Session T15: Focus Session: Spins in Semiconductors - III-V Magnetic Semiconductors

Valence-band structure of the ferromagnetic semiconductor GaMnAs investigated by resonant tunneling spectroscopy

The origin of ferromagnetism in the prototype ferromagnetic semiconductor GaMnAs is still controversial due to the insufficient understanding of its band structure and Fermi level position. Here, we investigate the valence-band (VB) structure of GaMnAs by analyzing the resonant tunneling levels of the GaMnAs quantum well (QW) in double-barrier heterostructures. The resonant levels including the heavy-hole first state (HH1) are clearly observed in the metallic GaMnAs QW with the Curie temperature (T{\_}C) of 60 K, which indicates that no holes reside in the VB of GaMnAs in the equilibrium condition. Clear enhancement of tunnel magnetoresistance induced by resonant tunneling is demonstrated. We find that the resonant levels formed in the GaMnAs QW are well explained by using the transfer matrix method with the 6x6 \textit{kp} Hamiltonian and small $p-d$ exchange Hamiltonian. The VB structure of GaMnAs is well reproduced by that of GaAs with a small exchange splitting energy of 3-5 meV and with the Fermi level lying at $\sim $30 meV higher than HH1 in the bandgap. Furthermore, we show our more recent results of resonant tunneling spectroscopy on various surface GaMnAs films (Mn concentration: 6-15{\%}, T{\_}C: 71-154 K) grown on an AlAs layer, where the resonant levels are formed by confinement of the VB holes by the surface Schottky barrier and the AlAs barrier. We systematically investigate the thickness dependence of the resonant levels in GaMnAs by precisely etching the surface of GaMnAs. We find that the p-d exchange interaction is negligibly small (3-5 meV) and that the Fermi level exists in the bandgap. This work was performed in collaboration with I. Muneta, P. N. Hai, K. Takata, and M. Tanaka, and partly supported by Grant-in-Aids for Scientific Research, the Special Coordination Programs for Promoting Science and Technology, and FIRST Program by JSPS.\\[4pt] [1] S. Ohya et al., Phys. Rev. Lett. 104, 167204 (2010).\\[0pt] [2] S. Ohya et al., arXiv:1009.2235.   

On magnetism and the insulator-to-metal transition in $p$-doped GaAs

Although Ga$_{1-x}$Mn$_{x}$As is often described as the prototypical ferromagnetic semiconductor, many aspects of the electronic structure and nature of mediating carriers remain open. A central question in this regard is whether the insulator-to-metal transition (IMT) in $p$-doped GaAs is significantly modified when dopants are magnetic. We address this through an infrared spectroscopic study of GaAs doped with either non-magnetic Be or magnetic Mn acceptors. Through our comparison, we are able to isolate effects of magnetic dopants in GaAs from those associated with disorder and proximity to the IMT. Here we show Mn-doped samples exhibit an unusual electronic transport regime, combining elements of both metallic and insulating behavior, at doping concentrations far beyond the onset of the IMT. Be-doped films however, reveal genuine metallicity just above the IMT boundary. These results underscore the pivotal role of magnetism in transport and optical phenomena of Ga$_{1-x}$Mn$_{x}$As.   

Magnetic properties of narrow gap In$_{1-x}$Mn$_{x}$Sb semiconductor films with x$>$0.10

Narrow gap In$_{1-x}$Mn$_{x}$Sb magnetic semiconductors with x $<$ 0.05 have been recently shown to have interesting magnetotransport properties at room temperature.\footnote{J. A. Peters et al, PRB \textbf{82} 2010.} Calculations based on the field dependence of the magnetoresistance indicate that the carriers are highly spin polarized. To increase both the saturation magnetization and potentially the Curie temperature Tc of the alloys, we have investigated MOVPE epitaxial layers with 0.10 $<$ x $<$ 0.22. Films were ferromagnetic at room temperature, showing clear hysteresis in field dependent measurements from 5 to 300 K. Alloys with magnetization values as high as 83 emu/cm$^{3}$ for x=0.22 were measured at 5 K. Temperature dependent magnetization indicated that the Curie temperature of the films was above 400 K. These measurements indicated the presence of two magnetic species both with Curie temperatures above 300 K. The high Tc is attributed to carrier mediated ferromagnetism involving Mn and its complexes that form shallow or resonant electronic states with the valence band through correlated substitution.   

Cyclotron Resonance in InMnAs and InMnSb Ferromagnetic Films

Ferromagnetic semiconductors are important materials for development of spintronic devices. While effort in this area was made primarily on GaMnAs, other ferromagnetic III-Mn-V alloys have also been developed, including the narrow gap ferromagnetic alloys such as InMnAs and InMnSb. Investigation of the electronic structure of III-Mn-V alloys by techniques such as the cyclotron resonance (CR) can shed important light on the origin of ferromagnetism and the p-d exchange interaction in III-Mn-V systems. In this work we report on CR experiments carried out on the ferromagnetic InMnAs and InMnSb films, on which clear resonance signals have been successfully observed in high magnetic fields generated by a single turn coil technique. The CR in ferromagnetic InMnSb was observed for the first time and we compare our observations with the Landau levels calculations on the basis of an 8-band $k\dot.p$ model.   

Visualizing Critical Correlations Near the Metal-Insulator Transition in Ga$_{1-x}$Mn$_x$As

Semiconductors have long been an ideal class of materials for studying the metal-insulator transition. Samples of the dilute magnetic semiconductor Ga$_{1-x}$Mn$_x$As with Mn doping levels near to the metal-insulator transition have been studied using low temperature cross-sectional scanning tunneling microscopy (STM). This allows us to visualize the electronic states near the Fermi level which display unique critical properties. Strong modifications to the density of states around the Fermi energy due to electron-electron interactions are observed. In this energy range, the electronic states show a diverging correlation length approaching E$_F$, where the correction to the density of states due to interactions is strongest. The distance dependence of the correlations at E$_F$ is consistent with a power law decay, expected for multifractal states near criticality in the metal-insulator transition, while away from E$_F$ the correlations fall off exponentially. These results highlight the importance of electron-electron interactions and represent some of the first experimental observations of states near the Mott-Anderson metal-insulator transition, where both disorder and interactions are important for the localization of electronic states.   

Time resolved spectroscopy of MOVPE grown narrow gap III-Mn-V ferromagnetic semiconductors

The emergence of III-Mn-V magnetic semiconductors, has led to a number of exciting results relevant to the spin and charge based applications. Important advances have now been made in the MOVPE growth of the narrow gap ferromagnetic structures with the Tc above room temperature. As the switching rates in electronic and optoelectronic devices are pushed to higher frequencies, understanding the dynamical behavior of non- equilibrium carriers/spins can provide valuable information about different scattering mechanisms, carrier phonon coupling, and band structures. In this work, we report several time- resolved and magneto-optical measurements on $In_{1-x}Mn_{x}As$ and $In_{1-x}Mn_{x}Sb$ ferromagnetic films with the Mn content of 4$\%$. Our measurements on the basis of several time resolved differential transmission techniques in NIR and MIR demonstrate unique and complex dynamics in these material systems where photo-induced absorption and bleaching can co- exist.   

Structural and Magnetic Characteristics of p-GaAs/MnAs Nanocluster Hybrids

A possible route towards semiconductor spintronic devices involves the controlled synthesis of hybrid materials that combine ferromagnetic (FM) nanoclusters within a doped semiconductor host lattice. We use molecular beam epitaxy of (Ga,Mn,Be)As followed by in situ annealing to synthesize a systematic set of samples wherein FM nanoclusters are embedded in a p-GaAs matrix. High resolution transmission electron microscopy (HRTEM) and magnetometry demonstrate our ability to reproducibly synthesize two distinct classes of materials: (a) type I samples consisting of uniformly distributed, small clusters ($\sim $6 nm); (b) type II samples consisting of a bimodal distribution of small ($\sim $6 nm) and large ($\sim $25 nm) clusters. HRTEM studies show that while the large clusters are clearly MnAs with NiAs structure, the smaller clusters are possibly zinc blende in structure but with a more complex composition. We analyze the magnetic behavior of these two classes of samples and show measurements of their transport properties. Supported by the ONR-MURI program.   

Magnetoresistance due to inelastic spin-flip cotunneling within Coulomb blockade regime in III-V semiconductor / MnAs nanoparticle heterostructures

Inelastic spin-flip cotunneling is a key to understand the spin-dependent single-electron transport in the ferromagnetic nanoparticles systems. We fabricated a heterostructure consisting of Al/ AlAs/ ferromagnetic zinc-blende (ZB) MnAs nanoparticles embedded in GaAs / GaAs:Be on a GaAs(001) substrate, where electrons are expected to go through only one nanoparticle during tunneling. By analyzing the $I-V$ data at various temperatures $T$, we found that inelastic cotunneling is dominant when $T<$60 K. The ratio of the inelastic cotunneling energy $E$ to the thermal energy \textit{kT}, estimated by the $I-V$ data, was remarkably increased with decreasing $T$. We observed clear magnetoresistance (MR) up to $\sim $3{\%} (at 1T), and MR was also increased with decreasing $T$. The shape of the \textit{MR-T }curve was quite similar to that of the \textit{E/kT - T} curve, which strongly suggests that MR is induced by the spin-flip process due to inelastic cotunneling. From the \textit{E/kT - T} curve, the energy needed for the spin-flip process is estimated to be $\sim $0.04 meV, which corresponds to $\sim $3.3{\%} of the inelastic cotunneling energy. This work was partly supported by the Grant-in-Aids for Scientific Research, Special Coordination Programs by JST, FIRST Program, and JSPS Fellowship.   

Magnetism in Cr doped Si nanowires

We carry out first principles calculations of magnetic and electronic structures of single and multiple Cr atom dopants in Si nanowires. Both unsupported isolated wires and supported wires on Si 110 surfaces are studied. The relative stability and underlying physical picture of the ferromagnetic and antiferromagnetic configurations of the local moments on the Cr atoms are studied. Results are also presented for fully noncollinear calculations.   

GaMnAs-based Core-Shell Nanowires Grown by Molecular Beam Epitaxy

We have successfully fabricated GaMnAs/GaAs core-shell nanowires (NWs) by molecular beam epitaxy (MBE), by first growing Au-assisted GaAs NWs, and subsequently depositing the GaMnAs shells on the Gaas NW side facets under low temperature conditions. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) show that GaMnAs grows epitaxially on the GaAs NWs, retaining good crystalline quality. SQUID magnetometry shows that the shells obtained so far are ferromagnetic below 20 K. Studies by high resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy are planned for the future, in order to allow us to relate the observed magnetic properties of these one-dimensional magnetic wires to their chemical and structure profiles, in the hope of designing strategies for increasing the Curie temperature of the GaMnAs shells.   

Magnetic Nanostructures in the Mn-Si system

Magnetic doping of group IV semiconductors is coveted for spintronics building blocks. Theoretical assessment of magnetism in Mn-Si is promising, but many of these structures have not been realized yet. Our STM study combines the study of Mn-nanostructure growth on Si(100)(2x1) with the investigation of the magnetic signature with X-ray magnetic circular dichroism and magnetometry. Mn self-assembles into monoatomic chains on the Si(100) surface. The mechanism of chain-formation and its competition with cluster growth will be presented. The nanostructures are capped with a 10 ML Si-or Ge- layer to form delta-doped layers, and protect the Mn-nanostructure. The Mn-chains are preserved, and the growth process for the cap was studied by STM and is now well understood. The magnetic signature is presented for nanowires and nanocluster structures below about 50 K, and a dense array of Mn-chains shows the highest saturation magnetization with 2-3 $\mu _{B}{\rm g}$er Mn. The hysteresis loops indicate a superparamagnetic behavior. We will discuss the relative spin-orbital contributions and the directional dependence of the magnetic signature in relation to the Mn-nanostructure geometry.