Intersubband magnetophonon resonances in quantum cascade structures

Abstract

We report on our magnetotransport measurements of GaAs/GaAlAs quantum cascade structures in a magnetic field of up to 62 T. We observe novel quantum oscillations in tunneling current that are periodic in reciprocal magnetic field. We explain these oscillations as intersubband magnetophonon resonance due to electron relaxation by emission of either single optical or acoustic phonons. Our work also provides a non-optical in situ measurement of intersubband separations in quantum cascade structures. 73.21.Fg,73.43.Qt,85.35.Be,42.55.Px Ever since the pioneering work on the magnetophonon effect by Gurevich and Firsov [1], high magnetic fields have been regarded as an important tool for investigation of the electron-optical-phonon interaction in semiconductor systems, particularly in confined structures. For the in-plane transport, quantization of the carrier motion in the plane into discrete Landau levels (LLs) of energies (N + 1/2)hω c , ω c = eB/m * is the cyclotron frequency, gives rise to quantum oscillations at elevated temperatures due to resonant phonon absorption. These magnetophonon oscillations are periodic in 1/B, and their strength is related to the electron-optical-phonon coupling, while the period gives either the effective mass or the energy of the participating optical phonons [2]. On the other hand, for perpendicular transport the magnetotunneling measurements in double barrier systems have allowed direct probing of the optical-phononassisted transitions from a quasi-two-dimensional (2D) emitter into empty LLs of the central well, as well as determining the effective mass carrier dynamics in these structures [3]. However, little is known from these double barrier studies about intersubband relaxation via opticalphonon emission in quantum wells (QWs). A particularly interesting and unexplored situation occurs when the cyclotron energy exceeds the optical phonon energy and/or the subband energy separation. The first situation is achieved in a GaAs 2D electron gas at magnetic fields above 22 T [2], where the in-plane electron wave function is localized on a scale of the magnetic length, l c = h/eB (3.2 nm at 62 T), and the electron behavior is essentially zero dimensional.Intersubband relaxation via optical phonon emission in quantum wells plays a key role in intersubband radiation sources like the quantum cascade lasers (QCLs) [4]. These systems consist of double-barrier-like structures with three subbands belonging to a central QW structure. When a high bias is applied to the QCL system, the upper subband is populated by tunneling injection. The electron relaxation in the central wells is then essentially governed by the optical phonon emission rate from both upper subbands. As we demonstrate below, the QCL is appropriate to study intersubband relaxation via optical phonon emission. Recently, intersubband relaxation effects were observed in a GaAs/GaAlAs QC structure from both magnetoresistance and and luminescence up to 8 tesla [5]. However, the QCL structure for these studies had intersubband sep...

Electron-phonon relaxation rates and optical gain in a quantum cascade laser in a magnetic field

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Magnetic-field tunable terahertz quantum well infrared photodetector

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Density matrix theory of transport and gain in quantum cascade lasers in a magnetic field

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