Mapping the wave function of transition metal acceptor states in the GaAs surface

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Mapping the wave function of transition metal acceptor states in the GaAs surface

Anthony Richardella, Dale Kitchen, and Ali Yazdani

Phys. Rev. B 80, 045318 – Published 23 July 2009

Abstract

We utilize a single-atom substitution technique with spectroscopic imaging in a scanning tunneling microscope to visualize the anisotropic spatial structure of magnetic and nonmagnetic transition metal acceptor states in the GaAs (110) surface. The character of the defect states play a critical role in the properties of the semiconductor, the localization of the states influencing such things as the onset of the metal-insulator transition and in dilute magnetic semiconductors the mechanism and strength of magnetic interactions that lead to the emergence of ferromagnetism. We study these states in the GaAs surface finding remarkable similarities between the shape of the acceptor-state wave function for Mn, Fe, Co, and Zn dopants, which is determined by the GaAs host and is generally reproduced by tight-binding calculations of Mn in bulk GaAs [J.-M. Tang and M. E. Flatté, Phys. Rev. Lett. 92, 047201 (2004)]. The similarities originate from the antibonding nature of the acceptor states that arise from the hybridization of the impurity $d$ levels with the host. A second deeper in-gap state is also observed for Fe and Co that can be explained by the symmetry breaking of the surface.

Received 16 March 2009

DOI:https://doi.org/10.1103/PhysRevB.80.045318

Authors & Affiliations

Anthony Richardella^1,2, Dale Kitchen^1,2,*, and Ali Yazdani¹

¹Department of Physics, Joseph Henry Laboratories, Princeton University, Princeton, New Jersey 08544, USA
²Department of Physics, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801, USA

^*Present address: Milliken Research Corporation, PO Box 1927, M-405, Spartanburg, SC 29304.

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Vol. 80, Iss. 4 — 15 July 2009

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Images

Figure 1
(Color) Two substitutions of cobalt adatoms into the lattice showing cobalt can desubstitute and come back to the surface. (a) Several cobalt adatoms that appear as depressions on the lattice. A subsurface Zn dopant can be seen in the upper left. (b) A voltage pulse from the STM tip, as described in the text, causes a cobalt atom in the lower right to substitute into the lattice, kicking a gallium atom to the surface. (c) The gallium from the first site is moved out of view and a second cobalt is substituted at the center. [(d) and (e)] The gallium adatom from the second site is manipulated to the location of the first site. (f) A voltage pulse causes the gallium to go in and the cobalt comes back to the surface. (All images are $120 Å \times 90 Å$ at $- 1.5 V$ )Reuse & Permissions
Figure 2
(Color) Topography of in-gap states ( $40 Å$ by $40 Å$ ) and insets of valence-band states ( $30 Å$ by $30 Å$ ). Topography in insets has been multiplied by two to enhance the contrast. (a) Zn (1.6V, inset: $- 1.4 V$ ), (b) Mn (1.6V, inset: $- 1.3 V$ ), (c) Fe (1.5V, inset: $- 1 V$ ), and (d) Co (1.5V, inset: $- 1.3 V$ ).Reuse & Permissions
Figure 3
(Color) (a) $d I / d V$ measurements over an Mn acceptor and Zn acceptor; each show a strong and broad in-gap resonance. The Zn resonance peaks near $0.65 \pm 0.05 V$ and the Mn resonance peaks near $0.85 \pm 0.03 V$ . (b) $d I / d V$ measurements near an Fe acceptor, with the tip between head and arm features (see inset), bring out two states near $0.87 \pm 0.05 V$ and $1.52 \pm 0.05 V$ . (c) Two $d I / d V$ measurements taken near a Co acceptor show two distinct states, with the tip centered over the ${Co}_{Ga}$ a lower energy resonance near $0.92 \pm 0.05 V$ and the tip over the arm feature a higher energy peak near $1.15 \pm 0.05 V$ .Reuse & Permissions
Figure 4
(Color) Differential conductance maps of Fe and Co acceptors ( $50 Å$ by $50 Å$ ). Energy maps near the ${Fe}_{Ga}$ show (a) the lower resonance state mapped at 0.88 V (inset: $30 Å$ by $30 Å$ topography taken at 1 V better shows spatial extent of the state), (b) the higher energy resonance mapped at 1.50 V. Energy maps of an isolated ${Co}_{Ga}$ , (c) near the lower resonance peak energy of 0.85 V, and (d) near the higher energy resonance of 1.15 V with a setup voltage of 1.05 V. The ${Fe}_{Ga}$ setup voltage was 1.45 V which created a larger shadow effect in (a) than in (c) since the set voltage for the ${Fe}_{Ga}$ map was nearer its high energy resonance. Both (b) and (d) exaggerate the depression in the center because the tip is further from the surface due to the tunneling contribution from the lower energy resonance.Reuse & Permissions
Figure 5
(Color) (a) Valence-band topography of two ${Mn}_{Ga}$ sites showing an enhancement of the adjacent arsenic atoms. ( $50 Å$ by $50 Å$ , $- 1.5 V$ ) (b) Low-bias topography of the same area shows an enhancement on only one arsenic and depression. Scan proceeds from right to left. A glitch occurs $\frac{3}{4}$ of the way through the measurement $(- 0.25 V)$ . (c) Result of glitch in (b), feature has switched to the mirror configuration. (d) Composite image made by combining (c) and (d) showing how the high-bias image is a mixture of the two configurations. (e) As the bias is increased the switching rate increases, appearing as noise in the topography. $(- 0.5 V)$ [(f)-(h)] Noise patterns were observed around (f) ${Fe}_{Ga}$ ( $45 Å$ by $45 Å$ , $- 0.3 V$ ) and (h) ${Zn}_{Ga}$ ( $32 Å$ by $32 Å$ , $- 0.5 V$ ) but not (g) ${Co}_{Ga}$ ( $32 Å$ by $32 Å$ , $- 0.3 V$ ).Reuse & Permissions
Figure 6
(Color) (a) Schematic of hybridization of an atom with an ideal gallium vacancy $(V_{Ga})$ leading to a shallow acceptor level such as for Zn. The valence levels of the impurity are higher in energy than the host atom resulting in a state just above the band edge that shares the valence band’s bonding character. (b)Hybridization of levels giving rise to the Mn acceptor following the model of Ref. 5. In contrast to (a) the acceptor is antibonding in character. States are labeled $+ (-)$ for majority(minority) spin. Colors represent the initial state the hybridized ${Mn}_{Ga}$ level most resembles in character. (c) Possible level ordering that would give rise to a symmetric acceptor state for an Mn under the influence of the symmetry breaking of the surface, as described below. (d) Splitting of a $t_{2}$ state by the surface. Removal of one neighboring As atom lowers the $T_{d}$ symmetry to $C_{3 v}$ , the rest of the missing atoms at the surface and the GaAs surface reconstruction lower the symmetry further to $C_{s}$ ( $(1 \bar{1} 0)$ mirror plane). For there to be a symmetric $(a^{'})$ acceptor level at the surface there can be at most three electrons in the vacancylike $t_{2}$ level. For Fe and Co, this would require that their additional electrons fill the impuritylike $e$ and $t_{2 -}$ levels according to their atomic configuration.Reuse & Permissions

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