Charged defects on Ge(111)-c(2×8): characterization using STM
Introduction
In the bulk or at the surface, defects including impurities have a large influence on the electronic and optoelectronic properties of semiconductors. The defects at surfaces play an important role in growth nucleation, carrier recombination, and Fermi-level pinning. Scanning tunneling microscopy (STM) is an effective tool for studying the surface defects and also the bulk defects due to the high resolution achieved in real space [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. To date, most studies of point defects using STM have been on heterogeneous III–V semiconductors [1], [2], [3], [4], [5], [6], [7], [8], [9]. In particular, the non-polar (110) cleavage face has been preferentially chosen as an ideal surface since it does not reconstruct, and the structural and electronic properties of its ideal periodic surface are generally well understood. The first pioneering work was reported by Stroscio et al. [1] in a study of adsorbed oxygen atoms on GaAs(110). It was experimentally demonstrated that the local state density near the defect (the adsorbed oxygen, in this case) is reflected in the STM images. Voltage-dependent STM images clearly revealed the bonding characteristics of an electronegative adsorbate atom (an increase in the state density below the Fermi level at the expense of a decrease in the unoccupied state density). In addition to this local chemical effect due to the bonding, a long-ranged screening effect was also observed in the form of delocalized contrast encompassing the local oxygen-related feature. The delocalized feature provides a microscopic insight into lateral band bending at the surface surrounding this point-charge defect. In ensuing STM studies, both anion and cation vacancies on doped GaAs(110) [2], [3], InP(110) and GaP(110) [4] surfaces have been identified. These vacancy defects were in most cases charged and accompanied by spatially delocalized variations over the background lattice in the STM image as well as a local relaxation of the neighboring atoms.
Bulk defects have also been probed by STM. The arsenic antisite in low-temperature-grown GaAs was identified to be in a tetrahedral environment [5]. The electronic structure arising from the tail of the antisite wave function was observed as a satellite feature near the defect in these images. Dopant (both donor and acceptor) atoms substituting for bulk atoms were also observed as delocalized features arising from its charged nature [6], [7], [8]. In both cases, the long-range electrostatic perturbation (band bending) caused by charged defects made the imaging of these defects in the subsurface region possible.
Defects on homogeneous semiconductors have been relatively less studied, although some studies have focused on defects related to dimers on the (100) surfaces of Si and Ge [10], [11], [12]. In this work, we have studied defects appearing on the Ge(111)-c(2×8) surface for which the STM has been useful in establishing its atomic structure [13], [14], [15]. The unperturbed Ge(111)-c(2×8) surface, a schematic of which is shown in Fig. 1, consists of adatoms and rest atoms (i.e. first-layer atoms not bonded to adatoms) [16]. Voltage-dependent STM imaging [17], [18], [19] revealed that an incomplete charge transfer occurs from the adatoms to the rest atoms, in agreement with a theoretical prediction [20]. Recently, voltage-dependent STM imaging has also proved to be useful in extracting spectroscopic information identifying a low-lying occupied surface state delocalized over the clean surface [21].
Defects on the Ge(111)-c(2×8) surface have been previously investigated by Molinas-Mata et al. [22]. Several point defects, appearing as localized features in the STM images, were reported and identified using dual-polarity STM imaging. In this paper, we report the STM observation of similar point defects and impurities present on the Ge(111)-c(2×8) surface re-examined by varying the bias voltage as well as the polarity. Preliminary experimental results of this work have been presented in the proceedings of a conference [21]. Here, we expand upon that work and offer new results and argumentation to support a coherent picture of the appearance of various surface defects on the Ge(111)-c(2×8) surface. The defects in the STM images exhibit significant voltage-dependent variations in brightness. Around each localized defect, a delocalized enhancement or depression in the STM topography is also observed. These delocalized features are particularly prominent in those images representing the unoccupied states of a sample at a low bias voltage. Based on this voltage-dependent behavior, the observed point defects are found to be either charged defects [relative to the unperturbed Ge(111)-c(2×8) surface] or neutral composites of defects with opposite charges. Their charged natures are discussed by comparing the charge state of each defect with those of the adatoms and rest atoms composing the Ge(111)-c(2×8) reconstructed surface.
Section snippets
Experimental
The experiment was carried out in an ultra-high vacuum with a base pressure of ∼2×10−10 mbar. The samples used were the commercial Ge(111) wafers (Fluoroware Inc.) of various doping types and concentrations [an undoped, a p-type (resistivity, ρ∼0.1–3.0 Ωcm), and an n-type (ρ∼0.1–3.0 Ωcm)]. Cycles of sputtering (substrate temperature ∼450°C) and annealing (∼650°C) routinely yield a clean and well-ordered Ge(111)-c(2×8) surface, as confirmed by both a low-energy electron diffraction (LEED) pattern
Results
Fig. 2 shows images of a sputter-annealed Ge(111)-c(2×8) surface under dual-polarity biases, representing a distribution of the unoccupied states and the occupied states, respectively. These images of the defect-rich part of the surface are intentionally chosen to display all of the defects of current interest. The overall defect density on the surface is normally much lower than that shown. On the defect-free part of the surface, only adatoms are visible in the empty-state image (Fig. 2a), but
Discussion
In Section 3, numerous defects observed in the STM images of the Ge(111)-c(2×8) surface were identified and their charged natures relative to the unperturbed surface assigned. The results are summarized in Table 1. The charged nature assigned to each defect was experimentally determined from the direction of the band bending inducing the characteristic images. These assignments can be justified by analyzing the charge state of each defect in comparison with those of an adatom and a rest atom of
Conclusion
In conclusion, we have reported STM images of numerous defects on Ge(111)-c(2×8) taken at various bias voltages. The images exhibited a significant variation in voltage-dependent brightness of these localized defects, reflecting the spectroscopic information. In addition, the long-ranged elevation or depression in the STM contour was also observed surrounding the defects. This delocalization was particularly prominent in the empty-state images at a low bias voltage. The delocalized variation in
Acknowledgements
This work was supported by the Office of Naval Research under Grant No. N00014-92-J-1479. One of the authors (G.L.) acknowledges the partial supports from the Korea Science and Engineering Foundation through the ASSRC at Yonsei University.
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Present address: Department of Electrical Engineering, Princeton University, PO Box 5263, Princeton, NJ 08544, USA.