• PDF / 506,963 Bytes
• 5 Pages / 612 x 792 pts (letter) Page_size
• 71 Downloads / 237 Views

DOWNLOAD

REPORT

---

Vacancy and oxygen behavior in carbon highly doped silicon

Pierre Lavéant, Peter Werner, Norbert Engler, and Ulrich Goesele, Max Planck Institute of Microstructure Physics Weinberg 2, D-06120 Halle, Germany

ABSTRACT Carbon doping of silicon has gained interest since in high concentrations, carbon can reduce or even suppress undesirable diffusion of the base dopant boron in silicon-based bipolar transistors. This behavior can only be understood in taking into account the silicon point defects i.e. vacancies and self-interstitials. In this work, we observe the oversaturation of vacancies produced by a high carbon concentration during annealing. Experiments with a vacancy diffusing dopant, Antimony, are shown and prove this effect : in a carbon rich sample, the antimony diffusion is enhanced about 8 times compared to samples with a much lower carbon concentration. We also investigate the carbon coprecipitation with oxygen. The carbon precipitation, as SiC, is facilitated with a high oxygen concentration. We explain this affinity by an exchange of point defects and a volume compensation. Finally, we show the precipitation of oxygen in relation to the vacancy oversaturation at 900°C. INTRODUCTION Considered as well understood and as a benign impurity, carbon gained some interest when incorporated in high concentration in the silicon lattice. Since carbon contracts the silicon lattice considerably it can be used for gap engineering or for lattice engineering, i.e. to elaborate new substrates for better epitaxy. But the main potential of high carbon concentrations may lie in the control of dopant diffusion. The suppression by carbon incorporation of the so-called “transient enhanced diffusion” of boron opened up the possibility to fabricate Si-Ge based heterobipolar transistors with a very narrow base region which is of enormous interest for ultra-high frequency applications [1]. This effect can only be fully understood and modeled in taking into account the silicon point defects i.e. vacancies (V) and self-interstitials (I) [2]. In the present study, we simulate the carbon diffusion and discuss its influence on the selfinterstitials and vacancies. MBE-grown samples with a high carbon concentration and doped with a vacancy diffusing element are annealed. We modeled the carbon out-diffusion by kick-out and Frank-Turnbull mechanism. In addition, the co-precipitation of carbon and oxygen is also investigated and we try to explain the effect of oxygen on the SiC precipitation and then the behavior of oxygen in a vacancy oversaturated layer. SIMS depth profile and cross-sectional TEM are also used. CARBON DIFFUSION Carbon is mostly substitutionally dissolved in the Si lattice, Cs. The solubility or thermal equilibrium concentration of this electrically neutral specie is given by [3] 24 -3 eq (1) C S = 4× 10 exp(−2.3 / k B T) cm Nevertheless, to understand the carbon diffusion one has to take into account the carbon interstitial, denoted here by Ci, which is orders of magnitudes more mobile than Cs [4]: J4.3.1

(2) cm /s D =