Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-04-23
2002-02-26
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370130, C250S370120, C257S428000
Reexamination Certificate
active
06350989
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to radiation detectors, particularly to semiconductor radiation detectors, and more particularly to wafer-fused semiconductor radiation detectors wherein an internal electrically conductive grid is located between ends of a pair of fused semiconductor pieces, and to a method for fabricating same.
Various types of radiation detectors have been developed for detecting gamma-rays and x-rays, among which are the planar semiconductor radiation detectors. Semiconductor radiation detectors generally operate by absorbing a quantum of gamma-ray or x-ray radiation and by converting the radiation energy into a number of electron-hole pairs that is proportional to the absorbed energy. After the conversion, the motion of the electrons and holes induce electrical signals on the detector electrodes. The electrical signals are also proportional to the energy of the absorbed radiation. Hence, by using a semiconductor radiation detector, one can detect gamma-ray and x-ray radiation and measure its energy spectrum.
The conventional planar semiconductor radiation detector is basically composed of a semiconductor material having a cathode on one surface and an anode on the opposite surface. A positive bias voltage is applied to the anode and a negative bias voltage is applied to the cathode. As x-rays or gamma-rays pass through the semiconductor material, electrons and holes are formed, and electrical signals are generated, and thus the energy of x-ray or gamma-ray radiation on the semiconductor material can be measured.
The conventional planar semiconductor radiation detector does not function well due to the poor electrical transport properties of the holes. Many of the common radiation detectors are made from CdZnTe or GaAs, with a cathode and anode made, for example, of gold, and for these semiconductors, the electrical signal due to the holes is typically much smaller than the electrical signal due to the electrons. These effects are due to the slower motion of the holes and greater probability of trapping of the holes within these materials. Because the total electrical signal is a sum of the signal due to the electrons and the holes, the signal due to the holes complicates the signal analysis and results in poor energy resolution and low efficiency for the detector.
The planar semiconductor radiation detector also suffers from a position dependence on the signal. For example, a signal due to electrons originating from radiation absorbed near the cathode will be larger than a signal originating from near the anode. Thus, the conventional planar semiconductor radiation detectors suffer from both poor electrical transport properties of the holes and from a position dependence of the signal.
Recent efforts have been directed to improve the energy resolution of the planar semiconductor radiation detectors and also to lessen the dependence of the signal on the position of the radiation absorption, and thus allow one to isolate the electrical signal due to the motion of electrons. These improved approaches are referred to as “electron-only devices” and have shown to give superior energy resolution for x-ray and gamma-ray radiation over the conventional planar semiconductor radiation detectors. The “electron-only devices” are exemplified by P. N. Luke, “Single-polarity charge sensing in ionization detectors using coplanar electrodes,” Appl. Phys. Lett. 65 (22), Nov. 28, 1994; E. Y. Lee, et al., “Device Simulation of an Unipolar Gamma-Ray Detector,” Mat. Res. Soc. Symp. Proc., 487, p. 537 (1998), U.S. Pat. No. 5,677,539, issued Oct. 14, 1997 to B. Apotovsky, et al., U.S. application Ser. No. 09/075,419 filed May 8, 1998, entitled, “Method and Apparatus for Electron-Only Radiation Detectors from Semiconductor Materials” by Lund, et al., now U.S. Pat. No. 6,069,360, and U.S. application Ser. No. 09/075,351 filed May 8, 1998, entitled, “High Resolution Ionization Detector and Array of Such Detectors” by McGregor, et al. now U.S. Pat. No. 6,175,120. These “electron-only devices” place a third metallic electrode, called a grid, on the surface of the detector near the anode to electrostatically shield the anode from the signal originating between the grid and the cathode. In these devices, all the signals from the anode originates from a motion of the electrons and holes moving between the anode and the grid. Since the electrons move toward the anode while the holes move away from the anode toward the cathode, due to their polarities, the signal on the anode will be dominated by the motion of the electrons. Furthermore, the signal will have much less position dependence, since electron trapping between the grid and anode will be unlikely.
In an “electron-only device” one can characterize the space between the grid and the cathode as a detection volume and the region between the grid and the anode as the measurement volume. Ideally, all radiation absorbed in the detection volume would give rise to electrical signals due only to the motion of the electrons in the measurement volume. However, there are several imperfections associated with the prior art of the “electron-only” detector, which are:
1. For the grid to shield the anode, the grid can not be placed too close to the anode. This decreases the measurement volume of the detector and therefore the radiation detection efficiency of the detector.
2. Many of the electrons created between the grid and the cathode are collected by the grid and produce no signal on the anode. Hence, these detectors have dead regions where no signals can be detected, leading to a loss of detector efficiency.
3. The internal electric field of the detector is highly non-uniform, due to the placement of the external grid. The electric field is uniform only very close to the cathode and the anode. The non-uniformity of the electric field causes variation in the charge collection time of the electrons. Since electron trapping does occur in the detector, this non-uniformity in the electric field results in variation of the signal strength with the position of the x-ray and gamma-ray absorption event, and hence in loss of the energy resolution. Attempts at correcting for the electron trapping by trying to deduce the position of the original radiation absorption are difficult due to the non-uniform internal electric field. This is commonly attempted by monitoring of the cathode signal and using it to correct the anode signal with electronic circuits external to the detector.
Another type of prior art radiation detectors utilize a grid called the Frisch grid. This type of detector contains a gas at a high pressure, and the Frisch grid comprises a metal mesh located between the cathode and the anode. The Frisch grid gives electron-only behavior and it has been the inspiration for a new class of electron-only detectors based on semiconductor materials. However, obviously one can not place a metal mesh through a solid semiconductor and hence it is not possible to directly implement the idea of the Frisch grid for semiconductor radiation detectors. Also, it is not possible to grow a metal mesh and then cover it up with a good quality semiconductor, since the resulting semiconductor overlayer always has poor electrical characteristics and low transmission through the interface, due to difficulties in the growth process. Simply pressing a metal mesh between two semiconductors does not make them a single piece, because in this device, electrons must cross the interface without becoming trapped by defects.
However, it has been recently discovered that, by applying high pressure and high temperature uniformly, it is possible to “bond” two clean semiconductor pieces or wafers together, without any glue, to form good interfaces. See Z. L. Leau, et al., Appl. Phys. Let. 56,737 (1990). This process is known as wafer bonding or wafer fusion. Wafer bonding has been successfully demonstrated in such semiconductor systems as GaAs/InP, GaN/InP, InGaAsP/Si, InP/SiO
2
/InP, LiTaO
3
/Si, Si/In(Sb), Si/SiO
2
, and LiTaO
3
/Si. See above referenced
James Ralph B.
Lee Edwin Y.
Carnahan L. E.
Epps Georgia
Evans T. P.
Hanig Richard
Sandia National Laboratories
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