Si Dopant Behavior in InGaAs

As devices continue to shrink there is interest in the possibility of using compound semiconductors to replace Si and Ge for certain parts of the transistor.   InAs and InGaAs are of particular interest.  The contact resistance issue continues to be one of the largest challenges to future devices.  In order to minimize contact resistance it is critical that the doping of the semiconductor be maximized in the contact regions. The best n-type dopant to date for InGaAs has been silicon.  We have studied a number of methods of introducing Si including implantation and MBE doping.  After annealing all samples saturate at the same doping level of around 1.5 x 10¹⁹/cm³.   Several models have been proposed in the past to explain the saturation of Si doping in GaAs.  These include the concept of a solid solubility and saturation through precipitate formation, the amphoteric nature of Si and self-compensation through the formation of Si_Ga - Si_As pairs and finally the possibility of compensation through the formation of vacancies, which bind to the active Si making a Si_Ga-V_Ga pair.

This talk will review a number of recent studies exploring the highly unusual doping and diffusion behavior of Si In InGaAs.  Epitaxial-grown [001] In_0.53Ga_0.47As samples were either implanted with silicon at 80°C, or the Si doping was grown into the InGaAs.  The peak concentration values were > 5×10¹⁹ cm^-3 in both cases. The samples were capped with 15nm of Al₂O₃ by ALD to prevent surface degradation and annealed between 550°C and 750°C. The activation and diffusion studies show that the precipitation model does not explain the observed diffusion behavior.   A set of co-implant experiments comparing Al and P co-implants show that there are no notable changes in activation from co-implanting elements.  The self-compensation model does not explain the observed results and it also does not explain the diffusion behavior.  In addition co-implants with Sulfur also do not support the self-compensation model. 

This leaves the third explanation of a vacancy complex. Prior InGaAs and GaAs DFT calculations suggest that the negatively charged vacancy-Si pair is the primary contributor for Si diffusion in InGaAs.  Modeling of the diffusion of Si in InGaAs requires an unusual point defect population that can be explained if there is a decrease in the formation energy for vacancies coupled with percolation theory for reduced migration energy of the vacancies. Vacancy formation can explain doping saturation, the diffusion behavior, the co-implant results.  Finally implant damage was studied by TEM.  It is shown that the implant defects are extrinsic and thus much like implants into bulk silicon there is an excess of interstitials associated with the implant process.  These interstitials are shown to reduce the diffusivity of Si in InGaAs.  As a final test defects are intentionally introduced into Si doped regions and their behavior is consistent with a vacancy rich region existing around the diffusing silicion. 

All these experiments are systematically compared to the proposed models and it is shown that only the vacancy complex model can explain all of the observed results.  THus it is suggested that the formation of vacancy complexes is the reason for satuation of Si doping in InGaAs.As devices continue to shrink there is interest in the possibility of using compound semiconductors to replace Si and Ge for certain parts of the transistor.   InAs and InGaAs are of particular interest.  The contact resistance issue continues to be one of the largest challenges to future devices.  In order to minimize contact resistance it is critical that the doping of the semiconductor be maximized in the contact regions. The best n-type dopant to date for InGaAs has been silicon.  We have studied a number of methods of introducing Si including implantation and MBE doping.  After annealing all samples saturate at the same doping level of around 1.5 x 10¹⁹/cm³.   Several model have been proposed in the past to explain the saturation of doping of Si in GaAs.  These include the concept of a solid solubility and saturation through precipitate formation, the amphoteric nature of Si and self-compensation through the formation of Si_Ga Si_As pairs and finally the possibility of compensation through the formation of vacancies, which bind to the active Si making a Si_Ga-V_Ga pair.

This talk will also review a number of recent studies exploring the highly unusual doping and diffusion behavior of Si In InGaAs.  Epitaxial-grown [001] In_0.53Ga_0.47As samples were either implanted with silicon at 80°C, or the Si doping was grown into the InGaAs.  The peak concentration values were > 5×10¹⁹ cm^-3 in both cases. The samples were capped with 15nm of Al₂O₃ by ALD to prevent surface degradation and annealed between 550°C and 750°C. The activation and diffusion studies show that the precipitation model does not explain the observed diffusion behavior.   A set of co-implant experiments comparing Al and P co-implants show that there are no notable changes in activation from co-implanting elements.  The self-compensation model does not explain the observed results and it also does not explain the diffusion behavior.  In addition co-implants with Sulfur also do not support the self-compensation model. 

This leaves the third explanation of a vacancy complex. Prior InGaAs and GaAs DFT calculations suggest that the negatively charged vacancy-Si pair is the primary contributor for Si diffusion in InGaAs.  Modeling of the diffusion of Si in InGaAs requires an unusual point defect population that can be explained if there is a decrease in the formation energy for vacancies coupled with percolation theory for reduced migration energy of the vacancies. Vacancy formation can explain doping saturation, the diffusion behavior, the co-implant results.  Finally implant damage was studied by TEM.  It is shown that the implant defects are extrinsic and thus much like implants into bulk silicon there is an excess of interstitials associated with the implant process.  These interstitials are shown to reduce the diffusivity of Si in InGaAs.  As a final test defects are intentionally introduced into Si doped regions and their behavior is consistent with a vacancy rich region existing around the diffusing silicion. 

All these experiments are systematically compared to the proposed models and it is shown that only the vacancy complex model can explain all of the observed results.  THus it is suggested that the formation of vacancy complexes is the reason for satuation of Si doping in InGaAs.