Mn Acceptor Research Articles

Measuring the hole chemical potential in ferromagnetic Ga1−xMnxAs∕GaAs heterostructures by photoexcited resonant tunneling

The authors investigate the optical and electrical properties of a p-i-n GaAs∕AlAs resonant tunneling diode in which the p-type layer is the ferromagnetic alloy semiconductor Ga1−xMnxAs (x=3%). The high density of Mn acceptors affects significantly the electrostatic potential profile of the heterostructure and inhibits hole tunneling from Ga1−xMnxAs.The authors use photoconductivity to probe this potential and measure the hole chemical potential in Ga1−xMnxAs relative to the band edges of the adjacent undoped GaAs layers.

Magnetization-Switched Metal-Insulator Transition in a (Ga,Mn)As Tunnel Device

We observe the occurrence of an Efros-Shklovskii gap in (Ga,Mn)As based tunnel junctions. The occurrence of the gap is controlled by the extent of the hole wave function on the Mn acceptor atoms. Using k.p-type calculations we show that this extent depends crucially on the direction of the magnetization in the (Ga,Mn)As (which has two almost equivalent easy axes). This implies one can reversibly tune the system into the insulating or metallic state by changing the magnetization.

Electronic conductivity and chemical diffusion in n-conducting barium titanate ceramics at high temperatures

The electronic conductivity as well as the chemical diffusion coefficient of barium titanate ceramics doped with Y and Mn (donor-doped and acceptor co-doped) have been determined by application of conductivity relaxation experiments. The equilibrium values of the electronic conductivity of n-conducting BaTiO 3 have been analyzed by application of a defect chemical model involving electrons and cation vacancies as the predominant defect species at oxidizing conditions (fairly high oxygen partial pressures). The relaxation curves of the electronic conductivity yield the chemical diffusion coefficient of the bulk by employing a spherical grain model where the appropriate diffusion length is the radius of grains (average grain size). The conductivity relaxation experiments have been performed as a function of temperature ranging from 1100 to 1250 °C at oxygen partial pressures between 0.01 and 1 bar. The kinetics of the oxygen exchange process can be interpreted in terms of extremely fast diffusion of oxygen via oxygen vacancies along the grain boundaries and slow diffusion of Ti (cation)-vacancies from the grain boundaries into the grains. The Ti-vacancy diffusion coefficients were extracted from the chemical diffusion coefficients as a function of temperature. Typical values for the Ti-vacancy diffusivity are around 10 − 15 cm 2 s − 1 with an activation energy of 3.9 ± 0.7 eV.

Atom-by-atom substitution of Mn in GaAs and visualization of their hole-mediated interactions

The discovery of ferromagnetism in Mn-doped GaAs has ignited interest in the development of semiconductor technologies based on electron spin and has led to several proof-of-concept spintronic devices. A major hurdle for realistic applications of Ga(1-x)Mn(x)As, or other dilute magnetic semiconductors, remains that their ferromagnetic transition temperature is below room temperature. Enhancing ferromagnetism in semiconductors requires us to understand the mechanisms for interaction between magnetic dopants, such as Mn, and identify the circumstances in which ferromagnetic interactions are maximized. Here we describe an atom-by-atom substitution technique using a scanning tunnelling microscope (STM) and apply it to perform a controlled study at the atomic scale of the interactions between isolated Mn acceptors, which are mediated by holes in GaAs. High-resolution STM measurements are used to visualize the GaAs electronic states that participate in the Mn-Mn interaction and to quantify the interaction strengths as a function of relative position and orientation. Our experimental findings, which can be explained using tight-binding model calculations, reveal a strong dependence of ferromagnetic interaction on crystallographic orientation. This anisotropic interaction can potentially be exploited by growing oriented Ga(1-x)Mn(x)As structures to enhance the ferromagnetic transition temperature beyond that achieved in randomly doped samples.

Electronic States and Spatial Charge Distribution of Single MnImpurity in Diluted Magnetic Semiconductors

The electronic and magnetic properties as well as the spatial charge distributionof single Mn impurity in III–V diluted magnetic semiconductors areobtained when the degeneracy of the p orbits contributed from the fournearest-neighbouring As(N) atoms is taken into account. We show thatin the ground state, the Mn spin is strongly antiferromagneticallycoupled to the surrounding As(N) atoms when the p−d hybridizationVpd is large and both the hole level Ev and the impurity levelEd are close to the Fermi energy. The spatial chargedistribution of the Mn acceptor in the (110) plane is non-sphericallysymmetric, in good agreement with the recent STM images.

Optical characterization of gallium antimonide highly doped with manganese

Abstract Gallium antimonide crystals highly doped with Mn were prepared by a liquid-phase-electroepitaxy growth method. The crystals exhibited high hole concentrations up to 6×10 18 cm −3 . Photoluminescence (PL) and transmission techniques were used for their investigation. Spectral line-shapes typical for highly doped semiconductors were observed. The lines revealed the features corresponding to band gap narrowing and valence-band filling phenomena. Values of the band-gap narrowing Δ E g and the degree of the valence-band filling Δ E F were estimated from the PL spectra. The ionization energy of the Mn acceptor E i was estimated to be approximately 15.1–15.6 meV. At low temperatures, the PL maxima shifted relatively strongly towards higher energy with temperature. The shifts most probably resulted from a dramatic change in the electron density of states near the bottom of the conduction band. The extent of low-energy tails of the PL bands correlates with the doping levels. The transmission spectra exhibited an absorption band centred at around 774–780 meV. The band most probably originated in electron transitions from the level of spin–orbit splitting to the top of the valence band.

Manganese as a fast charge carrier trapping center in InP

Significant influence of Mn centers on the radiative and nonradiative recombinations in Czochralski-grown InP:Mn crystals was observed. Time-resolved measurements showed that manganese recombination centers caused very fast, probably subpicosecond, decay of holes and excitons. This recombination was explained by the capture of holes on an excited state of the Mn acceptor. The holes trapping coefficient R Mn determined on the order of 10 −5 cm 3/s provided an estimation of the Mn cross-section for hole capture σ p of the order of 10 −12 cm 2. Electrical transport measurements showed that hopping conductivity dominated at low temperatures. From analysis of the hopping, the wavefunction radius of Mn-bound hole a f = 0.74 ± 0.1 nm was calculated. A model explaining the values of a f is presented.

Ferromagnetic Properties in Diluted Magnetic Semiconductors (Al,Mn)N grown by PEMBE

We present the structural, magnetic, and electrical properties in the (Al,Mn)N films with various Mn concentrations grown by plasma-enhanced molecular beam epitaxy. X-ray diffraction analyses reveal that the (Al,Mn)N films have the wurtzite structure without secondary phases. All (Al,Mn)N films showed the ferromagnetic ordering. Particularly, (<TEX>$Al_{1-x}Mn_{x}$</TEX>)N film with x = 0.028 exhibited the highest magnetic moment per Mn atom at room temperature. Since all the films exhibit the insulating characteristics, the origin of ferromagnetism in (Al,Mn)N might be attributed to either indirect exchange interaction caused by virtual electron excitations from Mn acceptor level to the valence band within the samples or a percolation of bound magnetic polarons arisen from exchange interaction of localized carriers with magnetic impurities in a low carrier density regime.

Hydrogen and Magnetism in Ga1‐xMnxAs

AbstractFor Abstract see ChemInform Abstract in Full Text.

Manganese on various lattice sites in (Ga,Mn)As investigated using electron paramagnetic resonance

The differences between variously doped and grown samples of manganese-doped GaAs have been studied and set in relation to the magnetic and electronic properties of these materials. For interion exchange and contrary to the resolved hyperfine and crystal field contributions from the ionized acceptor $\mathrm{Mn}_{\mathrm{Ga}}{}^{2+}$ in GaAs:Mn (doped at ${10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$), the spectrum of $\mathrm{Mn}_{\mathrm{Ga}}{}^{2+}$ in (Ga,Mn)As with a dopant concentration of ${10}^{20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ is almost unresolved. In addition, the electron paramagnetic resonance (EPR) spectra of (Ga,Mn)As exhibit no neutral manganese acceptor contribution. The latter is attributed to the strong compensation of low-temperature molecular-beam-epitaxy-grown (Ga,Mn)As, which is in part caused by interstitial manganese $({\mathrm{Mn}}_{\mathrm{I}})$. Contrary to Mn acceptors on Ga lattice sites, ${\mathrm{Mn}}_{\mathrm{I}}$ acts as a double donor. As the Curie temperature of (Ga,Mn)As films depends on the hole concentration, the compensation of these films also affects their magnetic properties. A comparison between the spectra corresponding to ${\mathrm{Mn}}_{\mathrm{I}}$ inside bulk-doped GaAs:Mn and inside epitaxial (Ga,Mn)As results in the identification of ${\mathrm{Mn}}_{\mathrm{I}}$ in (Ga,Mn)As at dopant concentrations as low as 0.5%. In this way EPR facilitates the identification of interstitial $({\mathrm{Mn}}_{\mathrm{I}})$ and the investigation of ionization and exchange of substitutional $({\mathrm{Mn}}_{\mathrm{Ga}})$ manganese in (Ga,Mn)As.

Analysis of magnetic field dependent mobility in ferromagnetic Ga 1− xMn xAs layers

The magneto-transport properties of ferromagnetic Ga 1− x Mn x As epilayers with Mn mole fractions in the range of x≈2.2–4.4% were investigated through Hall effect measurements. The magnetic field-dependent Hall mobility for a metallic sample with x≈2.2% in the temperature range of T=0–300 K was analyzed by magnetic field-dependent mobility model including an activation energy of Mn acceptor level. This model provides outstanding fits to the measured data up to T=300 K. It was found that the acceptor levels with activation energies of 112 meV at B=0 Oe decreased to 99 meV at B=5 kOe in the ferromagnetic region. The decrease in acceptor activation energy was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for increase in carrier concentration.

Mn Impurity in Bulk GaAs Crystals

Magnetic and electron transport properties of GaAs:Mn crystals grown by Czochralski method were studied. Electron spin resonance showed the presence of Mn acceptor A in two charge states: singly ionized A i in the form of Mn 2+ (d 5 ), and neutral A 0 in the form of Mn 2+ (d 5 ) plus a bound hole (h). It was possible to determine the relative concentration of both types of centers from intensity of the corresponding electron spin resonance lines. Magnetization measured as a function of magnetic fleld (up to 6 T) in the temperature range of 2{300 K revealed overall paramagnetic behavior of the samples. Efiective spin was found to be about 1.5 value, which was consistent with the presence of two types of Mn conflgurations. In most of the studied samples the dominance of Mn 2+ (d 5 ) + h conflguration was established and it increased after annealing of native donors. The total value of Mn content was obtained from fltting of magnetization curves with the use of parameters obtained from electron spin resonance. In electron transport, two mechanisms of conductivity were observed: valence band transport dominated above 70 K, and hopping conductivity within Mn impurity band at lower temperatures. From the analysis of the hopping conductivity and using the obtained values of the total Mn content, the efiective radius of Mn acceptor in GaAs was estimated as a = 11 § 3 ”

Infrared Spectroscopy of GaAs Doped with Mn

We found that the fine structure related to Lyman spectra of [Mn(d) + a hole] centers in GaAs was present only for samples with low Mn concentration. Such samples, at low temperature, did not show any hopping conductance within Mn impurity band. Magnetooptical measurements revealed that magnetic field induced splitting of the Lyman optical transitions was larger than Zeeman splitting observed for typical shallow acceptors in GaAs, like Be, Zn, and C. This experimental result proved that in the case of Mn acceptor impurity, the exchange coupling of a hole and the S = 5/2 Mn(d) core could not be neglected, which was in accordance with the [Mn(d) + a hole] model of the neutral Mn center in GaAs.

Interstitial manganese in (Ga,Mn)As detected by electron paramagnetic resonance

Various types of Mn in epitaxially grown (Ga,Mn)As have been investigated. Ionized interstitial manganese donors Mn I 2 + have been found to occur in (Ga,Mn)As at dopant concentrations as low as 0.5%. A comparison with spectra from interstitial Mn I 2 + inside bulk-doped GaAs:Mn yields a slight decrease of 1.5% in the hyperfine splitting with increasing dopant concentration. This is attributed to an increase in the lattice constant of (Ga,Mn)As with increasing manganese concentration. Contrary to Mn interstitials, Mn dopants on Ga lattice sites acts as acceptors. It is shown that Mn on Ga lattice sites Mn Ga 2 + is non-uniformly distributed. A low percentage of isolated Mn acceptors can be distinguished from the exchange broadened Mn Ga 2 + signal. Thus, electron paramagnetic resonance is a promising tool for the investigation of (Ga,Mn)As and the classification of various types of Mn dopants responsible for the magnetic properties of (Ga,Mn)As.

Mn-impurity-conduction-mediated ferromagnetic ordering in Ga 1− xMn xAs ( x = 0.034 and x = 0.050)

The conduction mechanism expected for a diluted magnetic semiconductor, Ga 1− x Mn x As ( x = 0.034 and x = 0.050), is presented. Infrared (IR) and far-infrared (FIR) spectroscopy reveal that the free carrier absorption spectrum cannot be observed, and that the broad absorption peak observed at 2000 cm −1 is ascribable to the optical transition from the ground states of the isolated acceptor, acceptor pair, triplet and cluster to their excited states and to the valence bands. The absorption peak exhibits Stokes shift from 2100 to 1900 cm −1 with decreasing temperature below the ferromagnetic ordering temperature T C = 43 K. These phenomena indicate that holes are redistributed in the spin splitting states leading to magnetization. Since Mn acceptors are in a random system, holes are weakly localized at Mn-sites and hop in the acceptor complexes. The absorption coefficient integrated from 50 to 150 cm −1 increases with decreasing temperature from T C and with increasing external magnetic field. These phenomena have been explained in terms of correlated many-hole hopping (CMHH). That is, the absorption is proportional to the number of holes which simultaneously hop in the Mn acceptor states. CMHH conduction depends mainly on the exchange energy between the hole spins and localized Mn spins. In addition, the many-body effects of CMHH conduction have been confirmed by IR and FIR reflection spectroscopy and analysis of the spectra. The effective mass ratio enhancement has been found to be as large as 1.7 at 10 K below T C. Thus, the authors have concluded that CMHH conduction mediates the ferromagnetic ordering in Ga 1− x Mn x As ( x = 0.034 and x = 0.050).

Molecular-beam epitaxial growth and characterization of (In0.5Al0.5)1−xMnxAs-(In0.5Ga0.5)1−xMnxAs: Thin films and superlattices

We describe the growth and properties of (In0.5Al0.5)1−xMnxAs and (In0.5Ga0.5)1−xMnxAs epilayers and superlattices. We find that the structural quality of the epilayers is similar to that of the more extensively studied In1−xMnxAs and Ga1−xMnxAs magnetic semiconductors and that we can incorporate significantly larger amounts of Mn (∼12%) without phase segregation. Magnetization measurements indicate that the Curie temperatures of (In0.5Ga0.5)1−xMnxAs and (In0.5Al0.5)1−xMnxAs(x∼0.11) epilayers are 95 and 25K, respectively. The Curie temperature of the (In0.5Ga0.5)1−xMnxAs∕(In0.5Al0.5)1−xMnxAs superlattices decreases with the increase of the Al∕(Al+Ga) ratio. We attribute this to a decreased overlap between the impurity band and the valence band because of an enhanced Mn acceptor activation energy.

Polarization of Valence Band Holes in the (Ga,Mn)As Diluted Magnetic Semiconductor

We report on direct measurements of the impurity band hole polarization in the diluted magnetic semiconductor (Ga,Mn)As. The polarization of impurity band holes in a magnetic field is strongly enhanced by antiferromagnetic exchange interaction with Mn ions. The temperature dependence of the hole polarization shows a strong increase of this polarization below the Curie temperature. We show that the ground state of the impurity band is formed by uniaxial stress split F=+/-1 states of antiferromagnetically coupled Mn ions (S=5/2) and valence band holes (J=3/2). The gap between the Mn acceptor related impurity band and the valence band is directly measured in a wide range of Mn content.

Voltage control of the magnetic properties of charged semiconductor quantum dots containing magnetic ions

We describe a model device allowing voltage control of the magnetic properties of magnetic ions in III-V self-assembled semiconductor quantum dots. The applied voltage, combined with Coulomb blockade, allows the control of the number of holes in the quantum dot. The spins of the holes interact with the spins of the magnetic ions via $\mathit{sp}\text{\ensuremath{-}}d$ exchange interactions. The spectrum of a Mn ion in a $p$-type InAs quantum disk in a magnetic field is calculated as a function of the number of holes described by the Luttinger-Kohn Hamiltonian. For a neutral Mn acceptor, the spin of the hole leads to an effective magnetic field which strongly modifies the magnetization of the ion. The magnetization can be modified further by charging the dot with an additional hole. The interacting holes form a singlet parity ground state, suppress the effective field and modify the magnetic moment of the charged complex.

Magnetotransport properties of ferromagnetic Ga1−xMnxAs layers on a (100) GaAs substrate

The magnetotransport properties of ferromagnetic Ga1−xMnxAs epilayers with Mn mole fractions in the range of x≈2.2%–4.4% were investigated using Hall effect measurements. The temperature-dependent Hall carrier concentration for a metallic sample with x≈2.2% was analyzed assuming an activation energy from two acceptor levels. It was found that the two acceptor levels with activation energies of 129.4 and 31.6 meV at B=0Oe decreased to 87.6 and 30.7 meV, respectively, at B=5kOe. The decrease in acceptor activation energy from 129.6 to 87.6 meV was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for the increase in carrier concentration. From magnetic-field-dependent Hall resistance data, the Curie temperature was estimated to be TC=60 and 70 K for Ga1−xMnxAs samples with x≈2.2 and x≈4.4%, respectively. The magnetoresistance measurements confirmed that the anomalous Hall effect existed in these samples that showed metallic and insulating behavior, respectively.

The Special Features of the Hall Effect in GaMnSb Layers Deposited from a Laser Plasma

Epitaxial GaMnSb films with Mn contents up to about 10 at. % were obtained by deposition from a laser plasma in vacuum. The growth temperature T s during deposition was varied from 440 to 200°C, which changed the concentration of holes from 3 × 1019 to 5 × 1020 cm−3, respectively. Structure studies showed that, apart from Mn ions substituting Ga, the GaMnSb layers contained ferromagnetic clusters with Mn and shallow acceptor defects of the GaSb type controlled by the T s value. Unlike single-phase GaMnSb systems studied earlier with negative anomalous Hall effect values and Curie temperatures not exceeding 30 K, the films obtained in this work exhibited a positive anomalous Hall effect, whose hysteresis character manifested itself up to room temperature and was the more substantial the higher the concentration of holes. The unusual behavior of this effect was interpreted in terms of the interaction of charge carriers with ferromagnetic clusters, which was to a substantial extent determined by the presence of Schottky barriers at the boundary between the clusters and the semiconducting matrix; this interaction increased as the concentration of holes grew. The absence of this effect in semiconducting compounds based on III–V Group elements with MnSb or MnAs ferromagnetic clusters was discussed in the literature; we showed that this absence was most likely related to the low hole concentrations in these objects.