Few Carriers Injection and Local Transport Characteristics of Si Based Quantum Dots Probed by Atomic Force Microscopy

Abstract

Electronic charged states of hemispherical Si dots with different sizes formed on ∼4 nm‐thick SiO2/p‐Si(100) have been studied using a non‐contact Atomic Force Microscopy / Kelvin probe technique. After electrons injection to and extraction from Si dots larger than 20 nm in height, a unique surface potential image was observable in each of dots namely torus‐shape image, in which the surface potential change caused by charging or discharging is much higher in the peripheral region rather than in the center of the dot. Since the observed surface potential change corresponds to a few electrons and holes retained in the dot, such a torus‐shape potential image is attributable by Columbic repulsion among the stored charges in the dot. Correspondingly, surface potential changes induced by electron charging and discharging at each of isolated Si quantum dot with Ge core also have been measured. The Surface potential change confirmed the injected electrons are stored in the Si clad and holes are stably retained in Ge core, as expected from the energy band diagram of Si/Ge heterostructure. Furthermore, the distinct current images through Si dots with and without Ge core have been measured using conductive AFM probe in both bias polarities. Under the positive bias condition, valence electrons can be extracted from Si clad and Ge core, resulting in the generation and collection of holes in the Ge core, and accumulated holes can recombine by electron tunneling from the Si substrate through the bottom SiO2 layer. For the negative tip bias, current is rate‐limited by electron tunneling through the bottom oxide from the conduction band of Si clad rather than electron injection from the tip to the Si clad.