F+ co-implantation at different doses and energies was performed into GaAs already implanted with Be+ at high dose (1015 cm−2) and low energy (20 keV), in order to reduce the beryllium diffusion during post-implant annealing. The redistribution behavior of Be and associated electrical effects were studied by secondary-ion mass spectrometry, transmission electron microscopy (TEM), Hall effect measurements, and current-voltage profiling. Be outdiffusion was reduced by co-implantation of F; more than 80% of the implanted Be was retained during rapid thermal annealing up to 850 °C. The dose and energy of the F implant strongly influenced Be electrical activation efficiency. High activation, up to 48.5%, was obtained when F was co-implanted at high dose (1015 cm−2) and low energy (10 keV). Hole profiles shown reduced electrical activation in the region where F and Be profiles overlapped and TEM studies indicated the formation of {111} coherent plates, possibly BeF2 precipitates, in the same region. The reduction of Be outdiffusion in F co-implanted samples led to high activation after annealing, and was believed to be due to chemical interaction between Be and F.
Thermally stable, low-resistance p-type ohmic contacts have been developed by depositing NiInW metals on GaAs substrates in which Be and F were coimplanted. The contacts provided resistances of about 1.4 Ω mm after annealing at temperatures in the range of 300–800 °C for short times. The electrical properties did not deteriorate after annealing at 400 °C for more than 100 h, which far exceeds the requirements for current GaAs device fabrication. The present study demonstrated for the first time that thermally stable, low-resistance ohmic contacts to both n- and p-type GaAs can be fabricated using the same metallurgy. In addition, NiInW ohmic contacts were prepared by simultaneous (one-step) annealing for ion-implant activation and contact formation, which simplifies significantly the device fabrication process. A factor-of-2 reduction of the contact resistances was achieved by slight etching of the GaAs surface prior to the contact metal deposition so that the metal/GaAs interface contacted the peak position of the Be concentration in the GaAs substrate. Another method used to reduce the contact resistance was to add a small amount of Mn to the NiInW metals: the resistance decreased with increasing amounts of Mn. The contacts had smooth morphology and shallow depth, less than 70 nm, which is desirable for very-large-scale integration device application.
For GaInAs/InP junction field effect transistors as well as heterojunction bipolar transistors, the achievement of very low resistivity P type ohmic contact is a very critical step because the Schottky barrier height on these materials is quite high. The realization of a highly doped P+ layer by Zn diffusion in a semi-closed box and the use of MnAu alloy contact have allowed to solve these difficulties : in fact, a contact resistivity as low as 10-7 Ω cm2 has been obtained
Recently, thermally stable, low resistance In-based ohmic contacts to n-type GaAs have been developed in our laboratories by depositing a small amount of In with refractory metals in a conventional evaporator, followed by rapid thermal annealing. By correlating the interfacial microstructure to the electrical properties, InxGa1-xAs phases grown epitaxially on the GaAs were found to be essential for reduction of the contact resistance (Rc). This low resistance was believed to be due to separation of the high barrier (φb) at the metal/GaAs contact into two low barriers at the metal/InxGa1-xAs and InxGa1-xAs/GaAs interfaces. In this paper the effects of the In concentration (x) in the InxGa1-xAs phases and addition of dopants to the contact metal are presented. High In concentration is desirable to reduce the φb at the metal/InxGa1-xAs interface. Such contacts were prepared by sputter-depositing InAs with other contact elements, but the low Rc values were not obtained. The reason was explained to be due to an increase in the φb at the InxGa1-xAs/GaAs interface due to the formation of misfit dislocations. However, addition of a small amount of Si to the contact metals reduced significantly the Rc value. This contact demonstrated excellent thermal stability: no deterioration was observed at 400°C for more than 100 hrs. In addition, the use of this Ni(Si)InW contact metal allowed us to fabricate the low resistance ohmic contacts by one-step (simultaneous) annealing for “implant-activation” and “ohmic contact formation”, which simplifies significantly GaAs device fabrication process steps. For p-type ohmic contacts, low resistance contacts were fabricated by depositing the same NilnW contact material to p-type GaAs. This contact was also thermally stable during subsequent annealing at 400°C. Within our knowledge this is believed to be the first demonstration of low resistance, thermally stable ohmic contact fabrication using the same materials for both n and p-type GaAs.
The effects of Si or Ge addition to NiInW ohmic contacts on their electrical behavior were studied, where the samples were prepared by evaporating Ni(Si) or Ni(Ge) pellets with In and W and annealed by a rapid thermal annealing method. An addition of Si affected the contact resistances of NiInW contacts: the resistances decreased with increasing the Si concentrations in the Ni(Si) pellets and the lowest value of ∼0.1 Ω mm was obtained in the contact prepared with the Ni-5 at. % Si pellets after annealing at temperatures around 800 °C. The contact resistances did not deteriorate during isothermal annealing at 400 °C for more than 100 h, far exceeding process requirements for self-aligned GaAs metal-semiconductor field-effect-transistor devices. In addition, the contacts were compatible with TiAlCu interconnects which have been widely used in the current Si process. Furthermore, the addition of Si to the NiInW contacts eliminated an annealing step for activation of implanted dopants and low resistance (∼0.2 Ω mm) contacts were fabricated for the first time by a ‘‘one-step’’ anneal. In contrast, an addition of Ge to the NiInW contacts did not significantly reduce the contact resistances.
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