Scanning tunneling microscopy and spectroscopy of semi-insulating GaAs

Abstract

We report on atomically resolved scanning tunneling microscopy images and tunneling spectra of ͑110͒ cleavage surfaces of semi-insulating GaAs without illumination at room temperature. With help of simple model calculations we extract the physical mechanisms involved in the tunneling processes from and into semi-insulating GaAs. Atomically resolved images can only be observed at negative voltages, while no tunneling into empty states is possible without illumination. This is explained, on the one hand, by the absence of a carrier inversion at the semiconductor surface without illumination under the nonequilibrium tunneling contact conditions. On the other hand, at negative voltages in the noncontact mode an accumulation at the surface occurs and leads to tunneling of electron from the valence band states into the empty tip states. This current is limited by the tunneling through the vacuum barrier and the scanning tunneling microscopy images are found to show the occupied dangling bond states above the arsenic atoms. In the point contact mode the current is limited by tunneling through the space charge region without and with illumination. The implications of the results for the investigation of low-conductivity materials by scanning tunneling microscopy are discussed.

Structure and electronic spectroscopy of steps on GaAs(110) surfaces

“…Nonequilibrium carrier occupation on the surface has been observed in prior STM/S studies, 16,17,18,19,20 and reduced κ values have been associated with this effect. 19,20 For the present situation, at negative sample voltages, the Fermi-energy at the surface will intersect the states induced by the compensating defects.…”

Scanning tunneling microscopy on unpinnedGaN(11¯00)surfaces: Invisibility of valence-band states

“…Thus, under tunneling conditions an inversion layer is instantly filled and removed. 10,14 Finally, there are nevertheless conditions under which one can determine properly the band gap and the band-edge position of the GaN͑1100͒ surfaces as experimentally shown in Ref. 7.…”

“…Therefore, carrier inversion does not occur under tunneling conditions on GaN͑1100͒, in analogy to GaAs͑110͒. 10,14 The tunneling current is computed following Feenstra and Stroscio 9 and Bono and Good. 15 This model is based on the approach by Tersoff-Hamann 1 and Selloni et al 16 to integrate over all states between the tip and the sample Fermi-level times the transmission coefficient.…”

A prospective: Quantitative scanning tunneling spectroscopy of semiconductor surfaces

“…7 Over the past decade, much work has been performed to enable quantitative interpretation of tunneling spectra of semiconductors. 8,9,10,11,12,13,14,15,16 What makes this task difficult? Primarily the fact that the semiconductor acts as a dielectric, with some of the applied voltage between sample and tip being dropped within the semiconductor itself, as pictured in Fig.…”

“…in which transport in the semiconductor itself is somewhat limited. 7,10,11,27 In those cases, a much more complicated theory is needed to describe the distribution of carriers and resulting charge densities on the surface and in the bulk of the semiconductor. Such a theory could prove to be useful in enabling the determination of transport parameters for the carriers, even in nanoscale situations as occur in the STM geometry.…”