Artificial atoms from magnetic semiconductors
Zusammenfassung der Projektergebnisse
This work combined the knowhow of two leading groups in their specific research field to deepen the knowledge on magnetic properties of artificial atoms. The German group, an expert on II-VI DMS RTD heterostructures, has developed a reliable fabrication method of artificial atoms from the (Zn,Cd,Be,Mn)Se material system with the aid of the Japanese group. In contrast to previously presented work on II-VI DMS micron sized RTDs, devices with mesa diameters as small as 250 nm have been reliably fabricated and fitted with a gating electrode. During the development process the line mesa for top contacting of artificial atoms showed to be unsuitable unlike for structures in III-V material systems. The implementation of air bridges reveals the importance of good electrical junctions between the bridge post and the QD’s top contact. The awareness of this point lead to a fabrication technology with a good yield. Furthermore the all II-VI DMS heterostructure imposes a challenge to a good gate alignment, necessary for an efficient manipulation of the QD’s potential landscape. Experiments on etching parameters and insulator growth rates enabled the fabrication of gateable QD’s and lead finally to the first observation of Coulomb blockade in artificial atoms from magnetic semiconductors. Low temperature electric measurements of QDs with mesa diameter of 250 nm enabled the observation of Coulomb diamonds revealing the zero dimensional quantization and proving the artificial atom characteristics. Simultaneously these devices show a high device resistance and a not yet determined intrinsic resistance hindering measurements at source-drain biases of a few µV. The evolution of the zero dimensional states in an applied external magnetic field shows Brillouin dependence corresponding to the giant Zeeman splitting in DMS. The energetic spectrum in an external magnetic field reveals interesting features, e.g. anticrossing of quantum states, promising interesting physical dependencies. A more thorough analysis of the spectrum is in process but complicated by the low current through the device and consequently the low signal to noise ratio. New samples incorporating improvements based on the above described insights from fabrication and measurements are in preparation and promising improved electrical properties. While the magnetic characterization is still obviously in its infancy, we believe the results of this project set the stage for a proper study of artificial atoms, and will stimulate much future work, both in our own lab, and across the field in general.
