From weak to strong localization in a ferromagnetic high mobility 2DHG

Manganese modulation-doped two-dimensional hole systems confined in strained InAs/InGaAs/InAlAs heterostructures were investigated by low-temperature magnetotransport experiments. The study demonstrates quantized transport phenomena in the high field region, weak anti-localization in the low-field region and ferromagnetic ordering in the separated and insulating manganese-doped layer. A significant amount of manganese in the channel of inverted modulation-doped structures causes a strong localization effect with hysteretic-like abrupt resistance changes over several orders of magnitude at very low temperatures. & 2009 Elsevier B.V. All rights reserved. Low-dimensional charge carrier systems confined in InGaAs or InAs quantum wells (QWs) hold advantageous properties such as large g-factor and significant spin-orbit coupling [1,2] to study spin-dependent transport phenomena in reduced dimensions. Diluted magnetic semiconductors highly doped with manganese (Mn) feature hole-mediated ferromagnetism [3]. For high doping concentration (42%) metallic behavior is achieved resulting in poor charger carrier mobility and short mean free path of only a few lattice constants [4]. Combining InAs-based heterostructures together with Mn modulation doping afford to explore the interaction of spins of highly mobile two-dimensional holes with magnetic moments of 5/2 provided by Mn ions. Therefore we have investigated ferromagnetic Mn modulationdoped two-dimensional hole gases (2DHGs) with low-temperature magnetotransport experiments. The samples were grown on strain-relaxed metamorphic buffers on semi-insulating (0 0 1) GaAs substrate by means of molecular beam epitaxy. The active layer consists of a 20 nm InGaAs QW with an inserted 4 nm InAs channel and a 7-nm-thick Mn-doped InAlAs layer that is 5 nm separated from the QW. The Mn acceptor concentration in the InAlAs doping layer is less than 2 10 cm 3 as estimated from flux calibration of the Mn effusion cell [5]. The In concentration in the active region amounts to 75% leading to a compressive strain in the InAs channel hosting the 2DHG. Magnetotransport measurements were performed on optically defined and wet chemical etched Hall bars aligned along the [1 1 0] crystallographic direction using standard four-terminal low-frequency ll rights reserved. ed Physics, Jungiusstr. 11 el.: +49 40 428 382040; de (U. Wurstbauer). lock-in technique with an excitation current of I=100 nA for resistance values less than 30 K. High resistance states were quantified in a two-terminal geometry applying a constant excitation voltage Ubias=0.5 V and measuring the flowing current through the device. The resistance is then calculated by R=Ubias/I. The difference of the two QW structures of interest in this paper is the position of the doping layer with respect to the growth direction. Schemes of the QW layer sequence are inserted in Fig. 1. In the normal structure (Fig. 1(a)) the Mn-doping layer is carried out after the InGaAs/InAs QW growth and in the inverted structure (Fig. 1(b)) the doping layer is grown before the QW. Asymmetric broadening of the doping layer causes a significant amount of Mn in the InAs channel of the inverted structure. This has been quantified by secondary ion mass spectroscopy (SIMS) measurements to about 1% of the Mn concentration in doping layer, whereas the quantum well region of the normal structure is free of Mn [5]. As demonstrated in Fig. 1, low-temperature magnetotransport measurements on both structures in the four-terminal geometry at T=200 mK reveal clear Shubnikov-de Haas (SdH) oscillations in the longitudinal resistance Rxx and quantum Hall plateaus in the transverse resistance Rxy. The positive Hall coefficient proves the transport in a hole systems and the vanishing longitudinal resistance at lower filling factors excludes the existence of a parallel conducting channel. The two-dimensional hole density p was ascertained from the classical Hall slope and confirmed from the 1/B periodicity of the SdH oscillations and constitutes p=4.3 10 cm 2 and p=4.4 10 cm 2 for the normal and inverted doped structure, respectively. The more pronounced SdH oscillations and quantum Hall plateaus reveal a higher mobility for the normal compared to the inverted doped structure. Substantial differences are visible in the low-field region. A