Charge-coupled devices (CCDs) of a type developed previously for imaging in ultraviolet light have been found to be useful as detectors of electrons and other charged particles with kinetic energies as low as about 100 eV. Heretofore, solid-state electronic devices have generally not been useful for detecting particles with kinetic energies below the keV range. The devices in question are back-side-illuminated silicon CCDs with p+ delta (δ) doping at their back-side interfaces between silicon and surface layers of silicon dioxide. Such a device at an earlier stage of development was described in "Growth of δ-Doped Layer on Silicon CCD" (NPO-18688) in the Laser Tech Briefs edition of NASA Tech Briefs, Vol. 19, No. 2 (February 1995), page 11.
When an energetic charged particle enters a detector, some of its kinetic energy is dissipated in the generation of electron/hole pairs; the problem is to collect and measure the electron/hole charges before the electrons and holes recombine. In the absence of δdoping, the inability of a silicon back-surface-illuminated device to detect either ultraviolet photons or low-energy charged particles is attributable to a "dead" layer that includes the surface SiO2 plus a potential well that extends about 0.5 μm into the silicon from the Si/SiO2 interface (see figure). The depth of penetration of the particles in the energy range of interest is less than the depth of the dead layer. Consequently, most of the electrons generated by impingement of charged particles become trapped in the potential well, where they eventually recombine with holes and thus go undetected.
Delta doping reduces the low-energy detection limit by effectively eliminating the back-side potential well. Delta doping is so named because its density-vs.-depth characteristic is reminiscent of the Dirac δfunction (impulse function); the dopant is highly concentrated in a very thin layer. Preferably, the dopant is concentrated in one or at most two atomic layers in a crystal plane and therefore δdoping is also known as atomic plane doping.
An experimental δ-doped CCD for detecting low-energy charged particles was made by subjecting a commercial CCD to the following additional fabrication steps: First, an atomically clean silicon back surface was prepared by use of a hydrogen-termination surface-cleaning procedure that involves temperatures no more than about 200 °C. Then residues of the cleaning procedure were outgassed in a vacuum as the temperature of the device was gradually increased to an epitaxial-deposition temperature of 450 °C. A 1-nm-thick layer of silicon doped with boron [an acceptor (p) dopant] to a density of 4 × 1020 atoms/cm3 was deposited epitaxially on the cleaned back surface. The silicon flux was interrupted briefly to enable the deposition of a δlayer of boron with an areal density of 2 × 1014 atoms/cm2. The silicon flux was resumed to deposit a 1.5-nm-thick cap layer of silicon. Finally, the cap layer was exposed to steam to form a protective oxide on the back surface.
In experiments, the device was found to detect incident electrons with energies as low as 50 eV. Quantitative analysis was performed for incident electrons in the 200-to-1,000-eV range. The signal monotonically increased with increasing energy of the incident electrons. On the basis of data acquired in the experiments, it has been estimated that eventually, cooled, δ-doped CCDs should be able to detect single electrons with energies as low as 100 eV, at a noise limit of 3 electrons per pixel.
This work was done by Shouleh Nikzad, Michael Hoenk, and Michael Hecht of Caltech; Amy Smith of MIT; and Qiuming Yu of Kansas State University for NASA's Jet Propulsion Laboratory.
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