Large splitting of the cyclotron-resonance line in AlxGa1-xN/GaN heterostructures

Ever since its first demonstration in 1953 [1], cyclotron resonance (CR) has become the most widely used technique to determine effective masses of carriers in semiconductors and their heterojunctions. In the presence of an external magnetic field, B, electrons of charge e are set into cyclotron motion whose frequency is given by ωc = eB/m ∗ c. An RF or far infra-red source is swept which generates a single absorption line at ωc, from which the effective mass m ∗ is deduced. CR has also been instrumental in detecting other excitations of the solid via their interaction with CR. They often take on the shape of a level anti-crossing from which the energy of the new excitation and its coupling to CR can be inferred. These are well-established, textbook phenomena, which make CR one of the best-understood tools in solid state research [2]. Yet, in our experiments on the comparatively new AlGaN/GaN heterostructures we observe huge splittings in the CR line, indicating a strong interaction with an excitation whose origin remains unknown. Our results provide a fundamental challenge to our apparently thorough understanding of CR in solids. For AlGaN/GaN heterostructures there exist only few CR data [3, 4, 5, 6]. Our study of CR in two-dimensional electron systems (2DESs) in a sequence of high-quality AlGaN/GaN structures with a wide range of carrier densities (1 - 4×10 12 cm −2 ) reveals splittings in the CR line reminiscent of level anti-crossings, reaching 20% of the resonance energy. While their origin is not established, they resemble splittings seen previously in the CR of AlGaAs/GaAs and Si systems. Our data do not support a universal ad-hoc model put forward in the AlGaAs/GaAs work and points to the lack of a theoretical understanding of this phenomenon, now seen in three different 2D systems.