Non-Alloyed Ohmic Contacts to n-GaAs using Epitaxial Ge Layers
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T, SAWADA*, W. X. CHEN-, E. D. MARSHALL*, K. L. KAVANAGH**, T. F. KUECH-**, C. S. PAI* AND S. S. LAU* * University of California at San Diego, La Jolla, CA 92093 ** Cornell University, Ithaca, N. Y. 14853 *** IBM Watson Research Center, Yorktown Heights, N.Y. 10598 ABSTRACT Alloyed ohmic contacts (i.e. Au-Ge-Ni) to n-GaAs lead to non-planar interfaces which are unsuitable for devices with shallow junctions and small dimensions. In this study, the fabrication of non-alloyed ohmic contacts (via solid state reactions) is investigated. A layered structure involving the solid phase epitaxy of Ge using a transport medium (PdGe) is shown to produce low (1 - 5 x l0-6fl cm2) and reproducible values of contact resistivity. The resultant interface is shown to be abrupt by cross-sectional transmission electron microscopy. INTRODUCTION The most commonly used ohmic contact to n-GaAs involves the alloyed Au-Ge-Ni systemt1 1 . The alloying process leads to a complex reacted structure with a non-planar interface 12,31.As device dimensions decrease and shallow junction devices become more prevalent, the desired ohmic contacts are laterally confined and do not penetrate into the substrate to cause electrical shorts through the junctions. Non-alloyed contacts in which the reaction with the substrate is non-existent or uniform and very limited in scope will satisfy these requirements. Previous schemes have included the degenerate doping of the GaAs surface and the use of heterostructures for bandgap engineeringt 4,51. These contact structures are designed to circumvent the largest limiting factor in the formation of an ohmic contact to n-GaAs: The barrier height of metal/n-GaAs (Gn) contact is pinned at approximately 0.8 eV[61. The tunneling current component can be enhanced by the formation of a 71 metal/GaAs (n++) structure by degenerately doping the GaAs surface during growthM or after a limited reaction between the metallization and the substratet8,'9,°, 1 1. Solid state reacted metal/Ge[ 81 t 9 10 t and germanide thin films , ,Il on n-GaAs have displayed ohmic behavior after relatively high temperature annealing ( > 450 0C). It is believed that these contacts become ohmic only after diffusion of Ge into GaAs191. The heterostructure contacts substitute a low energy barrier for the high barrier at the metal/GaAs contact junction. The lowest value of contact resistivity to n-GaAs ever quoted (pc < I0r- fl -cm 2) resulted from a Ge(n+, 2 x 1019 cm- 3 ) /GaAs(n÷, 2 x 1018 cm- 3 < 100>) heterostructure by MBE161. The success of this scheme lies in the reportedly low value of the Ge/GaAs< 100> conduction band discontinuity (AE,c < 0.3eVi3,141), the lower barrier height of metal overlayers on Ge ( < 0.6eVt l5 t ), and the high doping level obtained in the Ge epilayer. The tunneling current due to the metal/n+ Ge contact is greater than that through a metal/n+ GaAs contact. An alternative approach to ohmic contact formation to n-GaAs which does not require ultrahigh vacuum is the growth of a Ge layer on n-type GaAs< 100> by the method of s
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