SSRM analysis of dopant diffusion in InP based structures

We have successfully used scanning spreading resistance microscopy (SSRM) for two-dimensional (2D) analysis of MOCVD-grown InP-based semiconductor structures. SSRM is an analytical technique based on atomic force microscopy (AFM), where the instrument is equipped with a conductive tip that is biased relative to the sample. Ideally, the spreading resistance value at the tip contact region of the surface can be derived from the measured electrical current. The spreading resistance is a function, among others, of the local carrier concentration at the surface region surrounding the probe´s tip. While not yet quantitative, SSRM analysis, with its high spatial resolution of 10-30 nm, proved to effectively complement SIMS, an analytical technique whose spatial resolution is limited to dimensions larger by about three orders of magnitude. In particular, we found SSRM effective in detecting low level lateral dopant diffusion, and in assessing the effectiveness of barrier layers in blocking such diffusion. We have used both metal tips and Si tips coated with a thin doped-diamond layer. Differences in images obtained with these two types of tips, as well as in images obtained from different samples of a similar structure, support the proposition that besides the spreading resistance, there is a non-negligible contribution of the tip-surface resistance. Although this complicates the conversion of the SSRM output signal to carrier concentration, quantification could still be attainable via a specially designed calibration procedure. In this paper we present SSRM analysis of InP-based test structures, with n, p, and semi-insulating layers. We studied Fe diffusion from a semi-insulating InP(Fe) layer into an undoped Q layer, and lateral Zn-diffusion from a Zn-doped ridge into a semi-insulating layer through a vertical boundary layer