Micro-patterned RTDs: Fabrication details and device performance T. Borzenko * , T. Slobodskyy, D. Supp, C. Gould, G. Schmidt, L.W. Molenkamp Physikalisches Institut der Universita ¨t Wu ¨ rzburg, Am Hubland, 97074 Wu ¨ rzburg, Germany Available online 1 February 2007 Abstract Resonant tunnelling diode (RTD) structures of complex design can be fabricated from diluted magnetic semiconductors (ZnSe-based material) grown by molecular beam epitaxy. The heterostructures contain one or more pairs of tunnel barriers such as for example (Zn,Be)Se/(Zn,Mn)Se/(Zn,Be)Se in the growth sequence. The fabrication process for a double RTD considered in this work has pecu- liarities and also common approaches distinctive to this material. Process optimization confirmed the possibility to obtain complex devices from this material interesting both from fundamental and practical points of view. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Electron beam lithography; Resonant tunnelling diodes 1. Introduction Resonant tunnelling diode (RTD) structures from diluted magnetic semiconductor (DMS) materials are cur- rently of great interest because, due to the presence of mag- netic atoms in the active layers, both charge and spin play a role in determining the structures’ transport properties [1,2]. A further direction of research in this field is pursuing the properties of microstructured RTDs, which should act as artificial magnetic atoms. Previously, Tarucha and his collaborators investigated micro-structured RTDs from III–V materials as vertical quantum dots which in fact rep- resent artificial atoms, and also a gated micro-RTD as a single electron transistor, e.g. [3–5]. Experiments on the artificial atoms in electrical and magnetic fields give answers on many fundamental questions in solid state physics. Combining both practical and fundamental inter- est we thus have realized micro-RTD structures of varying complexity using II–VI (ZnSe-based) diluted magnetic semiconductors. The double RTD-microstructure pre- sented in this work is one of such structures intended for transport experiments. In this work we present some tech- nological aspects important for the device performance whereas transport characteristics of the devices will be pre- sented elsewhere. 2. Experiment and discussion 2.1. Layer sequence A standard layer sequence for RTDs starts from grow- ing a superlattice consisting of alternation of a Zn 0.92 Be 0.08 Se (20 nm)/ZnSe (50 nm) pair of undoped lay- ers on a Zn 0.97 Be 0.03 Se highly doped (1.5e19) layer (200 nm thick) which in turn is grown on a GaAs buffer (undoped) located on an undoped GaAs substrate. The superlattice is capped by 300 nm of undoped Zn 0.97 Be 0.03 Se, providing an appropriate template for the growth of the RTD. An example of a typical RTD- sequence is Zn 0.97 Be 0.03 Se (1.5e19, 300 nm)/ZnSe (1.5e19, 100 nm)/Zn 0.97 Be 0.03 Se (undoped, 10 nm)/Zn 0.75 Be 0.25 Se (undoped, 5 nm, barrier)/Zn 0.92 Mn 0.08 Se (undoped, 7 nm, well)/Zn 0.75 Be 0.25 Se (undoped, 5 nm, barrier)/ZnSe (undoped, 10 nm)/Zn 0.97 Be 0.03 Se (1e18, 15 nm)/ZnSe (1.5e19, 30 nm). This RTD structure is capped in situ (in a molecular beam epitaxy (MBE) chamber) by Al (10 nm)/Ti (10 nm)/Au (30 nm). 0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.095 * Corresponding author. Tel.: +49 931 8885876. E-mail address: borzenko@physik.uni-wuerzburg.de (T. Borzenko). www.elsevier.com/locate/mee Microelectronic Engineering 84 (2007) 1566–1569