Nanostructure and millikelvin laboratories

History, sponsors, collaboration, activities

In the recent years, a number of new phenomena demonstrating the quantum nature of electron transport in solids has been discovered in the millikelvin temperature range. The integer and fractional quantum Hall effect, disorder-induced quantum localization, universal conductance fluctuations, nonequilibrium and 1/f noise, quantization of the point contact resistance, the Coulomb blockade, the Aharonov-Bohm effect, and persistent currents in normal-metal rings constitute the celebrated examples [1,2]. For the presently available nanostructuring methods these effects show up at subkelvin temperatures, but in view of steady progress in device miniaturization they may start to perturb operation of the room-temperature devices in a near future. At the same time, however, some of them could constitute the principle of operation of the next generation devices [2].

This growing importance of quantum structures in today's science and technology has resulted in a common initiative of the Institute of Physics of the Polish Academy of Sciences and the Institute of Electron Technology to create a new laboratory capable of fabricating nanostructures by means of high-energy electron-beam lithography. This technique has been chosen from many other nanostructuring methods because of its versatility and usefulness for both basic and applied research. The laboratory has been rooted in two groups: Group of Low Temperature Physics at the Institute of Physics and Department of III-V Semiconductor Processing at the Institute of Electron Technology. The former is equipped with a dilution refrigerator and measuring apparatus, suitable for search for dimensional effects in charge transport, magnetic and optical properties down to 25 mK, up to 9 T and under hydrostatic pressure of 6 kbar. The latter, in addition to professional setups for photolithography and metal deposition, has reactors for wet, plasma, and reactive-ion etching.

The Nanostructure Laboratory is located in a newly constructed clean room with the purity class of 100 inside laminar-flow boxes, and temperature stability ±0.5°C. Its one part houses the electron-beam writer, placed on an antivibration base. The second is a chemical room for resist processing. The electron beam lithography system consists of a JEOL 40 kV scanning electron microscope (JSM 6400) coupled to a 2.6 MHz Raith pattern generator (Elphy-Plus). Such a system, together with appropriate resist processing, makes it possible to fabricate structures with critical dimensions down to 20 nm. Moreover, multi-level and large surface lithography with the linewidth down to 100 nm is possible with the use of a Raith laser-interferometer stage that completes the installed equipment. In the chemical part of the clean room, in addition to a standard equipment such as a fume cupboard and laminar box, there is a Brewer spinner with a hot plate and a multisubstrate chuck (Model 100CB). Thus, all resist processes, from coating and baking through exposure and development, can be made in one place and under controlled conditions.

In addition of serving as the electron-beam source for the lithography, the microscope is routinely used for surface visualization as well as for a nondestructive chemical analysis. The latter is possible due to a Link X-ray analyzer (Isis), equipped with a low-energy detector and a signal processor that provide quantitative information on the concentration of all elements from beryllium upwards. The construction of the clean room and the purchasing of the advanced equipment was possible due to harmonious collaboration between the two Institutes involved, and financial support from a number of organizations: Committee for Scientific Research (Poland), Polish-German Cooperation Foundation, Foundation for Polish Science, Austrian Ost-West Program, and II Joint Polish-American Maria Sklodowska-Curie Fund.

A long tradition in low-temperature research and rather unique equipment of the Group of Low Temperature Physics enabled the performance of various projects involving physicists from the Institute and outside. An example of successful collaboration can be the study of the effect of compensation upon localization in n-InSb, carried out in cooperation with the Kurchatov Institute [3]. Another example constitutes the observation of a substantial reduction of the Landau level spin-splitting in narrow quantum wires, a work involving physicists from the Boltzmann Institute, performed on GaAs/AlGaAs heterostructures grown in the Research Institute of the German Post and subsequently processed in IBM [4]. A continuation of the project with a group from the Hamburg University led to the discovery of the quenching of electron scattering by acoustic phonons in certain ranges of the magnetic fields [5].

Collaboration with a group of the Kepler University in Linz was particularly extensive. For example, noteworthy is the study aiming at elucidating the nature of resonant states in semiconductors. By means of a comprehensive optical and transport millikelvin study of CdSe:Sc (done in Linz and Warsaw, respectively) it was shown that the standard hydrogenic level constitutes the ground state of resonant impurities [6].

Other collaborative work involved quantum localization in short-period Si/SiGe:Sb superlattices, which were grown in the Daimler-Benz Laboratory. It has been found that localization is three-dimensional at subkelvin temperatures despite the lack of coherent vertical transport [7]. It has also been demonstrated that the metal-to-insulator transition induced by the magnetic field is driven by the destructive influence of the spin-splitting upon the anti-localizing terms of the disorder-modified electron--electron interactions [8]. In the framework of the most recent common project, quantum wires of IV-VI semiconductors, such as PbTe and PbSe, were patterned by photo- [9] or electron-beam lithography [10] and studied at millikelvin temperatures [10]. Because of a strong sensitivity of the conductivity in these multivalley semiconductors to strain, the results provided important information on the deformation of the wires which are patterned from epilayers grown on lattice mismatched substrates [10]. Another attractive property of this system is their high electron mobility, which is presently explored to examine the regime of ballistic transport.

The especially rewarding stream of low-temperature research has aimed at elucidating the influence of localized spins upon quantum transport in semimagnetic semiconductors. It has turned out that the spins constitute a novel tool to probe various aspects of quantum transport and, conversely, the phenomena of quantum transport provide important information on spin ordering and dynamics. In particular, detailed experimental studies of conductance fluctuations and noise in wires of n+Cd1-xMnxTe with diameter of the order of 0.3 m , were undertaken [11,12]. Similar measurements were carried out for wires [11,13] and films [13] of n+CdTe. The nanostructures were patterned by electron-beam lithography from layers obtained by MBE in the Laboratory of Growth and Physics of Low-Dimensional Crystals. The epilayers were doped with either indium or iodine. In the studied range of the Mn concentrations 0.01 Ł x Ł 0.2, the short-range antiferromagnetic spin--spin interactions lead to the spin-glass transition at 0.01 Ł T Ł 2.5 K, respectively. Because of a large difference between the relevant length scales, the studied wires are mesoscopic from the point of view of the electronic properties but macroscopic as far as the magnetic subsystem is concerned. Millikelvin studies of these structures demonstrated the existence of a new mechanism, by which the universal conductance fluctuations can be generated in mesoscopic systems containing magnetic ions [11]. Moreover, 1/f conductance noise as well as thermal and magnetic irreversibilities, predicted by Monte Carlo simulations [14], have been observed [12], providing important information on spin-glass dynamics.

[1] See, T. Dietl, G. Grabecki, J. Jaroszynski, Semicond. Sci. Technol. 8, S141 (1993), and references therein.
[2] See, T. Dietl, At the Limit of Device Miniaturization, in: From Quantum Mechanics to Technology, Eds. Z. Petrou et al., Springer, Berlin 1996, p. 75, and references therein.
[3] B.A. Aronzon, N.K. Chumakov, J. Wrobel, T. Dietl, Zh. Eksp. Teor. Fiz.105, 405. (1994) [Sov. Phys. JETP 78, 216. (1994)].
[4] J. Wrobel, F. Kuchar, K. Ismail, K.Y. Lee, H. Nickel, W. Schlapp, G.Grabecki, T. Dietl, Surf. Sci. 305, 615. (1994).
[5] J. Wrobel, T. Brandes, F. Kuchar, B. Kramer, K. Ismail, K.Y. Lee, H.Hillmer, W. Schlapp, T. Dietl, Europhys. Lett. 29, 481. (1995).
[6] P. Glod, T. Dietl, T. Fromherz, G. Bauer, I. Miotkowski, Phys. Rev. B 49, 7797. (1994).
[7] G. Stoger, G. Brunthaler, G. Bauer, J. Jaroszynski, M. Sawicki, T. Dietl, F. Schaffler, Solid State Electron. 40, 47. (1996).
[8] G. Brunthaler, T. Dietl, J. Jaroszynski, M. Sawicki, G. Stoger, A. Prinz, F. Schaffler, G. Bauer, Semicond. Sci. Technol. 11, 1624. (1996).
[9] G. Grabecki, S. Takeyama, S. Adachi, Y. Takagi, T. Dietl, E. Kaminska, A. Piotrowska, E. Papis, N. Frank, G. Bauer, Jpn. J. Appl. Phys. 34, 4433. (1995).
[10] G. Grabecki, J. Wrobel, T. Dietl, M. Sawicki, J. Domagala, T.Skoskiewicz, E. Papis, E. Kaminska, A. Piotrowska, M. Leszczynski, Y.Ueta, G. Springholtz, G. Bauer, Superlattices Microstruct. 22, 51. (1997).
[11] J. Jaroszynski, J. Wrobel, M. Sawicki, E. Kaminska, T. Skoskiewicz, G. Karczewski, T. Wojtowicz, A. Piotrowska, J. Kossut, T. Dietl, Phys. Rev. Lett. 75, 3170. (1995).
[12] J. Jaroszynski, J.Wrobel, G.Karczewski, T.Wojtowicz, T.Dietl, Phys. Rev. Lett. 80, 5635. (1998).
[13] J. Jaroszynski, J. Wrobel, R. Nowakowski, R. Dus, E. Papis, E.Kaminska, A.Piotrowska, G.Karczewski, T.Wojtowicz, M.Sawicki, T.Skoskiewicz, T.Dietl, Thin Solid Films. 306, 291. (1997).
[14] M.Cieplak, B.R.Bulka, T.Dietl, Phys. Rev. B 51, 8939. (1995).