may be used as semiconductor material 240 as a buffer between the HgI2 semiconductor material layer 230 and either of contacts 260 and 210 (or both as discussed below in relation to
Various thickness relationships between the two semiconductor materials 230 and 240 may be used. For example, first semiconductor material layer 240 may be thin and the semiconductor material layer thick relative to each other. In one embodiment, first semiconductor material 240 may have a thickness less than 250 microns (μm), for examples, 10, 40 or 150 microns. The second semiconductor material 230 may have a thickness greater than 250 microns, for examples, 200, 350 or 450 microns. Note that the specific thickness need not be matched up respectively, such that a 10 microns first semiconductor material thickness and 450 microns second semiconductor thickness may be used together. In another embodiment, two relatively medium thickness layers (such as two 150 micron layers for example) may be used. In an alternative embodiment, the first semiconductor material 240 may be thicker than second semiconductor material 230, for example, to further remove second semiconductor material 230 from contact 260. It should be noted that the semiconductor materials may have thickness outside the exemplary ranges provided above and, in particular, the primary detection material (e.g., semiconductor material 230) depending on the particular application (e.g., mammography, general radiography, industrial, etc.) in which photodetector 200 will be used. For example, first semiconductor material 240 may have a thickness on the order of Angstroms and the second semiconductor material 230 may have a thickness on the order of millimeters.
In addition, reactive problems with contact 210 may also be reduced with a sandwich approach by the use of an additional semiconductor material as illustrated in
In one embodiment, both layers 220 and layer 240 (e.g., the PbI2 layers) are thinner than layer 230 (e.g., the HgI2 layer). The resulting structure may be expected to have the properties of HgI2 for detection purposes, without the corresponding reactive properties of a single layer, HgI2, photodetector.
In alternative embodiments, semiconductor materials other than mercuric iodide and lead iodide may be used, such as other semiconductor halides, for example. In one embodiment, such alternative materials may be iodide compounds such as bismuth iodide (BiI2). Alternatively, non-iodide compounds and may be used, for example, thallium bromide (TlBr). The semiconductor materials selected for use may operate as a corrosion barrier layer to a contact and/or as part of the heterojunction structure to optimize the electric parameters of the detector (e.g., reduce dark currents). The other semiconductor materials that may be used for semiconductor materials 240 and/or 230 may have band gaps approximately the same or different than either of mercuric iodide (2.1 eV) and lead iodide (2.3 eV). For example, bismuth iodide has a band gap of 1.73 eV and thallium bromide has a band gap of 2.7 eV. As previously noted, yet other halides may also be used for the semiconductor material layers.
In an alternative embodiment, as discussed above, the photoconductor may include additional semiconductor materials. In one such embodiment, an additional semiconductor material is deposited above the semiconductor material 230, step 345, thereby sandwiching the semiconductor material 230.
It should be noted that the process illustrated is simplified, and may involve patterning (such as for isolation of individual conductors for example). Furthermore, a self-aligned process may be used, in which individual detectors are separated out through etching of some form after formation of layers on the substrate.
In one embodiment, an x-ray detector 576 may be constructed, for example, as a flat panel detector with a matrix of photodetectors 200 with readout electronics to transfer the light (e.g., x-ray) intensity of a pixel to a digital signal for processing. The readout electronics may be disposed around the edges of the detector to facilitate reception of incident x-rays on either surface of the detector. The flat panel detector may use, for example, TFT switch matrix coupled to the detectors 200 and capacitors to collect charge produced by the current from detectors 200. The charge is collected, amplified and processed as discussed below in relation to
In the foregoing detailed description, the method and apparatus of the present invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. In particular, the separate blocks of the various block diagrams represent functional blocks of methods or apparatuses and are not necessarily indicative of physical or logical separations or of an order of operation inherent in the spirit and scope of the present invention. Moreover, the foregoing materials are provided by way of example as they represent the materials used in photoconductors. It will be appreciated that other semiconducting materials or other materials may be used. Any material that has improved corrosion resistance and otherwise satisfies the desired electrical parameters may be used. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.
