A fundamental semiconductor device - the pn-diode - is formed by stacking a p-doped and a n-doped layer of the semiconductor material. The diode performance is controlled by the spatial distribution of dopants in the pn-interface region on the nanoscale. Since the late 80's Scanning Tunneling Microscopy (STM) has been employed to study the local properties of these pn-interfaces [1]. Being sensitive to electronic states close to the semiconductor band edges, the STM is the ideal tool for probing properties of diode structures. Because of the difficult tip-sample coarse positioning most studies were carried out on pn-superlattices. They were limited to intrinsic features accessible without applied bias voltage across the diode. Recently, surface photovoltage measurements on a working GaAs pn-diode were reported that could resolve the pn-interface with 10nm resolution [2].
Our work concentrates on the direct investigation of the interface of a GaAs pin-diode heterostructure with Cross Sectional Scanning Tunneling Spectroscopy (X-STS) at 6K. The sample contains one single p-i-n device. With external source and drain contacts the electric field across the junction is controlled during the measurement. Due to delta-doped sheets terminating the doped layers, the internal field is of the order of 108 V/m. Therefore the diode's active region is confined to approximately 10 - 30 nm and was mapped with atomic resolution. The variable field of the diode structure acts perpendicular to the tip induced field. The band-edge alignment is quantitatively controlled by applying a source drain voltage at the external contacts. The resulting change of the internal field is directly measured with spatially resolved I(V)-spectroscopy. At a fixed tunneling bias voltage the source drain control allows to shift the different surface resonances of the {110} cleavage surfaces in and out of the tunneling channel. The impact of the external field on individual dopant atoms in the pin-interface region is demonstrated.
This work was supported by the DFG, SFB 602, and the German National Academic Foundation.
[1] P. Muralt et al., Appl. Phys. Lett. 50, 1352 (1987)
[2] S. Yoshida et al., International Conference on Nano Science and Technology, Basel (2006)
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