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Type of Document Master's Thesis Author Rai, Shishir K. Author's Email Address shishir.rai@gmail.com URN etd-12152004-143702 Title Zinc Diffusion in GaAsSb from Spin-on Glass Dopant Sources Degree Master of Science in Electrical Engineering Department Electrical Engineering Advisory Committee
Advisor Name Title Patrick Fay Committee Chair Alan Seabaugh Committee Member Debdeep Jena Committee Member Keywords
- diffusion modeling
- dopant activation
- maximum oscillation frequency
Date of Defense 2004-10-15 Availability unrestricted Abstract The first study of p-type doping in GaAsSb lattice-matched to InP from Zn spin-on glass sources is reported. Shallow diffusion profiles suitable for aggressively-scaled heterojunction devices and good electronic transport properties in the doped films have been observed. Potential applications for spin-on doping of p-type GaAsSb include the optimization of the extrinsic base resistance and the contact resistance to the base in GaAsSb/InP HBTs for ultra-high-speed applications, without the use of regrowth or other complex processing.
Diffusions were carried out in a rapid thermal processor (RTP) using epitaxial heterostructures consisting of 2000 Å of p-type GaAs 0.51Sb0.49 grown on semi-insulating InP by MOCVD. The diffusions were performed at temperatures ranging from 350-625 °C and diffusion times of up to 30 minutes. SIMS depth profiling indicates that Zn starts to diffuse into GaAsSb layer at temperatures as low as 350 °C, forming very shallow diffusion profiles. For higher temperatures deep diffusion profiles extending up to 2500 Å were obtained. The Zn appears to remain largely electrically inactive for diffusions at temperatures below 500 °C, as indicated by Hall-effect measurements. The threshold temperature at which Zn starts to become electrically active is found to be in the range of 500-550 °C. Hall-effect measurements indicate that the average hole concentration increased by almost three orders of magnitude for the lowest-doped (2.05e16 /cm³) test structure, rising from 2.05e16 /cm³ to 1.67e19 /cm³ for a 30 minute, 600 °C diffusion. The measured sheet resistance decreased from 280 kΩ/sq. to 910 Ω/sq., while the hole mobility was reduced from 59 cm²/Vs as-grown to 26 cm²/Vs due to increased impurity scattering. With increase in as-grown background doping concentration, the diffusion rate was found to increase.
Zn distribution profiles measured by SIMS were modeled and a mechanism for
Zn diffusion in undoped GaAs 0.51Sb0.49 is proposed. Zn diffuses by an interstitial-substitutional mechanism in the region close to the surface but as it moves away from the surface and Zn concentration falls down, it diffuses by substitutional mechanism. Surface diffusivity of 1e13 cm²/s at 600 °C for Zn interstitials is extracted from the diffusion profiles. Zn diffusivity away from the surface is found to be 1.2e14 cm²/s at 600 °C. There is insuffcient evidence to tell whether this diffusivity is that of substitutional Zn or of a complex that Zn atoms form with the surrounding defects.
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