Formation of InAs quantum dots and wetting layers in GaAs and AlAs analyzed by cross-sectional scanning tunneling microscopy

We have used cross-sectional scanning-tunneling microscopy (X-STM) to compare the formation of self-assembled InAs quantum dots (QDs) and wetting layers on AlAs (1 0 0) and GaAs (1 0 0) surfaces. On AlAs we find a larger QD density and smaller QD size than for QDs grown on GaAs under the same growth conditions (500 °C substrate temperature and 1.9 ML indium deposition). The QDs grown on GaAs show both a normal and a lateral gradient in the indium distribution whereas the QDs grown on AlAs show only a normal gradient. The wetting layers on GaAs and AlAs do not show significant differences in their composition profiles. We suggest that the segregation of the wetting layer is mainly strain-driven, whereas the formation of the QDs is also determined by growth kinetics. We have determined the indium composition of the QDs by fitting it to the measured outward relaxation and lattice constant profile of the cleaved surface using a three-dimensional finite element calculation based on elasticity theory.     

Wave-function mapping of InAs quantum dots by scanning tunneling spectroscopy

Scanning tunneling spectroscopy is used to investigate the single-electron states and the corresponding squared wave functions of single and freestanding strain-induced InAs quantum dots grown on GaAs(001). Several peaks are found in dI/dV curves, which belong to different single-electron states. Spatially resolved dI/dV images reveal (000), (100), (010), (200), and (300) states, where the numbers describe the number of nodes in [11;0], [110], and [001] directions, respectively. The total number and energetic sequence of states is different for different dots. Interestingly, the (010) state is often missing, even when (200) and (300) states are present. We interpret this anisotropy in electronic structure as a consequence of the shape asymmetry of the dots.     

Formation and atomic structure of GaSb nanostructures in GaAs studied by cross-sectional scanning tunneling microscopy

GaSb nanostructures in GaAs, grown by metalorganic chemical vapor deposition, were studied with cross-sectional scanning tunneling microscopy. Three different samples were examined, containing a thin quantum well, a quantum well near the critical thickness for dot formation, and finally self-organized quantum dots with base lengths of 5–8 nm and heights of about 2 nm. The dots are intermixed with a GaSb content between 60% and 100%. Also small 3D and 2D islands were observed, possibly representing quantum dots in an early growth stage and quantum dot precursors. All GaSb layers exhibit gaps, which are indications of an island-like growth mode during epitaxy.     

Thermal and kinetic surface roughening studied by scanning tunneling microscopy and atomic force microscopy

Recent studies of thermal roughening on Si surfaces and kinetic roughening of some growing films, copper and tungsten, by using scanning tunneling microscopy and atomic force microscopy are reviewed. A logarithmic divergence of the surface height fluctuations of Si(111) vicinal surfaces is confirmed, in agreement with the theoretical prediction of rough surface in thermal equilibrium. For the kinetically formed rough surfaces, power law dependences of the interface width on the system size are clearly observed. Furthermore, the tungsten films show a short-range scaling regime and a long-range “smooth” regime. The roughness exponents α are compared with theoretical predictions: for the typical Cu electrode position condition (α=1/2), the exponent appears to be close to that found for local growth models, and for tungsten films (0.7～0.8), it is consistent with recent predictions for growth where surface diffusion is predominant.     

Mn-including InAs quantum dots fabricated by Mn implantation

Mn-including InAs quantum dots (QDs) were fabricated by Mn-ion implantation and subsequent annealing. The optical, compositional, and structural properties of the treated samples were analyzed by photoluminescence (PL) and microscopy. Energy dispersive X-ray (EDX) results indicate that Mn ions diffused from the bulk GaAs into the InAs QDs during annealing, and the diffusion appears to be driven by the strain in the InAs QDs. The temperature dependence of the PL of Mn-including InAs QD samples exhibits QDs PL characteristics. At the same time, the heavy Mn-including InAs QD samples have ferromagnetic properties and high T_c.     

In-plane and perpendicular tunneling through InAs quantum dots

A Schottky diode with InAs dots in the intrinsic GaAs region was used to investigate perpendicular tunneling (in growth direction) through InAs quantum dots (QDs). At forward bias conditions electrons tunnel from the ohmic back contact into the metal Schottky gate. Peaks appear in the differential conductance when a QD level comes into resonance with the Fermi-level of the n-doped region. The observed tunneling features are attributed to electron transport through the s- and p-shell of the InAs islands. In our in-plane tunneling experiments the islands were embedded in the channel region of an n-doped GaAs/AlGaAs HEMT-structure. In order to study tunneling through single InAs islands, a quantum point contact was defined by lithography with an atomic force microscope and subsequent wet-chemical etching. In contrast to unpatterned devices sharp peaks appear in the I–V characteristic of our samples reflecting the transport of electrons through the p-shell of a single InAs QD.     

Surface defects in semiconductor lasers studied with cross-sectional scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy is used to study defects on the surface of semiconductor laser devices. Step defects across the active region caused by the cleave process are identified. Curved blocking layers used in buried heterostructure lasers are shown to induce strain in the layers above them. Devices are also studied whilst powered to look at how the devices change during operation, with a numerical model that confirms the observed behavior. Whilst powered, low-doped blocking layers adjacent to the active region are found to change in real time, with dopant diffusion and the formation of surface states. A tunneling model which allows the inclusion of surface states and tip-induced band bending is applied to analyze the effects on the tunneling current, confirming that the doping concentration is reducing and defect surface states are being formed.     

Features of the formation of Mn doped InAs/GaAs quantum dots by vapor phase epitaxy

A self-organized InAs/GaAs quantum dot (QD) array is doped with Mn. The effect of the Mn concentration on the morphology and QD luminescence properties is investigated. It is found that Mn deltadoping of the GaAs buffer layer before QD growth with a layer concentration of 10¹⁴ cm^?2 leads to the formation of an array of large QDs with variable composition In _x Ga_{1 ? x} As. The effect is explained within a model of In and Ga atom interdiffusion.     

Charge manipulation and imaging of the Mn acceptor state in GaAs by cross-sectional scanning tunneling microscopy

An individual Mn acceptor in GaAs is mapped by cross-sectional scanning tunneling microscopy (X-STM) at room temperature and a strongly anisotropic shape of the acceptor state is observed. An acceptor state manifests itself as a cross-like feature which we attribute to a valence hole weakly bound to the Mn ion forming the (Mn²⁺3d⁵+hole) complex. We propose that the observed anisotropy of the Mn acceptor wavefunction is due to the d-wave present in the acceptor ground state.     

Electronic structure of MgO-supported Au clusters: quantum dots probed by scanning tunneling microscopy

We investigate via density functional theory (DFT) the appearance of small MgO-supported gold clusters with 8 to 20 atoms in a scanning tunneling microscope (STM) experiment. Comparison of simulations of ultrathin films on a metal support with a bulk MgO leads to similar results for the cluster properties relevant for STM. Simulated STM pictures show the delocalized states of the cluster rather than the atomic structure. This finding is due to the presence of s- derived delocalized states of the cluster near the Fermi energy. The properties of theses states can be understood from a jellium model for monovalent gold.     

Anomalous photoluminescence of InAs quantum dots implanted by Mn ions

The photoluminescence (PL) of Mn-implanted quantum dot (QD) samples after rapid annealing is studied. It is found that the blue shift of the PL peak of the QDs, introduced by the rapid annealing, decreases abnormally as the implantation dose increases. This anomaly is probably related to the migration of Mn atoms to the InAs QDs during annealing, which leads to strain relaxation when Mn atoms enter InAs QDs or to the suppression of the inter-diffusion of In and Ga atoms when Mn atoms surround QDs. Both effects will suppress the blue shift of the QD PL peaks. The temperature dependence of the PL intensity of the heavily implanted QDs confirms the existence of defect traps around the QDs.     

Atomic-scale surface science phenomena studied by scanning tunneling microscopy

Following the development of the scanning tunneling microscope (STM), the technique has become a very powerful and important tool for the field of surface science, since it provides direct real-space imaging of single atoms, molecules and adsorbate structures on surfaces. From a fundamental perspective, the STM has changed many basic conceptions about surfaces, and paved the way for a markedly better understanding of atomic-scale phenomena on surfaces, in particular in elucidating the importance of local bonding geometries, defects and resolving non-periodic structures and complex co-existing phases. The so-called “surface science approach”, where a complex system is reduced to its basic components and studied under well-controlled conditions, has been used successfully in combination with STM to study various fundamental phenomena relevant to the properties of surfaces in technological applications such as heterogeneous catalysis, tribology, sensors or medical implants. In this tribute edition to Gerhard Ertl, we highlight a few examples from the STM group at the University of Aarhus, where STM studies have revealed the unique role of surface defects for the stability and dispersion of Au nanoclusters on TiO₂, the nature of the catalytically active edge sites on MoS₂ nanoclusters and the catalytic properties of Au/Ni or Ag/Ni surfaces. Finally, we briefly review how reaction between complex organic molecules can be used to device new methods for self-organisation of molecular surface structures joined by comparatively strong covalent bonds.     

Visualizing interfacial structure at non-common-atom heterojunctions with cross-sectional scanning tunneling microscopy

We describe how cross-sectional scanning tunneling microscopy (STM) may be used to image the interfacial bonding across the nearly lattice-matched, non-common-atom GaSb/InAs heterojunction with atomic-scale precision. The method, which takes advantage of the length difference between interfacial and bulk bonds, appears equally applicable to AlSb/InAs and suggests how one might recover the complete structure of either heterojunction from atomic-resolution STM data.     

Superconducting Pb island nanostructures studied by scanning tunneling microscopy and spectroscopy

Superconductivity of nanosized Pb-island structures whose radius is 0.8 to 2.5 times their coherence length was studied under magnetic fields using low-temperature scanning tunneling microscopy and spectroscopy. Spatial profiles of superconductivity were obtained by conductance measurements at zero-bias voltage. Critical magnetic fields for vortex penetration and expulsion and for superconductivity breaking were measured for each island. The critical fields depending on the lateral size of the islands and existence of the minimum lateral size for vortex formation were observed.     

Interface behavior of Mn/PbTe(111) studied by scanning tunneling microscopy and X-ray photoemission spectroscopy

Mn overlayers growth on PbTe(111) have been investigated by using scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS). The strong chemical interactions were found during the formation of Mn/PbTe(111) interface. At the initial deposition of Mn, one part of Mn adatoms substitute Pb atoms on the PbTe(111) surface, forming a (√3 × √3)R30° MnTe phase, and the other part of Mn adatoms, together with the kicked-out Pb atoms, nucleate at the boundaries of the MnTe islands, forming loop islands around the MnTe islands as an intermediate state. Finally, they develop into regular 3D Pb capped Mn islands upon further Mn deposition. For Mn growth on the PbTe surface where Pb atoms are almost completely substituted by Mn, the deposited Mn atoms either cooperate into the 3D Pb capped Mn islands promoting the upright growth of the 3D Pb capped Mn islands, or nucleate and grow on the MnTe superstructure areas. Free Pb layer always floats on the top of surface, indicating that Pb layer has smaller surface energy, and Mn adatoms always exchange the positions with the underneath Pb atoms during the growth.     

Single InAs/GaAs quantum dots: Photocurrent and cross-sectional AFM analysis

Photocurrent (PC) spectroscopy is employed to study the carrier escape from self-assembled InAs/GaAs quantum dots (QDs) embedded in a Schottky photodiode structure. As a function of the applied field, we detect a shift of the exciton ground-state transition due to the quantum-confined Stark effect (). The tunneling time, which is directly related to the observed photocurrent linewidth due to τ/(2Γ), changes by a factor of five in the photocurrent regime. The measured linewidth dependency on the electric field is modeled by a simple 1D WKB approximation for the tunneling process, which shows that the energetic position of the wetting layer is important for the measured tunneling time out of the dot. In addition to that we present cross-sectional atomic force measurements (AFM) of the investigated photodiode structure. The method needs a minimum of time and sample preparation (cleaving and etching) to obtain the dot density, dot distribution, and give an estimate of the dot dimensions. Etching only the cleaved surface of the sample opens up the opportunity to determine the properties of a buried dot layer before or even after device fabrication.     

Determination of the outward relaxation of cleaved strained InAs structures by scanning tunneling microscopy

When a semi-conductor structure containing strained layers such as quantum wells (QWs) or quantum dot layers is cleaved, the surface will relax outward in order to release built-in strain. This outward relaxation is directly linked to the composition of the strained layers, and can thus provide accurate information about the local composition of these layers. By using cross-sectional scanning tunneling microscopy (X-STM) it is possible to measure this outward relaxation. The measured height profiles, however, are also dependent on the chemical composition of the measured surface, resulting in an extra height contrast in the images. In order to analyze only the outward relaxation, it is necessary to suppress this latter chemical component in the STM measurements. This can be achieved by choosing the proper tunnel conditions.