Abstract:
The present invention relates to an electrode forming process for forming an ohmic contact on a compound semiconductor crystal of a GaAs-based material having p-type conductivity. The process includes a first step of depositing a thin Pt layer having a thickness larger than 50 Å on the compound semiconductor crystal, and a second step of depositing a Ti/Pt/Au electrode on the Pt layer.

Description:
This application is a continuation of application Ser. No. 08/026,075 filed Mar. 4, 1993 abandoned, which is a continuation of application Ser. No. 07/674,201, filed Mar. 25, 1991, abandoned, which applications are entirely incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an electrode forming process for forming an ohmic electrode on a compound semiconductor having a substrate of a p-type GaAs-based material. 
     2. Related Background Art 
     Conventionally as ohmic contacts on p-type GaAs and p-type Al x  Ga 1-x  As, Ti/Pt/Au and Cr/Au electrode structures have been studied as the non-alloy type, and AuZn, AuMn, AuBe, Pt/Zn/Au and Pd/Zn/Pd/Au electrode structures have been studied as the alloy type. Specifically, for example, Japanese Patent Laid-Open Publication No. SHO59-189669 describes forming an ohmic contact by alloying of Pt and GaAs. Extended Abstract 5p-G-16 (The 49th Spring Meeting, October 1988); The Japan Society of Applied Physics and Related Societies) describes forming an ohmic contact being Pt/Zn ohmic contact. 
     Requirements of these electrodes are 1) exhibition of low resistances, 2) flat and smooth surfaces, 3) no diffusion of electrode materials into substrates, 4) long-lasting reliability, and others. 
     Generally the non-alloy ohmic contact satisfies the above-described requirements 2 to 4 but, in comparison with the alloy ohmic contact, does not exhibit sufficiently low contact resistance. However the alloy ohmic contact exhibits sufficiently low contact resistance owing to increased surface impurity concentrations due to diffusion of active p-type dopants contained in the electrode materials. Resultantly a sufficiently low contact resistance whose specific resistance is below 10 -6  Ω cm 2  can be obtained. However problems with the alloy ohmic contact are diffusion of impurities or electrode materials into the substrate, and low reliability and so on. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a process for forming an electrode which satisfies all the above-described four requirements. 
     A further object of this invention is to provide an electrode forming process for forming an ohmic contact on a compound semiconductor crystal of a GaAs-based material leaving p-type conductivity. This process comprises the first step of depositing a thin Pt layer having a thickness equal to or larger than 50 Å on a compound semiconductor crystal; and the second step of depositing a Ti/Pt/Au electrode on the Pt layer. This electrode forming process interposes a thin layer of Pt, whose work function is large, between the GaAs-based semiconductor and the Ti/Pt/Au electrode, whereby an ohmic electrode characterized by low contact resistance, flat smoothness, stability and high reliability can be obtained. In comparison with the art of interposing a Pt layer between a GaAs-based semiconductor and a ZnAu-based electrode, this process has an advantage that a semiconductor cap layer, e.g. InGaAs layer, enables the ohmic contact with an n-type GaAs-based semiconductor. 
     Another object of this invention is to provide an electrode forming process for forming an electrode having a Pt layer having a thickness larger than 50 Å and smaller than 200 Å (exclusive of 200 Å). 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of an electrode formed by the process according to this invention; 
     FIG. 2 is a graph showing changes in the contact resistance when the thickness of the Pt layer between the p +  -GaAs layer and the Ti/Pt/Au electrode is varied from 0 to 400 Å; 
     FIG. 3 is an Arrehenius plot for a life-time test with the average life set at a 1.5 time rise of a contact resistance value; and 
     FIG. 4 shows a μ-AES profile of the electrode of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of this invention will be explained with reference to the drawings attached hereto. 
     In the electrode structure according to this embodiment, a p +  -GaAs layer 2 is formed on a semi-conducting GaAs substrate 1, and on the p +  -GaAs layer 2 a Pt layer 3 is formed in contact therewith. On the Pt layer 3 there are formed a Ti electrode layer 41, a Pt electrode layer 42, and an Au electrode layer 43 in this stated order. The Pt layer 3, interposed between the GaAs layer 2 and the Ti/Pt/Au layer 4 consisting of the layers 41, 42 and 43, has a thickness t 1  equal to or larger than 50 Å and smaller than 400 Å. The semiconductor (p +  -GaAs) layer 2 can be formed by ion implantation, epitaxial growth or others and is effectively applicable to contact electrodes on p-type GaAs-based semiconductors composing p-channel FETs, diodes (including lasers), and bipolar transistors. It is also possible to use ohmic contacts for n-type GaAs-based semiconductors by providing an InGaAs cap layer below the electrode. The process per se for forming the InGaAs cap layer below the electrode is disclosed by Koichi Nagata et al. (IEEE TRANSACTIONS ON ELECTRON DEVICES, vol. ED-35, No. 1, p. 2, January, 1988). In contrast to this, the ZnAu electrode disclosed by Eiji Murata and Isao Kamo can be only used as ohmic contacts of p-type GaAs-based semiconductors (Japanese Patent Laid-Open Publication No. SHO 59-189669). 
     FIG. 2 shows changes of the contact resistance in the case where the film thickness of the Pt layer 3 between the p +  -GaAs layer 2 and the Ti/Pt/Au electrode 4 is varied from 0 to 400 Å. The contact resistance is lowered to below 1/3 by making the Pt layer 3 more than 50 Å thick, and when the thickness was above 50 Å, to 100 Å, the dispersion value became small. This proves that the insertion of the Pt layer 3 is sufficiently effective. 
     In terms of the contact resistance, a good result was obtained when the thickness of the Pt layer was 400 Å. However, it is preferable for the ohmic electrode in the field that this invention relates that the depth of the reaction of the Pt in the GaAs semiconductor is smaller than 500 Å, more preferably smaller than 400 Å. In view of this it is preferable that the Pt layer 3 has a thickness larger than 50 Å, but smaller than 200 Å exclusive of 200 Å. According to the Naotaka Uchitomi et al. report (Extended Abstract 7a-D-10 (The 29th Spring Meeting 1982; The Japan Society of Applied Physics and Related Societies) or the V Kumar report (J. Phys. Chem. Solids, 1975, vol. 36, pp.535-541), the solid phase reaction of Pt into the GaAs-based semiconductor advances up to a depth of about twice a deposition thickness of the Pt and stops there. Accordingly, when the required reaction depth of the Pt is 500 Å, the thickness of the Pt layer 3 to be deposited is 250 Å, and when the reaction depth of the Pt is 400 Å, the thickness of the Pt layer 3 is 200 Å. 
     The inventors have made a study of this point in connection with FIG. 3. FIG. 3 is an Arrhenius plot of degradation of the contact resistance in the case where the thickness of the Pt layer 3 on the surface of the GaAs-based semiconductor is varied. The temperature used in a life-time test is on the horizontal axis, and the mean time to failure is taken on the vertical axis. The mean time to failure is a time in which the resistance goes up to 1.5 times an initial contact resistance value. As seen from FIG. 3, when the thickness of the Pt layer 3 is smaller than 200 Å, the activation energy is so high that a long life-time at low temperatures can be expected. The mean time to failure at 300° C. on the extrapolated line in FIG. 3 shows a sufficiently practical life-time of about 10 years. On the other hand, when the thickness of the Pt layer 3 is larger than 200 Å, it is seen that the mean time is shorter compared with that when the thickness is smaller than 200 Å exclusive of 200 Å. The mean time was calculated at 150° C. This result is shown in TABLE 1. It is seen that the mean time becomes very short when the thickness of the Pt layer 3 is larger than 200 Å. The average mean time required for ohmic electrodes of this type is generally higher than 1×10 4  hrs., preferably 1×1O 5  hrs. 
     Furthermore, in the case where the thickness of the Pt layer at the GaAs/Pt interface is 100 Å, its profile with depth after alloying was studied. The result is shown in FIG. 4. The measurement of this profile was conducted by a μ-AES analysis. It is seen that the profile is steep as before the alloying. This shows that low contact resistance is obtained by the GaAs/Pt contact, and no reaction layer will not be necessary between the GaAs and the Pt. 
     An embodiment of the ohmic contact with respect to the p +  -GaAs layer 2 has been explained. However actually by providing a Pt layer with respect to a p +  -Al x  Ga 1-x  As layer, the contact resistance is lowered in accordance with the band theory, and it can be expected that more stable reaction product (PtAs 2 ) will be formed. 
     
                       TABLE 1______________________________________Extrapolated MTTF for various interfaceplatinum thickness at 150° C.InterfacePlatinumThickness           μ50 (150° C.)(Å)             (hour)______________________________________ 0                  3.0 × 10.sup.12 50                 1.2 × 10.sup.12100                 5.2 × 10.sup.11200                 3.0 × 10.sup.4400                 4.2 × 10.sup.4______________________________________ 
    
     From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.