Development of refractory ohmic contact materials for gallium arsenide compound semiconductors

Recent strong demands for optoelectronic communication and portable telephones have encouraged engineers to develop optoelectronic devices, microwave devices, and high-speed devices using hetero-structural GaAs-based compound semiconductors. Although the GaAs crystal growth techniques had reached a level to control the compositional stoichiometry and crystal defects on a nearly atomic scale by the advanced techniques such as molecular beam epitaxy and metal organic chemical vapor deposition techniques, development of ohmic contact materials (which play a key role to inject external electric current from the metals to the semiconductors) was still on a trial-and-error basis.Our research efforts have been focused to develop low resistance, refractory ohmic contact materials to n-type GaAs using the deposition and annealing techniques, and it was found the growth of homo-or hetero-epitaxial intermediate semiconductor layers (ISL) on the GaAs surface was essential for the low resistance ohmic contact formation. In this paper, two typical examples of ohmic contact materials developed by forming ISL were given. The one was refractory NiGe-based ohmic contact material, which was developed by forming the homo-epitaxial ISL doped heavily with donors. This heavily doped ISL was discovered to be formed through the regrowth mechanism of GaAs layers at the NiGe/GaAs interfaces during annealing at elevated temperatures. To reduce the contact resistance further down to a value required by the device designers, an addition of small amounts of third elements to NiGe, which have strong binding energy with Ga, was found to be essential. These third elements contributed to increase the carrier concentration in ISL. The low resistance ohmic contact materials developed by forming homo-epitaxial ISL were Ni/M/Ge where a slash '/' denotes the deposition sequence and M is an extremely thin (~5 nm) layer of Au, Ag, Pd, Pt or In. The other was refractory InxGa1−xAs-based ohmic contact materials which were developed by forming the hetero-epitaxial ISL with low Schottky barrier to the contacting metals by growing the InxGa1−xAs layers on the GaAs substrate by sputter-depositing InxGa1−xAs targets and subsequently annealing at elevated temperatures. To reduce the contact resistance, it was found that this InxGa1−xAs (ISL) layer had to have In compositional gradient normal to the GaAs surface: the In concentration being rich at the metal/InxGa1−xAs interface and poor close to the InxGa1−xAs/GaAs interface. This concentration graded ISL reduced both the barrier heights at the metal/ISL and ISL/GaAs interfaces and reduced the contact resistance. The ohmic contact materials developed by forming hetero-epitaxial ISL was In0.7Ga0.3As/Ni/WN2/W. These contact materials formed refractory compounds at the interfaces, which was also found to be essential to improve thermal stability of ohmic contacts used in the GaAs devices.