Nano and Giga Challenges in Microelectronics, Cracow, 2004 Spin Injection in Semiconductor Nanostructures Alexey Toropov Ioffe Institute, St.Petersburg, Russia

Nano and Giga Challenges in Microelectronics, Cracow, 2004OUTLINE * Brief historical introduction, motivation * Different approaches proposed to create spin-polarized carriers in semiconductors * Spin-dependent resonant tunneling in diluted magnetic semiconductor heterostructures (DMS) for spin injection and spin manipulation - In-depth optical studies of spin functionality in II-VI DMS - Hybrid III-V / II-VI DMS heterostructures – spin- dependent resonant tunneling through a III-V / II-VI heterovalent interface

Nano and Giga Challenges in Microelectronics, Cracow, 2004 SPIN INJECTION IN SEMICONDUCTORS A. G. Aronov, G. E. Pikus Spin injection appears when the current flows through a ferromagnetic-semiconductor contact. Depending on the type of the contact, either majority or minority carriers can be polarized. The magnetic field effect on the spin injection and possible experimental conditions are discussed.

Nano and Giga Challenges in Microelectronics, Cracow, 2004 First realization of the spin transistor? K. Yoh et al., PASPS III (2004), ICPS27 (2004) Hokkaido Uni./JST/Stanford Uni./Uni. of Michigan From spin transistor to spin quantum computer..

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Spin-polarized carriers in semiconductors - Spin injection from metals - Spin injection and manipulation in diluted magnetic semiconductors (DMS) III-V compounds II-VI compounds Wide-band-gap semiconductors (ZnO:Mn,Co..; GaN:Mn) - Spin polarization in non-magnetic semiconductors, Rashba effect

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Ferromagnetic metal versus diluted magnetic semiconductor … : numerous fruitless attempts to inject spins from ferromagnetic metals (Fe, Co,…) to semiconductors. Schmidt, et al., Phys. Rev. B (2000): metal-semiconductor spin injection is impossible due to the large conductivity mismatch. Fiederling, et al., Nature (1999): Successful spin injection (~80%) from a DMS semiconductor into a GaAs/AlGaAs light-emitting diode (LED)

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Ferromagnetic metals Rashba, Phys. Rev. B (2000): the metal-semiconductor conductivity mismatch problem can be overcome by injecting spins through a tunnel Schottky barrier Zhu et al, Phys. Rev. Lett (2001): Fe/GaAs interface, 2% injection efficiency Hanbicki et al, Appl. Phys.Lett (2002): Fe/AlGaAs interface, 30% injection efficiency (240K) Van’t Erve et al, Appl. Phys.Lett (2004): Fe/Al 2 O 3 /GaAs interface, 40% injection efficiency Problems : - limit current through a tunnel barrier - difficult formation of the high quality metal-semiconductor interface K. Yoh et al, (2004): up to 12% efficiency of the spin injection from Fe to InAs without any tunnel barrier!

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Ferromagnetic metals seem currently most promising for room temperature spin injection applications Advantages of DMS are: - Better technological compatibility with semiconductor electronics and optoelectronics; - Efficient electrical, optical and strain control over magnetization (sign reversal, “easy axis rotation”,..); - New spin functionalities (spin-dependent resonant tunneling [Waag et al., 2001 ], spin-relaxation engineering [Hall et al., 2003],..).Summarizing..

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Diluted magnetic semiconductors II-VI DMS (ZnMnSe, CdMnTe, …): - Unlimited solubility of Mn – giant spin splittings are available - Perfect optical quality – advantages of optical spectroscopy - Both n- and p-type doping is available BUT - Very small Curie Temperature Tc<4-5K - Paramagnetic DMS require external magnetic field and low temperatures (<10-15 K) - Relatively fast spin relaxation, low carriers mobility III-V DMS (GaMnAs, InMnAs, …): - Relatively high Curie Temperature ~180 K in bulk GaMnAs and ~250 K in Mn  -doped GaAs/p-AlGaAs structures (Nazmul et al., MBE-2004) - Long spin relaxation in n-type III-V compounds BUT - Low solubility of Mn ions - Pour optical quality even at negligible Mn concentration - Only p-type materials available, since Mn is an acceptor..

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Spin functionalities in II-VI DMS heterostructures * Samples with ZnMnSe quantum wells (QW) and superlatticies (SL) * Careful design, MBE growth and in-depth optical studies of spin- dependent phenomena Appl. Phys. Lett., Phys. Rev. B, 2003

Nano and Giga Challenges in Microelectronics, Cracow, 2004 II-VI DMS samples ZnMnSe/CdSe DMS SL  ZnCdSe QW ZnMnSe QW  ZnCdSe QW ZnMnSe(4nm) /CdSe(0.8 ML) DMS SL optical spin detector

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Spin injection in the II-VI DMS samples   

Nano and Giga Challenges in Microelectronics, Cracow, 2004 PL excitation spectra in the SL+QW sample Detection: ZnCdSe QW PL energy

Nano and Giga Challenges in Microelectronics, Cracow, 2004 PL excitation spectra in the QW+QW sample Detection: ZnCdSe QW PL energy

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Spin alignment versus spin switching QW+SL sample, spacer 8 nm QW+QW sample, spacer 5 nm 0.5 T 8nm

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Spin relaxation in the DMS SL versus magnetic field: spin injection experiment Relaxation processes: I.Spin relaxation within the hh exciton states of the DMS SL II.Spin injection to the QW from the lower spin sublevel of the SL hh exciton III.Spin injection to the QW from the upper spin sublevel of the SL hh exciton When the energy of the excitation photon (  - polarized) is tuned to the   active |+1/2,-3/2> spin state of the DMS exciton, the measured QW PL polarization is governed by the relative efficiency of the I and III competing processes

Nano and Giga Challenges in Microelectronics, Cracow, 2004 QW+SL sample, spacer 4 nm Spin relaxation in the DMS SL versus magnetic field: spin injection experiment Degree of circular polarization (%) Observation of a drastic increase in spin relaxation when the exciton spin splitting exceeds the LO phonon energy

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Spin relaxation in the DMS SL versus magnetic field: hot PL experiment A hot PL spectrum, obtained under the resonant  - optical pumping at the upper spin level of the DMS SL exciton, when the spin splitting of the two spin levels exceeds the LO phonon energy. The polarization of the hot PL line has the same sign as the lower spin state, which is opposite to that of the pumping light. This change of sign shows that energy relaxation from the optically pumped upper spin state to the lower spin state is accompanied by an exciton spin flip via the emission of one LO phonon. When the spin splitting of two spin levels of the DMS hh exciton is less than the LO phonon energy, the observed hot PL line retains the polarization of the pumping light.

Nano and Giga Challenges in Microelectronics, Cracow, 2004 I a - a nearly inelastic spin scattering I b - an LO-assisted, spin-preserving exciton kinetic energy relaxation I c - a single quantum process of LO- assisted elastic spin relaxation No theory is currently available to explain the observed LO-phonon assisted spin-flip of excitons in DMS This effect should be taken into account for design of spin-dependent resonant tunneling devices

Nano and Giga Challenges in Microelectronics, Cracow, 2004 III-V / II-VI-Mn coupled QWs * MBE growth and magnito-optical studies of coupled double QWs separated by a heterovalent III-V/II-VI interface PASPSIII, 2004 * Spin-polarized 2D electron gas in an optical quality Mn-free GaAs QW Goal: Means: * Control over the growth and properties of the heterovalent interface Problem:

Nano and Giga Challenges in Microelectronics, Cracow, 2004 III-V part: 3 – 8 nm wide nonmagnetic GaAs/AlGaAs QW (a reference GaAs/AlGaAs QW in some samples, 10 nm apart from the near- interface QW) II-VI part: 8-10 nm wide diluted magnetic semiconductor (DMS) ZnCdMnSe/ZnSe QW III-V/II-VI Double QWs

Nano and Giga Challenges in Microelectronics, Cracow, 2004 ElectronsHeavy Holes Band Line-ups and Wave Functions

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Cross Section TEM

Nano and Giga Challenges in Microelectronics, Cracow, nm GaAs QW: far from resonance

Nano and Giga Challenges in Microelectronics, Cracow, nm GaAs QW: around resonance

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Conventional GaAs/GaAlAs QW thinner than 5-6 nm g e >0, g h <0, |g e |<|g h | Snelling et al., Phys. Rev. B (1992) GaAs (3.4nm)/ZnCdMnSe coupled QWs Zeeman splitting

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Exciton energies: fitting Constant parameters QW widths: GaAs – 3.4 nm Barrier width: 2 nm AlGaAs nm ZnSe ZnCdMnSe – 10 nm Variable parameter GaAs/ZnSe conduction band offset (CBO) – can vary from ~100 meV (Zn- rich interface) to ~600 meV (Se-rich interface), depending on growth conditions. Fitting procedure gives CBO~200 meV, which corresponds to neither Zn- rich nor Se-rich interface. In the simulation, the exciton energies were calculated variationally. Diamagnetic shift and Zeeman splittings due to the intrinsic g factors were neglected and Stokes shift was determined as 0.6*  PL, where  PL is full width at half maximum of the PL peak.

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Electron levels versus magnetic field (3.4 nm GaAs QW)

Nano and Giga Challenges in Microelectronics, Cracow, 2004 * Optical-quality GaAs/AlGaAs/ZnSe/ZnCdMnSe double QWs are grown by MBE * Resonant electron coupling through a heterovalent AlGaAs/ZnSe interface is observed in the double QW with a DMS II-VI part * The structure design allows one to resonantly enhance penetration of the electron wave function from the nonmagnetic III-V QW into the DMS II-VI QW and hence to control the magnetic g factor of a GaAs QW excitonSummarizing..

Nano and Giga Challenges in Microelectronics, Cracow, 2004 Thorough control over tunneling and spin relaxation times allows different spin functionalities to be obtained in DMS QW structures (yet II-VI) - either spin alignment or spin switching The advantages of the two very well developed technologies can be combined in the III-V / II-VI DMS samples, providing new opportunities: - obtaining and studies of high-mobility spin-polarized 2D electron gas; - electrical spin manipulation in p-i-n diodes including the III-V / II-VI DMS coupled QW; - self-organized formation of InAs quantum dots coupled to the DMS spin-polarized QW – electrically and optically addressed qubits for quantum computers and spin-polarized single photon sources for quantum cryptography.Outlook

Nano and Giga Challenges in Microelectronics, Cracow, 2004Acknowledgements * All samples were grown by MBE in Ioffe Institute: Sergey Ivanov, Sergey Sorokin, Irina Sedova * Optical studies: Yakov Terent’ev Ioffe Institute, Russia Irina Buyanova, Weimin Chen Linkoping University, Sweden * Theory: Evgeniy Ivchenko Ioffe Institute