Magnetic barrier in a two-dimensional hole gas
Introduction
Two-dimensional hole gases (2DHGs) in Ga[Al]As - heterostructures show a variety of phenomena like strong interactions [1] or a pronounced spin–orbit coupling [2] relevant for the spin-Hall effect [3], edge spin accumulation [4] as well as for future spintronic devices. Many recently observed effects rely on the special properties of nanostructured 2DHGs in Ga[Al]As, like spin separation in cyclotron motion [5], the anisotropic modification of the effective g-factor by confinement [6], or the influence of spin–orbit interactions on Aharonov–Bohm oscillations in quantum rings [7], [8]. It is therefore essential to have technologies at hand which allow a–preferably tunable–confinement of Ga[Al]As—based 2DHGs. However, such nanostructures are notoriously difficult to be defined and tuned by top gates, since they show leakage currents and instabilities [9], [10]. This is particularly the case for shallow Ga[Al]As -2DHG systems with the hole gas residing at less than ≈100 nm below the surface, [11] which, on the other hand, is desirable for the definition of small nanostructures. The reason for these difficulties is that the metal–GaAs Schottky barrier in 2DHG systems is small and has to be operated in the forward direction for hole depletion. As a consequence, tunable nanostructures in Ga[Al]As–based 2DHGs [12], [5], [13], [7], [8], [6] have so far been fabricated by scanning probe lithography [14]. However, as compared to top gates, this technique does have the disadvantages of reduced tuneability and screening.
Here, we report the implementation of a magnetic nanostructure on a shallow 2DHG in a Ga[Al]As heterostructure. Our technique comprises the perpendicular fringe field of a magnetized ferromagnetic film residing on top of the semiconductor surface. In order to avoid leakage between the 2DHG and the ferromagnet, an AlOx layer is deposited on top of the semiconductor prior to the metallization step. This technology paves the way for further experiments which rely on special properties of Ga[Al]As—based 2DHGs in inhomogeneous magnetic fields, like the predicted effects of spin–orbit coupling on the transmission of magnetic barriers [15].
Section snippets
Sample preparation and experiments
Fig. 1(a) shows the scheme of the sample. We use a C-doped, p-type GaxAl1−xAs (100) heterostructure [16] with a 2DHG 45 nm below the surface. The Hall bar is prepared by optical lithography followed by wet chemical etching. The hole gas is accessed via Au/Zn ohmic contacts, fabricated by thermal evaporation of 10 nm Au followed by 50 nm Zn and 200 nm Au.
The Zn layer was evaporated at a rate of 3 nm/min from a closed boat with a pin-hole. The contacts were alloyed by the sequence 120 ∘C for
Simulations and discussion
To provide a quantitative numerical model of the magnetic barrier resistance [22] we simulated the process using the Landauer–Büttiker formalism [24], [25] in a 6 probe geometry. Due to strong inter-subbband scattering at 2 K, the two-band magnetoresistance is suppressed [26] and the drift velocities of the holes in the two subbands are similar. Therefore, it is justified to simplify the model and consider a single band with a single density determined by the Hall slope at low magnetic fields,
Acknowledgments
M.C. acknowledges the financial support from the Forschungs—Förderungsfonds of HHU Düsseldorf. D.R. and A.D.W. acknowledge the financial support from BMBF nanoQuit and SFB 491.
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