Use of scanning probe microscopies to study dopants at semiconductor surfaces

Abstract

Dopants, in semiconductors, are detected as either protrusions or depressions in scanning tunneling microscopy (STM) images. Measured dopant heights for layered semiconductors are considerably larger than for conventional semiconductors. This is interpreted as the influence of dopant induced electrostatic forces between the tip and the sample leading to a structural deformation of the surface around the dopant atoms. To investigate the influence of electrostatic forces, we performed STM measurements on p-type MoS2 at different bias voltages. The bias dependence of the STM images indicates the presence of electrostatic forces. Additional measurements with current imaging tunneling spectroscopy (CITS) show that changes in the density of states at dopant sites play only a minor role and cannot account for the large protrusions observed. Atomic force microscopy (AFM), with an applied D.C. voltage between the cantilever and sample, also confirms the role of electrostatic forces. Recently, we developed a new Tappingmode* AFM (TMAFM) based dopant profiling method based on an electrostatic mechanism similar to the STM imaging of dopants in layered semiconductors. TMAFM with an applied bias was used to spatially resolve areas of different doping type and density on silicon patterned via ion implantation. The application of a D.C. bias between the cantilever and sample during the measurement results in a Coulomb interaction between the tip and sample, whose magnitude depends on the spatial variation in the doping density. This effect was utilized to detect areas differing in doping by monitoring the phase angle between the drive frequency and cantilever response while scanning over areas of differing doping density. Measurements at various bias voltages are presented to demonstrate that the phase contrast observed between differently doped areas is directly connected to the bias induced surface potential (band bending) present on these areas. A quantitative investigation of the contrast mechanism was performed by measuring deflection (force), amplitude, and phase versus distance curves for a typical cantilever under an applied bias, on a gold film. Dopant profiles using BAAFM have been demonstrated on commercial integrated circuits in both a top-down and cross-sectional manner. Our experiments demonstrate that this method allows for distinguishing between p- and n-doped areas as well as distinguishing between areas of doping density ranging from at least 1016 to 1020 dopants/cm3. From measurements on several commercial devices, it was determined that BAAFM is capable of resolving regions differing by 1/10 an order of magnitude at a doping density of 1017 cm-3. Sample preparation is discussed and a comparison scanning capacitance microscopy (SCM) and BAAFM is made.