Pm Paul Koenraad

Single dopants in semiconductors

The sensitive dependence of a semiconductors electronic, optical and magnetic properties on dopants has provided an extensive range of tunable phenomena to explore and apply to devices. Recently it has become possible to move past the tunable properties of an ensemble of dopants to identify the effects of a solitary dopant on commercial device performance as well as locally on the fundamental properties of a semiconductor. New applications that require the discrete character of a single dopant, such as single-spin devices in the area of quantum information or single-dopant transistors, demand a further focus on the properties of a specific dopant. This article describes the huge advances in the past decade towards observing, controllably creating and manipulating single dopants, as well as their application in novel devices which allow opening the new field of solotronics (solitary dopant optoelectronics).

Determination of the shape and indium distribution of low-growth-rate InAs quantum dots by cross-sectional scanning tunneling microscopy

We present a cross-sectional scanning-tunneling microscopy investigation of the shape, size, and composition of InAs quantum dots in a GaAs matrix, grown by molecular beam epitaxy at low growth rate. From the dimensional analysis we conclude that the investigated quantum dots have an average height of 5 nm, a square base of 18 nm oriented along [010] and [100] and the shape of a truncated pyramid. From outward relaxation and lattice constant profiles we conclude that the dots consist of an InGaAs alloy and that the indium concentration increases linearly in the growth direction. Our results justify the predictions obtained from previous photocurrent measurements on similar structures and the used theoretical model.

Spatial structure of an individual Mn acceptor in GaAs.

The wave function of a hole bound to an individual Mn acceptor in GaAs is spatially mapped by scanning tunneling microscopy at room temperature and an anisotropic, crosslike shape is observed. The spatial structure is compared with that from an envelope-function, effective mass model and from a tight-binding model. This demonstrates that anisotropy arising from the cubic symmetry of the GaAs crystal produces the crosslike shape for the hole wave function. Thus the coupling between Mn dopants in GaMnAs mediated by such holes will be highly anisotropic.

Atomic-scale structure of self-assembled In(Ga)As quantum rings in GaAs

We present an atomic-scale analysis of the indium distribution of self-assembled In(Ga)As quantum rings (QRs) which are formed from InAs quantum dots by capping with a thin layer of GaAs and subsequent annealing. We find that the size and shape of QRs as observed by cross-sectional scanning tunneling microscopy (X-STM) deviate substantially from the ring-shaped islands as observed by atomic force microscopy on the surface of uncapped QR structures. We show unambiguously that X-STM images the remaining quantum dot material whereas the AFM images the erupted quantum dot material. The remaining dot material shows an asymmetric indium-rich crater-like shape with a depression rather than an opening at the center and is responsible for the observed electronic properties of QR structures. These quantum craters have an indium concentration of about 55% and a diameter of about 20nm which is consistent with the observed electronic radius of QR structures.

Capping process of InAs/GaAs quantum dots studied by cross-sectional scanning tunneling microscopy

The capping process of self-assembled InAs quantum dots (QDs) grown on GaAs(100) substrates by molecular-beam epitaxy is studied by cross-sectional scanning tunneling microscopy. GaAs capping at 500°C causes leveling of the QDs which is completely suppressed by decreasing the growth temperature to 300°C. At elevated temperature the QD leveling is driven in the initial stage of the GaAs capping process while it is quenched during continued overgrowth when the QDs become buried. For common GaAs growth rates, both phenomena take place on a similar time scale. Therefore, the size and shape of buried InAs QDs are determined by a delicate interplay between driving and quenching of the QD leveling during capping which is controlled by the GaAs growth rate and growth temperature.

Suppression of InAs/GaAs quantum dot decomposition by the incorporation of a GaAsSb capping layer

The influence of a GaAsSb capping layer on the structural properties of self-assembled InAs∕GaAs quantum dots (QDs) is studied on the atomic scale by cross-sectional scanning tunneling microscopy. QDs capped with GaAs0.75Sb0.25 exhibit a full pyramidal shape and a height more than twice that of the typical GaAs-capped QDs, indicating that capping with GaAsSb suppresses dot decomposition. This behavior is most likely related to the reduced lattice mismatch between the dot and the capping layer.

Formation of InAs wetting layers studied by cross-sectional scanning tunneling microscopy

We show that the composition of (segregated) InAs wetting layers (WLs) can be determined by either direct counting of the indium atoms or by analysis of the outward displacement of the cleaved surface as measured by cross-sectional scanning tunneling microscopy. We use this approach to study the effects of the deposited amount of indium, the InAs growth rate, and the host material on the formation of the WLs. We conclude that the formation of (segregated) WLs is a delicate interplay between surface migration, strain-driven segregation, and the dissolution of quantum dots during overgrowth.

Atomic scale analysis of self assembled GaAs/AlGaAs quantum dots grown by droplet epitaxy

In this letter we have performed a structural analysis at the atomic scale of GaAs/AlGaAs quantum dots grown by droplet epitaxy. The shape, composition, and strain of the quantum dots and the AlGaAs matrix are investigated. We show that the GaAs quantum dots have a Gaussian shape and that minor intermixing of Al with the GaAs quantum dot takes place. A wetting layer with a thickness of less than one bilayer was observed.

Stacked low-growth-rate InAs quantum dots studied at the atomic level by cross-sectional scanning tunneling microscopy

Structures containing stacked self-assembled InAs quantum dots within a GaAs matrix are studied by cross-sectional scanning tunneling microscopy. The dots consist of an InGaAs alloy with an increasing indium concentration in the growth direction. From comparison of the lattice constant profiles of stacked and unstacked dots, it is evident that the strain in the GaAs matrix around the dots is strongly affected by the stacking process. The results show an increasing deformation of the dots in the stack and a reduced growth rate of the GaAs spacer layers, resulting in the formation of terraces on the growth surface on which new dots form. If the total structure, containing the dot layers and the spacer layers, exceeds 30 nm, the local GaAs growth rate remains constant from this point on. The InAs dot growth rate remains constant throughout the entire stack.

Theory of electron energy spectrum and Aharonov-Bohm effect in self-assembled Inx Ga1-x As quantum rings in GaAs

We analyze theoretically the electron energy spectrum and the magnetization of an electron in a strained