Journal of Nanoscience and Nanotechnology

Vol. 9, No. 8, pp. 4700-4708, (2009).

Low-temperature scanning tunneling microscopy of Ring-Like Surface Electronic Structures Around Co Islands on InAs(110) Surfaces

D.A. Muzychenko¹, K. Schouteden², S.V. Savinov¹, N.S. Maslova¹, V. I. Panov¹ and C. Van Haesendonck²

¹ Faculty of Physics, Moscow State University, 119991 Moscow, Russia

² Laboratory of Solid-State Physics and Magnetism, BE-3001 Leuven, Belgium

Abstract

We report on the experimental observation by STM at low temperature of ring-like features that appear around Co metal clusters deposited on a clean (110) oriented surface of cleaved p-type InAs crystals. These features are visible in spectroscopic images within a certain range of negative tunneling bias voltages due to the presence of a negative differential conductance in the current-voltage dependence. A theoretical model is introduced, which takes into account non-equilibrium effects in the small tunneling junction area. In the framework of this model the appearance of the ring-like features is explained in terms of interference effects between electrons tunneling directly and indirectly (via a Co island) between the tip and the InAs surface.

Introduction

After the pioneering work of Tsui, two-dimensional (2D) electron systems have been intensively investigated. The 2D electron gas provides an excellent playing ground for studying the physical properties of low-dimensional systems. Here, we present the results of local STM/STS measurements of the electronic properties of a 2D system, which consists of metal Co islands deposited in situ on the (110) oriented surface of InAs that is obtained by cleavage in ultra high vacuum (UHV) of a single crystal.

Fig.1. Experimental topographical STM image of the Co-InAs(110) surface and maps of the measured differential conductance dI/dV for the same surface region at different tunneling bias voltage Set of I(Vt) curves extracted from the CITS data along the lines indicated on images.

Experiment and results

The investigated InAs samples are doped with Mn at a doping level of 5×10¹⁷ cm^-3. Mn in InAs is expected to act as a shallow acceptor with ionization energy around 28meV. InAs slabs with size 5×2×2 mm³ were cleaved in-situ at room temperature in the UHV preparation chamber (base pressure is about 5×10^-11 mbar). Co islands were deposited by means of electron-beam evaporation. All the reported STM/STS experiments are performed at liquid helium temperature (4.5 K). Tunneling bias voltage Vt refers to the sample voltage. According to our STM measurements, Co atoms tend to form small clusters on the InAs(110) surface despite the low substrate temperature during deposition. Surprisingly, some clusters are surrounded by ring-like features on the STM images. We have also performed spatially resolved spectroscopic measurements above the same surface area where the STM topography image was obtained (Fig. 1). In a fist set of experiments, we relied on imaging based on harmonic detection with a lock-in amplifier. The results are shown in Figs. 1. Each image corresponds to a map of tunneling conductance proportional to LDOS.

From our analysis of the results shown in Fig. 1 we conclude that:

1. sharply defined dark rings appear in the differential conductance (LDOS) images around some of the clusters within a certain range of negative tunneling bias voltages,

2. the size of the dark rings is shrinking when the absolute value of the tunneling bias voltage is decreased,

3. the LDOS has an almost constant value above the Co-InAs surface except for the clusters and the immediate vicinity of the dark rings,

4. the differential conductivity has negative (NDC) value on the dark rings.

Most probably the presence and the different diameters of the dark rings around the different clusters is caused by the different bonding of the clusters to the InAs substrate. The large grid size that has been used for acquiring the CITS data allows us to present the data in another interesting way. Figure 2 gives a 2D map of the variation of the normalized tunneling conductance data along the cross-section bb’ in Fig. 1(a). The horizontal axis corresponds to the spatial coordinate, while the vertical axis corresponds to the tunneling bias voltage, and the color intensity corresponds to LDOS. From Fig. 2 it is clear that a Co cluster affects the LDOS in the conduction and valence bands only very locally. Except for a limited voltage range near Vt=0, the influence of a cluster on the LDOS is very rapidly decaying outside the cluster. Inside the cluster, the conductivity in the valence band is considerably suppressed. Some of the energy levels of the Co cluster become visible in the voltage range corresponding to the band gap and the conduction band. In Fig. 2 there are two tilted dark lines that approximately start from the peak in the normalized tunneling conductance of the cluster and that extend over a distance of about 8nm into the defect free surface area. These two dark lines directly reflect the presence of the dark rings around the clusters as well as the dependence of the ring diameter on the tunneling bias voltage, which is clearly non-linear. Another important observation related to Fig. 2 is the influence of the Mn impurity atoms below the Co-InAs(110) surface on the tunneling spectra. In the vicinity of a Mn atom the LDOS is affected deep inside the conduction and valence bands. Additionally, an intense peak appears in Fig. 2 near the valence band top and most probably this peak reflects the energy position of the Mn acceptor band. We note that the spatial extent of this peak is relatively large, about 7 nm.

Fig. 2. 2D representation of the normalized differential tunneling conductance, extracted from the CITS data along the line indicated on left images. The horizontal axis corresponds to the spatial coordinate, while the vertical axis corresponds to the tunneling bias voltage. The color contrast in the image corresponds to the value of the differential tunneling conductance as given by the color scale on the right hand side

Discussion

In order to describe the observed effects consistently one needs to take into account the presence of non-equilibrium and interference effects in the small tunneling junction, which can be described using the Keldysh diagram technique. Two main peculiarities of the ring-like features observation have to be explained. First, the presence of the NDC region in the tunneling conductivity curves has to be accounted for. Second, the limited spatial extent of the circular features requires an explanation. The model structure of InAs-Co-STM tip structure is depicted in Fig 3. Co cluster on the surface acts as a donor, and consequently it is positively charged. In case of non-equilibrium, the energy e_a of the cluster level, which is participating in the tunneling (see Fig. 3), depends on the electron filling number <n_a> of this level: e_a = e₀ + U(n₀ - <n_a>), where e₀ is the unperturbed position of the cluster energy level, n₀ is the equilibrium cluster level filling number, and U is the Coulomb interaction energy of the localized charges. At certain conditions this can lead to the appearance of negative slope area on I(V) dependence. The most probable physical mechanism causing the formation of narrow (of the order of the interatomic distance) ring-like features in real space is some kind of resonant tunneling. When the STM tip changes its spatial position, localized energy levels that participate in the tunneling processes are coming out of resonance or even are falling outside the tunneling window [E_F; E_F - eV_t]. This leads to variations of the cluster level filling number, and consequently to changes in the cluster level energy.

Fig. 3. Schematic representation of the tunneling processes occurring in the system consisting of the STM tip, a Co cluster and the InAs(110) substrate. ea is the Co cluster energy level that is participating in the tunneling. E0 is the first 2D subband, which has a non-zero but narrow width, as indicated by the gray stripe. A gray stripe is also used to mark the presence of the higher 2D subbands, which form a continuum of states due to their broadening. The system is shown at non-zero bias voltage when ea and E0 are aligned and a dark ring appears around the Co cluster in the maps of the differential tunneling conductance.

Conclusion

In conclusion, by means of LT STM/STS measurements, we have been able to identify the presence of ring-like features around Co metal clusters on a p-type InAs(110). These features become clearly visible in the maps of the differential conductance for a certain range of negative tunneling bias voltages, due to the presence of a NDC region in the I(Vt) dependence. The diameter of the rings is decreasing when decreasing the absolute value of the tunneling bias voltage. A theoretical model was developed, which takes into account the non-equilibrium effects that occur in the small STM tunneling junction. In the framework of the model the appearance of the ring-like features is accounted for by the interference of direct and indirect (via a Co cluster) tunneling between the STM tip and the InAs surface.