Light emitting diode structure
BACKGROUND OF THE INVENTION 
     Small-area electroluminescent diodes that can be current-modulated at relatively high current densities and at room temperatures are finding increasing uses as transmitting sources for multimode optical fiber communication lines. Thanks to modern liquid phase epitaxy (LPE) deposition techniques and other techniques now highly developed for manufacture of integrated circuits and diffused-junction planar semiconductor devices, such light-emitting diode structures can now be made small enough for relatively efficient coupling to the end of an optical communication fiber whose active light-transmitting area may have a very small diameter ranging from a few microns up to 100 microns or so. 
     Numerous forms of light-emitting diodes (LED&#39;s) have been developed for such purposes and are described in the published art. For example, one available type of surface-emitting LED incorporates a double heterojunction structure as described in an article appearing in &#34;Optical Fiber Technology&#34;  on pages 306-308, copyright 1976 by IEEE Press, and in turn stated to be reprinted with permission from &#34;Optics Communications&#34;, Vol. 4, Dec. 1971, pages 307-309. This article, by C. A. Burrus and B. I. Miller of Bell Telephone Laboratories, describes briefly what has been sometimes called in the trade a &#34;Burrus diode&#34;. It is with this general class of LED&#39;s that our invention is concerned. 
     SUMMARY OF THE INVENTION 
     Our invention also relates to a small-area surface-emitting LED incorporating a double heterostructure that is similar in some respects to the above-mentioned &#34;Burrus diode&#34;. However, the &#34;Burrus diode&#34; requires extensive etching of a &#34;well&#34; through a relatively thick semiconductor substrate to permit the generated light energy to be emitted in a direction desired by the authors of the above-cited article. In our modified and improved structure, less etching is required and the current distribution is controlled to produce efficient light emission in a direction diametrically opposite to that of the &#34;Burrus diode&#34;. 
     Several modifications of our invention are also described below which permit even greater efficiencies and simplified manufacturing methods. These involve different LPE techniques and choices of layer compositions that are described in detail in the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical cross-section view, partially cut away, of one form of multi-layer surface-emitting LED structure made in accordance with our invention; 
     FIG. 2 is a similar cross-section view of a modification of the structure of FIG. 1; and 
     FIG. 3 is another similar cross-section view illustrating a further modification of our invention. 
     In all three figures of the drawings, layer thicknesses and structural dimensions are purely illustrative and not necessarily to scale. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The structure of our surface-emitting LED illustrated in FIG. 1 may be fabricated as follows: 
     (1) The starting material in this illustrative embodiment is a substrate layer 11 of n-type gallium arsenide (n-GaAs). 
     (2) A relatively thin blocking layer 12 of p-type gallium arsenide (p-GaAs) is first grown on the substrate 11 by liquid phase epitaxy (LPE) techniques well known to the art. A suitable thickness may be in the range of 0.3 to 5.0 microns. The layers 11-12 are then removed from the LPE furnace. 
     (3) Using known photoresist etching techniques, a small area is etched through layer 12 down to the surface of substrate layer 11. For example, it may be a small round hole of diameter A, as shown in FIG. 1, equal to about 50 microns. 
     (3) The etched structure is then replaced in the LPE furnace and a double heterostructure is grown on top of it. In a preferred embodiment of FIG. 1, these four layers may have the following compositions and physical characteristics: 
     (a) Layer 13 is of n-type gallium aluminum arsenide, for example of the formula n-Ga 1-x  Al x  As, where x≃0.3. Such a layer has low absorption at desired wavelengths of light to be emitted, e.g., in the range of about 0.75 to 0.90 microns. 
     (b) Layer 14 is the active emitting layer of p-type semiconductor materials containing gallium and arsenic, e.g., p-type gallium arsenide (p-GaAs) or p-type gallium aluminum arsenide of the formula p-Ga 1-y  Al y  As, where y≃0.05 to 0.10. 
     (c) Layer 15 is similar to layer 13 but of the opposite semiconductor type, e.g., p-type gallium aluminum arsenide of the formula p-Ga 1-x  Al x  As, where x≃0.3. As noted under (3) (a) above, such a layer has low absorption at the desired wavelengths of light to be emitted. The band gap of the layers 13 and 15 is effectively larger than that of the intermediate active layer 14. Hence, in the illustrated structure of FIG. 1, layer 15 is essentially transparent at the desired wavelengths. 
     (d) Layer 16 is the cap layer of the heterostructure. In FIG. 1 it is of p-type gallium arsenide (p-GaAs). 
     This completes the semiconductor layer structure and junctions prepared by LPE techniques in the embodiment of FIG. 1. 
     (4) The next step is to apply a metal layer 17 and a connection tab 18 to the top of the semiconductor wafer structure of FIG. 1. Again using conventional photoresist etching techniques, a small area is etched through the metal layer 17 directly above the hole A. It may, for example, be another small round hole of diameter B centered on the perpendicular axis 19 through the center of hole A. It is also preferably equal in size, or slightly larger, as compared to hole A. 
     (5) Next, in the fabrication of the LED of FIG. 1, preferential etching is used to etch further down from the hole in layer 17 through the p-GaAs layer 16, just stopping at the surface of layer 15. This will inherently undercut layer 17, resulting in the hole of diameter C, as illustrated, though ideally it should be no larger than B. 
     (6) In usual production techniques for LED&#39;s, a large number of devices 10 are deposited and etched simultaneously on relatively large substrates. These are then cut up into individual chips, each forming an individual LED structure as shown in FIG. 1. These techniques are well-known to the art and do not form part of our invention. 
     (7) The final steps are to metallize the lower surface of substrate 11 in FIG. 1 and to bond it to a metal header 20 in any suitable manner, as by the solder layer 21. Alternatively, the lower surface can be metallized before metal layer 17 is applied. Header 20 may conventionally serve as a heat sink as well as an electrical terminal for LED 10 and may have any appropriate size and shape; hence it is shown only in fragmentary cross-section in FIG. 1. 
     Turning next to the operation of the LED 10 of FIG. 1, assume that an appropriate unidirectional potential or dc voltage pulse is impressed between terminals 18 and 20. A novel feature of our invention is the current confinement provided by the blocking layer 12 so that current flow is concentrated through the hole A and the regions immediately above hole A. The active light-emitting recombination area in layer 14 is therefore as indicated schematically by the cross-hatched area 22 above hole A. This concentration of current flow permits the hole areas B and C to be formed without drastically affecting current flow through area 22 and the cap layer 16, and these in turn permit the light beam, represented by the dashed arrows 23 in FIG. 1, to be taken out through the top of the LED structure as shown. As previously mentioned, this direction of the beam is diametrically opposite to that of the so-called &#34;Burrus diode&#34; described in the referenced article by Messrs. Burrus and Miller, and it also eliminates the need for the deep etched well through the substrate, as described in that article. 
     Our improved structure therefore provides increased efficiency and lower current for a given light output, contributing to longer life. 
     The modified device 10A shown in FIG. 2 may, with one exception, be identical to that of FIG. 1 and fabricated by the same method steps. Corresponding elements and dimensions are identified by the same reference indicia. The exception is the semiconductor layer 16A which now forms the cap layer of the heterostructure and does not have a hole etched through it. In this embodiment it is grown as a p-type gallium aluminum arsenide layer having the formula p-Ga 1-y  Al y  As where y≃0.10 to 0.15. Such a layer has low absorption and is essentially transparent to the range of wavelength of the emitted light; hence need not be removed for light efficiency. 
     FIG. 3 shows another modified structure 10B that is similar to those of FIGS. 1 and 2, again with one major difference. As shown, the layer structure is very similar to that shown in FIG. 2 except that the active light-emitting layer is essentially limited to the disc-shaped structure 14A which has again been cross-hatched to indicate schematically that it is almost entirely light-emitting. The formulation of this thin, flattened spheroid 14A requires careful control in the LPE furnace so that the desired material is deposited in the slight depression in layer 13 that resulted from its growth over the hole area A in the blocking layer 12. The objective is to limit the growth of layer 14A to little more than the area immediately above hole area A, as shown in FIG. 3. 
     The remaining superstructure above the layer 14A is functionally the same as shown in FIG. 2, with layers 15A and 16B corresponding to layers 15 and 16A in FIG. 2 except for a possible slight variation in thickness above the area 14A. Alternatively, layer 16B may be of the same composition as cap layer 16 in FIG. 1 (i.e., of p-GaAs) with the hole C etched through it. 
     As one further general modification applicable to all of the illustrated structures 10, 10A and 10B, the semiconductor materials may all be doped, by known techniques, so that all of those layers described above as being of the n-types are of the p-types, and vice versa. The compositions of the layers 16A and 16B may also be varied, within limits, so that they can function as filters to produce a narrow spectrum output of light emitted through hole B. 
     Other structural modifications and alternative manufacturing techniques will also occur to those skilled in the art who read our foregoing disclosure. We therefore intend that such be included to the extent that they are within the spirit and scope of our invention as set forth in the appended claims.