Patent Document

This is a division of application Ser. No. 272,603, filed June 11, 1981, now abandoned. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention is directed to a novel planar structure GaAs electro-optic transmitter-receiver and to the method of fabrication. High speed low cost fiber optic communication systems will require a monolithic electro-optic transmitter-receiver. GaAs depletion mode MESFET logic is integratable with an AlGaAs-GaAs laser diode to form the transmitter of a fiber optic communication system. 
     In the prior art there has been described the growth of N type AlGaAs, GaAs, and AlGaAs layers successively on a semi insulating GaAs substrate for a semiconductor laser device such as shown in the U.S. Pat. Nos. 4,105,955, 4,166,278 and 4,183,038. 
     In the present invention a channel or depression is formed in the planar surface of a semi-insulating GaAs substrate; the channel is substantially filled with successive epitaxial layers of AlGaAs, GaAs, AlGaAs and GaAs which form the optoelectronic component (i.e. the transverse junction stripe laser or the detector), the surface of which top epitaxial layer is coplanar with (or substantially coplanar with) the substrate planar surface. The adjacent planar surface of the semi-insulating GaAs substrate contains cooperating MESFET electronics preferably fabricated by direct ion implantation. This placing of both the electrical and optical components on or at the planar surface simplifies processing steps and facilitates fabrication and testing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 describes a series of steps in the fabrication of the invention. 
     FIG. 2 is a cross-sectional representation of a portion of the structure. 
     FIG. 3 is a graphical presentation of the relative emission intensity of the diode versus wavelength (A°). 
     FIGS. 4 and 5 are photomicrograph presentations of the device cross-section. 
    
    
     DESCRIPTION 
     Referring now to FIG. 1 there is shown a progression of processing steps involved in the fabrication of the planar-structure GaAs electro-optic transmitter. FIG. 1a discloses a substrate 10 of semi-insulating GaAs, with an optional GaAs:Cr doped buffer 11, and having a planar surface 12. Etched into the planar surface of the substrate is a &#34;wide-well&#34; or channel 13 (for example 100-500 micron wide) to prepare a site for the laser structure. By way of example, the depth of the channel may be on the order of about 2-20 microns. Then by employing VPE (vapor phase epitaxy) or LPE (liquid phase epitaxy) growth the desired n-doped AlGaAs-GaAs-AlGaAs-GaAs epitaxial layers 14, 15, 16 and 17 are grown, as shown in FIG. 1b, over the planar surface 12 and the well 13. The thickness of the epitaxial layers 14-17 are adjusted such that a subsequent chemical-mechanical polishing step produces a new planar surface 18, shown in FIG. 1c, where the GaAs:Cr substrate is exposed and the layers 14, 15, 16 and 17 are preserved and remain only in the channel 13. These epitaxial layers grown in the wide well will provide the laser structure. The mechanical polishing exposes the GaAs:Cr substrate over the remainder of the planar surface 18. A mask of Si 3  N 4  over the planar surface 18 with a window over a portion of the well area provides for a selective diffusion 19 of Zn into the well structure, as shown in FIG. 1d, in order to make the required p+, p and n regions needed for a transverse junction stripe (TJS) laser structure in the well. In one preferred embodiment the Al x  Ga 1-x  As layers are x=0.35 and the GaAs layer 15 was Te-doped to a carrier concentration of approximately 2×10 18  cm -3  on a substrate of GaAs oriented &lt;100&gt;. The Zn diffusion extends down through the epitaxial layers 17, 16, 15 and 14 to provide a p-n junction in the active GaAs layer 15. 
     Prior to MESFET fabrication by direct ion implantation the Si 3  N 4  is removed and the surface is etched to remove surface damage. When the multiplexer and driver 20 are completed, FIG. 1e, the final processing steps are carried out to complete the laser. For illustrative purposes a TJS DBR (distributed Bragg reflector) laser 21 is shown in FIG. 1f. In the process steps outlined above, all contact lithographic processes are performed on planar surfaces to eliminate the problems encountered in processing non-planar surfaces such as non-uniform photoresist spin-on and mask alignment. Further, the temperature cycling, chemical etches and mechanical stress involved in the fabrication of the MESFETs and the lasers are compatible. Still further, since all the active components are at the surface, the electronic and the optical components can be developed independently during initial stages. The planar structure provides direct access to all the active components and facilitates testing and heat sinking. 
     FIG. 2 shows the details of a GaAs p-n homojunction emitter in the form of a Transverse junction stripe light emitting diode or laser diode fabricated in the well. The layer 10 shows the semi-insulating substrate, then the epitaxial layers including the AlGaAs confining layer 14, the GaAs active layer 15, the AlGaAs confining layer 16 and the GaAs cap layer 17. The layers 14-17 as deposited are n-layers. The cross-hatched portion of FIG. 2 represents the Zn diffused area, which is p doped, with the junction area being indicated by the dashed line 30. The heavy line 31 is indicative through the predominant current flow path through the diode. The junction can also be formed by the ion implantation of Be in place of the diffusion of Zn. FIG. 3 is a graphical presentation of spectral characteristics and plots the relative emission intensity of the diode on the ordinate axis versus wavelength (A°). It can be seen that in this AlGaAs-GaAs device the emission curve peaks at about 9000 A°. 
     FIGS. 4 and 5 are photomicrographs of a cross-section of a transverse junction structure grown in an etched wide well. FIG. 4 shows the &#34;as grown&#34; AlGaAs, GaAs, AlGaAs structure on the substrate. Prior to the heteroepitaxial growth, wide-wells of 450 microns×10 microns were formed by chemical etching in a &lt;100 &gt; oriented substrate structure. Subsequently a three layer TJS structure (Al x  Ga 1-x  As=0.35; GaAs active layer; Al x  Ga 1-x  As=0.35) was grown by liquid phase epitaxy on the entire surface as shown in FIG. 4. While the active layer is shown as GaAs throughout the description, it may also be Al x&#39;  Ga 1-x&#39;  As where the x&#39; is not equal to zero. The planar surface was then restored, as shown in FIG. 5, by a chemical-mechanical polish in which the substrate is again exposed except in the well where the layered structure remains. The Zn diffusion described above can be performed either before or after restoration of the planar surface, however, it is preferable to perform the Zn diffusion with the epitaxial layers on the wafer to help protect the GaAs:Cr substrate. 
     Although the planar structure solid state electro-optic element has been described as fabricated in a GaAs system it may also be fabricated in the same way in a InP (indium phosphide) system. In this case the substrate is InP and the epitaxial layers may be the quaternary semiconductor InGaAsP of the general composition In 1-x  Ga x  P 1-z  As z  and in which the respective values of x and z may differ from one layer to another and where 0≦x≦1 and where 0≦z≦1.

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