Princeton University

School of Engineering & Applied Science

Spatially resolved doping profiles of in-situ and ex-situ doped nanowires

Iddo Amit, Tel Aviv University, Department of Physical Electronics
Engineering Quadrangle, B418
Thursday, July 24, 2014 - 11:00am

Semiconductor nanowire-based electronics offers the opportunity to achieve tight control over a precisely defined component, often comprised of the nanowire (NW) itself. This, in turn, enables the enhancement of the device performance. Currently, one of the main challenges facing the fabrication process of NW-based electronics is to obtain well-defined doping profiles which are crucial for the optimization of such devices. 
However, the conventional doping mechanism, where dopants are introduced in-situ during growth, results in both axial and radial inhomogeneous doping profiles which stem from a surface doping mechanism known as vapor-solid (VS) doping. Moreover, at the interfaces between different types of doping, a region of diffuse boundaries is formed rather than an abrupt transition between the two segments. 
We use quantitative Kelvin probe force microscopy (KPFM) and nano-probe scanning Auger spectroscopy to measure both the longitudinal and the radial doping distribution in doped Si nanowires (SiNWs). Our findings shed light on the underlying mechanisms that produce these inhomogeneities by studying P doping profiles of axially modulation-doped SiNWs. We find that both the VLS and the VS mechanisms result in radially inhomogeneous doping, specifically, a lightly doped core surrounded by a heavily doped shell structure. By designing a modulated doping profile, the effects of the two mechanisms can be distinguished. We also discuss the influence of the reservoir effect that significantly broadens the axial doping junctions. 
These results are compared to measurements conducted on monolayer contact doped (MLCD) NWs. This ex-situ doping process takes advantage of the precision of electron beam lithography as well as the selectivity and controllability of chemical monolayer formation to produce tailor-made doping profiles by post-growth incorporation of doping atoms through the surface.