Monolithic semiconductor photodiodes that discriminate against infrared and visible light and that can be tailored to detect ultraviolet light in selected wavelength bands have been invented. These devices could serve as ultraviolet sensors in such instruments as hydrogen- and hydrocarbon-flame detectors, dosimeters for monitoring exposure to ultraviolet in industrial processes, and instruments for measuring ultraviolet in sunlight to monitor the ozone content of the upper atmosphere.
These photodetectors are made from silicon or combinations of silicon and silicon carbide, and therefore can readily be monolithically integrated with silicon-based electronic readout circuitry. They are relatively compact and inexpensive alternatives to previously developed ultraviolet detectors that (1) must be equipped with expensive, bulky monochromators or optical interference filters to obtain the desired wavelength selectivity, or (2) incorporate GaN- or GaP-based photodiodes, which are expensive to fabricate and cannot be monolithically integrated with silicon-based readout circuitry.
The upper part of the figure shows a simplified cross section of a representative device according to the invention. The device is fabricated on a silicon-on-insulator (SOI) or a SiC-on-SOI (SiC/SOI) substrate. A buried insulating layer made of silicon dioxide divides the substrate into a relatively thin active Si or SiC upper section and a relatively thick passive lower section made of silicon. The depth at which the insulating layer is buried determines the thickness of the active upper section and thus, as described below, affects the wavelength selectivity of the device.
A photodiode is fabricated in and on the upper section of the substrate. If, for example, the upper section of the substrate is made of p-doped silicon, then the photodiode includes a layer of n-doped silicon formed in the upper section. One of the ohmic contacts for the photodiode, in the form of a ring, is deposited on the n-doped silicon layer. The other ohmic contact - a wider ring that surrounds the first-mentioned ring - is formed on top of the p-doped upper section. Optionally, an antireflection coating is applied to the top surface of the photodiode in the area enclosed by the contact ring.
In operation, the absorption of incident photons in the photodiode gives rise to paired electrons and holes, which become separated at the p/n junction. Thus, photocurrent is generated across the junction. The photocurrent can be measured by use of external circuitry connected to the ohmic contacts. The long-wavelength cutoff (the maximum wavelength for photons to excite photocurrent) and the quantum efficiency of the device as a function of wavelength depend on the thickness (or the thicknesses of the sublayers) of the upper section of the substrate and on band gap(s) of the photodiode material(s) (Si and/or SiC). A monolithic array of such devices, with different upper-section thicknesses and thus different spectral responses, can be fabricated for use in a spectrometer.
Those photons that pass through the photodiode without being absorbed strike the insulating layer. Preferably, the device should be designed so that (1) photons with wavelengths greater than some cutoff wavelength (which may or may not be the same as the cutoff wavelength for excitation of photocurrent) pass through the insulating layer and are dissipated within the lower section of the substrate, while (2) photons with wavelengths shorter than the cutoff wavelength are reflected by the insulator back through the diode to provide another opportunity for absorption, thereby increasing the quantum efficiency and electrical output of the device.
This work was done by Nader M. Kalkhoran and Fereydoon Namavar of Spire Corp. for Stennis Space Center.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
Nader Kalkhoran, Spire Corporation, One Patriots Park, Bedford, MA 01730-2396; (781) 275-6000.
Refer to SSC-00072