Distinct local electronic structure and magnetism for Mn in amorphous Si and Ge

Li Zeng (曾立), J. X. Cao, E. Helgren, J. Karel, E. Arenholz, Lu Ouyang, David J. Smith, R. Q. Wu, and F. Hellman

Phys. Rev. B 82, 165202 – Published 14 October 2010

Abstract

Transition metals such as Mn generally have large local moments in covalent semiconductors due to their partially filled $d$ shells. However, Mn magnetization in group-IV semiconductors is more complicated than often recognized. Here we report a striking crossover from a quenched Mn moment $(< 0.1 μ_{B})$ in amorphous Si $(a -Si)$ to a large distinct local Mn moment $(\geq 3 μ_{B})$ in amorphous Ge $(a -Ge)$ over a wide range of Mn concentrations (0.005–0.20). Corresponding differences are observed in $d$ -shell electronic structure and the sign of the Hall effect. Density-functional-theory calculations show distinct local structures, consistent with different atomic density measured for $a -Si$ and $a -Ge$ , respectively, and the Mn coordination number $N_{c}$ is found to be the key factor. Despite the amorphous structure, Mn in $a -Si$ is in a relatively well-defined high coordination interstitial type site with broadened $d$ bands, low moment, and electron ( $n$ -type) carriers, while Mn in $a -Ge$ is in a low coordination substitutional type site with large local moment and holes ( $p$ -type) carriers. Moreover, the correlation between $N_{c}$ and the magnitude of the local moment is essentially independent of the matrix; the local Mn moments approach zero when $N_{c} > 7$ for both $a -Si$ and $a -Ge$ .

Received 15 April 2010

DOI:https://doi.org/10.1103/PhysRevB.82.165202

Authors & Affiliations

Li Zeng (曾立)^1,*, J. X. Cao², E. Helgren¹, J. Karel³, E. Arenholz⁴, Lu Ouyang⁵, David J. Smith⁵, R. Q. Wu², and F. Hellman^1,†

¹Department of Physics, University of California, Berkeley, California 94720, USA
²Department of Physics and Astronomy, University of California, Irvine, California 92697, USA
³Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
⁴Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁵Department of Physics, Arizona State University, Tempe, Arizona 85287, USA

^*Present address: NSF Nano-Scale Science and Engineering Center (NSEC), 3112 Etcheverry Hall, University of California, Berkeley, California 94720, USA.
^†fhellman@berkeley.edu

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Vol. 82, Iss. 16 — 15 October 2010

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Figure 1
(Color online) Temperature dependence of dc conductivity $σ (T)$ for $a {-Mn}_{x} {Si}_{1 - x}$ and $a {-Mn}_{x} {Ge}_{1 - x}$ for various concentrations $x$ across the $I − M$ transition. As $x$ increases for each, the magnitude of $σ (T)$ increases monotonically and an $I − M$ quantum phase transition is seen in both sets of samples, with critical concentrations $x_{c} \sim 0.14$ found by fits to low $T$ data. $σ$ increases with $T$ , a result of localization and Coulomb effects in disordered electronic systems.Reuse & Permissions
Figure 2
(Color online) Magnetic properties of $a {-Mn}_{x} {Si}_{1 - x}$ and $a {-Mn}_{x} {Ge}_{1 - x}$ . (a) zero-field-cooled and field-cooled magnetic susceptibility $χ (T)$ data for typical samples. For $a {-Mn}_{x} {Si}_{1 - x}$ (green solid triangles), $χ (T)$ is negligible compared to $χ (T)$ of $a {-Mn}_{x} {Ge}_{1 - x}$ (blue and red open circles and squares). Inset shows the same $a {-Mn}_{x} {Si}_{1 - x}$ data on expanded scale. The red dashed line is a Curie fit. (b) $M$ vs $H$ data measured at $T = 2 K$ normalized to the number of Mn atoms. The dashed lines are BFs for $J = S = 5 / 2$ and 1/2 and $g = 2$ (pure spin state). For $a {-Mn}_{x} {Ge}_{1 - x}$ (blue open symbols), all $M (H)$ data collapse, are suppressed well below either BF and show no sign of saturation to $H = 6 kOe$ . For $a {-Mn}_{x} {Si}_{1 - x}$ (red solid symbols), $M (H)$ data behave like free moments but with very small saturation moments (values dependent on $x$ and less than $0.3 μ_{B}$ ). (c) Concentration dependence of effective moment $p_{e f f}$ of $a {-Mn}_{x} {Ge}_{1 - x}$ (blue circles) and $a {-Mn}_{x} {Si}_{1 - x}$ (red squares), calculated from $χ (T)$ data (above $T_{f}$ for $a {-Mn}_{x} {Ge}_{1 - x}$ ) assuming all Mn are magnetically active. $p_{e f f}$ of $a {-Mn}_{x} {Si}_{1 - x}$ vanishes, distinct from the large $p_{e f f}$ of $a {-Mn}_{x} {Ge}_{1 - x}$ , which shows an increase with $x$ . For $a {-Mn}_{x} {Si}_{1 - x}$ , a better and consistent fit for $χ (T)$ and $M (H, T)$ is obtained by assuming $S = 5 / 2$ for a tiny fraction of noninteracting Mn $(\sim 10^{- 3})$ . For $a {-Mn}_{x} {Ge}_{1 - x}$ , the distinct local moments interact with strongly mixed AFM and FM interactions, suppressing $M (H, T)$ far below the BF.Reuse & Permissions
Figure 3
(Color online) X-ray absorption spectroscopy of $Mn L$ edges in different materials. $L_{3, 2}$ edges measure the transition from occupied $p$ states to unoccupied $d$ states. (a) $Mn L$ edges of $a {-Mn}_{x} {Ge}_{1 - x}$ resembles the spectra of Mn oxides, which show atomic multiplet features associated with localized $d$ electronic states. The dash-dotted line is a simulated spectrum for $3 d^{5}$ . On the other hand, Mn in $a {-Mn}_{x} {Si}_{1 - x}$ resembles Mn metal films with smooth and broad $L$ edges without any multiplet feature but is different from Mn metal films in peak positions and peak width. (b) $L$ edges of $a {-Mn}_{x} {Ge}_{1 - x}$ for different $x$ show the same multiplet lineshape. When normalized by the postedge jump intensity, all $a {-Mn}_{x} {Ge}_{1 - x}$ XA spectra collapse to the same curve. A typical XA spectrum of $a {-Mn}_{x} {Si}_{1 - x}$ (with two broad peaks) is also plotted here to show the difference in line shape. Due to different capping layer and surface sensitivity, $a {-Mn}_{x} {Si}_{1 - x}$ does not scale well with postedge jump intensity for small $x$ .Reuse & Permissions
Figure 4
(Color online) Structural analysis of $a {-Mn}_{x} {Si}_{1 - x}$ and $a {-Mn}_{x} {Ge}_{1 - x}$ . (a) and (b) High-resolution cross-sectional transmission electron micrographs for $a {-Mn}_{0.15} {Si}_{0.85}$ and $a {-Mn}_{0.19} {Ge}_{0.81}$ samples showing an amorphous structure with no indication of any second phase or clustering. (c) Total atomic number density as a function of concentration $x$ . Symbols are data from RBS and thickness measurements (red and blue for $a {-Mn}_{x} {Si}_{1 - x}$ and $a {-Mn}_{x} {Ge}_{1 - x}$ , respectively). The red and blue dotted (dashed) lines are number densities calculated for interstitial (substitutional) Mn in Si and Ge, respectively. For $a {-Mn}_{x} {Si}_{1 - x}$ samples, there is a strong concentration dependence matching closely the expectation for interstitial Mn in Si (with a constant $n_{Si}$ ), while for $a {-Mn}_{x} {Ge}_{1 - x}$ , there is no concentration dependence, suggesting substitutional Mn in Ge.Reuse & Permissions
Figure 5
(Color online) DFT results for magnetic moment distribution $ρ (m)$ . For $a {-Mn}_{x} {Si}_{1 - x}$ , $ρ (m)$ is centered at zero moment; thus the lack of magnetization is due to lack of local moments instead of any AFM coupling. For $a {-Mn}_{x} {Ge}_{1 - x}$ , significant moment distribution is between $1.5 - 3.0 μ_{B}$ with almost no distribution at low moment. Data are not shown for the lowest concentration $x = 0.0156$ due to poor statistics (only 20 data points from one Mn atom in each 20 configurations)Reuse & Permissions
Figure 6
(Color online) Pair correlation functions (PCFs), locally projected density of states (PDOS) on Mn, and statistics of local parameters as a function of the number of nearest neighbors for Mn concentration $x = 0.094$ . (a) Mn-Si (red solid line) and Mn-Ge (blue dashed line) PCFs. Inset: Si-Si (red dotted line) and Ge-Ge (blue dash-dotted line) PCFs. (b) Statistical counts of the 120 Mn atoms in all 20 configurations (red and blue bars for Si and Ge, respectively) and the average Mn-Si (red squares) and Mn-Ge distances (blue circles) $d$ as a function of the number of Si or Ge nearest neighbors $N_{c}$ . (c) Local Mn moment $M$ in each matrix vs $N_{c}$ . (d) PDOS for majority and minority spins of three Mn atoms with different $N_{c}$ in $a {-Mn}_{0.094} {Ge}_{0.906}$ .Reuse & Permissions

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