Zinc Diffusion in GaAsSb from Spin-on Glass Dopant Sources

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 ÌÉ of p-type GaAs _0.51Sb_0.49 grown on semi-insulating InP by MOCVD. The diffusions were performed at temperatures ranging from 350-625 å¡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 å¡C, forming very shallow diffusion profiles. For higher temperatures deep diffusion profiles extending up to 2500 ÌÉ were obtained. The Zn appears to remain largely electrically inactive for diffusions at temperatures below 500 å¡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 å¡C. Hall-effect measurements indicate that the average hole concentration increased by almost three orders of magnitude for the lowest-doped (2.05e16 /cmå_) test structure, rising from 2.05e16 /cmå_ to 1.67e19 /cmå_ for a 30 minute, 600 å¡C diffusion. The measured sheet resistance decreased from 280 kΩ/sq. to 910 Ω/sq., while the hole mobility was reduced from 59 cmå_/Vs as-grown to 26 cmå_/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 _0.51Sb_0.49 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 cmå_/s at 600 å¡C for Zn interstitials is extracted from the diffusion profiles. Zn diffusivity away from the surface is found to be 1.2e14 cmå_/s at 600 å¡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.