Optical and electrical study of cap layer effect in QHE devices with double-2DEG
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Optical and electrical study of cap layer effect in QHE devices with double-2DEG L. Zamora-Peredo1*, I. Cortes-Mestizo1, L. García-Gonzáez1, J. Hernández-Torres1, T. Hernandez-Quiroz1, M. Peres-Caro2, M. Ramirez-López2, I. Martinez-Veliz2, Y. L. CasallasMoreno2, S. Gallardo-Hernández2, A. Conde-Gallardo2, M. López-López2. 1
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Centro de Investigación en Micro y Nanotecnología, Universidad Veracruzana, Calzada Adolfo Ruiz Cortines # 455, Fracc. Costa Verde, C.P. 94292, Boca del Río, Veracruz, México. Departamento de Física, Centro de Investigaciones y de Estudios Avanzados - IPN, México D. F., México
ABSTRACT In this work we report on the characteristics of GaAs/AlGaAs heterostructures with a symmetric double two-dimensional electron gas (D-2DEG). Optical characterization was made by room temperature photoreflectance (PR) spectroscopy as well as electrical properties were determinated using the quantum Hall effect measurements at 2K. In order to study the surface effects on the conduction band profile, three samples with different GaAs cap layer thickness (25, 60 and 80 nm) were grown by the molecular beam epitaxy. Photoreflectance spectra at room temperature show the wide-period Franz-Keldysh oscillations between 1.42 and 1.70 eV originated by the surface electric field. The analysis of these oscillations shows that the surface electric field varies from 503 to 120 kV/cm whereas the thickness of the cap layer increases that was produced by the reduction of the depletion zone near the surface. Using QHE measurements we found that electron density increases if the surface electric field decreases. INTRODUCTION In the field of the electrical quantum metrology, the quantum Hall effect (QHE) in most devices is based on a two-dimensional electron gas (2DEG) realized in GaAs/AlGaAs heterostructures. The metrological calibrations are carried out by filling with factor i=2, thus providing a quantized resistance value of RK/2=h/2e2a12.9 k:, where RK is the von Klitzing constant, e is the electron’s charge, and h is the Planck constant [1, 2]. For practical applications, however, it is desired to have a set of resistance values besides RK/2. QHE circuits consisting of devices connected in parallel or in series have been proposed for tailoring of resistance to different values [3, 4]. For parallel QHE circuits, double 2DEGs structures with two parallel transport channels stacked in the as-grown heterostructure have substantial advantages. The number of Hall bars is halved and it is possible to use the higher values of the working current. Different type heterostructures have been proposed in order to design a double-2DEG system. Some symmetric heterostructures [5] with the electron density of 5.4x10-11 cm-2 was created and others asymmetric structures [6, 7] were created with the electron density varying between 4 and 9x10-11 cm-2. In this work, we studied a set of heterostructures by PR and QHE measurements in order to determinate the effects of the surface electric field over the conduction band behavior.
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