Microstructural analysis of Pd-based ohmic contacts to p -type GaAs
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G. Rajeswaran and L. R. Zheng Corporate Research Laboratory, Eastman Kodak Company, Rochester, New York 14650
M.W. Cole and R.T. Lareau United States Army Electronics Technology and Devices Laboratory, Fort Monmouth, New Jersey 07703 (Received 31 January 1990; accepted 13 November 1990)
As part of the investigation of the use of Pd-based ohmic contacts to p-type GaAs, cross-sectional transmission electron microscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy were used to explore the uniformity at the metal/GaAs interface and its composition profile after ohmic contact formation. Comparisons were made among Au:Be, Au:Be/Pd, and Au/Pd contacts. Regions otp+ were formed in n-type GaAs by a spin-on source which was rapid diffused at 950 °C for 6 s or by ion implantation at a dose of 3 x 1014 atoms/cm2 at 150 keV for 15 min. Metallizations were accomplished by evaporation with a base pressure of 3 x 10"6 Torr. Sintering of the metallizations was done in a furnace at 350 °C for 15 min. Cross-sectional transmission electron microscope studies revealed an absence of spiking when Be is present in the metallization scheme but a broad band diffused into GaAs. An improper metal/GaAs adhesion was observed when Pd is absent. Be assists in confining the reaction of Pd with GaAs and acts as a diffusion barrier to thep+ dopant. Electrical measurements, taken from transmission line and cross bridge Kelvin resistors, were best for the Pd/Au:Be, which yielded a contact resistance of 0.3
I. INTRODUCTION
GaAs and its related compounds are becoming widely used in modern optoelectronic devices such as semiconductor lasers. High current density, utilized in laser operations, requires high quality ohmic contacts. These contacts must exhibit low resistance, thermal and electrical stability, and be compatible in fabrication with other processing steps. In the past, much of the ohmic contact research focused primarily on n-type GaAs. However, becoming increasingly important are ohmic contacts to p-type GaAs, which are more difficult to achieve due to their higher barrier heights and larger effective mass of carriers.1 Both effects reduce the tunneling of charge carriers that is required for ohmic contacts. Conventional contact technology requires the deposition of a metal containing a dopant onto the GaAs. The complete structure is heated and interdiffusion between the metallization and the GaAs occurs. The dopant is believed to heavily dope a thin layer of GaAs immediately beneath the metallization forming a metal/p+-GaAs contact. Some ohmic contacts are based on a GaAs/X/Y/Z configuration. Here, X is a thin metal (e.g., Pt,Ti, Cr, Pd) used primarily for J. Mater. Res., Vol. 6, No. 3, Mar 1991 http://journals.cambridge.org
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adhesion purposes, and Y is an electrically conductive diffusion barrier layer consisting of a eutectic-type metal composition. A typical contact scheme for this layer would involve a metal (e.g., Au or Ag) combined with a dopant element such as Zn or Be. This layer not o
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