Possible applications of scanning frequency comb microscopy for carrier profiling in semiconductors

Abstract

In scanning frequency comb microscopy, a mode-locked ultrafast laser is focused on the tunneling junction of a scanning tunneling microscope to generate a microwave frequency comb (MFC) with hundreds of measurable harmonics that is superimposed on the dc tunneling current when the sample is metallic. With semiconductor samples, each laser pulse creates a surface charge that may have a radius of less than 1 nm, and this charge is rapidly dispersed by intense electrostatic repulsion. Time or frequency-domain measurements of the resulting pulse train with semiconductors, or hyperspectral measurements of the MFC with metals, may be used to determine the properties near the surface of a sample with atomic resolution.

"Inside-Out" Approach for Sub-nm Resolution Carrier Profiling in Semiconductors

“…However, the spreading capacitance [15] must also be considered at the high frequencies of the harmonics in the MFC. Simulations suggest that measurements of the attenuation of the MFC would be sensitive to the carrier density in a semiconductor [14].…”

“…We have modeled a method for carrier profiling that is related to SSRM but has the advantages just described in using an STM tunneling junction for one contact [13] as well as the advantages of the "inside-out" approach with the MFC [14].…”

Simplified calculations of the lateral distribution for the current in tunnelling junctions having general shapes

“…The surfaces of the sample and tip are given by (4) and (5) where A, B, C, and D are constants. This model is a first approximation to a sample with atomic structure hence we set A = 0.13 nm and B = 2π/(0.25 nm) to be consistent with typical atomic sizes and spacing.…”

“…Introduction: Others have used different methods to determine the lateral distribution of the tunnelling current in a scanning tunnelling microscope (STM) to understand the high-resolution in surface topography [1][2][3]. Our objective is to develop a fast and accurate method to do this for simulation and control in new methods for characterising metals [4] and semiconductors [5].…”

Resolution in Carrier Profiling Semiconductors by Scanning Spreading Resistance Microscopy and Scanning Frequency Comb Microscopy

“…These two procedures are generally used together because dopant profiling verifies that the design was followed and carrier profiling validates the dopant profile and verifies the activation of the dopant atoms. Atom probe tomography provides sub-nm resolution in dopant profiling (Kelly & Miller, 2007) and we consider new technology to complement this with finer resolution in carrier profiling (Hagmann et al, 2015). The present method of scanning spreading resistance microscopy (SSRM) as well as our new technology of scanning frequency comb microscopy (SFCM) are defined in the following two paragraphs.…”