Observation of single charge carriers by force microscopy

Abstract

The scanning force microscope is used to deposit charge carriers on insulating Si3N4 films and to monitor their recombination. The charge decay shows up as a discontinuous staircase, demonstrating singlecarrier resolution. The decay is found to be controlled by thermionic emission.PACS numbers: 73.25.+i, 61.16.Di, 73.40.Bf, 73.50.Gr The scanning force microscope (SFM) introduced by Binnig, Quate, and Gerber' is a remarkably useful instrument to map the surface topography of virtually any solid.Most noticeably, atomic resolution has been achieved on insulating crystals. Besides the repulsive contact force used in profilometry, attractive forces like the quantum-mechanical exchange, electrostatic, and magnetic dipolar forces were utilized to measure such quantities as metallic adhesion, electric surface potential variation, and magnetic domain structures.In two recent Letters, Stern et al. and Terris er al.have demonstrated that the SFM can be used to deposit and image charges on insulators, both by contact electrification (CE) as well as by corona discharge (CD).They speculated that single carriers could be observable.In this Letter the observation of single-charge recombination events is described. Ionized particles are deposited by a short CD (10 ms) onto silicon nitride (Si3N4) films prepared by plasma-enhanced chemical-vapor deposition (PECVD) onto degenerately doped (conducting) GaAs substrates. The detected charge signal following the CD was observed to decay within a few seconds in a staircase fashion, showing the charge quantization. The experiments described were performed in air at ambient conditions. The force is detected with a diA'erential interferometer described elsewhere. Figure 1 is a schematic of the tip apex, positioned at a distance d above an insulating film of thickness h. A voltage V applied to the tungsten tip (-) v" Insulator . :=. -, -'--==: Substrate~W~~~F IG. 1. Schematic explaining the detection of the excess charge q on an insulating film via the force Fl related to the image charge q;. results in the Coulomb force F=(V+&) G, where Gcontains the tip radius R, d, and h, as well as the relative dielectric constant s of the film. The term p is the contact potential between the tip and sample. Applying an ac voltage V= J2V", sin(cot) results in two measured oscillating force terms at m and 2', with the respective rms amplitudes F~=2V PG and F2 = V J2G from which G and p can be extracted. The tip-to-sample distance d is adjusted by a controller loop so that F2 and hence G remain constant. Since V and G are both constant, any contrast in F~is due to a varying contact potential p. A charge carrier q lying on the surface of the insulator induces image charges q; and q in the tip and the substrate, respectively (Fig. 1). Since charge conservation requires q;+q =q, the magnitude of q;/q is always less than unity, increasing when the tip-sample distance d is decreased with respect to the film thickness h.The value of F I is approximately given by F I =q; E" where E, is the rms electric field ...

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