Diffusion of Be in MBE-GaAs structures

Beryllium is a common p-type dopant in GaAs grown by molecular beam epitaxy (MBE) since it allows very high hole concentrations to be achieved without degradation of the surface morphology. This feature is very important in the fabrication of GaAs-based n/p/n heterojunction bipolar transistors (HBT) where highly doped p-type base layers are required to fully exploit the device capabilities. Unfortunately, the high Be diffusivity is a strong disadvantage in these devices since the undesired diffusion of Be during either growth or high temperature device fabrication results in a dramatic degradation of the device performances. However it has been shown that the combined use of reduced substrate temperatures and high As/Ga flux ratios during base layer deposition is effective in reducing Be diffusion during MBE growth as well as during post-growth annealings performed at 800 C for 7 s. The Authors have recently reported the results of an investigation performed by Secondary Ion Mass Spectrometry (SIMS) on p/p+ and p/p +/p GaAs structures which underwent rapid thermal annealing (RTA) experiments [1]. In particular it has been shown that after annealing at 850 C for 30 s i) Be diffusion is faster in p/p+/p structures than in p/p+ ones and ii) the As4/Ga flux ratio used during the MBE growth affects Be diffusion only in p/p+ structures. These results have been qualitatively discussed in the frame of the Substitutional-Interstitial Diffusion (SID) model. In order to give a more accurate description of these results, Be diffusion in GaAs structures has been modeled by following a modified version of an approach that has been first proposed for Zn and Be diffusion in GaAs [2] and has been successfully used to model Be diffusion in InGaAs/InP [3] and InGaAs/InGaAsP heterostructures [4]. In this communication, the results of SIMS measurements performed after having annealed at 850 C for 30 s the p/p+ and p/p+/p structures are briefly reviewed. The modeling procedure is then illustrated and the simulation results are reported and discussed, also considering the influence of the boundary conditions on the modelled Be profiles. It is shown that commonly-used hypotheses do not allow us either to simulate Be diffusion in both p/p+ and p/p+/p structures, or to account for the influence of the As/Ga flux ratio on Be diffusivity observed in p/p+ structures. In contrast, a satisfactory description of the experimental results can be achieved by accounting for non-equilibrium conditions. [1] R. Mosca et al., Mater. Sci. Eng. B 80, 32 (2001) [2] S. Yu et al., J. Appl. Phys. 69, 3547 (1991) [3] M. Ihaddadene et al., Mater. Sci. Eng. B 80, 73 (2001) [4] K. Ketata et al., Physica B 273-274, 823 (1999)