Spin polarized electric currents in semiconductor heterostructures induced by microwave radiation

We report on microwave (mw) radiation induced electric currents in (Cd,Mn)Te/(Cd,Mg)Te and InAs/(In,Ga)As quantum wells subjected to an external in-plane magnetic field. The current generation is attributed to the spin-dependent energy relaxation of electrons heated by mw radiation. The relaxation produces equal and oppositely directed electron flows in the spin-up and spin-down subbands yielding a pure spin current. The Zeeman splitting of the subbands in the magnetic field leads to the conversion of the spin flow into a spin-polarized electric current.Comment: 3 pages, 4 figure

Quantum Oscillations of Photocurrents in HgTe Quantum Wells with Dirac and Parabolic Dispersions

We report on the observation of magneto-oscillations of terahertz radiation induced photocurrent in HgTe/HgCdTe quantum wells (QWs) of different widths, which are characterized by a Dirac-like, inverted and normal parabolic band structure. The photocurrent data are accompanied by measurements of photoresistance (photoconductivity), radiation transmission, as well as magneto-transport. We develop a microscopic model of a cyclotron-resonance assisted photogalvanic effect, which describes main experimental findings. We demonstrate that the quantum oscillations of the photocurrent are caused by the crossing of Fermi level by Landau levels resulting in the oscillations of spin polarization and electron mobilities in spin subbands. Theory explains a photocurrent direction reversal with the variation of magnetic field observed in experiment. We describe the photoconductivity oscillations related with the thermal suppression of the Shubnikov-de Haas effect.Comment: 16 pages, 13 figure

Cyclotron Resonance Assisted Photocurrents in Surface States of a 3D Topological Insulator Based on a Strained High Mobility HgTe Film

We report on the observation of cyclotron resonance induced photocurrents, excited by continuous wave terahertz radiation, in a 3D topological insulator (TI) based on an 80 nm strained HgTe film. The analysis of the photocurrent formation is supported by complimentary measurements of magneto-transport and radiation transmission. We demonstrate that the photocurrent is generated in the topologically protected surface states. Studying the resonance response in a gated sample we examined the behavior of the photocurrent, which enables us to extract the mobility and the cyclotron mass as a function of the Fermi energy. For high gate voltages we also detected cyclotron resonance (CR) of bulk carriers, with a mass about two times larger than that obtained for the surface states. The origin of the CR assisted photocurrent is discussed in terms of asymmetric scattering of TI surface carriers in the momentum space. Furthermore, we show that studying the photocurrent in gated samples provides a sensitive method to probe the effective masses and the mobility of 2D Dirac surface states, when the Fermi level lies in the bulk energy gap or even in the conduction band

Spin-polarized electric currents in diluted magnetic semiconductor heterostructures induced by terahertz and microwave radiation

We report on the study of spin-polarized electric currents in diluted magnetic semiconductor (DMS) quantum wells subjected to an in-plane external magnetic field and illuminated by microwave or terahertz radiation. The effect is studied in (Cd,Mn)Te/(Cd,Mg)Te quantum wells (QWs) and (In,Ga)As/InAlAs:Mn QWs belonging to the well known II-VI and III-V DMS material systems, as well as, in heterovalent AlSb/InAs/(Zn,Mn)Te QWs which represent a promising combination of II-VI and III-V semiconductors. Experimental data and developed theory demonstrate that the photocurrent originates from a spin-dependent scattering of free carriers by static defects or phonons in the Drude absorption of radiation and subsequent relaxation of carriers. We show that in DMS structures the efficiency of the current generation is drastically enhanced compared to non-magnetic semiconductors. The enhancement is caused by the exchange interaction of carrier spins with localized spins of magnetic ions resulting, on the one hand, in the giant Zeeman spin-splitting, and, on the other hand, in the spin-dependent carrier scattering by localized Mn2+ ions polarized by an external magnetic field.Comment: 14 pages, 13 figure

Residual Metallic Contamination of Transferred Chemical Vapor Deposited Graphene

Integration of graphene with Si microelectronics is very appealing by offering potentially a broad range of new functionalities. New materials to be integrated with Si platform must conform to stringent purity standards. Here, we investigate graphene layers grown on copper foils by chemical vapor deposition and transferred to silicon wafers by wet etch and electrochemical delamination methods with respect to residual sub-monolayer metallic contaminations. Regardless of the transfer method and associated cleaning scheme, time-of-flight secondary ion mass spectrometry and total reflection x-ray fluorescence measurements indicate that the graphene sheets are contaminated with residual metals (copper, iron) with a concentration exceeding 10$^{13}$ atoms/cm$^{2}$. These metal impurities appear to be partly mobile upon thermal treatment as shown by depth profiling and reduction of the minority charge carrier diffusion length in the silicon substrate. As residual metallic impurities can significantly alter electronic and electrochemical properties of graphene and can severely impede the process of integration with silicon microelectronics these results reveal that further progress in synthesis, handling, and cleaning of graphene is required on the way to its advanced electronic and optoelectronic applications.Comment: 26 pages, including supporting informatio

Exchange interaction of electrons with Mn in hybrid AlSb/InAs/ZnMnTe structures

Diluted magnetic semiconductor heterovalent AlSb/InAs/ZnMnTe quantum well (QW) structures with an electron channel have been designed and grown applying molecular-beam epitaxy. The enhanced magnetic properties of QWs as a result of the exchange interaction with Mn2+ ions, are proved by measuring the microwave radiation induced spin polarized electric currents