Recent developments in scanning tunneling spectroscopy of semiconductor surfaces

Abstract

Several recent developments in scanning tunneling spectroscopy (STS) of semiconductor surfaces are reviewed. First, the normalization of spectra is discussed, which for the Si(111)2×1 surface is found to produce a small shift in the apparent position of band edges. With this correction, the surface band gap measured by STS is found to be in good agreement with that obtained by other experimental and theoretical techniques. Second, it is shown for the SiC(0001)√3×√3 surface that the tunneling spectra show a remarkable evolution with decreasing current, and at pA levels they reveal a Mott-Hubbard gap for the surface states, in agreement with that seen by other methods. Finally, a detailed discussion is presented on the absence of electronic effects for the tunnel current into empty states of III-V (110) cleaved surfaces. From this result it is demonstrated that one can use observed strain induced displacements of such surfaces to yield information on the chemical composition of the underlying material. I IntroductionSince the early days of the scanning tunneling microscopy (STM), spectroscopy of semiconductor surfaces has played an important role both in furthering our understanding of the the STM technique as well as in yielding new information about the surfaces themselves. For example, early voltage-dependent results on Si(111)2×1 and Si(111)7×7 surfaces enabled a determination of their structures and also illustrated the exquisite sensitivity of the STM to the surface states [1][2][3][4]. Complete conductance spectra as a function of tip-sample bias voltage clearly revealed the surface state bands. This early development of the scanning tunneling spectroscopy (STS) field has been reviewed [5].Slightly after the development of STS for semiconductor surface states, further studies were performed on cleaved GaAs(110) surface for which surface states do not play a large role in the results [6]. The difference in these two situations, with or without surface states, is quite dramatic. For Si(111)7×7 the surface state band is essentially metallic (i.e. no band gap), and the conductance dI/dV varies by less than three orders of magnitude over the voltage range −3 to +3 V. In contrast, for GaAs(110) a large band gap opens up in the spectrum and one requires 5-6 orders of magnitude in dynamic range to accurately measure the spectrum. Special techniques have been developed to achieve this high dynamic range [7]. For all of the direct gap III-V semiconductors, STS of the (110) surface can be used to reveal the bulk band gap as well as some other bulk critical point of the band structure [7]. Surface states make a small (but still noticeable) contribution of the spectra.Published in Appl. Phys. A 72 [Suppl.], S193 (2001).

Localized charge carriers in graphene nanodevices

“…324 Alternatively, optical emission experiments can be used where light with an energy higher than the band gap is shone on the crystal and the photons emitted by the semiconductor are investigated. 325 Band gaps can also be measured with ARPES 326 or STM/STS, 327 but experiments are typically challenging.…”

Electronic structure of cubic boron arsenide probed by scanning tunneling spectroscopy

“…In contrast to the reconstructed Si (111) surfaces, the dimensions of the unit cell on the naturally cleaved surfaces of III-V compounds remains similar to those of the bulk. As such, the surface states do not play a large role in several STS reports of III-V compounds [25][26][27]. Because STS measurements of these natural cleaved surfaces of III-V semiconductors yield information that is not particularly surface-specific [26,28], the results can be used to probe the bulk band gap and other features in the bulk band structure.…”

“…As such, the surface states do not play a large role in several STS reports of III-V compounds [25][26][27]. Because STS measurements of these natural cleaved surfaces of III-V semiconductors yield information that is not particularly surface-specific [26,28], the results can be used to probe the bulk band gap and other features in the bulk band structure. It is of interest to investigate whether this behavior is applicable to BAs or not.…”

Mapping the evolution of Bi/Ge(111) empty states: From the wetting layer to pseudo-cubic islands

“…26 Spin-resolved photoemission spectroscopy (SR-PES) is the technique of choice to map the electronic structure of Bi surfaces and thin films, being also sensitive to the peculiar spin texture stemming from the presence of SOC effects. Complementary to SR-PES are spin-resolved inverse photoemission spectroscopy (SR-IPES), 27 and scanning tunneling spectroscopy (STS), 28 providing information also on empty electronic states. Both techniques have been synergistically employed in the past for the characterization of Bi growth on III-V semiconductors such as, for instance, GaAs 29,30 and also found application in the study of a variety of homo-and hetero-structures showing SOC related effects.…”