US 3615856 A
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Description (OCR text may contain errors)
United States Patent Henry Stern Sommers, Jr. Princeton, NJ.
Apr. 14, 1969 Oct. 26, 197 1 RCA Corporation Inventor Appl. No. Filed Patented Assignee GERMANIUM-TIN ALLOY lNFRA-RED DETECTOR l Claini, 2 Drawing Figs.
U.S. Cl 136/89, 75/175,148/33.4,148/175 Int. Cl. "011 15/02 Field of Search 75/175;
Primary Examiner-Allen B. Curtis Attorney-Glenn H. Bruestle ABSTRACT: An infrared radiation detector useful at room temperature includes an epitaxial, germanium-tin alloy grown by liquid phase epitaxy with a concentration of tin greater than 2.5 atomic percent,
PAIENTEBnm 2s |97l lfB L75 WAVELENGTH Henry Stem Somemdz dfmll'l GERMANIUMITIN..ALLQX.J NE RA'RED DET T R BACKGROUND OF INVENTION The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Army. This invention relates in general to photosensitive translating devices and in particular to semiconductor detectors suitable for use in the near infrared spectrum at room temperature.
Radiation detectors are" known which have used germanium-tin alloys as the photosensitive elements thereof; however, these detectors have had essentially the same absorption characteristics as those of pure germanium. Germanium arid the prior germanium-tin alloys have adequate absorption coefficients for radiation out to about 1.55 micrometers at room temperature, but they have not been sufiiciently sensitive throughout the 1.5 to l.75-micrometer region, which has been found to be a good windr-";w in the earths atmosphere for the transmission of infrared radiation.
SUMMARY OF INVENTION BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional schematic view of suitable ap paratus for the liquid phase epitaxial growth of the present alloy.
FIG. 2 is a graph showing the absorption characteristics of a prior germanium detector and four examples of the present germanium-tin alloy detector.
DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a cross-sectional schematic representation of apparatus useful for the liquid phase epitaxial growth of the present germanium-tin alloy. Four representative epitaxial growth examples are provided and discussed below.
EXAMPLE I In 'this example, a mixture 11 of 20 grams of tin and II grams of germanium is placed in the lower end of a graphite boat 12. The boat 12 is placed in a furnace tube 13, and swept with an inert gas such as nitrogen or. argon, or a reducing gas such as hydrogen, in order to maintain a nonoxidizing ambient during the growth of the alloy. The mixture 11 is preheated to a temperature of 800 C. with the furnace tube 13 tilted as shown in FIG. 1 to keep the mixture 1 1 at one end of the boat 12. After the mixture 11 is completely molten and homogenized, it is cooled to room temperature, and the boat 12 is removed from the tube '13. A substrate wafer 14 capable of promoting epitaxial growth from the mixture 11, such as high resistivity N- or P-type germanium, is placed in the flat bottom of the upper end of the boat 12 with an exposed surface 15, and a clamp 16 is used to hold the wafer 14 in place. The boat 12 is then replaced in the furnace tube 13 and tilted so that the mixture 11 is at the lower end of the boat. The furnace tube 13 is again swept with an inert or reducing gas, while the tube and its contents are heated to a temperature sufficiently above the melting point of tin to melt the mixture 11, but below the melting point of the wafer 14. In the present example, a temperature of 700 C. is selected. The furnace tube 13 is then tipped to ahorizontal position so that the molten mixture of germanium-tin 11 is flooded over the exposed surface 15 of the wafer 14. The mixture 11 is cooled at a rate of about l$ C. per minute so that an epitaxial germanium-tin layer is grown on the surface 15 of the wafer 14. After about minutes, when the temperature falls to about 555 C. the heating is discontinued. At about 400 C. the furnace tube is again tipped-as shown in FIG. 1 to decant the remaining mixture 11, and the wafer 14 is allowed to cool to room temperature.
The resulting wafer 14 has an epitaxial germanium-tin alloy layer with a thickness of about 6 mils deposited on its exposed surface 15. The concentration of tin in the alloy is about 2.7 atomic percent as determined by known methods of chemical analysis. Other methods of analysis such as mass spectroscopy and X-ray analysis are known and these methods yield less accurate and slightly different results. For consistency, all measurements of tin concentration described herein are by chemical analysis unless stated otherwise.
The original substrate wafer 14 is next lapped off and the remaining alloy body is further processed to the desiredv size and shape for the type of detector selected. For instance, two electrodes may be attached to the alloy and it can be used in the photoconductivemode, where radiant energy striking the alloy creates electron-hole pairs which produce an electrical current through the two electrodes attached to the alloy. The alloy can also be used in the photovoltaic mode where a P-N- junction is introduced into the alloy by conventional methods, and radiant energy striking the alloy produces a voltage drop across the P-N-junction and the electrodes attached to opposite sides of the alloy.
The present germanium-tin alloy has an adequate absorption coefficient at significantly longer wavelengths than that of prior germanium detectors. FIG. 2 is a graph which qualitatively shows the absorption characteristics of a prior germanium detector and four examples of the present germaniumtin alloy detector at room temperature. The boundaries of the l .5 to l.75-micrometer region are illustrated by the dotted lines 11 and 12. Assuming 300 cm. shown by the dotted line IS in FIG. 2, as the minimum absorption coefficient for adequate detector operation, the prior germanium detector has an adequate absorption coefficient only out to about [.55 micrometers, as shown by the curve 20in FIG. 2. The present I germanium-tin alloy, however, is sensitive throughout the L5 to l.75micrometer-region and region and has an adequate absorption coefficient out to about 1.76 micrometers, as shown by the curve 31. It is suggested that reason for the significantly greater absorption at longer wavelengths is due to the relatively high concentration of tin. The maximum solubility of tin in germanium using methods other than liquid phase epitaxy is about l.2 atomic percent, and this value is not sufficient to appreciably alter the absorption from that of pure germanium.
EXAMPLE II In this example, an amount of pure tin is placed in the boat 12, and the tube 13 and its contents are heated to l,000 C. for 1 hour to purify the system. In this respect, the tin acts as a getter for contaminants within the tube 13. The tin is then removed and a mixture of 7 grams of tin and 3 grams of germanium is preheated to a temperature of 700 C. The homogenized mixture is then heated to a temperature of 650 C. at which point it is tipped onto the exposed substrate surface, which is substantially parallel to a l l 1 plane, and cooled for 30 minutes to a temperature of 6 l 5 C. The heating is then discontinued, and when the temperature falls to 400 C. the remaining mixture is decanted and the wafer is cooled to room temperature. The resulting epitaxial alloy layer is 4-mils thick and has a tin concentration of 2.9 atomic percent. The alloy is then further processed as described in example I. As shown by the curve 32 in FIG. 2, this alloy is also sensitive throughout the L5 to l.75micrometer region and has an adequate absorption coefficient out to about L micrometers.
EXAMPLE III In this example, 20 grams of tin and 3 grams of germanium are preheated to 750 C. The homogenized mixture is then heated and tipped onto the substrate at a temperature of 600 C. The heating is discontinued and the molten mixture is allowed to cool to 400 C. at which point the remaining mixture is decanted and the wafer is cooled to room temperature. The alloy layer is 3.3-mils thick and has a tin concentration of 4.0 atomic percent, as determined by X-ray analysis. Chemical analysis was not performed, and precise comparisons with the other alloys described herein should not be made. As shown by the curve 33 in FIG. 2, this alloy has an adequate absorption coefficient out to 1.9 micrometers.
EXAMPLE IV lower absorption curve 34 than the other three alloys; however, it still has substantial utility throughout the desired wavelength region with a significantly greater absorption than that of germanium. By appropriately adjusting the growth conditions, as illustrated by the above examples, the absorption characteristics of the alloy can be matched with any desired wavelength in the 1.5 to 1.75-microme'ter region.
1. A photosensitive translating device for converting radiant energy to electrical energy comprising a semiconductor body of epitaxial germanium-tin alloy, the concentration of the tin being greater than 2.5 atomic percent, said alloy having an absorption coefficient substantially greater than 300 cm. for radiation throughout substantially all of the 1.5 to 1.75- micrometer region of the spectrum at room temperature.