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With high current sensitivity, conductive atomic force microscopy (C-AFM) has been widely used to measure the conductive current on material surface. Typically, C-AFM system works in air and most of the C-AFM applications are focused on dielectric thin films, combining external dc bias to study the physical phenomena of materials. In this work, we have employed C-AFM working in humidity-controlled ambience to measure the current distribution on self-assembled Ge and InN dots, and investigated the characteristics of current images without external dc bias. The samples used in this work were self-organized Ge and InN dots grown on p-type silicon and intrinsic sapphire substrates, respectively. All the C-AFM images were acquired at room temperature in a handmade shielding box. The experimental results indicated that the phase response and the current intensity were independent of surface morphology. It is also noticed that the current intensity at dot area increases with environmental humidity for all samples. For Ge dots, the positive current signal occurs prior to the negative one. From the phase response of current signals, we can qualitatively observe the local behavior of energy band and the work function of the scanned area. Since there is no external dc bias during C-AFM measurements, it is believed that positive current was due to carrier transfer from the localized hole states in the dot to the conductive tip during thermal equilibrium. Similarly, negative current came from the localized electronic states in the strained Si under Ge dots. For InN dots, the negative current around the central area is very significant. This result is in agreement with the electron accumulation at the surface of InN thin film. In addition, the experiments further revealed that environmental humidity is the dominant factor for the zero-bias current although laser illumination can also enhance the current intensity at the dot region. The mechanism of humidity-dependent current image will be discussed in depth. Our experimental results indicate that humidity-controlled C-AFM without external dc bias could be an advanced method in studying the electronic properties of semiconductor nanostructures. |