Effects of Rapid Thermal Annealing on Heavily Boron Doped Silicon Epitaxial Layers
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ABSTRACT The effects of rapid thermal annealing (RTA) on the electrical properties and the strain relaxation of the heavily boron doped silicon layers grown by molecular beam epitaxy have been studied by Hall effect measurements and double crystal x-ray diffraction. After the RTA treatment at temperatures above I 0000 C, the increases of both carrier concentration and Hall mobility as well as the improvement of crystalline quality of epitaxial layers are acheived. It is verified that the lattice mismatch between the epitaxial layer and the Si substrate is proportional to the concentration of the substitutional boron rather than the total boron concentration. The lattice contraction coefficient 13 is determined to be 5.3 (in units 10- 24cm 3) concentration range of 0.75- 3x10 20 cm- 3 .
in the carrier
INTRODUCTION Boron is extensively employed as a p-type dopant in the Si molecular beam epitaxy (MBE). Due to the smaller atomic radius of boron, the normal lattice constant a(N) of the boron doped Si with dopant concentration N is smaller than the lattice constant ao of virgin Si. It is quite evident that the lattice contraction is induced in the heavily doped epitaxial layer. It has been reported that this contraction is one directional, i.e. along the surface normal of the epitaxial layer.' According to Yukota, 2 the lattice mismatch f depends linearly on N, f=(a(N)-ao)/ao=-13N,
(1)
where 03is the lattice contraction coefficient, which is in the order of 10"24cm 3 . 3 Besides acting as acceptors in the substitutional sites, boron atoms can exist in some electrically inactive forms in the heavily doped epitaxial layers, such as the interstitial boron and boron clusters if the incorporated boron concentration exceeds the solid solubility limit in bulk Si material. 4 All these forms of boron atoms will not only influence the electrical properties, but also play diffeient roles in the lattice distortions, which was suspected to be responsible to the serious discrepancy of measured values of 13reported from the maximum 3 of 6.2 to the minimum 5 of 2. These problems might be investigated in terms of converting the inactive forms into active one by thermal annealing treatments. In this work, we have made a scrutiny into the effects of rapid 637 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
thermal annealing (RTA) on heavily boron doped epitaxial layers by Hall effect measurements and double crystal x-ray diffraction (DCD). The results evince that although the strain relaxation of the epitaxial layers occurs, the increases of free carrier concentration and Hall mobility as well as the amelioration of crystalliniity are achieved by RTA at a temperature higher than 10000 C.
EXPERIMENTAL Heavily boron doped samples were grown on a p- type Si(100) substrate by MBE with coevaporation of B2 0 3 at a substrate temperature of 680°C and growth rate of 0.1 nm/s. High doping concentration was acheived by setting the temperature of the B2 0 3 source to 930 0 C, which is the highest temperature that kept
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