Systematic Study of Mn-Doping Trends in Optical Properties of (Ga,Mn)As

Abstract

We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible substitutional MnGa dopings. The growth and post-growth annealing procedures were optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the midinfrared absorption spectra whose position exhibits a prevailing blue-shift for increasing Mn-doping. In the visible range, a peak in the magnetic circular dichroism blue shifts with increasing Mndoping. These observed trends confirm that disorder-broadened valence band states provide a better one-particle representation for the electronic structure of high-doped (Ga,Mn)As with metallic conduction than an energy spectrum assuming the Fermi level pinned in a narrow impurity band.PACS numbers: 74.20. Mn, 74.25.Nf, 74.72.Bk, 74.76.Bz The discovery of ferromagnetism in (Ga,Mn)As above 100 K [1] opened an attractive prospect for exploring the physics of magnetic phenomena in doped semiconductors and for developing advanced concepts for spintronics. Assessment of a wide range of magnetic and transport properties of the material [2][3][4] showed that in ferromagnetic (Ga,Mn)As with Mn dopings x > 1%, disorderbroadened and shifted host Bloch bands represent a useful one-particle basis for describing this mixed-crystal degenerate semiconductor. The common kinetic-exchange model implementation of this valence band theory and the more microscopic tight-binding Anderson model or ab-initio density functional theory can all be shown [5] to be mutually consistent on the level of atomic and orbital resolved band structure. The main utility of valence band theories have been in providing a qualitative and often semi-quantitative description of phenomena originating from the exchange split and spin-orbit coupled electronic structure and in assisting the development of prototype spintronic devices [4]. Other basic physical properties of (Ga,Mn)As, namely those reflecting the vicinity of the metal-insulator transition and localization and electronelectron interaction effects, remain to be fully understood and require to go beyond the commonly employed perturbative or disorder averaged Bloch-band theories.In the insulator non-magnetic regime (x 1%), the system is readily described by localized Fermi level states residing inside a narrow impurity band separated from the valence band by an energy gap of magnitude close to the isolated Mn Ga impurity binding energy. Recently, a debate has been stirred by proposals, based in particular on optical spectroscopy measurements [6], that the narrow impurity band persists in high-doped (Ga,Mn)As with metallic conduction. Several phenomenological variants of the impurity band model have been proposed for the high-doped regime [6][7][8][9][10] which are mutually inconsistent from the perspective of the assumed atomic orbital nature of the impurity band states [5]. Further theoretical inconsistencies arise when recreating ...

Ferromagnetism and infrared electrodynamics of Ga1−xMnxAs

“…Earlier IR studies of Ga 1−x Mn x As revealed a systematic red-shift of ω 0 as a function of effective carriers 9,10 . However, other more recent experiments have reported ω 0 to blue-shift with increased Mndoping 11 . We note, data reported in Ref.…”

“…1b we plot T C as a function of dopant concentration x of all the Ga 1−x Mn x As samples in our analysis. The samples include as-grown films investigated by Singley et al 9 , as-grown and annealed films of Burch et al 10 , annealed films of Jungwirth et al 11 , and the non-rotated as-grown and annealed samples of this work (see Table I). For the later samples presented in Fig.…”

Quantum kinetic equations for the ultrafast spin dynamics of excitons in diluted magnetic semiconductor quantum wells after optical excitation

“…In DMS, a small fraction of magnetic ions, usually Manganese 23 , is introduced into a semiconductor. While III-V compounds such as Ga 1−x Mn x As are typically p-doped 8 and can thus exhibit carrier-mediated ferromagnetism 24 , II-VI materials such as Cd 1−x Mn x Te are found to be intrinsic and paramagnetic due to the isoelectrical incorporation of the Mn impurities.A lot of theoretical works on DMS has been devoted to the understanding of structural properties [25][26][27][28][29][30] . But in many experiments, also the spin dynamics studied via optical pump-probe experiments is of interest 17,18,31 .…”

The essential role of carefully optimized synthesis for elucidating intrinsic material properties of (Ga,Mn)As

“…Because of the highly non-equilibrium nature of the heavily-doped ferromagnetic (Ga,Mn)As, the growth and post-growth annealing procedures have to be individually optimized for each Mn-doping level to obtain films that are as close as possible to idealized uniform (Ga,Mn)As mixed crystals with the minimal density of compensating and other unintentional defects. An extensive set of characterization measurements has to accompany the synthesis to guarantee that the materials show systematic doping trends; monitoring the thermodynamic Curie point singularities is essential for assuring the uniformity and high magnetic quality of the materials 11,15,54,55 . When omitting the above procedures, extrinsic impurities and sample inhomogeneities can yield nonsystematic doping trends and conceal the intrinsic magnetic properties of (Ga,Mn)As.…”