A. Schramm

Lithographically defined metal-semiconductor-hybrid nanoscrolls

We report on two-layer metal-semiconductor-hybrid scrolls fabricated from rolled-up strained metal∕InGaAs-layers. As the central approach, the metallic layer itself acts as a stressor in contact with the semiconductor. Position and length of the scrolls can be precisely tuned by patterning the e-beam-evaporated metallic stressor with conventional lithographic techniques. The thickness of the metallization determines the radius of the resulting scrolls. This fabrication technique significantly improves the reliability and simplifies the fabrication of metal∕semiconductor three-dimensional objects which employ bending up layers. Even more important, using this technique the bending radius of such three-dimensional objects can easily be downsized to very small radii in the nanometer scale, e.g. in order to build nano-electro-mechanical systems.

InAs-coverage dependence of self-assembled quantum dot size, composition, and density

The authors report about the x-ray investigation of InAs∕GaAs(001) quantum dot systems grown with varying amount of deposited InAs. It is shown that the intermixing induced composition of investigated quantum dots remains constant within the whole probed InAs deposition region. It is found that the increase of deposited InAs entirely leads to a proportional increase of surface density of dots and does not significantly influence the dot size. The dot average chemical composition was quantitatively estimated by comparison to finite-element based calculations.

Electron emission from self-assembled quantum dots in strong magnetic fields

We probe with deep level transient spectroscopy electron states in self-assembled InAs quantum dots. Two pronounced maxima are observed that we associate with emission from different quantum-dot orbital states. Fine structure clearly establishes distinct emission rates for quantum dots with one or two electrons in the s state and up to four electrons in the p-like states. In order to confirm these assignments spectra have been recorded in strong magnetic fields. The observed magnetic field dispersion of the emission energies is described with a harmonic oscillator model using an effective electron mass of m*=0.03me.

Field effect enhanced carrier‐emission from InAs Quantum Dots

We probe the emission from the electron levels bound to MBE‐grown self‐assembled InAs/GaAs quantum dots with transient capacitance spectroscopy (DLTS). Emission from the s‐state occupied with one ore two electrons is reflected by two well separated peaks. An additional broad peak observed at lower temperature is associated to emission from the p‐state. We relate substructure in this peak to the p‐level occupation with up to 4 electrons. The observations establish a strong dependence of the emission energies on the electric field at the dots. From an analysis of our results we conclude that the field dependence originates from a thermally assisted tunneling process through an intermediate state in a continuous band. Dot‐binding energies calculated with this model are 165±2 meV for the s1‐state and 151±2 meV for the s2‐state in our sample and found to be nearly field independent. In addition we perform admittance spectroscopy on the same samples, which yields emission energies in good agreement with those f...

Constant Capacitance‐DLTS on InAs‐Quantum Dots embedded in Schottkydiodes

Electron escape from self‐assembled InAs quantum dots (QDs) in GaAs is studied. We apply constant capacitance deep level transient spectroscopy (CC‐DLTS) to investigate the electric field dependence of the activation energies. As a major advantage for interpretation of the results in contrast to conventional DLTS the electric field of the quantum dot layer remains constant in CC‐DLTS experiments. The results of conventional DLTS results are well reproduced.

Tunnelling Transient Spectroscopy on self‐assembled InAs Quantum Dots

We study electron tunnelling from self‐assembled InAs quantum dots with time‐resolved capacitance measurements at low temperature (T = 10 K). Within a simple WKB model the electric field dependence of the tunnelling rate is analyzed. The data reveal that tunnelling emission from s‐ and p‐like quantum dot states can clearly be resolved. The binding energies obtained from a triangular‐well model are in good agreement with values obtained with DLTS experiments.

X-ray study of temperature dependent growth of InAs/AlAs(0 0 1) quantum dots

We report on the x-ray analysis of non-capped InAs/AlAs(0 0 1) quantum dot systems grown at temperatures ranging from 480 up to 530 °C. A constant amount of InAs has been deposited resulting in a growth stage where coherently strained dots and plastically relaxed clusters coexist. It is found that with an increase in deposition temperature the average size of elastically strained dots increases without changes in their chemical composition and surface density. The observed process is in accordance with the InAs volume decrease stored in plastically relaxed clusters. The results establish the crucial role of strain-induced material intermixing between strained InAs dots and the AlAs substrate over the investigated growth temperature range.